If I select a digital HDTV channel and try to watch it on an old set,
whats going to happen ?? This is with video in, but how about
antenna in ??

Channel 6 here is crazy, broadcasting analog some days and
broadcasting digital other days-- while broadcasting analog at night--
every night. Geez, it's enough to make somebody really go off the deep
end!

But to answer your question, a digital signal going into an old TV or
VCR tuner gets you either a blank picture-- a nice blue screen if
you're lucky-- or more likely a lot of snow and static; but no picture
whatsoever. You either need a converter box or cable.

Ron