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Versions: (draft-weaver-cellar-flac) 00 01 02            Standards Track
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
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   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.






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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



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     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



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   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:





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   *  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.





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   *  *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






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   *  *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.





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   *  *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:











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   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.





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   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.




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   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).









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      -  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.














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      -  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



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      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.




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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







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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










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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.                                     |
       +--------+-------------------------------------------------+



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       | 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.)    |
           +-------+-------------------------------------------+



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           | 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.






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   *  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 |



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   |                 | 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|



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   |                     |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 |



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   |        | 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






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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           |
              +-------+-------------------------------------+




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                                  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                |
                +-------+--------------------------------+



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                | 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.













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   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                                             |
     +--------+-----------------------------------------------------+



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     | 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).
















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    +==========+======================================================+
    |    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.










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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.






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   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










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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                           |
   +----------+-------------------------------------------------------+



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                                 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. |
             +------+----------------------------------------+



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                                  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. |
         +---------+---------------------------------------------+




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                                  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





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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










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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









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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





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   [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





































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