doc/vorbis.html - platform/external/libvorbis - Git at Google

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 <h1>Ogg Vorbis encoding format documentation</h1>

 <p><img src="wait.png" alt="wait"/>As of writing, not all the below document
 links are live. They will be populated as we complete the documents.</p>

 <h2>Documents</h2>

 <ul>
 <li><a href="packet.html">Vorbis packet structure</a></li>
 <li><a href="envelope.html">Temporal envelope shaping and blocksize</a></li>
 <li><a href="mdct.html">Time domain segmentation and MDCT transform</a></li>
 <li><a href="resolution.html">The resolution floor</a></li>
 <li><a href="residuals.html">MDCT-domain fine structure</a></li>
 </ul>

 <ul>
 <li><a href="probmodel.html">The Vorbis probability model</a></li>
 <li><a href="bitpack.html">The Vorbis bitpacker</a></li>
 </ul>

 <ul>
 <li><a href="oggstream.html">Ogg bitstream overview</a></li>
 <li><a href="framing.html">Ogg logical bitstream and framing spec</a></li>
 <li><a href="vorbis-stream.html">Vorbis packet->Ogg bitstream mapping</a></li>
 </ul>

 <ul>
 <li><a href="programming.html">Programming with libvorbis</a></li>
 </ul>

 <h2>Description</h2>

 <p>Ogg Vorbis is a general purpose compressed audio format
 for high quality (44.1-48.0kHz, 16+ bit, polyphonic) audio and music
 at moderate fixed and variable bitrates (40-80 kb/s/channel). This
 places Vorbis in the same class as audio representations including
 MPEG-1 audio layer 3, MPEG-4 audio (AAC and TwinVQ), and PAC.</p>

 <p>Vorbis is the first of a planned family of Ogg multimedia coding
 formats being developed as part of the Xiph.org Foundation's Ogg multimedia
 project. See <a href="http://www.xiph.org/">http://www.xiph.org/</a>
 for more information.</p>

 <h2>Vorbis technical documents</h2>

 <p>A Vorbis encoder takes in overlapping (but contiguous) short-time
 segments of audio data. The encoder analyzes the content of the audio
 to determine an optimal compact representation; this phase of encoding
 is known as <em>analysis</em>. For each short-time block of sound,
 the encoder then packs an efficient representation of the signal, as
 determined by analysis, into a raw packet much smaller than the size
 required by the original signal; this phase is <em>coding</em>.
 Lastly, in a streaming environment, the raw packets are then
 structured into a continuous stream of octets; this last phase is
 <em>streaming</em>. Note that the stream of octets is referred to both
 as a 'byte-' and 'bit-'stream; the latter usage is acceptible as the
 stream of octets is a physical representation of a true logical
 bit-by-bit stream.</p>

 <p>A Vorbis decoder performs a mirror image process of extracting the
 original sequence of raw packets from an Ogg stream (<em>stream
 decomposition</em>), reconstructing the signal representation from the
 raw data in the packet (<em>decoding</em>) and them reconstituting an
 audio signal from the decoded representation (<em>synthesis</em>).</p>

 <p>The <a href="programming.html">Programming with libvorbis</a>
 documents discuss use of the reference Vorbis codec library
 (libvorbis) produced by the Xiph.org Foundation.</p>

 <p>The data representations and algorithms necessary at each step to
 encode and decode Ogg Vorbis bitstreams are described by the below
 documents in sufficient detail to construct a complete Vorbis codec.
 Note that at the time of writing, Vorbis is still in a 'Request For
 Comments' stage of development; despite being in advanced stages of
 development, input from the multimedia community is welcome.</p>

 <h3>Vorbis analysis and synthesis</h3>

 <p>Analysis begins by seperating an input audio stream into individual,
 overlapping short-time segments of audio data. These segments are
 then transformed into an alternate representation, seeking to
 represent the original signal in a more efficient form that codes into
 a smaller number of bytes. The analysis and transformation stage is
 the most complex element of producing a Vorbis bitstream.</p>

 <p>The corresponding synthesis step in the decoder is simpler; there is
 no analysis to perform, merely a mechanical, deterministic
 reconstruction of the original audio data from the transform-domain
 representation.</p>

 <ul>
 <li><a href="packet.html">Vorbis packet structure</a>:
 Describes the basic analysis components necessary to produce Vorbis
 packets and the structure of the packet itself.</li>
 <li><a href="envelope.html">Temporal envelope shaping and blocksize</a>:
 Use of temporal envelope shaping and variable blocksize to minimize
 time-domain energy leakage during wide dynamic range and spectral energy
 swings. Also discusses time-related principles of psychoacoustics.</li>
 <li><a href="mdct.html">Time domain segmentation and MDCT transform</a>:
 Division of time domain data into individual overlapped, windowed
 short-time vectors and transformation using the MDCT</li>
 <li><a href="resolution.html">The resolution floor</a>: Use of frequency
 doamin psychoacoustics, and the MDCT-domain noise, masking and resolution
 floors</li>
 <li><a href="residuals.html">MDCT-domain fine structure</a>: Production,
 quantization and massaging of MDCT-spectrum fine structure</li>
 </ul>

