Odd Frame Rate 29.68 fps in Live Photos

Question

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.

Answer 1

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.