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198 lines
8.5 KiB
HTML
<HTML><HEAD><TITLE>xiph.org: Ogg Vorbis documentation</TITLE>
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<BODY bgcolor="#ffffff" text="#202020" link="#006666" vlink="#000000">
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<nobr><img src="white-ogg.png"><img src="vorbisword2.png"></nobr><p>
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<h1><font color=#000070>
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Ogg Vorbis encoding format documentation
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</font></h1>
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<em>Last update to this document: July 2, 2002</em><br>
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<em>Last update to Vorbis documentation: July 2, 2002</em><p>
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<table><tr><td>
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<img src=wait.png>
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</td><td valign=center>
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As of writing, not all the below document
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links are live. They will be populated as we complete the
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documents.
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</td></tr></table>
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<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>
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<li><a href="envelope.html">Temporal envelope shaping and blocksize</a>
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<li><a href="mdct.html">Time domain segmentation and MDCT transform</a>
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<li><a href="resolution.html">The resolution floor</a>
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<li><a href="residuals.html">MDCT-domain fine structure</a><p>
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<li><a href="probmodel.html">The Vorbis probability model</a>
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<li><a href="bitpack.html">The Vorbis bitpacker</a><p>
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<li><a href="oggstream.html">Ogg bitstream overview</a>
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<li><a href="framing.html">Ogg logical bitstream and framing spec</a>
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<li><a href="vorbis-stream.html">Vorbis packet->Ogg bitstream
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mapping</a><p>
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<li><a href="programming.html">Programming with libvorbis</a><p>
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</ul>
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<h2>Description</h2>
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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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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.
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<h2>Vorbis technical documents</h2>
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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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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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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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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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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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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>: Describes the basic analysis components necessary to produce Vorbis packets and the structure of the packet itself.
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<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.
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<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
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<li><a href="resolution.html">The resolution floor</a>: Use of frequency doamin psychoacoustics, and the MDCT-domain noise, masking and resolution floors
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<li><a href="residuals.html">MDCT-domain fine structure</a>: Production, quantization and massaging of MDCT-spectrum fine structure
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</ul>
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<h3>Vorbis coding and decoding</h3>
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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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<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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<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>
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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.
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</ul>
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<h3>Vorbis streaming and stream decomposition</h3>
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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.
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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.
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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.
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</ul>
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<hr>
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<a href="http://www.xiph.org/">
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<img src="white-xifish.png" align=left border=0>
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</a>
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<font size=-2 color=#505050>
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Ogg is a <a href="http://www.xiph.org">Xiph.org Foundation</a> effort
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to protect essential tenets of Internet multimedia from corporate
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hostage-taking; Open Source is the net's greatest tool to keep
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everyone honest. See <a href="http://www.xiph.org/about.html">About
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the Xiph.org Foundation</a> for details.
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<p>
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Ogg Vorbis is the first Ogg audio CODEC. Anyone may freely use and
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distribute the Ogg and Vorbis specification, whether in a private,
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public or corporate capacity. However, the Xiph.org Foundation and
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the Ogg project (xiph.org) reserve the right to set the Ogg Vorbis
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specification and certify specification compliance.<p>
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Xiph.org's Vorbis software CODEC implementation is distributed under a
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BSD-like license. This does not restrict third parties from
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distributing independent implementations of Vorbis software under
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other licenses.<p>
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Ogg, Vorbis, Xiph.org Foundation and their logos are trademarks (tm)
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of the <a href="http://www.xiph.org/">Xiph.org Foundation</a>. These
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pages are copyright (C) 1994-2002 Xiph.org Foundation. All rights
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reserved.<p>
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</body>
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