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

[Francois Dion]

On 5/6/07, Charles Shannon Hendrix <shannon at widomaker.com> wrote:
> 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
> for -------- in this message.
>
> 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.


If you remember from a previous exchange last year, I had a related
issue. The Quadro FX 550 card I have would detect a problem @1600 and
force 1280x1024. I ended up using a dual link DVI cable and that
worked. I still really like my 204B.

Francois