Capture device playback dropped frames & clock/timestamp sync errors

[rarbee]

I find VLC on Mac cannot play video+audio from a Capture Device properly. I do File - Open Capture Device, and select my hdmi capture card's Video and Audio devices. The video will play but with lots of dropped frames (very choppy video). The audio is ok.

- Here are messages from the VLC log:
clock gap, unexpected stream discontinuity
feeding synchro with a new reference point trying to recover from clock gap
Timestamp conversion failed (delay 300000, buffering 0, bound 9000000)
Could not convert timestamp 2255674958149 for rawvideo

- The capture card is a UVC device playing 1080p@50fps content.

- I am using a USB 3 port and a USB 3 5Gpbs cable.

- I have an alternate player on OS X that works perfectly. I get full frame rate with perfect audio. This tells me it cannot be a hardware issue.

- I've tried two different capture cards. Same problem on both. (One Magewell, one EVGA XR1 lite). This tells me it's not specific to the card.

- The same capture cards work perfectly with VLC on Windows. This is a Mac specific issue.

- I tested both cards on a Mac Mini and a Macbook Pro, with Mojave and Catalina. Same problem on both.

- If I select only the video device in File - Open Capture Device (no audio) it plays perfectly (without sound). It seems this is a clock/timestamp issue synchronising the audio with the video.

I think VLC must have a problem playing content from AVFoundation/AVCapture.

[rarbee]

Edit: I'm using VLC 3.0.17.3 (Intel 64bit). I also tried the 3.x nightly and same problem.