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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>Page Multiplexing and Ordering in a Physical Ogg Stream</h1> |
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| 74 | |
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| 75 | <p>The low-level mechanisms of an Ogg stream (as described in the Ogg |
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| 76 | Bitstream Overview) provide means for mixing multiple logical streams |
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| 77 | and media types into a single linear-chronological stream. This |
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| 78 | document specifies the high-level arrangement and use of page |
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| 79 | structure to multiplex multiple streams of mixed media type within a |
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| 80 | physical Ogg stream.</p> |
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| 81 | |
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| 82 | <h2>Design Elements</h2> |
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| 83 | |
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| 84 | <p>The design and arrangement of the Ogg container format is governed by |
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| 85 | several high-level design decisions that form the reasoning behind |
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| 86 | specific low-level design decisions.</p> |
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| 87 | |
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| 88 | <h3>Linear media</h3> |
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| 89 | |
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| 90 | <p>The Ogg bitstream is intended to encapsulate chronological, |
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| 91 | time-linear mixed media into a single delivery stream or file. The |
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| 92 | design is such that an application can always encode and/or decode a |
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| 93 | full-featured bitstream in one pass with no seeking and minimal |
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| 94 | buffering. Seeking to provide optimized encoding (such as two-pass |
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| 95 | encoding) or interactive decoding (such as scrubbing or instant |
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| 96 | replay) is not disallowed or discouraged, however no bitstream feature |
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| 97 | must require nonlinear operation on the bitstream.</p> |
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| 98 | |
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| 99 | <h3>Multiplexing</h3> |
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| 100 | |
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| 101 | <p>Ogg bitstreams multiplex multiple logical streams into a single |
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| 102 | physical stream at the page level. Each page contains an abstract |
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| 103 | time stamp (the Granule Position) that represents an absolute time |
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| 104 | landmark within the stream. After the pages representing stream |
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| 105 | headers (all logical stream headers occur at the beginning of a |
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| 106 | physical bitstream section before any logical stream data), logical |
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| 107 | stream data pages are arranged in strict, monotonically increasing |
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| 108 | order of chronological absolute time as specified by the granule |
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| 109 | position.</p> |
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| 110 | |
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| 111 | <p>The only exception to arranging pages in strictly ascending time order |
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| 112 | by granule position is those pages that do not set the granule |
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| 113 | position value. This is a special case when exceptionally large |
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| 114 | packets span multiple pages; the specifics of handling this special |
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| 115 | case are described later under 'Continuous and Discontinuous |
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| 116 | Streams'.</p> |
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| 117 | |
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| 118 | <h3>Seeking</h3> |
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| 119 | |
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| 120 | <p>Ogg is designed to use a bisection search to implement exact |
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| 121 | positional seeking rather than building an index; an index requires |
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| 122 | two-pass encoding and as such is not acceptable given the requirement |
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| 123 | for full-featured linear encoding.</p> |
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| 124 | |
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| 125 | <p><i>Even making an index optional then requires an |
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| 126 | application to support multiple methods (bisection search for a |
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| 127 | one-pass stream, indexing for a two-pass stream), which adds no |
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| 128 | additional functionality as bisection search delivers the same |
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| 129 | functionality for both stream types.</i></p> |
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| 130 | |
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| 131 | <p>Seek operations are by absolute time; a direct bisection search must |
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| 132 | find the exact time position requested. Information in the Ogg |
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| 133 | bitstream is arranged such that all information to be presented for |
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| 134 | playback from the desired seek point will occur at or after the |
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| 135 | desired seek point. Seek operations are neither 'fuzzy' nor |
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| 136 | heuristic.</p> |
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| 137 | |
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| 138 | <p><i>Although key frame handling in video appears to be an exception to |
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| 139 | "all needed playback information lies ahead of a given seek", |
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| 140 | key frames can still be handled directly within this indexless |
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| 141 | framework. Seeking to a key frame in video (as well as seeking in other |
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| 142 | media types with analogous restraints) is handled as two seeks; first |
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| 143 | a seek to the desired time which extracts state information that |
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| 144 | decodes to the time of the last key frame, followed by a second seek |
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| 145 | directly to the key frame. The location of the previous key frame is |
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| 146 | embedded as state information in the granulepos; this mechanism is |
