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separate audio syncs when captured in real time - not when directly imported

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Mark Augustine
separate audio syncs when captured in real time - not when directly imported
on Feb 21, 2013 at 8:37:08 pm

Shot a 2 hour program with a card based camera and recorded the audio separately through the house sound system with a little tascam DR40.

When I directly copied and imported the .wav file from the DR40 to FCP, the audio sync drifts relative to the camera footage about 2 frames every 10 minutes.

But if I go to the trouble of playing the dr40 in real time and capturing its analog output into FCP - even with no video reference, everything stays in perfect sync.

Now, I don't want to spend the time or have a crappy analog signal in my edit so I'm wondering if there's a way to take that .wav file and apply some sort of time compression/expansion.

I know this has got to be some classic ratio between audio and video systems.

Thanks for any suggestions!


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Shane Ross
Re: separate audio syncs when captured in real time - not when directly imported
on Feb 21, 2013 at 9:01:17 pm

is the frequency of the recorded audio 44.1kHz? Because FCP works at 48kHz, and so when it is capturing it, it is doing so at that rate. That also happens to be the frequency of video...48kHz.

Shane
Little Frog Post
Read my blog, Little Frog in High Def


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Mark Augustine
Re: separate audio syncs when captured in real time - not when directly imported
on Feb 21, 2013 at 9:17:17 pm

Good thought, surprised I didn't check.

But both are 48KHz

Video frame rate was 29.97. I figure that's gotta be the difference - FCP thinks it's keeping the audio in sync by aligning 48,000 samples for every frame of video - which will understandably go out of sync because in reality there's 48,000 samples for every actual second, not 30 full frames of video, which takes a bit longer to play than a second.

I think I'm answering my own question but the math is making my head hurt.

There's gotta be a standard way to treat audio files for situations like this.


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Shane Ross
Re: separate audio syncs when captured in real time - not when directly imported
on Feb 21, 2013 at 9:57:12 pm

Recorders designed to work with video will record audio keeping in mind the 29.97fps frame rate. Audio doesn't really have a frame rate, but recorders do have settings to make sure that they record audio at the proper speed to match camera frame rates.

What recorder did you use for this?

Shane
Little Frog Post
Read my blog, Little Frog in High Def


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Mark Augustine
Re: separate audio syncs when captured in real time - not when directly imported
on Feb 22, 2013 at 12:04:27 am

I used a cheap Tascam DR40. No adustment for clock rates.
But I figured out how to do what I needed.

I calculated the drift, which was consistent and did the math. The speed difference was .011 - so I loaded the clip into Audacity and applied the "change speed" effect, entered .011 and exported the file. Sync was just perfect.

I wonder what that .011 difference is, it's not the same as compensating for 29.97 - but all that matters is that it works.


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Dave LaRonde
Re: separate audio syncs when captured in real time - not when directly imported
on Feb 22, 2013 at 10:32:27 pm

Mark Augustine: ...Shot a 2 hour program with a card based camera...

No offense, but that's not really illuminating information. Did you double-check the frame rate in a couple of applications?

I don't know of any card-based cameras that do 30 fps instead of 29.97 that don't cost as much as a brand-new Corvette.

Dave LaRonde
Former Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Nick Meyers
Re: separate audio syncs when captured in real time - not when directly imported
on Feb 23, 2013 at 12:58:59 am

FCP has an issue with importing audio.
it can be affected by your sequence preset.

check what ever you have selected as a sequence pre-set is the same frame rate as your footage.
if not, adjust it, and re-import

apparently you my need to import into a new project as the original one can "remember" or "hang on to" the older frame rate when importing the same audio files



nick


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