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Versions: (draft-weaver-cellar-flac) 00

cellar                                                        J. Coalson
Internet-Draft
Intended status: Standards Track
Expires: January 30, 2020                            Xiph.Org Foundation
                                                               A. Weaver
                                                           July 29, 2019


                       Free Lossless Audio Codec
                       draft-ietf-cellar-flac-00

Abstract

   This document defines FLAC, which stands for Free Lossless Audio
   Codec, a free, open source codec for lossless audio compression and
   decompression.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
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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 January 30, 2020.

Copyright Notice

   Copyright (c) 2019 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Notation and Conventions  . . . . . . . . . . . . . . . . . .   3
   3.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Blocking  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Interchannel Decorrelation  . . . . . . . . . . . . . . . . .   6
   9.  Prediction  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   10. Residual Coding . . . . . . . . . . . . . . . . . . . . . . .   7
   11. Format  . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     11.1.  Conventions  . . . . . . . . . . . . . . . . . . . . . .  11
     11.2.  STREAM . . . . . . . . . . . . . . . . . . . . . . . . .  12
     11.3.  METADATA_BLOCK . . . . . . . . . . . . . . . . . . . . .  12
     11.4.  METADATA_BLOCK_HEADER  . . . . . . . . . . . . . . . . .  12
     11.5.  BLOCK_TYPE . . . . . . . . . . . . . . . . . . . . . . .  12
     11.6.  METADATA_BLOCK_DATA  . . . . . . . . . . . . . . . . . .  13
     11.7.  METADATA_BLOCK_STREAMINFO  . . . . . . . . . . . . . . .  13
     11.8.  METADATA_BLOCK_PADDING . . . . . . . . . . . . . . . . .  14
     11.9.  METADATA_BLOCK_APPLICATION . . . . . . . . . . . . . . .  15
     11.10. METADATA_BLOCK_SEEKTABLE . . . . . . . . . . . . . . . .  15
     11.11. SEEKPOINT  . . . . . . . . . . . . . . . . . . . . . . .  15
     11.12. METADATA_BLOCK_VORBIS_COMMENT  . . . . . . . . . . . . .  16
     11.13. METADATA_BLOCK_CUESHEET  . . . . . . . . . . . . . . . .  16
     11.14. CUESHEET_TRACK . . . . . . . . . . . . . . . . . . . . .  18
     11.15. CUESHEET_TRACK_INDEX . . . . . . . . . . . . . . . . . .  19
     11.16. METADATA_BLOCK_PICTURE . . . . . . . . . . . . . . . . .  19
     11.17. PICTURE_TYPE . . . . . . . . . . . . . . . . . . . . . .  19
     11.18. FRAME  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     11.19. FRAME_HEADER . . . . . . . . . . . . . . . . . . . . . .  20
       11.19.1.  FRAME HEADER RESERVED . . . . . . . . . . . . . . .  21
       11.19.2.  BLOCKING STRATEGY . . . . . . . . . . . . . . . . .  21
       11.19.3.  INTERCHANNEL SAMPLE BLOCK SIZE  . . . . . . . . . .  22
       11.19.4.  SAMPLE RATE . . . . . . . . . . . . . . . . . . . .  22
       11.19.5.  CHANNEL ASSIGNMENT  . . . . . . . . . . . . . . . .  23
       11.19.6.  SAMPLE SIZE . . . . . . . . . . . . . . . . . . . .  23
       11.19.7.  FRAME HEADER RESERVED2  . . . . . . . . . . . . . .  24
       11.19.8.  CODED NUMBER  . . . . . . . . . . . . . . . . . . .  24
       11.19.9.  BLOCK SIZE INT  . . . . . . . . . . . . . . . . . .  24
       11.19.10. SAMPLE RATE INT . . . . . . . . . . . . . . . . . .  24
       11.19.11. FRAME CRC . . . . . . . . . . . . . . . . . . . . .  25
     11.20. FRAME_FOOTER . . . . . . . . . . . . . . . . . . . . . .  25
     11.21. SUBFRAME . . . . . . . . . . . . . . . . . . . . . . . .  25
     11.22. SUBFRAME_HEADER  . . . . . . . . . . . . . . . . . . . .  25
       11.22.1.  SUBFRAME TYPE . . . . . . . . . . . . . . . . . . .  25
       11.22.2.  WASTED BITS PER SAMPLE FLAG . . . . . . . . . . . .  26



