Audio hum entering through video feed

Conversations around originating a webcast for conference, including cameras and mixers.
kwillisjr
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Audio hum entering through video feed

Postby kwillisjr » Mon Oct 01, 2012 2:03 am

After much troubleshooting, I have determined that the hum in the communicator audio is originating from the video feed. If I unplug the video cable, I have a very clean audio signal. Any idea what could be causing hum in the video cable? We used a high grade video cable for the camera system (Belden 9259). Any suggestions for what to do to eliminate the hum from that cable? I'm assuming this is probably a ground loop issue.

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johnshaw
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Postby johnshaw » Mon Oct 01, 2012 7:16 am

“A long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom.”
― Thomas Paine, Common Sense

russellhltn
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Postby russellhltn » Mon Oct 01, 2012 10:01 am

Is this a low-pitched hum? Then I think you're right. And John's solution would be a good suggestion.

However, if the sound is more of a buzz that changes with the picture, then somehow the video is getting into the sound. I'm not sure what to suggest in that case.
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kwillisjr
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Noise source discovered

Postby kwillisjr » Tue Oct 09, 2012 6:19 am

After many hours and much troubleshooting, I finally was able to isolate the source of the noise (hum) in the webcast audio. It's coming from the satellite dish feed. If I disconnect that feed, the sound is clean and clear. I would assume the satellite cable is one piece from the closet to the dish. If this is the case, any idea what I need to look for at the dish? I'm thinking faulty ground at the dish but won't know for a couple of days.

One other thing I noticed is that the audio from the satellite receiver is also bleeding into the video feed. This and the hum are being transmitted through the metal cabinet which is grounded. I know this because I disconnected the video feed from the source within the satellite cabinet and laid the metal RCA connecter on the metal electric junction box that contains the ac power outlets for the equipment in the cabinet. This means that there should have been no signal at all coming through the cable. However, when I went back to the webcast location, I could still hear the hum (moderately) and the satellite audio (faintly) in the webcast audio. I went back to the satellite cabinet and moved the video cable so that the metal connector was hanging out in space. This completely eliminated any noise from the webcast audio. So, anything connected to the metal cabinet has this noise in it. Any ideas on how the satellite audio is bleeding into the metal cabinet?
Thanks,
Ken

russellhltn
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Postby russellhltn » Tue Oct 09, 2012 10:26 am

Either we're talking ground loop, or a connection from balanced to unbalanced audio has been done incorrectly.

With the sat dish, that smells like a ground loop. One thing you may want to do is pass the satellite dish feed though a small connector that grounds the coax to the cabinet instead of passing the ground though the receiver.
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kwillisjr
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grounding sat feed

Postby kwillisjr » Tue Oct 09, 2012 11:15 am

Tried that. It didn't remove the hum. The only thing so far that has removed the hum is to 'not' ground the sat coax to the cabinet 'and' disconnect the sat feed.

kwillisjr
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Re: Audio hum entering through video feed

Postby kwillisjr » Wed Dec 18, 2013 2:13 pm

This was the most bizarre thing I've ever encountered. The problem was that about 2.2 VAC was entering the metal framework of the satellite dish from the ground. This was being transmitted to the metal equipment cabinet inside the building via the shield of the coax wire which was grounded via a grounding block at the satellite dish frame. The voltage was getting into the webcast audio by way of the shield of the video cable which fed from the video amp inside the metal cabinet. The thing that finally fixed it was that we threw the main power disconnect for the building. When we powered back up, the voltage (and noise) was gone. I never did find out where the voltage leak originated. Weird.

russellhltn
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Re: Audio hum entering through video feed

Postby russellhltn » Wed Dec 18, 2013 2:59 pm

kwillisjr wrote:The problem was that about 2.2 VAC was entering the metal framework of the satellite dish from the ground.

OK, that's believable. And I can see how that could cause a problem.


kwillisjr wrote:The thing that finally fixed it was that we threw the main power disconnect for the building. When we powered back up, the voltage (and noise) was gone.

Wow. The only thing that comes to mind is that whatever you flipped also handled the neutral line and flipping it cleaned some contacts. Perhaps the source of the 2.2 VAC was the building neutral/grounding system and it was finding a "return" though the independent ground of the satellite dish. Might be worth inspecting that box for signs of overheating.

I wouldn't celebrate too early. It might come back. Flipping the power might have shut down the AC/Heat pump for awhile. (The noise disappearing is due to lower current draw.) Time will tell.
Have you searched the Wiki?

Try using a Google search by adding "site:tech.lds.org/wiki" to the search criteria.

kwillisjr
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Re: Audio hum entering through video feed

Postby kwillisjr » Wed Dec 18, 2013 6:16 pm

Actually it's been about 9 months. I didn't think about reporting this until I was on the forum today looking for solutions to another problem.


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