jean@maxwell.Concordia.CA ( JEAN GOULET | DCKS004 | | ) (04/18/89)
Hello. I've been looking into making a light pen for my computer, and I think
the design is complete, except for one important detail. My circuit needs
to be restarted every time the electron beam starts a new horizontal line, so
I figure the HSYNC signal is what I need. Unfortunately I don't have the
schematics for my computer, and I don't have it for my TV, so I'm wondering
if there is a way to find it in the TV 1) without an oscilloscope; and
2) without killing myself. Ideally the signal will be TTL, but if it isn't,
what are the voltages which signify HSYNC is happening (some kind of threshold,
right? - if the signal is higher than the threshold, then it's HSYNC?). If
that's the case, I could use a comparator with one input connected to that
constant threshold voltage, and the other to the right place in the TV. The
comparator output would be wired up to produce 0V or 5V.
On the other hand, would it be easier to extract HSYNC from the modulated
video signal? The connections would certainly be easier and safer since I
could tap it at the computer's RF output plug. What would the threshold
voltage be in this case? Would an LM339 comparator be sufficient for this
application?
Speaking of components, I also need a fast 10-bit binary counter, a fast
square wave generator (14.3 MHz), and a 10-bit latch. The counter needs to
work at 14.3 MHz. Do you have any suggestions? Thanks in advance.
Jean Goulet
Electrical Engineering
Class of '89
Concordia University
Montreal, Canadasgt@dukeac.UUCP (Stephen G. Tell) (04/23/89)
I would recommend using the National Semiconductor LM1881 chip to decode the sync from the baseband video signal before it gets to the modulator. It takes a video input, and gives you TTL outputs for HSync, VSync, blanking, field index, and burst gate, which tell youu just about everything you could possibly want to know about the video timing. For your circuit you would need just the Hsync and Vsync signals to reset your horizontal and vertical counters. I would strongly recommend staying out of your TV set; many connect the AC line to their power supply an chassis without any transformer, so it would be easy to fry your circuit, computer, and yourself to a crisp. The general principle is that if you have to ask if its dangerous to open up and hack on a TV, stay far away from it with your screwdriver. The modulated RF signal would have to be demodulated to baseband video before it could be used, so why not just pick off the signal before it gets modulated? In order to feed both your circuit and the modulator, you may need a video distribution amplifier (VDA) to split the signal. (Note that this is different from an RF DA used for splitting antenna signals.) Steve -- Steve Tell: senior, Duke University school of Engineering (please hire me). Former Chief Engineer, Cable 13 / Duke Union Community Television. sgt@dukeac.ac.duke.edu; !mcnc!ecsgate!dukeac!sgt
irwin@m.cs.uiuc.edu (04/25/89)
Re: Finding the HSYNC....... jean@maxwell.Concordia.CA Jean, can you e-mail me? I tried sending to you and it bounced. I have some ideas for you. irwin@m.cs.uiuc.edu