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Bring in flac, ogg, vorbis
author Chris Cannam
date Tue, 19 Mar 2013 17:37:49 +0000
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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>
71 </div>
72
73 <h1>Page Multiplexing and Ordering in a Physical Ogg Stream</h1>
74
75 <p>The low-level mechanisms of an Ogg stream (as described in the Ogg
76 Bitstream Overview) provide means for mixing multiple logical streams
77 and media types into a single linear-chronological stream. This
78 document specifies the high-level arrangement and use of page
79 structure to multiplex multiple streams of mixed media type within a
80 physical Ogg stream.</p>
81
82 <h2>Design Elements</h2>
83
84 <p>The design and arrangement of the Ogg container format is governed by
85 several high-level design decisions that form the reasoning behind
86 specific low-level design decisions.</p>
87
88 <h3>Linear media</h3>
89
90 <p>The Ogg bitstream is intended to encapsulate chronological,
91 time-linear mixed media into a single delivery stream or file. The
92 design is such that an application can always encode and/or decode a
93 full-featured bitstream in one pass with no seeking and minimal
94 buffering. Seeking to provide optimized encoding (such as two-pass
95 encoding) or interactive decoding (such as scrubbing or instant
96 replay) is not disallowed or discouraged, however no bitstream feature
97 must require nonlinear operation on the bitstream.</p>
98
99 <h3>Multiplexing</h3>
100
101 <p>Ogg bitstreams multiplex multiple logical streams into a single
102 physical stream at the page level. Each page contains an abstract
103 time stamp (the Granule Position) that represents an absolute time
104 landmark within the stream. After the pages representing stream
105 headers (all logical stream headers occur at the beginning of a
106 physical bitstream section before any logical stream data), logical
107 stream data pages are arranged in a physical bitstream in strict
108 non-decreasing order by chronological absolute time as
109 specified by the granule position.</p>
110
111 <p>The only exception to arranging pages in strictly ascending time order
112 by granule position is those pages that do not set the granule
113 position value. This is a special case when exceptionally large
114 packets span multiple pages; the specifics of handling this special
115 case are described later under 'Continuous and Discontinuous
116 Streams'.</p>
117
118 <h3>Seeking</h3>
119
120 <p>Ogg is designed to use an interpolated bisection search to
121 implement exact positional seeking. Interpolated bisection search is
122 a spec-mandated mechanism.</p>
123
124 <p><i>An index may improve objective performance, but it seldom
125 improves subjective performance outside of a few high-latency use
126 cases and adds no additional functionality as bisection search
127 delivers the same functionality for both one- and two-pass stream
128 types. For these reasons, use of indexes is discouraged, except in
129 cases where an index provides demonstrable and noticable performance
130 improvement.</i></p>
131
132 <p>Seek operations are by absolute time; a direct bisection search must
133 find the exact time position requested. Information in the Ogg
134 bitstream is arranged such that all information to be presented for
135 playback from the desired seek point will occur at or after the
136 desired seek point. Seek operations are neither 'fuzzy' nor
137 heuristic.</p>
138
139 <p><i>Although key frame handling in video appears to be an exception to
140 "all needed playback information lies ahead of a given seek",
141 key frames can still be handled directly within this indexless
142 framework. Seeking to a key frame in video (as well as seeking in other
143 media types with analogous restraints) is handled as two seeks; first
144 a seek to the desired time which extracts state information that
145 decodes to the time of the last key frame, followed by a second seek
146 directly to the key frame. The location of the previous key frame is
147 embedded as state information in the granulepos; this mechanism is
148 described in more detail later.</i></p>
149
150 <h3>Continuous and Discontinuous Streams</h3>
151
152 <p>Logical streams within a physical Ogg stream belong to one of two
153 categories, "Continuous" streams and "Discontinuous" streams.
