Video interference "60 cycle noise" or "hum bars" retail TV display

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.

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.

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!!!

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.

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.

Anyone have any ideas????


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Slow moving bars will just be 50Hz mains.

Yes. try earthing the tv set - at the aerial socket.

Many years ago I have a similar problem - but only when it rained. I eventually discovered that the aerial socket had a standing voltage of 120v.

During wet conditions this voltage flowed up the aerial lead into my aerial combiner, up to the lower frequency aerial and onto the metalwork of the support bracket and down the wall to ground. In the combiner, there was a

6v pd between the input and output on the 'earthy' side. This 6v became superimposed on the rf signal leading to the effect you describe.

the current involved was only about 100uA, so there shouldn't be any danger to life.

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In this case it's 60Hz, Guess I should have mentioned that I was in the US.

The TV's are double insulated, no ground conductor in the power cord. The HD distribution amps and the power are on same source and bonded together.

This one really has me stumped.



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Yes, mine had no earth either. The "earth" of the aerial (antenna) connector will be at half mains potential - in your case 55v.

Try connecting an a multimeter set to ac amps reading and measure current to a real "earth". If there is any - then my solution is likely to work


Try my suggestion.

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Classic description of hum bars which can be a bugger to eliminate.

Hope you're using good quality double-screened co-ax designed for video - if not get some.

Sometimes the only way is a "hum-bucking" coil at the end where it is being introduced. This consists of many turns of video co-ax wrapped around an iron coil. If you were in the UK I'd suggest looking at Canford Audio - they are on line. They may supply the US anyway.

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Core, sorry

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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

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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.

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).

Sounds to me as ground loop.

But other interfence kis possible but less likely.

Hand held scanners and two-way radios do not typically cause this kind of problems you described.

Analyze carefully the possibility of ground loop.

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Tomi Holger Engdahl

A slow moving horizontal bar is a definite ground loop. I don't know what your expertise is with ground loops, but it usually involves more than using a ground lifter at the power plug. When equipment is connected together using single ended coax, the shield of the coax carries ground current from one piece of equipment to the other if the chassis are not at the same exact potential. That ground current gets added to the video. There are balanced line drivers and receivers for video which means that you don't need to connect a shield at both ends. If there is one piece of equipment that seems to cause the hum bars, you might try balancing only that connection to keep the costs down. I think there might also be isolation transformers for video as well. Google has a few hits for balanced video, here's one

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