Audio playback out of sync

[Michael Prescott]

I just changed it to 99.9% and it worked. Thank you very much!

Could you just take a second to explain in detail why this workaround is necessary and whether this is a bug or a feature in FCP?

[Michael Gissing]

It’s a big nasty complicated story, but in a nutshell, .1% is the difference between real time and NTSC frame rates.

Technicians behind the development of NTSC cleverly used frame rates like 29.97 instead of 30fps to overcome a technical issue with color subcarrier around the middle of the last century. This .1% slowdown from real time continues to bug the minority of the broadcast world that clings to these frame rates. As the vast majority of the broadcast world is based on 25 fps, we don’t expect to have to compensate.

This forum is constantly dealing with these issues and sound sync is amongst the most common and fraught with fix ups for this .1% variance.

[Michael Prescott]

Thanks for the explanation. Just a quick follow up:

Why does FCP lock the timebase of audio clips to the initial Easy Setup parameters? Shouldn’t it be intuitive enough to play it at the same rate as the sequence settings timebase? It makes it very difficult to do an NTSC and PAL version in the same project file.

[Michael Gissing]

Prior to FCP6 (from memory) broadcast wav files with embedded timecode metadata were not correctly interpreted by FCP and people, quite rightly, wanted that fixed.

The fix however seems to have introduced the characteristic of frame rate stamping audio files, although I thought this was only a problem with wav files. It is a nuisance and as someone who works with sound post systems like Fairlight that don’t do this but do interpret embedded code in broadcast wav, mxf, mpeg and quicktime properly, I don’t know why FCP has this problem.

[Michael Prescott]

Actually, my saga is not over…
Even though setting the audio speed to 99.9% put everything back in sync, the producers do not want to do this. (It is a famous rock act and they don’t want the audio altered in any manner. I guess its already been tweaked too much as is.) So, I decided to perform a different approach:

1) Changed FCP Easy Setup to PAL 25fps 48khz.
2) Exported XML of PAL sequence
3) Deleted audio assets from timeline and browser
4) Closed FCP
5) Edited XML file and changed all tags to TRUE
6) Opened FCP and imported revised XML file
7) Set Sequence Settings to DV PAL 48khz 25fps

Upon import of the XML file, the following error message appeared: “Non-critical errors were found while processing XML document. Required element has a value that is out of range. Resulting sequence item definition may not behave correctly.”

Notwithstanding this error message, the audio was, in fact, now in sync. All seemed good. However, the Sequence Settings had the Timebase at 24.98.

Is this correct?
Why was the Timebase changed from 25fps?
Will this cause problems when I export to Compressor and, eventually, DVDSP to make the PAL DVD? And, most importantly, while the clip properties say 100%, has this process changed the speed or pitch of the audio file?

**I did go back and check the original, unedited XML file and noticed that all tags were set to 25. Could this have created the problem??

[Michael Prescott]

… that’s all “timebase” tags were set to 25.

[Michael Gissing]

I suspect that by slowing .1% you have put the audio back to its correct pitch and the NTSC version is actually running slightly slower. To put all that in perspective .1% is not detectable pitch variation.

It now appears you have changed the speed of your video instead of the sound, but that won’t make any difference as broadcast will be at 25 fps.

[Michael Prescott]

I agree with you that the .1% pitch variation is not detectable. I never should have told them I did it in the first place!

I’m still trying to understand why the timebase was changed to 24.98. The XML was adjusted by a search and replace of “all” instances of from FALSE to TRUE. If this correction was just limited to the audio files, would it have left the video timebase at 25fps? Or, is there some other reason?

Thanks again for all your patience and helping me get my head around this frustrating and complicated issue.