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     11.23. SUBFRAME_CONSTANT  . . . . . . . . . . . . . . . . . . .  26
     11.24. SUBFRAME_FIXED . . . . . . . . . . . . . . . . . . . . .  26
     11.25. SUBFRAME_LPC . . . . . . . . . . . . . . . . . . . . . .  27
     11.26. SUBFRAME_VERBATIM  . . . . . . . . . . . . . . . . . . .  27
     11.27. RESIDUAL . . . . . . . . . . . . . . . . . . . . . . . .  27
       11.27.1.  RESIDUAL_CODING_METHOD  . . . . . . . . . . . . . .  27
       11.27.2.  RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB . . .  28
       11.27.3.  RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB2  . .  29
       11.27.4.  ENCODED RESIDUAL  . . . . . . . . . . . . . . . . .  29
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     12.2.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   This is a detailed description of the FLAC format.  There is also a
   companion document that describes FLAC-to-Ogg mapping [1].

   For a user-oriented overview, see About the FLAC Format [2].

2.  Notation and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Acknowledgments

   FLAC owes much to the many people who have advanced the audio
   compression field so freely.  For instance: - A.  J.  Robinson [3]
   for his work on Shorten [4]; 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 [5] and
   Robert F.  Rice; their universal codes are used by FLAC's entropy
   coder.  - N.  Levinson and J.  Durbin; the reference encoder uses an
   algorithm developed and refined by them for determining the LPC
   coefficients from the autocorrelation coefficients.  - And of course,
   Claude Shannon [6]

4.  Scope

   It is a known fact that no algorithm can losslessly compress all
   possible input, so most compressors restrict themselves to a useful
   domain and try to work as well as possible within that domain.
   FLAC's domain is audio data.  Though it can losslessly code any
   input, only certain kinds of input will get smaller.  FLAC exploits



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   the fact that audio data typically has a high degree of sample-to-
   sample correlation.

   Within the audio domain, there are many possible subdomains.  For
   example: low bitrate speech, high-bitrate multi-channel music, etc.
   FLAC itself does not target a specific subdomain, but many of the
   default parameters of the reference encoder are tuned to CD-quality
   music data (i.e. 44.1 kHz, 2 channel, 16 bits per sample).  The
   effect of the encoding parameters on different kinds of audio data
   will be examined later.

5.  Architecture

   Similar to many audio coders, a FLAC encoder has the following
   stages:

   o  "Blocking" (see Section 7).  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.

   o  "Interchannel Decorrelation" (see Section 8).  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.

   o  "Prediction" (see Section 9).  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.

   o  "Residual Coding" (See Section 10).  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




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      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

   Many terms like "block" and "frame" are used to mean different things
   in different encoding schemes.  For example, a frame in MP3
   corresponds to many samples across several channels, whereas an S/
   PDIF frame represents just one sample for each channel.  The
   definitions we use for FLAC follow.  Note that when we talk about
   blocks and subblocks we are referring to the raw unencoded audio data
   that is the input to the encoder, and when we talk about frames and
   subframes, we are referring to the FLAC-encoded data.

   o  *Block*: One or more audio samples that span several channels.

   o  *Subblock*: One or more audio samples within a channel.  A block
      contains one subblock for each channel, and all subblocks contain
      the same number of samples.

   o  *Blocksize*: The number of samples in any of a block's subblocks.
      For example, a one second block sampled at 44.1 kHz has a
      blocksize of 44100, regardless of the number of channels.

   o  *Frame*: A frame header plus one or more subframes.

   o  *Subframe*: A subframe header plus one or more encoded samples
      from a given channel.  All subframes within a frame will contain
      the same number of samples.

   o  *Exponential-Golomb coding*: One of Robert Rice's universal coding
      schemes, FLAC's residual coder, compresses data by writing the
      number of bits to be read minus 1, before writing the actual
      value.

   o  *LPC*: Linear predictive coding [7].

