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I have a dual-screen system running on CentOS Linux using two industrial grade LCD touch screens. I currently have the following line set in my xorg.conf under the "Display" subsection of the Screen section:

Modes "1280x1024_60.00"

The Xorg.log also has the following:

(II) IEGD(1): Keeping user defined mode: 1280x1024_60.00

This all seems well and good, but I recently noticed in the screen's menu that it is running at 1280x1024 at 75Hz.

I have yet to dig up the documentation on the touch screen to work out if 75Hz is in fact its minimum refresh rate, but I was wondering - does this cause an issue? All signs point to Xorg and the graphics driver considering the refresh rate of the screen to be 60Hz. What happens if there is a discrepancy between the graphics driver's assumed refresh rate and the screen's actual refresh rate?

I can find plenty of stuff online that talks about differences between video game FPS and screen refresh rates. Does that apply in this instance too (i.e. is the effect of app : screen ratio the same as driver : screen ratio) ?

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up vote 1 down vote accepted

Just because Xorg keeps a user-supplied modeline doesn't mean that it will necessarily be used. If there is a better fit that is not overridden, Xorg will use it instead.

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I wondered that myself. Is there some string I can look for in the Xorg log that will tell me which mode it chooses? –  Bringer128 Sep 28 '11 at 5:15
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