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Exporting Audio - why different lengths?

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Ben Wharton
Exporting Audio - why different lengths?
on Mar 29, 2010 at 9:43:06 am

Hi

I feel really dumb about this, so please be gentle.

I'm trying to export individual audio tracks from FCP as WAV fles. A surround mix on an HD film needs to be sent as separate mono WAV files. This appears to be simple, selecting the individual tracks, and using QuickTime Conversion to export.

But when I then reimport these WAV files into my film timeline to check, the length of the audio files do not match the original. They're longer.

I've tried marking in and out points, which fixes the duration, but not the drift.

I've tried extracting the audio with just Quicktime Pro - same duration issue.

I've come across this before and the only way I managed to correctly extract the audio was by outputting the audio as a .mov file with a video track and then killing off the video track in QTPro - but then the audio is still a .mov...

The elephant in the room here is obviously Soundtrack Pro. Is this the only way to get correct exports out of FCP (which aren't AIFF / OMF)? Because I knolw nada about Soundtrack Pro... sigh!


Thanks for any tips.


Ben


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Ben Wharton
Re: Exporting Audio - why different lengths?
on Mar 29, 2010 at 10:40:27 am

As an addendum -

Could this be to do with 23.98 versus 24 frames per second?

The film is mastered at 23.98 but the exports are coming out about 5 seconds too long. And that goes from Soundtrack Pro too.

IS FCP counting at 24 and that's the issue?


Ben






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Ben Wharton
Re: Exporting Audio - why different lengths?
on Mar 29, 2010 at 11:05:21 am

Attendum 2:

I've just exported a WAV from a 24 fps timeline of the same project (NOT its native 23.98) and now the audio is one frame short.

So it's looking like there's some inherant time base error being introduced... anyone concur?

And more to the point, how do I get around it?! Please...


Ben




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Ben Wharton
Re: Exporting Audio - why different lengths?
on Mar 29, 2010 at 11:40:40 am

And finally

exported audio from a vanilla PAL DV25 timeline and no problems with audio drift/length at all...


Apologies not all of these tests were done before I first posted.


Ben




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John Fishback
Re: Exporting Audio - why different lengths?
on Mar 30, 2010 at 8:56:24 pm

What's the sample rate of the audio?

John

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Ben Wharton
Re: Exporting Audio - why different lengths?
on Mar 30, 2010 at 9:11:16 pm

Hi John

Thanks for the reply.

The audio is 8 tracks (Stereo Pair plus 6 tracks of Surround) of 24-bit Integer (Little Endian) 48,000Khz.


And I'm exporting it out exactly the same way.


It seems FCP doesn't like 23.98 for audio exports. Different media with the same spec does the same thing - render audio out too long.

Creating a 24fps timeline, putting the 23.98 master on that, then exporting audio at 24 appears to be the only way to get the correct output.

In the end I discovered we need to master to 24fps for this job - a theatrical movie server, but I'd rather FCP does what it's supposed to do.

Oh, unless this is something about drop frame? Grrrrr.

Am I missing something?


Thanks for any pointers!


Ben








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