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



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   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 stereo streams, many times there is an exploitable amount of
   correlation between the left and right channels.  FLAC allows the
   frames of stereo streams to have different channel assignments, and
   an encoder MAY choose to use the best representation on a frame-by-
   frame basis.

   o  *Independent*. The left and right channels are coded
      independently.

   o  *Mid-side*. The left and right channels are transformed into mid
      and side channels.  The mid channel is the midpoint (average) of
      the left and right signals, and the side is the difference signal
      (left minus right).

   o  *Left-side*. The left channel and side channel are coded.

   o  *Right-side*. The right channel and side channel are coded.

   Surprisingly, the left-side and right-side forms can be the most
   efficient in many frames, even though the raw number of bits per
   sample needed for the original signal is slightly more than that
   needed for independent or mid-side coding.

9.  Prediction

   FLAC uses four methods for modeling the input signal:

   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



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       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 [8] and shorten [9]).  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 [10] and shorten [11]).  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 [12] on
   Wikipedia.

   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.



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   (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
   [13] and pucrunch [14] for descriptions of several universal codes.

11.  Format

   This section specifies the FLAC bitstream format.  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.  All numbers MUST be unsigned unless otherwise specified.

   Before the formal description of the stream, an overview might be
   helpful.

   o  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.

   o  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



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         metadata is added, it will simply overwrite the padding (which
         is relatively quick) instead of having to insert it into the
         right place in the existing file (which would normally require
         rewriting the entire file).

      *  "APPLICATION": This block is for use by third-party
         applications.  The only mandatory field is a 32-bit identifier.
         This ID is granted upon request to an application by the FLAC
         maintainers.  The remainder is of the block is defined by the
         registered application.  Visit the registration page [15] 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 [16]
         (without the framing bit).  This is the only officially
         supported tagging mechanism in FLAC.  There MUST be only zero
         or one VORBIS_COMMENT blocks in a stream.  In some external
         documentation, Vorbis comments are called FLAC tags to lessen
         confusion.

      *  "CUESHEET": This block is for storing various information that
         can be used in a cue sheet.  It supports track and index
         points, compatible with Red Book CD digital audio discs, as
         well as other CD-DA metadata such as media catalog number and
         track ISRCs.  The CUESHEET block is especially useful for
         backing up CD-DA discs, but it can be used as a general purpose
         cueing mechanism for playback.

      *  "PICTURE": This block is for storing pictures associated with
         the file, most commonly cover art from CDs.  There MAY be more
         than one PICTURE block in a file.  The picture format is
         similar to the APIC frame in ID3v2 [17].  The PICTURE block has
         a type, MIME type, and UTF-8 description like ID3v2, and
         supports external linking via URL (though this is discouraged).



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         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.

   o  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.

   o  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.

   o  Again, since a decoder MAY start decoding at an arbitrary frame in
      the stream, each frame header MUST contain some basic information
      about the stream because the decoder MAY not have access to the
      STREAMINFO metadata block at the start of the stream.  This
      information includes sample rate, bits per sample, number of
      channels, etc.  Since the frame header is pure overhead, it has a
      direct effect on the compression ratio.  To keep the frame header
      as small as possible, FLAC uses lookup tables for the most
      commonly used values for frame parameters.  For instance, the
      sample rate part of the frame header is specified using 4 bits.
      Eight of the bit patterns correspond to the commonly used sample
      rates of 8, 16, 22.05, 24, 32, 44.1, 48 or 96 kHz.  However, odd
      sample rates can be specified by using one of the 'hint' bit
      patterns, directing the decoder to find the exact sample rate at
      the end of the frame header.  The same method is used for
      specifying the block size and bits per sample.  In this way, the
      frame header size stays small for all of the most common forms of
      audio data.



