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HTML
235 lines
8.0 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/>
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<title>Ogg Vorbis Documentation</title>
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</head>
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<body>
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<div id="xiphlogo">
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<a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.Org"/></a>
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</div>
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<h1>Ogg Vorbis encoding format documentation</h1>
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<p><img src="wait.png" alt="wait"/>As of writing, not all the below document
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links are live. They will be populated as we complete the documents.</p>
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<h2>Documents</h2>
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<ul>
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<li><a href="packet.html">Vorbis packet structure</a></li>
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<li><a href="envelope.html">Temporal envelope shaping and blocksize</a></li>
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<li><a href="mdct.html">Time domain segmentation and MDCT transform</a></li>
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<li><a href="resolution.html">The resolution floor</a></li>
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<li><a href="residuals.html">MDCT-domain fine structure</a></li>
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</ul>
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<ul>
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<li><a href="probmodel.html">The Vorbis probability model</a></li>
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<li><a href="bitpack.html">The Vorbis bitpacker</a></li>
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</ul>
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<ul>
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<li><a href="oggstream.html">Ogg bitstream overview</a></li>
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<li><a href="framing.html">Ogg logical bitstream and framing spec</a></li>
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<li><a href="vorbis-stream.html">Vorbis packet->Ogg bitstream mapping</a></li>
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</ul>
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<ul>
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<li><a href="programming.html">Programming with libvorbis</a></li>
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</ul>
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<h2>Description</h2>
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<p>Ogg Vorbis is a general purpose compressed audio format
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for high quality (44.1-48.0kHz, 16+ bit, polyphonic) audio and music
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at moderate fixed and variable bitrates (40-80 kb/s/channel). This
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places Vorbis in the same class as audio representations including
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MPEG-1 audio layer 3, MPEG-4 audio (AAC and TwinVQ), and PAC.</p>
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<p>Vorbis is the first of a planned family of Ogg multimedia coding
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formats being developed as part of the Xiph.Org Foundation's Ogg multimedia
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project. See <a href="http://www.xiph.org/">http://www.xiph.org/</a>
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for more information.</p>
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<h2>Vorbis technical documents</h2>
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<p>A Vorbis encoder takes in overlapping (but contiguous) short-time
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segments of audio data. The encoder analyzes the content of the audio
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to determine an optimal compact representation; this phase of encoding
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is known as <em>analysis</em>. For each short-time block of sound,
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the encoder then packs an efficient representation of the signal, as
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determined by analysis, into a raw packet much smaller than the size
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required by the original signal; this phase is <em>coding</em>.
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Lastly, in a streaming environment, the raw packets are then
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structured into a continuous stream of octets; this last phase is
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<em>streaming</em>. Note that the stream of octets is referred to both
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as a 'byte-' and 'bit-'stream; the latter usage is acceptible as the
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stream of octets is a physical representation of a true logical
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bit-by-bit stream.</p>
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<p>A Vorbis decoder performs a mirror image process of extracting the
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original sequence of raw packets from an Ogg stream (<em>stream
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decomposition</em>), reconstructing the signal representation from the
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raw data in the packet (<em>decoding</em>) and them reconstituting an
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audio signal from the decoded representation (<em>synthesis</em>).</p>
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<p>The <a href="programming.html">Programming with libvorbis</a>
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documents discuss use of the reference Vorbis codec library
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(libvorbis) produced by the Xiph.Org Foundation.</p>
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<p>The data representations and algorithms necessary at each step to
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encode and decode Ogg Vorbis bitstreams are described by the below
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documents in sufficient detail to construct a complete Vorbis codec.
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Note that at the time of writing, Vorbis is still in a 'Request For
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Comments' stage of development; despite being in advanced stages of
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development, input from the multimedia community is welcome.</p>
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<h3>Vorbis analysis and synthesis</h3>
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<p>Analysis begins by seperating an input audio stream into individual,
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overlapping short-time segments of audio data. These segments are
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then transformed into an alternate representation, seeking to
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represent the original signal in a more efficient form that codes into
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a smaller number of bytes. The analysis and transformation stage is
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the most complex element of producing a Vorbis bitstream.</p>
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<p>The corresponding synthesis step in the decoder is simpler; there is
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no analysis to perform, merely a mechanical, deterministic
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reconstruction of the original audio data from the transform-domain
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representation.</p>
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<ul>
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<li><a href="packet.html">Vorbis packet structure</a>:
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Describes the basic analysis components necessary to produce Vorbis
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packets and the structure of the packet itself.</li>
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<li><a href="envelope.html">Temporal envelope shaping and blocksize</a>:
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Use of temporal envelope shaping and variable blocksize to minimize
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time-domain energy leakage during wide dynamic range and spectral energy
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swings. Also discusses time-related principles of psychoacoustics.</li>
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<li><a href="mdct.html">Time domain segmentation and MDCT transform</a>:
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Division of time domain data into individual overlapped, windowed
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short-time vectors and transformation using the MDCT</li>
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<li><a href="resolution.html">The resolution floor</a>: Use of frequency
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doamin psychoacoustics, and the MDCT-domain noise, masking and resolution
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floors</li>
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<li><a href="residuals.html">MDCT-domain fine structure</a>: Production,
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quantization and massaging of MDCT-spectrum fine structure</li>
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</ul>
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<h3>Vorbis coding and decoding</h3>
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<p>Coding and decoding converts the transform-domain representation of
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the original audio produced by analysis to and from a bitwise packed
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raw data packet. Coding and decoding consist of two logically
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orthogonal concepts, <em>back-end coding</em> and <em>bitpacking</em>.</p>
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<p><em>Back-end coding</em> uses a probability model to represent the raw numbers
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of the audio representation in as few physical bits as possible;
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familiar examples of back-end coding include Huffman coding and Vector
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Quantization.</p>
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<p><em>Bitpacking</em> arranges the variable sized words of the back-end
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coding into a vector of octets without wasting space. The octets
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produced by coding a single short-time audio segment is one raw Vorbis
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packet.</p>
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<ul>
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<li><a href="probmodel.html">The Vorbis probability model</a></li>
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<li><a href="bitpack.html">The Vorbis bitpacker</a>: Arrangement of
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variable bit-length words into an octet-aligned packet.</li>
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</ul>
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<h3>Vorbis streaming and stream decomposition</h3>
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<p>Vorbis packets contain the raw, bitwise-compressed representation of a
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snippet of audio. These packets contain no structure and cannot be
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strung together directly into a stream; for streamed transmission and
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storage, Vorbis packets are encoded into an Ogg bitstream.</p>
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<ul>
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<li><a href="oggstream.html">Ogg bitstream overview</a>: High-level
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description of Ogg logical bitstreams, how logical bitstreams
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(of mixed media types) can be combined into physical bitstreams, and
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restrictions on logical-to-physical mapping. Note that this document is
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not specific only to Ogg Vorbis.</li>
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<li><a href="framing.html">Ogg logical bitstream and framing
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spec</a>: Low level, complete specification of Ogg logical
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bitstream pages. Note that this document is not specific only to Ogg
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Vorbis.</li>
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<li><a href="vorbis-stream.html">Vorbis bitstream mapping</a>:
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Specifically describes mapping Vorbis data into an
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Ogg physical bitstream.</li>
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</ul>
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<div id="copyright">
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The Xiph Fish Logo is a
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trademark (™) of Xiph.Org.<br/>
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These pages © 1994 - 2005 Xiph.Org. All rights reserved.
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</div>
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</body>
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</html>
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