Free Lossless Audio Codec
draft-ietf-cellar-flac-02
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 | Michael Richardson , Andrew Weaver | ||
| Last updated | 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-02
cellar M. Richardson
Internet-Draft
Intended status: Informational A. Weaver
Expires: 2 May 2022 29 October 2021
Free Lossless Audio Codec
draft-ietf-cellar-flac-02
Abstract
This document defines FLAC, which stands for Free Lossless Audio
Codec, a free, open source codec for lossless audio compression and
decompression.
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 2 May 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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.
Richardson & Weaver Expires 2 May 2022 [Page 1]
Internet-Draft FLAC October 2021
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Notation and Conventions . . . . . . . . . . . . . . . . . . 3
3. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 3
4. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 4
6. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. Blocking . . . . . . . . . . . . . . . . . . . . . . . . . . 7
8. Interchannel Decorrelation . . . . . . . . . . . . . . . . . 7
9. Prediction . . . . . . . . . . . . . . . . . . . . . . . . . 8
10. Residual Coding . . . . . . . . . . . . . . . . . . . . . . . 9
11. Format . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Principles . . . . . . . . . . . . . . . . . . . . . . . 10
11.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 11
11.3. Subset . . . . . . . . . . . . . . . . . . . . . . . . . 14
11.4. Conventions . . . . . . . . . . . . . . . . . . . . . . 15
11.5. STREAM . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.6. METADATA_BLOCK . . . . . . . . . . . . . . . . . . . . . 15
11.7. METADATA_BLOCK_HEADER . . . . . . . . . . . . . . . . . 16
11.8. BLOCK_TYPE . . . . . . . . . . . . . . . . . . . . . . . 16
11.9. METADATA_BLOCK_DATA . . . . . . . . . . . . . . . . . . 17
11.10. METADATA_BLOCK_STREAMINFO . . . . . . . . . . . . . . . 17
11.11. METADATA_BLOCK_PADDING . . . . . . . . . . . . . . . . . 18
11.12. METADATA_BLOCK_APPLICATION . . . . . . . . . . . . . . . 18
11.13. METADATA_BLOCK_SEEKTABLE . . . . . . . . . . . . . . . . 19
11.14. SEEKPOINT . . . . . . . . . . . . . . . . . . . . . . . 19
11.15. METADATA_BLOCK_VORBIS_COMMENT . . . . . . . . . . . . . 20
11.16. METADATA_BLOCK_CUESHEET . . . . . . . . . . . . . . . . 20
11.17. CUESHEET_TRACK . . . . . . . . . . . . . . . . . . . . . 21
11.18. CUESHEET_TRACK_INDEX . . . . . . . . . . . . . . . . . . 22
11.19. METADATA_BLOCK_PICTURE . . . . . . . . . . . . . . . . . 23
11.20. PICTURE_TYPE . . . . . . . . . . . . . . . . . . . . . . 24
11.21. FRAME . . . . . . . . . . . . . . . . . . . . . . . . . 25
11.22. FRAME_HEADER . . . . . . . . . . . . . . . . . . . . . . 25
11.22.1. FRAME HEADER RESERVED . . . . . . . . . . . . . . . 26
11.22.2. BLOCKING STRATEGY . . . . . . . . . . . . . . . . . 26
11.22.3. INTERCHANNEL SAMPLE BLOCK SIZE . . . . . . . . . . 27
11.22.4. SAMPLE RATE . . . . . . . . . . . . . . . . . . . . 27
11.22.5. CHANNEL ASSIGNMENT . . . . . . . . . . . . . . . . 28
11.22.6. SAMPLE SIZE . . . . . . . . . . . . . . . . . . . . 30
11.22.7. FRAME HEADER RESERVED2 . . . . . . . . . . . . . . 30
11.22.8. CODED NUMBER . . . . . . . . . . . . . . . . . . . 30
11.22.9. BLOCK SIZE INT . . . . . . . . . . . . . . . . . . 31
11.22.10. SAMPLE RATE INT . . . . . . . . . . . . . . . . . . 31
11.22.11. FRAME CRC . . . . . . . . . . . . . . . . . . . . . 31
11.23. FRAME_FOOTER . . . . . . . . . . . . . . . . . . . . . . 31
11.24. SUBFRAME . . . . . . . . . . . . . . . . . . . . . . . . 32
Richardson & Weaver Expires 2 May 2022 [Page 2]
Internet-Draft FLAC October 2021
11.25. SUBFRAME_HEADER . . . . . . . . . . . . . . . . . . . . 32
11.25.1. SUBFRAME TYPE . . . . . . . . . . . . . . . . . . . 32
11.25.2. WASTED BITS PER SAMPLE FLAG . . . . . . . . . . . . 33
11.26. SUBFRAME_CONSTANT . . . . . . . . . . . . . . . . . . . 33
11.27. SUBFRAME_FIXED . . . . . . . . . . . . . . . . . . . . . 34
11.28. SUBFRAME_LPC . . . . . . . . . . . . . . . . . . . . . . 34
11.29. SUBFRAME_VERBATIM . . . . . . . . . . . . . . . . . . . 34
11.30. RESIDUAL . . . . . . . . . . . . . . . . . . . . . . . . 35
11.30.1. RESIDUAL_CODING_METHOD . . . . . . . . . . . . . . 35
11.30.2. RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB . . . 35
11.30.3. RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB2 . . 36
11.30.4. ENCODED RESIDUAL . . . . . . . . . . . . . . . . . 37
12. Security Considerations . . . . . . . . . . . . . . . . . . . 38
13. Normative References . . . . . . . . . . . . . . . . . . . . 38
14. Informative References . . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
1. Introduction
This is a detailed description of the FLAC format. There is also a
companion document that describes FLAC-to-Ogg mapping
(https://xiph.org/flac/ogg_mapping.html).
