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00073 <h1>Ogg Vorbis encoding format documentation</h1>
00074
00075 <p><img src="wait.png" alt="wait"/>As of writing, not all the below document
00076 links are live. They will be populated as we complete the documents.</p>
00077
00078 <h2>Documents</h2>
00079
00080 <ul>
00081 <li><a href="packet.html">Vorbis packet structure</a></li>
00082 <li><a href="envelope.html">Temporal envelope shaping and blocksize</a></li>
00083 <li><a href="mdct.html">Time domain segmentation and MDCT transform</a></li>
00084 <li><a href="resolution.html">The resolution floor</a></li>
00085 <li><a href="residuals.html">MDCT-domain fine structure</a></li>
00086 </ul>
00087
00088 <ul>
00089 <li><a href="probmodel.html">The Vorbis probability model</a></li>
00090 <li><a href="bitpack.html">The Vorbis bitpacker</a></li>
00091 </ul>
00092
00093 <ul>
00094 <li><a href="oggstream.html">Ogg bitstream overview</a></li>
00095 <li><a href="framing.html">Ogg logical bitstream and framing spec</a></li>
00096 <li><a href="vorbis-stream.html">Vorbis packet->Ogg bitstream mapping</a></li>
00097 </ul>
00098
00099 <ul>
00100 <li><a href="programming.html">Programming with libvorbis</a></li>
00101 </ul>
00102
00103 <h2>Description</h2>
00104
00105 <p>Ogg Vorbis is a general purpose compressed audio format
00106 for high quality (44.1-48.0kHz, 16+ bit, polyphonic) audio and music
00107 at moderate fixed and variable bitrates (40-80 kb/s/channel). This
00108 places Vorbis in the same class as audio representations including
00109 MPEG-1 audio layer 3, MPEG-4 audio (AAC and TwinVQ), and PAC.</p>
00110
00111 <p>Vorbis is the first of a planned family of Ogg multimedia coding
00112 formats being developed as part of the Xiph.org Foundation's Ogg multimedia
00113 project. See <a href="http:
00114 for more information.</p>
00115
00116 <h2>Vorbis technical documents</h2>
00117
00118 <p>A Vorbis encoder takes in overlapping (but contiguous) short-time
00119 segments of audio data. The encoder analyzes the content of the audio
00120 to determine an optimal compact representation; this phase of encoding
00121 is known as <em>analysis</em>. For each short-time block of sound,
00122 the encoder then packs an efficient representation of the signal, as
00123 determined by analysis, into a raw packet much smaller than the size
00124 required by the original signal; this phase is <em>coding</em>.
00125 Lastly, in a streaming environment, the raw packets are then
00126 structured into a continuous stream of octets; this last phase is
00127 <em>streaming</em>. Note that the stream of octets is referred to both
00128 as a 'byte-' and 'bit-'stream; the latter usage is acceptible as the
00129 stream of octets is a physical representation of a true logical
00130 bit-by-bit stream.</p>
00131
00132 <p>A Vorbis decoder performs a mirror image process of extracting the
00133 original sequence of raw packets from an Ogg stream (<em>stream
00134 decomposition</em>), reconstructing the signal representation from the
00135 raw data in the packet (<em>decoding</em>) and them reconstituting an
00136 audio signal from the decoded representation (<em>synthesis</em>).</p>
00137
00138 <p>The <a href="programming.html">Programming with libvorbis</a>
00139 documents discuss use of the reference Vorbis codec library
00140 (libvorbis) produced by the Xiph.org Foundation.</p>
00141
00142 <p>The data representations and algorithms necessary at each step to
00143 encode and decode Ogg Vorbis bitstreams are described by the below
00144 documents in sufficient detail to construct a complete Vorbis codec.