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| 147 | described in more detail later.</i></p> |
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| 148 | |
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| 149 | <h3>Continuous and Discontinuous Streams</h3> |
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| 150 | |
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| 151 | <p>Logical streams within a physical Ogg stream belong to one of two |
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| 152 | categories, "Continuous" streams and "Discontinuous" streams. |
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| 153 | Although these are discussed in more detail later, the distinction is |
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| 154 | important to a high-level understanding of how to buffer an Ogg |
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| 155 | stream.</p> |
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| 156 | |
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| 157 | <p>A stream that provides a gapless, time-continuous media type with a |
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| 158 | fine-grained timebase is considered to be 'Continuous'. A continuous |
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| 159 | stream should never be starved of data. Clear examples of continuous |
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| 160 | data types include broadcast audio and video.</p> |
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| 161 | |
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| 162 | <p>A stream that delivers data in a potentially irregular pattern or with |
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| 163 | widely spaced timing gaps is considered to be 'Discontinuous'. A |
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| 164 | discontinuous stream may be best thought of as data representing |
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| 165 | scattered events; although they happen in order, they are typically |
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| 166 | unconnected data often located far apart. One possible example of a |
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| 167 | discontinuous stream types would be captioning. Although it's |
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| 168 | possible to design captions as a continuous stream type, it's most |
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| 169 | natural to think of captions as widely spaced pieces of text with |
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| 170 | little happening between.</p> |
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| 171 | |
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| 172 | <p>The fundamental design distinction between continuous and |
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| 173 | discontinuous streams concerns buffering.</p> |
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| 174 | |
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| 175 | <h3>Buffering</h3> |
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| 176 | |
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| 177 | <p>Because a continuous stream is, by definition, gapless, Ogg buffering |
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| 178 | is based on the simple premise of never allowing any active continuous |
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| 179 | stream to starve for data during decode; buffering proceeds ahead |
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| 180 | until all continuous streams in a physical stream have data ready to |
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| 181 | decode on demand.</p> |
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| 182 | |
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| 183 | <p>Discontinuous stream data may occur on a fairly regular basis, but the |
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| 184 | timing of, for example, a specific caption is impossible to predict |
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| 185 | with certainty in most captioning systems. Thus the buffering system |
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| 186 | should take discontinuous data 'as it comes' rather than working ahead |
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| 187 | (for a potentially unbounded period) to look for future discontinuous |
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| 188 | data. As such, discontinuous streams are ignored when managing |
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| 189 | buffering; their pages simply 'fall out' of the stream when continuous |
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| 190 | streams are handled properly.</p> |
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| 191 | |
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| 192 | <p>Buffering requirements need not be explicitly declared or managed for |
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| 193 | the encoded stream; the decoder simply reads as much data as is |
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| 194 | necessary to keep all continuous stream types gapless (also ensuring |
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| 195 | discontinuous data arrives in time) and no more, resulting in optimum |
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| 196 | implicit buffer usage for a given stream. Because all pages of all |
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| 197 | data types are stamped with absolute timing information within the |
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| 198 | stream, inter-stream synchronization timing is always explicitly |
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| 199 | maintained without the need for explicitly declared buffer-ahead |
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| 200 | hinting.</p> |
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| 201 | |
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| 202 | <p>Further details, mechanisms and reasons for the differing arrangement |
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| 203 | and behavior of continuous and discontinuous streams is discussed |
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| 204 | later.</p> |
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| 205 | |
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| 206 | <h3>Whole-stream navigation</h3> |
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| 207 | |
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| 208 | <p>Ogg is designed so that the simplest navigation operations treat the |
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| 209 | physical Ogg stream as a whole summary of its streams, rather than |
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| 210 | navigating each interleaved stream as a separate entity.</p> |
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| 211 | |
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| 212 | <p>First Example: seeking to a desired time position in a multiplexed (or |
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| 213 | unmultiplexed) Ogg stream can be accomplished through a bisection |
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| 214 | search on time position of all pages in the stream (as encoded in the |
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| 215 | granule position). More powerful searches (such as a key frame-aware |
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| 216 | seek within video) are also possible with additional search |
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| 217 | complexity, but similar computational complexity.</p> |
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| 218 | |
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| 219 | <p>Second Example: A bitstream section may consist of three multiplexed |
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| 220 | streams of differing lengths. The result of multiplexing these |
