You guys are on to something. I had a chance to test your audio theory and it works. More time and tests need to expand the use cases, but reducing audio output to 2 channels over HDMI immediately cleared up the judder on my go-to test sequence (a long, slow, regular pan). I tried a few different files that I'd tried before and sure engouh, gone there, too. As kro mentioned, I saw the odd judder when subs hadn't been on in a while, but so far I've seen nothing like the judder before. Now to find out what's causing it...Great job narrowing it down guys.
Just to add another voice to the pile: I'm experiencing exactly what krobolbus describes, particularly the part about it not showing in logs at all and being sporadic in nature. I keep using the same sequence, a very regular pan, to test, and sometimes it doesn't judder at all. Frame matching to video is on, matching to display is off (was never needed on Pi). Files can be 24 flat or 23.976, doesn't matter, there's irregular judder.
I'll try playing with the audio settings kro, thanks for that idea.
Anyone else having playback issues with the pi4 (1080p, h264/h265, adjust refresh rate on start/stop)?
Some files play without problem, but i have also encountered dropped frames every few seconds in some files. It is quite noticable. I started watching my h264 encode of Gladiator and it stutters every 2-3 seconds (so it drops some frames)
I also get the infamous "drop frames before first appearance of srt subtitles" in a lot of files. This happens every time a new block of subtitles appears after some time without dialog.
Are that known issues?
I don't know if it's a known issue, but I came here for the same reason. Like you say, it's only in some files, and I can't find the constant. h264/h265, 24/23.98, some fluid, some jerky. I'll try to produce some logs tonight, otherwise the good people working on this have nothing to go on. Just wanted to let you know you're not the only one seeing this.