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When video drivers lie

A couple of days ago, I was running one of my side projects that uses several of VirtualDub's core libraries, including the display code, and discovered to my annoyance that the full-screen mode was not working properly. The program displays video at lock-step 60 fps, so it's sensitive to timing and the glitches are very apparent when it misses frames. This was a particularly bad case, though, because it was missing very badly -- enough that the audio was breaking up -- and this was on a CPU that was about 50% faster than the previous one I had, which ran this program perfectly. The windowed mode wasn't syncing very well either, but that's understandable given the non-exclusive nature of windowed mode, and so clearly the speed of the program core wasn't the problem.

I eventually tracked it down to something odd in the NVIDIA driver for my Quadro NVS 140M. For some strange reason, the driver was lying to me and actually running the full-screen display at 50Hz, but lying to my program and saying it was 60Hz, which of course totally trashed the program timing. I didn't notice because I'm on an LCD, which doesn't show a 50Hz refresh rate by flickering like a CRT would. The desktop was also suffering from the same problem, which was even more bizarre. It turned out that the culprit in the windowed case was a setting in the NVIDIA control panel called "Extend battery charge by using less power for the display"; clearing that restored the desktop at least to 60Hz and gave me clean frame sync again. I still can't figure out how to get the full screen mode back to 60Hz, though. The DirectX APIs are all reporting only 60Hz being available for refresh rate on all available display modes, but every single one of them is actually 50Hz. I tried turning off all the PowerMizer options, but to no avail. How annoying.

Anyway, if you're getting a lot of tearing when you try to play video on your laptop, it may not be the video player. The video driver may be fooling with it a bit.


This blog was originally open for comments when this entry was first posted, but was later closed and then removed due to spam and after a migration away from the original blog software. Unfortunately, it would have been a lot of work to reformat the comments to republish them. The author thanks everyone who posted comments and added to the discussion.