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.