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Double system audio drifting

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Brian Yankou
Double system audio drifting
on Nov 8, 2010 at 11:35:48 pm

Hey everyone! This is my first C.C. post.

I'm having some trouble with audio drifting after syncing my double system audio tracks. I'm a novice editor so I'm sure I'm making some novice mistake.

I shot on an HVX and recorded scratch in-camera audio at 48 KHz/ 16 bit integer. The audio recordings were done on a digital multitrack recorder at 48 KHz/ 24 bit integer. I imported the video and scratch tracks and started creating an assembly of my best takes in a sequence. I then went back and started to sync up my footage to the audio from the multitrack recorder by marking in points where I slated and "merging clips".

It appeared to work until I got to the end of some of the longer takes (around 10 minutes) and realized that my audio was lagging behind my video. Does this have to do with my audio formats for scratch tracks and "real" audio being different? Does this have to do with the sequence settings on the sequence I made my assembly on? It made itself "32 bit floating point" when I put the first clips with scratch tracks in there. I would think that different sample rates would cause this problem but not different audio formats.

Here's the weird thing: Out of curiosity I exported that sequence with some of the clips with out-of-sync audio into soundtrack pro and they magically were synced up! I exported that audio back in to FCP and it was still in sync on the .aif tracks that came from soundtrack pro.

So what do I do from here? Should I continue merging clips in the same way, deal with the fact that my audio is out of sync while editing (ugh - please no) and then just know that soundtrack pro will figure out how to put it back in sync?

Someone please point out the very simple, obvious solution that I am not seeing/ hoping exists. Thanks, experts!

-Brian


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Michael Gissing
Re: Double system audio drifting
on Nov 8, 2010 at 11:45:53 pm

Some basic info is needed. What frame rate did you shoot with (camera and codec info might be useful to know as well).

How did you import the audio files into FCP (file format etc). Did the files have embedded timecode (ie: broadcast wav files). If so what frame rate or timecode type was used.

For your info , bit depth doesn't matter. 24 or 26 bit files work in the same timeline. The sequence setting of 32 bit floating point merely refers to the internal mix processing in FCP and is correct. FCP timelines only accept 16 or 24 bit files and hate anything other than PCM 48khz so you are all good so far.


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Brian Yankou
Re: Double system audio drifting
on Nov 9, 2010 at 12:51:42 am

I shot on an HVX 200 at 23.98 fps, DVCPRO HD 720p 60, data rate was 5.9 MB/sec. The files I imported were quicktime .mov files.

The audio files were .wav files with a data rate of 562.5 K/sec. There wasn't any embedded timecode (I don't think).


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Michael Gissing
Re: Double system audio drifting
on Nov 9, 2010 at 9:10:55 am

Brian, heaps of people have reported problems with wav files in FCP getting stamped with the wrong timecode. Search for Jeremy Garchow's method of deleting wrongly stamped files and reimporting them with the right easy setup and sequence settings.

The other possibility is that the files are broadcast wav and timestamped at 24 fps not 23.98. A quick way to test is to change the speed to 99.9% and see if they still drift. If they sync then you might have to convert the files to the correct speed. You can batch this in Compressor


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Brian Yankou
Re: Double system audio drifting
on Nov 9, 2010 at 5:05:23 pm

Hey Michael, thanks for the advice! I couldn't find that Jeremy Garchow method you were talking about. Was it an article, forum post or something else? Would it be possible to link me to it? I know this should be findable but my searches haven't yielded what you were talking about.

Also, I am still curious about the whole soundtrack pro part of this problem. How did it know how to sync the audio?


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Steve Eisen
Re: Double system audio drifting
on Nov 9, 2010 at 7:09:42 pm

Keep searching, it's not that hard.

Steve Eisen
Eisen Video Productions
Vice President
Chicago Final Cut Pro Users Group


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