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+<!-- Copyright (c) 2000,2001,2002,2003,2004,2005,2006,2007,2008,2009 Josh Coalson -->
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<br />
<a name="definitions"><font size="+1"><b><u>Definitions</u></b></font></a><br />
<br />
- Many terms like "block" and "frame" are used to mean different things in differenct 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 refering to the raw unencoded audio data that is the input to the encoder, and when we talk about frames and subframes, we are refering to the FLAC-encoded data.
+ Many terms like "block" and "frame" are used to mean different things in differenct 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.
<ul>
<li>
<b>Block</b>: One or more audio samples that span several channels.
<br />
<a name="interchannel"><font size="+1"><b><u>Interchannel Decorrelation</u></b></font></a><br />
<br />
- In stereo streams, in many cases 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.
+ 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.
<ul>
<li>
<b>Independent</b>. The left and right channels are coded independently.
<b>Fixed linear predictor</b>. FLAC uses a class of computationally-efficient fixed linear predictors (for a good description, see <a href="http://www.hpl.hp.com/techreports/1999/HPL-1999-144.pdf">audiopak</a> and <a href="http://svr-www.eng.cam.ac.uk/~ajr/GroupPubs/Robinson94-tr156/index.html">shorten</a>). 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.
</li>
<li>
- <b>FIR Linear prediction</b>. For more accurate modeling (at a cost of slower encoding), FLAC supports up to 32nd order FIR linear prediction (again, for info on linear prediction, see <a href="http://www.hpl.hp.com/techreports/1999/HPL-1999-144.pdf">audiopak</a> and <a href="http://svr-www.eng.cam.ac.uk/~ajr/GroupPubs/Robinson94-tr156/index.html">shorten</a>). 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.
+ <b>FIR Linear prediction</b>. For more accurate modeling (at a cost of slower encoding), FLAC supports up to 32nd order FIR linear prediction (again, for info
rmation on linear prediction, see <a href="http://www.hpl.hp.com/techreports/1999/HPL-1999-144.pdf">audiopak</a> and <a href="http://svr-www.eng.cam.ac.uk/~ajr/GroupPubs/Robinson94-tr156/index.html">shorten</a>). 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.
</li>
</ul>
<a name="residualcoding"><font size="+1"><b><u>Residual Coding</u></b></font></a><br />
<li><a name="def_APPLICATION"><b>APPLICATION</b></a>: 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 <a href="id.html">registration page</a> if you would like to register an ID for your application with FLAC.</li>
<li><a name="def_PADDING"><b>PADDING</b></a>: 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).</li>
<li><a name="def_SEEKTABLE"><b>SEEKTABLE</b></a>: 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.</li>
- <li><a name="def_VORBIS_COMMENT"><b>VORBIS_COMMENT</b></a>: 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 <a href="http://xiph.org/vorbis/doc/v-comment.html">Vorbis comment specification</a>. This is the only officially supported tagging mechanism in FLAC. There may be only one VORBIS_COMMENT block in a stream. In some external documentation, Vorbis comments are called FLAC tags to lessen confusion.</li>
+ <li><a name="def_VORBIS_COMMENT"><b>VORBIS_COMMENT</b></a>: 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 <a href="http://xiph.org/vorbis/doc/v-comment.html">Vorbis comment specification</a> (without the framing bit). This is the only officially supported tagging mechanism in FLAC. There may be only one VORBIS_COMMENT block in a stream. In some external documentation, Vorbis comments are called FLAC tags to lessen confusion.</li>
<li><a name="def_CUESHEET"><b>CUESHEET</b></a>: 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.</li>
<li><a name="def_PICTURE"><b>PICTURE</b></a>: 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 <a href="http://www.id3.org/id3v2.4.0-frames.txt">APIC frame in ID3v2</a>. 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.</li>
</ul>
</li>
<li>
- The audio data is composed of one or more audio frames. Each frame consists of a frame header, which contains a sync code, info 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.
+ 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.
</li>
<li>
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.
<n>
</td>
<td>
- Also known as FLAC tags, the contents of a vorbis comment packet as specified <a href="http://www.xiph.org/vorbis/doc/v-comment.html">here</a>, including the vendor string. 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.
+ Also known as FLAC tags, the contents of a vorbis comment packet as specified <a href="http://www.xiph.org/vorbis/doc/v-comment.html">here</a> (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.
</td>
</tr>
</table>
<1>
</td>
<td>
- Reserved:<br />
+ Reserved: <a href="#frame_header_notes">[1]</a><br />
<ul>
<li>
<tt>0</tt> : mandatory value
<1>
</td>
<td>
- Blocking strategy:<br />
+ Blocking strategy: <a href="#frame_header_notes">[2]</a> <a href="#frame_header_notes">[3]</a><br />
<ul>
<li>
<tt>0</tt> : fixed-blocksize stream; frame header encodes the frame number
</td>
<td>
if(variable blocksize)<br />
- <8-56>:"UTF-8" coded sample number (decoded number is 36 bits)<br />
+ <8-56>:"UTF-8" coded sample number (decoded number is 36 bits) <a href="#frame_header_notes">[4]</a><br />
else<br />
- <8-48>:"UTF-8" coded frame number (decoded number is 31 bits)
+ <8-48>:"UTF-8" coded frame number (decoded number is 31 bits) <a href="#frame_header_notes">[4]</a>
</td>
</tr>
<tr>
<td>
</td>
<td bgcolor="#F4F4CC">
- <font size="+1">NOTES</font><br />
- <ul>
+ <a name="frame_header_notes"><font size="+1">NOTES</font></a><br />
+ <ol>
+ <li>
+ This bit must remain reserved for <tt>0</tt> in order for a FLAC frame's initial 15 bits to be distinguishable from the start of an MPEG audio frame (<a href="http://lists.xiph.org/pipermail/flac-dev/2008-December/002607.html">see also</a>).
+ </li>
<li>
The "blocking strategy" bit must be the same throughout the entire stream.
</li>
<li>
The "UTF-8" coding used for the sample/frame number is the same variable length code used to store compressed UCS-2, extended to handle larger input.
</li>
- </ul>
+ </ol>
</td>
</tr>
</table>
<table>
<tr>
<td align="left">
- Copyright (c) 2000,2001,2002,2003,2004,2005,2006,2007 Josh Coalson
+ Copyright (c) 2000,2001,2002,2003,2004,2005,2006,2007,2008,2009 Josh Coalson
</td>
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