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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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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>
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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">
9 body {
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25 #xiphlogo {
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55 #copyright {
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60 color: #888888;
61 clear: both;
62 }
63 </style>
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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 bitstream framing</h1>
74
75 <h2>Ogg bitstreams</h2>
76
77 <p>The Ogg transport bitstream is designed to provide framing, error
78 protection and seeking structure for higher-level codec streams that
79 consist of raw, unencapsulated data packets, such as the Vorbis audio
80 codec or Theora video codec.</p>
81
82 <h2>Application example: Vorbis</h2>
83
84 <p>Vorbis encodes short-time blocks of PCM data into raw packets of
85 bit-packed data. These raw packets may be used directly by transport
86 mechanisms that provide their own framing and packet-separation
87 mechanisms (such as UDP datagrams). For stream based storage (such as
88 files) and transport (such as TCP streams or pipes), Vorbis uses the
89 Ogg bitstream format to provide framing/sync, sync recapture
90 after error, landmarks during seeking, and enough information to
91 properly separate data back into packets at the original packet
92 boundaries without relying on decoding to find packet boundaries.</p>
93
94 <h2>Design constraints for Ogg bitstreams</h2>
95
96 <ol>
97 <li>True streaming; we must not need to seek to build a 100%
98 complete bitstream.</li>
99 <li>Use no more than approximately 1-2% of bitstream bandwidth for
100 packet boundary marking, high-level framing, sync and seeking.</li>
101 <li>Specification of absolute position within the original sample
102 stream.</li>
103 <li>Simple mechanism to ease limited editing, such as a simplified
104 concatenation mechanism.</li>
105 <li>Detection of corruption, recapture after error and direct, random
106 access to data at arbitrary positions in the bitstream.</li>
107 </ol>
108
109 <h2>Logical and Physical Bitstreams</h2>
110
111 <p>A <em>logical</em> Ogg bitstream is a contiguous stream of
112 sequential pages belonging only to the logical bitstream. A
113 <em>physical</em> Ogg bitstream is constructed from one or more
114 than one logical Ogg bitstream (the simplest physical bitstream
115 is simply a single logical bitstream). We describe below the exact
116 formatting of an Ogg logical bitstream. Combining logical
117 bitstreams into more complex physical bitstreams is described in the
118 <a href="oggstream.html">Ogg bitstream overview</a>. The exact
119 mapping of raw Vorbis packets into a valid Ogg Vorbis physical
120 bitstream is described in the Vorbis I Specification.</p>
121
122 <h2>Bitstream structure</h2>
123
124 <p>An Ogg stream is structured by dividing incoming packets into
125 segments of up to 255 bytes and then wrapping a group of contiguous
126 packet segments into a variable length page preceded by a page
127 header. Both the header size and page size are variable; the page
128 header contains sizing information and checksum data to determine
129 header/page size and data integrity.</p>
130
131 <p>The bitstream is captured (or recaptured) by looking for the beginning
132 of a page, specifically the capture pattern. Once the capture pattern
133 is found, the decoder verifies page sync and integrity by computing
134 and comparing the checksum. At that point, the decoder can extract the
135 packets themselves.</p>
136
137 <h3>Packet segmentation</h3>
138
139 <p>Packets are logically divided into multiple segments before encoding
140 into a page. Note that the segmentation and fragmentation process is a
141 logical one; it's used to compute page header values and the original
142 page data need not be disturbed, even when a packet spans page
143 boundaries.</p>
144
145 <p>The raw packet is logically divided into [n] 255 byte segments and a
146 last fractional segment of &lt; 255 bytes. A packet size may well
147 consist only of the trailing fractional segment, and a fractional
148 segment may be zero length. These values, called "lacing values" are
149 then saved and placed into the header segment table.</p>
150
151 <p>An example should make the basic concept clear:</p>
152
153 <pre>
154 <tt>
155 raw packet:
156 ___________________________________________
157 |______________packet data__________________| 753 bytes
158
159 lacing values for page header segment table: 255,255,243
160 </tt>
161 </pre>
162
163 <p>We simply add the lacing values for the total size; the last lacing
164 value for a packet is always the value that is less than 255. Note
165 that this encoding both avoids imposing a maximum packet size as well
166 as imposing minimum overhead on small packets (as opposed to, eg,
167 simply using two bytes at the head of every packet and having a max
168 packet size of 32k. Small packets (&lt;255, the typical case) are
169 penalized with twice the segmentation overhead). Using the lacing
170 values as suggested, small packets see the minimum possible
171 byte-aligned overheade (1 byte) and large packets, over 512 bytes or
172 so, see a fairly constant ~.5% overhead on encoding space.</p>
173
174 <p>Note that a lacing value of 255 implies that a second lacing value
175 follows in the packet, and a value of &lt; 255 marks the end of the
176 packet after that many additional bytes. A packet of 255 bytes (or a
177 multiple of 255 bytes) is terminated by a lacing value of 0:</p>
178
179 <pre><tt>
180 raw packet:
181 _______________________________
182 |________packet data____________| 255 bytes
183
184 lacing values: 255, 0
185 </tt></pre>
186
187 <p>Note also that a 'nil' (zero length) packet is not an error; it
188 consists of nothing more than a lacing value of zero in the header.</p>
189
190 <h3>Packets spanning pages</h3>
191
192 <p>Packets are not restricted to beginning and ending within a page,
193 although individual segments are, by definition, required to do so.