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| 221 | streams should be thought of as a single mixed stream with a length |
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| 222 | equal to the longest of the three component streams. Although it is |
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| 223 | also possible to think of the multiplexed results as three concurrent |
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| 224 | streams of different lengths and it is possible to recover the three |
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| 225 | original streams, it will also become obvious that once multiplexed, |
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| 226 | it isn't possible to find the internal lengths of the component |
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| 227 | streams without a linear search of the whole bitstream section. |
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| 228 | However, it is possible to find the length of the whole bitstream |
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| 229 | section easily (in near-constant time per section) just as it is for a |
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| 230 | single-media unmultiplexed stream.</p> |
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| 231 | |
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| 232 | <h2>Granule Position</h2> |
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| 233 | |
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| 234 | <h3>Description</h3> |
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| 235 | |
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| 236 | <p>The Granule Position is a signed 64 bit field appearing in the header |
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| 237 | of every Ogg page. Although the granule position represents absolute |
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| 238 | time within a logical stream, its value does not necessarily directly |
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| 239 | encode a simple timestamp. It may represent frames elapsed (as in |
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| 240 | Vorbis), a simple timestamp, or a more complex bit-division encoding |
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| 241 | (such as in Theora). The exact encoding of the granule position is up |
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| 242 | to a specific codec.</p> |
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| 243 | |
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| 244 | <p>The granule position is governed by the following rules:</p> |
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| 245 | |
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| 246 | <ul> |
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| 247 | |
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| 248 | <li>Granule Position must always increase forward or remain equal from |
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| 249 | page to page, be unset, or be zero for a header page. The absolute |
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| 250 | time to which any correct sequence of granule position maps must |
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| 251 | similarly always increase forward or remain equal. <i>(A codec may |
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| 252 | make use of data, such as a control sequence, that only affects codec |
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| 253 | working state without producing data and thus advancing granule |
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| 254 | position and time. Although the packet sequence number increases in |
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| 255 | this case, the granule position, and thus the time position, do |
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| 256 | not.)</i></li> |
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| 257 | |
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| 258 | <li>Granule position may only be unset if there no packet defining a |
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| 259 | time boundary on the page (that is, if no packet in a continuous |
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| 260 | stream ends on the page, or no packet in a discontinuous stream begins |
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| 261 | on the page. This will be discussed in more detail under Continuous |
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| 262 | and Discontinuous streams).</li> |
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| 263 | |
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| 264 | <li>A codec must be able to translate a given granule position value |
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| 265 | to a unique, deterministic absolute time value through direct |
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| 266 | calculation. A codec is not required to be able to translate an |
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| 267 | absolute time value into a unique granule position value.</li> |
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| 268 | |
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| 269 | <li>Codecs shall choose a granule position definition that allows that |
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| 270 | codec means to seek as directly as possible to an immediately |
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| 271 | decodable point, such as the bit-divided granule position encoding of |
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| 272 | Theora allows the codec to seek efficiently to key frame without using |
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| 273 | an index. That is, additional information other than absolute time |
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| 274 | may be encoded into a granule position value so long as the granule |
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| 275 | position obeys the above points.</li> |
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| 276 | |
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| 277 | </ul> |
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| 278 | |
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| 279 | <h4>Example: timestamp</h4> |
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| 280 | |
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| 281 | <p>In general, a codec/stream type should choose the simplest granule |
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| 282 | position encoding that addresses its requirements. The examples here |
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| 283 | are by no means exhaustive of the possibilities within Ogg.</p> |
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| 284 | |
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| 285 | <p>A simple granule position could encode a timestamp directly. For |
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| 286 | example, a granule position that encoded milliseconds from beginning |
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| 287 | of stream would allow a logical stream length of over 100,000,000,000 |
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| 288 | days before beginning a new logical stream (to avoid the granule |
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| 289 | position wrapping).</p> |
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| 290 | |
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| 291 | <h4>Example: framestamp</h4> |
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| 292 | |
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| 293 | <p>A simple millisecond timestamp granule encoding might suit many stream |
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| 294 | types, but a millisecond resolution is inappropriate to, eg, most |
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| 295 | audio encodings where exact single-sample resolution is generally a |
