[theora-dev] A/V sync in Theora

David Kuehling dvdkhlng at gmx.de
Thu Jun 12 12:29:15 PDT 2003

>>>>> "Ralph" == Ralph Giles <giles at xiph.org> writes:

> I'm surprised you describe this as a problem with DVD though. I didn't
> think players were smart enough to adjust playback to match an
> external sync.

What do you mean with "external sync"?  

The way I understand mplayer, mplayer continously calculates the A/V
delay using either heuristics or any kind of timing information from the
stream.  It then slighly adjusts playback speed to keep the delay down. 

I'm not sure about what Dan said about MPEG2.  Exact time stamps might
not be required to get A/V sync right.  I'm not concerned about absolut
correctness in the time domain, just relative A/V delay.  Maybe audio
fragments in MPEG2 can somehow be related to the frames they belong to,
either by position in the bitstream or any kind of additional

Imagine how many people would be annoyed, if the first beta of Theora
with the first simple transcoders produced desynced output.  How long
will it take until someone implements a transcoder which is smart enough
to minimize A/V sync by scanning the source material and adjusting
numerator/denumerator correctly?  

That technology is also IMO too restricted.  How am I supposed to record
television flawlessly with Theora?  My sound card usually reports a
sample rate of 44101 when set to 44100.  But mencoder will still assume
that the sample rate is 44100, introducing an error that will amount to
163ms after 2 hours.  I start to notice that something is wrong after
about 50ms.  I have a Sound Blaster PCI 128 which I would consider quite
high quality.  What about even cheaper sound cards?  Remember that this
is the error reported by the sound card, due to limits in the timing
chip, not taking into account the actual inaccuracy of the timer itself.
Note also that Linux won't report fractual sample rates.  So the average
error after 2 hours of recording with 22050 Hz sample rate would as well
be 163ms, even if the soundcard's timer is perfect.

You cannot tell people that they can use Theora for one thing but not
another.  Sorry for not having more constructive comments at the moment.
You won't be willing to throw away the flawed Ogg concept and stick to
Matroska instead? ;-)


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