[geeks] Samsung 204B LCD bug, and a fix for it

[Charles Shannon Hendrix]

I have a Samsung 204B monitor, and it works great. However, when I
changed video cards, I started having two annoying problems: flickering
pixels on black areas, and screen blanking (screen goes off and back on,
randomly).

If you have this problem and just want to jump to the solution, search
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I thought this was a bad video card, but it turned out to be an issue
with some LCDs not having enough bandwidth for a full DVI signal.

Samsung, nVidia, and EVGA were universally unhelpful and all three said
it was not possible to diagnose this very unusual problem.

Those two symptoms indicate an LCD is unable to handle the pixel clock
in the DVI signal. The 204B has this problem, and so do a few others
here and there. 

Samsung made the 204B with no wiggle room in the signal, and some of
them just can't handle it. Other LCDs here and there have the problem
too.

The other thing is that video cards default to sending older CRT based
signals even in DVI mode. It wasn't until 2003 that the industry
approved the CVT signal standard, and tightened standards on DVI timing.
It took awhile for everything to comply, and some things still haven't.

1600x1200 at 32 bits is 162MHz of DVI bandwith, very close to the
maximum. Add the blanking interval and you are hitting the wall, which
some LCDs won't tolerate. The Samsung 204B is very close to the edge.
Some of them can handle it, some of them cannot.

It's partly a quality control issue, and also partly the lack of strong
standards on DVI timing.

The solution is to tell your video card to stop sending the blanking
intervals. It will greatly reduce the bandwidth you need, eliminating
the problem, and helping fix other issues related to bandwidth.

I don't really know for sure, but it seems like it might even be a good
idea to do this even if you don't have any problems. It seems that
reducing the bandwidth you need for your chosen resolution might be
helpful in other ways.

-------------------------

Here's how you do it in X11's Xorg server, and Windows XP:

*** UNIX/X11 ***

Get the crt.c program and compile it.  You can get it here:

	http://www.uruk.org/~erich/projects/cvt/

This program will generate a modeline that forces "reduced blanking"
signals, eliminating the CRT gun retraces, and reducing overall
bandwidth needed.

For example, I use this line now in my xorg.conf file:

	# 1600x1200 @ 60.00 Hz Reduced Blank (CVT)
	#   field rate 59.92 Hz; hsync: 74.01 kHz; pclk: 130.25 MHz
    Modeline "1600x1200_60.00_rb"  130.25  1600 1648 1680 1760 1200 1203 120 7 1235  +HSync -Vsync

Also, add this line to the Device section:

	Option "ExactModeTimingsDVI" "yes"

This causes X to generate reduced blanking signals and reduces signal
bandwidth by quite a bit.  In my case, 30MHz.

For Windows, I only know how to fix this problem with the newer nVidia
drivers.  For older drivers or ATI video cards, I have no idea, but I'm
sure it can be done.

You have to use the new nVidia control panel, or at least I did.

You want this:

	Control->Display->Manage custom timings

In there you should see a way to create your own timings.  Choose the
resolution you want and be sure to select reduced blanking and whatever
else you need.

Save your new timings and your video card should have reduced the
bandwidth it is using, but you'll get the same display.

Use your LCD's signal information display to see if it has worked. You
should see a reduction in horizontal signal, and sometimes you'll see
the refresh rate drop a bit. Most LCDs can't show fractions of Hz so it
will probably still say 60Hz, or whatever yours is by default.

** UPDATE **

Samsung is admitting to this problem now. Their "real" support have
discovered a bad part in the video decoder board. Some of them evidently
can't handle a full signal. They've also said that firmware updates can
fix some LCDs. Unfortunately, I can't find any information on how to
update firmeware. Maybe Samsung has to do it.

Samsung will swap your LCD for free. They ship you a new one, and you
ship the old one back. However, this may not solve your problem as some
people report newer ones have the issue too.

>From what I've read, you want a Samsung LCD made after January 2007,
which appears to be when they fixed this problem. 



-- 
shannon           | The trade of governing has always been monopolized by the
                  | most ignorant and the most rascally individuals of
                  | mankind.