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> 4 5<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/> 6<title>Ogg Vorbis Documentation</title> 7 8<style type="text/css"> 9body { 10 margin: 0 18px 0 18px; 11 padding-bottom: 30px; 12 font-family: Verdana, Arial, Helvetica, sans-serif; 13 color: #333333; 14 font-size: .8em; 15} 16 17a { 18 color: #3366cc; 19} 20 21img { 22 border: 0; 23} 24 25#xiphlogo { 26 margin: 30px 0 16px 0; 27} 28 29#content p { 30 line-height: 1.4; 31} 32 33h1, h1 a, h2, h2 a, h3, h3 a { 34 font-weight: bold; 35 color: #ff9900; 36 margin: 1.3em 0 8px 0; 37} 38 39h1 { 40 font-size: 1.3em; 41} 42 43h2 { 44 font-size: 1.2em; 45} 46 47h3 { 48 font-size: 1.1em; 49} 50 51li { 52 line-height: 1.4; 53} 54 55#copyright { 56 margin-top: 30px; 57 line-height: 1.5em; 58 text-align: center; 59 font-size: .8em; 60 color: #888888; 61 clear: both; 62} 63</style> 64 65</head> 66 67<body> 68 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 logical and physical bitstream overview</h1> 74 75<h2>Ogg bitstreams</h2> 76 77<p>Ogg codecs use octet vectors of raw, compressed data 78(<em>packets</em>). These compressed packets do not have any 79high-level structure or boundary information; strung together, they 80appear to be streams of random bytes with no landmarks.</p> 81 82<p>Raw packets may be used directly by transport mechanisms that provide 83their own framing and packet-separation mechanisms (such as UDP 84datagrams). For stream based storage (such as files) and transport 85(such as TCP streams or pipes), Vorbis and other future Ogg codecs use 86the Ogg bitstream format to provide framing/sync, sync recapture 87after error, landmarks during seeking, and enough information to 88properly separate data back into packets at the original packet 89boundaries without relying on decoding to find packet boundaries.</p> 90 91<h2>Logical and physical bitstreams</h2> 92 93<p>Raw packets are grouped and encoded into contiguous pages of 94structured bitstream data called <em>logical bitstreams</em>. A 95logical bitstream consists of pages, in order, belonging to a single 96codec instance. Each page is a self contained entity (although it is 97possible that a packet may be split and encoded across one or more 98pages); that is, the page decode mechanism is designed to recognize, 99verify and handle single pages at a time from the overall bitstream.</p> 100 101<p>Multiple logical bitstreams can be combined (with restrictions) into a 102single <em>physical bitstream</em>. A physical bitstream consists of 103multiple logical bitstreams multiplexed at the page level and may 104include a 'meta-header' at the beginning of the multiplexed logical 105stream that serves as identification magic. Whole pages are taken in 106order from multiple logical bitstreams and combined into a single 107physical stream of pages. The decoder reconstructs the original 108logical bitstreams from the physical bitstream by taking the pages in 109order from the physical bitstream and redirecting them into the 110appropriate logical decoding entity. The simplest physical bitstream 111is a single, unmultiplexed logical bitstream with no meta-header; this 112is referred to as a 'degenerate stream'.</p> 113 114<p><a href="framing.html">Ogg Logical Bitstream Framing</a> discusses 115the page format of an Ogg bitstream, the packet coding process 116and logical bitstreams in detail. The remainder of this document 117specifies requirements for constructing finished, physical Ogg 118bitstreams.</p> 119 120<h2>Mapping Restrictions</h2> 121 122<p>Logical bitstreams may not be mapped/multiplexed into physical 123bitstreams without restriction. Here we discuss design restrictions 124on Ogg physical bitstreams in general, mostly to introduce 125design rationale. Each 'media' format defines its own (generally more 126restrictive) mapping. An 'Ogg Vorbis Audio Bitstream', for example, has a 127specific physical bitstream structure. 128An 'Ogg A/V' bitstream (not currently specified) will also mandate a 129specific, restricted physical bitstream format.</p> 130 131<h3>additional end-to-end structure</h3> 132 133<p>The <a href="framing.html">framing specification</a> defines 134'beginning of stream' and 'end of stream' page markers via a header 135flag (it is possible for a stream to consist of a single page). A 136stream always consists of an integer number of pages, an easy 137requirement given the variable size nature of pages.