A/V sync problems with CoreAudio driver
A/V sync problems with CoreAudio driver
- Subject: A/V sync problems with CoreAudio driver
- From: "Sean McBride" <email@hidden>
- Date: Tue, 12 Aug 2003 16:13:08 -0400
- Organization: Matrox Electronic Systems Ltd.
Hi,
We have written a CoreAudio driver that works well with most apps, but
has a/v sync problems in certain cases.
Our tests were done with a DV clip that has an audible beep and a visible
flash at the same time. We tested the sync by eye (which does not prove
perfect sync but is good enough to detect very bad sync). We tried
playing the clip with the following apps, with the following results:
QT Player with built-in audio GOOD
QT Player with ourSoundOut GOOD
iMovie with built-in audio GOOD
iMovie with ourSoundOut GOOD
FCP3 with built-in audio GOOD
FCP3 with ourSoundOut GOOD
FCP4 with built-in audio GOOD
FCP4 with ourSoundOut audio late
The results are consistently reproducible on several machines. In all
apps the audio is played at the right time, except with FCP4 where it is
played late relative to the video.
What might explain this? The only thing we can think of is that
QuickTime is doing something somewhere. FCP4 does sound 'the CoreAudio
way' and FCP3 does it 'the QT way'.
Our CoreAudio driver uses the setSampleLatency API. One API we are not
using is setSampleOffest(), which we can't find much info on. Could not
using it create this problem? The docs say 'If a device is block based
or if there is a need to keep the CoreAudio.framework a certain number of
samples ahead of (or behind for input) the I/O head, that value should be
specified using setSampleOffset().' What does 'block based' mean in this
context?
Thanks for any help!
--
____________________________________________________________
Sean McBride, B. Eng email@hidden
Mac Software Designer +1-514-822-6000
Matrox Electronic Systems Ltd. Montrial, Quibec, Canada
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