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25 Jan 2021
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Video and audio from external source desyncing over time? 14 Sep 2021 12:45 #116246

Hello everyone!

I am quite sorry if this isn't the correct place to post, no problem whatsoever if a moderator changes its place!

I am currently having a problem when I import to Final Cut Pro X a video source from, let's say my iPhone 12, shot with Filmic Pro, and the audio from another source, in this example a H2n Zoom audio recorder. The beginning of the video is correctly synced, with the help of a clap, but over time, the audio and video start desyncing, and I have trouble understanding why. Could it be something related with the frequency of my video shoot (29.97 f/s)?

I am wondering what can I do to sync video and audio correctly without having, for each clip of the video, to move the audio track until it is synced again.

Thanks a lot for your help!

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Video and audio from external source desyncing over time? 14 Sep 2021 17:22 #116247

  • joema
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I don't think an iOS device -- even using Filmic Pro -- can shoot constant frame rate material. It is variable frame rate that might mostly stay at (say) 30 fps, but it's not guaranteed. Thus it would not always stay persistently in sync with other cameras or audio recorders.

But in your case it's not two cameras it's an iOS camera plus external audio. FCP does audio sync at a single location. I think the 3rd-party sync tool Plural Eyes can do continuous drift correction, but I haven't tested that between iOS A/V and external audio. www.maxon.net/en/red-giant-complete/pluraleyes

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Video and audio from external source desyncing over time? 14 Sep 2021 17:33 #116248

  • VTC
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My vote goes to Pluraleyes for correcting drift sync probs or even syncs in general. I will have at times 3 other non-camera audio sources that can't be ganged using timecode alone because the H4n or H6n don't dance that way. PE hasn't failed me yet.

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Video and audio from external source desyncing over time? 15 Sep 2021 00:15 #116254

  • Ben Balser
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I shoot with my iPhone all the time and it syncs audio with my GH5 and other cameras just fine. But then, it's all 48kHz audio. I keep reading about VFR with iPhones but have never experienced the problem.

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Video and audio from external source desyncing over time? 15 Sep 2021 05:40 #116255

… keep reading about VFR with iPhones but have never experienced the problem.

Ben,
can you share a little more in depth info about your settings?
Do you use 3rd party tools as FILMIC PRO to 'cement' the frame rate?
'Even'/straight frame-rates (24/30), not these 'comma' 29.97 NTSC-thingie? ;)
Low-light situations? (an Iphone has no aperture, therefor adjusts exposure per frame rate)?

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Video and audio from external source desyncing over time? 15 Sep 2021 12:45 #116258

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4K 30 with both the Apple camera app and with Filmic Pro, I use each about 50/50. Nothing fancy or special. I use second highest quality setting in Filmic, can’t remember off-hand what they call that. I don’t shoot in very low light. Nothing good ever comes of that. Through my last three iPhones, I’ve never had the issues I read about others having. Curious what makes the difference. Would like to continue avoiding those.

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Video and audio from external source desyncing over time? 15 Sep 2021 16:24 #116264

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To my knowledge even Filmic Pro cannot "lock in" a constant frame rate (CFR) on iPhone, it is always variable frame rate (VFR). If you shoot in good light it will maintain the frame rate but in lower light it will drop the frame rate to maintain exposure. There apparently isn't even an API to request CFR.

That was always the situation previously. I don't know about the new iPhone 13 Pro. A guaranteed constant frame rate regardless of exposure would obviously be important for serious filmmakers. It wouldn't make sense to put ProRes video on the iPhone 13 then not give a constant frame rate option.

That assumes the sync drift is somehow related to VFR. Another possibility is the usual suspects, such as a drifting time base between the two devices.

@Hikenkami: It would help to know how rapidly it drifts out of sync. IOW after 60 sec, or 5 min, how far out is it? E.g, a drift rate of 99.9% is the difference between 29.97 and 30.0 fps, so in some cases rate conforming might cause that. A drift rate of 91.89% is the difference between 44.1 Khz and 48 Khz. Other drift rates have different implications. One way to check is this procedure:

(1) Decide roughly how many sec is needed to show the problem: 60 sec? 5 min? 10 min?

(2) Set a marker at that point (M key) to easily find it

(3) Expand the audio waveforms vertically and horizontally to improve visibility

(4) Enable subframe audio with View>Zoom to Samples

(5) Press & hold the R key then drag a range over the separate *audio* lane (not in the primary storyline) from the waveform peak position to the corresponding position of the drifted waveform in the main storyline. The length by default is in frames and fractions of a frame, e.g, 1.43 means 1 and 43/100ths of a frame.

Tell us (1) The project frame rate as shown in the project inspector (2) the clip frame rate as shown in the clip inspector, (3) How far out it is in frames and fractions of a frame (4) The number of sec from the "in sync" point. E.g, if started in sync then by 5 min it was out by 2.43 frames, tell us that.

Then repeat the above for another region further down the timeline to verify it is drifting at the same rate each time vs an unpredictable rate.

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