</p> 138 139<p>In addition to the header flag marking the first and last pages of a 140logical bitstream, the first page of an Ogg bitstream obeys 141additional restrictions. Each individual media mapping specifies its 142own implementation details regarding these restrictions.</p> 143 144<p>The first page of a logical Ogg bitstream consists of a single, 145small 'initial header' packet that includes sufficient information to 146identify the exact CODEC type and media requirements of the logical 147bitstream. The intent of this restriction is to simplify identifying 148the bitstream type and content; for a given media type (or across all 149Ogg media types) we can know that we only need a small, fixed 150amount of data to uniquely identify the bitstream type.</p> 151 152<p>As an example, Ogg Vorbis places the name and revision of the Vorbis 153CODEC, the audio rate and the audio quality into this initial header, 154thus simplifying vastly the certain identification of an Ogg Vorbis 155audio bitstream.</p> 156 157<h3>sequential multiplexing (chaining)</h3> 158 159<p>The simplest form of logical bitstream multiplexing is concatenation 160(<em>chaining</em>). Complete logical bitstreams are strung 161one-after-another in order. The bitstreams do not overlap; the final 162page of a given logical bitstream is immediately followed by the 163initial page of the next. Chaining is the only logical->physical 164mapping allowed by Ogg Vorbis.</p> 165 166<p>Each chained logical bitstream must have a unique serial number within 167the scope of the physical bitstream.</p> 168 169<h3>concurrent multiplexing (grouping)</h3> 170 171<p>Logical bitstreams may also be multiplexed 'in parallel' 172(<em>grouped</em>). An example of grouping would be to allow 173streaming of separate audio and video streams, using different codecs 174and different logical bitstreams, in the same physical bitstream. 175Whole pages from multiple logical bitstreams are mixed together.</p> 176 177<p>The initial pages of each logical bitstream must appear first; the 178media mapping specifies the order of the initial pages. For example, 179Ogg A/V will eventually specify an Ogg video bitstream with 180audio. The mapping may specify that the physical bitstream must begin 181with the initial page of a logical video bitstream, followed by the 182initial page of an audio stream. Unlike initial pages, terminal pages 183for the logical bitstreams need not all occur contiguously (although a 184specific media mapping may require this; it is not mandated by the 185generic Ogg stream spec). Terminal pages may be 'nil' pages, 186that is, pages containing no content but simply a page header with 187position information and the 'last page of bitstream' flag set in the 188page header.</p> 189 190<p>Each grouped bitstream must have a unique serial number within the 191scope of the physical bitstream.</p> 192 193<h3>sequential and concurrent multiplexing</h3> 194 195<p>Groups of concurrently multiplexed bitstreams may be chained 196consecutively. Such a physical bitstream obeys all the rules of both 197grouped and chained multiplexed streams; the groups, when unchained , 198must stand on their own as a valid concurrently multiplexed 199bitstream.</p> 200 201<h3>multiplexing example</h3> 202 203<p>Below, we present an example of a grouped and chained bitstream:</p> 204 205<p><img src="stream.png" alt="stream"/></p> 206 207<p>In this example, we see pages from five total logical bitstreams 208multiplexed into a physical bitstream. Note the following 209characteristics:</p> 210 211<ol> 212<li>Grouped bitstreams begin together; all of the initial pages 213must appear before any data pages. When concurrently multiplexed 214groups are chained, the new group does not begin until all the 215bitstreams in the previous group have terminated.</li> 216 217<li>The pages of concurrently multiplexed bitstreams need not conform 218to a regular order; the only requirement is that page <tt>n</tt> of a 219logical bitstream follow page <tt>n-1</tt> in the physical bitstream. 220There are no restrictions on intervening pages belonging to other 221logical bitstreams. (Tying page appearance to bitrate demands is one 222logical strategy, ie, the page appears at the chronological point 223where decode requires more information).</li> 224</ol> 225 226<div id="copyright"> 227 The Xiph Fish Logo is a 228 trademark (™) of Xiph.Org.<br/> 229 230 These pages © 1994 - 2005 Xiph.Org. All rights reserved. 231</div> 232 233</body> 234</html> 235