194 Packets are not restricted to a maximum size, although excessively
195 large packets in the data stream are discouraged; the Ogg
196 bitstream specification strongly recommends nominal page size of
197 approximately 4-8kB (large packets are foreseen as being useful for
198 initialization data at the beginning of a logical bitstream).</p>
199
200 <p>After segmenting a packet, the encoder may decide not to place all the
201 resulting segments into the current page; to do so, the encoder places
202 the lacing values of the segments it wishes to belong to the current
203 page into the current segment table, then finishes the page. The next
204 page is begun with the first value in the segment table belonging to
205 the next packet segment, thus continuing the packet (data in the
206 packet body must also correspond properly to the lacing values in the
207 spanned pages. The segment data in the first packet corresponding to
208 the lacing values of the first page belong in that page; packet
209 segments listed in the segment table of the following page must begin
210 the page body of the subsequent page).</p>
211
212 <p>The last mechanic to spanning a page boundary is to set the header
213 flag in the new page to indicate that the first lacing value in the
214 segment table continues rather than begins a packet; a header flag of
215 0x01 is set to indicate a continued packet. Although mandatory, it
216 is not actually algorithmically necessary; one could inspect the
217 preceding segment table to determine if the packet is new or
218 continued. Adding the information to the packet_header flag allows a
219 simpler design (with no overhead) that needs only inspect the current
220 page header after frame capture. This also allows faster error
221 recovery in the event that the packet originates in a corrupt
222 preceding page, implying that the previous page's segment table
223 cannot be trusted.</p>
224
225 <p>Note that a packet can span an arbitrary number of pages; the above
226 spanning process is repeated for each spanned page boundary. Also a
227 'zero termination' on a packet size that is an even multiple of 255
228 must appear even if the lacing value appears in the next page as a
229 zero-length continuation of the current packet. The header flag
230 should be set to 0x01 to indicate that the packet spanned, even though
231 the span is a nil case as far as data is concerned.</p>
232
233 <p>The encoding looks odd, but is properly optimized for speed and the
234 expected case of the majority of packets being between 50 and 200
235 bytes (note that it is designed such that packets of wildly different
236 sizes can be handled within the model; placing packet size
237 restrictions on the encoder would have only slightly simplified design
238 in page generation and increased overall encoder complexity).</p>
239
240 <p>The main point behind tracking individual packets (and packet
241 segments) is to allow more flexible encoding tricks that requiring
242 explicit knowledge of packet size. An example is simple bandwidth
243 limiting, implemented by simply truncating packets in the nominal case
244 if the packet is arranged so that the least sensitive portion of the
245 data comes last.</p>
246
247 <h3>Page header</h3>
248
249 <p>The headering mechanism is designed to avoid copying and re-assembly
250 of the packet data (ie, making the packet segmentation process a
251 logical one); the header can be generated directly from incoming
252 packet data. The encoder buffers packet data until it finishes a
253 complete page at which point it writes the header followed by the
254 buffered packet segments.</p>
255
256 <h4>capture_pattern</h4>
257
258 <p>A header begins with a capture pattern that simplifies identifying
259 pages; once the decoder has found the capture pattern it can do a more
260 intensive job of verifying that it has in fact found a page boundary
261 (as opposed to an inadvertent coincidence in the byte stream).</p>
262
263 <pre><tt>
264 byte value
265
266 0 0x4f 'O'
267 1 0x67 'g'
268 2 0x67 'g'
269 3 0x53 'S'
270 </tt></pre>
271
272 <h4>stream_structure_version</h4>
273
274 <p>The capture pattern is followed by the stream structure revision:</p>
275
276 <pre><tt>
277 byte value
278
279 4 0x00
280 </tt></pre>
281
282 <h4>header_type_flag</h4>
283
284 <p>The header type flag identifies this page's context in the bitstream:</p>
285
286 <pre><tt>
287 byte value
288
289 5 bitflags: 0x01: unset = fresh packet
290 set = continued packet
291 0x02: unset = not first page of logical bitstream
292 set = first page of logical bitstream (bos)
293 0x04: unset = not last page of logical bitstream
294 set = last page of logical bitstream (eos)
295 </tt></pre>
296
297 <h4>absolute granule position</h4>
298
299 <p>(This is packed in the same way the rest of Ogg data is packed; LSb
300 of LSB first. Note that the 'position' data specifies a 'sample'
301 number (eg, in a CD quality sample is four octets, 16 bits for left
302 and 16 bits for right; in video it would likely be the frame number.