 <h3>Vorbis coding and decoding</h3>

 <p>Coding and decoding converts the transform-domain representation of
 the original audio produced by analysis to and from a bitwise packed
 raw data packet. Coding and decoding consist of two logically
 orthogonal concepts, <em>back-end coding</em> and <em>bitpacking</em>.</p>

 <p><em>Back-end coding</em> uses a probability model to represent the raw numbers
 of the audio representation in as few physical bits as possible;
 familiar examples of back-end coding include Huffman coding and Vector
 Quantization.</p>

 <p><em>Bitpacking</em> arranges the variable sized words of the back-end
 coding into a vector of octets without wasting space. The octets
 produced by coding a single short-time audio segment is one raw Vorbis
 packet.</p>

 <ul>
 <li><a href="probmodel.html">The Vorbis probability model</a></li>
 <li><a href="bitpack.html">The Vorbis bitpacker</a>: Arrangement of
 variable bit-length words into an octet-aligned packet.</li>
 </ul>

 <h3>Vorbis streaming and stream decomposition</h3>

 <p>Vorbis packets contain the raw, bitwise-compressed representation of a
 snippet of audio. These packets contain no structure and cannot be
 strung together directly into a stream; for streamed transmission and
 storage, Vorbis packets are encoded into an Ogg bitstream.</p>

 <ul>
 <li><a href="oggstream.html">Ogg bitstream overview</a>: High-level
 description of Ogg logical bitstreams, how logical bitstreams
 (of mixed media types) can be combined into physical bitstreams, and
 restrictions on logical-to-physical mapping. Note that this document is
 not specific only to Ogg Vorbis.</li>
 <li><a href="framing.html">Ogg logical bitstream and framing
 spec</a>: Low level, complete specification of Ogg logical
 bitstream pages. Note that this document is not specific only to Ogg
 Vorbis.</li>
 <li><a href="vorbis-stream.html">Vorbis bitstream mapping</a>:
 Specifically describes mapping Vorbis data into an
 Ogg physical bitstream.</li>
 </ul>

 <div id="copyright">
   The Xiph Fish Logo is a
   trademark (&trade;) of Xiph.Org.<br/>

   These pages &copy; 1994 - 2005 Xiph.Org. All rights reserved.
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	</div>

	<h1>Ogg Vorbis encoding format documentation</h1>

	<p><img src="wait.png" alt="wait"/>As of writing, not all the below document
	links are live. They will be populated as we complete the documents.</p>

	<h2>Documents</h2>

	<ul>
	<li><a href="packet.html">Vorbis packet structure</a></li>
	<li><a href="envelope.html">Temporal envelope shaping and blocksize</a></li>
	<li><a href="mdct.html">Time domain segmentation and MDCT transform</a></li>
	<li><a href="resolution.html">The resolution floor</a></li>
	<li><a href="residuals.html">MDCT-domain fine structure</a></li>
	</ul>

	<ul>
	<li><a href="probmodel.html">The Vorbis probability model</a></li>
	<li><a href="bitpack.html">The Vorbis bitpacker</a></li>
	</ul>

	<ul>
	<li><a href="oggstream.html">Ogg bitstream overview</a></li>
	<li><a href="framing.html">Ogg logical bitstream and framing spec</a></li>
	<li><a href="vorbis-stream.html">Vorbis packet->Ogg bitstream mapping</a></li>
	</ul>

	<ul>
	<li><a href="programming.html">Programming with libvorbis</a></li>
	</ul>

	<h2>Description</h2>

	<p>Ogg Vorbis is a general purpose compressed audio format
	for high quality (44.1-48.0kHz, 16+ bit, polyphonic) audio and music
	at moderate fixed and variable bitrates (40-80 kb/s/channel). This
	places Vorbis in the same class as audio representations including
	MPEG-1 audio layer 3, MPEG-4 audio (AAC and TwinVQ), and PAC.</p>

	<p>Vorbis is the first of a planned family of Ogg multimedia coding
	formats being developed as part of the Xiph.org Foundation's Ogg multimedia
	project. See <a href="http://www.xiph.org/">http://www.xiph.org/</a>
	for more information.</p>

	<h2>Vorbis technical documents</h2>

	<p>A Vorbis encoder takes in overlapping (but contiguous) short-time
	segments of audio data. The encoder analyzes the content of the audio
	to determine an optimal compact representation; this phase of encoding
	is known as <em>analysis</em>. For each short-time block of sound,
	the encoder then packs an efficient representation of the signal, as
	determined by analysis, into a raw packet much smaller than the size
	required by the original signal; this phase is <em>coding</em>.
	Lastly, in a streaming environment, the raw packets are then
	structured into a continuous stream of octets; this last phase is
	<em>streaming</em>. Note that the stream of octets is referred to both
	as a 'byte-' and 'bit-'stream; the latter usage is acceptible as the
	stream of octets is a physical representation of a true logical
	bit-by-bit stream.</p>