For a user-oriented overview, see About the FLAC Format
(https://xiph.org/flac/documentation_format_overview.html).
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.
3. Acknowledgments
FLAC owes much to the many people who have advanced the audio
compression field so freely. For instance: - A. J. Robinson
(http://svr-www.eng.cam.ac.uk/~ajr/) for his work on Shorten
(http://svr-www.eng.cam.ac.uk/reports/abstracts/robinson_tr156.html);
his paper 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 Exponential-Golomb 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
Richardson & Weaver Expires 2 May 2022 [Page 3]
Internet-Draft FLAC October 2021
determining the LPC coefficients from the autocorrelation
coefficients. - And of course, Claude Shannon
(http://en.wikipedia.org/wiki/Claude_Shannon)
4. Scope
FLAC stands for Free Lossless Audio Codec: it 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.
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 FLAC format is suited 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 reading and writing 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.
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.
The coding methods provided by the FLAC format works 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.
5. Architecture
Similar to many audio coders, a FLAC encoder has the following
stages:
Richardson & Weaver Expires 2 May 2022 [Page 4]
Internet-Draft FLAC October 2021
* Blocking (see section on Blocking (#blocking)). The input is
broken 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. Though FLAC allows the block size
to vary within a stream, the reference encoder uses a fixed block
size.
* Interchannel Decorrelation (see section on Interchannel
Decorrelation (#interchannel-decorrelation)). In the case of
stereo streams, the encoder will create mid and side signals based
on the average and difference (respectively) of the left and right
channels. The encoder will then pass the best form of the signal
to the next stage.
* Prediction (see section on Prediction (#prediction)). The block
is passed through a prediction stage where the encoder tries to
find a mathematical description (usually an approximate one) of
the signal. This description is typically much smaller than the
raw signal itself. 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. FLAC currently uses
four different classes of predictors, but the format has reserved
space for additional methods. FLAC allows the class of predictor
to change from block to block, or even within the channels of a
block.
* Residual Coding (See section on Residual Coding (#residual-
coding)). If 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
currently uses only one method for encoding the residual, but the
format has reserved space for additional methods. FLAC allows the
residual coding method to change from block to block, or even
within the channels of a block.
In addition, FLAC specifies a metadata system, which allows arbitrary
information about the stream to be included at the beginning of the
stream.
6. Definitions
* *Block*: A (short) section of linear pulse-code modulated audio,
with one or more channels.
Richardson & Weaver Expires 2 May 2022 [Page 5]
Internet-Draft FLAC October 2021
* *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).
* *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
Richardson & Weaver Expires 2 May 2022 [Page 6]
Internet-Draft FLAC October 2021
* *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.
7. 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.
Currently the reference encoder uses a fixed block size, optimized on
the sample rate of the input. Future versions MAY vary the block
size depending on the characteristics of the signal.
Blocked data is passed to the predictor stage one subblock (channel)
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, one channel of a stereo frame MAY be encoded as a
constant subframe, and the other an LPC subframe.
8. 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.
Richardson & Weaver Expires 2 May 2022 [Page 7]
Internet-Draft FLAC October 2021
* *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
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 left 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 left one bit. The left 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.
9. Prediction
FLAC uses four methods for modeling the input signal:
Richardson & Weaver Expires 2 May 2022 [Page 8]
Internet-Draft FLAC October 2021
1. *Verbatim*. This is essentially a zero-order predictor of the
signal. The predicted signal is zero, meaning the residual is
the signal itself, and the compression is zero. This is the
baseline against which the other predictors are measured. If you
feed random data to the encoder, the verbatim predictor will
probably be used for every subblock. Since the raw signal is not
actually passed through the residual coding stage (it is added to
the stream 'verbatim'), the encoding results will not be the same
as a zero-order linear predictor.
2. *Constant*. This predictor is used whenever the subblock is pure
DC ("digital silence"), i.e. a constant value throughout. The
signal is run-length encoded and added to the stream.
3. *Fixed linear predictor*. FLAC uses a class of computationally-
efficient fixed linear predictors (for a good description, see
audiopak (http://www.hpl.hp.com/techreports/1999/HPL-
1999-144.pdf) and shorten (http://svr-
www.eng.cam.ac.uk/reports/abstracts/robinson_tr156.html)). FLAC
adds a fourth-order predictor to the zero-to-third-order
predictors used by Shorten. Since the predictors are fixed, the
predictor order is the only parameter that needs to be stored in
the compressed stream. The error signal is then passed to the
residual coder.
4. *FIR Linear prediction*. For more accurate modeling (at a cost of
slower encoding), FLAC supports up to 32nd order FIR linear
prediction (again, for information on linear prediction, see
audiopak (http://www.hpl.hp.com/techreports/1999/HPL-
1999-144.pdf) and shorten (http://svr-
www.eng.cam.ac.uk/reports/abstracts/robinson_tr156.html)). The
reference encoder uses the Levinson-Durbin method for calculating
the LPC coefficients from the autocorrelation coefficients, and
the coefficients are quantized before computing the residual.
