twitching image

[Alex Deucher]

On Sat, Sep 12, 2009 at 1:11 PM, Csillag
Kristof<csillag.kristof at united-consult.hu> wrote:
> 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
> here?
>
> 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?

This is likely display watermark or pll related.  Please try
xf86-video-ati from git master as that has proper watermark setup
implemented and support for fractional feedback dividers.  If you are
still having problems with master, please try:
Option "DisplayPriority" "HIGH"
in the device section of your config.

Alex