I am trying to build a circuit that detects the presence of video (Vs.
a blank screen) by monitoring the R,G, and B outputs of a graphics
card.  It should be able to detect the presence of a single pixel at
65 MHz, which would mean detecting a 15 NS pulse.  It should also be
able to tell the difference between a blank screen (about 300 mv)
and a dim screen (say, around 310 mv).  Oh yes, it also needs to be
cheap.  

My first circuit was a dismal failure.  I used 3 compariators; each
compariator had the + input going to one of the guns, the - input
went to a reference created by a voltage divider(a potentiometer).

The first problem was that the compariator was way too slow.. I
needed to get several pixels in a row before it would fire the
compariators, so I could have a whole screen full of text, but my
circuit would not detect it.   

The second problem is that there was more noise on the reference then
the smallest difference between a blank screen and a dim screen.  In
fact the difference between completely black and completely white is
only 650 mv.  I am wondering if I am going to have to amplify the
video signals to make this work.  

There are faster compariators, but they are expensive, and require 
split supplies.   I would need to replace my .49 quad compariator
with three 1.89 compariators, and create a whole new power supply
circuit.  

At this point, I think what I need is some sort of transistor
circuit.  Transistors are fast and cheap and should do the trick...

Unfortunately, I am way out of my league when It comes to designing
transistor circuits, so I am appealing to the net for help.  Any
ideas, tips, circuits, pointers, references, etc. would be greatly
appreciated.  

Oh yes, I only sample the output of this thing every second or so, so
I don't need a fast response time at all, however, I haven't found a
way to take advantage of that fact.

Thanks a lot for any help anybody might be able to give. Of course,
you will have my undying gratitude.


Steve Verity