Whereas encoders such as Shorten used a fixed quantization for
the entire input, FLAC allows the quantized coefficient precision
to vary from subframe to subframe. The FLAC reference encoder
estimates the optimal precision to use based on the block size
and dynamic range of the original signal.
10. Residual Coding
FLAC uses Exponential-Golomb (a variant of Rice) coding as its
residual encoder. You can learn more about exp-golomb coding
(https://en.wikipedia.org/wiki/Exponential-Golomb_coding) on
Wikipedia.
Richardson & Weaver Expires 2 May 2022 [Page 9]
Internet-Draft FLAC October 2021
FLAC currently defines two similar methods for the coding of the
error signal from the prediction stage. The error signal is coded
using Exponential-Golomb codes in one of two ways:
1. the encoder estimates a single exp-golomb parameter based on the
variance of the residual and exp-golomb codes the entire residual
using this parameter;
2. the residual is partitioned into several equal-length regions of
contiguous samples, and each region is coded with its own exp-
golomb parameter based on the region's mean.
(Note that the first method is a special case of the second method
with one partition, except the exp-golomb parameter is based on the
residual variance instead of the mean.)
The FLAC format has reserved space for other coding methods. Some
possibilities for volunteers would be to explore better context-
modeling of the exp-golomb parameter, or Huffman coding. See LOCO-I
(http://www.hpl.hp.com/techreports/98/HPL-98-193.html) and pucrunch (
http://web.archive.org/web/20140827133312/http://www.cs.tut.fi/~alber
t/Dev/pucrunch/packing.html) for descriptions of several universal
codes.
11. Format
This section specifies the FLAC bitstream format.
11.1. 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.
Richardson & Weaver Expires 2 May 2022 [Page 10]
Internet-Draft FLAC October 2021
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
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.
11.2. Overview
Before the formal description of the stream, an overview 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).
Richardson & Weaver Expires 2 May 2022 [Page 11]
Internet-Draft FLAC October 2021
- 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.
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.
Richardson & Weaver Expires 2 May 2022 [Page 12]
Internet-Draft FLAC October 2021
- 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.
* 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
Richardson & Weaver Expires 2 May 2022 [Page 13]
Internet-Draft FLAC October 2021
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
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.
11.3. 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 in the FRAME_HEADER (see FRAME_HEADER section
(#frameheader)) 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 in the FRAME_HEADER MUST be 0b0001-0b1110.
* The bits-per-sample bits in the FRAME_HEADER MUST be 0b001-0b111.
* If the sample rate is <= 48000 Hz, the filter order in LPC
subframes (see SUBFRAME_LPC section (#subframelpc)) MUST be less
than or equal to 12, i.e. the subframe type bits in the
SUBFRAME_HEADER (see SUBFRAME_HEADER section (#subframeheader))
SHOULD NOT be 0b101100-0b111111.
* The Rice partition order (see Coded residual section (#coded-
residual)) MUST be less than or equal to 8.
Richardson & Weaver Expires 2 May 2022 [Page 14]
Internet-Draft FLAC October 2021
11.4. Conventions
The following tables constitute a formal description of the FLAC
format. Values expressed as u(n) represent unsigned big-endian
integer using n bits. 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.
11.5. STREAM
+===========================+=====================================+
| Data | Description |
+===========================+=====================================+
| u(32) | "fLaC", the FLAC stream marker in |
| | ASCII, meaning byte 0 of the stream |
| | is 0x66, followed by 0x4C 0x61 0x43 |
+---------------------------+-------------------------------------+
| METADATA_BLOCK_STREAMINFO | This is the mandatory STREAMINFO |
| | metadata block that has the basic |
| | properties of the stream. |
+---------------------------+-------------------------------------+
| METADATA_BLOCK* | Zero or more metadata blocks |
+---------------------------+-------------------------------------+
| FRAME+ | One or more audio frames |
+---------------------------+-------------------------------------+
Table 1
11.6. METADATA_BLOCK
+=======================+========================================+
| Data | Description |
+=======================+========================================+
| METADATA_BLOCK_HEADER | A block header that specifies the type |
| | and size of the metadata block data. |
+-----------------------+----------------------------------------+
| METADATA_BLOCK_DATA | |
+-----------------------+----------------------------------------+
Table 2
Richardson & Weaver Expires 2 May 2022 [Page 15]
Internet-Draft FLAC October 2021
11.7. METADATA_BLOCK_HEADER
+=======+=========================================================+
| Data | Description |
+=======+=========================================================+
| u(1) | Last-metadata-block flag: '1' if this block is the last |
| | metadata block before the audio blocks, '0' otherwise. |
+-------+---------------------------------------------------------+
| u(7) | BLOCK_TYPE |
+-------+---------------------------------------------------------+
| u(24) | Length (in bytes) of metadata to follow (does not |
| | include the size of the METADATA_BLOCK_HEADER) |
+-------+---------------------------------------------------------+
Table 3
11.8. BLOCK_TYPE
+=========+====================================================+
| Value | Description |
+=========+====================================================+
| 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 4
Richardson & Weaver Expires 2 May 2022 [Page 16]
Internet-Draft FLAC October 2021
11.9. METADATA_BLOCK_DATA
+===================================================+==============+
| Data | Description |
+===================================================+==============+
| METADATA_BLOCK_STREAMINFO || | The block |
| METADATA_BLOCK_PADDING || | data MUST |
| METADATA_BLOCK_APPLICATION || | match the |
| METADATA_BLOCK_SEEKTABLE || | block type |
| METADATA_BLOCK_VORBIS_COMMENT || | in the block |
| METADATA_BLOCK_CUESHEET || METADATA_BLOCK_PICTURE | header. |
+---------------------------------------------------+--------------+
Table 5
11.10. METADATA_BLOCK_STREAMINFO
+========+=================================================+
| Data | Description |
+========+=================================================+
| u(16) | The minimum block size (in samples) used in the |
| | stream. |
+--------+-------------------------------------------------+
| u(16) | The maximum block size (in samples) used in the |
| | stream. (Minimum blocksize == maximum |
| | blocksize) implies a fixed-blocksize 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. |
+--------+-------------------------------------------------+
Richardson & Weaver Expires 2 May 2022 [Page 17]
Internet-Draft FLAC October 2021
| 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. |
+--------+-------------------------------------------------+
Table 6
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 blocksize and maximum blocksize 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.
