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author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 18 Oct 2016 13:40:26 +0100
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cannam@86 1 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
cannam@86 2 <html>
cannam@86 3 <head>
cannam@86 4
cannam@86 5 <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-15"/>
cannam@86 6 <title>Ogg Vorbis Documentation</title>
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cannam@86 63 </style>
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cannam@86 65 </head>
cannam@86 66
cannam@86 67 <body>
cannam@86 68
cannam@86 69 <div id="xiphlogo">
cannam@86 70 <a href="http://www.xiph.org/"><img src="fish_xiph_org.png" alt="Fish Logo and Xiph.Org"/></a>
cannam@86 71 </div>
cannam@86 72
cannam@86 73 <h1>Ogg logical and physical bitstream overview</h1>
cannam@86 74
cannam@86 75 <h2>Ogg bitstreams</h2>
cannam@86 76
cannam@86 77 <p>Ogg codecs use octet vectors of raw, compressed data
cannam@86 78 (<em>packets</em>). These compressed packets do not have any
cannam@86 79 high-level structure or boundary information; strung together, they
cannam@86 80 appear to be streams of random bytes with no landmarks.</p>
cannam@86 81
cannam@86 82 <p>Raw packets may be used directly by transport mechanisms that provide
cannam@86 83 their own framing and packet-separation mechanisms (such as UDP
cannam@86 84 datagrams). For stream based storage (such as files) and transport
cannam@86 85 (such as TCP streams or pipes), Vorbis and other future Ogg codecs use
cannam@86 86 the Ogg bitstream format to provide framing/sync, sync recapture
cannam@86 87 after error, landmarks during seeking, and enough information to
cannam@86 88 properly separate data back into packets at the original packet
cannam@86 89 boundaries without relying on decoding to find packet boundaries.</p>
cannam@86 90
cannam@86 91 <h2>Logical and physical bitstreams</h2>
cannam@86 92
cannam@86 93 <p>Raw packets are grouped and encoded into contiguous pages of
cannam@86 94 structured bitstream data called <em>logical bitstreams</em>. A
cannam@86 95 logical bitstream consists of pages, in order, belonging to a single
cannam@86 96 codec instance. Each page is a self contained entity (although it is
cannam@86 97 possible that a packet may be split and encoded across one or more
cannam@86 98 pages); that is, the page decode mechanism is designed to recognize,
cannam@86 99 verify and handle single pages at a time from the overall bitstream.</p>
cannam@86 100
cannam@86 101 <p>Multiple logical bitstreams can be combined (with restrictions) into a
cannam@86 102 single <em>physical bitstream</em>. A physical bitstream consists of
cannam@86 103 multiple logical bitstreams multiplexed at the page level and may
cannam@86 104 include a 'meta-header' at the beginning of the multiplexed logical
cannam@86 105 stream that serves as identification magic. Whole pages are taken in
cannam@86 106 order from multiple logical bitstreams and combined into a single
cannam@86 107 physical stream of pages. The decoder reconstructs the original
cannam@86 108 logical bitstreams from the physical bitstream by taking the pages in
cannam@86 109 order from the physical bitstream and redirecting them into the
cannam@86 110 appropriate logical decoding entity. The simplest physical bitstream
cannam@86 111 is a single, unmultiplexed logical bitstream with no meta-header; this
cannam@86 112 is referred to as a 'degenerate stream'.</p>
cannam@86 113
cannam@86 114 <p><a href="framing.html">Ogg Logical Bitstream Framing</a> discusses
cannam@86 115 the page format of an Ogg bitstream, the packet coding process
cannam@86 116 and logical bitstreams in detail. The remainder of this document
cannam@86 117 specifies requirements for constructing finished, physical Ogg
cannam@86 118 bitstreams.</p>
cannam@86 119
cannam@86 120 <h2>Mapping Restrictions</h2>
cannam@86 121
cannam@86 122 <p>Logical bitstreams may not be mapped/multiplexed into physical
cannam@86 123 bitstreams without restriction. Here we discuss design restrictions
cannam@86 124 on Ogg physical bitstreams in general, mostly to introduce
cannam@86 125 design rationale. Each 'media' format defines its own (generally more
cannam@86 126 restrictive) mapping. An 'Ogg Vorbis Audio Bitstream', for example, has a
cannam@86 127 specific physical bitstream structure.
