Live Performance Audio Drifting Badly

[tsugi]

Hi all,

I filmed a live event, the engineer recorded the playback live as well.


I've edited the footage, and all is well. I bounced the master audio - touching nothing - from Pro Tools and it's drifting like mad. It's noticeably out after about 15 seconds - - like a small delay. By 12 minutes it's a full second out.

Pro Tools bounce is ahead of the camera sound.

These were recorded at the same time!

Any ideas on what is happening, and how to fix it?

It seems PT session was 30fps and cameras were 25fps. But recorded simultaneously - one camera taking a direct feed!

[tsugi]

FWIW, the cameras were at 48KHz 24-bit, and the audio is 32-bit floating, apparently.

Any thoughts?

[Top Jimmy]

When recording digital, if the sample clocks aren’t synchronized, there’s no guarantee of a match. This is what you are seeing.

Import the final video and the audio mix into a new session. Enable elastic audio varispeed mode on the audio. Park the audio in time at the beginning while elasticing the end into time. Export video with the new audio. Bob’s your uncle.

[Darryl Ramm]

Sample depth, 32 float, has nothing to do with time.

Well is 30fps actually 30 or 29.92? In the camera? Editor? You need to work this out.

“Touched nothing” may not be true, you made assumptions about frame rates when processing this video and audio.

[tsugi]

Originally Posted by Darryl Ramm

Sample depth, 32 float, has nothing to do with time.

Well is 30fps actually 30 or 29.92? In the camera? Editor? You need to work this out.

“Touched nothing” may not be true, you made assumptions about frame rates when processing this video and audio.

Indeed as regards bit rate - straw clutching going on.
Cameras 25fps, PT session 30fps
Frame rate was agreed in advance. Not sure why it was recorded at 30.

I've run the camera sound that came from the desk alongside the PT sound - same drift. Everything I've tried - Bounced audio into FCP; exported video into PT; Bounce audio then import into 25fps session; import session data into new 25fps session, etc, etc.

I think it's elastic audio to try!

The bizarre thing for my addled brain is how far out of sync it goes - a full second after 12 minutes! Hey ho.

Thank you for the thoughts and ideas.

[tsugi]

Originally Posted by Top Jimmy

When recording digital, if the sample clocks aren’t synchronized, there’s no guarantee of a match. This is what you are seeing.

Import the final video and the audio mix into a new session. Enable elastic audio varispeed mode on the audio. Park the audio in time at the beginning while elasticing the end into time. Export video with the new audio. Bob’s your uncle.

Many thanks - I think that's going to have to be the route. Hopefully it won't sound too odd. each recording was around 15 minutes, so at least they're back in sync with a new recording, so I'm not stretching it over the hour!

Thanks again.

[Darryl Ramm]

BTW ~1 sec in 12 mins is in the right ballpark for a 30fps 29.97fps confusion caused offset but also might be within differences with badly performing local clocks.

(30-29.97)/30 * 12 * 60 = 0.72 sec

It's such a common mistake to confuse these frame rates even when the numbers are staring folks in the face, so I'll go with that's the cause here until proven not. All this will drive you mad...

And as James says get stuff locked up if you can. You'll likely need an external timecode/wordclock box(s).

[tsugi]

Originally Posted by Darryl Ramm

BTW ~1 sec in 12 mins is in the right ballpark for a 30fps 29.97fps confusion caused offset but also might be within differences with badly performing local clocks.

(30-29.97)/30 * 12 * 60 = 0.72 sec

It's such a common mistake to confuse these frame rates even when the numbers are staring folks in the face, so I'll go with that's the cause here until proven not. All this will drive you mad...

And as James says get stuff locked up if you can. You'll likely need an external timecode/wordclock box(s).

Many thanks.

Like I said, the camreras are all 25. The PT session I was sent says 30 - how do I sort that or verify it's not 30. I tried the stretching and it tied in at the end, but splayed out farther in the middle! That said, I tried it in FCP - just as an experiment. Hopefully elastic in PT will make it a smoother stretch.

I'll keep you updated as a matter of interest and amusement!

[tsugi]

OK - you want something weird?

Put audio in as varispeed elastic. Dragged the end of the take - around 13 minutes - to match the camera audio. Got it spot on.

The start, however, still heads out at 12 seconds, and gets gradually worse, starting to head back in sync at 12 minutes.

Erm..

That makes absolutely no sense to me at all.

[Darryl Ramm]

Yep if it's sliding around noticeably then its just bad clock drift. Portable sync boxes are cheap and amazing, e.g. Atomos Ultra Sync One or Ambient The Lockit, etc. ... if your camera and audio recorders have the suitable sync/gen/wordclock interfaces.