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   o  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.

   o  "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 Section 11.19)
         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 Section 11.25) MUST be less than or equal to
         12, i.e. the subframe type bits in the "SUBFRAME_HEADER" (see
         Section 11.22) SHOULD NOT be 0b101100-0b111111.

      *  The Rice partition order in an "exp-golomb coded residual
         section" (see Section 11.27.2) MUST be less than or equal to 8.

11.1.  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



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   occurrences of the value.  The name of a value followed by a plus
   sign "+" indicates one or more occurrences of the value.

11.2.  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            |
   +-----------------------------+-------------------------------------+

11.3.  METADATA_BLOCK

   +-------------------------+-----------------------------------------+
   | Data                    | Description                             |
   +-------------------------+-----------------------------------------+
   | "METADATA_BLOCK_HEADER" | A block header that specifies the type  |
   |                         | and size of the metadata block data.    |
   | "METADATA_BLOCK_DATA"   |                                         |
   +-------------------------+-----------------------------------------+

11.4.  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")        |
   +---------+---------------------------------------------------------+

11.5.  BLOCK_TYPE










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

11.6.  METADATA_BLOCK_DATA

   +-------------------------------------------------+-----------------+
   | Data                                            | Description     |
   +-------------------------------------------------+-----------------+
   | "METADATA_BLOCK_STREAMINFO" ||                  | The block data  |
   | "METADATA_BLOCK_PADDING" ||                     | MUST match the  |
   | "METADATA_BLOCK_APPLICATION" ||                 | block type in   |
   | "METADATA_BLOCK_SEEKTABLE" ||                   | the block       |
   | "METADATA_BLOCK_VORBIS_COMMENT" ||              | header.         |
   | "METADATA_BLOCK_CUESHEET" ||                    |                 |
   | "METADATA_BLOCK_PICTURE"                        |                 |
   +-------------------------------------------------+-----------------+

11.7.  METADATA_BLOCK_STREAMINFO






















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

   NOTE

   o  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.

11.8.  METADATA_BLOCK_PADDING

            +--------+----------------------------------------+
            | Data   | Description                            |
            +--------+----------------------------------------+
            | "u(n)" | n '0' bits (n MUST be a multiple of 8) |
            +--------+----------------------------------------+





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11.9.  METADATA_BLOCK_APPLICATION

   +---------+---------------------------------------------------------+
   | Data    | Description                                             |
   +---------+---------------------------------------------------------+
   | "u(32)" | Registered application ID. (Visit the registration page |
   |         | [18] to register an ID with FLAC.)                      |
   | "u(n)"  | Application data (n MUST be a multiple of 8)            |
   +---------+---------------------------------------------------------+

11.10.  METADATA_BLOCK_SEEKTABLE

                +--------------+--------------------------+
                | Data         | Description              |
                +--------------+--------------------------+
                | "SEEKPOINT"+ | One or more seek points. |
                +--------------+--------------------------+

   NOTE - The number of seek points is implied by the metadata header
   'length' field, i.e. equal to length / 18.

11.11.  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.                  |
   +---------+---------------------------------------------------------+

   NOTES

   o  For placeholder points, the second and third field values are
      undefined.

   o  Seek points within a table MUST be sorted in ascending order by
      sample number.

   o  Seek points within a table MUST be unique by sample number, with
      the exception of placeholder points.

   o  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.



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11.12.  METADATA_BLOCK_VORBIS_COMMENT

   +--------+----------------------------------------------------------+
   | Data   | Description                                              |
   +--------+----------------------------------------------------------+
   | "u(n)" | Also known as FLAC tags, the contents of a vorbis        |
   |        | comment packet as specified here [19] (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.                            |
   +--------+----------------------------------------------------------+

11.13.  METADATA_BLOCK_CUESHEET
































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






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11.14.  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   |
   |                         | 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 [20] and    |
   |                         | here [21]. 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 [22].           |
   | "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.  |
   +-------------------------+-----------------------------------------+



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11.15.  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 |
   |          | 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.                |
   +----------+--------------------------------------------------------+

11.16.  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.                               |
   +----------+--------------------------------------------------------+

11.17.  PICTURE_TYPE









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

   Other values are reserved and SHOULD NOT be used.  There MAY only be
   one each of picture type 1 and 2 in a file.

