Re: Monitor vertical refresh rate setting in X.

[Stephen Powell <zlinuxman@wowway.com>]

On Mon, 22 Nov 2010 12:14:45 -0500 (EST), Sthu Deus wrote:
> 
> I can not set a desired V-frequency for a Samsung CRT monitor (an old
> one). - As far as I understand from its own menu, it supports
> resolution 800x600@85 Hz, then 1024x768@85 Hz and 1024x768@70 Hz.
> 
> I specify in xorg.conf to use:
> 
> resolution 1024x768
> VertRefresh 85.0
> 
> then X-server uses 800x600@85 Hz mode,
> 
> if
> 
> resolution 1024x768
> VertRefresh 70.0-85.0
> 
> then X-server uses 1024x768@70 Hz mode.
> 
> How I can make it using 1024x768@85 Hz mode?

Usually, in situations like this, the limiting factor is the maximum
pixel clock rate of the video card.  Assuming a non-interlaced video
mode is being used, the general formulae which govern trade-offs are
as follows:

   pixel_clock_rate / horizontal_frame_length = horizontal_refresh_rate
   horizontal_refresh_rate / vertical_frame_length = vertical_refresh_rate

Note that I used the variables horizontal_frame_length and
vertical_frame_length, not horizontal_resolution and vertical_resolution.
They are not the same.  The former include the sync pulses and guard
times, the latter do not.  But in general, as horizontal_resolution
increases, so does horizontal_frame_length, and as vertical_resolution
increases, so does vertical_frame_length.  Sometimes, a given video
card just doesn't have enough bandwidth (maximum pixel clock rate)
to drive a monitor at its maximum vertical refresh rate at its
maximum resolution.  If you will post your /etc/X11/xorg.conf file
and the resulting /var/log/Xorg.0.log file, I will take a look at it
for you to see if this is the case.

-- 
  .''`.     Stephen Powell    
 : :'  :
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