11.11. METADATA_BLOCK_PADDING
+======+========================================+
| Data | Description |
+======+========================================+
| u(n) | n '0' bits (n MUST be a multiple of 8) |
+------+----------------------------------------+
Table 7
11.12. METADATA_BLOCK_APPLICATION
+=======+===========================================+
| Data | Description |
+=======+===========================================+
| u(32) | Registered application ID. (Visit the |
| | registration page (https://xiph.org/flac/ |
| | id.html) to register an ID with FLAC.) |
+-------+-------------------------------------------+
Richardson & Weaver Expires 2 May 2022 [Page 18]
Internet-Draft FLAC October 2021
| u(n) | Application data (n MUST be a multiple of |
| | 8) |
+-------+-------------------------------------------+
Table 8
11.13. METADATA_BLOCK_SEEKTABLE
+============+==========================+
| Data | Description |
+============+==========================+
| SEEKPOINT+ | One or more seek points. |
+------------+--------------------------+
Table 9
NOTE - The number of seek points is implied by the metadata header
'length' field, i.e. equal to length / 18.
11.14. 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 10
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.
Richardson & Weaver Expires 2 May 2022 [Page 19]
Internet-Draft FLAC October 2021
* 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.
11.15. METADATA_BLOCK_VORBIS_COMMENT
+======+===========================================================+
| Data | Description |
+======+===========================================================+
| u(n) | Also known as FLAC tags, the contents of a vorbis comment |
| | packet as specified here (http://www.xiph.org/vorbis/doc/ |
| | v-comment.html) (without the framing bit). Note that the |
| | vorbis comment spec allows for on the order of 2^64 bytes |
| | of data where as 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 little-endian coded according to the |
| | vorbis spec, as opposed to the usual big-endian coding of |
| | fixed-length integers in the rest of FLAC. |
+------+-----------------------------------------------------------+
Table 11
11.16. METADATA_BLOCK_CUESHEET
+=================+================================================+
| Data | Description |
+=================+================================================+
| u(128*8) | Media catalog number, in ASCII printable |
| | characters 0x20-0x7E. In general, the media |
| | catalog number SHOULD be 0 to 128 bytes long; |
| | any unused characters SHOULD be right-padded |
| | with NUL characters. For CD-DA, this is a |
| | thirteen digit number, followed by 115 NUL |
| | bytes. |
+-----------------+------------------------------------------------+
| u(64) | The number of lead-in samples. This field 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 the |
| | Red Book, the lead-in MUST be silence and CD |
| | grabbing software does not usually store it; |
| | additionally, the lead-in MUST be at least two |
Richardson & Weaver Expires 2 May 2022 [Page 20]
Internet-Draft FLAC October 2021
| | 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. |
+-----------------+------------------------------------------------+
| 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) | The number of tracks. Must be at least 1 |
| | (because of the requisite lead-out track). |
| | For CD-DA, this number MUST be no more than |
| | 100 (99 regular tracks and one lead-out |
| | track). |
+-----------------+------------------------------------------------+
| CUESHEET_TRACK+ | One or more tracks. A CUESHEET block is |
| | REQUIRED to have a lead-out track; it is |
| | always the last track in the CUESHEET. For |
| | CD-DA, the lead-out track number MUST be 170 |
| | as specified by the Red Book, otherwise it |
| | MUST be 255. |
+-----------------+------------------------------------------------+
Table 12
11.17. CUESHEET_TRACK
+=====================+=================================================+
|Data |Description |
+=====================+=================================================+
|u(64) |Track offset in samples, relative to the |
| |beginning of the FLAC audio stream. It is the |
| |offset to the first index point of the track. |
| |(Note how this differs from 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 offset MUST be evenly divisible by 588 |
| |samples (588 samples = 44100 samples/s * 1/75 s).|
+---------------------+-------------------------------------------------+
|u(8) |Track number. 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|
Richardson & Weaver Expires 2 May 2022 [Page 21]
Internet-Draft FLAC October 2021
| |non-CD-DA, the track number MUST for 255 for the |
| |lead-out. It is not REQUIRED but encouraged to |
| |start with track 1 and increase sequentially. |
| |Track numbers MUST be unique within a CUESHEET. |
+---------------------+-------------------------------------------------+
|u(12*8) |Track ISRC. This is a 12-digit alphanumeric |
| |code; see here (http://isrc.ifpi.org/) and here |
| |(http://www.disctronics.co.uk/technology/cdaudio/|
| |cdaud_isrc.htm). A value of 12 ASCII NUL |
| |characters MAY be used to denote absence of an |
| |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; see here |
| |(http://www.chipchapin.com/CDMedia/cdda9.php3). |
+---------------------+-------------------------------------------------+
|u(6+13*8) |Reserved. All bits MUST be set to zero. |
+---------------------+-------------------------------------------------+
|u(8) |The number of track index points. There MUST be |
| |at least one index in every track in a CUESHEET |
| |except for the lead-out track, which MUST have |
