Free Lossless Audio Codec
draft-ietf-cellar-flac-03
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Active".
|
|
|---|---|---|---|
| Authors | Martijn van Beurden , Andrew Weaver | ||
| Last updated | 2022-04-23 (Latest revision 2021-11-01) | ||
| Replaces | draft-weaver-cellar-flac | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-11)
by Reese Enghardt
Ready w/nits
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Associated WG milestone |
|
||
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-cellar-flac-03
cellar M.Q.C. van Beurden
Internet-Draft
Intended status: Standards Track A. Weaver
Expires: 25 October 2022 23 April 2022
Free Lossless Audio Codec
draft-ietf-cellar-flac-03
Abstract
This document defines the Free Lossless Audio Codec (FLAC) format.
FLAC is designed to reduce the amount of computer storage space
needed to store digital audio signals without needing to remove
information in doing so (i.e. lossless). FLAC is free in the sense
that its specification is open, its reference implementation is open-
source and it is not encumbered by any known patent. Compared to
other lossless (audio) coding formats, FLAC is a format with low
complexity and can be coded to and from with little computing
resources. Decoding of FLAC has seen many independent
implementations on many different platforms, and both encoding and
decoding can be implemented without needing floating-point
arithmetic.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 25 October 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
van Beurden & Weaver Expires 25 October 2022 [Page 1]
Internet-Draft FLAC April 2022
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Notation and Conventions . . . . . . . . . . . . . . . . . . 4
3. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 4
4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Conceptual overview . . . . . . . . . . . . . . . . . . . . . 7
5.1. Blocking . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Interchannel Decorrelation . . . . . . . . . . . . . . . 8
5.3. Prediction . . . . . . . . . . . . . . . . . . . . . . . 9
5.4. Residual Coding . . . . . . . . . . . . . . . . . . . . . 10
6. Format principles . . . . . . . . . . . . . . . . . . . . . . 11
7. Format lay-out . . . . . . . . . . . . . . . . . . . . . . . 12
8. Format subset . . . . . . . . . . . . . . . . . . . . . . . . 15
9. File-level metadata . . . . . . . . . . . . . . . . . . . . . 15
9.1. Metadata block header . . . . . . . . . . . . . . . . . . 16
9.2. Streaminfo . . . . . . . . . . . . . . . . . . . . . . . 16
9.3. Padding . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.4. Application . . . . . . . . . . . . . . . . . . . . . . . 18
9.5. Seektable . . . . . . . . . . . . . . . . . . . . . . . . 19
9.5.1. Seekpoint . . . . . . . . . . . . . . . . . . . . . . 19
9.6. Vorbis comment . . . . . . . . . . . . . . . . . . . . . 20
9.6.1. Standard field names . . . . . . . . . . . . . . . . 20
9.6.2. Channel mask . . . . . . . . . . . . . . . . . . . . 21
9.7. Cuesheet . . . . . . . . . . . . . . . . . . . . . . . . 23
9.7.1. Cuesheet track . . . . . . . . . . . . . . . . . . . 25
9.8. Picture . . . . . . . . . . . . . . . . . . . . . . . . . 26
10. Frame structure . . . . . . . . . . . . . . . . . . . . . . . 29
10.1. Frame header . . . . . . . . . . . . . . . . . . . . . . 29
10.1.1. Blocksize bits . . . . . . . . . . . . . . . . . . . 29
10.1.2. Sample rate bits . . . . . . . . . . . . . . . . . . 30
10.1.3. Channels bits . . . . . . . . . . . . . . . . . . . 31
10.1.4. Bit depth bits . . . . . . . . . . . . . . . . . . . 32
10.1.5. Coded number . . . . . . . . . . . . . . . . . . . . 32
10.1.6. Uncommon blocksize . . . . . . . . . . . . . . . . . 33
10.1.7. Uncommon sample rate . . . . . . . . . . . . . . . . 33
10.1.8. Frame header CRC . . . . . . . . . . . . . . . . . . 33
10.2. Subframes . . . . . . . . . . . . . . . . . . . . . . . 33
10.2.1. Subframe header . . . . . . . . . . . . . . . . . . 33
van Beurden & Weaver Expires 25 October 2022 [Page 2]
Internet-Draft FLAC April 2022
10.2.2. Wasted bits per sample . . . . . . . . . . . . . . . 34
10.2.3. Constant subframe . . . . . . . . . . . . . . . . . 35
10.2.4. Verbatim subframe . . . . . . . . . . . . . . . . . 35
10.2.5. Fixed predictor subframe . . . . . . . . . . . . . . 35
10.2.6. Linear predictor subframe . . . . . . . . . . . . . 37
10.2.7. Coded residual . . . . . . . . . . . . . . . . . . . 38
10.3. Frame footer . . . . . . . . . . . . . . . . . . . . . . 40
11. Implementation status . . . . . . . . . . . . . . . . . . . . 40
12. Security Considerations . . . . . . . . . . . . . . . . . . . 41
13. Normative References . . . . . . . . . . . . . . . . . . . . 41
14. Informative References . . . . . . . . . . . . . . . . . . . 42
Appendix A. Numerical considerations . . . . . . . . . . . . . . 43
A.1. Determining necessary data type size . . . . . . . . . . 43
A.2. Stereo decorrelation . . . . . . . . . . . . . . . . . . 44
A.3. Prediction . . . . . . . . . . . . . . . . . . . . . . . 44
A.4. Rice coding . . . . . . . . . . . . . . . . . . . . . . . 46
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 46
B.1. Decoding example 1 . . . . . . . . . . . . . . . . . . . 47
B.1.1. Example file 1 in hexadecimal representation . . . . 47
B.1.2. Example file 1 in binary representation . . . . . . . 47
B.1.3. Signature and streaminfo . . . . . . . . . . . . . . 47
B.1.4. Audio frames . . . . . . . . . . . . . . . . . . . . 49
B.2. Decoding example 2 . . . . . . . . . . . . . . . . . . . 51
B.2.1. Example file 2 in hexadecimal representation . . . . 51
B.2.2. Example file 2 in binary representation (only audio
frames) . . . . . . . . . . . . . . . . . . . . . . . 52
B.2.3. Signature and streaminfo . . . . . . . . . . . . . . 53
B.2.4. Seektable . . . . . . . . . . . . . . . . . . . . . . 53
B.2.5. Vorbis comment . . . . . . . . . . . . . . . . . . . 54
B.2.6. Padding . . . . . . . . . . . . . . . . . . . . . . . 55
B.2.7. First audio frame . . . . . . . . . . . . . . . . . . 56
B.2.8. Second audio frame . . . . . . . . . . . . . . . . . 62
B.2.9. MD5 checksum verification . . . . . . . . . . . . . . 64
B.3. Decoding example 3 . . . . . . . . . . . . . . . . . . . 64
B.3.1. Example file 3 in hexadecimal representation . . . . 64
B.3.2. Example file 3 in binary representation (only audio
frame) . . . . . . . . . . . . . . . . . . . . . . . 64
B.3.3. Signature and streaminfo . . . . . . . . . . . . . . 64
B.3.4. Audio frame . . . . . . . . . . . . . . . . . . . . . 65
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 70
van Beurden & Weaver Expires 25 October 2022 [Page 3]
Internet-Draft FLAC April 2022
1. Introduction
This document defines the FLAC format. FLAC files and streams can
code for pulse-code modulated (PCM) audio with 1 to 8 channels,
sample rates from 1 to 1048576 Hertz and bit depths between 4 and 32
bits. Most tools for coding to and decoding from the FLAC format
have been optimized for CD-audio, which is PCM audio with 2 channels,
a sample rate of 44.1 kHz and a bit depth of 16 bits.
FLAC is able to achieve lossless compression because samples in audio
signals tend to be highly correlated with their close neighbors. In
contrast with general purpose compressors, which often use
dictionaries, do run-length coding or exploit long-term repetition,
FLAC removes redundancy solely in the very short term, looking back
at most 32 samples.
The coding methods provided by the FLAC format work best on PCM audio
signals of which the samples have a signed representation and are
centered around zero. Audio signals in which samples have an
unsigned representation must be transformed to a signed
representation as described in this document in order to achieve
reasonable compression. The FLAC format is not suited to compress
audio that is not PCM. Pulse-density modulated audio, e.g. DSD,
cannot be compressed by FLAC.
2. Notation and Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Values expressed as u(n) represent unsigned big-endian integer using
n bits. Values expressed as s(n) represent signed big-endian integer
using n bits, signed two's complement. n may be expressed as an
equation using * (multiplication), / (division), + (addition), or -
(subtraction). An inclusive range of the number of bits expressed
may be represented with an ellipsis, such as u(m...n). The name of a
value followed by an asterisk * indicates zero or more occurrences of
the value. The name of a value followed by a plus sign + indicates
one or more occurrences of the value.
3. Acknowledgments
FLAC owes much to the many people who have advanced the audio
compression field so freely. For instance:
van Beurden & Weaver Expires 25 October 2022 [Page 4]
Internet-Draft FLAC April 2022
* A. J. Robinson (https://web.archive.org/web/20160315141134/
http://mi.eng.cam.ac.uk/~ajr/) for his work on Shorten; his paper
([robinson-tr156]) is a good starting point on some of the basic
methods used by FLAC. FLAC trivially extends and improves the
fixed predictors, LPC coefficient quantization, and Rice coding
used in Shorten.
* S. W. Golomb
(https://web.archive.org/web/20040215005354/http://csi.usc.edu/
faculty/golomb.html) and Robert F. Rice; their universal codes
are used by FLAC's entropy coder.
* N. Levinson and J. Durbin; the reference encoder uses an
algorithm developed and refined by them for determining the LPC
coefficients from the autocorrelation coefficients.
* And of course, Claude Shannon (https://en.wikipedia.org/wiki/
Claude_Shannon)
4. Definitions
* *Lossless compression*: reducing the amount of computer storage
space needed to store data without needing to remove or
irreversibly alter any of this data in doing so. In other words,
decompressing losslessly compressed information returns exactly
the original data.
* *Lossy compression*: like lossless compression, but instead
removing, irreversibly altering or only approximating information
for the purpose of further reducing the amount of computer storage
space needed. In other words, decompressing lossy compressed
information returns an approximation of the original data.
* *Block*: A (short) section of linear pulse-code modulated audio,
with one or more channels.
* *Subblock*: All samples within a corresponding block for 1
channel. One or more subblocks form a block, and all subblocks in
a certain block contain the same number of samples.
* *Frame*: A frame header plus one or more subframes. It encodes
the contents of a corresponding block.
* *Subframe*: An encoded subblock. All subframes within a frame
code for the same number of samples. A subframe MAY correspond to
a subblock, else it corresponds to either the addition or
subtraction of two subblocks, see section on interchannel
decorrelation (#interchannel-decorrelation).
van Beurden & Weaver Expires 25 October 2022 [Page 5]
Internet-Draft FLAC April 2022
* *Blocksize*: The total number of samples contained in a block or
coded in a frame, divided by the number of channels. In other
words, the number of samples in any subblock of a block, or any
subframe of a frame. This is also called *interchannel samples*.
* *Bit depth* or *bits per sample*: the number of bits used to
contain each sample. This MUST be the same for all subblocks in a
block but MAY be different for different subframes in a frame
because of interchannel decorrelation (#interchannel-
decorrelation).
* *Predictor*: a model used to predict samples in an audio signal
based on past samples. FLAC uses such predictors to remove
redundancy in a signal in order to be able to compress it.
* *Linear predictor*: a predictor using linear prediction
(https://en.wikipedia.org/wiki/Linear_prediction). This is also
called *linear predictive coding (LPC)*. With a linear predictor
each prediction is a linear combination of past samples, hence the
name. A linear predictor has a causal discrete-time finite
impulse response (https://en.wikipedia.org/wiki/
Finite_impulse_response).
* *Fixed predictor*: a linear predictor in which the model
parameters are the same across all FLAC files, and thus not need
to be stored.
* *Predictor order*: the number of past samples that a predictor
uses. For example, a 4th order predictor uses the 4 samples
directly preceding a certain sample to predict it. In FLAC,
samples used in a predictor are always consecutive, and are always
the samples directly before the sample that is being predicted
* *Residual*: The audio signal that remains after a predictor has
been subtracted from a subblock. If the predictor has been able
to remove redundancy from the signal, the samples of the remaining
signal (the *residual samples*) will have, on average, a smaller
numerical value than the original signal.
* *Rice code*: A variable-length code
(https://en.wikipedia.org/wiki/Variable-length_code) which
compresses data by making use of the observation that, after using
an effective predictor, most residual samples are closer to zero
than the original samples, while still allowing for a small part
of the samples to be much larger.
van Beurden & Weaver Expires 25 October 2022 [Page 6]
Internet-Draft FLAC April 2022
5. Conceptual overview
Similar to many audio coders, a FLAC file is encoded following the
steps below. On decoding a FLAC file, these steps are undone in
reverse order, i.e. from bottom to top.
* Blocking (see section on Blocking (#blocking)). The input is
split up into many contiguous blocks. With FLAC, the blocks MAY
vary in size. The optimal size of the block is usually affected
by many factors, including the sample rate, spectral
characteristics over time, etc. However, as finding the optimal
block size arrangement is a rather complex problem, the FLAC
format allows for a constant block size throughout a stream as
well.
* Interchannel Decorrelation (see section on Interchannel
Decorrelation (#interchannel-decorrelation)). In the case of
stereo streams, the FLAC format allows for transforming the left-
right signal into a mid-side signal to remove redundancy, if there
is any. Besides coding as left-right and mid-side, it is also
possible to code left-side and side-right, whichever ordering
results in the highest compression. Choosing between any of these
transformation is done independently for each block.
* Prediction (see section on Prediction (#prediction)). To remove
redundancy in a signal, a predictor is stored for each subblock or
its transformation as formed in the previous step. A predictor
consists of a simple mathematical description that can be used, as
the name implies, to predict a certain sample from the samples
that preceded it. As this prediction is rarely exact, the error
of this prediction is passed to the next stage. The predictor of
each subblock is completely independent from other subblocks.
Since the methods of prediction are known to both the encoder and
decoder, only the parameters of the predictor need be included in
the compressed stream. In case no usable predictor can be found
for a certain subblock, the signal is stored instead of compressed
and the next stage is skipped.
* Residual Coding (See section on Residual Coding (#residual-
coding)). As the predictor does not describe the signal exactly,
the difference between the original signal and the predicted
signal (called the error or residual signal) MUST be coded
losslessly. If the predictor is effective, the residual signal
will require fewer bits per sample than the original signal. FLAC
uses Rice coding, a subset of Golomb coding, with either 4-bit or
5-bit parameters to code the residual signal.
van Beurden & Weaver Expires 25 October 2022 [Page 7]
Internet-Draft FLAC April 2022
In addition, FLAC specifies a metadata system (see section on File-
level metadata (#file-level-metadata)), which allows arbitrary
information about the stream to be included at the beginning of the
stream.
5.1. Blocking
The size used for blocking the audio data has a direct effect on the
compression ratio. If the block size is too small, the resulting
large number of frames mean that excess bits will be wasted on frame
headers. If the block size is too large, the characteristics of the
signal may vary so much that the encoder will be unable to find a
good predictor. In order to simplify encoder/decoder design, FLAC
imposes a minimum block size of 16 samples, and a maximum block size
of 65535 samples. This range covers the optimal size for all of the
audio data FLAC supports.
While the block size MAY vary in a FLAC file, it is often difficult
to find the optimal arrangement of block sizes for maximum
compression. Because of this the FLAC format explicitly stores
whether a file has a constant or a variable blocksize throughout the
stream, and stores a block number instead of a sample number to
slighly improve compression in case a stream has a constant block
size.
Blocked data is passed to the predictor stage one subblock at a time.
Each subblock is independently coded into a subframe, and the
subframes are concatenated into a frame. Because each channel is
coded separately, subframes MAY use different predictors, even within
a frame.
5.2. Interchannel Decorrelation
In many audio files, channels are correlated. The FLAC format can
exploit this correlation in stereo files by not directly coding
subblocks into subframes, but instead coding an average of all
samples in both subblocks (a mid channel) or the difference between
all samples in both subblocks (a side channel). The following
combinations are possible:
* *Independent*. All channels are coded independently. All non-
stereo files MUST be encoded this way.
