[Novalug] screen resolution, continued

Igor Birman igor_birman@yahoo.com
Tue May 19 12:38:05 EDT 2009


Awesome, great explanation, thanks!

This is worth putting on a web page somewhere..

Igor



________________________________
From: Nino R. Pereira <pereira@speakeasy.net>
To: Gregory Maxwell <gmaxwell@gmail.com>; macoafi@gmail.com
Cc: NOVALUG <novalug@calypso.tux.org>
Sent: Tuesday, May 19, 2009 12:13:56 PM
Subject: Re: [Novalug] screen resolution, continued

Thanks to all who thought about this problem.

> > Any idea what I do wrong? If not in /etc/X11/xorg.conf, where does
> > the screen resolution come from?
> 
> xorg.conf is becoming increasingly deprecated, especially for things
> which need to be changeable on the fly like resolution.
> 
> You can use the xrandr command to control your display resolution from
> the command line. The various desktop environments include display
> control programs that use the same API to setup the display
> resolution.

The way to a solution I stumbled upon is as follows:

There are some standard synchronization frequencies for the monitors.
The basic one is the vertical refresh rate, the lowest frequency. 
You get the ones you can use with 'xrandr'. 
On http://www.debian-administration.org/articles/201
I see two principal choices, 75 Hz and 60 Hz, but on my Ubuntu 
system xrandr gives a larger table that's not so clear. On my Debian
system xrandr gives 60 Hz, but before I fixed the problem 
it gave 75 Hz.

>From the vertical refresh rate and the monitor resolution
you have to calculate the horizontal frequency. The program 'gtf' 
tells you what it is, and 'man gtf' tells you how to run it:
you give the horizontal and vertical resolutions you want, presumably,
provided that the hardware supports these. Mine are 1680x1050, so I do:

gtf 1680 1050 75

and I get the horizontal frequency, 82.20 kHz.

Now, these numbers must be put in the appropriate sections 
of /etc/X11/xorg.conf. The section "Monitor" now looks like

Section "Monitor"
        Identifier "Generic Monitor"
        Option     "DPMS"
        HorizSync   82.20
        VertRefresh 75
EndSection

In the section "Display" I have, unchanged from earlier struggles,
...
        SubSection "Display"
            Depth               24
            Modes              "1680x1050"
        EndSubSection

A reboot gives me a nicer screen, but still not the right one:
it turns out (by doing 'xrandr', or by using the GUI 
'system' > preferences > screen resolution, to be 1400x1050.

Trying the other vertical refresh rate, 60 Hz, in gtf gives
as the corresponding horizontal synchronization frequency 
65.22 kHz, lower than the 82.20 kHz that matches 75 Hz vertical.
Putting these two numbers into /etc/X11/xorg.conf, like so,

Section "Monitor"
        Identifier "Generic Monitor"
        Option     "DPMS"
        HorizSync   65.22
        VertRefresh 60
EndSection

finally gives what I want and had earlier, 
a screen with 1680x1050 resolution.

Nino


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