| |zero. For CD-DA, this number SHOULD NOT be more |
| |than 100. |
+---------------------+-------------------------------------------------+
|CUESHEET_TRACK_INDEX+|For all tracks except the lead-out track, one or |
| |more track index points. |
+---------------------+-------------------------------------------------+
Table 13
11.18. CUESHEET_TRACK_INDEX
+========+=========================================================+
| Data | Description |
+========+=========================================================+
| u(64) | Offset in samples, relative to the track offset, of the |
| | index point. For CD-DA, the 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. |
+--------+---------------------------------------------------------+
| u(8) | The index point number. For CD-DA, an index number of |
| | 0 corresponds to the track pre-gap. The first index in |
Richardson & Weaver Expires 2 May 2022 [Page 22]
Internet-Draft FLAC October 2021
| | a track MUST have a number of 0 or 1, and subsequently, |
| | index numbers MUST increase by 1. Index numbers MUST |
| | be unique within a track. |
+--------+---------------------------------------------------------+
| u(3*8) | Reserved. All bits MUST be set to zero. |
+--------+---------------------------------------------------------+
Table 14
11.19. METADATA_BLOCK_PICTURE
+========+==================================================+
| Data | Description |
+========+==================================================+
| u(32) | The PICTURE_TYPE according to the ID3v2 APIC |
| | frame. |
+--------+--------------------------------------------------+
| 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 15
Richardson & Weaver Expires 2 May 2022 [Page 23]
Internet-Draft FLAC October 2021
11.20. PICTURE_TYPE
+=======+=====================================+
| Value | Description |
+=======+=====================================+
| 0 | Other |
+-------+-------------------------------------+
| 1 | 32x32 pixels 'file icon' (PNG only) |
+-------+-------------------------------------+
| 2 | Other file icon |
+-------+-------------------------------------+
| 3 | Cover (front) |
+-------+-------------------------------------+
| 4 | Cover (back) |
+-------+-------------------------------------+
| 5 | Leaflet page |
+-------+-------------------------------------+
| 6 | Media (e.g. label side of CD) |
+-------+-------------------------------------+
| 7 | Lead artist/lead performer/soloist |
+-------+-------------------------------------+
| 8 | Artist/performer |
+-------+-------------------------------------+
| 9 | Conductor |
+-------+-------------------------------------+
| 10 | Band/Orchestra |
+-------+-------------------------------------+
| 11 | Composer |
+-------+-------------------------------------+
| 12 | Lyricist/text writer |
+-------+-------------------------------------+
| 13 | Recording Location |
+-------+-------------------------------------+
| 14 | During recording |
+-------+-------------------------------------+
| 15 | During performance |
+-------+-------------------------------------+
| 16 | Movie/video screen capture |
+-------+-------------------------------------+
| 17 | A bright colored fish |
+-------+-------------------------------------+
| 18 | Illustration |
+-------+-------------------------------------+
| 19 | Band/artist logotype |
+-------+-------------------------------------+
| 20 | Publisher/Studio logotype |
+-------+-------------------------------------+
Richardson & Weaver Expires 2 May 2022 [Page 24]
Internet-Draft FLAC October 2021
Table 16
Other values are reserved and SHOULD NOT be used. There MAY only be
one each of picture type 1 and 2 in a file.
11.21. FRAME
+==============+=================================+
| Data | Description |
+==============+=================================+
| FRAME_HEADER | |
+--------------+---------------------------------+
| SUBFRAME+ | One SUBFRAME per channel. |
+--------------+---------------------------------+
| u(?) | Zero-padding to byte alignment. |
+--------------+---------------------------------+
| FRAME_FOOTER | |
+--------------+---------------------------------+
Table 17
11.22. FRAME_HEADER
+=======+================================+
| Data | Description |
+=======+================================+
| u(14) | Sync code '0b11111111111110' |
+-------+--------------------------------+
| u(1) | FRAME HEADER RESERVED |
+-------+--------------------------------+
| u(1) | BLOCKING STRATEGY |
+-------+--------------------------------+
| u(4) | INTERCHANNEL SAMPLE BLOCK SIZE |
+-------+--------------------------------+
| u(4) | SAMPLE RATE |
+-------+--------------------------------+
| u(4) | CHANNEL ASSIGNMENT |
+-------+--------------------------------+
| u(3) | SAMPLE SIZE |
+-------+--------------------------------+
| u(1) | FRAME HEADER RESERVED2 |
+-------+--------------------------------+
| u(?) | CODED NUMBER |
+-------+--------------------------------+
| u(?) | BLOCK SIZE INT |
+-------+--------------------------------+
| u(?) | SAMPLE RATE INT |
+-------+--------------------------------+
Richardson & Weaver Expires 2 May 2022 [Page 25]
Internet-Draft FLAC October 2021
| u(8) | FRAME CRC |
+-------+--------------------------------+
Table 18
11.22.1. FRAME HEADER RESERVED
+=======+=========================+
| Value | Description |
+=======+=========================+
| 0 | mandatory value |
+-------+-------------------------+
| 1 | reserved for future use |
+-------+-------------------------+
Table 19
FRAME HEADER RESERVED MUST remain reserved for 0 in order for a FLAC
frame's initial 15 bits to be distinguishable from the start of an
MPEG audio frame (see also (http://lists.xiph.org/pipermail/flac-
dev/2008-December/002607.html)).
