1 Topic: 2 3 Sample granularity editing of a Vorbis file; inferred arbitrary sample 4 length starting offsets / PCM stream lengths 5 6 Overview: 7 8 Vorbis, like mp3, is a frame-based* audio compression where audio is 9 broken up into discrete short time segments. These segments are 10 'atomic' that is, one must recover the entire short time segment from 11 the frame packet; there's no way to recover only a part of the PCM time 12 segment from part of the coded packet without expanding the entire 13 packet and then discarding a portion of the resulting PCM audio. 14 15 * In mp3, the data segment representing a given time period is called 16 a 'frame'; the roughly equivalent Vorbis construct is a 'packet'. 17 18 Thus, when we edit a Vorbis stream, the finest physical editing 19 granularity is on these packet boundaries (the mp3 case is 20 actually somewhat more complex and mp3 editing is more complicated 21 than just snipping on a frame boundary because time data can be spread 22 backward or forward over frames. In Vorbis, packets are all 23 stand-alone). Thus, at the physical packet level, Vorbis is still 24 limited to streams that contain an integral number of packets. 25 26 However, Vorbis streams may still exactly represent and be edited to a 27 PCM stream of arbitrary length and starting offset without padding the 28 beginning or end of the decoded stream or requiring that the desired 29 edit points be packet aligned. Vorbis makes use of Ogg stream 30 framing, and this framing provides time-stamping data, called a 31 'granule position'; our starting offset and finished stream length may 32 be inferred from correct usage of the granule position data. 33 34 Time stamping mechanism: 35 36 Vorbis packets are bundled into into Ogg pages (note that pages do not 37 necessarily contain integral numbers of packets, but that isn't 38 inportant in this discussion. More about Ogg framing can be found in 39 ogg/doc/framing.html). Each page that contains a packet boundary is 40 stamped with the absolute sample-granularity offset of the data, that 41 is, 'complete samples-to-date' up to the last completed packet of that 42 page. (The same mechanism is used for eg, video, where the number 43 represents complete 2-D frames, and so on). 44 45 (It's possible but rare for a packet to span more than two pages such 46 that page[s] in the middle have no packet boundary; these packets have 47 a granule position of '-1'.) 48 49 This granule position mechaism in Ogg is used by Vorbis to indicate when the 50 PCM data intended to be represented in a Vorbis segment begins a 51 number of samples into the data represented by the first packet[s] 52 and/or ends before the physical PCM data represented in the last 53 packet[s]. 54 55 File length a non-integral number of frames: 56 57 A file to be encoded in Vorbis will probably not encode into an 58 integral number of packets; such a file is encoded with the last 59 packet containing 'extra'* samples. These samples are not padding; they 60 will be discarded in decode. 61 62 *(For best results, the encoder should use extra samples that preserve 63 the character of the last frame. Simply setting them to zero will 64 introduce a 'cliff' that's hard to encode, resulting in spread-frame 65 noise. Libvorbis extrapolates the last frame past the end of data to 66 produce the extra samples. Even simply duplicating the last value is 67 better than clamping the signal to zero). 68 69 The encoder indicates to the decoder that the file is actually shorter 70 than all of the samples ('original' + 'extra') by setting the granule 71 position in the last page to a short value, that is, the last 72 timestamp is the original length of the file discarding extra samples. 73 The decoder will see that the number of samples it has decoded in the 74 last page is too many; it is 'original' + 'extra', where the 75 granulepos says that through the last packet we only have 'original' 76 number of samples. The decoder then ignores the 'extra' samples. 77 This behavior is to occur only when the end-of-stream bit is set in 78 the page (indicating last page of the logical stream). 79 80 Note that it not legal for the granule position of the last page to 81 indicate that there are more samples in the file than actually exist, 82 however, implementations should handle such an illegal file gracefully 83 in the interests of robust programming. 84 85 Beginning point not on integral packet boundary: 86 87 It is possible that we will the PCM data represented by a Vorbis 88 stream to begin at a position later than where the decoded PCM data 89 really begins after an integral packet boundary, a situation analagous 90 to the above description where the PCM data does not end at an 91 integral packet boundary. The easiest example is taking a clip out of 92 a larger Vorbis stream, and choosing a beginning point of the clip 93 that is not on a packet boundary; we need to ignore a few samples to 94 get the desired beginning point. 95 96 The process of marking the desired beginning point is similar to 97 marking an arbitrary ending point. If the encoder wishes sample zero 98 to be some location past the actual beginning of data, it associates a 99 'short' granule position value with the completion of the second* 100 audio packet. The granule position is associated with the second 101 packet simply by making sure the second packet completes its page. 102 103 *(We associate the short value with the second packet for two reasons. 104 a) The first packet only primes the overlap/add buffer. No data is 105 returned before decoding the second packet; this places the decision 106 information at the point of decision. b) Placing the short value on 107 the first packet would make the value negative (as the first packet 108 normally represents position zero); a negative value would break the 109 requirement that granule positions increase; the headers have 110 position values of zero) 111 112 The decoder sees that on the first page that will return 113 data from the overlap/add queue, we have more samples than the granule 114 position accounts for, and discards the 'surplus' from the beginning 115 of the queue. 116 117 Note that short granule values (indicating less than the actually 118 returned about of data) are not legal in the Vorbis spec outside of 119 indicating beginning and ending sample positions. However, decoders 120 should, at minimum, tolerate inadvertant short values elsewhere in the 121 stream (just as they should tolerate out-of-order/non-increasing 122 granulepos values, although this too is illegal). 123 124 Beginning point at arbitrary positive timestamp (no 'zero' sample): 125 126 It's also possible that the granule position of the first page of an 127 audio stream is a 'long value', that is, a value larger than the 128 amount of PCM audio decoded. This implies only that we are starting 129 playback at some point into the logical stream, a potentially common 130 occurence in streaming applications where the decoder may be 131 connecting into a live stream. The decoder should not treat the long 132 value specially. 133 134 A long value elsewhere in the stream would normally occur only when a 135 page is lost or out of sequence, as indicated by the page's sequence 136 number. A long value under any other situation is not legal, however 137 a decoder should tolerate both possibilities. 138 139 140
