[ltp] 1600 gray vertical bars on external monitor at 1600x1200

[Ben Pearre]

Hi!

R40 (2897gwu), latest Debian unstable, ATI Radeon Mobility 7500 (Xorg
radeon driver)... more details below.

I'm trying to use an external monitor--a Dell 2000FP.

At 1280x1024 and lower, everything is great.

At 1600x1200 (native resolution of my external monitor, being driven,
in this case, by the Thinkpad D-sub analog out), I get a weird pattern
of gray vertical bars (1600 of them, I suspect, although I have not
counted) over the whole monitor.  It's most visible on a solid
background (especially white or black), but it's omnipresent.

Even weirder, the monitor's built-in zoom function magnifies these
lines.  xmag, however, does not.  So the signal is probably coming
from the DAC.  However, I really hope against reason that I'm missing
something and there's a software fix.

The monitor is ok: driving it at lower resolution (with or without
internal scaling) is fine, as is driving at 1600x1200 with a friend's
T41 (same video cable), or the DVI connection from my workstation.

I'm using this mode (from /var/log/Xorg.0.log):
(**) RADEON(0): *Default mode "1600x1200": 162.0 MHz, 75.0 kHz, 60.0 Hz
(II) RADEON(0): Modeline "1600x1200"  162.00  1600 1664 1856 2160 1200 1201 1204 1250 +hsync +vsync

I've put my xorg.conf, my Xorg.0.log, and photos of the offending
streaks here:

http://ml.cs.colorado.edu/~ben/monitor

The first picture (2.jpg) shows a closeup of the screen.  It's hard to
tell what the problem is, but note that the horizontal bar in the
fraction seems to have every green subpixel turned on.

The second picture (6.jpg) is of the monitor's zoom function.  Of
course, the subpixels are not zoomed, but the black lines are--they
must be coming from the computer.  Now there are about 19 of them
across the whole image.  No, that's not a Moire pattern!

The third picture (9.jpg) is a closeup of the OSD, to demonstrate what
this _should_ look like.  The black is black, the white is white, and
there are no gray lines.  It's hard to tell from the closeup, but
there's a world of difference from normal viewing position between
this and the monitor as driven.

Is this just due to a crappy DAC?  Is there any software fix?

Many thanks for reading.  Any clues would be immensely appreciated!

Cheers,
-Ben

--
Ben Pearre     http://ml.cs.colorado.edu/~ben      PGP: CFDA6CDA
Don't let Bush read your email!             http://www.gnupg.org