11.22.2. BLOCKING STRATEGY
+=======+==================================+
| Value | Description |
+=======+==================================+
| 0 | fixed-blocksize stream; frame |
| | header encodes the frame number |
+-------+----------------------------------+
| 1 | variable-blocksize stream; frame |
| | header encodes the sample number |
+-------+----------------------------------+
Table 20
The BLOCKING STRATEGY bit MUST be the same throughout the entire
stream.
Richardson & Weaver Expires 2 May 2022 [Page 26]
Internet-Draft FLAC October 2021
The BLOCKING STRATEGY bit determines how to calculate the sample
number of the first sample in the frame. If the bit is 0 (fixed-
blocksize), the frame header encodes the frame number as above, and
the frame's starting sample number will be the frame number times the
blocksize. If it is 1 (variable-blocksize), the frame header encodes
the frame's starting sample number itself. (In the case of a fixed-
blocksize stream, only the last block MAY be shorter than the stream
blocksize; its starting sample number will be calculated as the frame
number times the previous frame's blocksize, or zero if it is the
first frame).
11.22.3. INTERCHANNEL SAMPLE BLOCK SIZE
+=================+=========================================+
| Value | Description |
+=================+=========================================+
| 0b0000 | reserved |
+-----------------+-----------------------------------------+
| 0b0001 | 192 samples |
+-----------------+-----------------------------------------+
| 0b0010 - 0b0101 | 576 * (2^(n-2)) samples, i.e. 576, |
| | 1152, 2304 or 4608 |
+-----------------+-----------------------------------------+
| 0b0110 | get 8 bit (blocksize-1) from end of |
| | header |
+-----------------+-----------------------------------------+
| 0b0111 | get 16 bit (blocksize-1) from end of |
| | header |
+-----------------+-----------------------------------------+
| 0b1000 - 0b1111 | 256 * (2^(n-8)) samples, i.e. 256, 512, |
| | 1024, 2048, 4096, 8192, 16384 or 32768 |
+-----------------+-----------------------------------------+
Table 21
11.22.4. SAMPLE RATE
+========+=====================================================+
| Value | Description |
+========+=====================================================+
| 0b0000 | get from STREAMINFO metadata block |
+--------+-----------------------------------------------------+
| 0b0001 | 88.2 kHz |
+--------+-----------------------------------------------------+
| 0b0010 | 176.4 kHz |
+--------+-----------------------------------------------------+
| 0b0011 | 192 kHz |
+--------+-----------------------------------------------------+
Richardson & Weaver Expires 2 May 2022 [Page 27]
Internet-Draft FLAC October 2021
| 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 | get 8 bit sample rate (in kHz) from end of header |
+--------+-----------------------------------------------------+
| 0b1101 | get 16 bit sample rate (in Hz) from end of header |
+--------+-----------------------------------------------------+
| 0b1110 | get 16 bit sample rate (in daHz) from end of header |
+--------+-----------------------------------------------------+
| 0b1111 | invalid, to prevent sync-fooling string of 1s |
+--------+-----------------------------------------------------+
Table 22
11.22.5. CHANNEL ASSIGNMENT
Values 0b0000-0b0111 represent the (number of independent channels)-1
coded independently, channel order follows SMPTE/ITU-R
recommendations. Values 0b1000-0b1010 represent 2 channel (stereo)
audio where the signal has been mapped to a different representation,
see section on Interchannel Decorrelation (#interchannel-
decorrelation).
Richardson & Weaver Expires 2 May 2022 [Page 28]
Internet-Draft FLAC October 2021
+==========+======================================================+
| Value | Description |
+==========+======================================================+
| 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 | left/side stereo: channel 0 is the left channel, |
| | channel 1 is the side(difference) channel |
+----------+------------------------------------------------------+
| 0b1001 | right/side stereo: channel 0 is the side(difference) |
| | channel, channel 1 is the right channel |
+----------+------------------------------------------------------+
| 0b1010 | mid/side stereo: channel 0 is the mid(average) |
| | channel, channel 1 is the side(difference) channel |
+----------+------------------------------------------------------+
| 0b1011 - | reserved |
| 0b1111 | |
+----------+------------------------------------------------------+
Table 23
Please note that the actual coded subframe order for right/side
stereo is side-right.
Richardson & Weaver Expires 2 May 2022 [Page 29]
Internet-Draft FLAC October 2021
11.22.6. SAMPLE SIZE
+=======+====================================+
| Value | Description |
+=======+====================================+
| 0b000 | get from 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 24
For subframes that encode a difference channel, the sample size is
one bit larger than the sample size of the frame, in order to be able
to encode the difference between extreme values.
