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Syncing problems- total mystery

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tom bailey
Syncing problems- total mystery
on Nov 30, 2011 at 7:56:59 pm

Hi,

I'm syncing up RED footage with stand alone sound recorded to card. Everything works fine except a handful of shots. Despite syncing with a clapperboard the audio becomes increasingly out of time with the shot.
We thought it was something to do with frame rates as the majority of the footage we are having problems with was shot at 27fps. However some of this footage does sync and all of the footage has been transcoded to Apple ProRes 422 and brought in at 25fps. The clip information in the browser window says 25fps as well.

The sound was all recorded at 48 khz, 25fps. It seems to make sense to me that it has something to do with the frame rates, but this is only a guess. This is definitely not a playback error.

Any suggestions on what to try?

Many thanks,
Bailey.


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Neil Patience
Re: Syncing problems- total mystery
on Nov 30, 2011 at 9:30:42 pm

Just curious as to why things were shot at 27fps - this is a non standard frame rate that doesnt fit any spec that I have heard of.

Obviously transcoding these to 25 fps is going to make them show up as 25fps in the browser but 27fps to 25 fps is bound to cause sync issues but I cant explain why some fit thats very odd.

I dont think its an elegant solution but do they fit if you speed them up by 8% ? (think thats the right maths)

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


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tom bailey
Re: Syncing problems- total mystery
on Dec 1, 2011 at 4:01:01 pm

Hi Neil,

These were shot at 27fps to avoid a flickering light I believe, some kind of cinematographer trick. We weren't shooting under a HMI but it was an huge industrial work light with its own generator and I think had a similar problem.

We double checked the shots that seemed to sync up and we think we were wrong- it just had footsteps in a wide frame and its a bit ambiguous so we didn't notice but it is out.

Good news- speeding up to 108% on the video HAS worked, but its not good practice obviously and we would like to find a more elegant solution.
Something that might help our thinking- you said in your post "27fps to 25 fps is bound to cause sync issue"- could you elaborate on why this is? Is it a problem in our logic or is it that the Log and Transfer tool doesn't do that properly?

Is it possible that FCP's Log and Transfer tool is not actually reducing the frame rate?
Is there a program that will drop or merge those two extra frames... but without going all jittery?

Thanks this has been really helpful so far!

Bailey.


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Neil Patience
Re: Syncing problems- total mystery
on Dec 1, 2011 at 9:54:51 pm

[tom bailey] "Is there a program that will drop or merge those two extra frames... but without going all jittery?"

The problem you have is that by turning 27fps into 25 fps its not that you have 2 spare frames the the other 25 match each other.

Each of your 25 frames is in effect 1.08 frames of your 27 frames, so the 2 "extra frames" are kind of spread across the 25.

Thats why it is slightly slowed down. Its like if you shot at 50 fps and then played back at 25 it would be half speed.

I have to be honest I have never in 20 years of editing been given 27 frame material as it, as far as I know, does not conform to any technical standard in any television system or film standard.
So I am not quite sure what you would use to do this standard conversion as everything is geared around going from 30 to 25 or 24 to 25 etc but 27 is an odd one.

I understand the idea of using it as a way of avoiding lighting flicker and the small change in frame rate is probably largely inperceptable in terms of motion.
Maybe they would overcrank film cameras back in the day to achieve this but a more usual way with video is to use a shutter that negates the flicker.

If the pictures look smooth as they are how about attacking it the other way and slowing the audio down for those shots ?

Timestretch in Soundtrack Pro could be helpful
or you could slow the audio down in FCP by I think 92.59% then use pitch shift to make it sound correct. Try about -80 ish as a pitch filter value but you will need to experiment.

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


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tom bailey
Re: Syncing problems- total mystery
on Dec 1, 2011 at 11:06:20 pm

Hi Neil,

That makes total sense about 25 frames = 1.08 of my 27 frames, I was being a bit slow there.
We will try these ideas to see what works best, from what my editor said it sounds like speeding up the footage has done the trick for now.

Next time if i hear the DoP wanting to shoot at 27fps I'll stop them and suggest the shutter!

Thanks for your time and helping us get our thinking on the right path.
Cheers,
Bailey.


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tom bailey
Re: Syncing problems- total mystery
on Dec 6, 2011 at 4:16:19 pm

Neil, thanks a million, thats worked perfectly.

It seems slowing it down 92.64% works, but thanks for the maths! Would have really been stuck on that.


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tom bailey
Re: Syncing problems- total mystery
on Dec 6, 2011 at 5:50:51 pm

Hi Neil, or anyone else.

I just wondered if you know how to make audio speed settings a favourite effect, so i can marry it to a button?

I understand the process of setting it on the keyboard, but I can't add it to favourite effects if I can't put it in the folder. Can one of the effects in Audio Filters folder change speed?

Thanks,

Bailey.


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