libvorbis/doc/vorbis.html

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<h1>Ogg Vorbis encoding format documentation</h1>

<p><img src="wait.png" alt="wait"/>As of writing, not all the below document
links are live. They will be populated as we complete the documents.</p>

<h2>Documents</h2>

<ul>
<li><a href="packet.html">Vorbis packet structure</a></li>
<li><a href="envelope.html">Temporal envelope shaping and blocksize</a></li>
<li><a href="mdct.html">Time domain segmentation and MDCT transform</a></li>
<li><a href="resolution.html">The resolution floor</a></li>
<li><a href="residuals.html">MDCT-domain fine structure</a></li>
</ul>

<ul>
<li><a href="probmodel.html">The Vorbis probability model</a></li>
<li><a href="bitpack.html">The Vorbis bitpacker</a></li>
</ul>

<ul>
<li><a href="oggstream.html">Ogg bitstream overview</a></li>
<li><a href="framing.html">Ogg logical bitstream and framing spec</a></li>
<li><a href="vorbis-stream.html">Vorbis packet->Ogg bitstream mapping</a></li>
</ul>

<ul>
<li><a href="programming.html">Programming with libvorbis</a></li>
</ul>

<h2>Description</h2>

<p>Ogg Vorbis is a general purpose compressed audio format
for high quality (44.1-48.0kHz, 16+ bit, polyphonic) audio and music
at moderate fixed and variable bitrates (40-80 kb/s/channel). This
places Vorbis in the same class as audio representations including
MPEG-1 audio layer 3, MPEG-4 audio (AAC and TwinVQ), and PAC.</p>

<p>Vorbis is the first of a planned family of Ogg multimedia coding
formats being developed as part of the Xiph.org Foundation's Ogg multimedia
project. See <a href="http://www.xiph.org/">http://www.xiph.org/</a>
for more information.</p>

<h2>Vorbis technical documents</h2>

<p>A Vorbis encoder takes in overlapping (but contiguous) short-time
segments of audio data. The encoder analyzes the content of the audio
to determine an optimal compact representation; this phase of encoding
is known as <em>analysis</em>. For each short-time block of sound,
the encoder then packs an efficient representation of the signal, as
determined by analysis, into a raw packet much smaller than the size
required by the original signal; this phase is <em>coding</em>.
Lastly, in a streaming environment, the raw packets are then
structured into a continuous stream of octets; this last phase is
<em>streaming</em>. Note that the stream of octets is referred to both
as a 'byte-' and 'bit-'stream; the latter usage is acceptible as the
stream of octets is a physical representation of a true logical
bit-by-bit stream.</p>

<p>A Vorbis decoder performs a mirror image process of extracting the
original sequence of raw packets from an Ogg stream (<em>stream
decomposition</em>), reconstructing the signal representation from the
raw data in the packet (<em>decoding</em>) and them reconstituting an
audio signal from the decoded representation (<em>synthesis</em>).</p>

<p>The <a href="programming.html">Programming with libvorbis</a>
documents discuss use of the reference Vorbis codec library
(libvorbis) produced by the Xiph.org Foundation.</p>

<p>The data representations and algorithms necessary at each step to
encode and decode Ogg Vorbis bitstreams are described by the below
documents in sufficient detail to construct a complete Vorbis codec.
Note that at the time of writing, Vorbis is still in a 'Request For
Comments' stage of development; despite being in advanced stages of
development, input from the multimedia community is welcome.</p>

<h3>Vorbis analysis and synthesis</h3>

<p>Analysis begins by seperating an input audio stream into individual,
overlapping short-time segments of audio data. These segments are
then transformed into an alternate representation, seeking to
represent the original signal in a more efficient form that codes into
a smaller number of bytes. The analysis and transformation stage is
the most complex element of producing a Vorbis bitstream.</p>

<p>The corresponding synthesis step in the decoder is simpler; there is
no analysis to perform, merely a mechanical, deterministic
reconstruction of the original audio data from the transform-domain
representation.</p>

<ul>
<li><a href="packet.html">Vorbis packet structure</a>:
Describes the basic analysis components necessary to produce Vorbis
packets and the structure of the packet itself.</li>
<li><a href="envelope.html">Temporal envelope shaping and blocksize</a>:
Use of temporal envelope shaping and variable blocksize to minimize
time-domain energy leakage during wide dynamic range and spectral energy
swings. Also discusses time-related principles of psychoacoustics.</li>
<li><a href="mdct.html">Time domain segmentation and MDCT transform</a>:
Division of time domain data into individual overlapped, windowed
short-time vectors and transformation using the MDCT</li>
<li><a href="resolution.html">The resolution floor</a>: Use of frequency
doamin psychoacoustics, and the MDCT-domain noise, masking and resolution
floors</li>
<li><a href="residuals.html">MDCT-domain fine structure</a>: Production,
quantization and massaging of MDCT-spectrum fine structure</li>
</ul>

<h3>Vorbis coding and decoding</h3>

<p>Coding and decoding converts the transform-domain representation of
the original audio produced by analysis to and from a bitwise packed
raw data packet. Coding and decoding consist of two logically
orthogonal concepts, <em>back-end coding</em> and <em>bitpacking</em>.</p>

<p><em>Back-end coding</em> uses a probability model to represent the raw numbers
of the audio representation in as few physical bits as possible;
familiar examples of back-end coding include Huffman coding and Vector
Quantization.</p>

<p><em>Bitpacking</em> arranges the variable sized words of the back-end
coding into a vector of octets without wasting space. The octets
produced by coding a single short-time audio segment is one raw Vorbis
packet.</p>

<ul>
<li><a href="probmodel.html">The Vorbis probability model</a></li>
<li><a href="bitpack.html">The Vorbis bitpacker</a>: Arrangement of
variable bit-length words into an octet-aligned packet.</li>
</ul>

<h3>Vorbis streaming and stream decomposition</h3>

<p>Vorbis packets contain the raw, bitwise-compressed representation of a
snippet of audio. These packets contain no structure and cannot be
strung together directly into a stream; for streamed transmission and
storage, Vorbis packets are encoded into an Ogg bitstream.</p>

<ul>
<li><a href="oggstream.html">Ogg bitstream overview</a>: High-level
description of Ogg logical bitstreams, how logical bitstreams
(of mixed media types) can be combined into physical bitstreams, and
restrictions on logical-to-physical mapping. Note that this document is
not specific only to Ogg Vorbis.</li>
<li><a href="framing.html">Ogg logical bitstream and framing
spec</a>: Low level, complete specification of Ogg logical
bitstream pages. Note that this document is not specific only to Ogg
Vorbis.</li>
<li><a href="vorbis-stream.html">Vorbis bitstream mapping</a>:
Specifically describes mapping Vorbis data into an
Ogg physical bitstream.</li>
</ul>

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