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Problems syncing audio with H264 video @ 25p

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Ariel Ritchin
Problems syncing audio with H264 video @ 25p
on Mar 11, 2012 at 9:24:46 pm

I'm having difficulty syncing audio recorded with a zoom zh4n recorder. The video was shot in H264 at 25p.
I've tried converting the footage using MPEG Streamclip to Apple ProRes 422, and even tried altering the frame rate to 23.98, 24, 29.98 & 30 as well as using the original footage (H264, 25p).
I can match the audio and video at the start of the clip, but as it goes on, the syncing gets worse.
Any advice?


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Mark Glenister
Re: Problems syncing audio with H264 video @ 25p
on Mar 12, 2012 at 2:21:36 pm

Hey Ariel,

I have this absolute nightmare too. We currently change the audio speed to 99.89 (or something like that) to bring it back in sync.
We record all audio at 48,000 on the deck and camera, but it always slips out of sync. I've tried several times to get an answer for this but noone has had a decent one yet?!


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Neil Patience
Re: Problems syncing audio with H264 video @ 25p
on Mar 12, 2012 at 4:41:08 pm

Just wondering how quickly does it start to become a noticable drift and roughly how far out does it get in say 5 mins or 10 mins ?

I am only asking as I did a job recently that was recorded on 2 5Ds with a Rode NTG3 that was split through a Zoom and to the main camera.
So the camera was recording the Rode as a syncing guide.

They were talking head interviews and we recorded in about 8 min chunks on both cameras so I only had 2 or 3 "joins" per interview rushes as each one ran around 20 mins or so in total.

As they were questions and answers it was easy to stop and start both cameras about the same time, they didnt stop the zoom at all.

I actually just matched the waveforms by eye from a clap and found that they pretty much stayed in sync throughout each clip.

I could not get the samples to match exactly on the clapper, they were a tiny bit out and although the samples had drifted more at the end of the clips the difference seemed to be less than a frame in total.

I was fully expecting them to run out quite a bit more.

Maybe I was lucky but the sound ran the same for both cameras.

Sounds like you experienced much worse ?

best wishes
Neil
http://www.patience.tv


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Ariel Ritchin
Re: Problems syncing audio with H264 video @ 25p
on Mar 14, 2012 at 2:28:32 am

Hey Neil,

Thanks for taking the time to respond. It starts becoming noticeable after about a minute or so, but since the audio is 45 minutes long, by the end it's wayyyy off.

I don't know how he figured this out, but Mark Glenister responded to my question as well, and told me that if you slow down the audio track to 99.89% it all lines up. He was right! Again, I have no idea why this magic number works, but if you run into the same problem give it a shot.

All the best,

Ariel


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Neil Patience
Re: Problems syncing audio with H264 video @ 25p
on Mar 14, 2012 at 11:35:36 pm

Yes I had heard that speed change worked and was about to do it before I even tried syncing.
However since everything was in nice, neat little chunks and easy to sync I thought I would look first.

I wasnt too suprised that you couldnt totally frame match the audio as obviously the Zoom is not locked to anything but as I said the error never exceeded a full frame. But the clips were way shorter than yours.

I have not had to deal with that much DSLR footage where the audio mattered but ss soon as I get one that drifts at least I know the "magic number"

best wishes
Neil
http://www.patience.tv


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Ariel Ritchin
Re: Problems syncing audio with H264 video @ 25p
on Mar 14, 2012 at 2:24:02 am

Hey Mark,

THANK YOU SO MUCH! I have no idea why you would have to slow down the audio to 99.89%, or how you figured that out for that matter... but it worked!

Again, thanks so much,

Ariel


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