154 Although these are discussed in more detail later, the distinction is
155 important to a high-level understanding of how to buffer an Ogg
156 stream.</p>
157
158 <p>A stream that provides a gapless, time-continuous media type with a
159 fine-grained timebase is considered to be 'Continuous'. A continuous
160 stream should never be starved of data. Clear examples of continuous
161 data types include broadcast audio and video.</p>
162
163 <p>A stream that delivers data in a potentially irregular pattern or with
164 widely spaced timing gaps is considered to be 'Discontinuous'. A
165 discontinuous stream may be best thought of as data representing
166 scattered events; although they happen in order, they are typically
167 unconnected data often located far apart. One possible example of a
168 discontinuous stream types would be captioning. Although it's
169 possible to design captions as a continuous stream type, it's most
170 natural to think of captions as widely spaced pieces of text with
171 little happening between.</p>
172
173 <p>The fundamental design distinction between continuous and
174 discontinuous streams concerns buffering.</p>
175
176 <h3>Buffering</h3>
177
178 <p>Because a continuous stream is, by definition, gapless, Ogg buffering
179 is based on the simple premise of never allowing any active continuous
180 stream to starve for data during decode; buffering proceeds ahead
181 until all continuous streams in a physical stream have data ready to
182 decode on demand.</p>
183
184 <p>Discontinuous stream data may occur on a fairly regular basis, but the
185 timing of, for example, a specific caption is impossible to predict
186 with certainty in most captioning systems. Thus the buffering system
187 should take discontinuous data 'as it comes' rather than working ahead
188 (for a potentially unbounded period) to look for future discontinuous
189 data. As such, discontinuous streams are ignored when managing
190 buffering; their pages simply 'fall out' of the stream when continuous
191 streams are handled properly.</p>
192
193 <p>Buffering requirements need not be explicitly declared or managed for
194 the encoded stream; the decoder simply reads as much data as is
195 necessary to keep all continuous stream types gapless (also ensuring
196 discontinuous data arrives in time) and no more, resulting in optimum
197 implicit buffer usage for a given stream. Because all pages of all
198 data types are stamped with absolute timing information within the
199 stream, inter-stream synchronization timing is always explicitly
200 maintained without the need for explicitly declared buffer-ahead
201 hinting.</p>
202
203 <p>Further details, mechanisms and reasons for the differing arrangement
204 and behavior of continuous and discontinuous streams is discussed
205 later.</p>
206
207 <h3>Whole-stream navigation</h3>
208
209 <p>Ogg is designed so that the simplest navigation operations treat the
210 physical Ogg stream as a whole summary of its streams, rather than
211 navigating each interleaved stream as a separate entity.</p>
212
213 <p>First Example: seeking to a desired time position in a multiplexed (or
214 unmultiplexed) Ogg stream can be accomplished through a bisection
215 search on time position of all pages in the stream (as encoded in the
216 granule position). More powerful searches (such as a key frame-aware
217 seek within video) are also possible with additional search
218 complexity, but similar computational complexity.</p>
219
220 <p>Second Example: A bitstream section may consist of three multiplexed
221 streams of differing lengths. The result of multiplexing these
222 streams should be thought of as a single mixed stream with a length
223 equal to the longest of the three component streams. Although it is
224 also possible to think of the multiplexed results as three concurrent
225 streams of different lengths and it is possible to recover the three
226 original streams, it will also become obvious that once multiplexed,
227 it isn't possible to find the internal lengths of the component
228 streams without a linear search of the whole bitstream section.