* *Mid-side*. A left and right subblock are converted to mid and
side subframes. To calculate a sample for a mid subframe, the
corresponding left and right samples are summed and the result is
shifted right by 1 bit. To calculate a sample for a side
subframe, the corresponding right sample is subtracted from the
van Beurden & Weaver Expires 25 October 2022 [Page 8]
Internet-Draft FLAC April 2022
corresponding left sample. On decoding, the mid channel has to be
shifted left by 1 bit. Also, if the side channel is uneven, 1 has
to be added to the mid channel after the left shift. To
reconstruct the left channel, the corresponding samples in the mid
and side subframes are added and the result shifted right by 1
bit, while for the right channel the side channel has to be
subtracted from the mid channel and the result shifted right by 1
bit.
* *Left-side*. The left subblock is coded and the left and right
subblock are used to code a side subframe. The side subframe is
constructed in the same way as for mid-side. To decode, the right
subblock is restored by subtracting the samples in the side
subframe from the corresponding samples the left subframe.
* *Right-side*. The right subblock is coded and the left and right
subblock are used to code a side subframe. Note that the actual
coded subframe order is side-right. The side subframe is
constructed in the same way as for mid-side. To decode, the left
subblock is restored by adding the samples in the side subframe to
the corresponding samples in the right subframe.
The side channel needs one extra bit of bit depth as the subtraction
can produce sample values twice as large as the maximum possible in
any given bit depth. The mid channel in mid-side stereo does not
need one extra bit, as it is shifted right one bit. The right shift
of the mid channel does not lead to non-lossless behavior, because an
uneven sample in the mid subframe must always be accompanied by a
corresponding uneven sample in the side subframe, which means the
lost least significant bit can be restored by taking it from the
sample in the side subframe.
5.3. Prediction
The FLAC format has four methods for modeling the input signal:
1. *Verbatim*. Samples are stored directly, without any modelling.
This method is used for inputs with little correlation like white
noise. Since the raw signal is not actually passed through the
residual coding stage (it is added to the stream 'verbatim'), the
method is different from using a zero-order fixed predictor.
2. *Constant*. A single sample value is stored. This method is used
whenever a signal is pure DC ("digital silence"), i.e. a constant
value throughout.
van Beurden & Weaver Expires 25 October 2022 [Page 9]
Internet-Draft FLAC April 2022
3. *Fixed predictor*. Samples are predicted with one of five fixed
(i.e. predefined) predictors, the error of this prediction is
processed by the residual coder. These fixed predictors are well
suited for predicting simple waveforms. Since the predictors are
fixed, no predictor coefficients are stored. From a mathematical
point of view, the predictors work by extrapolating the signal
from the previous samples. The number of previous samples used
is equal to the predictor order. For more information see the
section on the fixed predictor subframe (#fixed-predictor-
subframe)
4. *Linear predictor*. Samples are predicted using past samples and
a set of predictor coefficients, the error of this prediction is
processed by the residual coder. Compared to a fixed predictor,
using a generic linear predictor adds overhead as predictor
coefficients need to be stored. Therefore, this method of
prediction is best suited for predicting more complex waveforms,
where the added overhead is offset by space savings in the
residual coding stage resulting from more accurate prediction. A
linear predictor in FLAC has two parameters besides the predictor
coefficients and the predictor order: the number of bits with
which each coefficient is stored (the coefficient precision) and
a prediction right shift. A prediction is formed by taking the
sum of multiplying each predictor coefficient with the
corresponding past sample, and dividing that sum by applying the
specified right shift. For more information see the section on
the linear predictor subframe (#linear-predictor-subframe)
For more information on fixed and linear predictors, see
[HPL-1999-144] and [robinson-tr156].
5.4. Residual Coding
In case a subframe uses a predictor to approximate the audio signal,
a residual needs to be stored to 'correct' the approximation to the
exact value. When an effective predictor is used, the average
numerical value of the residual samples is smaller than that of the
samples before prediction. While having smaller values on average,
it is possible a few 'outlier' residual samples are much larger than
any of the original samples. Sometimes these outliers even exceed
the range the bit depth of the original audio offers.
To be able to efficiently code such a stream of relatively small
numbers with an occasional outlier, Rice coding (a subset of Golomb
coding) is used. Depending on how small the numbers are that have to
be coded, a Rice parameter is chosen. The numerical value of each
residual sample is split in two parts by dividing it with 2^(Rice
parameter), creating a quotient and a remainder. The quotient is
van Beurden & Weaver Expires 25 October 2022 [Page 10]
Internet-Draft FLAC April 2022
stored in unary form, the remainder in binary form. If indeed most
residual samples are close to zero and the Rice parameter is chosen
right, this form of coding, a so-called variable-length code, needs
less bits to store than storing the residual in unencoded form.
As Rice codes can only handle unsigned numbers, signed numbers are
zigzag encoded to a so-called folded residual. For more information
see section coded residual (#coded-residual) for a more thorough
explanation.
Quite often the optimal Rice parameter varies over the course of a
subframe. To accommodate this, the residual can be split up into
partitions, where each partition has its own Rice parameter. To keep
overhead and complexity low, the number of partitions used in a
subframe is limited to powers of two.
The FLAC format uses two forms of Rice coding, which only differ in
the number of bits used for encoding the Rice parameter, either 4 or
5 bits.
6. Format principles
FLAC has no format version information, but it does contain reserved
space in several places. Future versions of the format MAY use this
reserved space safely without breaking the format of older streams.
Older decoders MAY choose to abort decoding or skip data encoded with
newer methods. Apart from reserved patterns, in places the format
specifies invalid patterns, meaning that the patterns MAY never
appear in any valid bitstream, in any prior, present, or future
versions of the format. These invalid patterns are usually used to
make the synchronization mechanism more robust.
All numbers used in a FLAC bitstream MUST be integers; there are no
floating-point representations. All numbers MUST be big-endian
coded, except the length field used in Vorbis comments, which MUST be
little-endian coded. All numbers MUST be unsigned except linear
predictor coefficients, the linear prediction shift and numbers which
directly represent samples, which MUST be signed. None of these
restrictions apply to application metadata blocks.
All samples encoded to and decoded from the FLAC format MUST be in a
signed representation.
There are several ways to convert unsigned sample representations to
signed sample representations, but the coding methods provided by the
FLAC format work best on audio signals of which the numerical values
of the samples are centered around zero, i.e. have no DC offset. In
most unsigned audio formats, signals are centered around halfway the
van Beurden & Weaver Expires 25 October 2022 [Page 11]
Internet-Draft FLAC April 2022
range of the unsigned integer type used. If that is the case, all
sample representations SHOULD be converted by first copying the
number to a signed integer with sufficient range and then subtracting
half of the range of the unsigned integer type, which should result
in a signal with samples centered around 0.
7. Format lay-out
Before the formal description of the stream, an overview of the lay-
out of FLAC file might be helpful.
* A FLAC bitstream consists of the "fLaC" (i.e. 0x664C6143) marker
at the beginning of the stream, followed by a mandatory metadata
block (called the STREAMINFO block), any number of other metadata
blocks, then the audio frames.
* FLAC supports up to 128 kinds of metadata blocks; currently the
following are defined:
- STREAMINFO: This block has information about the whole stream,
like sample rate, number of channels, total number of samples,
etc. It MUST be present as the first metadata block in the
stream. Other metadata blocks MAY follow, and ones that the
decoder doesn't understand, it will skip.
- PADDING: This block allows for an arbitrary amount of padding.
The contents of a PADDING block have no meaning. This block is
useful when it is known that metadata will be edited after
encoding; the user can instruct the encoder to reserve a
PADDING block of sufficient size so that when metadata is
added, it will simply overwrite the padding (which is
relatively quick) instead of having to insert it into the right
place in the existing file (which would normally require
rewriting the entire file).
- APPLICATION: This block is for use by third-party applications.
The only mandatory field is a 32-bit identifier. This ID is
granted upon request to an application by the FLAC maintainers.
The remainder is of the block is defined by the registered
application. Visit the registration page
(https://xiph.org/flac/id.html) if you would like to register
an ID for your application with FLAC.
- SEEKTABLE: This is an OPTIONAL block for storing seek points.
It is possible to seek to any given sample in a FLAC stream
without a seek table, but the delay can be unpredictable since
the bitrate MAY vary widely within a stream. By adding seek
points to a stream, this delay can be significantly reduced.
van Beurden & Weaver Expires 25 October 2022 [Page 12]
Internet-Draft FLAC April 2022
Each seek point takes 18 bytes, so 1% resolution within a
stream adds less than 2K. There can be only one SEEKTABLE in a
stream, but the table can have any number of seek points.
There is also a special 'placeholder' seekpoint which will be
ignored by decoders but which can be used to reserve space for
future seek point insertion.
- VORBIS_COMMENT: This block is for storing a list of human-
readable name/value pairs. Values are encoded using UTF-8. It
is an implementation of the Vorbis comment specification
(http://xiph.org/vorbis/doc/v-comment.html) (without the
framing bit). This is the only officially supported tagging
mechanism in FLAC. There MUST be only zero or one
VORBIS_COMMENT blocks in a stream. In some external
documentation, Vorbis comments are called FLAC tags to lessen
confusion.
- CUESHEET: This block is for storing various information that
can be used in a cue sheet. It supports track and index
points, compatible with Red Book CD digital audio discs, as
well as other CD-DA metadata such as media catalog number and
track ISRCs. The CUESHEET block is especially useful for
backing up CD-DA discs, but it can be used as a general purpose
cueing mechanism for playback.
- PICTURE: This block is for storing pictures associated with the
file, most commonly cover art from CDs. There MAY be more than
one PICTURE block in a file. The picture format is similar to
the APIC frame in ID3v2 (http://www.id3.org/id3v2.4.0-frames).
The PICTURE block has a type, MIME type, and UTF-8 description
like ID3v2, and supports external linking via URL (though this
is discouraged). The differences are that there is no
uniqueness constraint on the description field, and the MIME
type is mandatory. The FLAC PICTURE block also includes the
resolution, color depth, and palette size so that the client
can search for a suitable picture without having to scan them
all.
van Beurden & Weaver Expires 25 October 2022 [Page 13]
Internet-Draft FLAC April 2022
* The audio data is composed of one or more audio frames. Each
frame consists of a frame header, which contains a sync code,
information about the frame like the block size, sample rate,
number of channels, et cetera, and an 8-bit CRC. The frame header
also contains either the sample number of the first sample in the
frame (for variable-blocksize streams), or the frame number (for
fixed-blocksize streams). This allows for fast, sample-accurate
seeking to be performed. Following the frame header are encoded
subframes, one for each channel, and finally, the frame is zero-
padded to a byte boundary. Each subframe has its own header that
specifies how the subframe is encoded.
* Since a decoder MAY start decoding in the middle of a stream,
there MUST be a method to determine the start of a frame. A
14-bit sync code begins each frame. The sync code will not appear
anywhere else in the frame header. However, since it MAY appear
in the subframes, the decoder has two other ways of ensuring a
correct sync. The first is to check that the rest of the frame
header contains no invalid data. Even this is not foolproof since
valid header patterns can still occur within the subframes. The
decoder's final check is to generate an 8-bit CRC of the frame
header and compare this to the CRC stored at the end of the frame
header.
* Again, since a decoder MAY start decoding at an arbitrary frame in
the stream, each frame header MUST contain some basic information
about the stream because the decoder MAY not have access to the
STREAMINFO metadata block at the start of the stream. This
information includes sample rate, bits per sample, number of
channels, etc. Since the frame header is pure overhead, it has a
direct effect on the compression ratio. To keep the frame header
as small as possible, FLAC uses lookup tables for the most
commonly used values for frame parameters. For instance, the
sample rate part of the frame header is specified using 4 bits.
Eight of the bit patterns correspond to the commonly used sample
rates of 8, 16, 22.05, 24, 32, 44.1, 48 or 96 kHz. However, odd
sample rates can be specified by using one of the 'hint' bit
patterns, directing the decoder to find the exact sample rate at
the end of the frame header. The same method is used for
specifying the block size and bits per sample. In this way, the
frame header size stays small for all of the most common forms of
audio data.
* Individual subframes (one for each channel) are coded separately
within a frame, and appear serially in the stream. In other
words, the encoded audio data is NOT channel-interleaved. This
reduces decoder complexity at the cost of requiring larger decode
buffers. Each subframe has its own header specifying the
van Beurden & Weaver Expires 25 October 2022 [Page 14]
Internet-Draft FLAC April 2022
attributes of the subframe, like prediction method and order,
residual coding parameters, etc. The header is followed by the
encoded audio data for that channel.
8. Format subset
FLAC specifies a subset of itself as the Subset format. The purpose
of this is to ensure that any streams encoded according to the Subset
are truly "streamable", meaning that a decoder that cannot seek
within the stream can still pick up in the middle of the stream and
start decoding. It also makes hardware decoder implementations more
practical by limiting the encoding parameters such that decoder
buffer sizes and other resource requirements can be easily
determined. *flac* generates Subset streams by default unless the "--
lax" command-line option is used. The Subset makes the following
limitations on what MAY be used in the stream:
* The blocksize bits (#blocksize-bits) in the frame header MUST be
0b0001-0b1110. The blocksize MUST be <= 16384; if the sample rate
is <= 48000 Hz, the blocksize MUST be <= 4608 = 2^9 * 3^2.
* The sample rate bits (#sample-rate-bits) in the frame header MUST
be 0b0001-0b1110.
* The bits depth bits (#bit-depth-bits) in the frame header MUST be
0b001-0b111.
* If the sample rate is <= 48000 Hz, the filter order in linear
subframes (see section linear predictor subframe (#linear-
predictor-subframe)) MUST be less than or equal to 12, i.e. the
subframe type bits in the subframe header (see subframe header
section (#subframe-header)) SHOULD NOT be 0b101100-0b111111.
* The Rice partition order (see coded residual section (#coded-
residual)) MUST be less than or equal to 8.
9. File-level metadata
At the start of a FLAC file or stream, following the fLaC ASCII file
signature, one or more metadata blocks MUST be present before any
audio frames appear. The first metadata block MUST be a streaminfo
block.
van Beurden & Weaver Expires 25 October 2022 [Page 15]
Internet-Draft FLAC April 2022
9.1. Metadata block header
Each metadata block starts with a 4 byte header. The first bit in
this header flags whether a metadata block is the last one, it is a 0
when other metadata blocks follow, otherwise it is a 1. The 7
remaining bits of the first header byte contain the type of the
metadata block as an unsigned number between 0 and 126 according to
the following table. A value of 127 (i.e. 0b1111111) is invalid.
The three bytes that follow code for the size of the metadata block
in bytes excluding the 4 header bytes as an unsigned number coded
big-endian.
+=========+====================================================+
| Value | Metadata block type |
+=========+====================================================+
| 0 | Streaminfo |
+---------+----------------------------------------------------+
| 1 | Padding |
+---------+----------------------------------------------------+
| 2 | Application |
+---------+----------------------------------------------------+
| 3 | Seektable |
+---------+----------------------------------------------------+
| 4 | Vorbis comment |
+---------+----------------------------------------------------+
| 5 | Cuesheet |
+---------+----------------------------------------------------+
| 6 | Picture |
+---------+----------------------------------------------------+
| 7 - 126 | reserved |
+---------+----------------------------------------------------+
| 127 | invalid, to avoid confusion with a frame sync code |
+---------+----------------------------------------------------+
Table 1
9.2. Streaminfo
The streaminfo metadata block contains technical information about
the FLAC stream relevant for decoding. Decoder behavior in case of
incorrect or incomplete information is left unspecified (i.e. up to
the decoder implementation). A decoder MAY choose to stop further
decoding in case the information supplied by the streaminfo metadata
block turns out to be incorrect or invalid. A decoder accepting
information from the streaminfo block (most significantly the maximum
frame size, maximum block size, number of audio channels, number of
bits per sample and total number of samples) without doing further
checks during decoding of audio frames could be vulnerable to buffer
van Beurden & Weaver Expires 25 October 2022 [Page 16]
Internet-Draft FLAC April 2022
overflows. See also the section on security considerations
(#security-considerations).
