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Current stable PortAudio source
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 18 Oct 2016 13:11:05 +0100
parents 98c1576536ae
children
rev   line source
cannam@86 1 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
cannam@86 2 <html>
cannam@86 3 <head>
cannam@86 4
cannam@86 5 <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/>
cannam@86 6 <title>Ogg Vorbis Documentation</title>
cannam@86 7
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cannam@86 63 </style>
cannam@86 64
cannam@86 65 </head>
cannam@86 66
cannam@86 67 <body>
cannam@86 68
cannam@86 69 <div id="xiphlogo">
cannam@86 70 <a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.Org"/></a>
cannam@86 71 </div>
cannam@86 72
cannam@86 73 <h1>Ogg Vorbis encoding format documentation</h1>
cannam@86 74
cannam@86 75 <p><img src="wait.png" alt="wait"/>As of writing, not all the below document
cannam@86 76 links are live. They will be populated as we complete the documents.</p>
cannam@86 77
cannam@86 78 <h2>Documents</h2>
cannam@86 79
cannam@86 80 <ul>
cannam@86 81 <li><a href="packet.html">Vorbis packet structure</a></li>
cannam@86 82 <li><a href="envelope.html">Temporal envelope shaping and blocksize</a></li>
cannam@86 83 <li><a href="mdct.html">Time domain segmentation and MDCT transform</a></li>
cannam@86 84 <li><a href="resolution.html">The resolution floor</a></li>
cannam@86 85 <li><a href="residuals.html">MDCT-domain fine structure</a></li>
cannam@86 86 </ul>
cannam@86 87
cannam@86 88 <ul>
cannam@86 89 <li><a href="probmodel.html">The Vorbis probability model</a></li>
cannam@86 90 <li><a href="bitpack.html">The Vorbis bitpacker</a></li>
cannam@86 91 </ul>
cannam@86 92
cannam@86 93 <ul>
cannam@86 94 <li><a href="oggstream.html">Ogg bitstream overview</a></li>
cannam@86 95 <li><a href="framing.html">Ogg logical bitstream and framing spec</a></li>
cannam@86 96 <li><a href="vorbis-stream.html">Vorbis packet->Ogg bitstream mapping</a></li>
cannam@86 97 </ul>
cannam@86 98
cannam@86 99 <ul>
cannam@86 100 <li><a href="programming.html">Programming with libvorbis</a></li>
cannam@86 101 </ul>
cannam@86 102
cannam@86 103 <h2>Description</h2>
cannam@86 104
cannam@86 105 <p>Ogg Vorbis is a general purpose compressed audio format
cannam@86 106 for high quality (44.1-48.0kHz, 16+ bit, polyphonic) audio and music
cannam@86 107 at moderate fixed and variable bitrates (40-80 kb/s/channel). This
cannam@86 108 places Vorbis in the same class as audio representations including
cannam@86 109 MPEG-1 audio layer 3, MPEG-4 audio (AAC and TwinVQ), and PAC.</p>
cannam@86 110
cannam@86 111 <p>Vorbis is the first of a planned family of Ogg multimedia coding
cannam@86 112 formats being developed as part of the Xiph.Org Foundation's Ogg multimedia
cannam@86 113 project. See <a href="http://www.xiph.org/">http://www.xiph.org/</a>
cannam@86 114 for more information.</p>
cannam@86 115
cannam@86 116 <h2>Vorbis technical documents</h2>
cannam@86 117
cannam@86 118 <p>A Vorbis encoder takes in overlapping (but contiguous) short-time
cannam@86 119 segments of audio data. The encoder analyzes the content of the audio
cannam@86 120 to determine an optimal compact representation; this phase of encoding
cannam@86 121 is known as <em>analysis</em>. For each short-time block of sound,
cannam@86 122 the encoder then packs an efficient representation of the signal, as
cannam@86 123 determined by analysis, into a raw packet much smaller than the size
cannam@86 124 required by the original signal; this phase is <em>coding</em>.
cannam@86 125 Lastly, in a streaming environment, the raw packets are then
cannam@86 126 structured into a continuous stream of octets; this last phase is
cannam@86 127 <em>streaming</em>. Note that the stream of octets is referred to both
cannam@86 128 as a 'byte-' and 'bit-'stream; the latter usage is acceptible as the
cannam@86 129 stream of octets is a physical representation of a true logical
cannam@86 130 bit-by-bit stream.</p>
cannam@86 131
cannam@86 132 <p>A Vorbis decoder performs a mirror image process of extracting the
cannam@86 133 original sequence of raw packets from an Ogg stream (<em>stream
cannam@86 134 decomposition</em>), reconstructing the signal representation from the
cannam@86 135 raw data in the packet (<em>decoding</em>) and them reconstituting an
cannam@86 136 audio signal from the decoded representation (<em>synthesis</em>).</p>
cannam@86 137
cannam@86 138 <p>The <a href="programming.html">Programming with libvorbis</a>
cannam@86 139 documents discuss use of the reference Vorbis codec library
cannam@86 140 (libvorbis) produced by the Xiph.Org Foundation.</p>
cannam@86 141
cannam@86 142 <p>The data representations and algorithms necessary at each step to
cannam@86 143 encode and decode Ogg Vorbis bitstreams are described by the below
cannam@86 144 documents in sufficient detail to construct a complete Vorbis codec.
