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	Sun, 29 Jun 1997 19:43:54 -0500 (EST)
From: "John S. Dyson" <toor@dyson.iquest.net>
Message-Id: <199706300043.TAA09721@dyson.iquest.net>
Subject: Re: Monitor shadows?
In-Reply-To: <19970629195452.32243@vinyl.quickweb.com> from Mark Mayo at "Jun 29, 97 07:54:52 pm"
To: mark@quickweb.com (Mark Mayo)
Date: Sun, 29 Jun 1997 19:43:54 -0500 (EST)
Cc: multimedia@FreeBSD.ORG, hackers@FreeBSD.ORG
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> Hi. I'm noticing a strange thing happening on my computer. At higher refresh
> rates (above 72Hz), I see what I can only describe as "shadows". For example,
> the borders of windows seem to have a little grey fading border on the right
> side. If the mouse is over a simple green background, I can really see the
> "shadow". It's quite annoying - even the black letters on the white background
> of my rxvt leave a little grey silhouette to the right.
> 
> I'm guessing it happens (or more likely is visible) whenver you have sharp
> contrasting colors next to each other.
> 
What you are seeing is a transient response problem.  That problem can be
due to many causes.  It can even happen with a supurb monitor and an
excellent display adaptor, if you have a lousy transmission line between
them.  The problem becomes more pronouced at higher and higher freqs.
(So high scan rates will show the problem more than lower ones.)

Most often, we have seen the problem with transmission line problems
(the cable between the CRT and adapter.)  However, I usually use good
monitors, and good display adapters -- so the only thing left is a
lousy cable.  The next most likely cause would be a monitor limitation.

Note that cables cause problems only because people don't think that
the cable is very critical.  Well, the cables are critical -- and I
am not talking about guilded audio cables (which are mostly hype, IMO).
Video is a very different animal.  You can get by with fairly long cables,
IF and only IF you match the impedance of the cable and driving/receiving
devices.  Cheap cables have widely varying and uncontrolled impedance
characteristics.  Such attributes can lead to reflections (similar to
SWR in Ham/CB radio parlance), and that can ringing or delayed rise/fall
times.  Those delayed rise/fall times can make the video look smeary.

After this long explanation, your problem could still be a bad video
card, or a CRT that just can't handle the bandwidth also.  Note that
a reasonable CRT mostly gets soft looking though.  Smearing biases
my guess about the problem a little bit more towards the cable.

John