Ground Loop Hum for Four TVs

[hcproductions]

Hey guys,

So I'm having a little video issue with 4 televisions. The four TVs are high quality Samsung 55" LED tvs that were donated. All four tvs are being run from a Mac Pro using Qlab. The feed from the Mac Pro is run through VGA, a little over 75 feet to the tv's. Some of the cable is run near some power cable, which I should probably check. I'm sure that's causing a problem as well, because all of the TV's have a cloudy signal to it. Two of the TVs are run to a Sound outlet backstage using an extension cord with two outlets on it, and the other two are also run the same way to the same sound outlet. The thing is, two tvs that are run together on the same extension cord seem to be humming those vertical lines much worse than the other two, and once automation occurs it gets much worse.

I'm somewhat familiar with the ground loop hum, and I've managed to eliminate it once before on a projector that was doing it, but I was using a BNC run, and I just popped on a ground loop isolator and it eliminated the horizontal lines and worked fine.

Is there anything similar for VGA? Should I try making sure that the Computer is plugged into the same outlet? Or should I try and get it away from automation as much as possible? Or maybe it's a combination of a lot of things? Any help would be appreciated, the tv's are really nice and are completely black when they are on and the computer is off, but they look cloudy and terrible otherwise. Thanks,

 

[museav]

Sounds like you may be identifying two issues, hum or noise and general image quality (the 'cloudy' images). Is that correct?
How are you getting the signal to the four different displays? What are you using for the VGA cable?

This sounds like a permanent installation, in which case you should not be using extension cords, code restricts those to temporary use.

 

[hcproductions]

The tvs are temporary just for a show that's running for a month, so we are using extension cords.

The TVs are setup next to each other, so I'll number them 1-4. Tvs 1 and 2 are on the same extension cord and have a very slight hum that becomes more prevalent when the automation for the set occurs. Tvs 3 and 4 constantly have the hum and are both on the same extension cord. All the images are pretty cloudy.

I'm not sure what kind of VGA cable it is, we rented it from a company for the duration of the show. Is there something specific I should look for to help solve the issue?

 

[Studio]

The tvs are temporary just for a show that's running for a month, so we are using extension cords.
The TVs are setup next to each other, so I'll number them 1-4. Tvs 1 and 2 are on the same extension cord and have a very slight hum that becomes more prevalent when the automation for the set occurs. Tvs 3 and 4 constantly have the hum and are both on the same extension cord. All the images are pretty cloudy.
I'm not sure what kind of VGA cable it is, we rented it from a company for the duration of the show. Is there something specific I should look for to help solve the issue?

Are you using any kind of VGA splitter? If so Active or Passive (Passive usually dosen't work well). The Ground Hum also looks like something a passive splitter would do.

 

[hcproductions]

No, the Mac pro has four separate outputs that we are using to each of the tvs. So there are four separate VGA runs, and we just run the cable to the tvs. Thanks for the help though, I'm kind of on a time budget so it's nice to get the speedy responses. Has anyone else dealt with similar problems?

 

[hcproductions]

do they make ground loop isolators for vga connections? or do I have to convert the vga to bnc or something else?

 

[ruinexplorer]

I don't know of any product that is built for VGA to only eliminate a ground loop. Generally, they also are a DA or other functionality. Since you are running into a time crunch, I would go with adapting the VGA to RGBHV for the isolator (hum bucker) and then back. This, of course if you have no other recourse of isolating your power.

 

[museav]

I'm still not clear if the 'cloudy' images are directly related to or independent of the hum or if the hum is a ground loop or induced. I'm also not sure why automation is affecting it.

A fairly simple test would be to try running the displays, one or more of them, and the computer off the same outlet, which should eliminate ground loops due to the power. Does that resolve all of the problems, some of them or none?

 

[hcproductions]

So here's the deal:

I have these four tvs, (samsung un55c6400), LED tvs. They are all mounted 1 2 3 4 right next to each other. Tvs 1 and 4 have a constant hum, the other two do not. TVs 1 and 2 are run together, plugged into a double extension cord and plugged into one outlet. TVs 3 and 4 are plugged into a double headed extension cord and run into another outlet. The computer is also plugged into another outlet. All of these outlets are on the same box, all in sound power, which should be an isolated ground. I even tried plugging all of these things into a surge protector, then plugging them all into the same outlet on sound power. None of this helped. The tvs are using 125v little D cables, that do not have a ground pin. These are the plugs that came with the TVs, so I'm assuming that's right.

Right now I'm not concerned with the cloudy image. It's the hum bars that are irritating. My solution for this was to take the vga cable, plug it into an ez pc to tv, which converts the signal to composite, then use a ground loop isolater that is bnc to the tv. The only problem is, those boxes are causing the tvs not to sync up properly when a cue fires, making it look really messy. We troubleshot that it was in fact the boxes causing it. So I'm now trying to run directly vga into the tv without all the conversion, but again the bars are preventing me from doing this. It seems like it's either the bars or the delay at this point, and I'd much rather have the delay, but other designers would not.

Is there anything else I could try and test?

 

[museav]

Is there a way to try plugging the displays and computer into the same circuit?

Isolated ground simply means a dedicated ground path back to the service entrance. It minimizes but does not eliminate the chance of varying potential on the grounds at different locations and it is also easily corrupted, for example a supposedly isolated bus bar in a distribution panel being bonded to the panel. For example, if it is a 20' run to one receptacle and 300' to the other then there is still the chance of having a ground potential difference between them even with an isolated ground scheme.

 

[ruinexplorer]

Since the TVs will take the VGA signal, I would recommend against downconverting them to use the composite since it will degrade the quality of the image. Instead, as I suggested, just use an adapter cable so that you can use something like this.

 

[ccm1495]

how long are the vga runs and what is the quality of the cable. IE Thin Missing pins stretched or pinched

 

[hcproductions]

so interestingly enough...i think i discovered the problem.

the local monitor i was using to look at the computer desktop also had a PIP of a surveillance monitor to see the stage. After I unplugged it, the problem seemed to go away. Could it be that this was dirtying the signal to the rest of the tvs?

 

[ruinexplorer]

Since you have your ground loop isolator, try putting that on the line from the surveillance camera and reconnect it to the monitor. It could be the problem. Are you still having the increase in hum with automation movement?