VCR modulators and similar cheap ones invade the lower adjacent channel, as they don't supress some of the lower sideband. Analog NTSC RF TV has the picture carrier 1.25MHz above the bottom end of the TV channel's 6Mhz of bandwidth. The sound is an FM carrier 1.25 + 4.5 MHz above the bottom edge of the TV channel. The video is AM modulated on teh picture carrier, with the sync tip at max modulation, black level a little lower than that, to white which is about 10% of sync tip. Real TV stations filter the lower sideband to stop at the bottom of the TV channel. Which requires a special filter, and this is the part VCR modulators leave out. As VCR modulators on channel 3 expect to feed nothing but a TV set tuned to channel 3, and not channel 2 when the modulator is on. But if you fed the VCR modulator into a splitter to combine it with a cable or antenna feed that has a channel 2 but an empty channel 3, oh, you would get a usable reception on channel 3, but channel 2 would be trashed by the unsupressed lower sideband of the channel 3 modulator.
Assuming that you can lay hands on some simple cheap Tv modulators that you can change the picture carrier frequency on, you'd have to put each on every other TV channel. Like 2,4,6,7,9,11,13. 6 is not adjacent to 7. To create your own analog CATV head end to feed your analog TV sets via coax.
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