Newsgroups: sci.electronics.misc
Subject: Re: Unstable video on images with lots of white
References:  <[email protected]> 
eug k  writes:

> Laurence Taylor  wrote:
> > eug k wrote:
> > > 
> > > hi,
> > > 
> > > I've got a problem with a projection system. The setup is like
> > > this:
> > > 
> > > PC TV out --> video amp --> approx 30m RG59 --> projector
> > > 
> > > On certain images, the image gets unstable and rolls. It's not a
> > > continuous roll but more like *quick roll*stable for 0.3s*quick roll*.
> > > 
> > > I noticed it happens mainly on images with plenty of white. Is it
> > > just the PC's tv out not conforming to specs, or too much loss in the
> > > cable, or a cheap video amp... ?
> > 
> > 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.
> 
> When I run it without the amp, it's really dim. 

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..)

> 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 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. 

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

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.

> > > There's a cat5 cable running to the projector so i could put on a
> > > balun and use that instead of the RG59 if that would help. 
> > 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.
> 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.

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. 

> 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? 

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 
http://www.epanorama.net/documents/groundloop/index.html

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. 

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

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. 

> > > would a TBC do the job?
> > It might, if it was just in front of the projector, but you'd simply
> > be curing the symptom, not the cause.
> Good point... always better to have a good source. :)

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/