+========+===================================================+
| Data | Description |
+========+===================================================+
| u(16) | The minimum block size (in samples) used in the |
| | stream, excluding the last block. |
+--------+---------------------------------------------------+
| u(16) | The maximum block size (in samples) used in the |
| | stream. |
+--------+---------------------------------------------------+
| u(24) | The minimum frame size (in bytes) used in the |
| | stream. A value of 0 signifies that the value is |
| | not known. |
+--------+---------------------------------------------------+
| u(24) | The maximum frame size (in bytes) used in the |
| | stream. A value of 0 signifies that the value is |
| | not known. |
+--------+---------------------------------------------------+
| u(20) | Sample rate in Hz. Though 20 bits are available, |
| | the maximum sample rate is limited by the |
| | structure of frame headers to 655350 Hz. Also, a |
| | value of 0 is invalid. |
+--------+---------------------------------------------------+
| u(3) | (number of channels)-1. FLAC supports from 1 to |
| | 8 channels |
+--------+---------------------------------------------------+
| u(5) | (bits per sample)-1. FLAC supports from 4 to 32 |
| | bits per sample. Currently the reference encoder |
| | and decoders only support up to 24 bits per |
| | sample. |
+--------+---------------------------------------------------+
| u(36) | Total samples in stream. 'Samples' means inter- |
| | channel sample, i.e. one second of 44.1 kHz audio |
| | will have 44100 samples regardless of the number |
| | of channels. A value of zero here means the |
| | number of total samples is unknown. |
+--------+---------------------------------------------------+
| u(128) | MD5 signature of the unencoded audio data. This |
| | allows the decoder to determine if an error |
| | exists in the audio data even when the error does |
| | not result in an invalid bitstream. A value of 0 |
| | signifies that the value is not known. |
+--------+---------------------------------------------------+
Table 2
van Beurden & Weaver Expires 25 October 2022 [Page 17]
Internet-Draft FLAC April 2022
The minimum block size is excluding the last block of a FLAC file,
which may be smaller. If the minimum block size is equal to the
maximum block size, the file contains a fixed block size stream.
Note that the actual maximum block size might be smaller than the
maximum block size listed in the streaminfo block, and the actual
smallest block size excluding the last block might be larger than the
minimum block size listed in the streaminfo block. This is because
the encoder has to write these fields before receiving any input
audio data, and cannot know beforehand what block sizes it will use,
only between what bounds these will be chosen.
FLAC specifies a minimum block size of 16 and a maximum block size of
65535, meaning the bit patterns corresponding to the numbers 0-15 in
the minimum block size and maximum block size fields are invalid.
The MD5 signature is made by performing an MD5 transformation on the
samples of all channels interleaved, represented in signed, little-
endian form. This interleaving is on a per-sample basis, so for a
stereo file this means first the first sample of the first channel,
then the first sample of the second channel, then the second sample
of the first channel etc. Before performing the MD5 transformation,
all samples must be byte-aligned. So, in case the bit depth is not a
whole number of bytes, additional zero bits are inserted at the most-
significant position until each sample representation is a whole
number of bytes.
9.3. Padding
+======+========================================+
| Data | Description |
+======+========================================+
| u(n) | n '0' bits (n MUST be a multiple of 8) |
+------+----------------------------------------+
Table 3
9.4. Application
+=======+===========================================+
| Data | Description |
+=======+===========================================+
| u(32) | Registered application ID. (Visit the |
| | registration page (https://xiph.org/flac/ |
| | id.html) to register an ID with FLAC.) |
+-------+-------------------------------------------+
| u(n) | Application data (n MUST be a multiple of |
| | 8) |
+-------+-------------------------------------------+
van Beurden & Weaver Expires 25 October 2022 [Page 18]
Internet-Draft FLAC April 2022
Table 4
9.5. Seektable
+============+==========================+
| Data | Description |
+============+==========================+
| SEEKPOINT+ | One or more seek points. |
+------------+--------------------------+
Table 5
NOTE - The number of seek points is implied by the metadata header
'length' field, i.e. equal to length / 18.
9.5.1. Seekpoint
+=======+==========================================================+
| Data | Description |
+=======+==========================================================+
| u(64) | Sample number of first sample in the target frame, or |
| | 0xFFFFFFFFFFFFFFFF for a placeholder point. |
+-------+----------------------------------------------------------+
| u(64) | Offset (in bytes) from the first byte of the first frame |
| | header to the first byte of the target frame's header. |
+-------+----------------------------------------------------------+
| u(16) | Number of samples in the target frame. |
+-------+----------------------------------------------------------+
Table 6
NOTES
* For placeholder points, the second and third field values are
undefined.
* Seek points within a table MUST be sorted in ascending order by
sample number.
* Seek points within a table MUST be unique by sample number, with
the exception of placeholder points.
* The previous two notes imply that there MAY be any number of
placeholder points, but they MUST all occur at the end of the
table.
van Beurden & Weaver Expires 25 October 2022 [Page 19]
Internet-Draft FLAC April 2022
9.6. Vorbis comment
A vorbis comment metadata block contains human-readable information
coded in UTF-8. The name vorbis comment points to the fact that the
vorbis codec stores such metadata in almost the same way. A vorbis
comment metadata block consists of a vendor string optionally
followed by a number of fields, which are pairs of field names and
field contents. Many users refer to these fields as FLAC tags or
simply as tags. A FLAC file MUST NOT contain more than one vorbis
comment metadata block.
In a vorbis comment metadata block, the metadata block header is
directly followed by 4 bytes containing the length in bytes of the
vendor string as an unsigned number coded little-endian. The vendor
string follows UTF-8 coded, and is not terminated in any way.
Following the vendor string are 4 bytes containing the number of
fields that are in the vorbis comment block, stored as an unsigned
number, coded little-endian. If this number is non-zero, it is
followed by the fields themselves, each field stored with a 4 byte
length. First, the 4 byte field length in bytes is stored as an
unsigned number, coded little-endian. The field itself is, like the
vendor string, UTF-8 coded, not terminated in any way.
Each field consists of a field name and a field content, separated by
an = character. The field name MUST only consist of UTF-8 code
points U+0020 through U+0074, excluding U+003D, which is the =
character. In other words, the field name can contain all printable
ASCII characters except the equals sign. The evaluation of the field
names MUST be case insensitive, so U+0041 through 0+005A (A-Z) MUST
be considered equivalent to U+0061 through U+007A (a-z) respectively.
The field contents can contain any UTF-8 character.
Note that the vorbis comment as used in vorbis allows for on the
order of 2^64 bytes of data whereas the FLAC metadata block is
limited to 2^24 bytes. Given the stated purpose of vorbis comments,
i.e. human-readable textual information, this limit is unlikely to be
restrictive. Also note that the 32-bit field lengths are coded
little-endian, as opposed to the usual big-endian coding of fixed-
length integers in the rest of the FLAC format.
9.6.1. Standard field names
Except the one defined in the section channel mask (#channel-mask),
no standard field names are defined. In general, most software
recognizes the following field names
* Title: name of the current work
van Beurden & Weaver Expires 25 October 2022 [Page 20]
Internet-Draft FLAC April 2022
* Artist: name of the artist generally responsible for the current
work. For orchestral works this is usually the composer,
otherwise is it often the performer
* Album: name of the collection the current work belongs to
For a more comprehensive list of possible field names, the list of
tags used in the MusicBrainz project (http://picard-
docs.musicbrainz.org/en/variables/variables.html) is recommended.
9.6.2. Channel mask
Besides fields containing information about the work itself, one
field is defined for technical reasons, of which the field name is
WAVEFORMATEXTENSIBLE_CHANNEL_MASK. This field contains information
on which channels the file contains. Use of this field is
RECOMMENDED in case these differ from the channels defined in the
section channels bits (#channels-bits).
The channel mask consists of flag bits indicating which channels are
present, stored in a hexadecimal representation preceded by 0x. The
flags only signal which channels are present, not in which order, so
in case a file has to be encoded in which channels are ordered
differently, they have to be reordered. Please note that a file in
which the channel order is defined through the
WAVEFORMATEXTENSIBLE_CHANNEL_MASK is not streamable, i.e. non-subset,
as the field is not found in each frame header. The mask bits can be
found in the following table
van Beurden & Weaver Expires 25 October 2022 [Page 21]
Internet-Draft FLAC April 2022
+============+=============================+
| Bit number | Channel description |
+============+=============================+
| 0 | Front left |
+------------+-----------------------------+
| 1 | Front right |
+------------+-----------------------------+
| 2 | Front center |
+------------+-----------------------------+
| 3 | Low-frequency effects (LFE) |
+------------+-----------------------------+
| 4 | Back left |
+------------+-----------------------------+
| 5 | Back right |
+------------+-----------------------------+
| 6 | Front left of center |
+------------+-----------------------------+
| 7 | Front right of center |
+------------+-----------------------------+
| 8 | Back center |
+------------+-----------------------------+
| 9 | Side left |
+------------+-----------------------------+
| 10 | Side right |
+------------+-----------------------------+
| 11 | Top center |
+------------+-----------------------------+
| 12 | Top front left |
+------------+-----------------------------+
| 13 | Top front center |
+------------+-----------------------------+
| 14 | Top front right |
+------------+-----------------------------+
| 15 | Top rear left |
+------------+-----------------------------+
| 16 | Top rear center |
+------------+-----------------------------+
| 17 | Top rear right |
+------------+-----------------------------+
Table 7
Following are 3 examples:
* if a file has a single channel, being a LFE channel, the vorbis
comment field is WAVEFORMATEXTENSIBLE_CHANNEL_MASK=0x8
van Beurden & Weaver Expires 25 October 2022 [Page 22]
Internet-Draft FLAC April 2022
* if a file has 4 channels, being front left, front right, top front
left and top front right, the vorbis comment field is
WAVEFORMATEXTENSIBLE_CHANNEL_MASK=0x5003
* if an input has 4 channels, being back center, top front center,
front center and top rear center in that order, they have to be
reordered to front center, back center, top front center and top
rear center. The vorbis comment field added is
WAVEFORMATEXTENSIBLE_CHANNEL_MASK=0x12004.
WAVEFORMATEXTENSIBLE_CHANNEL_MASK fields MAY be padded with zeros,
for example, 0x0008 for a single LFE channel. Parsing of
WAVEFORMATEXTENSIBLE_CHANNEL_MASK fields MUST be case-insensitive for
both the field name and the field contents.
9.7. Cuesheet
To either store the track and index point structure of a CD-DA along
with its audio or to provide a mechanism to store locations of
interest within a FLAC file, a cuesheet metadata block can be used.
Certain aspects of this metadata block follow directly from the CD-DA
specification, called Red Book, which is standardized as
[IEC.60908.1999]. For more information on the function and history
of these aspects, please refer to [IEC.60908.1999].
The structure of a cuesheet metadata block is enumerated in the
following table.
van Beurden & Weaver Expires 25 October 2022 [Page 23]
Internet-Draft FLAC April 2022
+============+================================================+
| Data | Description |
+============+================================================+
| u(128*8) | Media catalog number, in ASCII printable |
| | characters 0x20-0x7E. |
+------------+------------------------------------------------+
| u(64) | Number of lead-in samples. |
+------------+------------------------------------------------+
| u(1) | 1 if the cuesheet corresponds to a Compact |
| | Disc, else 0. |
+------------+------------------------------------------------+
| u(7+258*8) | Reserved. All bits MUST be set to zero. |
+------------+------------------------------------------------+
| u(8) | Number of tracks in this cuesheet. |
+------------+------------------------------------------------+
| Cuesheet | A number of structures as specified in the |
| tracks | section cuesheet track (#cuesheet-track) equal |
| | to the number of tracks specified previously. |
+------------+------------------------------------------------+
Table 8
If the media catalog number is less than 128 bytes long, it SHOULD be
right-padded with NUL characters. For CD-DA, this is a thirteen
digit number, followed by 115 NUL bytes.
The number of lead-in samples has meaning only for CD-DA cuesheets;
for other uses it SHOULD be 0. For CD-DA, the lead-in is the TRACK
00 area where the table of contents is stored; more precisely, it is
the number of samples from the first sample of the media to the first
sample of the first index point of the first track. According to
[IEC.60908.1999], the lead-in MUST be silence and CD grabbing
software does not usually store it; additionally, the lead-in MUST be
at least two seconds but MAY be longer. For these reasons the lead-
in length is stored here so that the absolute position of the first
track can be computed. Note that the lead-in stored here is the
number of samples up to the first index point of the first track, not
necessarily to INDEX 01 of the first track; even the first track MAY
have INDEX 00 data.
The number of tracks MUST be at least 1, as a cuesheet block MUST
have a lead-out track. For CD-DA, this number MUST be no more than
100 (99 regular tracks and one lead-out track). The lead-out track
is always the last track in the cuesheet. For CD-DA, the lead-out
track number MUST be 170 as specified by [IEC.60908.1999], otherwise
it MUST be 255.
van Beurden & Weaver Expires 25 October 2022 [Page 24]
Internet-Draft FLAC April 2022
9.7.1. Cuesheet track
+===========+=======================================================+
| Data | Description |
+===========+=======================================================+
| u(64) | Track offset of first index point in |
| | samples, relative to the beginning of the |
| | FLAC audio stream. |
+-----------+-------------------------------------------------------+
| u(8) | Track number. |
+-----------+-------------------------------------------------------+
| u(12*8) | Track ISRC. |
+-----------+-------------------------------------------------------+
| u(1) | The track type: 0 for audio, 1 for non- |
| | audio. This corresponds to the CD-DA |
| | Q-channel control bit 3. |
+-----------+-------------------------------------------------------+
| u(1) | The pre-emphasis flag: 0 for no pre- |
| | emphasis, 1 for pre-emphasis. This |
| | corresponds to the CD-DA Q-channel control |
| | bit 5. |
+-----------+-------------------------------------------------------+
| u(6+13*8) | Reserved. All bits MUST be set to zero. |
+-----------+-------------------------------------------------------+
| u(8) | The number of track index points. |
+-----------+-------------------------------------------------------+
| Cuesheet | For all tracks except the lead-out track, a |
| track | number of structures as specified in the |
| index | section cuesheet track index point |
| points | (#cuesheet-track-index-point) equal to the |
| | number of index points specified previously. |
+-----------+-------------------------------------------------------+
Table 9
Note that the track offset differs from the one in CD-DA, where the
track's offset in the TOC is that of the track's INDEX 01 even if
there is an INDEX 00. For CD-DA, the track offset MUST be evenly
divisible by 588 samples (588 samples = 44100 samples/s * 1/75 s).
A track number of 0 is not allowed to avoid conflicting with the CD-
DA spec, which reserves this for the lead-in. For CD-DA the number
MUST be 1-99, or 170 for the lead-out; for non-CD-DA, the track
number MUST for 255 for the lead-out. It is RECOMMENDED to start
with track 1 and increase sequentially. Track numbers MUST be unique
within a cuesheet.
van Beurden & Weaver Expires 25 October 2022 [Page 25]
Internet-Draft FLAC April 2022
The track ISRC (International Standard Recording Code) is a 12-digit
alphanumeric code; see [ISRC-handbook]. A value of 12 ASCII NUL
characters MAY be used to denote absence of an ISRC.
There MUST be at least one index point in every track in a cuesheet
except for the lead-out track, which MUST have zero. For CD-DA, the
number of index points SHOULD NOT be more than 100.
9.7.1.1. Cuesheet track index point
+========+====================================+
| Data | Description |
+========+====================================+
| u(64) | Offset in samples, relative to the |
| | track offset, of the index point. |
+--------+------------------------------------+
| u(8) | The track index point number. |
+--------+------------------------------------+
| u(3*8) | Reserved. All bits MUST be set to |
| | zero. |
+--------+------------------------------------+
Table 10
For CD-DA, the track index point offset MUST be evenly divisible by
588 samples (588 samples = 44100 samples/s * 1/75 s). Note that the
offset is from the beginning of the track, not the beginning of the
audio data.
For CD-DA, an track index point number of 0 corresponds to the track
pre-gap. The first index point in a track MUST have a number of 0 or
1, and subsequently, index point numbers MUST increase by 1. Index
point numbers MUST be unique within a track.
9.8. Picture
The picture metadata block contains image data of a picture in some
way belonging to the audio contained in the FLAC file. Its format is
derived from the APIC frame in the ID3v2 specification. However,
contrary to the APIC frame in ID3v2, the MIME type and description
are prepended with a 4-byte length field instead of being null
delimited strings. A FLAC file MAY contain one or more picture
metadata blocks.
