Audio sync problems..

B

BajotierraTV

Member
I recently did a 5 camera shoot of a concert that I'll be editing for a DVD, we shoot at 24p (Not advanced). Captured the footage at DV NTSC 29.97 48khz using FCP. The audio was recorded with ProTools at 48khz. When I try to sync up the video with the clean audio it start drifting from the footage after a while.
 
Am I using the wrong settings? Why is the audio going out of sync after a while?
 
Re: Audio sync problems..

How much out of sync is it drifting? If it's something like two frames every minute, then it might be attributable to the 29.97 vs. 30 base frame rate... in that case try telling FCP that the audio file is actually 48048hz, maybe that'll fix it?

I know nothing about FCP so maybe someone more knowledgeable will write in and be able to help you.
 
Re: Audio sync problems..

It is something like 2 frames every minute. Hmmm, that might work even though I don't know how to do that. Thanx
 
Re: Audio sync problems..

Okay, sounds like we're on the right track.

Here's the thing: NTSC video runs 1/10 of 1% slower than its nominal speed, so it's not 30 frames per second, it's 29.97 frames per second.

So when recording on a DAT or Nagra, you usually have the ability to set the clock to either 30fps or 29.97fps, even though the sampling rate is still 48000 hz.

The two are not completely compatible.

So on Adobe Premiere 4.2, which didn't properly support 29.97 but instead operated internally at 30, we used to have to go into Sound Forge or another sound editing program and "lie" to the sound file, telling it to play back 0.1% slower.  The way to do that was to change the sampling frequency to 48048Hz, which, when played back at 48000Hz, will take 1/10 of 1% longer to play back. Note: we did not tell it to re-sample the file to become 48048Hz, we told it that it should treat the data that it already had as if it was recorded at 48048Hz. That way it would play the data back 0.1% slower.

I don't know ProTools, but I'll bet you a nickel that there's an option in there somewhere for converting a file to 48048Hz, for that exact reason.

Good luck!
 
Re: Audio sync problems..

Fixed the problem.. Barry Green pointed me in the right direction.
There is an option in my audio editing program (peak) to convert the 48khz audio to match with drop frame footage. I had to convert the audio to 47952.1khz (1% slower) and bring it into FCP again. It matched perfectly. Very strange, I had never heard of this before but it worked.

Thanks Barry
 
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