QuickTime Player slows down during video playback

coder4life wrote:

What if the video is 60fps or 120fps on a Dell monitor?

The monitor should not matter.

QuickTime Player (in Sequoia, for example) plays middle part of high frame rate movies (seems to be ≈85 fps or more) in slow-motion.

That can be prevented by adding or editing FullFrameRatePlaybackIntent tag to value 1 with:

exiftool -m -P -overwrite_original -ext mov -Keys:FullFrameRatePlaybackIntent=1 movie.mov

Or by playing such movie with Sequoia QuickLook or some app like IINA that does not honor that tag. Then the whole high frame rate movie is played in normal speed (if the device can handle that).

That tag can be set back to the "middle part slow-motion" option with:

exiftool -m -P -overwrite_original -ext mov -Keys:FullFrameRatePlaybackIntent=0 movie.mov

Or deleting that tag with:

exiftool -m -P -overwrite_original -ext mov -Keys:FullFrameRatePlaybackIntent= movie.mov

* Sequoia Photos.app plays the middle part of high frame rate movies in slow motion no matter what.

So for Photos those movies must be re-encoded at a slower frame rate if slow-motion is undesirable. So export such movie as original out from Photos and re-encode with some 3rd party app. 30 fps is a good starting point because it is an even integer of 240.

For example with ffmpeg H.265 defaults and '-r 30' for 30 fps with something like (this preserves metadata date but other metadata must be copied from the original which is best done with exiftool -- sadly ffmpeg versions 4-8 do not properly handle movie metadata):

ffmpeg -i input.mov -c:v libx265 -r 30 -crf 28 -preset medium -timecode 00:00:00:00 -tag:v hvc1 -map_metadata 0 -c:a copy output.mp4

Handbrake does about the same with its default setting (maybe force 30 fps via Video > Framerate > 30, Constant framerate).

Could it be that this video was recorded with the slo-mo option on iPhone (intentionally or accidentally)? Then the video would be recorded at high frame-rate, so that a section can be slowed down after recording. A shared clip may use a default slow-down section, at 30 fps throughout, while the original export transfer would be editable for speed in whole or in part, then exportable with those transitions.

The secondary timeline in the editable version, when opened in QuickTime Player, would have 2 moveable markers for up to two speed transitions, to create three sections: regular speed, slow-motion, and back to regular speed again.

Record a slow-motion video - iPhone user guide - Apple Support

iOS/iPadOS 18 introduced QuickTime FullFrameRatePlaybackIntent tag:

"key that represents whether this movie should play at full frame rate

Some apps play movies recorded at frame rates of 120fps or higher in slow motion. If your app records high-frame-rate movies, you can add this movie-level metadata to indicate whether the movie intends to play at the full frame rate (1) or at a slow motion rate (0). Apps that play movies may use this metadata, when present, to guide their behavior."

quickTimeMetadataKeyFullFrameRatePlaybackIntent | Apple Developer Documentation

macOS 15 Sequoia QuickTime Player plays middle part of high frame rate movies (seems to be ≈85 fps or more) in slow-motion without that tag (earlier macOS like Ventura seem to behave the same).

High frame rate playback option in iOS/iP… - Apple Community