Matrox G200 and the SyncMaster 730B at 800x600
The Matrox G200 video card came out before DVI and when my nice looking 19" Sony Trinitron monitor died I went for a 17" LCD. I'm running Linux and everything looked like it was working just fine until I switched into the 800x600 resolution mode. The screen would look fine, then it would go to black for a second or so, then it would look fine for a while. Normally I'm at the native 1280x1024 but occationally I find a reason to switch down to a smaller resolution.

X windows has the ability to take a mode line. It used to be that you had to pretty much define them yourself, now they have an internal list that they try by default, but in my case it was failing pretty miserably, but only for one mode. So I got out my calculator and came up with a mode line that worked. Just put it in the Monitor section on one line and change all the references from 800x600 to 800x600x60 in the Screen section.

ModeLine "800x600x60" 41.02 800 840 968 1056 600 610 615 644 +hsync +vsync

The original was the following, which didn't work for me.
"800x600" 40.00 800 840 968 1056 600 601 605 628 +hsync +vsync

Basically I increased the vertical blanking period and increased the dot clock to keep the frame rate at close to 60Hz. The math to figure out the horizontal sync is 41.02*1000/1056 = 38.845 kHz and the vertical is 41.02*1000/1056*1000/644 = 60.32 in Hz. The on screen display of the monitor lists 63.9kHz and 60Hz.

Interestingly enough another computer with the same LCD had the same original mode line but with a 39.97 Hz dot clock and it didn' thave any problems. The graphics card in that system is a SiS 315 PRO.


Written by David Fries <david@fries.net>

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