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Audio Drift

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David Neidert
Audio Drift
on May 13, 2010 at 5:31:29 pm

I am having a problem with my audio drifting out of sync with the video.

I recorded with a dual system using a Canon 7D and an external audio recorder. I recorded the audio in 48Khz 32 bit .mp3's. The footage is 16bit audio.

I am trying to sync the camera audio with the external audio, but what is synced perfectly at the beginning of the clip is completely out of sync by the end. Is the problem the 32 vs 16 bit audio? Is there a solution?

David



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cowcowcowcowcow
Jean-Christophe Boulay
Re: Audio Drift
on May 13, 2010 at 7:25:50 pm

Hi David,

Bit depth has nothing to do with sync. Sample rate does, though. Your two sources were maybe recorded at different SR, probably 44.1k and 48k, and imported into the same session without SR conversion? That's a classic mistake everyone has done at least once. The solution would be ensuring your two sound files are of the same sample rate and convert whichever one doesn't match your editing project.

If things are "completely out of sync", it's probably SRs, but recording sync audio to mp3 is a no-no, especially if the recorder does real-time mp3 conversion. The conversion process can affect sync, but the effect is pretty minimal. This is probably not your problem.

In most serious audio work, 32-bit audio is frowned on (except if you're a Cakewalk user). 32 is a bigger number than 24 or 16, but that doesn't make it better. I won't go into full-integer vs floating-point calculations, but 24-bit/48kHz is the standard most sound-for-picture is recorded to and edited at. Recording directly to your editing format will bypass conversions that can introduce artifacts and smudge sync. These last two points are probably not the source of your drifting, but things to look out for in the future.

IHTH

JC Boulay
Technical Director
Audio Z
Montreal, Canada
http://www.audioz.com


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Terry Mikkelsen
Re: Audio Drift
on May 13, 2010 at 7:38:59 pm

No the bit depth is not your problem. It comes from using two separate pieces of gear that were not locked together. No VITC, LTC, S/PDIF, AES3, etc.... So each piece of equipment runs at its own speed, what it thinks is "48kHz" for example. Each is just slightly off from each other. Who's right / who's wrong doesn't really matter, they just don't match.

You now get the lovely job of rematching it manually. The good thing is that is should be consistent. If you find that it takes about 10 minutes to get out of synch by 1 frame, you can then bet that you need to do that every 10 minutes.

Now for your next shoot:
What you have done is actually a good thing (if you obtained better audio quality than your camera got). You just need to synch the two. If that isn't possible, get a clapperboard (or just use your hands in a giant clapping motion) and use it before each shot.

Tech-T Productions
http://www.technical-t.com


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David Neidert
Re: Audio Drift
on May 13, 2010 at 8:15:55 pm

Thanks Gentlemen,

I did in fact use a sync point, in this case a iPhone Slate application.

The problem was the SR. The 7D is at 44.1 and my external audio is coming in a 48kHz. I'll just re-sample the audio from the external source. I'll know better next time!

Thanks for the tip about the .mp3 format. I know that I have some others to choose from.

I am going to do a drift test as well to see how long it takes the audio to get out of sync and I'll no to watch for that next time.

I appreciate all the help.



David



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Ty Ford
Re: Audio Drift
on May 13, 2010 at 10:09:20 pm

Hello David and welcome to the Cow AUdio Forum.

In addition to the good warnings you've read here. I'll emphasize that even if you DO set both devices to 48 kHz sample rate, you can still expect drift. I just proved that agin here in reviewing a small sony audio recorder and my Canon Xl2. After 58 minutes the Sony was 6 frames fast.

The point is, you just don't know until you test two devices against each other. If your shots are short enough, in the best of cases, it won't matter, but if one clock is slow relative to absolute time and the other clock is fast, you'll see/hear the effect of drift pretty quickly.

Remember the old addage, "A man with one watch always knows what time it us. A man with two watches never does."

Regards,

Ty Ford

Want better production audio?: Ty Ford's Audio Bootcamp Field Guide






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Richard Crowley
Re: Audio Drift
on May 14, 2010 at 1:20:40 am

Since you didn't say how long it was between "beginning" and "end", nor whether "completely out of sync" means 5 frames or 5 seconds, its hard to attribute the cause to anything specific.

You did't mention what NLE you were using, either, but many of them will automatically convert the audio sample rate to conform to the project standard.

Without knowing the scale of the problem it isn't really posible to say with any certainty that the problem was the sample rate, using two non-sync pieces of gear, or even using a lossy compressed format like MP3.

I have never found any significant problem with "pulling-up" sync when I have sound that was recorded on a separate (non-sync) device. Most modern equipment (even pro-sumer and even consumer gear) stays reasonably close over a 10 minute span that it won't drift more than a few frames. More drift over longer time spans, of course, but one rarely uses even a 10-minute uninterrupted clip when editing video. And any point where the video is edited (to go to another shot, etc.) is an opportunity to "pull-up" the video to match the audio.

I would have to agree, however, that using any lossy compressed format (like MP3) is never a good choice for production work. For several reasons. If I had a choice, I would record audio at the same sample-rate that the camera is using (if that is 44.1K, then so be it.)


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John Fishback
Re: Audio Drift
on May 14, 2010 at 1:45:23 pm

I agree with Richard, but I suggest changing the camera to 48k (if that's possible) as that sample rate is what most editors (sound & video) would expect to see. And definitely record wav or aif audio files.

John

MacPro 8-core 2.8GHz 8 GB RAM OS 10.5.8 QT7.6.4 Kona 3 Dual Cinema 23 ATI Radeon HD 3870, 24" TV-Logic Monitor, ATTO ExpressSAS R380 RAID Adapter, PDE enclosure with 8-drive 6TB RAID 5
FCS 3 (FCP 7.0.2, Motion 4.0.2, Comp 3.5.2, DVDSP 4.2.2, Color 1.5.2)

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