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From: jpexg@mit-hermes.ARPA (John Purbrick)
Newsgroups: net.micro.atari16
Subject: Re: Composite video output from ST
Message-ID: <2588@mit-hermes.ARPA>
Date: Sat, 22-Mar-86 17:44:16 EST
Article-I.D.: mit-herm.2588
Posted: Sat Mar 22 17:44:16 1986
Date-Received: Tue, 25-Mar-86 04:12:48 EST
References: <2587@mit-hermes.ARPA>
Distribution: net
Organization: The MIT AI Lab, Cambridge, MA
Lines: 6

Since noone has answered my question yet, here it is for anyone who's 
interested (the question was how do the new ST's generate 60Hz and 70Hz
video simultaneously): 

In fact they don't. The 70Hz monitor, if used, is detected and causes the
ST to generate the appropriate video signals. Otherwise 60 Hz (only) is
generated, at least I think this is what happens.