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[Steve Swaffer]

   We had a a video projector installed last spring by an outside contractor that specializes in audio/video.  We noticed then some interference in the form of dark bars that are obviously related to the audio program, but we have not used the projector for much video. Now, we need to run video with during a loud part of a program, and it is creating great difficulties.  We have found that the intereference is coming from the monitor amp. If I turn the amp off, or turn down volume to that amp, the problem goes away irregardless of what else I do.  Interefernce is also visible on a "local" LCD monitor, both monitor and projector aere fed from a video amp, projector feed is about 100'.

1.  Sound system-A&H GL3800, DBX 231 EQ, Ashley crossover on biamped FOH, Peavey Architectural Sound 400 on FOH, QSC RMX 1450 on monitors (One channel is monitor, other is overflow speakers)  Problem originated when I had a Peavey Architectural 800 in place of the QSC-it died last week.

2.  I have tried different supply circuits for the projector, amp, and DVD player with no change in interference.

3.  The closest the monitor and video wiring come is in the sound booth-one is at one end of the mixer , the other at the other end.  Video goes up through the ceiling and monitor down under the floor to reach the platform.

4.  Intereference is present whether DVD audio is connected or not.


I know this headache will have a simple solution-eventually!  Right now I do not know what to try next??

Steve Swaffer

[DaveGetting]

It sounds power related to me.  Since you see it on the local LCD I'd lean towards isolating the video amp and monitor amp power sources.

What about plugging the DVD directly into the local LCD - bypassing the vid amp?

[Steve Swaffer]

Originally, I had DVD direct to monitor-added the amp to try to fix the problem.  I fed the DVD player from a completely separate circuit-I am a licensed master electrician, and I know the electrician that wired the sound booth, I know it was done right.  Not saying it is not power-almost has to be, but I am at a total loss as to what to do to fix it-maybe an emi/rfi filter?  The service to the church is 700 amp 3 phase-I can't see one amp pulling that down!

Steve Swaffer

[Arnold B. Krueger]

Steve Swaffer wrote on Sat, 13 December 2008 05:15

Originally, I had DVD direct to monitor-added the amp to try to fix the problem.  I fed the DVD player from a completely separate circuit-I am a licensed master electrician, and I know the electrician that wired the sound booth, I know it was done right.  Not saying it is not power-almost has to be, but I am at a total loss as to what to do to fix it-maybe an emi/rfi filter?  The service to the church is 700 amp 3 phase-I can't see one amp pulling that down! Steve Swaffer

Done right according to a master electrician and done right according to an A/V specialist may be two different things.

As far as using other circuits not helping, your goal might be to keep most of the media booth on one circuit, and putting your AV equipment on another. The ideal situation is to have the video source equipment and the video display equipment on the same circuit.

The basic symptom, dark bars, virtually screams power problems to people who are experienced with video. The probable source of the situation is called "a ground potential difference". Basically, protective ground for the video projector is at a slightly different potential than it is for the source.

A difference of a small fraction of a volt is more than sufficient to cause visible bars. If you've got the time, you can even measure this voltage. You want it to be like a few millivolts or less. In your situation, you need to measure it with your sound system running full tilt.

I finessed this issue by running my projector from the same circuit as supplies the video equipment in my A/V booth, 150 feet of 12-3 notwithstanding.

In your case, it sounds like the bars are worse when the sound is loud. This means that the ground potential difference is getting larger when more power is being drawn from the power line. This is a bit unusual, and suggests that additional current is flowing into the safety ground when the music is loud. I'd be concerned. It sounds like safety ground and neutral are somewhat confused as to who is who.

There are two basic approaches - either reduce or eliminate the ground potential difference, or make it irrelevant. I've already covered reducing it.

I understand that most video cable extenders and video baluns afford a degree of ground isolation that may be beneficial in your case.

But first I'd take a second look at the wiring for your media booth. Even experts make mistakes. There is only One who is perfect!

[Steve Swaffer]

I am a master electrician out of necessity, "sound man" by choice, AV tech by default!  I hadn't thought of the ground potential difference but it certainly makes sense-but I am seeing the intereference on the local monitor with the same ground.  Would running the DVD player and monitor temporarily on a UPS power supply tell me anything?

[Arnold B. Krueger]

Steve Swaffer wrote on Tue, 16 December 2008 02:01

I am a master electrician out of necessity, "sound man" by choice, AV tech by default!  I hadn't thought of the ground potential difference but it certainly makes sense-but I am seeing the intereference on the local monitor with the same ground.  Would running the DVD player and monitor temporarily on a UPS power supply tell me anything?

You mean unplug the UPS and run the gear on the UPS's bettery?

Yeah, that could work.

[Steve Swaffer]

Neil has a similar problem posted on the audio side, I spoke to him today, here is a bit more info on my problem-which is still not really resolved!

I found out that if I turn my platform monitors down, I can eliminate the interefernce completely-and that has been my workaround so far.  I can turn them down by the fader on the mixer or by the pot on the amp, or my switching the amp off-as long as no audio is being produced from the platform monitors I am OK.  I was looking hard at the amp-but here is the puzzler.  That amp has two channels-one is my platform monitor, the other is an "overflow" fill speaker.  I recently ran a skit video where I ran everything HARD-except the monitor, but including the overflow and I had no intereference-how can it be that amp?  The speaker wiring starts about 4' from the DVD player and video cabling and goes the opposite direction (down and west) from the video cabling (up and east).

The intereference is visible both locally and on the projector.

Steve Swaffer

[ProSoundWeb Community]