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Audio Sync Drift with MXF Files

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Ben Williams-Butt
Audio Sync Drift with MXF Files
on Jan 8, 2016 at 12:21:36 pm

Dear all,

I've just started working on editing a short doc that was shot about 3 or 4 years ago and has only just been pushed through to post by the production team. They don't have records of the production notes (i.e. fps, aspect ratio etc. etc.) to hand but in attempting to get underway with it, I ran the files through MediaInfo and got that data.

Basically, all files are 24fps and the separate sound is recorded at 48khz. My sequence and import settings on FCP7 are set for this.

On syncing the sound though, there is sync drift between video and audio and the audio is actually running slower than the video. I've checked all the settings, as well as the frame rate that the sound was imported at and everything matches up.

I'm a long time reader of Creative Cow and have been browsing for a few hours for answers to this but all seem to revolve around the incorrect conversion of a file or the frame rates/audio samples not matching the sequence.

Does anyone have any idea?

Any help is much appreciated.

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Dave LaRonde
Re: Audio Sync Drift with MXF Files
on Jan 8, 2016 at 4:10:31 pm
Last Edited By Dave LaRonde on Jan 8, 2016 at 5:45:17 pm

Is the frame rate REALLY 24, or is it 23.976, aka 23.98? That could account for drift... especially if there's video shot at 23.976, but some genius assigned a frame rate of 24 to the external audio recording device.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA

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John Rofrano
Re: Audio Sync Drift with MXF Files
on Jan 9, 2016 at 2:11:09 pm

Is the audio immediately slower than the video or does it start out in sync and just drift eventually over time?

If it's immediately slow then what Dave has suggested it probably the cause, i.e., the settings are incorrect between 24.0 and 23.976.

If the audio drifts over time, this could be cause by the clocks of the camera and audio recorder were not genlocked. Unless you shoot with an external clock and genlock the equipment, they will drift because the internal clock crystals will not be synchronized. In this case, split the audio every so often and change the speed to get it back in sync.


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