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| 296 | requirement. A millisecond is both too large a granule and often does |
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| 297 | not represent an integer number of samples.</p> |
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| 298 | |
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| 299 | <p>In the event that audio frames are always encoded as the same number of |
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| 300 | samples, the granule position could simply be a linear count of frames |
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| 301 | since beginning of stream. This has the advantages of being exact and |
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| 302 | efficient. Position in time would simply be <tt>[granule_position] * |
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| 303 | [samples_per_frame] / [samples_per_second]</tt>.</p> |
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| 304 | |
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| 305 | <h4>Example: samplestamp (Vorbis)</h4> |
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| 306 | |
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| 307 | <p>Frame counting is insufficient in codecs such as Vorbis where an audio |
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| 308 | frame [packet] encodes a variable number of samples. In Vorbis's |
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| 309 | case, the granule position is a count of the number of raw samples |
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| 310 | from the beginning of stream; the absolute time of |
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| 311 | a granule position is <tt>[granule_position] / |
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| 312 | [samples_per_second]</tt>.</p> |
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| 313 | |
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| 314 | <h4>Example: bit-divided framestamp (Theora)</h4> |
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| 315 | |
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| 316 | <p>Some video codecs may be able to use the simple framestamp scheme for |
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| 317 | granule position. However, most modern video codecs introduce at |
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| 318 | least the following complications:</p> |
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| 319 | |
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| 320 | <ul> |
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| 321 | |
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| 322 | <li>video frames are relatively far apart compared to audio samples; |
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| 323 | for this reason, the point at which a video frame changes to the next |
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| 324 | frame is usually a strictly defined offset within the frame 'period'. |
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| 325 | That is, video at 50fps could just as easily define frame transitions |
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| 326 | <.015, .035, .055...> as at <.00, .02, .04...>.</li> |
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| 327 | |
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| 328 | <li>frame rates often include drop-frames, leap-frames or other |
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| 329 | rational-but-non-integer timings.</li> |
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| 330 | |
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| 331 | <li>Decode must begin at a 'key frame' or 'I frame'. Keyframes usually |
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| 332 | occur relatively seldom.</li> |
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| 333 | |
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| 334 | </ul> |
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| 335 | |
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| 336 | <p>The first two points can be handled straightforwardly via the fact |
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| 337 | that the codec has complete control mapping granule position to |
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| 338 | absolute time; non-integer frame rates and offsets can be set in the |
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| 339 | codec's initial header, and the rest is just arithmetic.</p> |
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| 340 | |
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| 341 | <p>The third point appears trickier at first glance, but it too can be |
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| 342 | handled through the granule position mapping mechanism. Here we |
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| 343 | arrange the granule position in such a way that granule positions of |
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| 344 | key frames are easy to find. Divide the granule position into two |
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| 345 | fields; the most-significant bits are an absolute frame counter, but |
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| 346 | it's only updated at each key frame. The least significant bits encode |
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| 347 | the number of frames since the last key frame. In this way, each |
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| 348 | granule position both encodes the absolute time of the current frame |
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| 349 | as well as the absolute time of the last key frame.</p> |
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| 350 | |
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| 351 | <p>Seeking to a most recent preceding key frame is then accomplished by |
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| 352 | first seeking to the original desired point, inspecting the granulepos |
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| 353 | of the resulting video page, extracting from that granulepos the |
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| 354 | absolute time of the desired key frame, and then seeking directly to |
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| 355 | that key frame's page. Of course, it's still possible for an |
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| 356 | application to ignore key frames and use a simpler seeking algorithm |
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| 357 | (decode would be unable to present decoded video until the next |
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| 358 | key frame). Surprisingly many player applications do choose the |
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| 359 | simpler approach.</p> |
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| 360 | |
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| 361 | <h3>granule position, packets and pages</h3> |
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| 362 | |
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| 363 | <p>Although each packet of data in a logical stream theoretically has a |
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| 364 | specific granule position, only one granule position is encoded |
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| 365 | per page. It is possible to encode a logical stream such that each |
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| 366 | page contains only a single packet (so that granule positions are |
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| 367 | preserved for each packet), however a one-to-one packet/page mapping |
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| 368 | is not intended to be the general case.</p> |
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| 369 | |
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| 370 | <p>Because Ogg functions at the page, not packet, level, this |