Note that while the length fields for MIME type, description and
picture data are 4 bytes in length and could in theory code for a
size up to 4 GiB, the total metadata block size cannot exceed what
can be described by the metadata block header, i.e. 16 MiB.
van Beurden & Weaver Expires 25 October 2022 [Page 26]
Internet-Draft FLAC April 2022
+========+==================================================+
| Data | Description |
+========+==================================================+
| u(32) | The picture type according to next table |
+--------+--------------------------------------------------+
| u(32) | The length of the MIME type string in bytes. |
+--------+--------------------------------------------------+
| u(n*8) | The MIME type string, in printable ASCII |
| | characters 0x20-0x7E. The MIME type MAY also be |
| | --> to signify that the data part is a URL of |
| | the picture instead of the picture data itself. |
+--------+--------------------------------------------------+
| u(32) | The length of the description string in bytes. |
+--------+--------------------------------------------------+
| u(n*8) | The description of the picture, in UTF-8. |
+--------+--------------------------------------------------+
| u(32) | The width of the picture in pixels. |
+--------+--------------------------------------------------+
| u(32) | The height of the picture in pixels. |
+--------+--------------------------------------------------+
| u(32) | The color depth of the picture in bits-per- |
| | pixel. |
+--------+--------------------------------------------------+
| u(32) | For indexed-color pictures (e.g. GIF), the |
| | number of colors used, or 0 for non-indexed |
| | pictures. |
+--------+--------------------------------------------------+
| u(32) | The length of the picture data in bytes. |
+--------+--------------------------------------------------+
| u(n*8) | The binary picture data. |
+--------+--------------------------------------------------+
Table 11
The following table contains all defined picture types. Values other
than those listed in the table are reserved and SHOULD NOT be used.
There MAY only be one each of picture type 1 and 2 in a file. In
general practice, many decoders display the contents of a picture
metadata block with picture type 3 (front cover) during playback, if
present.
van Beurden & Weaver Expires 25 October 2022 [Page 27]
Internet-Draft FLAC April 2022
+=======+=================================================+
| Value | Picture type |
+=======+=================================================+
| 0 | Other |
+-------+-------------------------------------------------+
| 1 | PNG file icon of 32x32 pixels |
+-------+-------------------------------------------------+
| 2 | General file icon |
+-------+-------------------------------------------------+
| 3 | Front cover |
+-------+-------------------------------------------------+
| 4 | Back cover |
+-------+-------------------------------------------------+
| 5 | Liner notes page |
+-------+-------------------------------------------------+
| 6 | Media label (e.g. CD, Vinyl or Cassette label) |
+-------+-------------------------------------------------+
| 7 | Lead artist, lead performer or soloist |
+-------+-------------------------------------------------+
| 8 | Artist or performer |
+-------+-------------------------------------------------+
| 9 | Conductor |
+-------+-------------------------------------------------+
| 10 | Band or orchestra |
+-------+-------------------------------------------------+
| 11 | Composer |
+-------+-------------------------------------------------+
| 12 | Lyricist or text writer |
+-------+-------------------------------------------------+
| 13 | Recording location |
+-------+-------------------------------------------------+
| 14 | During recording |
+-------+-------------------------------------------------+
| 15 | During performance |
+-------+-------------------------------------------------+
| 16 | Movie or video screen capture |
+-------+-------------------------------------------------+
| 17 | A bright colored fish |
+-------+-------------------------------------------------+
| 18 | Illustration |
+-------+-------------------------------------------------+
| 19 | Band or artist logotype |
+-------+-------------------------------------------------+
| 20 | Publisher or studio logotype |
+-------+-------------------------------------------------+
Table 12
van Beurden & Weaver Expires 25 October 2022 [Page 28]
Internet-Draft FLAC April 2022
10. Frame structure
Directly after the last metadata block, one or more frames follow.
Each frame consists of a frame header, one or more subframes, padding
zero bits to achieve byte-alignment and a frame footer. The number
of subframes in each frame is equal to the number of audio channels.
10.1. Frame header
Each frame starts with the 15-bit frame sync code 0b111111111111100.
Following the sync code is the blocking strategy bit, which MUST NOT
change during the audio stream. The blocking strategy bit is 0 for a
fixed blocksize stream or 1 for variable blocksize stream. If the
blocking strategy is known, a decoder can search for a 16-bit sync
code, either 0xF8 for a fixed blocksize stream or 0xF9 for a variable
blocksize stream. To ease the search for the sync code and further
reduction of false positives, all frames MUST start on a byte
boundary.
10.1.1. Blocksize bits
Following the frame sync code and blocksize strategy bit are 4 bits
referred to as the blocksize bits. Their value relates to the
blocksize according to the following table, where v is the value of
the 4 bits as an unsigned number. Uncommon blocksizes are stored
after the coded number.
+=================+===========================================+
| Value | Blocksize |
+=================+===========================================+
| 0b0000 | reserved |
+-----------------+-------------------------------------------+
| 0b0001 | 192 |
+-----------------+-------------------------------------------+
| 0b0010 - 0b0101 | 144 * (2^v), i.e. 576, 1152, 2304 or 4608 |
+-----------------+-------------------------------------------+
| 0b0110 | uncommon blocksize minus 1 stored as an |
| | 8-bit number |
+-----------------+-------------------------------------------+
| 0b0111 | uncommon blocksize minus 1 stored as a |
| | 16-bit number |
+-----------------+-------------------------------------------+
| 0b1000 - 0b1111 | 2^v, i.e. 256, 512, 1024, 2048, 4096, |
| | 8192, 16384 or 32768 |
+-----------------+-------------------------------------------+
Table 13
van Beurden & Weaver Expires 25 October 2022 [Page 29]
Internet-Draft FLAC April 2022
10.1.2. Sample rate bits
The next 4 bits, referred to as the sample rate bits, contain the
sample rate according to the following table
+========+=======================================================+
| Value | Sample rate |
+========+=======================================================+
| 0b0000 | sample rate only stored in streaminfo metadata block |
+--------+-------------------------------------------------------+
| 0b0001 | 88.2 kHz |
+--------+-------------------------------------------------------+
| 0b0010 | 176.4 kHz |
+--------+-------------------------------------------------------+
| 0b0011 | 192 kHz |
+--------+-------------------------------------------------------+
| 0b0100 | 8 kHz |
+--------+-------------------------------------------------------+
| 0b0101 | 16 kHz |
+--------+-------------------------------------------------------+
| 0b0110 | 22.05 kHz |
+--------+-------------------------------------------------------+
| 0b0111 | 24 kHz |
+--------+-------------------------------------------------------+
| 0b1000 | 32 kHz |
+--------+-------------------------------------------------------+
| 0b1001 | 44.1 kHz |
+--------+-------------------------------------------------------+
| 0b1010 | 48 kHz |
+--------+-------------------------------------------------------+
| 0b1011 | 96 kHz |
+--------+-------------------------------------------------------+
| 0b1100 | uncommon sample rate in kHz stored as an 8-bit number |
+--------+-------------------------------------------------------+
| 0b1101 | uncommon sample rate in Hz stored as a 16-bit number |
+--------+-------------------------------------------------------+
| 0b1110 | uncommon sample rate in Hz divided by 10, stored as a |
| | 16-bit number |
+--------+-------------------------------------------------------+
| 0b1111 | invalid |
+--------+-------------------------------------------------------+
Table 14
van Beurden & Weaver Expires 25 October 2022 [Page 30]
Internet-Draft FLAC April 2022
10.1.3. Channels bits
The next 4 bits (the first 4 bits of the fourth byte of each frame),
referred to as the channel bits, code for both the number of channels
as well as any stereo decorrelation used according to the following
table, where v is the value of the 4 bits as an unsigned number. See
also the section on interchannel decorrelation (#interchannel-
decorrelation).
+==========+====================================================+
| Value | Channels |
+==========+====================================================+
| 0b0000 | 1 channel: mono |
+----------+----------------------------------------------------+
| 0b0001 | 2 channels: left, right |
+----------+----------------------------------------------------+
| 0b0010 | 3 channels: left, right, center |
+----------+----------------------------------------------------+
| 0b0011 | 4 channels: front left, front right, back left, |
| | back right |
+----------+----------------------------------------------------+
| 0b0100 | 5 channels: front left, front right, front center, |
| | back/surround left, back/surround right |
+----------+----------------------------------------------------+
| 0b0101 | 6 channels: front left, front right, front center, |
| | LFE, back/surround left, back/surround right |
+----------+----------------------------------------------------+
| 0b0110 | 7 channels: front left, front right, front center, |
| | LFE, back center, side left, side right |
+----------+----------------------------------------------------+
| 0b0111 | 8 channels: front left, front right, front center, |
| | LFE, back left, back right, side left, side right |
+----------+----------------------------------------------------+
| 0b1000 | 2 channels, stored as left/side stereo |
+----------+----------------------------------------------------+
| 0b1001 | 2 channels, stored as right/side stereo |
+----------+----------------------------------------------------+
| 0b1010 | 2 channels, stored as mid/side stereo |
+----------+----------------------------------------------------+
| 0b1011 - | reserved |
| 0b1111 | |
+----------+----------------------------------------------------+
Table 15
van Beurden & Weaver Expires 25 October 2022 [Page 31]
Internet-Draft FLAC April 2022
10.1.4. Bit depth bits
The next 3 bits code for the bit depth of the samples in the subframe
according to the following table.
+=======+====================================================+
| Value | Bit depth |
+=======+====================================================+
| 0b000 | bit depth only stored in streaminfo metadata block |
+-------+----------------------------------------------------+
| 0b001 | 8 bits per sample |
+-------+----------------------------------------------------+
| 0b010 | 12 bits per sample |
+-------+----------------------------------------------------+
| 0b011 | reserved |
+-------+----------------------------------------------------+
| 0b100 | 16 bits per sample |
+-------+----------------------------------------------------+
| 0b101 | 20 bits per sample |
+-------+----------------------------------------------------+
| 0b110 | 24 bits per sample |
+-------+----------------------------------------------------+
| 0b111 | reserved |
+-------+----------------------------------------------------+
Table 16
The next bit is reserved and MUST be zero.
10.1.5. Coded number
Following the reserved bit (starting at the fifth byte of the frame)
is either a sample or a frame number, which will be referred to as
the coded number. When dealing with variable blocksize streams, the
sample number of the first sample in the frame is encoded. When the
file contains a fixed blocksize stream, the frame number is encoded.
The coded number is stored in a variable length code like UTF-8, but
extended to a maximum of 36 bits unencoded, 7 byte encoded. When a
frame number is encoded, the value MUST NOT be larger than what fits
a value 31 bits unencoded or 6 byte encoded. Please note that most
general purpose UTF-8 encoders and decoders will not be able to
handle these extended codes.
van Beurden & Weaver Expires 25 October 2022 [Page 32]
Internet-Draft FLAC April 2022
10.1.6. Uncommon blocksize
If the blocksize bits defined earlier in this section were 0b0110 or
0b0111 (uncommon blocksize minus 1 stored), this follows the coded
number as either an 8-bit or a 16-bit unsigned number coded big-
endian.
10.1.7. Uncommon sample rate
Following either the coded number or an uncommon blocksize is the
sample rate, if the sample rate bits were 0b1100, 0b1101 or 0b1110
(uncommon sample rate stored), as either an 8-bit or a 16-bit
unsigned number coded big-endian.
10.1.8. Frame header CRC
Finally, after either the frame/sample number, the blocksize or the
sample rate, is a 8-bit CRC. This CRC is initialized with 0 and has
the polynomial x^8 + x^2 + x^1 + x^0. This CRC covers the whole
frame header before the CRC, including the sync code.
10.2. Subframes
Following the frame header are a number subframes equal to the number
of audio channels.
10.2.1. Subframe header
Each subframe starts with a header. The first bit of the header is
always 0, followed by 6 bits describing which subframe type is used
according to the following table, where v is the value of the 6 bits
as an unsigned number.
van Beurden & Weaver Expires 25 October 2022 [Page 33]
Internet-Draft FLAC April 2022
+=====================+=====================================+
| Value | Subframe type |
+=====================+=====================================+
| 0b000000 | Constant subframe |
+---------------------+-------------------------------------+
| 0b000001 | Verbatim subframe |
+---------------------+-------------------------------------+
| 0b000010 - 0b000111 | reserved |
+---------------------+-------------------------------------+
| 0b001000 - 0b001100 | Subframe with a fixed predictor |
| | v-8, i.e. 0, 1, 2, 3 or 4 |
+---------------------+-------------------------------------+
| 0b001101 - 0b011111 | reserved |
+---------------------+-------------------------------------+
| 0b100000 - 0b111111 | Subframe with a linear predictor |
| | v-31, i.e. 1 through 32 (inclusive) |
+---------------------+-------------------------------------+
Table 17
Following the subframe type bits is a bit that flags whether the
subframe has any wasted bits. If it is 0, the subframe doesn't have
any wasted bits and the subframe header is complete. If it is 1, the
subframe does have wasted bits and the number of wasted bits follows
unary coded.
10.2.2. Wasted bits per sample
Certain file formats, like AIFF, can store audio samples with a bit
depth that is not an integer number of bytes by padding them with
least significant zero bits to a bit depth that is an integer number
of bytes. For example, shifting a 14-bit sample right by 2 pads it
to a 16-bit sample, which then has two zero least-significant bits.
In this specification, these least-significant zero bits are referred
to as wasted bits-per-sample or simply wasted bits. They are wasted
in a sense that they contain no information, but are stored anyway.
The wasted bits-per-sample flag in a subframe header is set to 1 if a
certain number of least-significant bits of all samples in the
current subframe are zero. If this is the case, the number of wasted
bits-per-sample (k) minus 1 follows the flag in an unary encoding.
For example, if k is 3, 0b001 follows. If k = 0, the wasted bits-
per-sample flag is 0 and no unary coded k follows.
In case k is not equal to 0, samples are coded ignoring k least-
significant bits. For example, if the preceding frame header
specified a sample size of 16 bits per sample and k is 3, samples in
the subframe are coded as 13 bits per sample. A decoder MUST add k
van Beurden & Weaver Expires 25 October 2022 [Page 34]
Internet-Draft FLAC April 2022
least-significant zero bits by shifting left (padding) after decoding
a subframe sample. In case the frame has left/side, right/side or
mid/side stereo, padding MUST happen to a sample before it is used to
reconstruct a left or right sample.
Besides audio files that have a certain number of wasted bits for the
whole file, there exist audio files in which the number of wasted
bits varies. There are DVD-Audio discs in which blocks of samples
have had their least-significant bits selectively zeroed, as to
slightly improve the compression of their otherwise lossless Meridian
Lossless Packing codec. There are also audio processors like
lossyWAV that enable users to improve compression of their files by a
lossless audio codec in a non-lossless way. Because of this the
number of wasted bits k MAY change between frames and MAY differ
between subframes.
10.2.3. Constant subframe
In a constant subframe only a single sample is stored. This sample
is stored as a integer number coded big-endian, signed two's
complement. The number of bits used to store this sample depends on
the bit depth of the current subframe. The bit depth of a subframe
is equal to the bit depth as coded in the frame header (#bit-depth-
bits), minus the number of wasted bits coded in the subframe header
(#wasted-bits-per-sample). In case a subframe is a side subframe
(see the section on interchannel decorrelation (#interchannel-
decorrelation), the bit depth of that subframe is increased by 1 bit.
10.2.4. Verbatim subframe
A verbatim subframe stores all samples unencoded in sequential order.
See section on Constant subframe (#constant-subframe) on how a sample
is stored unencoded. The number of samples that need to be stored in
a subframe is given by the blocksize in the frame header.
10.2.5. Fixed predictor subframe
Five different fixed predictors are defined in the following table,
one for each prediction order 0 through 4. In the table is also a
derivation, which explains the rationale for choosing these fixed
predictors.
van Beurden & Weaver Expires 25 October 2022 [Page 35]
Internet-Draft FLAC April 2022
+=======+==================================+======================+
| Order | Prediction | Derivation |
+=======+==================================+======================+
| 0 | 0 | N/A |
+-------+----------------------------------+----------------------+
| 1 | s(n-1) | N/A |
+-------+----------------------------------+----------------------+
| 2 | 2 * s(n-1) - s(n-2) | s(n-1) + s'(n-1) |
+-------+----------------------------------+----------------------+
| 3 | 3 * s(n-1) - 3 * s(n-2) + s(n-3) | s(n-1) + s'(n-1) + |
| | | s''(n-1) |
+-------+----------------------------------+----------------------+
| 4 | 4 * s(n-1) - 6 * s(n-2) + 4 * | s(n-1) + s'(n-1) + |
| | s(n-3) - s(n-4) | s''(n-1) + s'''(n-1) |
+-------+----------------------------------+----------------------+
Table 18
Where
* n is the number of the sample being predicted
* s(n) is the sample being predicted
* s(n-1) is the sample before the one being predicted
* s'(n-1) is the difference between the previous sample and the
sample before that, i.e. s(n-1) - s(n-2). This is the closest
available first-order discrete derivative
* s''(n-1) is s'(n-1) - s'(n-2) or the closest available second-
order discrete derivative
* s'''(n-1) is s''(n-1) - s''(n-2) or the closest available third-
order discrete derivative
To encode a signal with a fixed predictor, each sample has the
corresponding prediction subtracted and sent to the residual coder.
