Unstable video on images with lots of white

[eug k]

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... ?

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.


would a TBC do the job?


thanks!

 

[Laurence Taylor]

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.

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.

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.

--

rgds
LAurence

....Licat volare si super tergum Aquila volat

 

[CWatters]

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.

 

[Fred Abse]

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... ?

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.

 

[eug k]

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.

 

[eug k]

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.
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.

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.

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..

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.

 

[CWatters]

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.

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.

 

[Tomi Holger Engdahl]

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.

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.