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234 lines
8.0 KiB
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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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<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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