Logic Pro audio sync drift with video

[Beardy]

Hi Sound On Sound gang! Hope you’re all well.

I have a question for you. I have a Logic recording of a concert that I recently did. It was pretty cool because I was performing with Mainstage and then used Rogue Amoeba’s LoopBack to route the multi-output MainStage session into Logic, and it worked brilliantly. Combined with the 2 live vocals through a UAD Apollo Twin, we now have a multitrack recording of the show, all from inside the same laptop that’s playing the show. Epic.

The gig was also filmed and as we near completion of the video, the editor has sent me the latest cut so I can mix the gig.

I can actually import this movie file into the same Logic session that I recorded the gig with. All seems great to start with, but then as the 45 minute video progresses, slowly but surely the audio starts to drift out of sync. By the end is totally unwatchable.

I’m using the same sample rate (48khz) and the same frame rate as the movie. And I’m completely baffled as to what is going on! I have done lots of TV commercials / short film scores etc and never encountered this! Confusingly the stereo audio rough mix files that I send to the editor sync up perfectly with no issues. I have read some stuff about this but not found anything that specifically address it in this precise way, so I’m throwing it out here hoping that someone can educate me and also maybe a Logic Pro solution?

Many thanks. Cheers from Sydney!

pb

[Drew Stephenson]

Could be the battery in the video camera getting tired. That can lead to significant drift over much shorter timeframes.

[Hugh Robjohns]

All seems great to start with, but then as the 45 minute video progresses, slowly but surely the audio starts to drift out of sync.

That's not technically accurate, if we assume that the audio is the master and your mix is the same duration as the original recording. In that scenario, the video is really drifting out of sync with the video, and the reason is that the camera's frame rate wasn't explicitly synchronised to the audio word-clock at the time of recording.

This is a common problem, and a lot of cameras don't even have facilities to accept or provide a synchronising signal of some suitable type.

I’m using the same sample rate (48kHz) and the same frame rate as the movie. And I’m completely baffled as to what is going on!

The camera frame rate and, separately, the computer's audio sample rate are usually generated from the vibrations of a crystal oscillator. However, while these are usually very accurate, they will never be exactly the same. While both set to 48kHz / 25fps, one might be slightly fast and the other slightly slow, and across the space of 20 or 30 minutes they can easily drift apart by several frames.

The situation is often worse if camera battery voltage droops or there are significant temperature changes. Hence the need to physically link the clocking systems of separate devices in some way to share a common clock for guaranteed frame accuracy over long periods.

I have done lots of TV commercials / short film scores etc and never encountered this!

Programme durations of less than 15 minutes rarely drift enough to become obvious.

Confusingly the stereo audio rough mix files that I send to the editor sync up perfectly with no issues.

That is worrying! That suggests you've done something in your remix that has affected the running time in some way. Edited a section out? Slipped a section backwards or forwards during a cross fade? Pitch corrected a section without maintaining speed/duration?

[Beardy]

Hey Hugh thanks for the fast and detailed response Consider me learned!

The mix is actually really simple, all one take, straight through no edits, no tuning and no moving stuff about. Just EQ, compression reverb / volume automationaaaa

[sonics]

Confusingly the stereo audio rough mix files that I send to the editor sync up perfectly with no issues.

That is worrying! That suggests you've done something in your remix that has affected the running time in some way.

I'm with Hugh here. Unless you are mistaken in thinking the sync was good on the rough mix, you must have changed something.

If it's still an issue, the simple fix is to finish your mix and then do a quick timestretch sync to picture as the final process.

[Hugh Robjohns]

Rather than timestretch the audio (with its potential for unwanted artefacts), if you're seeking a quick fix I'd simply edit the video at camera cutaways to slip the pics back into sync. Four or five little nips and tucks across 40 minutes will sort it out imperceptibly....

However, the elephant in the room is why there's a syncing error between the rough mix and the final. Something is not right there and the cause needs to be found.

[MOF]

Hugh mentions 25fps but you didn’t, are you USA based and have you or the editor changed from one of the 30@fps variants to another?

[merlyn]

... Confusingly the stereo audio rough mix files that I send to the editor sync up perfectly with no issues. ...

It doesn't seem like too much of a problem then. It may be nice but you don't actually need the video to mix the audio. If on the editor's side everything is fine, that would suggest that it's Logic's playback that is the problem. Does Logic use Quicktime to play video? It could be that, and if it is, there's not a lot you can do about it.

It is kind of a sync issue, but it sounds like the internal sync of Core Audio and whatever plays video. Opening the audio and video in a video editor app may shed light on the matter.

[ManFromGlass]

I may have missed something but the files you send to the editor sync up fine in his machine, but it’s just in your machine where the problem is?

[Hugh Robjohns]

Hugh mentions 25fps but you didn’t, are you USA based and have you or the editor changed from one of the 30@fps variants to another?

He signed off, from Sydney... and Australia uses 25fps as standard.

But you're right in that if there's 29.97 drop frame shenanigans its anyone's guess where the problem lies!

[MOF]

He signed off, from Sydney... and Australia uses 25fps as standard.

Thanks Hugh, I didn’t join up the dots on that one.

[sonics]

Rather than timestretch the audio (with its potential for unwanted artefacts), if you're seeking a quick fix I'd simply edit the video at camera cutaways to slip the pics back into sync. Four or five little nips and tucks across 40 minutes will sort it out imperceptibly....

Agreed, but I was assuming a scenario where that no further edits were possible. Audio artefacts these days are just as impeceptible IMO.