229 However, it is possible to find the length of the whole bitstream
230 section easily (in near-constant time per section) just as it is for a
231 single-media unmultiplexed stream.</p>
232
233 <h2>Granule Position</h2>
234
235 <h3>Description</h3>
236
237 <p>The Granule Position is a signed 64 bit field appearing in the header
238 of every Ogg page. Although the granule position represents absolute
239 time within a logical stream, its value does not necessarily directly
240 encode a simple timestamp. It may represent frames elapsed (as in
241 Vorbis), a simple timestamp, or a more complex bit-division encoding
242 (such as in Theora). The exact encoding of the granule position is up
243 to a specific codec.</p>
244
245 <p>The granule position is governed by the following rules:</p>
246
247 <ul>
248
249 <li>Granule Position must always increase forward or remain equal from
250 page to page, be unset, or be zero for a header page. The absolute
251 time to which any correct sequence of granule position maps must
252 similarly always increase forward or remain equal. <i>(A codec may
253 make use of data, such as a control sequence, that only affects codec
254 working state without producing data and thus advancing granule
255 position and time. Although the packet sequence number increases in
256 this case, the granule position, and thus the time position, do
257 not.)</i></li>
258
259 <li>Granule position may only be unset if there no packet defining a
260 time boundary on the page (that is, if no packet in a continuous
261 stream ends on the page, or no packet in a discontinuous stream begins
262 on the page. This will be discussed in more detail under Continuous
263 and Discontinuous streams).</li>
264
265 <li>A codec must be able to translate a given granule position value
266 to a unique, deterministic absolute time value through direct
267 calculation. A codec is not required to be able to translate an
268 absolute time value into a unique granule position value.</li>
269
270 <li>Codecs shall choose a granule position definition that allows that
271 codec means to seek as directly as possible to an immediately
272 decodable point, such as the bit-divided granule position encoding of
273 Theora allows the codec to seek efficiently to key frame without using
274 an index. That is, additional information other than absolute time
275 may be encoded into a granule position value so long as the granule
276 position obeys the above points.</li>
277
278 </ul>
279
280 <h4>Example: timestamp</h4>
281
282 <p>In general, a codec/stream type should choose the simplest granule
283 position encoding that addresses its requirements. The examples here
284 are by no means exhaustive of the possibilities within Ogg.</p>
285
286 <p>A simple granule position could encode a timestamp directly. For
287 example, a granule position that encoded milliseconds from beginning
288 of stream would allow a logical stream length of over 100,000,000,000
289 days before beginning a new logical stream (to avoid the granule
290 position wrapping).</p>
291
292 <h4>Example: framestamp</h4>
293
294 <p>A simple millisecond timestamp granule encoding might suit many stream
295 types, but a millisecond resolution is inappropriate to, eg, most
296 audio encodings where exact single-sample resolution is generally a
297 requirement. A millisecond is both too large a granule and often does
298 not represent an integer number of samples.</p>
299
300 <p>In the event that audio frames are always encoded as the same number of
301 samples, the granule position could simply be a linear count of frames
302 since beginning of stream. This has the advantages of being exact and
303 efficient. Position in time would simply be <tt>[granule_position] *
304 [samples_per_frame] / [samples_per_second]</tt>.</p>
305
306 <h4>Example: samplestamp (Vorbis)</h4>
307
308 <p>Frame counting is insufficient in codecs such as Vorbis where an audio
309 frame [packet] encodes a variable number of samples. In Vorbis's
310 case, the granule position is a count of the number of raw samples
311 from the beginning of stream; the absolute time of
312 a granule position is <tt>[granule_position] /
313 [samples_per_second]</tt>.</p>
314
315 <h4>Example: bit-divided framestamp (Theora)</h4>
316
317 <p>Some video codecs may be able to use the simple framestamp scheme for
318 granule position. However, most modern video codecs introduce at
319 least the following complications:</p>
320
321 <ul>
322
323 <li>video frames are relatively far apart compared to audio samples;
324 for this reason, the point at which a video frame changes to the next
325 frame is usually a strictly defined offset within the frame 'period'.