00145 Note that at the time of writing, Vorbis is still in a 'Request For
00146 Comments' stage of development; despite being in advanced stages of
00147 development, input from the multimedia community is welcome.</p>
00148
00149 <h3>Vorbis analysis and synthesis</h3>
00150
00151 <p>Analysis begins by seperating an input audio stream into individual,
00152 overlapping short-time segments of audio data. These segments are
00153 then transformed into an alternate representation, seeking to
00154 represent the original signal in a more efficient form that codes into
00155 a smaller number of bytes. The analysis and transformation stage is
00156 the most complex element of producing a Vorbis bitstream.</p>
00157
00158 <p>The corresponding synthesis step in the decoder is simpler; there is
00159 no analysis to perform, merely a mechanical, deterministic
00160 reconstruction of the original audio data from the transform-domain
00161 representation.</p>
00162
00163 <ul>
00164 <li><a href="packet.html">Vorbis packet structure</a>:
00165 Describes the basic analysis components necessary to produce Vorbis
00166 packets and the structure of the packet itself.</li>
00167 <li><a href="envelope.html">Temporal envelope shaping and blocksize</a>:
00168 Use of temporal envelope shaping and variable blocksize to minimize
00169 time-domain energy leakage during wide dynamic range and spectral energy
00170 swings. Also discusses time-related principles of psychoacoustics.</li>
00171 <li><a href="mdct.html">Time domain segmentation and MDCT transform</a>:
00172 Division of time domain data into individual overlapped, windowed
00173 short-time vectors and transformation using the MDCT</li>
00174 <li><a href="resolution.html">The resolution floor</a>: Use of frequency
00175 doamin psychoacoustics, and the MDCT-domain noise, masking and resolution
00176 floors</li>
00177 <li><a href="residuals.html">MDCT-domain fine structure</a>: Production,
00178 quantization and massaging of MDCT-spectrum fine structure</li>
00179 </ul>
00180
00181 <h3>Vorbis coding and decoding</h3>
00182
00183 <p>Coding and decoding converts the transform-domain representation of
00184 the original audio produced by analysis to and from a bitwise packed
00185 raw data packet. Coding and decoding consist of two logically
00186 orthogonal concepts, <em>back-end coding</em> and <em>bitpacking</em>.</p>
00187
00188 <p><em>Back-end coding</em> uses a probability model to represent the raw numbers
00189 of the audio representation in as few physical bits as possible;
00190 familiar examples of back-end coding include Huffman coding and Vector
00191 Quantization.</p>
00192
00193 <p><em>Bitpacking</em> arranges the variable sized words of the back-end
00194 coding into a vector of octets without wasting space. The octets
00195 produced by coding a single short-time audio segment is one raw Vorbis
00196 packet.</p>
00197
00198 <ul>
00199 <li><a href="probmodel.html">The Vorbis probability model</a></li>
00200 <li><a href="bitpack.html">The Vorbis bitpacker</a>: Arrangement of
00201 variable bit-length words into an octet-aligned packet.</li>
00202 </ul>
00203
00204 <h3>Vorbis streaming and stream decomposition</h3>
00205
00206 <p>Vorbis packets contain the raw, bitwise-compressed representation of a
00207 snippet of audio. These packets contain no structure and cannot be
00208 strung together directly into a stream; for streamed transmission and
00209 storage, Vorbis packets are encoded into an Ogg bitstream.</p>
00210
00211 <ul>
00212 <li><a href="oggstream.html">Ogg bitstream overview</a>: High-level
00213 description of Ogg logical bitstreams, how logical bitstreams
00214 (of mixed media types) can be combined into physical bitstreams, and
00215 restrictions on logical-to-physical mapping. Note that this document is
00216 not specific only to Ogg Vorbis.</li>
00217 <li><a href="framing.html">Ogg logical bitstream and framing
00218 spec</a>: Low level, complete specification of Ogg logical
00219 bitstream pages. Note that this document is not specific only to Ogg
00220 Vorbis.</li>
00221 <li><a href="vorbis-stream.html">Vorbis bitstream mapping</a>:
00222 Specifically describes mapping Vorbis data into an
00223 Ogg physical bitstream.</li>
00224 </ul>
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