twitching image

Csillag Kristof csillag.kristof at
Sat Sep 12 10:11:28 PDT 2009

Dear all,

Now that the CustomEDID support is (mostly) in place,
I can force my card to output the proper data,
even when the monitor is connected via DVI.

According to the X log, the selected mode is correct
(ie. it is the same as the mode I get when I connect
via VGA.) The OSD menu of the monitor confirms this.

However, despite the fact the the output mode seems to be correct,
the image on the display is twitching. It's only happening
once or twice a minute, but it's still annoying.

Furthermore, the first time I have tried the CustomEDID
code, there were some random red dots scattered on the screen,
appearing and disappearing back to nothingness, really fast.
(Something like static on the TV screen, but only in red,
with fewer artifacts, scattered over the normal image.)

I have changed the DVI cable, and the red dots went away,
but the twitching has remained.

What could be the reason for this?

I know the monitor can be driven via DVI, since it works flawlessy
with my office notebook (which has an NVIDIA chip built in.)

   * * *

So here is what I know:

 - VGA connection with the radeon card works
 - DVI connection with the NVIDIA card works
 - DVI connection with the radeon card works, but with twitching.

All above cases use the same modeline

If the monitor is OK (as proven by the NVIDIA case,
and the mode is right (as proven by the NVIDIA case and the VGA case),
what can be wrong?

One thing that bothers me is that the bandwidth required by the mode
(1600x1200, 60Hz) is 161 Mhz, dangerously close to the 165 Mhz
maximum bandwidth of a single-link DVI connection.

Could it be that I am running into low-level signal transmission issues

Previously I thought that DVI belongs to the digital world, so
things are black and white: either we have a connection,
or we don't have one, with no third choice in-between.

Maybe this was a false assumption, and as we get close to the maximum
bandwidth, the analog hell creeps back?

I have run some tests, and I have weed out more than half
of my el-cheapo DVI cable collection I usually use,
and the twitching definitly got better, but it is still there
sometimes. (Strangely, the picture data seems to influence
the frequency.) The red dots disappeared completely.

I have also noticed that removing the display switch
from the settings improves the situation, but does
not rectify it completely.

   * * *

But then again, the NVIDIA card could drive the monitor properly,
using the same cables, switch and everything.

What should I think of that, then?

This is pure speculation (and I don't have an oscilloscope
to check it ouy), but could it be that the NVIDIA operates
with somewhat higher signal voltages (or sharper signals),
which get through the poor-quality cabling, where the weeker
signals from the radeon get sometimes mingled?

Is there any way I can influence the lowest level
of the signal transmission done by the card?
(Signal voltage, timings, etc.)

Do you have any advice?

Thank you:

    Kristof Csillag

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