Path: utzoo!dptcdc!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!sunkisd!maxwell!jean
From: jean@maxwell.Concordia.CA ( JEAN GOULET | DCKS004 |              | )
Newsgroups: sci.electronics
Subject: Finding the HSYNC signal
Summary: Getting a TTL signal for a light pen
Message-ID: <772@sunkisd.CS.Concordia.CA>
Date: 17 Apr 89 21:44:48 GMT
References: <8903310230.AA08623@maxwell> <16552@cup.portal.com>
Sender: usenet@Sunkisd.CS.Concordia.CA
Reply-To: jean@maxwell.Concordia.Ca ( JEAN GOULET | DCKS004 |              | )
Organization: Concordia University, Montreal, Canada
Lines: 28

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, Canada