To decode a signal with a fixed predictor, first the residual has to
be decoded, after which for each sample the prediction can be added.
This means that decoding MUST be a sequential process within a
subframe, as for each sample, enough fully decoded previous samples
are needed to calculate the prediction.
van Beurden & Weaver Expires 25 October 2022 [Page 36]
Internet-Draft FLAC April 2022
For fixed predictor order 0, the prediction is always 0, thus each
residual sample is equal to its corresponding input or decoded
sample. The difference between a fixed predictor with order 0 and a
verbatim subframe, is that a verbatim subframe stores all samples
unencoded, while a fixed predictor with order 0 has all its samples
processed by the residual coder.
The first order fixed predictor is comparable to how DPCM encoding
works, as the resulting residual sample is the difference between the
corresponding sample and the sample before it. The higher fixed
predictors can be understood as polynomials fitted to the previous
samples.
As the fixed predictors are specified, they do not have to be stored.
The fixed predictor order specifies which predictor is used. To be
able to predict samples, warm-up samples are stored, as the predictor
needs previous samples in its prediction. The number of warm-up
samples is equal to the predictor order. See section on Constant
subframe (#constant-subframe) on how samples are stored unencoded.
Directly following the warm-up samples is the coded residual.
10.2.6. Linear predictor subframe
Whereas fixed predictors are well suited for simple signals, using a
(non-fixed) linear predictor on more complex signals can improve
compression by making the residual samples even smaller. There is a
certain trade-off however, as storing the predictor coefficients
takes up space as well.
In the FLAC format, a predictor is defined by up to 32 predictor
coefficients and a shift. To form a prediction, each coefficient is
multiplied with its corresponding past sample, the results are added
and this addition is then shifted. To encode a signal with a linear
predictor, each sample has the corresponding prediction subtracted
and sent to the residual coder. To decode a signal with a linear
predictor, first the residual has to be decoded, after which for each
sample the prediction can be added. This means that decoding MUST be
a sequential process within a subframe, as for each sample, enough
fully decoded previous samples are needed to calculate the
prediction.
The table below defines how a linear predictor subframe appears in
the bitstream
van Beurden & Weaver Expires 25 October 2022 [Page 37]
Internet-Draft FLAC April 2022
+==========+===============================================+
| Data | Description |
+==========+===============================================+
| s(n) | Unencoded warm-up samples (n = frame's bits- |
| | per-sample * lpc order). |
+----------+-----------------------------------------------+
| u(4) | (Predictor coefficient precision in bits)-1 |
| | (NOTE: 0b1111 is invalid). |
+----------+-----------------------------------------------+
| s(5) | Prediction right shift needed in bits. |
+----------+-----------------------------------------------+
| s(n) | Unencoded predictor coefficients (n = |
| | predictor coefficient precision * lpc order). |
+----------+-----------------------------------------------+
| Coded | Encoded residual |
| residual | |
+----------+-----------------------------------------------+
Table 19
See section on Constant subframe (#constant-subframe) on how the
warm-up samples are stored unencoded. The unencoded predictor
coefficients are stored the same way as the warm-up samples, but the
number of bits needed for each coefficient is defined by the
predictor coefficient precision. While the prediction right shift is
signed two's complement, this number MUST be positive.
Please note that the order in which the predictor coefficients appear
in the bitstream corresponds to which *past* sample they belong. In
other words, the order of the predictor coefficients is opposite to
the chronological order of the samples. So, the first predictor
coefficient has to be multiplied with the sample directly before the
sample that is being predicted, the second predictor coefficient has
to be multiplied with the sample before that etc.
10.2.7. Coded residual
The first two bits in a coded residual indicate which coding method
is used. See the table below
van Beurden & Weaver Expires 25 October 2022 [Page 38]
Internet-Draft FLAC April 2022
+=============+=============================================+
| Value | Description |
+=============+=============================================+
| 0b00 | partitioned Rice code with 4-bit parameters |
+-------------+---------------------------------------------+
| 0b01 | partitioned Rice code with 5-bit parameters |
+-------------+---------------------------------------------+
| 0b10 - 0b11 | reserved |
+-------------+---------------------------------------------+
Table 20
Both defined coding methods work the same way, but differ in the
number of bits used for rice parameters. The 4 bits that directly
follow the coding method bits form the partition order, which is an
unsigned number. The rest of the coded residual consists of
2^(partition order) partitions. For example, if the 4 bits are
0b1000, the partition order is 8 and the residual is split up into
2^8 = 256 partitions.
Each partition contains a certain amount of residual samples. The
number of residual samples in the first partition is equal to
(blocksize >> partition order) - predictor order, i.e. the blocksize
divided by the number of partitions minus the predictor order. In
all other partitions the number of residual samples is equal to
(blocksize >> partition order).
The partition order MUST be so that the blocksize is evenly divisible
by the number of partitions. This means for example that for all odd
blocksizes, only partition order 0 is allowed. The partition order
also MUST be so that the (blocksize >> partition order) is larger
than the predictor order. This means for example that with a
blocksize of 4096 and a predictor order of 4, partition order cannot
be larger than 9.
In case the coded residual of a subframe is one with a 4-bit Rice
parameter (see table at the start of this section), the first 4 bits
of each partition are either a rice parameter or an escape code.
These 4 bits indicate an escape code if they are 0b1111, otherwise
they contain the rice parameter as an unsigned number. In case the
coded residual of the current subframe is one with a 5-bit Rice
parameter, the first 5 bits indicate an escape code if they are
0b11111, otherwise they contain the rice parameter as an unsigned
number as well.
In case an escape code was used, the partition does not contain a
variable-length rice coded residual, but a fixed-length unencoded
residual. Directly following the escape code are 5 bits containing
van Beurden & Weaver Expires 25 October 2022 [Page 39]
Internet-Draft FLAC April 2022
the number of bits with which each residual sample is stored, as an
unsigned number. The residual samples themselves are stored signed
two's complement.
In case a rice parameter was provided, the partition contains a rice
coded residual. The residual samples, which are signed numbers, are
represented by unsigned numbers in the rice code. For positive
numbers, the representation is the number doubled, for negative
numbers, the representation is the number multiplied by -2 and has 1
subtracted. This representation of signed numbers is also known as
zigzag encoding and the zigzag encoded residual is called the folded
residual. The folded residual samples are then each divided by the
rice parameter. The result of each division rounded down (the
quotient) is stored unary, the remainder is stored binary.
Decoding the coded residual thus involves selecting the right coding
method, finding the number of partitions, reading unary and binary
parts of each codeword one-by-one and keeping track of when a new
partition starts and thus when a new rice parameter needs to be read.
10.3. Frame footer
Following the last subframe is the frame footer. If the last
subframe is not byte aligned (i.e. the bits required to store all
subframes put together are not divisible by 8), zero bits are added
until byte alignment is reached. Following this is a 16-bit CRC,
initialized with 0, with polynomial x^16 + x^15 + x^2 + x^0. This
CRC covers the whole frame excluding the 16-bit CRC, including the
sync code.
11. Implementation status
This section records the status of known implementations of the FLAC
format, and is based on a proposal described in [RFC7942]. Please
note that the listing of any individual implementation here does not
imply endorsement by the IETF. Furthermore, no effort has been spent
to verify the information presented here that was supplied by IETF
contributors. This is not intended as, and must not be construed to
be, a catalog of available implementations or their features.
Readers are advised to note that other implementations may exist.
A reference encoder and decoder implementation of the FLAC format
exists, known as libFLAC, maintained by Xiph.Org. It can be found at
https://xiph.org/flac/ (https://xiph.org/flac/) Note that while all
libFLAC components are licensed under 3-clause BSD, the flac and
metaflac command line tools often supplied together with libFLAC are
licensed under GPL.
van Beurden & Weaver Expires 25 October 2022 [Page 40]
Internet-Draft FLAC April 2022
Another completely independent implementation of both encoder and
decoder of the FLAC format is available in libavcodec, maintained by
FFmpeg, licensed under LGPL 2.1 or later. It can be found at
https://ffmpeg.org/ (https://ffmpeg.org/)
A list of other implementations and an overview of which parts of the
format they implement can be found here: https://github.com/ietf-wg-
cellar/flac-specification/wiki/Implementations (https://github.com/
ietf-wg-cellar/flac-specification/wiki/Implementations)
12. Security Considerations
Like any other codec (such as [RFC6716]), FLAC should not be used
with insecure ciphers or cipher modes that are vulnerable to known
plaintext attacks. Some of the header bits as well as the padding
are easily predictable.
Implementations of the FLAC codec need to take appropriate security
considerations into account. Those related to denial of service are
outlined in Section 2.1 of [RFC4732]. It is extremely important for
the decoder to be robust against malicious payloads. Malicious
payloads MUST NOT cause the decoder to overrun its allocated memory
or to take an excessive amount of resources to decode. An overrun in
allocated memory could lead to arbitrary code execution by an
attacker. The same applies to the encoder, even though problems in
encoders are typically rarer. Malicious audio streams MUST NOT cause
the encoder to misbehave because this would allow an attacker to
attack transcoding gateways. An example is allocating more memory
than available especially with blocksizes of more than 10000 or with
big metadata blocks, or not allocating enough memory before copying
data, which lead to execution of malicious code, crashes, freezes or
reboots on some known implementations. See the FLAC decoder
testbench (https://wiki.hydrogenaud.io/
index.php?title=FLAC_decoder_testbench) for a non-exhaustive list of
FLAC files with extreme configurations which lead to crashes or
reboots on some known implementations.
None of the content carried in FLAC is intended to be executable.
13. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
van Beurden & Weaver Expires 25 October 2022 [Page 41]
Internet-Draft FLAC April 2022
[RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
Denial-of-Service Considerations", RFC 4732,
DOI 10.17487/RFC4732, December 2006,
<https://www.rfc-editor.org/info/rfc4732>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
14. Informative References
[HPL-1999-144]
Hans, M. and RW. Schafer, "Lossless Compression of Digital
Audio", DOI 10.1109/79.939834, November 1999,
<https://www.hpl.hp.com/techreports/1999/HPL-
1999-144.pdf>.
[IEC.60908.1999]
International Electrotechnical Commission, "Audio
recording - Compact disc digital audio system",
IEC International standard 60908 second edition, 1999.
[ISRC-handbook]
"International Standard Recording Code (ISRC) Handbook,
4th edition", 2021, <https://www.ifpi.org/isrc_handbook/>.
[RFC6716] Valin, JM., Vos, K., and T. Terriberry, "Definition of the
Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716,
September 2012, <https://www.rfc-editor.org/info/rfc6716>.
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
[robinson-tr156]
Robinson, T., "SHORTEN: Simple lossless and near-lossless
waveform compression", December 1994,
<https://mi.eng.cam.ac.uk/reports/abstracts/
robinson_tr156.html>.
van Beurden & Weaver Expires 25 October 2022 [Page 42]
Internet-Draft FLAC April 2022
Appendix A. Numerical considerations
In order to maintain lossless behavior, all arithmetic used in
encoding and decoding sample values MUST be done with integer data
types to eliminate the possibility of introducing rounding errors
associated with floating-point arithmetic. Use of floating-point
representations in analysis (e.g. finding a good predictor or rice
parameter) is not a concern, as long as the process of using the
found predictor and rice parameter to encode audio samples is
implemented with only integer math.
Furthermore, the possibility of integer overflow MUST be eliminated
by using data types large enough to never overflow. Choosing a
64-bit signed data type for all arithmetic involving sample values
would make sure the possibility for overflow is eliminated, but
usually smaller data types are chosen for increased performance,
especially in embedded devices. This section will provide guidelines
for choosing the right data type in each step of encoding and
decoding FLAC files.
A.1. Determining necessary data type size
To find the smallest data type size that is guaranteed not to
overflow for a certain sequence of arithmetic operations, the
combination of values producing the largest possible result should be
considered.
If for example two 16-bit signed integers are added, the largest
possible result forms if both values are the largest number that can
be represented with a 16-bit signed integer. To store the result, an
signed integer data type with at least 17 bits is needed. Similarly,
when adding 4 of these values, 18 bits are needed, when adding 8, 19
bits are needed etc. In general, the number of bits necessary when
adding numbers together is increased by the log base 2 of the number
of values rounded up to the nearest integer. So, when adding 18
unknown values stored in 8 bit signed integers, we need a signed
integer data type of at least 13 bits to store the result, as the log
base 2 of 18 rounded up is 5.
In case of multiplication, the number of bits needed for the result
is the size of the first variable plus the size of the second
variable, but counting only one sign bit if working with signed data
types. If for example a 16-bit signed integer is multiplied by a
16-bit signed integer, the result needs at least 31 bits to store
without overflowing.
van Beurden & Weaver Expires 25 October 2022 [Page 43]
Internet-Draft FLAC April 2022
A.2. Stereo decorrelation
When stereo decorrelation is used, the side channel will have one
extra bit of bit depth, see section on Interchannel Decorrelation
(#interchannel-decorrelation).
This means that while 16-bit signed integers have sufficient range to
store samples from a fully decoded FLAC frame with a bit depth of 16
bit, the decoding of a side subframe in such a file will need a data
type with at least 17 bit to store decoded subframe samples before
undoing stereo decorrelation.
Most FLAC decoders store decoded (subframe) samples as 32-bit values,
which is sufficient for files with bit depths up to (and including)
31 bit.
A.3. Prediction
A prediction (which is used to calculate the residual on encoding or
added to the residual to calculate the sample value on decoding) is
formed by multiplying and summing preceding sample values. In order
to eliminate the possibility of integer overflow, the combination of
preceding sample values and predictor coefficients producing the
largest possible value should be considered.
To determine the size of the data type needed to calculate either a
residual sample (on encoding) or an audio sample value (on decoding)
in a fixed predictor subframe, the maximal possible value for these
is calculated as described in the previous subsection (#determining-
necessary-data-type-size) in the following table. For example: if a
frame codes for 16-bit audio and has some form of stereo
decorrelation, the subframe coding for the side channel would need
16+1+3 bits in case a third order fixed predictor is used.
van Beurden & Weaver Expires 25 October 2022 [Page 44]
Internet-Draft FLAC April 2022
+=======+==============================+===============+=======+
| Order | Calculation of residual | Sample values | Extra |
| | | summed | bits |
+=======+==============================+===============+=======+
| 0 | s(n) | 1 | 0 |
+-------+------------------------------+---------------+-------+
| 1 | s(n) - s(n-1) | 2 | 1 |
+-------+------------------------------+---------------+-------+
| 2 | s(n) - 2 * s(n-1) + s(n-2) | 4 | 2 |
+-------+------------------------------+---------------+-------+
| 3 | s(n) - 3 * s(n-1) + 3 * | 8 | 3 |
| | s(n-2) - s(n-3) | | |
+-------+------------------------------+---------------+-------+
| 4 | s(n) - 4 * s(n-1) + 6 * | 16 | 4 |
| | s(n-2) - 4 * s(n-3) + s(n-4) | | |
+-------+------------------------------+---------------+-------+
Table 21
Where
* n is the number of the sample being predicted
* s(n) is the sample being predicted
* s(n-1) is the sample before the one being predicted, s(n-2) is the
sample before that etc.
For subframes with a linear predictor, calculation is a little more
complicated. Each prediction is a sum of several multiplications.
Each of these multiply a sample value with a predictor coefficient.
The extra bits needed can be calculated by adding the predictor
coefficient precision (in bits) to the bit depth of the audio
samples. As both are signed numbers and only one 'sign bit' is
necessary, 1 bit can be subtracted. To account for the summing of
these multiplications, the log base 2 of the predictor order rounded
up is added.
For example, if the sample bitdepth of the source is 24, the current
subframe encodes a side channel (see the section on interchannel
decorrelation (#interchannel-decorrelation)), the predictor order is
12 and the predictor coefficient precision is 15 bits, the minimum
required size of the used signed integer data type is at least (24 +
1) + (15 - 1) + ceil(log2(12)) = 43 bits. As another example, with a
side-channel subframe bit depth of 16, a predictor order of 8 and a
predictor coefficient precision of 12 bits, the minimum required size
of the used signed integer data type is (16 + 1) + (12 - 1) +
ceil(log2(8)) = 31 bits.
van Beurden & Weaver Expires 25 October 2022 [Page 45]
Internet-Draft FLAC April 2022
After the prediction has been shifted right, the number of bits
needed is reduced by the amount of right shift and increased by one
bit for the subtraction from the current sample on encoding. On
decoding, the data type size needed to store the result of the
addition of the residual and the prediction should fit the subframe
bit depth, assuming all calculations were done correctly.
