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Audio Drifting in FCP

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David Jones
Audio Drifting in FCP
on Feb 8, 2011 at 9:32:59 pm

Hi Everyone...

Yet another question about audio sync :-)

My project was shot on super16mm @24fps, transferred directly to a harddrive (converted to quicktime movie ProRes 442HQ); audio is DAT 16bit/44.1kHz. Not sure why, but the audio was converted to a quicktime movie file as well, transferred to the same drive and, as far as I know, was converted to 48kHz.

My question is: can I convert it to an audio file of some sort, or what do I need to do to fix the drifting?

Dave J


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Michael Gissing
Re: Audio Drifting in FCP
on Feb 8, 2011 at 10:14:58 pm

I presume the telecine was 23.976 so you have the audio playing short by .1% . If that is correct then it should be possible to use Compressor to make an audio file with that speed correction and output as a wav or aif.

No need to worry about pitch as .1% is imperceptible.


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David Jones
Re: Audio Drifting in FCP
on Feb 9, 2011 at 12:47:56 am

Thanks Michael!

Yeah, it was 23.976.

Dave J


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Jeff Beaumont
Re: Audio Drifting in FCP
on Feb 9, 2011 at 5:52:15 pm

How long are the takes?. How soon does it drift? If it's just the difference between 24fps and 23.98 then it might be easily fixable. If you have long takes there maybe be other issues too.
For example converting the sample rate will almost always create some drift.
Also how are you capturing the audio from DAT? If you are going analog that's another potential problem.
The fact that the audio files where saved as movies indicates that the person who did the transfer didn't know what they were doing. Go back to the source files.
In the future ALWAYS use 48Khz (24bit if possible) NEVER sample rate convert. I would recommend against DAT. A solid state recorder (Sound Devices, Fostex, etc.) is going to be more reliable and easier to use. especially in post.
And if you are shooting film you really should be using some sort of a timecode sync system.

Jeff Beaumont


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David Jones
Re: Audio Drifting in FCP
on Feb 9, 2011 at 6:11:12 pm

Thanks everyone for all the help!

I did just need to go from 24fps to 23.976 (changing the speed .1%). Converted it to an AIFF file as well, all in Compressor. Everything's working (or syncing) great! Not to mention merging of clips; FCP doesn't like merging two video clips...lol.

Yeah, when this documentary film began shooting (10 years ago!), DAT was still it. Glad we're on to sold state and CP cards!

Dave J


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Michael Gissing
Re: Audio Drifting in FCP
on Feb 10, 2011 at 12:08:05 am

DATs are predominantly 48khz although many pro machines offered both sample rates. Sample rate conversion, in my experience as a sound post specialist, has never changed length so it isn't a factor in sync issues.

However, on the fly software conversion in playback by NLEs can create sync drift issues and FCP is particularly poor at it. If you use software to sample rate convert a file before bringing into FCP then you will not create a sync problem.


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Jeff Beaumont
Re: Audio Drifting in FCP
on Feb 10, 2011 at 1:23:44 pm

In my experience sample rate conversion does alter the length of audio, but if you are using professional tools the difference is only a few samples on shorter clips. On a concert length recording though it is pretty significant drift.
The problem is most people who are doing it wrong aren't using professional tools to sample rate convert. When ever someone brings me video that was shot double system with sync issues, the sample rate conversion was always part of the problem.

Jeff Beaumont


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Michael Gissing
Re: Audio Drifting in FCP
on Feb 10, 2011 at 12:09:14 am

DATs are predominantly 48khz although many pro machines offered both sample rates. Sample rate conversion, in my experience as a sound post pro, has never changed length so it isn't a factor in sync issues.

However, on the fly software conversion in playback by NLEs can create sync drift issues and FCP is particularly poor at it. If you use software to sample rate convert a file before bringing into FCP then you will not create a sync problem.


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Michael Gissing
Re: Audio Drifting in FCP
on Feb 10, 2011 at 5:19:42 am

Sorry about the double post. The first one was sent to moderation because of "Spechialist" which when spelt correctly triggers the spam police.


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