Topic: Sample granularity editing of a Vorbis file; inferred arbitrary sample length starting offsets / PCM stream lengths Overview: Vorbis, like mp3, is a frame-based* audio compression where audio is broken up into discrete short time segments. These segments are 'atomic' that is, one must recover the entire short time segment from the frame packet; there's no way to recover only a part of the PCM time segment from part of the coded packet without expanding the entire packet and then discarding a portion of the resulting PCM audio. * In mp3, the data segment representing a given time period is called a 'frame'; the roughly equivalent Vorbis construct is a 'packet'. Thus, when we edit a Vorbis stream, the finest physical editing granularity is on these packet boundaries (the mp3 case is actually somewhat more complex and mp3 editing is more complicated than just snipping on a frame boundary because time data can be spread backward or forward over frames. In Vorbis, packets are all stand-alone). Thus, at the physical packet level, Vorbis is still limited to streams that contain an integral number of packets. However, Vorbis streams may still exactly represent and be edited to a PCM stream of arbitrary length and starting offset without padding the beginning or end of the decoded stream or requiring that the desired edit points be packet aligned. Vorbis makes use of Ogg stream framing, and this framing provides time-stamping data, called a 'granule position'; our starting offset and finished stream length may be inferred from correct usage of the granule position data. Time stamping mechanism: Vorbis packets are bundled into into Ogg pages (note that pages do not necessarily contain integral numbers of packets, but that isn't inportant in this discussion. More about Ogg framing can be found in ogg/doc/framing.html). Each page that contains a packet boundary is stamped with the absolute sample-granularity offset of the data, that is, 'complete samples-to-date' up to the last completed packet of that page. (The same mechanism is used for eg, video, where the number represents complete 2-D frames, and so on). (It's possible but rare for a packet to span more than two pages such that page[s] in the middle have no packet boundary; these packets have a granule position of '-1'.) This granule position mechaism in Ogg is used by Vorbis to indicate when the PCM data intended to be represented in a Vorbis segment begins a number of samples into the data represented by the first packet[s] and/or ends before the physical PCM data represented in the last packet[s]. File length a non-integral number of frames: A file to be encoded in Vorbis will probably not encode into an integral number of packets; such a file is encoded with the last packet containing 'extra'* samples. These samples are not padding; they will be discarded in decode. *(For best results, the encoder should use extra samples that preserve the character of the last frame. Simply setting them to zero will introduce a 'cliff' that's hard to encode, resulting in spread-frame noise. Libvorbis extrapolates the last frame past the end of data to produce the extra samples. Even simply duplicating the last value is better than clamping the signal to zero). The encoder indicates to the decoder that the file is actually shorter than all of the samples ('original' + 'extra') by setting the granule position in the last page to a short value, that is, the last timestamp is the original length of the file discarding extra samples. The decoder will see that the number of samples it has decoded in the last page is too many; it is 'original' + 'extra', where the granulepos says that through the last packet we only have 'original' number of samples. The decoder then ignores the 'extra' samples. This behavior is to occur only when the end-of-stream bit is set in the page (indicating last page of the logical stream). Note that it not legal for the granule position of the last page to indicate that there are more samples in the file than actually exist, however, implementations should handle such an illegal file gracefully in the interests of robust programming. Beginning point not on integral packet boundary: It is possible that we will the PCM data represented by a Vorbis stream to begin at a position later than where the decoded PCM data really begins after an integral packet boundary, a situation analagous to the above description where the PCM data does not end at an integral packet boundary. The easiest example is taking a clip out of a larger Vorbis stream, and choosing a beginning point of the clip that is not on a packet boundary; we need to ignore a few samples to get the desired beginning point. The process of marking the desired beginning point is similar to marking an arbitrary ending point. If the encoder wishes sample zero to be some location past the actual beginning of data, it associates a 'short' granule position value with the completion of the second* audio packet. The granule position is associated with the second packet simply by making sure the second packet completes its page. *(We associate the short value with the second packet for two reasons. a) The first packet only primes the overlap/add buffer. No data is returned before decoding the second packet; this places the decision information at the point of decision. b) Placing the short value on the first packet would make the value negative (as the first packet normally represents position zero); a negative value would break the requirement that granule positions increase; the headers have position values of zero) The decoder sees that on the first page that will return data from the overlap/add queue, we have more samples than the granule position accounts for, and discards the 'surplus' from the beginning of the queue. Note that short granule values (indicating less than the actually returned about of data) are not legal in the Vorbis spec outside of indicating beginning and ending sample positions. However, decoders should, at minimum, tolerate inadvertant short values elsewhere in the stream (just as they should tolerate out-of-order/non-increasing granulepos values, although this too is illegal). Beginning point at arbitrary positive timestamp (no 'zero' sample): It's also possible that the granule position of the first page of an audio stream is a 'long value', that is, a value larger than the amount of PCM audio decoded. This implies only that we are starting playback at some point into the logical stream, a potentially common occurence in streaming applications where the decoder may be connecting into a live stream. The decoder should not treat the long value specially. A long value elsewhere in the stream would normally occur only when a page is lost or out of sequence, as indicated by the page's sequence number. A long value under any other situation is not legal, however a decoder should tolerate both possibilities.