140 lines
6.9 KiB
Plaintext
Executable File
140 lines
6.9 KiB
Plaintext
Executable File
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.
|
|
|
|
|