Taking the last example where 31 bits were needed for the prediction,
the required data type size for the residual samples in case of a
right shift of 10 bits would be 31 - 10 + 1 = 22 bits.
A.4. Rice coding
When folding (i.e. zig-zag encoding) the residual sample values, no
extra bits are needed when the absolute value of each residual sample
is first stored in an unsigned data type of the size of the last
step, then doubled and then has one subtracted depending on whether
the residual sample was positive or negative. Many implementations
however choose to require one extra bit of data type size so zig-zag
encoding can happen in one step and without a cast instead of the
procedure described in the previous sentence.
Appendix B. Examples
This informational appendix contains short example FLAC files and
short parts of FLAC files which are decoded step by step. These
examples provide a more engaging way to understand the FLAC format
than the formal specification. The text explaining these examples
assumes the reader has at least cursory read the specification and
that the reader refers to the specification for explanation of the
terminology used. These examples mostly focus on the lay-out of
several metadata blocks and subframe types and the implications of
certain aspects (for example wasted bits and stereo decorrelation) on
this lay-out.
The examples feature (parts of) files generated by various FLAC
encoders. These are presented in hexadecimal or binary format,
followed by tables and text referring to various features by their
starting bit positions in these representations. Each starting
position (shortened to 'start' in the tables) is a hexadecimal byte
position and a start bit within that byte, separated by a plus sign.
Counts for these start at zero. For example, a feature starting at
the 3rd bit of the 17th byte is referred to as starting at 0x10+2.
All data in this appendix has been thoroughly verified. However, as
this appendix is informational, in case any information here
conflicts with statements in the formal specification, the latter
takes precedence.
van Beurden & Weaver Expires 25 October 2022 [Page 46]
Internet-Draft FLAC April 2022
B.1. Decoding example 1
This very short example FLAC file codes for PCM audio that has two
channels, each containing 1 sample. The focus of this example is on
the essential parts of a FLAC file.
B.1.1. Example file 1 in hexadecimal representation
00000000: 664c 6143 8000 0022 1000 1000 fLaC..."....
0000000c: 0000 0f00 000f 0ac4 42f0 0000 ........B...
00000018: 0001 3e84 b418 07dc 6903 0758 ..>.....i..X
00000024: 6a3d ad1a 2e0f fff8 6918 0000 j=......i...
00000030: bf03 58fd 0312 8baa 9a ..X......
B.1.2. Example file 1 in binary representation
00000000: 01100110 01001100 01100001 01000011 fLaC
00000004: 10000000 00000000 00000000 00100010 ..."
00000008: 00010000 00000000 00010000 00000000 ....
0000000c: 00000000 00000000 00001111 00000000 ....
00000010: 00000000 00001111 00001010 11000100 ....
00000014: 01000010 11110000 00000000 00000000 B...
00000018: 00000000 00000001 00111110 10000100 ..>.
0000001c: 10110100 00011000 00000111 11011100 ....
00000020: 01101001 00000011 00000111 01011000 i..X
00000024: 01101010 00111101 10101101 00011010 j=..
00000028: 00101110 00001111 11111111 11111000 ....
0000002c: 01101001 00011000 00000000 00000000 i...
00000030: 10111111 00000011 01011000 11111101 ..X.
00000034: 00000011 00010010 10001011 10101010 ....
00000038: 10011010
B.1.3. Signature and streaminfo
The first 4 bytes of the file contain the fLaC file signature.
Directly following it is a metadata block. The signature and the
first metadata block header are broken down in the following table
van Beurden & Weaver Expires 25 October 2022 [Page 47]
Internet-Draft FLAC April 2022
+========+========+============+===========================+
| Start | Length | Contents | Description |
+========+========+============+===========================+
| 0x00+0 | 4 byte | 0x664C6143 | fLaC |
+--------+--------+------------+---------------------------+
| 0x04+0 | 1 bit | 0b1 | Last metadata block |
+--------+--------+------------+---------------------------+
| 0x04+1 | 7 bit | 0b0000000 | Streaminfo metadata block |
+--------+--------+------------+---------------------------+
| 0x05+0 | 3 byte | 0x000022 | Length 34 byte |
+--------+--------+------------+---------------------------+
Table 22
As the header indicates that this is the last metadata block, the
position of the first audio frame can now be calculated as the
position of the first byte after the metadata block header + the
length of the block, i.e. 8+34 = 42 or 0x2a. As can be seen 0x2a
indeed contains the frame sync code for fixed blocksize streams,
0xfff8.
The streaminfo metadata block contents are broken down in the
following table
+========+=========+====================+=========================+
| Start | Length | Contents | Description |
+========+=========+====================+=========================+
| 0x08+0 | 2 byte | 0x1000 | Min. blocksize 4096 |
+--------+---------+--------------------+-------------------------+
| 0x0a+0 | 2 byte | 0x1000 | Max. blocksize 4096 |
+--------+---------+--------------------+-------------------------+
| 0x0c+0 | 3 byte | 0x00000f | Min. frame size 15 byte |
+--------+---------+--------------------+-------------------------+
| 0x0f+0 | 3 byte | 0x00000f | Max. frame size 15 byte |
+--------+---------+--------------------+-------------------------+
| 0x12+0 | 20 bit | 0x0ac4, 0b0100 | Sample rate 44100 Hertz |
+--------+---------+--------------------+-------------------------+
| 0x14+4 | 3 bit | 0b001 | 2 channels |
+--------+---------+--------------------+-------------------------+
| 0x14+7 | 5 bit | 0b01111 | Sample bit depth 16 |
+--------+---------+--------------------+-------------------------+
| 0x15+4 | 36 bit | 0b0000, 0x00000001 | Total no. of samples 1 |
+--------+---------+--------------------+-------------------------+
| 0x1a | 16 byte | (...) | MD5 signature |
+--------+---------+--------------------+-------------------------+
Table 23
van Beurden & Weaver Expires 25 October 2022 [Page 48]
Internet-Draft FLAC April 2022
The minimum and maximum blocksize are both 4096. This was apparently
the blocksize the encoder planned to use, but as only 1 interchannel
sample was provided, no frames with 4096 samples are actually present
in this file.
Note that anywhere a number of samples is mentioned (blocksize, total
number of samples, sample rate), interchannel samples are meant.
The MD5 sum (starting at 0x1a) is 0x3e84 b418 07dc 6903 0758 6a3d
ad1a 2e0f. This will be validated after decoding the samples.
B.1.4. Audio frames
The frame header starts at position 0x2a and is broken down in the
following table.
+========+========+=================+==============================+
| Start | Length | Contents | Description |
+========+========+=================+==============================+
| 0x2a+0 | 15 bit | 0xff, 0b1111100 | frame sync |
+--------+--------+-----------------+------------------------------+
| 0x2b+7 | 1 bit | 0b0 | blocksize strategy |
+--------+--------+-----------------+------------------------------+
| 0x2c+0 | 4 bit | 0b0110 | 8-bit blocksize further down |
+--------+--------+-----------------+------------------------------+
| 0x2c+4 | 4 bit | 0b1001 | sample rate 44.1kHz |
+--------+--------+-----------------+------------------------------+
| 0x2d+0 | 4 bit | 0b0001 | stereo, no decorrelation |
+--------+--------+-----------------+------------------------------+
| 0x2d+4 | 3 bit | 0b100 | bit depth 16 bit |
+--------+--------+-----------------+------------------------------+
| 0x2d+7 | 1 bit | 0b0 | mandatory 0 bit |
+--------+--------+-----------------+------------------------------+
| 0x2e+0 | 1 byte | 0x00 | frame number 0 |
+--------+--------+-----------------+------------------------------+
| 0x2f+0 | 1 byte | 0x00 | blocksize 1 |
+--------+--------+-----------------+------------------------------+
| 0x30+0 | 1 byte | 0xbf | frame header CRC |
+--------+--------+-----------------+------------------------------+
Table 24
As the stream is a fixed blocksize stream, the number at 0x2e
contains a frame number. As the value is smaller than 128, only 1
byte is used for the encoding.
At byte 0x31 the subframe header of the first subframe starts, it is
broken down in the following table.
van Beurden & Weaver Expires 25 October 2022 [Page 49]
Internet-Draft FLAC April 2022
+========+========+================+=========================+
| Start | Length | Contents | Description |
+========+========+================+=========================+
| 0x31+0 | 1 bit | 0b0 | mandatory 0 bit |
+--------+--------+----------------+-------------------------+
| 0x31+1 | 6 bit | 0b000001 | verbatim subframe |
+--------+--------+----------------+-------------------------+
| 0x31+7 | 1 bit | 0b1 | wasted bits present |
+--------+--------+----------------+-------------------------+
| 0x32+0 | 2 bit | 0b01 | 2 wasted bits |
+--------+--------+----------------+-------------------------+
| 0x32+2 | 14 bit | 0b011000, 0xfd | 14-bit unencoded sample |
+--------+--------+----------------+-------------------------+
Table 25
As the wasted bits flag is 1 in this subframe, an unary coded number
follows. Starting at 0x32, we see 0b01, which unary codes for 1,
meaning we have 2 wasted bits in this subframe.
As this is a verbatim subframe, the subframe only contains unencoded
sample values. With a blocksize of 1, it contains only a single
sample. The bit depth of the audio is 16 bit, but as the subframe
header signals 2 wasted bits, only 14 bits are stored. As no stereo
decorrelation is used, a bit depth increase for the side channel is
not applicable. So, the next 14 bit (starting at position 0x32+2)
contain the unencoded sample coded big-endian, signed two's
complement. The value reads 0b011000 11111101, or 6397. This value
needs to be shifted left by 2 bits, to account for the wasted bits.
The value is then 0b011000 11111101 00, or 25588.
The second subframe starts at 0x34, it is broken down in the
following table.
van Beurden & Weaver Expires 25 October 2022 [Page 50]
Internet-Draft FLAC April 2022
+========+========+==============+=========================+
| Start | Length | Contents | Description |
+========+========+==============+=========================+
| 0x34+0 | 1 bit | 0b0 | mandatory 0 bit |
+--------+--------+--------------+-------------------------+
| 0x34+1 | 6 bit | 0b000001 | verbatim subframe |
+--------+--------+--------------+-------------------------+
| 0x34+7 | 1 bit | 0b1 | wasted bits present |
+--------+--------+--------------+-------------------------+
| 0x35+0 | 4 bit | 0b0001 | 4 wasted bits |
+--------+--------+--------------+-------------------------+
| 0x35+4 | 12 bit | 0b0010, 0x8b | 12-bit unencoded sample |
+--------+--------+--------------+-------------------------+
Table 26
Here the wasted bits flag is also one, but the unary coded number
that follows it is 4 bit long, indicating 4 wasted bits. This means
the sample is stored in 12 bits. The sample value is 0b0010
10001011, or 651. This value now has to be shifted left by 4 bits,
i.e. 0b0010 10001011 0000 or 10416.
At this point, we would do stereo decorrelation if that was
applicable.
As the last subframe ends byte-aligned, no padding bits were
inserted. The next 2 bytes, starting at 0x38, contain the frame CRC.
As this is the only frame in the file, the file ends with the CRC.
To validate the MD5, we line up the samples interleaved, byte-
aligned, little endian, signed two's complement. The first sample,
the value of which was 25588 translates to 0xf463, the second sample
had a value of 10416 which translates to 0xb028. When MD5 summing
0xf463b028, we get the MD5 sum found in the header, so decoding was
lossless.
B.2. Decoding example 2
This FLAC file is larger than the first example, but still contains
very little audio. The focus of this example is on decoding a
subframe with a fixed predictor and a coded residual, but it also
contains a very short seektable, vorbis comment and padding metadata
block.
B.2.1. Example file 2 in hexadecimal representation
van Beurden & Weaver Expires 25 October 2022 [Page 51]
Internet-Draft FLAC April 2022
00000000: 664c 6143 0000 0022 0010 0010 fLaC..."....
0000000c: 0000 1700 0044 0ac4 42f0 0000 .....D..B...
00000018: 0013 d5b0 5649 75e9 8b8d 8b93 ....VIu.....
00000024: 0422 757b 8103 0300 0012 0000 ."u{........
00000030: 0000 0000 0000 0000 0000 0000 ............
0000003c: 0000 0010 0400 003a 2000 0000 .......: ...
00000048: 7265 6665 7265 6e63 6520 6c69 reference li
00000054: 6246 4c41 4320 312e 332e 3320 bFLAC 1.3.3
00000060: 3230 3139 3038 3034 0100 0000 20190804....
0000006c: 0e00 0000 5449 544c 453d d7a9 ....TITLE=..
00000078: d79c d795 d79d 8100 0006 0000 ............
00000084: 0000 0000 fff8 6998 000f 9912 ......i.....
00000090: 0867 0162 3d14 4299 8f5d f70d .g.b=.B..]..
0000009c: 6fe0 0c17 caeb 2100 0ee7 a77a o.....!....z
000000a8: 24a1 590c 1217 b603 097b 784f $.Y......{xO
000000b4: aa9a 33d2 85e0 70ad 5b1b 4851 ..3...p.[.HQ
000000c0: b401 0d99 d2cd 1a68 f1e6 b810 .......h....
000000cc: fff8 6918 0102 a402 c382 c40b ..i.........
000000d8: c14a 03ee 48dd 03b6 7c13 30 .J..H...|.0
B.2.2. Example file 2 in binary representation (only audio frames)
00000088: 11111111 11111000 01101001 10011000 ..i.
0000008c: 00000000 00001111 10011001 00010010 ....
00000090: 00001000 01100111 00000001 01100010 .g.b
00000094: 00111101 00010100 01000010 10011001 =.B.
00000098: 10001111 01011101 11110111 00001101 .]..
0000009c: 01101111 11100000 00001100 00010111 o...
000000a0: 11001010 11101011 00100001 00000000 ..!.
000000a4: 00001110 11100111 10100111 01111010 ...z
000000a8: 00100100 10100001 01011001 00001100 $.Y.
000000ac: 00010010 00010111 10110110 00000011 ....
000000b0: 00001001 01111011 01111000 01001111 .{xO
000000b4: 10101010 10011010 00110011 11010010 ..3.
000000b8: 10000101 11100000 01110000 10101101 ..p.
000000bc: 01011011 00011011 01001000 01010001 [.HQ
000000c0: 10110100 00000001 00001101 10011001 ....
000000c4: 11010010 11001101 00011010 01101000 ...h
000000c8: 11110001 11100110 10111000 00010000 ....
000000cc: 11111111 11111000 01101001 00011000 ..i.
000000d0: 00000001 00000010 10100100 00000010 ....
000000d4: 11000011 10000010 11000100 00001011 ....
000000d8: 11000001 01001010 00000011 11101110 .J..
000000dc: 01001000 11011101 00000011 10110110 H...
000000e0: 01111100 00010011 00110000 |.0
van Beurden & Weaver Expires 25 October 2022 [Page 52]
Internet-Draft FLAC April 2022
B.2.3. Signature and streaminfo
Most of the streaminfo block is the same as in example 1, so only
parts that are different are listed in the following table
+========+========+============+=============================+
| Start | Length | Contents | Description |
+========+========+============+=============================+
| 0x04+0 | 1 bit | 0b0 | Not the last metadata block |
+--------+--------+------------+-----------------------------+
| 0x08+0 | 2 byte | 0x0010 | Min. blocksize 16 |
+--------+--------+------------+-----------------------------+
| 0x0a+0 | 2 byte | 0x0010 | Max. blocksize 16 |
+--------+--------+------------+-----------------------------+
| 0x0c+0 | 3 byte | 0x000017 | Min. frame size 23 byte |
+--------+--------+------------+-----------------------------+
| 0x0f+0 | 3 byte | 0x000044 | Max. frame size 68 byte |
+--------+--------+------------+-----------------------------+
| 0x15+4 | 36 bit | 0b0000, | Total no. of samples 19 |
| | | 0x00000013 | |
+--------+--------+------------+-----------------------------+
| 0x1a | 16 | (...) | MD5 signature |
| | byte | | |
+--------+--------+------------+-----------------------------+
Table 27
This time, the minimum and maximum blocksizes are reflected in the
file: there is one block of 16 samples, but the last block (which has
3 samples) is excluded from this number. The MD5 signature is 0xd5b0
5649 75e9 8b8d 8b93 0422 757b 8103, this will be verified at the end
of this example.
