1 <HTML><HEAD><TITLE>xiph.org: Ogg Vorbis documentation</TITLE>
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7 Ogg Vorbis encoding format documentation
10 <em>Last update to this document: July 15, 1999</em><br>
11 <em>Last update to Vorbis documentation: July 21, 1999</em><p>
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16 As of writing, not all the below document
17 links are live. They will be populated as we complete the
24 <li><a href="packet.html">Vorbis packet structure</a>
25 <li><a href="envelope.html">Temporal envelope shaping and blocksize</a>
26 <li><a href="mdct.html">Time domain segmentation and MDCT transform</a>
27 <li><a href="resolution.html">The resolution floor</a>
28 <li><a href="residuals.html">MDCT-domain fine structure</a><p>
30 <li><a href="probmodel.html">The Vorbis probability model</a>
32 <li><a href="bitpack.html">The Vorbis bitpacker</a><p>
34 <li><a href="oggstream.html">Ogg bitstream overview</a>
35 <li><a href="framing.html">Ogg logical bitstream and framing spec</a>
36 <li><a href="vorbis-stream.html">Vorbis packet->Ogg bitstream
39 <li><a href="programming.html">Programming with libvorbis</a><p>
43 Ogg Vorbis is a general purpose compressed audio format
44 for high quality (44.1-48.0kHz, 16+ bit, polyphonic) audio and music
45 at moderate fixed and variable bitrates (40-80 kb/s/channel). This
46 places Vorbis in the same class as audio representations including
47 MPEG-1 audio layer 3, MPEG-4 audio (AAC and TwinVQ), and PAC.<p>
49 Vorbis is the first of a planned family of Ogg multimedia coding
50 formats being developed as part of Xiphophorus's Ogg multimedia
51 project. See <a href="http://www.xiph.org/">http://www.xiph.org/</a>
54 <h2>Vorbis technical documents</h2>
56 A Vorbis encoder takes in overlapping (but contiguous) short-time
57 segments of audio data. The encoder analyzes the content of the audio
58 to determine an optimal compact representation; this phase of encoding
59 is known as <em>analysis</em>. For each short-time block of sound,
60 the encoder then packs an efficient representation of the signal, as
61 determined by analysis, into a raw packet much smaller than the size
62 required by the original signal; this phase is <em>coding</em>.
63 Lastly, in a streaming environment, the raw packets are then
64 structured into a continuous stream of octets; this last phase is
65 <em>streaming</em>. Note that the stream of octets is referred to both
66 as a 'byte-' and 'bit-'stream; the latter usage is acceptible as the
67 stream of octets is a physical representation of a true logical
70 A Vorbis decoder performs a mirror image process of extracting the
71 original sequence of raw packets from an Ogg stream (<em>stream
72 decomposition</em>), reconstructing the signal representation from the
73 raw data in the packet (<em>decoding</em>) and them reconstituting an
74 audio signal from the decoded representation (<em>synthesis</em>).<p>
76 The <a href="programming.html">Programming with libvorbis</a>
77 documents discuss use of the reference Vorbis codec library
78 (libvorbis) produced by Xiphophorus.<p>
80 The data representations and algorithms necessary at each step to
81 encode and decode Ogg Vorbis bitstreams are described by the below
82 documents in sufficient detail to construct a complete Vorbis codec.
83 Note that at the time of writing, Vorbis is still in a 'Request For
84 Comments' stage of development; despite being in advanced stages of
85 development, input from the multimedia community is welcome.<p>
87 <h3>Vorbis analysis and synthesis</h3>
89 Analysis begins by seperating an input audio stream into individual,
90 overlapping short-time segments of audio data. These segments are
91 then transformed into an alternate representation, seeking to
92 represent the original signal in a more efficient form that codes into
93 a smaller number of bytes. The analysis and transformation stage is
94 the most complex element of producing a Vorbis bitstream.<p>
96 The corresponding synthesis step in the decoder is simpler; there is
97 no analysis to perform, merely a mechanical, deterministic
98 reconstruction of the original audio data from the transform-domain
102 <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.
103 <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.
104 <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
105 <li><a href="resolution.html">The resolution floor</a>: Use of frequency doamin psychoacoustics, and the MDCT-domain noise, masking and resolution floors
106 <li><a href="residuals.html">MDCT-domain fine structure</a>: Production, quantization and massaging of MDCT-spectrum fine structure
109 <h3>Vorbis coding and decoding</h3>
111 Coding and decoding converts the transform-domain representation of
112 the original audio produced by analysis to and from a bitwise packed
113 raw data packet. Coding and decoding consist of two logically
114 orthogonal concepts, <em>back-end coding</em> and <em>bitpacking</em>.<p>
116 <em>Back-end coding</em> uses a probability model to represent the raw numbers
117 of the audio representation in as few physical bits as possible;
118 familiar examples of back-end coding include Huffman coding and Vector
121 <em>Bitpacking</em> arranges the variable sized words of the back-end
122 coding into a vector of octets without wasting space. The octets
123 produced by coding a single short-time audio segment is one raw Vorbis
128 <li><a href="probmodel.html">The Vorbis probability model</a>
130 <li><a href="bitpack.html">The Vorbis bitpacker</a>: Arrangement of
131 variable bit-length words into an octet-aligned packet.
135 <h3>Vorbis streaming and stream decomposition</h3>
137 Vorbis packets contain the raw, bitwise-compressed representation of a
138 snippet of audio. These packets contain no structure and cannot be
139 strung together directly into a stream; for streamed transmission and
140 storage, Vorbis packets are encoded into an Ogg bitstream.<p>
144 <li><a href="oggstream.html">Ogg bitstream overview</a>: High-level
145 description of Ogg logical bitstreams, how logical bitstreams
146 (of mixed media types) can be combined into physical bitstreams, and
147 restrictions on logical-to-physical mapping. Note that this document is
148 not specific only to Ogg Vorbis.
150 <li><a href="framing.html">Ogg logical bitstream and framing
151 spec</a>: Low level, complete specification of Ogg logical
152 bitstream pages. Note that this document is not specific only to Ogg
155 <li><a href="vorbis-stream.html">Vorbis bitstream mapping</a>:
156 Specifically describes mapping Vorbis data into an
157 Ogg physical bitstream.
163 <a href="http://www.xiph.org/">
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168 Ogg is a <a href="http://www.xiph.org">Xiphophorus</a> effort to
169 protect essential tenets of Internet multimedia from corporate
170 hostage-taking; Open Source is the net's greatest tool to keep
171 everyone honest. See <a href="http://www.xiph.org/about.html">About
172 Xiphophorus</a> for details.
175 Ogg Vorbis is the first Ogg audio CODEC. Anyone may
176 freely use and distribute the Ogg and Vorbis specification,
177 whether in a private, public or corporate capacity. However,
178 Xiphophorus and the Ogg project (xiph.org) reserve the right to set
179 the Ogg/Vorbis specification and certify specification compliance.<p>
181 Xiphophorus's Vorbis software CODEC implementation is distributed
182 under the Lesser/Library GNU Public License. This does not restrict
183 third parties from distributing independent implementations of Vorbis
184 software under other licenses.<p>
186 OggSquish, Vorbis, Xiphophorus and their logos are trademarks (tm) of
187 <a href="http://www.xiph.org/">Xiphophorus</a>. These pages are
188 copyright (C) 1994-2000 Xiphophorus. All rights reserved.<p>