cannam@86 128 An 'Ogg A/V' bitstream (not currently specified) will also mandate a
cannam@86 129 specific, restricted physical bitstream format.</p>
cannam@86 130
cannam@86 131 <h3>additional end-to-end structure</h3>
cannam@86 132
cannam@86 133 <p>The <a href="framing.html">framing specification</a> defines
cannam@86 134 'beginning of stream' and 'end of stream' page markers via a header
cannam@86 135 flag (it is possible for a stream to consist of a single page). A
cannam@86 136 stream always consists of an integer number of pages, an easy
cannam@86 137 requirement given the variable size nature of pages.</p>
cannam@86 138
cannam@86 139 <p>In addition to the header flag marking the first and last pages of a
cannam@86 140 logical bitstream, the first page of an Ogg bitstream obeys
cannam@86 141 additional restrictions. Each individual media mapping specifies its
cannam@86 142 own implementation details regarding these restrictions.</p>
cannam@86 143
cannam@86 144 <p>The first page of a logical Ogg bitstream consists of a single,
cannam@86 145 small 'initial header' packet that includes sufficient information to
cannam@86 146 identify the exact CODEC type and media requirements of the logical
cannam@86 147 bitstream. The intent of this restriction is to simplify identifying
cannam@86 148 the bitstream type and content; for a given media type (or across all
cannam@86 149 Ogg media types) we can know that we only need a small, fixed
cannam@86 150 amount of data to uniquely identify the bitstream type.</p>
cannam@86 151
cannam@86 152 <p>As an example, Ogg Vorbis places the name and revision of the Vorbis
cannam@86 153 CODEC, the audio rate and the audio quality into this initial header,
cannam@86 154 thus simplifying vastly the certain identification of an Ogg Vorbis
cannam@86 155 audio bitstream.</p>
cannam@86 156
cannam@86 157 <h3>sequential multiplexing (chaining)</h3>
cannam@86 158
cannam@86 159 <p>The simplest form of logical bitstream multiplexing is concatenation
cannam@86 160 (<em>chaining</em>). Complete logical bitstreams are strung
cannam@86 161 one-after-another in order. The bitstreams do not overlap; the final
cannam@86 162 page of a given logical bitstream is immediately followed by the
cannam@86 163 initial page of the next. Chaining is the only logical->physical
cannam@86 164 mapping allowed by Ogg Vorbis.</p>
cannam@86 165
cannam@86 166 <p>Each chained logical bitstream must have a unique serial number within
cannam@86 167 the scope of the physical bitstream.</p>
cannam@86 168
cannam@86 169 <h3>concurrent multiplexing (grouping)</h3>
cannam@86 170
cannam@86 171 <p>Logical bitstreams may also be multiplexed 'in parallel'
cannam@86 172 (<em>grouped</em>). An example of grouping would be to allow
cannam@86 173 streaming of separate audio and video streams, using different codecs
cannam@86 174 and different logical bitstreams, in the same physical bitstream.
cannam@86 175 Whole pages from multiple logical bitstreams are mixed together.</p>
cannam@86 176
cannam@86 177 <p>The initial pages of each logical bitstream must appear first; the
cannam@86 178 media mapping specifies the order of the initial pages. For example,
cannam@86 179 Ogg A/V will eventually specify an Ogg video bitstream with
cannam@86 180 audio. The mapping may specify that the physical bitstream must begin
cannam@86 181 with the initial page of a logical video bitstream, followed by the
cannam@86 182 initial page of an audio stream. Unlike initial pages, terminal pages
cannam@86 183 for the logical bitstreams need not all occur contiguously (although a
cannam@86 184 specific media mapping may require this; it is not mandated by the
cannam@86 185 generic Ogg stream spec). Terminal pages may be 'nil' pages,
cannam@86 186 that is, pages containing no content but simply a page header with
cannam@86 187 position information and the 'last page of bitstream' flag set in the
cannam@86 188 page header.</p>
cannam@86 189
cannam@86 190 <p>Each grouped bitstream must have a unique serial number within the
cannam@86 191 scope of the physical bitstream.</p>
cannam@86 192
cannam@86 193 <h3>sequential and concurrent multiplexing</h3>
cannam@86 194
cannam@86 195 <p>Groups of concurrently multiplexed bitstreams may be chained
cannam@86 196 consecutively. Such a physical bitstream obeys all the rules of both
cannam@86 197 grouped and chained multiplexed streams; the groups, when unchained ,
cannam@86 198 must stand on their own as a valid concurrently multiplexed
cannam@86 199 bitstream.</p>
cannam@86 200
cannam@86 201 <h3>multiplexing example</h3>
cannam@86 202
cannam@86 203 <p>Below, we present an example of a grouped and chained bitstream:</p>
cannam@86 204
cannam@86 205 <p><img src="stream.png" alt="stream"/></p>
cannam@86 206
cannam@86 207 <p>In this example, we see pages from five total logical bitstreams
cannam@86 208 multiplexed into a physical bitstream. Note the following
cannam@86 209 characteristics:</p>
cannam@86 210
cannam@86 211 <ol>
cannam@86 212 <li>Grouped bitstreams begin together; all of the initial pages
cannam@86 213 must appear before any data pages. When concurrently multiplexed
cannam@86 214 groups are chained, the new group does not begin until all the
cannam@86 215 bitstreams in the previous group have terminated.</li>
cannam@86 216
cannam@86 217 <li>The pages of concurrently multiplexed bitstreams need not conform
cannam@86 218 to a regular order; the only requirement is that page <tt>n</tt> of a
cannam@86 219 logical bitstream follow page <tt>n-1</tt> in the physical bitstream.
cannam@86 220 There are no restrictions on intervening pages belonging to other
cannam@86 221 logical bitstreams. (Tying page appearance to bitrate demands is one
cannam@86 222 logical strategy, ie, the page appears at the chronological point
cannam@86 223 where decode requires more information).</li>
cannam@86 224 </ol>
cannam@86 225
cannam@86 226 <div id="copyright">
cannam@86 227 The Xiph Fish Logo is a
cannam@86 228 trademark (&trade;) of Xiph.Org.<br/>
cannam@86 229
cannam@86 230 These pages &copy; 1994 - 2005 Xiph.Org. All rights reserved.
cannam@86 231 </div>
cannam@86 232
cannam@86 233 </body>
cannam@86 234 </html>