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| 70 | <a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.org"/></a> |
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| 71 | </div> |
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| 72 | |
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| 73 | <h1>Ogg Vorbis encoding format documentation</h1> |
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| 74 | |
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| 75 | <p><img src="wait.png" alt="wait"/>As of writing, not all the below document |
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| 76 | links are live. They will be populated as we complete the documents.</p> |
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| 77 | |
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| 78 | <h2>Documents</h2> |
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| 79 | |
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| 80 | <ul> |
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| 81 | <li><a href="packet.html">Vorbis packet structure</a></li> |
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| 82 | <li><a href="envelope.html">Temporal envelope shaping and blocksize</a></li> |
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| 83 | <li><a href="mdct.html">Time domain segmentation and MDCT transform</a></li> |
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| 84 | <li><a href="resolution.html">The resolution floor</a></li> |
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| 85 | <li><a href="residuals.html">MDCT-domain fine structure</a></li> |
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| 86 | </ul> |
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| 87 | |
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| 88 | <ul> |
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| 89 | <li><a href="probmodel.html">The Vorbis probability model</a></li> |
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| 90 | <li><a href="bitpack.html">The Vorbis bitpacker</a></li> |
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| 91 | </ul> |
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| 92 | |
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| 93 | <ul> |
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| 94 | <li><a href="oggstream.html">Ogg bitstream overview</a></li> |
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| 95 | <li><a href="framing.html">Ogg logical bitstream and framing spec</a></li> |
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| 96 | <li><a href="vorbis-stream.html">Vorbis packet->Ogg bitstream mapping</a></li> |
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| 97 | </ul> |
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| 98 | |
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| 99 | <ul> |
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| 100 | <li><a href="programming.html">Programming with libvorbis</a></li> |
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| 101 | </ul> |
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| 102 | |
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| 103 | <h2>Description</h2> |
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| 104 | |
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| 105 | <p>Ogg Vorbis is a general purpose compressed audio format |
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| 106 | for high quality (44.1-48.0kHz, 16+ bit, polyphonic) audio and music |
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| 107 | at moderate fixed and variable bitrates (40-80 kb/s/channel). This |
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| 108 | places Vorbis in the same class as audio representations including |
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| 109 | MPEG-1 audio layer 3, MPEG-4 audio (AAC and TwinVQ), and PAC.</p> |
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| 110 | |
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| 111 | <p>Vorbis is the first of a planned family of Ogg multimedia coding |
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| 112 | formats being developed as part of the Xiph.org Foundation's Ogg multimedia |
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| 113 | project. See <a href="http://www.xiph.org/">http://www.xiph.org/</a> |
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| 114 | for more information.</p> |
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| 115 | |
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| 116 | <h2>Vorbis technical documents</h2> |
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| 117 | |
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| 118 | <p>A Vorbis encoder takes in overlapping (but contiguous) short-time |
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| 119 | segments of audio data. The encoder analyzes the content of the audio |
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| 120 | to determine an optimal compact representation; this phase of encoding |
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| 121 | is known as <em>analysis</em>. For each short-time block of sound, |
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| 122 | the encoder then packs an efficient representation of the signal, as |
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| 123 | determined by analysis, into a raw packet much smaller than the size |
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| 124 | required by the original signal; this phase is <em>coding</em>. |
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| 125 | Lastly, in a streaming environment, the raw packets are then |
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| 126 | structured into a continuous stream of octets; this last phase is |
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| 127 | <em>streaming</em>. Note that the stream of octets is referred to both |
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| 128 | as a 'byte-' and 'bit-'stream; the latter usage is acceptible as the |
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| 129 | stream of octets is a physical representation of a true logical |
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| 130 | bit-by-bit stream.</p> |
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| 131 | |
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| 132 | <p>A Vorbis decoder performs a mirror image process of extracting the |
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| 133 | original sequence of raw packets from an Ogg stream (<em>stream |
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| 134 | decomposition</em>), reconstructing the signal representation from the |
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| 135 | raw data in the packet (<em>decoding</em>) and them reconstituting an |
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| 136 | audio signal from the decoded representation (<em>synthesis</em>).</p> |
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| 137 | |
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| 138 | <p>The <a href="programming.html">Programming with libvorbis</a> |
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| 139 | documents discuss use of the reference Vorbis codec library |
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| 140 | (libvorbis) produced by the Xiph.org Foundation.</p> |
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| 141 | |
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| 142 | <p>The data representations and algorithms necessary at each step to |
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| 143 | encode and decode Ogg Vorbis bitstreams are described by the below |
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| 144 | documents in sufficient detail to construct a complete Vorbis codec. |
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| 145 | Note that at the time of writing, Vorbis is still in a 'Request For |
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| 146 | Comments' stage of development; despite being in advanced stages of |
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| 147 | development, input from the multimedia community is welcome.</p> |
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| 148 | |
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| 149 | <h3>Vorbis analysis and synthesis</h3> |
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| 150 | |
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| 151 | <p>Analysis begins by seperating an input audio stream into individual, |
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| 152 | overlapping short-time segments of audio data. These segments are |
