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Path: utzoo!utgpu!utcsri!uthub!ecf!drascic
From: drascic@ecf.UUCP
Newsgroups: sci.electronics,comp.graphics
Subject: Mixing computer and video signals
Message-ID: <173@mv06.ecf.toronto.edu>
Date: Fri, 5-Jun-87 12:14:39 EDT
Article-I.D.: mv06.173
Posted: Fri Jun  5 12:14:39 1987
Date-Received: Sat, 6-Jun-87 10:48:09 EDT
Organization: Engineering Computing Facility, University of Toronto
Lines: 14
Keywords: video overlaying, keying
Xref: utgpu sci.electronics:701 comp.graphics:684

I want to overlay the graphics output of a computer onto a video image
of the real world, to implement a cursor.  This means that I have to
sync the camera to the computer's video, which is reasonably easy to do.
But once I have the two signals, I have to combine.  Someone suggested
that "keying" was the route.  This involves switching between the two
signals, depending on the level of one of them.  Unfortunately, this
seems to be very expensive.

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.

	David Drascic    UUCP: drascic@ecf.toronto.edu  OR drascic@utecfb