326 That is, video at 50fps could just as easily define frame transitions
327 &lt;.015, .035, .055...&gt; as at &lt;.00, .02, .04...&gt;.</li>
328
329 <li>frame rates often include drop-frames, leap-frames or other
330 rational-but-non-integer timings.</li>
331
332 <li>Decode must begin at a 'key frame' or 'I frame'. Keyframes usually
333 occur relatively seldom.</li>
334
335 </ul>
336
337 <p>The first two points can be handled straightforwardly via the fact
338 that the codec has complete control mapping granule position to
339 absolute time; non-integer frame rates and offsets can be set in the
340 codec's initial header, and the rest is just arithmetic.</p>
341
342 <p>The third point appears trickier at first glance, but it too can be
343 handled through the granule position mapping mechanism. Here we
344 arrange the granule position in such a way that granule positions of
345 key frames are easy to find. Divide the granule position into two
346 fields; the most-significant bits are an absolute frame counter, but
347 it's only updated at each key frame. The least significant bits encode
348 the number of frames since the last key frame. In this way, each
349 granule position both encodes the absolute time of the current frame
350 as well as the absolute time of the last key frame.</p>
351
352 <p>Seeking to a most recent preceding key frame is then accomplished by
353 first seeking to the original desired point, inspecting the granulepos
354 of the resulting video page, extracting from that granulepos the
355 absolute time of the desired key frame, and then seeking directly to
356 that key frame's page. Of course, it's still possible for an
357 application to ignore key frames and use a simpler seeking algorithm
358 (decode would be unable to present decoded video until the next
359 key frame). Surprisingly many player applications do choose the
360 simpler approach.</p>
361
362 <h3>granule position, packets and pages</h3>
363
364 <p>Although each packet of data in a logical stream theoretically has a
365 specific granule position, only one granule position is encoded
366 per page. It is possible to encode a logical stream such that each
367 page contains only a single packet (so that granule positions are
368 preserved for each packet), however a one-to-one packet/page mapping
369 is not intended to be the general case.</p>
370
371 <p>Because Ogg functions at the page, not packet, level, this
372 once-per-page time information provides Ogg with the finest-grained
373 time information is can use. Ogg passes this granule positioning data
374 to the codec (along with the packets extracted from a page); it is the
375 responsibility of codecs to track timing information at granularities
376 finer than a single page.</p>
377
378 <h3>start-time and end-time positioning</h3>
379
380 <p>A granule position represents the <em>instantaneous time location
381 between two pages</em>. However, continuous streams and discontinuous
382 streams differ on whether the granulepos represents the end-time of
383 the data on a page or the start-time. Continuous streams are
384 'end-time' encoded; the granulepos represents the point in time
385 immediately after the last data decoded from a page. Discontinuous
386 streams are 'start-time' encoded; the granulepos represents the point
387 in time of the first data decoded from the page.</p>
388
389 <p>An Ogg stream type is declared continuous or discontinuous by its
390 codec. A given codec may support both continuous and discontinuous
391 operation so long as any given logical stream is continuous or
392 discontinuous for its entirety and the codec is able to ascertain (and
393 inform the Ogg layer) as to which after decoding the initial stream
394 header. The majority of codecs will always be continuous (such as
395 Vorbis) or discontinuous (such as Writ).</p>
396
397 <p>Start- and end-time encoding do not affect multiplexing sort-order;
398 pages are still sorted by the absolute time a given granulepos maps to
399 regardless of whether that granulepos represents start- or
400 end-time.</p>
401
402 <h2>Multiplex/Demultiplex Division of Labor</h2>
403
404 <p>The Ogg multiplex/demultiplex layer provides mechanisms for encoding
405 raw packets into Ogg pages, decoding Ogg pages back into the original
406 codec packets, determining the logical structure of an Ogg stream, and
407 navigating through and synchronizing with an Ogg stream at a desired
408 stream location. Strict multiplex/demultiplex operations are entirely
409 in the Ogg domain and require no intervention from codecs.</p>
410
411 <p>Implementation of more complex operations does require codec
412 knowledge, however. Unlike other framing systems, Ogg maintains
413 strict separation between framing and the framed bitstream data; Ogg
414 does not replicate codec-specific information in the page/framing
415 data, nor does Ogg blur the line between framing and stream
416 data/metadata. Because Ogg is fully data-agnostic toward the data it
417 frames, operations which require specifics of bitstream data (such as
418 'seek to key frame') also require interaction with the codec layer
419 (because, in this example, the Ogg layer is not aware of the concept
420 of key frames). This is different from systems that blur the
421 separation between framing and stream data in order to simplify the
422 separation of code. The Ogg system purposely keeps the distinction in
423 data simple so that later codec innovations are not constrained by
424 framing design.</p>
425
426 <p>For this reason, however, complex seeking operations require
427 interaction with the codecs in order to decode the granule position of
428 a given stream type back to absolute time or in order to find
429 'decodable points' such as key frames in video.</p>
430
431 <h2>Unsorted Discussion Points</h2>
432
433 <p>flushes around key frames? RFC suggestion: repaginating or building a
434 stream this way is nice but not required</p>
435
436 <h2>Appendix A: multiplexing examples</h2>
437
438 <div id="copyright">
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