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| 371 | once-per-page time information provides Ogg with the finest-grained |
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| 372 | time information is can use. Ogg passes this granule positioning data |
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| 373 | to the codec (along with the packets extracted from a page); it is the |
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| 374 | responsibility of codecs to track timing information at granularities |
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| 375 | finer than a single page.</p> |
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| 376 | |
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| 377 | <h3>start-time and end-time positioning</h3> |
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| 378 | |
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| 379 | <p>A granule position represents the <em>instantaneous time location |
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| 380 | between two pages</em>. However, continuous streams and discontinuous |
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| 381 | streams differ on whether the granulepos represents the end-time of |
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| 382 | the data on a page or the start-time. Continuous streams are |
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| 383 | 'end-time' encoded; the granulepos represents the point in time |
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| 384 | immediately after the last data decoded from a page. Discontinuous |
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| 385 | streams are 'start-time' encoded; the granulepos represents the point |
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| 386 | in time of the first data decoded from the page.</p> |
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| 387 | |
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| 388 | <p>An Ogg stream type is declared continuous or discontinuous by its |
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| 389 | codec. A given codec may support both continuous and discontinuous |
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| 390 | operation so long as any given logical stream is continuous or |
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| 391 | discontinuous for its entirety and the codec is able to ascertain (and |
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| 392 | inform the Ogg layer) as to which after decoding the initial stream |
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| 393 | header. The majority of codecs will always be continuous (such as |
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| 394 | Vorbis) or discontinuous (such as Writ).</p> |
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| 395 | |
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| 396 | <p>Start- and end-time encoding do not affect multiplexing sort-order; |
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| 397 | pages are still sorted by the absolute time a given granulepos maps to |
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| 398 | regardless of whether that granulepos represents start- or |
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| 399 | end-time.</p> |
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| 400 | |
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| 401 | <h2>Multiplex/Demultiplex Division of Labor</h2> |
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| 402 | |
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| 403 | <p>The Ogg multiplex/demultiplex layer provides mechanisms for encoding |
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| 404 | raw packets into Ogg pages, decoding Ogg pages back into the original |
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| 405 | codec packets, determining the logical structure of an Ogg stream, and |
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| 406 | navigating through and synchronizing with an Ogg stream at a desired |
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| 407 | stream location. Strict multiplex/demultiplex operations are entirely |
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| 408 | in the Ogg domain and require no intervention from codecs.</p> |
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| 409 | |
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| 410 | <p>Implementation of more complex operations does require codec |
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| 411 | knowledge, however. Unlike other framing systems, Ogg maintains |
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| 412 | strict separation between framing and the framed bitstream data; Ogg |
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| 413 | does not replicate codec-specific information in the page/framing |
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| 414 | data, nor does Ogg blur the line between framing and stream |
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| 415 | data/metadata. Because Ogg is fully data-agnostic toward the data it |
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| 416 | frames, operations which require specifics of bitstream data (such as |
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| 417 | 'seek to key frame') also require interaction with the codec layer |
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| 418 | (because, in this example, the Ogg layer is not aware of the concept |
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| 419 | of key frames). This is different from systems that blur the |
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| 420 | separation between framing and stream data in order to simplify the |
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| 421 | separation of code. The Ogg system purposely keeps the distinction in |
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| 422 | data simple so that later codec innovations are not constrained by |
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| 423 | framing design.</p> |
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| 424 | |
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| 425 | <p>For this reason, however, complex seeking operations require |
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| 426 | interaction with the codecs in order to decode the granule position of |
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| 427 | a given stream type back to absolute time or in order to find |
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| 428 | 'decodable points' such as key frames in video.</p> |
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| 429 | |
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| 430 | <h2>Unsorted Discussion Points</h2> |
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| 431 | |
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| 432 | <p>flushes around key frames? RFC suggestion: repaginating or building a |
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| 433 | stream this way is nice but not required</p> |
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| 434 | |
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| 435 | <h2>Appendix A: multiplexing examples</h2> |
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| 436 | |
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| 437 | <div id="copyright"> |
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| 438 | The Xiph Fish Logo is a |
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| 439 | trademark (™) of Xiph.Org.<br/> |
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| 440 | |
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| 441 | These pages © 1994 - 2005 Xiph.Org. All rights reserved. |
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| 442 | </div> |
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| 443 | |
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| 444 | </body> |
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| 445 | </html> |
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