With what app do you play the video ? Kodi ? VLC ? MXplayer ?Just downloaded samples from gsmarena iPhone XS review page, and test it.
Minix do plays smooth 4k 30fps sample but not the 4k 60fps sample which was played with many stops (motion with judder).
So I think there is a problem.
No I didn't try that. I tried the file that you can directly download (4k@60fps 148mb 11s) from the same page.
Also tried the sample 4K, Quad-Full-HD (3840x2160) 60 fps
you can download with torrent from Big Buck Bunny short film page
http://bbb3d.renderfarming.net/download.html
I had the same results.
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The file I mentioned, I think it is a video taken with an iPhone. It is a .mov file which you can download it from this link and play it if you want.
Of course the Big Buck Bunny film wasn't taken with iPhone.
I think that the problem with iPhone 4k@60fps video is they have very high bitrate over 100mb/s.
those files at 4kmedia.org have bitrate around 50 to 80 mb/s and 59.94fps. Those files play fine with u9.