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| 153 | then transformed into an alternate representation, seeking to |
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| 154 | represent the original signal in a more efficient form that codes into |
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| 155 | a smaller number of bytes. The analysis and transformation stage is |
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| 156 | the most complex element of producing a Vorbis bitstream.</p> |
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| 157 | |
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| 158 | <p>The corresponding synthesis step in the decoder is simpler; there is |
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| 159 | no analysis to perform, merely a mechanical, deterministic |
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| 160 | reconstruction of the original audio data from the transform-domain |
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| 161 | representation.</p> |
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| 162 | |
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| 163 | <ul> |
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| 164 | <li><a href="packet.html">Vorbis packet structure</a>: |
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| 165 | Describes the basic analysis components necessary to produce Vorbis |
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| 166 | packets and the structure of the packet itself.</li> |
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| 167 | <li><a href="envelope.html">Temporal envelope shaping and blocksize</a>: |
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| 168 | Use of temporal envelope shaping and variable blocksize to minimize |
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| 169 | time-domain energy leakage during wide dynamic range and spectral energy |
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| 170 | swings. Also discusses time-related principles of psychoacoustics.</li> |
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| 171 | <li><a href="mdct.html">Time domain segmentation and MDCT transform</a>: |
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| 172 | Division of time domain data into individual overlapped, windowed |
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| 173 | short-time vectors and transformation using the MDCT</li> |
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| 174 | <li><a href="resolution.html">The resolution floor</a>: Use of frequency |
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| 175 | doamin psychoacoustics, and the MDCT-domain noise, masking and resolution |
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| 176 | floors</li> |
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| 177 | <li><a href="residuals.html">MDCT-domain fine structure</a>: Production, |
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| 178 | quantization and massaging of MDCT-spectrum fine structure</li> |
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| 179 | </ul> |
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| 180 | |
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| 181 | <h3>Vorbis coding and decoding</h3> |
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| 182 | |
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| 183 | <p>Coding and decoding converts the transform-domain representation of |
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| 184 | the original audio produced by analysis to and from a bitwise packed |
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| 185 | raw data packet. Coding and decoding consist of two logically |
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| 186 | orthogonal concepts, <em>back-end coding</em> and <em>bitpacking</em>.</p> |
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| 187 | |
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| 188 | <p><em>Back-end coding</em> uses a probability model to represent the raw numbers |
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| 189 | of the audio representation in as few physical bits as possible; |
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| 190 | familiar examples of back-end coding include Huffman coding and Vector |
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| 191 | Quantization.</p> |
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| 192 | |
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| 193 | <p><em>Bitpacking</em> arranges the variable sized words of the back-end |
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| 194 | coding into a vector of octets without wasting space. The octets |
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| 195 | produced by coding a single short-time audio segment is one raw Vorbis |
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| 196 | packet.</p> |
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| 197 | |
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| 198 | <ul> |
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| 199 | <li><a href="probmodel.html">The Vorbis probability model</a></li> |
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| 200 | <li><a href="bitpack.html">The Vorbis bitpacker</a>: Arrangement of |
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| 201 | variable bit-length words into an octet-aligned packet.</li> |
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| 202 | </ul> |
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| 203 | |
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| 204 | <h3>Vorbis streaming and stream decomposition</h3> |
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| 205 | |
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| 206 | <p>Vorbis packets contain the raw, bitwise-compressed representation of a |
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| 207 | snippet of audio. These packets contain no structure and cannot be |
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| 208 | strung together directly into a stream; for streamed transmission and |
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| 209 | storage, Vorbis packets are encoded into an Ogg bitstream.</p> |
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| 210 | |
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| 211 | <ul> |
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| 212 | <li><a href="oggstream.html">Ogg bitstream overview</a>: High-level |
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| 213 | description of Ogg logical bitstreams, how logical bitstreams |
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| 214 | (of mixed media types) can be combined into physical bitstreams, and |
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| 215 | restrictions on logical-to-physical mapping. Note that this document is |
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| 216 | not specific only to Ogg Vorbis.</li> |
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| 217 | <li><a href="framing.html">Ogg logical bitstream and framing |
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| 218 | spec</a>: Low level, complete specification of Ogg logical |
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| 219 | bitstream pages. Note that this document is not specific only to Ogg |
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| 220 | Vorbis.</li> |
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| 221 | <li><a href="vorbis-stream.html">Vorbis bitstream mapping</a>: |
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| 222 | Specifically describes mapping Vorbis data into an |
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| 223 | Ogg physical bitstream.</li> |
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| 224 | </ul> |
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| 225 | |
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| 226 | <div id="copyright"> |
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| 227 | The Xiph Fish Logo is a |
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| 228 | trademark (™) of Xiph.Org.<br/> |
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| 229 | |
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| 230 | These pages © 1994 - 2005 Xiph.Org. All rights reserved. |
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| 231 | </div> |
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| 232 | |
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| 233 | </body> |
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| 234 | </html> |
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