[theora-dev] A/V sync in Theora
dan at on2.com
Thu Jun 12 16:50:27 PDT 2003
as you say, Mplayer and other AVI apps slightly adjust framerate *after* the capture is complete. If this is done correctly, it will work well in most cases. The exception is if you are capturing off an old analog videotape, where tape stretch can easily make the sync drift up and down. DVD or broadcast TV capture, or mini-DV type stuff will work fine (because the playback machine has a stable digital clock).
Theora will work just as well as AVI. There are no arbitrary timestamps in AVI or MPEG; there are frame numbers and a global framerate (MPEG doesn't even have the option of a fractional framerate other than drop frame = 23.997).
From: David Kuehling [mailto:dvdkhlng at gmx.de]
Sent: Thu 6/12/2003 3:29 PM
To: theora-dev at xiph.org
Cc: theora-dev at xiph.org
Subject: Re: [theora-dev] A/V sync in Theora
>>>>> "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
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? ;-)
GnuPG public key: http://user.cs.tu-berlin.de/~dvdkhlng/dk.gpg
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