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Why 29.68 fps?

Is it possible to set up a 30 fps or 29.97 fps frame rate for Live Photos?

The problem is a video part of Live Photos in iPhone X in iOS 11.3 uses quite strange variable frame rate – 29.68 fps, that is not suitable for editing presets in FCP or DaVinci Resolve. If I convert Live Photos' frame rate to 30 fps or even 25 fps, a sound slightly or noticeably drifts away from sync at some point.

P.S. Live Photos in iPhone 7 (iOS 10) work with 29.97 fps.

enter image description here

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    Blame the Americans. They invented drop-frame to compensate for their atrocious NTSC [Never Twice the Same Colour] television standard, so what ought to have been 30fps couldn't quite keep up.. Move to the light side & go with the European PAL standard, 25 fps. Solid. (yes, I'm joking, kind of ;-)) – Tetsujin Apr 29 '18 at 16:31
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    @Tetsujin NTSC's CFR 29.97 fps isn't the same as VFR 29.68 fps)) – user243793 Apr 29 '18 at 17:49
  • Just a question. Re your statement: If I convert Live Photos' frame rate to 30 fps or even 25 fps, a sound slightly or noticeably drifts away from sync at some point, can you clarify how you're converting the frame rate? In FCP? Something else? What settings? – Monomeeth May 5 '18 at 10:39
  • @Monomeeth For instance, I set up my project in FCPX as Custom format 1440x1080, 30p, Render Sequence ProRes422, Audio 48kHz. Then, I distribute 2 formats: H.264 30fps, audio 48kHz, and H.264 25fps audio 48kHz. – user243793 May 6 '18 at 20:18
  • @Andy Hmm, considering that Live Photos are only 3 secs long, I wonder whether something else is at play here? Since Live Photos were first introduced the frame rate has varied between iPhone models (e.g. on an iPhone 8 it's 29.79 fps). But this is just me thinking out aloud. If I get a chance I will try and replicate your issue. In doing so, are you joining multiple Live Photos together in your project, or...? – Monomeeth May 7 '18 at 8:11
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I'll try to answer my own question. I'm not sure this is 100% right.

I suppose frame rate's issue depends on the architecture of Live Photos itself. Total number of frames inside each Live Photo in iPhoneX is 85 frames (look at the picture above):

42frames + 1frame + 42frames = 85frames

Frame rate for all these 85 frames is supposedly 30 fps, but considering just a video part (without a HEIC file), frame rate is 29.68 fps.

Here's a math:

//let's assume that 1/85th is 0.011764, 
//truncate that floating point number to 0.011
1 / 85 = 0.011

//let's assume that video frame rate of all 85 frames is 30 fps 
//so each frame's contribution is approximately 0.33 fps
30 fps * 0.011 = 0.33 fps

//thus H.265 video portion without HEIC frame will be 29.67 fps
30 fps - 0.33 fps = 29.67 fps

Value 29.67 fps is very close to 29.68 fps.

I reckon that altogether all 85 frames of Live Photos have frame rate 30 fps, but two H.265 video portions (without HEIC part) have frame rate 29.68 fps.

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