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5 <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/>
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6 <title>Ogg Vorbis Documentation</title>
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63 </style>
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65 </head>
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66
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67 <body>
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68
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69 <div id="xiphlogo">
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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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