11.18.  FRAME

           +----------------+---------------------------------+
           | Data           | Description                     |
           +----------------+---------------------------------+
           | "FRAME_HEADER" |                                 |
           | "SUBFRAME"+    | One SUBFRAME per channel.       |
           | "u(?)"         | Zero-padding to byte alignment. |
           | "FRAME_FOOTER" |                                 |
           +----------------+---------------------------------+

11.19.  FRAME_HEADER










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              +---------+----------------------------------+
              | 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"                |
              | "u(8)"  | "FRAME CRC"                      |
              +---------+----------------------------------+

11.19.1.  FRAME HEADER RESERVED

                    +-------+-------------------------+
                    | Value | Description             |
                    +-------+-------------------------+
                    |     0 | mandatory value         |
                    |     1 | reserved for future use |
                    +-------+-------------------------+

   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 [23]).

11.19.2.  BLOCKING STRATEGY

   +-------+-----------------------------------------------------------+
   | Value | Description                                               |
   +-------+-----------------------------------------------------------+
   |     0 | fixed-blocksize stream; frame header encodes the frame    |
   |       | number                                                    |
   |     1 | variable-blocksize stream; frame header encodes the       |
   |       | sample number                                             |
   +-------+-----------------------------------------------------------+

   The "BLOCKING STRATEGY" bit MUST be the same throughout the entire
   stream.

   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



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   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).

11.19.3.  INTERCHANNEL SAMPLE BLOCK SIZE

   +-------------+-----------------------------------------------------+
   |       Value | Description                                         |
   +-------------+-----------------------------------------------------+
   |      0b0000 | reserved                                            |
   |      0b0001 | 192 samples                                         |
   |    0b0010 - | 576 * (2^(n-2)) samples, i.e. 576, 1152, 2304 or    |
   |      0b0101 | 4608                                                |
   |      0b0110 | get 8 bit (blocksize-1) from end of header          |
   |      0b0111 | get 16 bit (blocksize-1) from end of header         |
   |    0b1000 - | 256 * (2^(n-8)) samples, i.e. 256, 512, 1024, 2048, |
   |      0b1111 | 4096, 8192, 16384 or 32768                          |
   +-------------+-----------------------------------------------------+

11.19.4.  SAMPLE RATE

     +--------+-----------------------------------------------------+
     |  Value | Description                                         |
     +--------+-----------------------------------------------------+
     | 0b0000 | get from STREAMINFO metadata block                  |
     | 0b0001 | 88.2 kHz                                            |
     | 0b0010 | 176.4 kHz                                           |
     | 0b0011 | 192 kHz                                             |
     | 0b0100 | 8 kHz                                               |
     | 0b0101 | 16 kHz                                              |
     | 0b0110 | 22.05 kHz                                           |
     | 0b0111 | 24 kHz                                              |
     | 0b1000 | 32 kHz                                              |
     | 0b1001 | 44.1 kHz                                            |
     | 0b1010 | 48 kHz                                              |
     | 0b1011 | 96 kHz                                              |
     | 0b1100 | 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       |
     +--------+-----------------------------------------------------+







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11.19.5.  CHANNEL ASSIGNMENT

   For values 0b0000-0b0111, the value represents the (number of
   independent channels)-1.  Where defined, the channel order follows
   SMPTE/ITU-R recommendations.

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

11.19.6.  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                           |
              +-------+------------------------------------+





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   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.

11.19.7.  FRAME HEADER RESERVED2

                    +-------+-------------------------+
                    | Value | Description             |
                    +-------+-------------------------+
                    |     0 | mandatory value         |
                    |     1 | reserved for future use |
                    +-------+-------------------------+

11.19.8.  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.

