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Audio and video out sync - delay

octium8's picture
Last seen: 2 years 3 months ago
Joined: 06/11/2012 - 5:47am

Hello

I am recording some videos with iMovie HD with Mac's iSight camera. The sound is bad so I am using a H4Zoom microphone for a better sound quality.

The problem comes up when I want to put the two together. I can match them up at the start of the video, but for 1 hr of video there is about 2-5 seconds delay in the end of it.

I tried all sorts of things and its still not yelding.. Been banging my head on the walls.
Tried using FCPX delay audio function and IN the software, it works, I get it right, but the exported file, the final video is still delayed...

Please help

Thank you



Jack Wolcott's picture
Last seen: 8 months 1 week ago
Joined: 01/02/2008 - 11:51pm
Plus Member

How to correct the obvious differences in recording speed between the camera and Zoom I don't know, and it may not be possible. If the differential is only 1 frame per minute, in 30 minutes you'll be out of sync by a full second.

However, in post, try this: Sync the audio and video and play until you notice the beginning of sync loss. Back up a few seconds and render this portion of the material. Re-sync and repeat from this point, repeating as often as necessary. Hopefully you'll wind up with several clips, all rendered and all in sync. Combine them and you're in business.

Jack


Rick Crampton's picture
Last seen: 2 months 4 weeks ago
Joined: 08/20/2009 - 1:08pm

It's an extreme act of faith to assume that even the best of gear will hold sync for an extended period of time. I'd be surprised, given the tolerances of consumer and semi-professional gear internal crystal or PLL references, if the audio DID hold perfectsync in post. Shooting double system in the wonderful world of professional film or video demands a common sync reference and time code being recorded on the audio and video gear; and then a method to resolve the two in post.

Rick Crampton


Harry Brooks's picture
Last seen: 10 months 4 weeks ago
Joined: 06/16/2009 - 6:28pm

make sure the zoom is set to 48k and not 44.1 which is the cd standard. they should hold sync both being digital.

Harry

 

 

 

Give the Bride What She Wants!


Eric Schwartz's picture
Last seen: 2 years 6 months ago
Joined: 03/07/2012 - 5:19pm

As gldnears suggests, it is very likely a matter of internal clock drift, and the only way to avoid it is to use word clock sync either with an external word clock signal or often digital signals will have embedded sync in the audio stream. If you're using them side by side independently with no connection, there will be drift over that period of time. It is simply a matter of inherent variation in the crystal clock circuits -- 44.1 on one device is never exactly the same as 44.1 on another device, and while the variation is relatively tiny it compounds over time. Consumer level products will typically have greater drift than top of the line pro gear, but there is always some amount of variability in internal clocks.

It has nothing to do with sample rate -- if you import 48kHz audio into a 44.1kHz project without sample rate conversion (usually the editing software will convert automatically) then you will notice a major difference as the audio will be pitched significantly up or down (depending on going from 44.1 -> 48 or vice versa) and the timing offset will be way more drastic than a few seconds after an hour.

If you're using the zoom H4n, the best thing to do would be to use it in audio interface mode, connected to the computer via USB and recording the audio directly into the editing software. Connected as an interface the zoom should use the computer's internal clock as a master and the audio should remain perfectly in sync.

Alternatively, to fix the problem, you'll just have to manually pick spots in the audio and make a cut, and either move it forward or backward very slightly in time (depending on which is delayed). You could do this every 10 or 15 minute segments that would probably be fine. If you have to push them forward just take some of the extra room tone you captured to "glue" over the gaps in the the audio if needed.

Eric

www.schwartzsound.com


Rick Crampton's picture
Last seen: 2 months 4 weeks ago
Joined: 08/20/2009 - 1:08pm

As an experiment, earlier this year, I shot a live concert of folk-style music using my HVX-200A and a Korg MR-1000 digital audio recorder which was connected to an aux out on the PA console. In post (CS-4 )I monitored the camera audio as a reference and adjusted the external digital audio for sync. I was surprised to find that over the first hour of the show ( continuous run )the syncdrifted out a total of 3 or 4 frames, easily pulled up a frame at a time. I was prepared for a much bigger hassle but was relieved that the sync held as well as it did running wild!

Rick Crampton