Unstable video on images with lots of white

Sounds like poor high frequency response (the sync pulses are losing their shape), and the projector gets confused between the white-black transition and the black-sync transition.

First thing to try, lose the amplifier. Also, make sure that everything is correctly terminated - all inputs should have 75 ohms to earth, and outputs 75 ohms in series with the source. If it's a cheap amp, they may well have skimped here.

Probably cause more problems than it would solve. Might be worth a try if you've got the bits handy, to eliminate the cable from your enquiries.

It might, if it was just in front of the projector, but you'd simply be curing the symptom, not the cause.

What Laurence says plus...

Watch out for a possible earth loop problem. If the projector it isn't a "double insulated" product it's possible that you have a big (60M+) earth loop.

Consumer unit -> earth in ring main -> Earth pin on PC -> 0V outer on video cable -> earth pin on projector -> earth in ring main -> Consumer unit.

or something like that. or involving the audio amp. or both.

If the building has an audio loop for the hard of hearing then that can couple into the PC-Projector earth loop and cause problems - although a noise image is the most likely effect.

Overdriven amplifier being driven into nonlinearity on peak white, and shifting its operating point so as to squash sync pulses, maybe.

Try reducing the gain (contrast), and see if it goes away.

on the old crappy amp, reducing the gain would fix the problem, but then the image would be really dim.

On the new amp, reducing the brightness does help somewhat, but it can't be turned down as much as the old one, so it still happens.

Without the amp, the image is really dim so that won't really work. :(

When I run it without the amp, it's really dim. Actually, thinking about it again, I don't think it worked at all without the amp.. the projector just displays nothing. I'll try it again though, I had to do it in a rush the other day so i might have gotten my wires mixed up!

Is that 75 DC ohms?

Would putting the amp halfway along the cable instead of at the start help? I'd imagine that would reduce the gain required on the amp, hopefully resulting in more "shapely" sync pulses. :)

I'll try borrow a scope and see if the pulses are getting screwed up.

I don't have the baluns so i can't try it yet. the projector's really high up too so i'll probably only try it if it'll definitely help.

I was planning on switching to cat5 to help reduce or eliminate interference from the lights in the hall. They're on dimmers so at certain levels, patterns appear on the video. Being balanced, i figured it should help. Am I correct? Are there any disadvantages?

i'd also imagine if it was an earth loop problem like Colin mentioned, the baluns should take care of it..

Good point... always better to have a good source. :)

It's hard to diagnose the problem without being there with the right kit to measure things. Could you afford to hire a CCTV engineer for half a day?

Perhaps you could try a wireless link? eg Video sender. The picture quality of these isn't always great but they aren't very expensive. Some have a back channel for IR signals which might be useful if you want to control the PC from the projector location.

Sounds linke you have some problem in the signal the video source originally gives or the projector settings. That 30 meters of RG-59 cable sould not make the image very dim..

Check your graphics card output settings and the projector setings. Usually one of them or both of them have controls for brightness, contrast etc.. Setting those right usually helps to get picture right.

And if the source / projector cannot be corrected then there is also possiblity to add some video procesign equipment that can do image processind needed (for example brigness / contrast / color / cable equalize controls etc..)

I don't think in general putting the amplifier in the middle of the cable will not change mught, if anythign at all. The attenuation of RG-59 cable is around 2.5 db/100m at 5 MHz. With the 30 meter cable you have we are talking about 1 dB attenuation at high frequencies of compoiste video, and less at lower frequencies. Quite a low loss.

Getting a scope it a good idea. You might see tha the signal levels really are, and if there are considerable signal reflections because of wrong impedances etc.

Nowadays with right baluns CAT5 cabling can be used to transport video from place to another. Generally going from RG-59 to CAT5 wirign does not fundamentally change things with video... If things are not working well in the beginning, going top CAT5 has quite lob propablity of making thing any way better.

Interference from the lighting in the hall could be related to ground loop problem. There are ways to solve this kind of problems using the existing wiring. For more details take a look at

CAT5 wiring is a balaced medium so it does not pick certain interference as easily as unblanced interface. The disadvanteges of using CAT5 are the cost of the adapters, the balancing on many adapters is far from ideal and there could be that the imepdance matching (75 ohm video vs. 100 ohm CAT5) is not perfect on many adapters.

Balun could help in this or not. Depends on the balun design used, the ground loop related problem could affect the balun operation or not. Different designs have their advantages and disadvantages.