235 lines
8.4 KiB
HTML
235 lines
8.4 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/>
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<title>Ogg Documentation</title>
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<style type="text/css">
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body {
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margin: 0 18px 0 18px;
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padding-bottom: 30px;
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font-family: Verdana, Arial, Helvetica, sans-serif;
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color: #333333;
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font-size: .8em;
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}
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a {
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color: #3366cc;
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}
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img {
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border: 0;
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}
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#xiphlogo {
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margin: 30px 0 16px 0;
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}
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#content p {
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line-height: 1.4;
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}
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h1, h1 a, h2, h2 a, h3, h3 a {
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font-weight: bold;
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color: #ff9900;
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margin: 1.3em 0 8px 0;
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}
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h1 {
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font-size: 1.3em;
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}
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h2 {
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font-size: 1.2em;
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}
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h3 {
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font-size: 1.1em;
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}
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li {
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line-height: 1.4;
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}
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#copyright {
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margin-top: 30px;
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line-height: 1.5em;
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text-align: center;
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font-size: .8em;
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color: #888888;
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clear: both;
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}
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</style>
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</head>
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<body>
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<div id="xiphlogo">
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<a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.org"/></a>
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</div>
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<h1>Ogg logical and physical bitstream overview</h1>
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<h2>Ogg bitstreams</h2>
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<p>Ogg codecs use octet vectors of raw, compressed data
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(<em>packets</em>). These compressed packets do not have any
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high-level structure or boundary information; strung together, they
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appear to be streams of random bytes with no landmarks.</p>
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<p>Raw packets may be used directly by transport mechanisms that provide
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their own framing and packet-separation mechanisms (such as UDP
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datagrams). For stream based storage (such as files) and transport
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(such as TCP streams or pipes), Vorbis and other future Ogg codecs use
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the Ogg bitstream format to provide framing/sync, sync recapture
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after error, landmarks during seeking, and enough information to
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properly separate data back into packets at the original packet
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boundaries without relying on decoding to find packet boundaries.</p>
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<h2>Logical and physical bitstreams</h2>
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<p>Raw packets are grouped and encoded into contiguous pages of
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structured bitstream data called <em>logical bitstreams</em>. A
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logical bitstream consists of pages, in order, belonging to a single
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codec instance. Each page is a self contained entity (although it is
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possible that a packet may be split and encoded across one or more
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pages); that is, the page decode mechanism is designed to recognize,
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verify and handle single pages at a time from the overall bitstream.</p>
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<p>Multiple logical bitstreams can be combined (with restrictions) into a
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single <em>physical bitstream</em>. A physical bitstream consists of
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multiple logical bitstreams multiplexed at the page level and may
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include a 'meta-header' at the beginning of the multiplexed logical
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stream that serves as identification magic. Whole pages are taken in
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order from multiple logical bitstreams and combined into a single
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physical stream of pages. The decoder reconstructs the original
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logical bitstreams from the physical bitstream by taking the pages in
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order from the physical bitstream and redirecting them into the
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appropriate logical decoding entity. The simplest physical bitstream
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is a single, unmultiplexed logical bitstream with no meta-header; this
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is referred to as a 'degenerate stream'.</p>
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<p><a href="framing.html">Ogg Logical Bitstream Framing</a> discusses
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the page format of an Ogg bitstream, the packet coding process
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and logical bitstreams in detail. The remainder of this document
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specifies requirements for constructing finished, physical Ogg
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bitstreams.</p>
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<h2>Mapping Restrictions</h2>
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<p>Logical bitstreams may not be mapped/multiplexed into physical
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bitstreams without restriction. Here we discuss design restrictions
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on Ogg physical bitstreams in general, mostly to introduce
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design rationale. Each 'media' format defines its own (generally more
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restrictive) mapping. An 'Ogg Vorbis Audio Bitstream', for example, has a
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specific physical bitstream structure.
