Newsgroups: alt.engineering.electrical
Subject: Re: Video interference "60 cycle noise" or "hum bars" retail TV display
References: <[email protected]>
Jim  writes:

> I've have a retail customer with a large TV display.  The content they
> display is generated from a multimedia server with both an NTSC and
> ATSC outputs.  The ATSC signal then feeds a high definition receiver
> which feeds a daisy chain of HD distribution amps.  The NTSC signal
> feeds other TV's which are just used for general information.
> 
> The TV's are displaying a horizontal bar that moves from bottom to top
> very slowly.  On the HD sets the bar is colored and on the analog sets
> it's just a dark bar.

Sounds like a "ground loop" problem situation.
The dark bar on analog sets are typical noise 
I have seen. I don't remeber seeiing any colourful bars on 
HD system but I would thing that kind of interference pattern
is possible with some HD interfaces in use. 

What signal interface yype you use for HD signal ? 
(analog component video, analog RGB, VGA, DVI, HDMI)      
 
  
For more information on ground loops read my document here:

Ground loop problems and how to get rid of them
http://www.epanorama.net/documents/groundloop/index.html
 
> Every componant has been changed out (server, video output card, amps,
> modulator, combiner, HD receiver, distribution amps and cables), and
> the bars are still there.

It is a typival for a ground loop problems that chaning 
different components on the system does not solve it. 
Ground loop problem is a system level problem. 
The question lies how different components are interconnected 
(what interfaces used, what kind of wiring, cabling routes) and
how the power distribution to equipment is done (power grounding 
most important issue).  

As the source and destination of a video signal can be at differing ac
or dc earth potentials, earth loop currents flow and cause
longitudinal hum to be introduced into the video signal. Video hum is
low frequency (50 or 60 Hz mains frequency or it's harmonics) noise
from the ground lines which has influenced the video signal, causing
degradation of the displayed signal. Video hum is usually observed as
bars rolling vertically through the video image, video hum may also
cause video distortion or even tearing of the picture in severe
cases. Video hum may be a problem in any system where video sources
and display devices are connected to different A/C power sources with
varying grounding potentials.

>  Even went as far as having a new server brought in and set it up right
> on the sales floor.  The bars are still there!!!!!  When I was feeding
> this from the back room the bars appeared in about 25% of the TV's,
> when I moved the server out on the floor, they appear in all of the
> TV's.
> 
> When I disconnect the HD receiver and connect a DVD player to the same
> HD distribution amps, the bars are gone!!!

Definately your servers are grounded.
 
It seems that your HD receiver gets the other ground connection 
to your system, the one at different potential. That ground connection 
on HD receiver can come through mains power (if grounded power plug) 
and/or the TV antenna cable (through CATV or antenna network connection).     
 
> At first I was trying to case this down as a ground loop problem, now
> I'm not sure.  I'm thinking more like RF or EMI.

Sounds to me as ground loop. 

But other interfence kis possible but less likely. 
 
> Store does have a ton electronics and some RF equipment (hand held
> scanners, two-way radios, etc).  No transformers close by, electric
> room is 100' away.

Hand held scanners and two-way radios do not typically cause 
this kind of problems you described. 
 
> Anyone have any ideas????

Analyze carefully the possibility of ground loop.  

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/