🚀 go-pugleaf

jimbo@socrates.umd.edu (Jim Bogard) writes:
>What's the best way to detect the presence/lack of a square wave?
>I'm trying to build a 'valid video present' indicator, and I have
>some chip I got from DIGI-KEY (number not handy) which strips
>horiz/vert sync and provides them seperately.  Now I'm wrestling
>with the problem of getting a TTL out from the mess indicating 
>what's going on.  Anybody got any ideas?  Or is there perhaps a 
>chip which will do this for me and I'm being stupid?

      I'd check the current National Semiconductor catalog; they have
a line of video sync extractors.  The ones in the 1989 book don't have
a "valid video output" pin, though.  You do need that; most sync extractors
produce reasonable sync outputs even when no input is present, since 
TV sets aren't usually designed to stop sweeping when no signal is present.

      You could also get a color decoder IC; those usually have a pin which
indicates whether a color burst is present.  That's usually a solid indication
that a valid color TV signal is present.  

					John Nagle