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Joined: Sep 2002
Posts: 37
the situation is this:
I have a church with catwalk above main santuary where all wiring is in romex (yes, I Know) and there are 2 new LCD Projectors hung from ceiling to show on screens. We put in 4- 575 watt theater lights with local dimmers which have a remote controllers through signal wiring in the sound booth. When the dimmer is in "black out" mode everything is great, but when I raise the lights I get a scrolling line on the projectors. I have made sure the computer and projectors are on the same phase, properly grounded, seperate neutrals for everything, the lighting comes from a different panel than the projectors, the 2 panels are fed from the main service seperately, the low volt communication cables are far enough from line volt cables not to be an issue.

Sooo yesterday we put a shielded transformer 240v primary 240v secondary and moved sound, computer, projectors to a new panel fed from transformer. The neutral doesn't connect from line to load on transformer and the ground goes to local building steel.

I still have scrolling lines as soon as I put up the theater lights and now I don't know what to tell the customer. And I'm out a bunch of money for x-former and sub panel because it didn't make the problem go away.

I have looked at some of the threads about harmonics but I thought that the transformer would solve the problem.
What should I do next??

[This message has been edited by falcondfb (edited 04-25-2005).]

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Joined: Oct 2004
Posts: 806

A few possibilities come to mind in your situation:

First, what is the physical proximity of the projector(s) to any lighting instrument or cables? Dimmers produce so much noise and harmonics that the instruments themselves can radiate enough EMI to affect the projectors. If possible, temporarily disconnect or de-lamp any instruments within 10 feet of the projectors. See if that cures the problem.

Also try powering up only one projector at a time. Unplug the unpowered one for the test. It is possible you have a ground loop between the two projectors through the video or even the control lines.

All of your video and control lines to the projectors should be installed in EMT or metal flex. Even if they are, try to maintain at least a foot of distance from any lighting power cables/instruments. Always cross the video/control cables at a 90 degree or so angle over any power cables.

Make sure the controller is away from the lighting control board and if the controller is AC powered, it will need to be on a separate circuit from the dimming console. Same rules above apply for cabling from the controller in the sound booth.

What type of video cable are you using? The junk from "The R-Shack" won't cut it. Your feed should be at least with S-Video line, not the "Composite" (Yellow RCA jack) signal. It will not carry more than about 20 feet without serious noise issues. S-Video can go as far as 100 feet with premium grade cable. I just noticed you mentioned computer in your post. Are you feeding a "VGA" signal? (With the 15-pin DB connectors?) Again, you need to use quality cabling. Check with a local true electronic supply house, ask for Belden, Canare or Liberty brand cables. They are lees expensive than the Monster cable brand but perform very well.

What model/brand are the projectors?

Please feel free to email me directly with more information or if you need additional help.

[This message has been edited by mxslick (edited 04-25-2005).]

Stupid should be painful.
Joined: Sep 2002
Posts: 37
Thanks for the info, I am going to work on the problem tomorrow.
The lights are 15 feet from the projs.
The projs are run by VGA signal with 15 pin DB conns through about 100' cables and good quality.
Most power and control wire is NM and therefore not shielded so that might be the biggest problem.
I think I will First try to eliminate the ground loop possibility between the projs, then feed by other means the lighting and hook up the lighting controller locally to the dimmer packs, feed the projs temp from other source to make sure noise is not getting in by laying wire parallel.
All projs and light circuits are run the length of the church to switches in the control area.....I think I just answered my own question.

Maybe the noise generated by the dimmer circuits is getting into the projector circuits not through the control and signal wiring, but through the power wiring. DOH

I'll let you know how I make out.

Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and

Sounds like the interference is coming from the Dimmers.

It could be "Falling Into" the branch circuit(s) which the Projectors are powered from, could be "Bursted" throughout the System from the Dimmers, or could be both items together.

Shielding the Video inputs should be done - and drive them to Ground at one end only. Ground at the "Far End" where the display is.
Along with the shielding, it might be necessary to couple the Line Inputs to Ground (at the Far End), via Capacitors.

Shielding of the A.C. Branch Circuit(s) may also be required.
This may be done by filtering the Circuit at the Display, along with actual shields and Bypass Capacitors.

You may be able to "Trap" most - possibly all - of the offending noise at the source.
Try filtering the Circuitry at the Dimmers, and be sure the Dimmers are in Grounded Metallic Enclosures.

Another tweek is to place Reactors on the LOAD side of the Dimmers - which reduce "Lamp Singing", but may also reduce the Capacitive Pulses leaving the Dimmers.

Good Luck!


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Nov 2002
Posts: 134
Isolate your video grounds at ONLY one end as Scott35 mentions. My bet is this will solve the problem.

If you have a small UPS available you may want to try powering the projector/PC off of it to make certain you aren't picking anything up through the air. Doubtful but possible. Charge up the UPS and run the PC/projector setup under AC loss conditions. Depending upon the UPS AC line shouldn't have any lines on the screen. Then plug in the UPS to AC. And try the dimmers.....Some of the better UPS system out there convert the AC bus into a DC buss and then make their own AC using internal switching power suppliers. This can solve alot of problems (it can also create them. Some UPS can create very clean AC.

Hey it's just a thought. It'll at least let you isolate things and do some troubleshooting.

Can you change the scan frequency on the projector? If it's a weird harmonic all you may need to do is move away from the nth-order one.....

If you are actually picking up the noise through the air then perhaps the projector need to be shielded. Not an easy task since they need to be cool.

Mxslick also has a good point. You need good cables. Bad cables work fine when there's no noise around. As soon as you have any EMI/RFI you'll know it. I've gone as far as to use fiber convertors to run cables across a room just to get the optical isolation benefits. BlackBox makes a lot of weird convertors that do this type of stuff.

You need to know where the noise is coming from before you can get rid of it. Ideally you'd have an o-scope and look at your sine wave to see what's happening.

Any chance the control signals are causing this. Are they just a 0-10 vdc signal for 0 to 100% dimming range. That's what I'm familiar with. If it's some type of serial communications then there's something else to look for. Can you locally bump the dimmer?

Anyway. I hope I've given you some ideas to try. I understand your frustration..


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