From: dino@inqmind.bison.mb.ca (Tony stewart)
Subject: Re: Need help with video detection circuit
Organization: The Inquiring Mind BBS  1 204 488-1607
Lines: 64

verity@jack.sns.com (Steve Verity) writes:

> 
> 
> 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
> 
> 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
> 
> 
> 
> 
> -- 
> ..........>.........>........>......>...>...>..>..>..>..>.>.>.>>>>>>>>+ .   
> Steve Verity                  +   +             ...Maxed on MIDI        + .
>                                     +      verity@jack.sns.com      + ..   +

IN ORDER to get the 15 nS response time you need, you are better off 
going to an ECL slicer which can run off a single +5 or -5.2 V supply, 
you just need to bias or do some level shifting perhaps to get it in the 
range. Check out the ECL 10K books for a simple cheap solution.

dino@inqmind.bison.mb.ca
The Inquiring Mind BBS, Winnipeg, Manitoba  204 488-1607