cannam@86 145 Note that at the time of writing, Vorbis is still in a 'Request For
cannam@86 146 Comments' stage of development; despite being in advanced stages of
cannam@86 147 development, input from the multimedia community is welcome.</p>
cannam@86 148
cannam@86 149 <h3>Vorbis analysis and synthesis</h3>
cannam@86 150
cannam@86 151 <p>Analysis begins by seperating an input audio stream into individual,
cannam@86 152 overlapping short-time segments of audio data. These segments are
cannam@86 153 then transformed into an alternate representation, seeking to
cannam@86 154 represent the original signal in a more efficient form that codes into
cannam@86 155 a smaller number of bytes. The analysis and transformation stage is
cannam@86 156 the most complex element of producing a Vorbis bitstream.</p>
cannam@86 157
cannam@86 158 <p>The corresponding synthesis step in the decoder is simpler; there is
cannam@86 159 no analysis to perform, merely a mechanical, deterministic
cannam@86 160 reconstruction of the original audio data from the transform-domain
cannam@86 161 representation.</p>
cannam@86 162
cannam@86 163 <ul>
cannam@86 164 <li><a href="packet.html">Vorbis packet structure</a>:
cannam@86 165 Describes the basic analysis components necessary to produce Vorbis
cannam@86 166 packets and the structure of the packet itself.</li>
cannam@86 167 <li><a href="envelope.html">Temporal envelope shaping and blocksize</a>:
cannam@86 168 Use of temporal envelope shaping and variable blocksize to minimize
cannam@86 169 time-domain energy leakage during wide dynamic range and spectral energy
cannam@86 170 swings. Also discusses time-related principles of psychoacoustics.</li>
cannam@86 171 <li><a href="mdct.html">Time domain segmentation and MDCT transform</a>:
cannam@86 172 Division of time domain data into individual overlapped, windowed
cannam@86 173 short-time vectors and transformation using the MDCT</li>
cannam@86 174 <li><a href="resolution.html">The resolution floor</a>: Use of frequency
cannam@86 175 doamin psychoacoustics, and the MDCT-domain noise, masking and resolution
cannam@86 176 floors</li>
cannam@86 177 <li><a href="residuals.html">MDCT-domain fine structure</a>: Production,
cannam@86 178 quantization and massaging of MDCT-spectrum fine structure</li>
cannam@86 179 </ul>
cannam@86 180
cannam@86 181 <h3>Vorbis coding and decoding</h3>
cannam@86 182
cannam@86 183 <p>Coding and decoding converts the transform-domain representation of
cannam@86 184 the original audio produced by analysis to and from a bitwise packed
cannam@86 185 raw data packet. Coding and decoding consist of two logically
cannam@86 186 orthogonal concepts, <em>back-end coding</em> and <em>bitpacking</em>.</p>
cannam@86 187
cannam@86 188 <p><em>Back-end coding</em> uses a probability model to represent the raw numbers
cannam@86 189 of the audio representation in as few physical bits as possible;
cannam@86 190 familiar examples of back-end coding include Huffman coding and Vector
cannam@86 191 Quantization.</p>
cannam@86 192
cannam@86 193 <p><em>Bitpacking</em> arranges the variable sized words of the back-end
cannam@86 194 coding into a vector of octets without wasting space. The octets
cannam@86 195 produced by coding a single short-time audio segment is one raw Vorbis
cannam@86 196 packet.</p>
cannam@86 197
cannam@86 198 <ul>
cannam@86 199 <li><a href="probmodel.html">The Vorbis probability model</a></li>
cannam@86 200 <li><a href="bitpack.html">The Vorbis bitpacker</a>: Arrangement of
cannam@86 201 variable bit-length words into an octet-aligned packet.</li>
cannam@86 202 </ul>
cannam@86 203
cannam@86 204 <h3>Vorbis streaming and stream decomposition</h3>
cannam@86 205
cannam@86 206 <p>Vorbis packets contain the raw, bitwise-compressed representation of a
cannam@86 207 snippet of audio. These packets contain no structure and cannot be
cannam@86 208 strung together directly into a stream; for streamed transmission and
cannam@86 209 storage, Vorbis packets are encoded into an Ogg bitstream.</p>
cannam@86 210
cannam@86 211 <ul>
cannam@86 212 <li><a href="oggstream.html">Ogg bitstream overview</a>: High-level
cannam@86 213 description of Ogg logical bitstreams, how logical bitstreams
cannam@86 214 (of mixed media types) can be combined into physical bitstreams, and
cannam@86 215 restrictions on logical-to-physical mapping. Note that this document is
cannam@86 216 not specific only to Ogg Vorbis.</li>
cannam@86 217 <li><a href="framing.html">Ogg logical bitstream and framing
cannam@86 218 spec</a>: Low level, complete specification of Ogg logical
cannam@86 219 bitstream pages. Note that this document is not specific only to Ogg
cannam@86 220 Vorbis.</li>
cannam@86 221 <li><a href="vorbis-stream.html">Vorbis bitstream mapping</a>:
cannam@86 222 Specifically describes mapping Vorbis data into an
cannam@86 223 Ogg physical bitstream.</li>
cannam@86 224 </ul>
cannam@86 225
cannam@86 226 <div id="copyright">
cannam@86 227 The Xiph Fish Logo is a
cannam@86 228 trademark (&trade;) of Xiph.Org.<br/>
cannam@86 229
cannam@86 230 These pages &copy; 1994 - 2005 Xiph.Org. All rights reserved.
cannam@86 231 </div>
cannam@86 232
cannam@86 233 </body>
cannam@86 234 </html>