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An 'Ogg A/V' bitstream (not currently specified) will also mandate a
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specific, restricted physical bitstream format.</p>
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<h3>additional end-to-end structure</h3>
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<p>The <a href="framing.html">framing specification</a> defines
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'beginning of stream' and 'end of stream' page markers via a header
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flag (it is possible for a stream to consist of a single page). A
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stream always consists of an integer number of pages, an easy
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requirement given the variable size nature of pages.</p>
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<p>In addition to the header flag marking the first and last pages of a
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logical bitstream, the first page of an Ogg bitstream obeys
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additional restrictions. Each individual media mapping specifies its
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own implementation details regarding these restrictions.</p>
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<p>The first page of a logical Ogg bitstream consists of a single,
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small 'initial header' packet that includes sufficient information to
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identify the exact CODEC type and media requirements of the logical
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bitstream. The intent of this restriction is to simplify identifying
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the bitstream type and content; for a given media type (or across all
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Ogg media types) we can know that we only need a small, fixed
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amount of data to uniquely identify the bitstream type.</p>
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<p>As an example, Ogg Vorbis places the name and revision of the Vorbis
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CODEC, the audio rate and the audio quality into this initial header,
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thus simplifying vastly the certain identification of an Ogg Vorbis
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audio bitstream.</p>
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<h3>sequential multiplexing (chaining)</h3>
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<p>The simplest form of logical bitstream multiplexing is concatenation
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(<em>chaining</em>). Complete logical bitstreams are strung
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one-after-another in order. The bitstreams do not overlap; the final
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page of a given logical bitstream is immediately followed by the
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initial page of the next. Chaining is the only logical->physical
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mapping allowed by Ogg Vorbis.</p>
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<p>Each chained logical bitstream must have a unique serial number within
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the scope of the physical bitstream.</p>
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<h3>concurrent multiplexing (grouping)</h3>
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<p>Logical bitstreams may also be multiplexed 'in parallel'
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(<em>grouped</em>). An example of grouping would be to allow
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streaming of separate audio and video streams, using different codecs
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and different logical bitstreams, in the same physical bitstream.
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Whole pages from multiple logical bitstreams are mixed together.</p>
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<p>The initial pages of each logical bitstream must appear first; the
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media mapping specifies the order of the initial pages. For example,
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Ogg A/V will eventually specify an Ogg video bitstream with
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audio. The mapping may specify that the physical bitstream must begin
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with the initial page of a logical video bitstream, followed by the
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initial page of an audio stream. Unlike initial pages, terminal pages
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for the logical bitstreams need not all occur contiguously (although a
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specific media mapping may require this; it is not mandated by the
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generic Ogg stream spec). Terminal pages may be 'nil' pages,
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that is, pages containing no content but simply a page header with
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position information and the 'last page of bitstream' flag set in the
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page header.</p>
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<p>Each grouped bitstream must have a unique serial number within the
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scope of the physical bitstream.</p>
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<h3>sequential and concurrent multiplexing</h3>
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<p>Groups of concurrently multiplexed bitstreams may be chained
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consecutively. Such a physical bitstream obeys all the rules of both
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grouped and chained multiplexed streams; the groups, when unchained ,
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must stand on their own as a valid concurrently multiplexed
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bitstream.</p>
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<h3>multiplexing example</h3>
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<p>Below, we present an example of a grouped and chained bitstream:</p>
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<p><img src="stream.png" alt="stream"/></p>
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<p>In this example, we see pages from five total logical bitstreams
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multiplexed into a physical bitstream. Note the following
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characteristics:</p>
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<ol>
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<li>Grouped bitstreams begin together; all of the initial pages
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must appear before any data pages. When concurrently multiplexed
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groups are chained, the new group does not begin until all the
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bitstreams in the previous group have terminated.</li>
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<li>The pages of concurrently multiplexed bitstreams need not conform
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to a regular order; the only requirement is that page <tt>n</tt> of a
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logical bitstream follow page <tt>n-1</tt> in the physical bitstream.
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There are no restrictions on intervening pages belonging to other
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logical bitstreams. (Tying page appearance to bitrate demands is one
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logical strategy, ie, the page appears at the chronological point
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where decode requires more information).</li>
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</ol>
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<div id="copyright">
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The Xiph Fish Logo is a
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trademark (™) of Xiph.Org.<br/>
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These pages © 1994 - 2005 Xiph.Org. All rights reserved.
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</div>
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</body>
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</html>
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