303 It is up to the specific codec in use to define the semantic meaning
304 of the granule position value). The position specified is the total
305 samples encoded after including all packets finished on this page
306 (packets begun on this page but continuing on to the next page do not
307 count). The rationale here is that the position specified in the
308 frame header of the last page tells how long the data coded by the
309 bitstream is. A truncated stream will still return the proper number
310 of samples that can be decoded fully.</p>
311
312 <p>A special value of '-1' (in two's complement) indicates that no packets
313 finish on this page.</p>
314
315 <pre><tt>
316 byte value
317
318 6 0xXX LSB
319 7 0xXX
320 8 0xXX
321 9 0xXX
322 10 0xXX
323 11 0xXX
324 12 0xXX
325 13 0xXX MSB
326 </tt></pre>
327
328 <h4>stream serial number</h4>
329
330 <p>Ogg allows for separate logical bitstreams to be mixed at page
331 granularity in a physical bitstream. The most common case would be
332 sequential arrangement, but it is possible to interleave pages for
333 two separate bitstreams to be decoded concurrently. The serial
334 number is the means by which pages physical pages are associated with
335 a particular logical stream. Each logical stream must have a unique
336 serial number within a physical stream:</p>
337
338 <pre><tt>
339 byte value
340
341 14 0xXX LSB
342 15 0xXX
343 16 0xXX
344 17 0xXX MSB
345 </tt></pre>
346
347 <h4>page sequence no</h4>
348
349 <p>Page counter; lets us know if a page is lost (useful where packets
350 span page boundaries).</p>
351
352 <pre><tt>
353 byte value
354
355 18 0xXX LSB
356 19 0xXX
357 20 0xXX
358 21 0xXX MSB
359 </tt></pre>
360
361 <h4>page checksum</h4>
362
363 <p>32 bit CRC value (direct algorithm, initial val and final XOR = 0,
364 generator polynomial=0x04c11db7). The value is computed over the
365 entire header (with the CRC field in the header set to zero) and then
366 continued over the page. The CRC field is then filled with the
367 computed value.</p>
368
369 <p>(A thorough discussion of CRC algorithms can be found in <a
370 href="http://www.ross.net/crc/download/crc_v3.txt">"A
371 Painless Guide to CRC Error Detection Algorithms"</a> by Ross
372 Williams <a href="mailto:ross@ross.net">ross@ross.net</a>.)</p>
373
374 <pre><tt>
375 byte value
376
377 22 0xXX LSB
378 23 0xXX
379 24 0xXX
380 25 0xXX MSB
381 </tt></pre>
382
383 <h4>page_segments</h4>
384
385 <p>The number of segment entries to appear in the segment table. The
386 maximum number of 255 segments (255 bytes each) sets the maximum
387 possible physical page size at 65307 bytes or just under 64kB (thus
388 we know that a header corrupted so as destroy sizing/alignment
389 information will not cause a runaway bitstream. We'll read in the
390 page according to the corrupted size information that's guaranteed to
391 be a reasonable size regardless, notice the checksum mismatch, drop
392 sync and then look for recapture).</p>
393
394 <pre><tt>
395 byte value
396
397 26 0x00-0xff (0-255)
398 </tt></pre>
399
400 <h4>segment_table (containing packet lacing values)</h4>
401
402 <p>The lacing values for each packet segment physically appearing in
403 this page are listed in contiguous order.</p>
404
405 <pre><tt>
406 byte value
407
408 27 0x00-0xff (0-255)
409 [...]
410 n 0x00-0xff (0-255, n=page_segments+26)
411 </tt></pre>
412
413 <p>Total page size is calculated directly from the known header size and
414 lacing values in the segment table. Packet data segments follow
415 immediately after the header.</p>
416
417 <p>Page headers typically impose a flat .25-.5% space overhead assuming
418 nominal ~8k page sizes. The segmentation table needed for exact
419 packet recovery in the streaming layer adds approximately .5-1%
420 nominal assuming expected encoder behavior in the 44.1kHz, 128kbps
421 stereo encodings.</p>
422
423 <div id="copyright">
424 The Xiph Fish Logo is a
425 trademark (&trade;) of Xiph.Org.<br/>
426
427 These pages &copy; 1994 - 2005 Xiph.Org. All rights reserved.
428 </div>
429
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431 </html>