iPhone 12 ProMax slowmo jitter

I just got some high framerate clips with the new iPhone and something weird is happening with some of them, but not all. There couldn't have been anything different in camera ,as all of them were shot on the native camera app so all slowmotion was shot with the basic slowmotion setting.

Most regular speed and slowmo clips have no issues, but many of them this cubic articacting and jitter happening every few seconds.

Am I missing a something basic?

https://drive.google.com/file/d/1MzXR6a ... sp=sharing
here is a sample

Most smartphones don't hold a constant frame rate, which is likely the issue.

A sample of a source file that shows the error would be better

But yes, it looks like a macroblock/reference error in the Resolve H264 encoder.
So debugging:
Assumption: It's the delivery that's the problem
Does it happen with Cineform delivery? Does it happen with software and hardware H264 encoding?
Assumption: It's the source file that the problem
Does it happen if you convert to cineform first?

The staff will ask you to make a diagnostics log so do that as well (Help -> create diagnostics log) and read the sticky help post.

Can you try this?
Key Frames : Every 1
and deselect "Frame reordering"

For the blocky artifacts, I almost always got them when the Key frames was on "Automatic".
Since then, I put it on 1 (=0), and I never encountered this problem (when I had this problem I was exporting a DNxHR file and used Handbrake to convert it to a mp4 file)

I read this about Frame Reordering, maybe it's related when I see your footage going back and forth :
Frame reordering is the concept of allowing frames to be decompressed in a different order than their display order. For almost all cases, leave this box checked for H.264 encoding. The only time you would uncheck this box is if you are creating an H.264 movie that needs to be played back by an application that does not understand frame reordering

Try to export one video with just the Key Frames at 1 (and Frame reordering untouched) to see if it changes something, and a second one with both changed.

Hey guys. Thanks for the help so far.

Maybe I should have gotten filmic pro as soon as I got this iPhone. The issue is on the original. You don't see the problem if you play it on the phone, but I tried copying the file over again and it still plays with the issue on my PC, on the windows player and in Davinci, before and after render. I tried the render settings suggested without the auto keyframes, and instead every frame, with frame reordering unchecked, but no dice.

What is weird is that only a few clips have this issue, and what is even more weird is that you can't see the issue on the phone... is there a kosher way of transfering files to a PC? I tired using the stupid "transfer photos and videos" thing that pops up when I plugged it in but it would freeze like ALL the DANG time, so I tried copying the files over, but if I tired more than a few at a time it would get hung up... took me forever to transfer everything. But it would be so weird that this would come about from copying files over... still, worth asking.

Is it possible for you to share one of your original file you edited that has this problem after your rendering?

This is what I wrote to another iPhone user:
Unfortunately, even with FiLMiC Pro you can't be sure to get constant frame rates on an iPhone.
My suggestion: use FCP-X. Apple is selling many more telephones than computers these days, and they seem to take care that it works.
DaVinci Resolve is coming from totally different background, the world of professional film and TV. In that world, a constant frame rate and clean timecode is mandatory. Actually, DR is handling phone footage (or screen recordings, for that matter) to some degree. But you can never be sure, it'll be a game of chance.
If you want to stay on a PC with DR, use ffmpeg (or any of the free GUIs for it) and transcode the footage to an intermediate codec, like Cineform, DNxHR or ProRes in a fixed frame rate. That will also make it run smoother on a PC in the free version.