11.22.7. FRAME HEADER RESERVED2
+=======+=========================+
| Value | Description |
+=======+=========================+
| 0 | mandatory value |
+-------+-------------------------+
| 1 | reserved for future use |
+-------+-------------------------+
Table 25
11.22.8. CODED NUMBER
Frame/Sample numbers are encoded using the UTF-8 format, from BEFORE
it was limited to 4 bytes by RFC3629, this variant supports the
original 7 byte maximum.
Richardson & Weaver Expires 2 May 2022 [Page 30]
Internet-Draft FLAC October 2021
Note to implementors: All Unicode compliant UTF-8 decoders and
encoders are limited to 4 bytes, it's best to just write your own one
off solution.
if(variable blocksize)
`u(8...56)`: "UTF-8" coded sample number (decoded number is 36 bits)
else
`u(8...48)`: "UTF-8" coded frame number (decoded number is 31 bits)
11.22.9. BLOCK SIZE INT
if(`INTERCHANNEL SAMPLE BLOCK SIZE` == 0b0110)
8 bit (blocksize-1)
else if(`INTERCHANNEL SAMPLE BLOCK SIZE` == 0b0111)
16 bit (blocksize-1)
11.22.10. SAMPLE RATE INT
if(`SAMPLE RATE` == 0b1100)
8 bit sample rate (in kHz)
else if(`SAMPLE RATE` == 0b1101)
16 bit sample rate (in Hz)
else if(`SAMPLE RATE` == 0b1110)
16 bit sample rate (in daHz)
11.22.11. FRAME CRC
CRC-8 (polynomial = x^8 + x^2 + x^1 + x^0, initialized with 0) of
everything before the CRC, including the sync code
11.23. FRAME_FOOTER
+=======+===================================================+
| Data | Description |
+=======+===================================================+
| u(16) | CRC-16 (polynomial = x^16 + x^15 + x^2 + x^0, |
| | initialized with 0) of everything before the CRC, |
| | back to and including the frame header sync code |
+-------+---------------------------------------------------+
Table 26
Richardson & Weaver Expires 2 May 2022 [Page 31]
Internet-Draft FLAC October 2021
11.24. SUBFRAME
+========================================+======================+
| Data | Description |
+========================================+======================+
| SUBFRAME_HEADER | |
+----------------------------------------+----------------------+
| SUBFRAME_CONSTANT || SUBFRAME_FIXED || | The SUBFRAME_HEADER |
| SUBFRAME_LPC || SUBFRAME_VERBATIM | specifies which one. |
+----------------------------------------+----------------------+
Table 27
11.25. SUBFRAME_HEADER
+========+========================================================+
| Data | Description |
+========+========================================================+
| u(1) | Zero bit padding, to prevent sync-fooling string of 1s |
+--------+--------------------------------------------------------+
| u(6) | SUBFRAME TYPE (see section on SUBFRAME TYPE |
| | (#subframe-type)) |
+--------+--------------------------------------------------------+
| u(1+k) | WASTED BITS PER SAMPLE FLAG (see section on WASTED |
| | BITS PER SAMPLE FLAG (#wasted-bits-per-sample-flag)) |
+--------+--------------------------------------------------------+
Table 28
11.25.1. SUBFRAME TYPE
+==========+=======================================================+
| Value | Description |
+==========+=======================================================+
| 0b000000 | SUBFRAME_CONSTANT |
+----------+-------------------------------------------------------+
| 0b000001 | SUBFRAME_VERBATIM |
+----------+-------------------------------------------------------+
| 0b00001x | reserved |
+----------+-------------------------------------------------------+
| 0b0001xx | reserved |
+----------+-------------------------------------------------------+
| 0b001xxx | if(xxx <= 4) SUBFRAME_FIXED, xxx=order; else reserved |
+----------+-------------------------------------------------------+
| 0b01xxxx | reserved |
+----------+-------------------------------------------------------+
| 0b1xxxxx | SUBFRAME_LPC, xxxxx=order-1 |
+----------+-------------------------------------------------------+
Richardson & Weaver Expires 2 May 2022 [Page 32]
Internet-Draft FLAC October 2021
Table 29
11.25.2. WASTED BITS PER SAMPLE FLAG
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
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.
