Video slowing down, then speeding up (What is my bottleneck?)

AM Academy

New Member
Hi everyone.
We've somewhat recently started doing short streams and recording videos. However, all my knowledge on this stuff is based on streaming 8 years ago so it's all a bit scuffed for the time being I guess.
Now we have a Logitech Rally camera as a video input. This camera is theoretically capable of a 4k resolution. But when I have the camera set up at that resolution, the video "lags". It slows down and then speeds up to catch up, if that makes sense. Here is a video we did recently, where this happened a number of times: I have tried a number of different settings, but haven't been able to identify the bottleneck. CPU and GPU both seem fine. The camera is connected using a USB 3 cable to a USB 3 port so that shouldn't necessarily be the issue either.
We don't stream in 4k, so I thought I could just set the camera to 1920x1080 - but for some reason that massively drops the quality. The camera then looks more like 720p. So the downscaling of the camera seems to not be as good. I would rather keep it at 4k because then the 1920x1080 stream looks sharp. It just has these lags which are really weird.

Basically the question is: What is my bottleneck? What is causing this slowdown and speedup? Is it the cable? My settings? The CPU/GPU?
I have attached the log file from that same video I linked. I will edit later with some more PC specs and maybe a task manager screenshot during streaming.
Any help will be greatly appreciated!

PS: The lags keep happening in the Preview WIndow even when I am not recording or streaming. Makes me think it's something with the camera connection? But I just can't be sure. We've added a fairly high-end USB card to try and eliminate that possibility, but it seems to not have done any good.


  • 2021-06-11 08-02-11.txt
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AM Academy

New Member
Computer specs:
Processor: Intel® Xeon QuadCore E3-1270 v5 3,60 GHz
SSD Hard drive
GPU: nVidia Quadro M4000 8,0 GB


Active Member
Can you provide timestamps in the yt-video please, when this happens?
Just to mention: With the Logitech Rally cam i once had a sobering effect against its price tag: Sometimes it seems to have a very low framerate in reality. Especially for going with 4k with that camera it may seem usual. You go 60fps with the stream, but when the obs rendering engine needs the next frame while the logitech driver didn't have it (either due to the usb-connection or slow camera electronics) its going to use the last one, again. This is one reason for going choppy sequence. So - possibly (i may be wrong on this) - with this camera type you can't guarantee a fixed, steady framerate... (These PTZ cameras aren't developed with a camcorder in mind.)

Just a try: You are working with a screen in the back. But: What happens with the rallys framerate if you push the brightness in the room far out? Try at very high luminance to see if the behaviour of the camera changes..


Active Member
Aaah, i found a point where this happened at 5:10 ff., right?
Seems the camera (or its transmission electronics) is relaxing/sleeping a moment, isn't it?

So please forget what i wrote above about the brightness/luminance thing. That doesn't fit the issue.

Can you fix a readout framerate in the DShow devices' properties?
Did you tried 1080p30 instead of p60 to slow down all demands?