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Sync drift on imported audio

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Kino Haitsma
Sync drift on imported audio
on Aug 31, 2016 at 9:17:26 am

Hi all, I have been having some problems with FCP7 and imported audio, namely getting it to sync up...

This is how I work: I shoot video (see example below) with my iPhone. I give sync clap every new take.
I record my playing (I'm a pianist, making tutorial movies) and a speech microphone into my Mac Pro, while I shoot video. (in this case I work with Ableton Live).
I transfer the movies from the phone to the Mac, convert them to ProRes in MPEF Streamclip, and import them into FCP7.
I then import the audio from Ableton as a wav file into FCP.
Here's the problem. The audio drifts out of sync.
So, even when I line up the audio exactly with my clap at the start of a take, after a little while, say, 20 seconds I start to notice the audio is not in sync with picture anymore.
In addition, I also use an application (Synthesia) that transforms MIDI notes (recorded while shooting video) into actual notes on keys. In a separate process, I make screen captures from that application (with Monosnap) and after converting those clips to ProRes again, also at 30 fps, I layer them on the piano clips in FCP. They also start to drift out of sync after a little while.
I can't figure out what I am doing wrong. iPhone video converted to 30 fps ProRes, screen captures converted to 30 fps ProRes, and still no accurate sync.
Any tips or advice is greatly appreciated!

https://www.youtube.com/watch?v=4Tnm8LxaYlk


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Dave LaRonde
Re: Sync drift on imported audio
on Aug 31, 2016 at 3:24:13 pm
Last Edited By Dave LaRonde on Aug 31, 2016 at 11:26:20 pm

I am not one bit surprised you're having problems. Here's the best advice: don't shoot with an iPhone and expect lip sync.

Those gizmos have the oh-so-nasty habit of shooting in a variable frame rate, which is an absolutely-alien concept for editing. A variable frame rate totally screws up audio-video sync in an application that relies 100% on a consistent frame rate.... such as just about any video editing application on Planet Earth.

Using MPEG Streamclip doesn't help except to make it ProRes. It forces the variable-duration video frames from the iPhone into a consistent frame rate. And what does that do? It changes the the original speed of the video. Try syncing audio to video that's speeding up and slowing down all the time. That's your problem.

Shooting on iPhones may look cool in the iPhone commercials, but it really sucks for FCP... and you can bet your bottom dollar those iPhone spots were shot under highly-controlled conditions to yield the best images. You'll notice none of them use lip-sync audio, either.

Maybe there's a setting in the phone that lets you shoot at a constant frame rate, but I don't have an iPhone so I don't know that.

Bottom line -- you're hosed with the video you've already shot. You must start over.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Nick Meyers
Re: Sync drift on imported audio
on Sep 1, 2016 at 10:25:08 am

a few things to check:

1. are you altering the frame rate when you convert in Mpeg Streamclip?
probably not, it defaults to the existing frame rate, i think

2. FCP has a drifting sync issue.
you need to set you sequence pre-set to match the frame-rate your are working at.
do this in the Audio/Video Settings (found under the Final Cut Pro Menu)
if you are working at 30fs, turn you need to chose a sequence preset that is 30fps.
as there ISNT one, you'll have to create one.

if your sequence preset has been set to 29.97
then when you import audio it takes on that frame rate, and can drift out of sync with your 30fps video.


fingers crossed that's al it is :)
nick


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Robert Withers
Re: Sync drift on imported audio
on Sep 1, 2016 at 5:22:42 pm

As Dave says, the iphone shoots with a variable frame rate that FCP and others convert to a fixed frame rate but there will be slippage with a separately recorded track.

There are special movie apps you can load into your iphone for more control over video shooting that may produce fixed frame rate. I use one called MoviePro, though haven't checked out the frame rate effects lately. That might be a solution.

If the iphone is recording sound with picture, I suppose one could import the sound as well as the picture into your NLE and use one of those programs that syncs your separate audio recording to the camera audio track. But this might produce some unwelcome sound anomalies of pitch, tempo, etc.

I think a special iphone movie app will probably be the solution. Needs researching and tests.

Cheers,

Robert Withers

Independent/personal/avant-garde cinema, New York City


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Kino Haitsma
Re: Sync drift on imported audio
on Sep 1, 2016 at 5:46:01 pm

Thanks for all these comments and tips!
I talked to an editor who said the problem was audio related, because I said I got a RED render line each time I imported an audio track.
Also I read online that you don't need MPEG streamclip to convert the iPhone footage - FCP can do it from the Media Manager.
So I changed my workflow a bit.

Import iPhone clips into FCP.
Use media manager to recompress/change codec to ProRes 422 LT 1920x1080 at 30p.
(The only thing I find strange is after having converted the clip, and looking at the settings, it says 29,79 - not 30)
I find that strange because the codec clearly says 30p. Anyone has clarity on that?

Anyway, the big mistake I made was to simultaneously record the audio in Ableton, at 44.1...
I did a test recording at 48 (same as the ProRes default) import the audio, and boom, no render line at all...

I also wonder why would one use MPEG streamclip, when FCP does a good job too?


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Nick Meyers
Re: Sync drift on imported audio
on Sep 1, 2016 at 11:09:13 pm

"I also wonder why would one use MPEG streamclip, when FCP does a good job too?"

there are lots of ways to recompress footage.
MPeg Stream Clip and Compressor give you more options, plus Compressor can, if set up properly, access more processing power from your computer/s, and work very quickly.

using FCP theres no way to simply by-pass the frame-rate , or to use the existing one,
you may permanently damage your clips by doing this.
i woud recommend AGAINST using FCP for this.


(The only thing I find strange is after having converted the clip, and looking at the settings, it says 29,79 - not 30)
I find that strange because the codec clearly says 30p. Anyone has clarity on that?

2 possibilities:

1. does it say 29.79 or 29.97?
29.7 is a common frame rate, and maybe your iphone is showing at that.
30p is probably a shorthand way to saying 29.97.

I'm no export here, as i live in PAL land, and don't have to deal with such frame-rates,
but i know that 24p is shorthand for 23.976


2. if it really is 29.79, then i guess it's a by-product of the weird frame rate iPhone shoots at.
you're probably best running those clips though CinemaTools to conform their frame rates back to a true 30fps.
if you've only got a few clips, just drag them onto ht CinemaTOols icon, and when they open, click the "Conform" button lower right.
if there are lots, then open CT. When it asks what you want to do, hit continue, then escape from the Open Database window,
then use the Batch Conform command from under the File Menu,
navigate to your clips and click on any one.

and in future get one of those apps that makes iPhone shot at a more respectable frame-rate!



nick


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John Rofrano
Re: Sync drift on imported audio
on Sep 4, 2016 at 4:52:42 am
Last Edited By John Rofrano on Sep 4, 2016 at 5:11:16 am

You might consider getting FiLMiC Pro for your iPhone. It will record at a constant frame rate and give you much more control over frame rates, bit rates, resolution, etc. This is actually what they use to shoot some of those iPhone commercials. If you have an Apple Watch (like I do) you can use it as a remote viewfinder and start and stop the recording from your watch. So if you are recording tutorials of yourself this should give you complete remote control of your iPhone recordings.

As you have figured out, audio for video is usually 48K/16-bit. Always record your audio at that bit rate.

Finally, you should expect some drift over time regardless of what you do. Two separate recording devices have two separate clock crystals and unless they are gen-locked to an external world clock source, you will always get drift after some time. How much time depends on how different the crystals are but seeing drift after 10 - 15 minutes is normal.

~jr

http://www.johnrofrano.com
http://www.vasstsoftware.com



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