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Bring in flac, ogg, vorbis
author Chris Cannam
date Tue, 19 Mar 2013 17:37:49 +0000
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1 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
2 <html>
3 <head>
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5 <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/>
6 <title>Ogg Vorbis Documentation</title>
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65 </head>
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67 <body>
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69 <div id="xiphlogo">
70 <a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.Org"/></a>
71 </div>
72
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>
225
226 <div id="copyright">
227 The Xiph Fish Logo is a
228 trademark (&trade;) of Xiph.Org.<br/>
229
230 These pages &copy; 1994 - 2005 Xiph.Org. All rights reserved.
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