Inconsistent audio-video synchronization with third-party video apps and variable frame rate on iPhone 15 Pro Max

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Level 1

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Inconsistent audio-video synchronization with third-party video apps and variable frame rate on iPhone 15 Pro Max

I’m using my iPhone (15 pro max, last OS) to record covers of songs. My workflow is first recording the live performance of the song (guitar covers) played using my amp over a back track which is the real track, played using Bluetooth also on the amp. Next, I try to superimpose the real track and the mic recorded performance in DaVinci Resolve.

The first frustrating thing is : the audio track is never synchronized with the video, it’s frustrating but 2 min of works and it’s done. The delay is most of the time, one or two frames.

The second thing really frustrating is that when adding the same track I used in Bluetooth for recording to DaVinci, it is impossible to synchronize. The first second is ok, and is slowly drift, it may come back synchronized, and drift again, out of synchronization at then end…..

I know that apple is using variable frame rate. It can’t be detected if the video is played alone but it is in the case I’m talking about. I did not notice that with my old 14 pro. In the preferences, I did disable this option and the problem is still there.

My questions are :

Is BlackMagic (free app) or moviePro (paid app) overrides the option and always have it on? (No option about that in BlackMagic or moviePro, just selecting a "constant" framerate)

Or

Is it a bug of iOS, that propose to change the settings and in reality, nothing happens when changing the value?

I'm at the point i'm now using my friend's S24 ultra which have no troubles, but a not as good video quality.

[Re-Titled by Moderator]

Original Title: variable frame rate always on with third party video apps even if turned off in the settings

iPhone 15 Pro Max, iOS 18

Posted on Jul 9, 2025 6:41 AM

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Answer 1

Top-ranking reply

AppleAnswerDude

Level 3

951 points

Jul 9, 2025 6:55 AM in response to yann296

Possible Problem:

iPhones (even with "Lock Camera FPS" or "Use Constant Frame Rate" toggled off) often still record in VFR, especially when the lighting or scene changes.

When this VFR footage is edited in DaVinci Resolve — which prefers Constant Frame Rate (CFR) — you get:

Slight drift (1–2 frames initially),
Then significant desync over time between mic-recorded audio and the track added later,
Even if the original video playback seems fine.

Possible Solution:

Transcode to Constant Frame Rate Before Editing

Use a tool like HandBrake to convert your iPhone footage to CFR before importing to Resolve:

In HandBrake:
- Choose a preset like “Fast 1080p30”
- Go to Video tab
Under “Frame Rate (FPS),” select:
- - Same as source
And then set “Constant Framerate” (not variable)
Export and then import that into Resolve

Why it could help: Forces the video to maintain a consistent frame rate for proper A/V sync.

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Answer 2

yann296 Author

Level 1

18 points

Jul 9, 2025 8:01 AM in response to AppleAnswerDude

Thank you for your reply.

I understand that Apple don't authorize third party apps to lock the FPS. I have to convert my file before using my editing software. This trouble is known for years and to the best of my knowledge, on my friend's S24 ultra (which is also using VFR) Blackmagic camera is able to lock the FPS using the pro settings.

I found an option in Davinci Resolve which can help : Used Optimized Media, This tells Resolve to create optimized versions of your media with a fixed frame rate... I will have a look at it.

Thanks again

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Inconsistent audio-video synchronization with third-party video apps and variable frame rate on iPhone 15 Pro Max

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