11.26. SUBFRAME_CONSTANT
+======+========================================+
| Data | Description |
+======+========================================+
| u(n) | Unencoded constant value of the |
| | subblock, n = frame's bits-per-sample. |
+------+----------------------------------------+
Richardson & Weaver Expires 2 May 2022 [Page 33]
Internet-Draft FLAC October 2021
Table 30
11.27. SUBFRAME_FIXED
+==========+========================================+
| Data | Description |
+==========+========================================+
| u(n) | Unencoded warm-up samples (n = frame's |
| | bits-per-sample * predictor order). |
+----------+----------------------------------------+
| RESIDUAL | Encoded residual |
+----------+----------------------------------------+
Table 31
11.28. SUBFRAME_LPC
+==========+========================================================+
| Data | Description |
+==========+========================================================+
| u(n) | Unencoded warm-up samples (n = frame's bits- |
| | per-sample * lpc order). |
+----------+--------------------------------------------------------+
| u(4) | (quantized linear predictor coefficients' |
| | precision in bits)-1 (NOTE: 0b1111 is invalid). |
+----------+--------------------------------------------------------+
| u(5) | Quantized linear predictor coefficient shift |
| | needed in bits (NOTE: this number is signed |
| | two's-complement). |
+----------+--------------------------------------------------------+
| u(n) | Unencoded predictor coefficients (n = qlp coeff |
| | precision * lpc order) (NOTE: the coefficients |
| | are signed two's-complement). |
+----------+--------------------------------------------------------+
| RESIDUAL | Encoded residual |
+----------+--------------------------------------------------------+
Table 32
11.29. SUBFRAME_VERBATIM
+=========+=============================================+
| Data | Description |
+=========+=============================================+
| u(n\*i) | Unencoded subblock, where n is frame's |
| | bits-per-sample and i is frame's blocksize. |
+---------+---------------------------------------------+
Richardson & Weaver Expires 2 May 2022 [Page 34]
Internet-Draft FLAC October 2021
Table 33
11.30. RESIDUAL
+================================================+======================+
|Data |Description |
+================================================+======================+
|u(2) |RESIDUAL_CODING_METHOD|
+------------------------------------------------+----------------------+
|RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB ||| |
|RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB2 | |
+------------------------------------------------+----------------------+
Table 34
11.30.1. RESIDUAL_CODING_METHOD
+=======+========================================================+
| Value | Description |
+=======+========================================================+
| 0b00 | partitioned Exp-Golomb coding with 4-bit Exp-Golomb |
| | parameter; |
| | RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB follows |
+-------+--------------------------------------------------------+
| 0b01 | partitioned Exp-Golomb coding with 5-bit Exp-Golomb |
| | parameter; |
| | RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB2 follows |
+-------+--------------------------------------------------------+
| 0b10 | reserved |
| - | |
| 0b11 | |
+-------+--------------------------------------------------------+
Table 35
11.30.2. RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB
+=======================+===================================+
| Data | Description |
+=======================+===================================+
| u(4) | Partition order. |
+-----------------------+-----------------------------------+
| EXP_GOLOMB_PARTITION+ | There will be 2^order partitions. |
+-----------------------+-----------------------------------+
Table 36
Richardson & Weaver Expires 2 May 2022 [Page 35]
Internet-Draft FLAC October 2021
11.30.2.1. EXP_GOLOMB_PARTITION
+==========+====================================================+
| Data | Description |
+==========+====================================================+
| u(4(+5)) | EXP-GOLOMB PARTITION ENCODING PARAMETER (see |
| | section on EXP-GOLOMB PARTITION ENCODING PARAMETER |
| | (#exp-golomb-partition-encoding-parameter)) |
+----------+----------------------------------------------------+
| u(?) | ENCODED RESIDUAL (see section on ENCODED RESIDUAL |
| | (#encoded-residual)) |
+----------+----------------------------------------------------+
Table 37
11.30.2.2. EXP GOLOMB PARTITION ENCODING PARAMETER
+==========+==========================================+
| Value | Description |
+==========+==========================================+
| 0b0000 - | Exp-golomb parameter. |
| 0b1110 | |
+----------+------------------------------------------+
| 0b1111 | Escape code, meaning the partition is in |
| | unencoded binary form using n bits per |
| | sample; n follows as a 5-bit number. |
+----------+------------------------------------------+
Table 38
11.30.3. RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB2
+========================+===================================+
| Data | Description |
+========================+===================================+
| u(4) | Partition order. |
+------------------------+-----------------------------------+
| EXP-GOLOMB2_PARTITION+ | There will be 2^order partitions. |
+------------------------+-----------------------------------+
Table 39
Richardson & Weaver Expires 2 May 2022 [Page 36]
Internet-Draft FLAC October 2021
11.30.3.1. EXP_GOLOMB2_PARTITION
+==========+=====================================================+
| Data | Description |
+==========+=====================================================+
| u(5(+5)) | EXP-GOLOMB2 PARTITION ENCODING PARAMETER (see |
| | section on EXP-GOLOMB2 PARTITION ENCODING PARAMETER |
| | (#expgolomb2-partition-encoding-parameter)) |
+----------+-----------------------------------------------------+
| u(?) | ENCODED RESIDUAL (see section on ENCODED RESIDUAL |
| | (#encoded-residual)) |
+----------+-----------------------------------------------------+
Table 40
11.30.3.2. EXP-GOLOMB2 PARTITION ENCODING PARAMETER
+===========+==========================================+
| Value | Description |
+===========+==========================================+
| 0b00000 - | Exp-golomb parameter. |
| 0b11110 | |
+-----------+------------------------------------------+
| 0b11111 | Escape code, meaning the partition is in |
| | unencoded binary form using n bits per |
| | sample; n follows as a 5-bit number. |
+-----------+------------------------------------------+
Table 41
11.30.4. ENCODED RESIDUAL
The number of samples (n) in the partition is determined as follows:
* if the partition order is zero, n = frame's blocksize - predictor
order
* else if this is not the first partition of the subframe, n =
(frame's blocksize / (2^partition order))
* else n = (frame's blocksize / (2^partition order)) - predictor
order
Richardson & Weaver Expires 2 May 2022 [Page 37]
Internet-Draft FLAC October 2021
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>.
[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
Richardson & Weaver Expires 2 May 2022 [Page 38]
Internet-Draft FLAC October 2021
[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>.
Authors' Addresses
Michael Richardson
Email: mcr@sandelman.ca
Andrew Weaver
Email: theandrewjw@gmail.com
Richardson & Weaver Expires 2 May 2022 [Page 39]