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Old 04-29-09, 09:33 AM   #12
pgs
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Join Date: Apr 2006
Posts: 73
Default Re: How do I get 23.976Hz?

Quote:
Originally Posted by lixe86 View Post
Ok, finally I got it working, but I still have A/V sync issues. How is that possible. If I play a video with 23.976 fps at 23.976Hz the playback should be synchronic, shouldn't it? I'm using XBMC, maybe that's the problem?
Because the audio and video could be (or they always are) asynchronous.

If the audio is 48000Hz or 44100Hz, they will not have any integer ratio with a video frame rate of 23.976Hz, so one frame worth of data may contain different amount of audio (which could still be OK).

In addition to this, the audio card can be completely un-synchronized with the video output.

So trying to keep the correct audio rate and video rate together might be difficult.

This is why the frame drop/repeat mechanism is implemented in many video backends. Basically the audio runs at its own rate and the video tries to follow. If it is too fast, frame drop occurs, if it is too slow, frame repeat.

Of course, another option would be to resample properly the audio, but this might be to expensive.
Or to use one single common clock to derive all timings, which might not be feasible on a PC (each peripheral seems to have its own clock).

In this situation, 24Hz or 23.976Hz are, basically, the same.

pgs
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