20 Oct
2006
20 Oct
'06
3:01 p.m.
On Fri, Oct 20, 2006 at 03:51:09PM +0100, Dave Griffiths wrote:
It *is* possible to get it exactly right (to within one audio sample),
assuming
- you know the audio round-trip latency of your sound card (jdelay
will measure it to less than a microsec precision),
- you can send triggers from the video to the audio processes with
a delay less than say 1/4 of the frame time, (not difficult),
Input the vertical video sync signal via the audio card and analyse
its timing in terms of audio samples (e.g. using a DLL). This will
enable you to predict where the next sync will be in the audio input.
Using the known round trip audio delay, you also know to which sample
that corresponds in the audio output. Now if the video process sends
its triggers a few frames ahead then the audio process will be able
to work out exactly where to put the corresponding samples.
--
FA
Lascia la spina, cogli la rosa.
The tricky bit
is of course getting a flash that's totally synchronous
with the beep. Absolute synchrony is not achievable without dedicated
hardware, but we need to get an approximation that's within the few ms
range.
I do things a bit like this for audiovisual performances. It is impossible
to get it precisely right as you say, but what I tend to do is run
everything slightly ahead of realtime - so you timestamp events to happen
in the future. Of course you can't do this if human input is involved, but
if the timing is machine generated you can tune the audio and visual
independantly so they are close enough for most purposes.