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