Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!orchid!mwtilden
From: mwtilden@orchid.UUCP
Newsgroups: sci.electronics,comp.graphics
Subject: Re: Mixing computer and video signals
Message-ID: <8037@orchid.UUCP>
Date: Sat, 6-Jun-87 12:48:41 EDT
Article-I.D.: orchid.8037
Posted: Sat Jun  6 12:48:41 1987
Date-Received: Sun, 7-Jun-87 02:08:20 EDT
References: <173@mv06.ecf.toronto.edu>
Reply-To: mwtilden@orchid.UUCP (M.W. Tilden, Hardware)
Organization: U. of Waterloo, Ontario
Lines: 28
Keywords: video overlaying, keying
Xref: utgpu sci.electronics:706 comp.graphics:691

In article <173@mv06.ecf.toronto.edu> drascic@ecf.toronto.edu (David Drascic) writes:
>Is it possible to simply add the two signals through appropriate
>resistors?  Or are there chips available suitable for building a
>simple keyer?  Any hints, suggestions, or suppliers would be much
>appreciated.
>
When I did this a few months ago, I used a MC14052 analog mux/demux
to select between the cursor-present signal and the normal video.  A high
grade amplifier has to re-boost the signal to 75 Ohm status but otherwise
there is no distortion at all.  These are all NTSC signals of course.

The problem with mixing computer with video through resistors is computers
generate single repeated sync signals of 262 scan lines (normally) and not
the 262.5 demanded by NTSC standards.  A bad jitter in your picture signal is
the result along with partial to complete color loss.

If you only have RGB coming out of your computer then I would recommend the 
MC1488 chip.  Works fine with a minimum of components (the funny coils
they insist you use can be easily faked with RC equivalents).  I have
still to get a reasonable circuit to color lock the video with this 
chip but I'm not worried, grey scale looks good for now.


-- 
Mark Tilden: _-_-_-__--__--_      /(glitch!)  M.F.C.F Hardware Design Lab.
-_-___       |              \  /\/            Un. of Waterloo. Canada, N2L-3G1  
     |__-_-_-|               \/               work: (519)-885-1211 ext.2457,
"MY OPINIONS, YOU HEAR!? MINE! MINE! MINE! MINE! MINE! AH HAHAHAHAHAHAHAHAHA!!"