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<HTML><HEAD><TITLE>xiph.org: Ogg Vorbis documentation</TITLE>
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<BODY bgcolor="#ffffff" text="#202020" link="#006666" vlink="#000000">
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<nobr><img src="white-ogg.png"><img src="vorbisword2.png"></nobr><p>
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<h1><font color=#000070>
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Ogg Vorbis I format specification: embedding Vorbis into an Ogg stream
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</font></h1>
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<em>Last update to this document: July 14, 2002</em><br>
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<h1>Overview</h1>
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This document describes using Ogg logical and physical transport
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streams to encapsulate Vorbis compressed audio packet data into file
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form.<p>
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_<a href="vorbis-spec-intro.html">Ogg Vorbis I format specification:
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high-level description</a>_ provides an overview of the construction
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of Vorbis audio packets.<p> The _<a href="oggstream.html">Ogg
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bitstream overview</a>_ and <a href="framing.html">Ogg logical
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bitstream and framing spec</a>_ provide detailed descriptions of Ogg
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transport streams. This specification document assumes a working
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knowledge of the concepts covered in these named backround
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documents. Please read them first.<p>
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<h2>Restrictions</h2>
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The Ogg/Vorbis I specification currently dictates that Ogg/Vorbis
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streams use Ogg transport streams in degenerate, unmultiplexed
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form only. That is:
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<ul>
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<li>A meta-headerless Ogg file encapsulates the Vorbis I packets
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<li>The Ogg stream may be chained, i.e. contain multiple, contigous logical streams (links).
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<li>The Ogg stream must be unmultiplexed (only one stream, a Vorbis audio stream, per link)
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</ul>
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This is not to say that it is not currently possible to multiplex
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Vorbis with other media types into a multi-stream Ogg file. At the
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time this document was written, Ogg was becoming a popular container
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for low-bitrate movies consisting of DiVX video and Vorbis audio.
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However, a 'Vorbis I audio file' is taken to imply Vorbis audio
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existing alone within a degenerate Ogg stream. A compliant 'Vorbis
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audio player' is not required to implement Ogg support beyond the
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specific support of Vorbis within a degenrate ogg stream (naturally,
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application authors are encouraged to support full multiplexed Ogg
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handling).
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<p>
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<h2>MIME type</h2>
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The correct MIME type of any Ogg file is <tt>application/x-ogg</tt>.
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However, if a file is a Vorbis I audio file (which implies a
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degenerate Ogg stream including only unmultiplexed Vorbis audio), the
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mime type <tt>audio/x-vorbis</tt> is also allowed.
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<h1>Encapsulation</h1>
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Ogg encapsulation of a Vorbis packet stream is straightforward.<p>
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<ul>
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<li>The first Vorbis packet [the indentification header], which
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uniquely identifies a stream as Vorbis audio, is placed alone in the
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first page of the logical Ogg stream. This results in a first Ogg
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page of exactly 58 bytes at the very beginning of the logical stream.<p>
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<li>This first page is marked 'beginning of stream' in the page flags.<p>
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<li>The second and third vorbis packets [comment and setup
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headers] may span one or more pages beginning on the second page of
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the logical stream. However many pages they span, the third header
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packet finishes the page on which it ends. The next [first audio] packet
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must begin on a fresh page.<p>
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<li>The granule position of these first pages containing only headers is
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zero.<p>
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<li>The first audio packet of the logical stream begins a fresh Ogg page.<p>
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<li>Packets are placed into ogg pages in order until the end of stream.<p>
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<li>The last page is marked 'end of stream' in the page flags.<p>
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<li>Vorbis packets may span page boundaries. <p>
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<li>The granule position of pages contining Vorbis audio is in units
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of PCM audio samples (per channel; a stereo stream's granule position
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does no increment at twice the speed of a mono stream).<p>
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<li>The granule position of a page represents the end PCM sample
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position of the last packet <em>completed</em> on that page. A page
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that is entirely spanned by a single packet (that completes on a
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subsequent page) has no granule position, and the granule position is
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set to '-1'.<p>
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<li>The granule (PCM) position of the first page need not indicate
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that the stream started at position zero. Although the granule
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position belongs to the last completed packet on the page and a
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valid granule position must be positive, by
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inference it may indicate that the PCM position of the beginning
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of audio is positive or negative.<p>
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<ul>
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<li>A positive starting value simply indicates that this stream begins at
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some positive time offset, potentially within a larger
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program. This is a common case when connecting to the middle
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of broadcast stream.<p> <li>A negative value indicates that
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output samples preceeding time zero should be discarded during
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decoding; this technique is used to allow sample-granularity
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editing of the stream start time of already-encoded Vorbis
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streams. The number of samples to be discarded must not exceed
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the overlap-add span of the first two audio packets.<p>
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</uL>
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In both of these cases in which the initial audio PCM starting
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offset is nonzero, the second finished audio packet must flush the
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page on which it appears and the third packet begin a fresh page.
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This allows the decoder to always be able to perform PCM position
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adjustments before needing to return any PCM data from synthesis,
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resulting in correct positioning information without any aditional
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seeking logic.<p>
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(Note however that failure to do so should, at worst, cause a
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decoder implementation to return incorrect positioning information
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for seeking operations at the very beginning of the stream.)<p>
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<li> A granule position on the final page in a stream that indicates
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less audio data than the final packet would normally return is used to
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end the stream on other than even frame boundaries. The difference
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between the actual available data returned and the declared amount
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indicates how many trailing samples to discard from the decoding
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process.<p>
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</ul>
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<hr>
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<a href="http://www.xiph.org/">
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<img src="white-xifish.png" align=left border=0>
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</a>
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<font size=-2 color=#505050>
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Ogg is a <a href="http://www.xiph.org">Xiph.org Foundation</a> effort
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to protect essential tenets of Internet multimedia from corporate
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hostage-taking; Open Source is the net's greatest tool to keep
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everyone honest. See <a href="http://www.xiph.org/about.html">About
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the Xiph.org Foundation</a> for details.
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<p>
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Ogg Vorbis is the first Ogg audio CODEC. Anyone may freely use and
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distribute the Ogg and Vorbis specification, whether in a private,
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public or corporate capacity. However, the Xiph.org Foundation and
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the Ogg project (xiph.org) reserve the right to set the Ogg Vorbis
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specification and certify specification compliance.<p>
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Xiph.org's Vorbis software CODEC implementation is distributed under a
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BSD-like license. This does not restrict third parties from
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distributing independent implementations of Vorbis software under
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other licenses.<p>
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Ogg, Vorbis, Xiph.org Foundation and their logos are trademarks (tm)
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of the <a href="http://www.xiph.org/">Xiph.org Foundation</a>. These
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pages are copyright (C) 1994-2002 Xiph.org Foundation. All rights
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reserved.<p>
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</body>
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