These kinds of problems is why I prefer coax for video connections where practical. You get a decent ground along with the signal. In the olden days, many analog video inputs also had differential inputs for canceling the bit of potential that is almost always on the shield. The balanced approach is nice when properly implemented though, as you can go further with less loss, and CAT 5 is cheaper than good video coax.
I have seen problems with neutral potentials, strangely enough, more in TV stations than in other places. One case was caused by a senior broadcast engineer who wouldn't let anyone else be involved in the design of a new building. others were caused by excessive neutral currents due to switching power supplies that didn't exist when the building was built.
In any case, this was an interesting story, and one that teaches us to think out of the box when problems get really strange.
On one long commercial airlines flight there was an in-flight movie that was displayed on a number of regular NTSC sceens around the plane. There was also a very audible 400Hz tone and that unique multiple hum bar display on at least one of the screens. So someplace there had to be a bit of connection to the plane's electrical system. I neve did get to investigate much more, since airlines are quite picky about that sort of thing, but on other flights with them I noticed a completely different in-flight entertainment system.
As a Broadcast Video Design engineer I have seen this problem many times. Today because the screens are all based in some way on a computer screen, grounds are not cared for. I have seen grounds on mounting sockets, frames and the input BNC or RCA Phono Plug that are all different potential from the local ground. There ar a couple of solutions.
1). Use active UTP transmitter and reciver pairs, not baulns, to get a true differential signal with full bandwidth video ( very important with large sceens!).
2) Another step is to make sure the monitor has only one ground, at the plug or from a ground screw to earth. Then check for grounding on the mounts as this creates ground loops.
3) The problem also occurs when you feed from a single source at one ground to multiple monitors with each on it's own ground. The grounds all begin to talk to each other back throught the wiring.
There are products that can fix these problems but there is always a cost.
With three of us in the house, we have dozens of switcher power supplies for everything from lap tops to iPods. I became aware of how many when making a family trip to UK I had to check the voltage range on these things. My wife bought three Chinese aftermarkey power supplies for her IBM thinkpad over the years so she could plug in wherever she was in the house.
Interestingly, while in Edinburgh, I plugged one of the Chinese ones I brought to charge my ham radio battery and it blew up upon seeing its first burst of 240 V 50 Hz. I spent an hour in London finding a Maplin store to get a replacement which sadly won't phyisicall plug in back in the US but worked for our stay.
I have on my to-do list to measure the leakage on each and every one of these "warts".
The problem that comes with isolation transformers is that as the isolation increases the efficiency goes down. Which means that heat goes up. And I really do wonder about that quality in some of those switchmode power supplies. Hospital Grade ones from reputable makers seem to be able to provide good isolation, while some of the others seem to only be good for powering lightbulbs or heating elements.
Yes the hum bars went away when the power cord was unplugged and the PC reverted to battery. Yes there is supposed to be a lot of isolation, yet apparently the AC input is not totally balanced.
The radio broadcast industry has seen this problem in rack mounted audio gear. You can buy toroidial isolation transformers to mount in the rack. The neutral and hot are totally isolated from each other going in and out of the transformer chassis. Only the safety ground passes through.
RFI, I am amazed that reversing a plug on an external computer power supply had any effect, since there is supposed to be a lot of isolation between input power and output power. I wonder if the hum bars were gone when the computer was running on internal battery power.
The story is interesting, and I wonder about not using an RF link when it was available, except that possibly cat5 cable is cheaper. It is easier to run, I know. But the grounding of the set, or lack of grounding, causing hum bars does indicate a fundamental flaw in the design, or else faults in the construction of the sets, or, most probably, something deffective in the design of the cable adapters that provided an unbalanced connection through an allegedly balanced circuit.
It would be interesting to check to see if the polarization of the outlet the set was plugged into was incorrect. If the set had a two prong plug with polarized blades which many do, everytime it was plugged in the neutral and hot might be reversed. The design of the TV power supply and chassis may favor the neutral being near ground potential.
I ran into a problem at the office where a vendor brought his laptop and hooked it to our TV to do a presentation. Hum bars and rolling picture resulted. Instinctively I jumped up and reversed the plug on his lap top power supply (this prongs were not polarized) and the problem ceased.
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