With cable --

When you choose the channels you modulate onto, don't forget that the digital channels are somewhere in the spectrum! They are usually in the channels above the high 70s, at least throughout southern California. You can't tell where they are until you try to use a channel for a modulator, mix it into the cable signal, and find the signal is messed up. The cable box doesn't tell you what channel it is really on and an analog TV won't likely even see the digital channel to tell you to expect interference.

I would start trying channels at 100 and go up from there. You might have to even buy a low-pass filter that removes some upper cable channels in order to get a modulated signal onto the system. But that removes some digital channels, so....