[Techtalk] FPD vs CRT adjustment

H. Peter Anvin hpa at zytor.com
Thu Jan 3 01:39:15 UTC 2008

Kelly Jones wrote:
> I recently switched from a CRT to an FPD, and noticed:
>  % "Afterimages": I have dark text on a light background. I see "ghost
>  characters" where-ever there's a space, and especially at the end of a
>  line. So "N" on a line by itself looks like "N [faint image of N]" to
>  me.
>  % The OSD "phase" setting on my FPD controls how "sharp" my
>  characters look. But, when the characters look really good and
>  crisp/sharp, the screen flickers. When I make the characters fuzzier,
>  the flickering goes away.
> Is this normal, or have I gotten stuck w/ a bad FPD?
> Are there any tests I can run to confirm it's a bad FPD, and not just me?

First of all, if you can switch from using VGA to using DVI, do so.  All 
the artifacts you're describing above is due to digital -> analog -> 
digital conversion.

If you can't do that:

- make sure you drive the output at 60 Hz, not faster.  You can even go 
slower with an LCD, but most of them are optimized for 60 Hz.

- replace the VGA cable (and make sure it's properly secured at both 
ends.)  The ghosting you're seeing is reflections on the cable.  Do not 
use extension cables; VGA connectors are not impedance-matched, and will 
cause reflections (they were expected to carry a 25 MHz signal, not the 
100+ MHz of modern screens.)


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