release/src/router/libvorbis/doc/vorbis.html

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