RRe: Shuddering Video

[Keith Lofstrom]

On Monday 10 November 2003 22:58, Res wrote:
> Using the onboard video from an MX45U2 with a SiS650 chipset
> on Fedora, the GUI shudders badly, RH 7/9 did not have this issue,
> has anyone else come accross this and solved it?
                                                                                
Andy Green wrote:
> Lastly, could it be that with the refresh rate that has been selected, the
> video card is exceeding the bandwidth of your monit-KABOOM!!!

That is pretty close.  As I wrote for a previous posting today, the faster
you sweep, the more voltage you need.  The voltage is probably set by an
amplifier working off an unregulated supply, with 2x mains frequency 
ripple on the supply.  If you ask for too much voltage, the amplifier
pegs against the ripple, and the screen wobbles horizontally or vertically
(depending on which amplifier pegs first).  And before teh amplifier pegs,
it will tend to get "soft", and so the edges of the display will feather
a bit from gain variation caused by lack of enough voltage.

Check the bandwidth of your monitor; there are probably specifications on
websites somewhere.  You are looking for maximum vertical and horizontal
scan rate;  make sure those numbers are in the monitor description   
for XFree86, probably in /etc/X11/XF86Config.  For example,  my Hitachi
SuperScan Elite 751 monitor has a section reading:

Section "Monitor"
	Identifier   "Monitor 0"
	VendorName   "Monitor Vendor"
	ModelName    "htcac13"
	HorizSync    31.0 - 94.0
	VertRefresh  50.0 - 160.0
        Option       "dpms"
EndSection

This says that the monitor will work with horizontal frequencies from
31.0 to 94.0 kHz, and vertical frequencies from 50 to 160 Hz.  

If your monitor still looks funny with the proper numbers in there, you
probably want to reduce the top-end numbers until it stops looking funny.
There may be a flaw in the tables that the X configuration script is
working from, or there may be a flaw in your monitor, or the manufacturer
of your monitor is lying about its capabilities.

Sometimes there is not enough time for retrace, which happens when you
ask for too many pixels.  You might try temporarily going to a lower
resolution and see what happens. 

Keith

-- 
Keith Lofstrom           keithl at ieee.org         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs