Video loses sync on export

Question

Video loses sync on export

I recorded a video in AVC HD at 1080P 24 fps. The clip shows 23.98 fps in FCP. I took the microphone audio file and am merging it into my project, because the on-cam mic sucked.

Audio is in stereo at 48kHz. Now, initially, I lost audio sync in the timeline as the video played. By the end of the hour long video, I lost approx. 2 seconds. After converting the audio file multiple times with no solve, I used markers to sync the audio to the video at the beginning and end of the clips, and then used the retime editor to line them up, stretching the audio very slightly. This solved the sync issue in the timeline (however, I admit, it may have not been the "correct" way to solve).

The problem now comes when exporting the file. I've chosen settings to export Video and Audio in H.264, Roles as QT movie. I've also tried format "Apple Devices", H.264. The exporting fps is still at 23.98 and audio is at 48 kHz. The audio loses sync in both cases.

Any idea why the video plays perfectly with the audio in the timeline and loses sync in the exported video?

Thanks very much,

Jeremy

Posted on Dec 3, 2012 1:40 PM

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Answer 1

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jeremyzimmermann Author

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8 points

Dec 4, 2012 8:05 AM in response to jeremyzimmermann

UPDATE: I resolved the issue by returning the timeline to out of sync (by undoing the audio stretch that I had performed) and instead stretching the video out. Because it is about a 1% difference over the hour long video, it rendered and looks great both and the timeline and in the export. The export settings, as I've found, really shouldn't affect the sync.

A note about the original audio stretch that didn't pan out - when I used the retime editor, FCP did not have anything to render. I'm wondering if such a slight (though necessary) adjustment caused FCP to ignore the correction?

Edit: Perhaps I lost the sync from 23.98 fps, should've been 24 fps, the .02 fps lost = 1.2 fpm = ~72 frames over the duration of the video / 24 fps that the audio was going at = 3 seconds of loss. Just a hunch.

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