	<p>A Vorbis decoder performs a mirror image process of extracting the
	original sequence of raw packets from an Ogg stream (<em>stream
	decomposition</em>), reconstructing the signal representation from the
	raw data in the packet (<em>decoding</em>) and them reconstituting an
	audio signal from the decoded representation (<em>synthesis</em>).</p>

	<p>The <a href="programming.html">Programming with libvorbis</a>
	documents discuss use of the reference Vorbis codec library
	(libvorbis) produced by the Xiph.org Foundation.</p>

	<p>The data representations and algorithms necessary at each step to
	encode and decode Ogg Vorbis bitstreams are described by the below
	documents in sufficient detail to construct a complete Vorbis codec.
	Note that at the time of writing, Vorbis is still in a 'Request For
	Comments' stage of development; despite being in advanced stages of
	development, input from the multimedia community is welcome.</p>

	<h3>Vorbis analysis and synthesis</h3>

	<p>Analysis begins by seperating an input audio stream into individual,
	overlapping short-time segments of audio data. These segments are
	then transformed into an alternate representation, seeking to
	represent the original signal in a more efficient form that codes into
	a smaller number of bytes. The analysis and transformation stage is
	the most complex element of producing a Vorbis bitstream.</p>

	<p>The corresponding synthesis step in the decoder is simpler; there is
	no analysis to perform, merely a mechanical, deterministic
	reconstruction of the original audio data from the transform-domain
	representation.</p>

	<ul>
	<li><a href="packet.html">Vorbis packet structure</a>:
	Describes the basic analysis components necessary to produce Vorbis
	packets and the structure of the packet itself.</li>
	<li><a href="envelope.html">Temporal envelope shaping and blocksize</a>:
	Use of temporal envelope shaping and variable blocksize to minimize
	time-domain energy leakage during wide dynamic range and spectral energy
	swings. Also discusses time-related principles of psychoacoustics.</li>
	<li><a href="mdct.html">Time domain segmentation and MDCT transform</a>:
	Division of time domain data into individual overlapped, windowed
	short-time vectors and transformation using the MDCT</li>
	<li><a href="resolution.html">The resolution floor</a>: Use of frequency
	doamin psychoacoustics, and the MDCT-domain noise, masking and resolution
	floors</li>
	<li><a href="residuals.html">MDCT-domain fine structure</a>: Production,
	quantization and massaging of MDCT-spectrum fine structure</li>
	</ul>

	<h3>Vorbis coding and decoding</h3>

	<p>Coding and decoding converts the transform-domain representation of
	the original audio produced by analysis to and from a bitwise packed
	raw data packet. Coding and decoding consist of two logically
	orthogonal concepts, <em>back-end coding</em> and <em>bitpacking</em>.</p>

	<p><em>Back-end coding</em> uses a probability model to represent the raw numbers
	of the audio representation in as few physical bits as possible;
	familiar examples of back-end coding include Huffman coding and Vector
	Quantization.</p>

	<p><em>Bitpacking</em> arranges the variable sized words of the back-end
	coding into a vector of octets without wasting space. The octets
	produced by coding a single short-time audio segment is one raw Vorbis
	packet.</p>

	<ul>
	<li><a href="probmodel.html">The Vorbis probability model</a></li>
	<li><a href="bitpack.html">The Vorbis bitpacker</a>: Arrangement of
	variable bit-length words into an octet-aligned packet.</li>
	</ul>

	<h3>Vorbis streaming and stream decomposition</h3>

	<p>Vorbis packets contain the raw, bitwise-compressed representation of a
	snippet of audio. These packets contain no structure and cannot be
	strung together directly into a stream; for streamed transmission and
	storage, Vorbis packets are encoded into an Ogg bitstream.</p>

	<ul>
	<li><a href="oggstream.html">Ogg bitstream overview</a>: High-level
	description of Ogg logical bitstreams, how logical bitstreams
	(of mixed media types) can be combined into physical bitstreams, and
	restrictions on logical-to-physical mapping. Note that this document is
	not specific only to Ogg Vorbis.</li>
	<li><a href="framing.html">Ogg logical bitstream and framing
	spec</a>: Low level, complete specification of Ogg logical
	bitstream pages. Note that this document is not specific only to Ogg
	Vorbis.</li>
	<li><a href="vorbis-stream.html">Vorbis bitstream mapping</a>:
	Specifically describes mapping Vorbis data into an
	Ogg physical bitstream.</li>
	</ul>

	<div id="copyright">
	The Xiph Fish Logo is a
	trademark (™) of Xiph.Org.<br/>

	These pages © 1994 - 2005 Xiph.Org. All rights reserved.
	</div>

	</body>
	</html>