   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)

11.19.9.  BLOCK SIZE INT

            if(`INTERCHANNEL SAMPLE BLOCK SIZE` == 0b0110)
              8 bit (blocksize-1)
            else if(`INTERCHANNEL SAMPLE BLOCK SIZE` == 0b0111)
              16 bit (blocksize-1)

11.19.10.  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)








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11.19.11.  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

11.20.  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             |
   +---------+---------------------------------------------------------+

11.21.  SUBFRAME

   +-------------------------------------------+-----------------------+
   | Data                                      | Description           |
   +-------------------------------------------+-----------------------+
   | "SUBFRAME_HEADER"                         |                       |
   | "SUBFRAME_CONSTANT" || "SUBFRAME_FIXED"   | The SUBFRAME_HEADER   |
   | || "SUBFRAME_LPC" || "SUBFRAME_VERBATIM"  | specifies which one.  |
   +-------------------------------------------+-----------------------+

11.22.  SUBFRAME_HEADER

   +----------+--------------------------------------------------------+
   | Data     | Description                                            |
   +----------+--------------------------------------------------------+
   | "u(1)"   | Zero bit padding, to prevent sync-fooling string of 1s |
   | "u(6)"   | "SUBFRAME TYPE" (see Section 11.22.1)                  |
   | "u(1+k)" | "WASTED BITS PER SAMPLE FLAG" (see Section 11.22.2)    |
   +----------+--------------------------------------------------------+

11.22.1.  SUBFRAME TYPE















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

11.22.2.  WASTED BITS PER SAMPLE FLAG

   +-------+-----------------------------------------------------------+
   | Value | Description                                               |
   +-------+-----------------------------------------------------------+
   |     0 | no wasted bits-per-sample in source subblock, k=0         |
   |     1 | k wasted bits-per-sample in source subblock, k-1 follows, |
   |       | unary coded; e.g. k=3 => 0b001 follows, k=7 => 0b0000001  |
   |       | follows.                                                  |
   +-------+-----------------------------------------------------------+

   The size of the samples stored in the subframe is the subframe sample
   size reduced by k bits.  Decoded samples must be shifted left by k
   bits.

11.23.  SUBFRAME_CONSTANT

   +--------+----------------------------------------------------------+
   | Data   | Description                                              |
   +--------+----------------------------------------------------------+
   | "u(n)" | Unencoded constant value of the subblock, n = frame's    |
   |        | bits-per-sample.                                         |
   +--------+----------------------------------------------------------+

11.24.  SUBFRAME_FIXED

   +------------+------------------------------------------------------+
   | Data       | Description                                          |
   +------------+------------------------------------------------------+
   | "u(n)"     | Unencoded warm-up samples (n = frame's bits-per-     |
   |            | sample * predictor order).                           |
   | "RESIDUAL" | Encoded residual                                     |
   +------------+------------------------------------------------------+





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11.25.  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                                     |
   +------------+------------------------------------------------------+

11.26.  SUBFRAME_VERBATIM

   +-----------+-------------------------------------------------------+
   | Data      | Description                                           |
   +-----------+-------------------------------------------------------+
   | "u(n\*i)" | Unencoded subblock, where "n" is frame's bits-per-    |
   |           | sample and "i" is frame's blocksize.                  |
   +-----------+-------------------------------------------------------+

11.27.  RESIDUAL

   +-------------------------------------------+-----------------------+
   | Data                                      | Description           |
   +-------------------------------------------+-----------------------+
   | "u(2)"                                    | "RESIDUAL_CODING_METH |
   |                                           | OD"                   |
   | "RESIDUAL_CODING_METHOD_PARTITIONED_EXP_G |                       |
   | OLOMB" || "RESIDUAL_CODING_METHOD_PARTITI |                       |
   | ONED_EXP_GOLOMB2"                         |                       |
   +-------------------------------------------+-----------------------+

11.27.1.  RESIDUAL_CODING_METHOD











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

11.27.2.  RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB

      +-------------------------+-----------------------------------+
      | Data                    | Description                       |
      +-------------------------+-----------------------------------+
      | "u(4)"                  | Partition order.                  |
      | "EXP_GOLOMB_PARTITION"+ | There will be 2^order partitions. |
      +-------------------------+-----------------------------------+