B.2.4. Seektable
The seektable metadata block only holds one entry. It is not really
useful here, as it points to the first frame, but it is enough for
this example. The seektable metadata block is broken down in the
following table.
van Beurden & Weaver Expires 25 October 2022 [Page 53]
Internet-Draft FLAC April 2022
+========+========+====================+================+
| Start | Length | Contents | Description |
+========+========+====================+================+
| 0x2a+0 | 1 bit | 0b0 | Not the last |
| | | | metadata block |
+--------+--------+--------------------+----------------+
| 0x2a+1 | 7 bit | 0b0000011 | Seektable |
| | | | metadata block |
+--------+--------+--------------------+----------------+
| 0x2b+0 | 3 byte | 0x000012 | Length 18 byte |
+--------+--------+--------------------+----------------+
| 0x2e+0 | 8 byte | 0x0000000000000000 | Seekpoint to |
| | | | sample 0 |
+--------+--------+--------------------+----------------+
| 0x36+0 | 8 byte | 0x0000000000000000 | Seekpoint to |
| | | | offset 0 |
+--------+--------+--------------------+----------------+
| 0x3e+0 | 2 byte | 0x0010 | Seekpoint to |
| | | | blocksize 16 |
+--------+--------+--------------------+----------------+
Table 28
B.2.5. Vorbis comment
The vorbis comment metadata block contains the vendor string and a
single comment. It is broken down in the following table.
van Beurden & Weaver Expires 25 October 2022 [Page 54]
Internet-Draft FLAC April 2022
+========+=========+============+===============================+
| Start | Length | Contents | Description |
+========+=========+============+===============================+
| 0x40+0 | 1 bit | 0b0 | Not the last metadata block |
+--------+---------+------------+-------------------------------+
| 0x40+1 | 7 bit | 0b0000100 | Vorbis comment metadata block |
+--------+---------+------------+-------------------------------+
| 0x41+0 | 3 byte | 0x00003a | Length 58 byte |
+--------+---------+------------+-------------------------------+
| 0x44+0 | 4 byte | 0x20000000 | Vendor string length 32 byte |
+--------+---------+------------+-------------------------------+
| 0x48+0 | 32 byte | (...) | Vendor string |
+--------+---------+------------+-------------------------------+
| 0x68+0 | 4 byte | 0x01000000 | Number of fields 1 |
+--------+---------+------------+-------------------------------+
| 0x6c+0 | 4 byte | 0x0e000000 | Field length 14 byte |
+--------+---------+------------+-------------------------------+
| 0x70+0 | 14 byte | (...) | Field contents |
+--------+---------+------------+-------------------------------+
Table 29
The vendor string is reference libFLAC 1.3.3 20190804, the field
contents of the only field is TITLE=שלום (U+05E9 U+05DC U+05D5
U+05DD). The vorbis comment field is 14 bytes but only 10 characters
in size, because it contains four 2-byte characters.
B.2.6. Padding
The last metadata block is a (very short) padding block.
+========+========+================+========================+
| Start | Length | Contents | Description |
+========+========+================+========================+
| 0x7e+0 | 1 bit | 0b1 | Last metadata block |
+--------+--------+----------------+------------------------+
| 0x7e+1 | 7 bit | 0b0000001 | Padding metadata block |
+--------+--------+----------------+------------------------+
| 0x7f+0 | 3 byte | 0x000006 | Length 6 byte |
+--------+--------+----------------+------------------------+
| 0x82+0 | 6 byte | 0x000000000000 | Padding bytes |
+--------+--------+----------------+------------------------+
Table 30
van Beurden & Weaver Expires 25 October 2022 [Page 55]
Internet-Draft FLAC April 2022
B.2.7. First audio frame
The frame header starts at position 0x88 and is broken down in the
following table.
+========+========+=================+==============================+
| Start | Length | Contents | Description |
+========+========+=================+==============================+
| 0x88+0 | 15 bit | 0xff, 0b1111100 | frame sync |
+--------+--------+-----------------+------------------------------+
| 0x89+7 | 1 bit | 0b0 | blocksize strategy |
+--------+--------+-----------------+------------------------------+
| 0x8a+0 | 4 bit | 0b0110 | 8-bit blocksize further down |
+--------+--------+-----------------+------------------------------+
| 0x8a+4 | 4 bit | 0b1001 | sample rate 44.1kHz |
+--------+--------+-----------------+------------------------------+
| 0x8b+0 | 4 bit | 0b1001 | right-side stereo |
+--------+--------+-----------------+------------------------------+
| 0x8b+4 | 3 bit | 0b100 | bit depth 16 bit |
+--------+--------+-----------------+------------------------------+
| 0x8b+7 | 1 bit | 0b0 | mandatory 0 bit |
+--------+--------+-----------------+------------------------------+
| 0x8c+0 | 1 byte | 0x00 | frame number 0 |
+--------+--------+-----------------+------------------------------+
| 0x8d+0 | 1 byte | 0x0f | blocksize 16 |
+--------+--------+-----------------+------------------------------+
| 0x8e+0 | 1 byte | 0x99 | frame header CRC |
+--------+--------+-----------------+------------------------------+
Table 31
The first subframe starts at byte 0x8f, it is broken down in the
following table excluding the coded residual. As this subframe codes
for a side channel, the bit depth is increased by 1 bit from 16 bit
to 17 bit. This is most clearly present in the unencoded warm-up
sample.
van Beurden & Weaver Expires 25 October 2022 [Page 56]
Internet-Draft FLAC April 2022
+========+========+=============+===========================+
| Start | Length | Contents | Description |
+========+========+=============+===========================+
| 0x8f+0 | 1 bit | 0b0 | mandatory 0 bit |
+--------+--------+-------------+---------------------------+
| 0x8f+1 | 6 bit | 0b001001 | fixed subframe, 1st order |
+--------+--------+-------------+---------------------------+
| 0x8f+7 | 1 bit | 0b0 | no wasted bits present |
+--------+--------+-------------+---------------------------+
| 0x90+0 | 17 bit | 0x0867, 0b0 | unencoded warm-up sample |
+--------+--------+-------------+---------------------------+
Table 32
The coded residual is broken down in the following table. All
quotients are unary coded, all remainders are unencoded with a number
of bits specified by the rice parameter.
+========+========+=================+=================+
| Start | Length | Contents | Description |
+========+========+=================+=================+
| 0x92+1 | 2 bit | 0b00 | Rice code with |
| | | | 4-bit parameter |
+--------+--------+-----------------+-----------------+
| 0x92+3 | 4 bit | 0b0000 | Partition order |
| | | | 0 |
+--------+--------+-----------------+-----------------+
| 0x92+7 | 4 bit | 0b1011 | Rice parameter |
| | | | 11 |
+--------+--------+-----------------+-----------------+
| 0x93+3 | 4 bit | 0b0001 | Quotient 3 |
+--------+--------+-----------------+-----------------+
| 0x93+7 | 11 bit | 0b00011110100 | Remainder 244 |
+--------+--------+-----------------+-----------------+
| 0x95+2 | 2 bit | 0b01 | Quotient 1 |
+--------+--------+-----------------+-----------------+
| 0x95+4 | 11 bit | 0b01000100001 | Remainder 545 |
+--------+--------+-----------------+-----------------+
| 0x96+7 | 2 bit | 0b01 | Quotient 1 |
+--------+--------+-----------------+-----------------+
| 0x97+1 | 11 bit | 0b00110011000 | Remainder 408 |
+--------+--------+-----------------+-----------------+
| 0x98+4 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+-----------------+-----------------+
| 0x98+5 | 11 bit | 0b11101011101 | Remainder 1885 |
+--------+--------+-----------------+-----------------+
| 0x9a+0 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+-----------------+-----------------+
van Beurden & Weaver Expires 25 October 2022 [Page 57]
Internet-Draft FLAC April 2022
| 0x9a+1 | 11 bit | 0b11101110000 | Remainder 1904 |
+--------+--------+-----------------+-----------------+
| 0x9b+4 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+-----------------+-----------------+
| 0x9b+5 | 11 bit | 0b10101101111 | Remainder 1391 |
+--------+--------+-----------------+-----------------+
| 0x9d+0 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+-----------------+-----------------+
| 0x9d+1 | 11 bit | 0b11000000000 | Remainder 1536 |
+--------+--------+-----------------+-----------------+
| 0x9e+4 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+-----------------+-----------------+
| 0x9e+5 | 11 bit | 0b10000010111 | Remainder 1047 |
+--------+--------+-----------------+-----------------+
| 0xa0+0 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+-----------------+-----------------+
| 0xa0+1 | 11 bit | 0b10010101110 | Remainder 1198 |
+--------+--------+-----------------+-----------------+
| 0xa1+4 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+-----------------+-----------------+
| 0xa1+5 | 11 bit | 0b01100100001 | Remainder 801 |
+--------+--------+-----------------+-----------------+
| 0xa3+0 | 13 bit | 0b0000000000001 | Quotient 12 |
+--------+--------+-----------------+-----------------+
| 0xa4+5 | 11 bit | 0b11011100111 | Remainder 1767 |
+--------+--------+-----------------+-----------------+
| 0xa6+0 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+-----------------+-----------------+
| 0xa6+1 | 11 bit | 0b01001110111 | Remainder 631 |
+--------+--------+-----------------+-----------------+
| 0xa7+4 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+-----------------+-----------------+
| 0xa7+5 | 11 bit | 0b01000100100 | Remainder 548 |
+--------+--------+-----------------+-----------------+
| 0xa9+0 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+-----------------+-----------------+
| 0xa9+1 | 11 bit | 0b01000010101 | Remainder 533 |
+--------+--------+-----------------+-----------------+
| 0xaa+4 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+-----------------+-----------------+
| 0xaa+5 | 11 bit | 0b00100001100 | Remainder 268 |
+--------+--------+-----------------+-----------------+
Table 33
At this point, the decoder should know it is done decoding the coded
residual, as it received 16 samples: 1 warm-up sample and 15 residual
samples. Each residual sample can be calculated from the quotient
van Beurden & Weaver Expires 25 October 2022 [Page 58]
Internet-Draft FLAC April 2022
and remainder, and undoing the zig-zag encoding. For example, the
value of the first zig-zag encoded residual sample is 3 * 2^11 + 244
= 6388. As this is an even number, the zig-zag encoding is undone by
dividing by 2, the residual sample value is 3194. This is done for
all residual samples in the next table
+==========+===========+=================+=======================+
| Quotient | Remainder | Zig-zag encoded | Residual sample value |
+==========+===========+=================+=======================+
| 3 | 244 | 6388 | 3194 |
+----------+-----------+-----------------+-----------------------+
| 1 | 545 | 2593 | -1297 |
+----------+-----------+-----------------+-----------------------+
| 1 | 408 | 2456 | 1228 |
+----------+-----------+-----------------+-----------------------+
| 0 | 1885 | 1885 | -943 |
+----------+-----------+-----------------+-----------------------+
| 0 | 1904 | 1904 | 952 |
+----------+-----------+-----------------+-----------------------+
| 0 | 1391 | 1391 | -696 |
+----------+-----------+-----------------+-----------------------+
| 0 | 1536 | 1536 | 768 |
+----------+-----------+-----------------+-----------------------+
| 0 | 1047 | 1047 | -524 |
+----------+-----------+-----------------+-----------------------+
| 0 | 1198 | 1198 | 599 |
+----------+-----------+-----------------+-----------------------+
| 0 | 801 | 801 | -401 |
+----------+-----------+-----------------+-----------------------+
| 12 | 1767 | 26343 | -13172 |
+----------+-----------+-----------------+-----------------------+
| 0 | 631 | 631 | -316 |
+----------+-----------+-----------------+-----------------------+
| 0 | 548 | 548 | 274 |
+----------+-----------+-----------------+-----------------------+
| 0 | 533 | 533 | -267 |
+----------+-----------+-----------------+-----------------------+
| 0 | 268 | 268 | 134 |
+----------+-----------+-----------------+-----------------------+
Table 34
It can be calculated that using a Rice code is in this case more
efficient than storing values unencoded. The rice code (excluding
the partition order and parameter) is 199 bits in length. The
largest residual value (-13172) would need 15 bits to be stored
unencoded, so storing all 15 samples with 15 bits results in a
sequence with a length of 225 bits.
van Beurden & Weaver Expires 25 October 2022 [Page 59]
Internet-Draft FLAC April 2022
The next step is using the predictor and the residuals to restore the
sample values. As this subframe uses a fixed predictor with order 1,
this means adding the residual value to the value of the previous
sample.
+===========+==============+
| Residual | Sample value |
+===========+==============+
| (warm-up) | 4302 |
+-----------+--------------+
| 3194 | 7496 |
+-----------+--------------+
| -1297 | 6199 |
+-----------+--------------+
| 1228 | 7427 |
+-----------+--------------+
| -943 | 6484 |
+-----------+--------------+
| 952 | 7436 |
+-----------+--------------+
| -696 | 6740 |
+-----------+--------------+
| 768 | 7508 |
+-----------+--------------+
| -524 | 6984 |
+-----------+--------------+
| 599 | 7583 |
+-----------+--------------+
| -401 | 7182 |
+-----------+--------------+
| -13172 | -5990 |
+-----------+--------------+
| -316 | -6306 |
+-----------+--------------+
| 274 | -6032 |
+-----------+--------------+
| -267 | -6299 |
+-----------+--------------+
| 134 | -6165 |
+-----------+--------------+
Table 35
With this, decoding of the first subframe is complete. Decoding of
the second subframe is very similar, as it also uses a fixed
predictor of order 1, so this is left as an exercise for the reader,
results are in the next table. The next step is stereo
decorrelation, which is done in the following table. As the stereo
van Beurden & Weaver Expires 25 October 2022 [Page 60]
Internet-Draft FLAC April 2022
decorrelation is right-side, in which the actual ordering of the
subframes is side-right, the samples in the right channel come
directly from the second subframe, while the samples in the left
channel are found by adding the values of both subframes for each
sample.
+============+============+========+=======+
| Subframe 1 | Subframe 2 | Left | Right |
+============+============+========+=======+
| 4302 | 6070 | 10372 | 6070 |
+------------+------------+--------+-------+
| 7496 | 10545 | 18041 | 10545 |
+------------+------------+--------+-------+
| 6199 | 8743 | 14942 | 8743 |
+------------+------------+--------+-------+
| 7427 | 10449 | 17876 | 10449 |
+------------+------------+--------+-------+
| 6484 | 9143 | 15627 | 9143 |
+------------+------------+--------+-------+
| 7436 | 10463 | 17899 | 10463 |
+------------+------------+--------+-------+
| 6740 | 9502 | 16242 | 9502 |
+------------+------------+--------+-------+
| 7508 | 10569 | 18077 | 10569 |
+------------+------------+--------+-------+
| 6984 | 9840 | 16824 | 9840 |
+------------+------------+--------+-------+
| 7583 | 10680 | 18263 | 10680 |
+------------+------------+--------+-------+
| 7182 | 10113 | 17295 | 10113 |
+------------+------------+--------+-------+
| -5990 | -8428 | -14418 | -8428 |
+------------+------------+--------+-------+
| -6306 | -8895 | -15201 | -8895 |
+------------+------------+--------+-------+
| -6032 | -8476 | -14508 | -8476 |
+------------+------------+--------+-------+
| -6299 | -8896 | -15195 | -8896 |
+------------+------------+--------+-------+
| -6165 | -8653 | -14818 | -8653 |
+------------+------------+--------+-------+
Table 36
As the second subframe ends byte-aligned, no padding bits follow it.
Finally, the last 2 bytes in the frame is the frame CRC.
van Beurden & Weaver Expires 25 October 2022 [Page 61]
Internet-Draft FLAC April 2022
B.2.8. Second audio frame
The second audio frame is very similar to the frame decoded in the
first example, but this time not 1 but 3 samples are present.
The frame header starts at position 0xcc and is broken down in the
following table.
