SHARP 37" LCD monitor has horrible shadow

  • Thread starter Thread starter Plastic Man
  • Start date Start date
P

Plastic Man

I've had time finally to get back to the LCD display. Bob and John
suggested the extra-long VGA cable as the culprit. Sure enough with a
shorter cable connected the ghosting/shadow problem is solved.

We have no choice but to use about 20 feet of VGA cable on this
particular screen. Is ghosting a common problem with this length of
cable, or can I expect a better picture with decent quality cable?

Plastic Man
 
Plastic said:
I've had time finally to get back to the LCD display. Bob and John
suggested the extra-long VGA cable as the culprit. Sure enough with a
shorter cable connected the ghosting/shadow problem is solved.

We have no choice but to use about 20 feet of VGA cable on this
particular screen. Is ghosting a common problem with this length of
cable, or can I expect a better picture with decent quality cable?
Click to expand...

A really good cable with impedences perfectly mathched might do the job.
 
Plastic Man said:
I've had time finally to get back to the LCD display. Bob and John
suggested the extra-long VGA cable as the culprit. Sure enough with a
shorter cable connected the ghosting/shadow problem is solved.

We have no choice but to use about 20 feet of VGA cable on this
particular screen. Is ghosting a common problem with this length of
cable, or can I expect a better picture with decent quality cable?

Plastic Man
Click to expand...

Long cables are DEFINITELY the culprit - the transit times extend
to "multi-pixel" level, which significantly increases the probability of
ghosting due to impedance discontinuities.
Problem is SOME of the discontinuities are NOT inside the cable,
but where the cable connects - so getting a "super cable" will not be
any help, as the connectors are NOT likely to be "super".
You may look into an "active driver" (AKA KVM) if you really
need to have the 20 ft of cable. Even then it's suggested to arrange
a trial if possible to see if it helps the ghosting. Sorry, I have no
direct experience with KVMs but their intent is exactly for the
application you mention.

HTH,
HGA
 
J. Clarke said:
A really good cable with impedences perfectly mathched might do the job.
Click to expand...

Agreed. And I want to add that it's not really the length of the
cable that's the problem here, it's the impedance mismatch at
either end. The added cable length really just introduces more delay,
which shifts the position of the reflection (the "ghost" image) farther
to the right of the edge that caused it in the first place, and thus
generally makes that reflection more visible and objectionable.
A better cable can help, but mostly from the improvement that
you're likely to see in the quality of the connector attachment at
either end (cheap VGA cables often have truly hideous connectors,
which have been attached to the cable itself with leads flying all
over the place, making for a really bad impedance discontinuity).
If, on the other hand, the mismatch is actually at the monitor input
(and the graphics card - it takes two bounces to make the ghost
visible!), then a better cable might actually make the thing look
worse (less loss in the cable making the reflection more visible)!

Bob M.
 
The maximum recommended length for high quality VGA cable is 10 feet.
Otherwise you experience image corruption.
 
DaveW said:
The maximum recommended length for high quality VGA cable is 10 feet.
Otherwise you experience image corruption.
Click to expand...

_May_ experience. It is not certain.
 
Bob said:
Seconded - and it has very little to do with the length of the
cable (which, with any decent cable - and I mean the basic
cable stock itself, the "wires" - is basically just a loss
issue) relative to the impact of the connectors and the
quality of their termination (attachment) to the cable.

For one data point - I have successfully run what were
essentially "VGA" signals, at 1280 x 1024 pixels, over
100 METERS of cable - but it was Really Truly Very Nice
Cable, and I did NOT use ordinary VGA connectors to do this...
Click to expand...

Have you described what you did in that case before?
 
J. Clarke said:
_May_ experience. It is not certain.
Click to expand...

Seconded - and it has very little to do with the length of the
cable (which, with any decent cable - and I mean the basic
cable stock itself, the "wires" - is basically just a loss
issue) relative to the impact of the connectors and the
quality of their termination (attachment) to the cable.

For one data point - I have successfully run what were
essentially "VGA" signals, at 1280 x 1024 pixels, over
100 METERS of cable - but it was Really Truly Very Nice
Cable, and I did NOT use ordinary VGA connectors to do this...

Bob M.
 
J. Clarke said:
Have you described what you did in that case before?
Click to expand...

No, because I doubt that it would be of much use to the readers
of this group; but just for the sake of satisfying curiousities, this
was an experiment performed for a customer who (obviously)
interested in doing some SERIOUS "remoting" of monitors, for
reasons that are unimportant here. We managed to get a very
usable image at the end of a 100 meter length of cable, but
the cable in question was some very, VERY nice low-loss
coax that had been special-ordered from Belden. It's not
something that most people would be able to do, of course, but
I offer it as an example that simple length alone is not THAT
big a deal - it's a matter of cable quality, losses, the ambient
environment, etc..

Bob M.
 
Back
Top