11.27.2.1.  EXP_GOLOMB_PARTITION

   +------------+------------------------------------------------------+
   | Data       | Description                                          |
   +------------+------------------------------------------------------+
   | "u(4(+5))" | "EXP-GOLOMB PARTITION ENCODING PARAMETER" (see       |
   |            | Section 11.27.2.2)                                   |
   | "u(?)"     | "ENCODED RESIDUAL" (see Section 11.27.4)             |
   +------------+------------------------------------------------------+

11.27.2.2.  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.                                          |
   +----------+--------------------------------------------------------+








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11.27.3.  RESIDUAL_CODING_METHOD_PARTITIONED_EXP_GOLOMB2

     +--------------------------+-----------------------------------+
     | Data                     | Description                       |
     +--------------------------+-----------------------------------+
     | "u(4)"                   | Partition order.                  |
     | "EXP-GOLOMB2_PARTITION"+ | There will be 2^order partitions. |
     +--------------------------+-----------------------------------+

11.27.3.1.  EXP_GOLOMB2_PARTITION

   +------------+------------------------------------------------------+
   | Data       | Description                                          |
   +------------+------------------------------------------------------+
   | "u(5(+5))" | "EXP-GOLOMB2 PARTITION ENCODING PARAMETER" (see      |
   |            | Section 11.27.3.2)                                   |
   | "u(?)"     | "ENCODED RESIDUAL" (see Section 11.27.4)             |
   +------------+------------------------------------------------------+

11.27.3.2.  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.                                          |
   +----------+--------------------------------------------------------+

11.27.4.  ENCODED RESIDUAL

   The number of samples (n) in the partition is determined as follows:

   o  if the partition order is zero, n = frame's blocksize - predictor
      order

   o  else if this is not the first partition of the subframe, n =
      (frame's blocksize / (2^partition order))

   o  else n = (frame's blocksize / (2^partition order)) - predictor
      order







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12.  References

12.1.  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>.

12.2.  URIs

   [1] https://xiph.org/flac/ogg_mapping.html

   [2] https://xiph.org/flac/documentation_format_overview.html

   [3] http://svr-www.eng.cam.ac.uk/~ajr/

   [4] http://svr-www.eng.cam.ac.uk/reports/abstracts/
       robinson_tr156.html

   [5] https://web.archive.org/web/20040215005354/http://csi.usc.edu/
       faculty/golomb.html

   [6] http://en.wikipedia.org/wiki/Claude_Shannon

   [7] https://en.wikipedia.org/wiki/Linear_predictive_coding

   [8] http://www.hpl.hp.com/techreports/1999/HPL-1999-144.pdf

   [9] http://svr-www.eng.cam.ac.uk/reports/abstracts/
       robinson_tr156.html

   [10] http://www.hpl.hp.com/techreports/1999/HPL-1999-144.pdf

   [11] http://svr-www.eng.cam.ac.uk/reports/abstracts/
        robinson_tr156.html

   [12] https://en.wikipedia.org/wiki/Exponential-Golomb_coding

   [13] http://www.hpl.hp.com/techreports/98/HPL-98-193.html

   [14] http://web.archive.org/web/20140827133312/http://www.cs.tut.fi/~
        albert/Dev/pucrunch/packing.html

   [15] https://xiph.org/flac/id.html

   [16] http://xiph.org/vorbis/doc/v-comment.html




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   [17] http://www.id3.org/id3v2.4.0-frames

   [18] https://xiph.org/flac/id.html

   [19] http://www.xiph.org/vorbis/doc/v-comment.html

   [20] http://isrc.ifpi.org/

   [21] http://www.disctronics.co.uk/technology/cdaudio/cdaud_isrc.htm

   [22] http://www.chipchapin.com/CDMedia/cdda9.php3

   [23] http://lists.xiph.org/pipermail/flac-
        dev/2008-December/002607.html

Authors' Addresses

   Josh Coalson


   Xiph.Org Foundation


   Andrew Weaver

   Email: theandrewjw@gmail.com

























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