+========+========+=================+==============================+
| Start | Length | Contents | Description |
+========+========+=================+==============================+
| 0xcc+0 | 15 bit | 0xff, 0b1111100 | frame sync |
+--------+--------+-----------------+------------------------------+
| 0xcd+7 | 1 bit | 0b0 | blocksize strategy |
+--------+--------+-----------------+------------------------------+
| 0xce+0 | 4 bit | 0b0110 | 8-bit blocksize further down |
+--------+--------+-----------------+------------------------------+
| 0xce+4 | 4 bit | 0b1001 | sample rate 44.1kHz |
+--------+--------+-----------------+------------------------------+
| 0xcf+0 | 4 bit | 0b0001 | stereo, no decorrelation |
+--------+--------+-----------------+------------------------------+
| 0xcf+4 | 3 bit | 0b100 | bit depth 16 bit |
+--------+--------+-----------------+------------------------------+
| 0xcf+7 | 1 bit | 0b0 | mandatory 0 bit |
+--------+--------+-----------------+------------------------------+
| 0xd0+0 | 1 byte | 0x01 | frame number 1 |
+--------+--------+-----------------+------------------------------+
| 0xd1+0 | 1 byte | 0x02 | blocksize 3 |
+--------+--------+-----------------+------------------------------+
| 0xd2+0 | 1 byte | 0xa4 | frame header CRC |
+--------+--------+-----------------+------------------------------+
Table 37
The first subframe starts at 0xd3+0 and is broken down in the
following table.
van Beurden & Weaver Expires 25 October 2022 [Page 62]
Internet-Draft FLAC April 2022
+========+========+==========+=========================+
| Start | Length | Contents | Description |
+========+========+==========+=========================+
| 0xd3+0 | 1 bit | 0b0 | mandatory 0 bit |
+--------+--------+----------+-------------------------+
| 0xd3+1 | 6 bit | 0b000001 | verbatim subframe |
+--------+--------+----------+-------------------------+
| 0xd3+7 | 1 bit | 0b0 | no wasted bits present |
+--------+--------+----------+-------------------------+
| 0xd4+0 | 16 bit | 0xc382 | 16-bit unencoded sample |
+--------+--------+----------+-------------------------+
| 0xd6+0 | 16 bit | 0xc40b | 16-bit unencoded sample |
+--------+--------+----------+-------------------------+
| 0xd8+0 | 16 bit | 0xc14a | 16-bit unencoded sample |
+--------+--------+----------+-------------------------+
Table 38
The second subframe starts at 0xda+0 and is broken down in the
following table
+========+========+===================+=========================+
| Start | Length | Contents | Description |
+========+========+===================+=========================+
| 0xda+0 | 1 bit | 0b0 | mandatory 0 bit |
+--------+--------+-------------------+-------------------------+
| 0xda+1 | 6 bit | 0b000001 | verbatim subframe |
+--------+--------+-------------------+-------------------------+
| 0xda+7 | 1 bit | 0b1 | wasted bits present |
+--------+--------+-------------------+-------------------------+
| 0xdb+0 | 1 bit | 0b1 | 1 wasted bit |
+--------+--------+-------------------+-------------------------+
| 0xdb+1 | 15 bit | 0b110111001001000 | 15-bit unencoded sample |
+--------+--------+-------------------+-------------------------+
| 0xdd+0 | 15 bit | 0b110111010000001 | 15-bit unencoded sample |
+--------+--------+-------------------+-------------------------+
| 0xde+7 | 15 bit | 0b110110110011111 | 15-bit unencoded sample |
+--------+--------+-------------------+-------------------------+
Table 39
As this subframe has wasted bits, the 15-bit unencoded samples need
to be shifted left by 1 bit. For example, sample 1 is stored as
-4536 and becomes -9072 after shifting left 1 bit.
As the last subframe does not end on byte alignment, 2 padding bits
are added before the 2 byte frame CRC follows at 0xe1+0.
van Beurden & Weaver Expires 25 October 2022 [Page 63]
Internet-Draft FLAC April 2022
B.2.9. MD5 checksum verification
All samples in the file have been decoded, we can now verify the MD5
sum. All sample values must be interleaved and stored signed, coded
little-endian. The result of this follows in groups of 12 samples
(i.e. 6 interchannel samples)
0x8428 B617 7946 3129 5E3A 2722 D445 D128 0B3D B723 EB45 DF28
0x723f 1E25 9D46 4929 B841 7026 5747 B829 8F43 8127 AEC7 14DF
0x9FC4 41DD 54C7 E4DE A5C4 40DD 1EC6 33DE 82C3 90DC 0BC4 02DD
0x4AC1 3EDB
The MD5sum of this is indeed the same as the one found in the
streaminfo metadata block.
B.3. Decoding example 3
This example is once again a very short FLAC file. The focus of this
example is on decoding a subframe with a linear predictor and a coded
residual with more than one partition.
B.3.1. Example file 3 in hexadecimal representation
00000000: 664c 6143 8000 0022 1000 1000 fLaC..."....
0000000c: 0000 1f00 001f 07d0 0070 0000 .........p..
00000018: 0018 f8f9 e396 f5cb cfc6 dc80 ............
00000024: 7f99 7790 6b32 fff8 6802 0017 ..w.k2..h...
00000030: e944 004f 6f31 3d10 47d2 27cb .D.Oo1=.G.'.
0000003c: 6d09 0831 452b dc28 2222 8057 m..1E+.("".W
00000048: a3 .
B.3.2. Example file 3 in binary representation (only audio frame)
0000002a: 11111111 11111000 01101000 00000010 ..h.
0000002e: 00000000 00010111 11101001 01000100 ...D
00000032: 00000000 01001111 01101111 00110001 .Oo1
00000036: 00111101 00010000 01000111 11010010 =.G.
0000003a: 00100111 11001011 01101101 00001001 '.m.
0000003e: 00001000 00110001 01000101 00101011 .1E+
00000042: 11011100 00101000 00100010 00100010 .(""
00000046: 10000000 01010111 10100011 .W.
B.3.3. Signature and streaminfo
Most of the streaminfo block is the same as in example 1, so only
parts that are different are listed in the following table
van Beurden & Weaver Expires 25 October 2022 [Page 64]
Internet-Draft FLAC April 2022
+========+=========+====================+=========================+
| Start | Length | Contents | Description |
+========+=========+====================+=========================+
| 0x0c+0 | 3 byte | 0x00001f | Min. frame size 31 byte |
+--------+---------+--------------------+-------------------------+
| 0x0f+0 | 3 byte | 0x00001f | Max. frame size 31 byte |
+--------+---------+--------------------+-------------------------+
| 0x12+0 | 20 bit | 0x07d0, 0x0000 | Sample rate 32000 Hertz |
+--------+---------+--------------------+-------------------------+
| 0x14+4 | 3 bit | 0b000 | 1 channel |
+--------+---------+--------------------+-------------------------+
| 0x14+7 | 5 bit | 0b00111 | Sample bit depth 8 bit |
+--------+---------+--------------------+-------------------------+
| 0x15+4 | 36 bit | 0b0000, 0x00000018 | Total no. of samples 24 |
+--------+---------+--------------------+-------------------------+
| 0x1a | 16 byte | (...) | MD5 signature |
+--------+---------+--------------------+-------------------------+
Table 40
B.3.4. Audio frame
The frame header starts at position 0x2a and is broken down in the
following table.
van Beurden & Weaver Expires 25 October 2022 [Page 65]
Internet-Draft FLAC April 2022
+========+========+=================+==============================+
| Start | Length | Contents | Description |
+========+========+=================+==============================+
| 0x2a+0 | 15 bit | 0xff, 0b1111100 | Frame sync |
+--------+--------+-----------------+------------------------------+
| 0x2b+7 | 1 bit | 0b0 | Blocksize strategy |
+--------+--------+-----------------+------------------------------+
| 0x2c+0 | 4 bit | 0b0110 | 8-bit blocksize further down |
+--------+--------+-----------------+------------------------------+
| 0x2c+4 | 4 bit | 0b1000 | Sample rate 32kHz |
+--------+--------+-----------------+------------------------------+
| 0x2d+0 | 4 bit | 0b0000 | Mono audio (1 channel) |
+--------+--------+-----------------+------------------------------+
| 0x2d+4 | 3 bit | 0b001 | Bit depth 8 bit |
+--------+--------+-----------------+------------------------------+
| 0x2d+7 | 1 bit | 0b0 | Mandatory 0 bit |
+--------+--------+-----------------+------------------------------+
| 0x2e+0 | 1 byte | 0x00 | Frame number 0 |
+--------+--------+-----------------+------------------------------+
| 0x2f+0 | 1 byte | 0x17 | Blocksize 24 |
+--------+--------+-----------------+------------------------------+
| 0x30+0 | 1 byte | 0xe9 | Frame header CRC |
+--------+--------+-----------------+------------------------------+
Table 41
The first and only subframe starts at byte 0x31, it is broken down in
the following table, without the coded residual.
van Beurden & Weaver Expires 25 October 2022 [Page 66]
Internet-Draft FLAC April 2022
+========+========+==========+=====================+
| Start | Length | Contents | Description |
+========+========+==========+=====================+
| 0x31+0 | 1 bit | 0b0 | Mandatory 0 bit |
+--------+--------+----------+---------------------+
| 0x31+1 | 6 bit | 0b100010 | Linear prediction |
| | | | subframe, 3rd order |
+--------+--------+----------+---------------------+
| 0x31+7 | 1 bit | 0b0 | No wasted bits |
| | | | present |
+--------+--------+----------+---------------------+
| 0x32+0 | 8 bit | 0x00 | Unencoded warm-up |
| | | | sample 0 |
+--------+--------+----------+---------------------+
| 0x33+0 | 8 bit | 0x4f | Unencoded warm-up |
| | | | sample 79 |
+--------+--------+----------+---------------------+
| 0x34+0 | 8 bit | 0x6f | Unencoded warm-up |
| | | | sample 111 |
+--------+--------+----------+---------------------+
| 0x35+0 | 4 bit | 0b0011 | Coefficient |
| | | | precision 4 bit |
+--------+--------+----------+---------------------+
| 0x35+4 | 5 bit | 0b00010 | Prediction right |
| | | | shift 2 |
+--------+--------+----------+---------------------+
| 0x36+1 | 4 bit | 0b0111 | Predictor |
| | | | coefficient 7 |
+--------+--------+----------+---------------------+
| 0x36+5 | 4 bit | 0b1010 | Predictor |
| | | | coefficient -6 |
+--------+--------+----------+---------------------+
| 0x37+1 | 4 bit | 0b0010 | Predictor |
| | | | coefficient 2 |
+--------+--------+----------+---------------------+
Table 42
The data stream continues with the coded residual, which is broken
down in the following table. Residual partition 3 and 4 are left as
an exercise for the reader.
+========+========+==========+======================================+
| Start | Length | Contents | Description |
+========+========+==========+======================================+
| 0x37+5 | 2 bit | 0b00 | Rice-coded residual, |
| | | | 4-bit parameter |
+--------+--------+----------+--------------------------------------+
van Beurden & Weaver Expires 25 October 2022 [Page 67]
Internet-Draft FLAC April 2022
| 0x37+7 | 4 bit | 0b0010 | Partition order 2 |
+--------+--------+----------+--------------------------------------+
| 0x38+3 | 4 bit | 0b0011 | Rice parameter 3 |
+--------+--------+----------+--------------------------------------+
| 0x38+7 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+----------+--------------------------------------+
| 0x39+0 | 3 bit | 0b110 | Remainder 6 |
+--------+--------+----------+--------------------------------------+
| 0x39+3 | 1 bit | 0b1 | Quotient 0 |
+--------+--------+----------+--------------------------------------+
| 0x39+4 | 3 bit | 0b001 | Remainder 1 |
+--------+--------+----------+--------------------------------------+
| 0x39+7 | 4 bit | 0b0001 | Quotient 3 |
+--------+--------+----------+--------------------------------------+
| 0x3a+3 | 3 bit | 0b001 | Remainder 1 |
+--------+--------+----------+--------------------------------------+
| 0x3a+6 | 4 bit | 0b1111 | No rice parameter, |
| | | | escape code |
+--------+--------+----------+--------------------------------------+
| 0x3b+2 | 5 bit | 0b00101 | Partition encoded |
| | | | with 5 bits |
+--------+--------+----------+--------------------------------------+
| 0x3b+7 | 5 bit | 0b10110 | Residual -10 |
+--------+--------+----------+--------------------------------------+
| 0x3c+4 | 5 bit | 0b11010 | Residual -6 |
+--------+--------+----------+--------------------------------------+
| 0x3d+1 | 5 bit | 0b00010 | Residual 2 |
+--------+--------+----------+--------------------------------------+
| 0x3d+6 | 5 bit | 0b01000 | Residual 8 |
+--------+--------+----------+--------------------------------------+
| 0x3e+3 | 5 bit | 0b01000 | Residual 8 |
+--------+--------+----------+--------------------------------------+
| 0x3f+0 | 5 bit | 0b00110 | Residual 6 |
+--------+--------+----------+--------------------------------------+
| 0x3f+5 | 4 bit | 0b0010 | Rice parameter 2 |
+--------+--------+----------+--------------------------------------+
| 0x40+1 | 22 bit | (...) | Residual partition 3 |
+--------+--------+----------+--------------------------------------+
| 0x42+7 | 4 bit | 0b0001 | Rice parameter 1 |
+--------+--------+----------+--------------------------------------+
| 0x43+3 | 23 bit | (...) | Residual partition 4 |
+--------+--------+----------+--------------------------------------+
Table 43
The frame ends with 6 padding bits and a 2 byte frame CRC
van Beurden & Weaver Expires 25 October 2022 [Page 68]
Internet-Draft FLAC April 2022
To decode this subframe, 21 predictions have to be calculated and
added to their corresponding residuals. This is a sequential
process: as each prediction uses previous samples, it is not possible
to start this decoding halfway a subframe or decode a subframe with
parallel threads.
+===========+=====================+===========+==============+
| Residual | Predictor w/o shift | Predictor | Sample value |
+===========+=====================+===========+==============+
| (warm-up) | N/A | N/A | 0 |
+-----------+---------------------+-----------+--------------+
| (warm-up) | N/A | N/A | 79 |
+-----------+---------------------+-----------+--------------+
| (warm-up) | N/A | N/A | 111 |
+-----------+---------------------+-----------+--------------+
| 3 | 303 | 75 | 78 |
+-----------+---------------------+-----------+--------------+
| -1 | 38 | 9 | 8 |
+-----------+---------------------+-----------+--------------+
| -13 | -190 | -48 | -61 |
+-----------+---------------------+-----------+--------------+
| -10 | -319 | -80 | -90 |
+-----------+---------------------+-----------+--------------+
| -6 | -248 | -62 | -68 |
+-----------+---------------------+-----------+--------------+
| 2 | -58 | -15 | -13 |
+-----------+---------------------+-----------+--------------+
| 8 | 137 | 34 | 42 |
+-----------+---------------------+-----------+--------------+
| 8 | 236 | 59 | 67 |
+-----------+---------------------+-----------+--------------+
| 6 | 191 | 47 | 53 |
+-----------+---------------------+-----------+--------------+
| 0 | 53 | 13 | 13 |
+-----------+---------------------+-----------+--------------+
| -3 | -93 | -24 | -27 |
+-----------+---------------------+-----------+--------------+
| -5 | -161 | -41 | -46 |
+-----------+---------------------+-----------+--------------+
| -4 | -134 | -34 | -38 |
+-----------+---------------------+-----------+--------------+
| -1 | -44 | -11 | -12 |
+-----------+---------------------+-----------+--------------+
| 1 | 52 | 13 | 14 |
+-----------+---------------------+-----------+--------------+
| 1 | 94 | 23 | 24 |
+-----------+---------------------+-----------+--------------+
| 4 | 60 | 15 | 19 |
van Beurden & Weaver Expires 25 October 2022 [Page 69]
Internet-Draft FLAC April 2022
+-----------+---------------------+-----------+--------------+
| 2 | 17 | 4 | 6 |
+-----------+---------------------+-----------+--------------+
| 2 | -24 | -6 | -4 |
+-----------+---------------------+-----------+--------------+
| 2 | -26 | -7 | -5 |
+-----------+---------------------+-----------+--------------+
| 0 | 1 | 0 | 0 |
+-----------+---------------------+-----------+--------------+
Table 44
Lining all these samples up, we get the following input for the MD5
summing process.
0x004F 6F4E 08C3 A6BC F32A 4335 0DE5 D2DA F40E 1813 06FC FB00
Which indeed results in the MD5 signature found in the streaminfo
metadata block.
Authors' Addresses
Martijn van Beurden
Netherlands
Email: mvanb1@gmail.com
Andrew Weaver
Email: theandrewjw@gmail.com
van Beurden & Weaver Expires 25 October 2022 [Page 70]