[vorbis-dev] Major quality decrease in RC3 compared to RC2
Daniel B. Miller
dan at on2.com
Sun Jan 6 11:49:51 PST 2002
___ Dan Miller
(++,) Founder, CTO, On2.com
On Sun, 6 Jan 2002, Jack Moffitt wrote:
> > now that we have managed bitrates, I imagine it shouldn't be difficult to
> > build an ACM component that would work properly with AVI files, right?
> > I realize the quality will suffer.
> That's the idea, although I'm not sure how CBR the acm codecs have to
> be. Hopefully someone will give it a shot a report the results.
My understanding: while it is possible to create an ACM that is not CBR,
seeking with different applications will produce unpredictable results.
Basically, your audio and video will not play in sync unless you just play
from the beginning without ff/rew etc. The reason is that AVI files do
not have any agreed-upon way to specify a changing ratio of audio samples
per chunks of data, nor is there any agreement on what it means to change
chunk size for a fixed number of samples. The original AVI spec
implicitly assumes CBR for audio.
At On2, we use a convention that every chunk will contain a specific
number of samples and vary the chunk size; but this only works reliably
within our own applications. For general use, it is strongly advisable to
make your ACM CBR to some degree. The more 'CBR' it is, the better your
sync will be on playback from arbitrary points in the file under various
I believe MP3 ACM's generally use a CBR mode, though I don't know what the
constraints are exactly. I've heard that the whole MPEG audio transport
is jammed into the audio stream, with sync headers etc. I wonder what
that means for us?
hope this helps
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