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Sync drift Issues when all seems right. Any ideas?

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Tangier Clarke
Sync drift Issues when all seems right. Any ideas?
on Dec 5, 2017 at 7:13:07 pm

Folks I have footage and audio from two Canon C100s and a Sound Devices recorder.

The cameras sync together just fine when trying to make a multicam just from the cameras.
The audio drifts out of sync increasingly over time when syncing the audio to the two cameras.
Footage is 23.98 with 48khz scratch audio
Sound Devices audio is 48Khz (also shows that in Wave Agent)

We recently thought it was one camera causing issue; being that pins on the C100 handle were broken and we had it repaired. After the repair we're still having sync drift problems.

We're using FCP X 10.3.4. Any ideas?


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Joe Marler
Re: Sync drift Issues when all seems right. Any ideas?
on Dec 5, 2017 at 9:20:10 pm

FCPX simply syncs at one matching point in the waveforms, it does not do continuous drift correction like Plural Eyes (which I think you have but had some problems with it).

This means the only thing maintaining sync over a long take is the inherent timebase stability in each camera. This implies your two C100s have a low drift rate, at least with respect to each other, and the Sound Devices field recorder (for whatever reason) does not.

The root source of the timebase is an inexpensive quartz oscillator, just like a cheap quartz wristwatch. Oscillators don't have perfect frequency stability, which is why two quartz watches will gradually drift apart. Actual surveys show some quartz watches drift by 2 sec per day (much worse than the spec), so two watches with opposite drift could move apart at 4 sec per day. For two audio recorders this equates to 100 milliseconds per hour, easily enough to cause a lip sync issue on longer programs.

It is possible to make precision temperature-controlled crystal oscillators that have low drift, or use GPS-corrected timebases, but most cameras and audio recorders don't use those. That said, I've seen inexpensive field recorders with a low drift rate. Maybe most of them are fairly stable but older or sick units develop drift.

I'd suggest testing your Sound Devices recorder individually vs each C100 or vs other cameras to see if it's the "odd man out".

Sound Devices makes good recorders but from a FCPX standpoint I actually prefer the Zoom F4 and F8 because they have iXML and produce a single polyphonic .wav file. This makes track identification easier in the editor. I don't know why so few field recorders support iXML. This video shows the benefit:







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Tangier Clarke
Re: Sync drift Issues when all seems right. Any ideas?
on Dec 5, 2017 at 11:10:07 pm

Joe thanks for the reply. I did test against each camera separately and the audio still had sync drift. I did have to use PluralEyes to remedy this simply because it has sync drift correction. It worked about 85% of the time. Essentially I had to locate in the Finder each matching angle and audio, drag that into PluralEyes, sync, then send to FCP X, repeat. Not fun, but it helped. I am not a huge fan of PluralEyes, but it does come in handy some time. I think perhaps my expectations of any app to be five multiples audio clips and audio and generate multicam/synchronized clips separately (rather than a synced timeline of clips) in the browser or bin or media pool is too heft a request; perhaps on the app memory side of things I am not sure.

One of these days I'll get around to using Tentacle Sync devices

It would definitely be a nice feature for FCP X to accommodate sync drift in the application. I too like the F8 for the same features you mentioned.

Thanks again.


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Tangier Clarke
Re: Sync drift Issues when all seems right. Any ideas?
on Dec 6, 2017 at 11:25:31 pm

As an aside, something that I just noticed: The syncdrift-corrected audio from PluralEyes boiled all of the channels from the original audio file into one. Definitely not something I'd want it to do. So now I have correctly synced multicam clips but without the character, MixL, MixR, and boom channels.


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