TONY,
why do you say use channel 4 on your box? do you mean cable box? we have an over-the-air channel 4 here in L.A., so we use channel 3 to avoid interference from accidental ingress.

For a moment I thought you were talking about modulating two channels a couple of channels apart in the VHF band, then i realized you probably did not.

Even this has problems: if the house has an antenna system, you have to know what actual UHF channels the local DTV stations are on. You can get this by going to www.antennaweb.com and pretending you need an antenna for that site. If it is cable, digital cable signals will muck up the upper cable channels, so you will have to modulate channels above 100 or get a filter, as from Channel Plus, that kills the digital cable signals.

By the way, your post and this one do not address the subject of this thread, combining channels 3 and 4. Hey, Albert, did you want to know what we would do if we could not combine channels 3 and 4, or did you just want to know about your initial subject?