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Audio drifting out of sync... Very troubling

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Teddy Wells
Audio drifting out of sync... Very troubling
on Nov 9, 2011 at 5:07:51 am

I just shot on film (24fps) and sent it to the lab for HD telecine, giving me video in 23.98. So far so good.
The audio from location was recorded in 48K, 30fps. Cool.

However, now that I'm trying to sync up my dailies in FCP, all of the audio is drifting out of sync. I sync up the clappers, and then immediately the audio begins to drift, becoming more and more exaggerated over the clip's duration. It is not a typical 0.1% difference, it is a dramatic difference. The audio becomes basically synced with the image if I speed it up to 103%, which is obviously not ideal due to the change in pitch.

Any ideas? Should the lab be transferring the footage to a different frame rate? Is this even a frame rate issue?


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Michael Gissing
Re: Audio drifting out of sync... Very troubling
on Nov 9, 2011 at 6:43:23 am

Why is the location audio recorded at a different frame rate? Uncool.

I presume you now have broadcast wavs with embedded timecode. Did you use clapper boards to sync? If so then you can salvage the audio by converting it to aifs and sample rate convert to 48048khz but set the flag to 48000. This slows the audio .1% which is all the drift you should have from location to telecine.

Bring it into FCP making sure that first you have your Easy Setup correctly set to 23.98 as FCP will stamp a frame rate on imported audio based on the default project rate. Now the audio should be in sync


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Teddy Wells
Re: Audio drifting out of sync... Very troubling
on Nov 9, 2011 at 1:50:01 pm

Michael,

Thank you for your response. I will try the method you advised, but I have my doubts only because I tried slowing the speed of the audio .1% in Final Cut, resulting in zero difference. The audio drifts heavily out of sync even 10 seconds into the clip. I have checked and tripled checked and I am definitely using the correct audio for the takes. I did some tests with speed and the audio falls in sync when it's sped up to around 103.25%. Very, very odd. Also, the video plays with normal motion, and the audio sounds completely fine.
My sequence setting is 23.98 and I am using clappers for sync.

Thanks,
Brian


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Michael Gissing
Re: Audio drifting out of sync... Very troubling
on Nov 10, 2011 at 2:17:11 am

If you import audio assets into FCP with the default sequence setting different to the frame rate you want the audio to be, it stamps a timecode on the files. That would account for the large speed difference. Alternatively the files have 29.97 frame rate stamped in if they are broadcast wav files. Either way the sync is being put way out because of metadata - not because of crystal sync between the camera and the recorder.

So when you bring in new aifs (no timecode metadata) and have the Easy Setup matvhing the correct frame rate, the sync difference should be .1% unless the frame rate on the camera was not 24.

It is a good idea to delete the old files and bring the new ones in with a different file name. FCP has a habit of remembering file names and it may restamp the wrong frame rate. So new files without timecode and slightly changed names. Do a test with one file


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