Re: "Can the Challenger drive a compostite video monitor directly or is it
only designed to drive an RF modulator?"
Since the input to a modulator is a composite video signal, the answer is
that both of your "or" options are the same thing.
However, some of the OSI video cards were 5-signal outputs (red, blue,
green, H sync, V sync) which, in terms of your question, would be "neither".
It's not difficult, however, to mix the two sync signals with one of the
video signals and then drive a monochrome monitor or a modulator.