Actually a modulator modulates a signal be it data or base band video and base band audio onto a carrier. So if you received an off air on channel 3 and you wanted it to be on channel 29 you would use a receiver to get the channel 3 then you would demodulate it and send the base band video and audio to a modulator for channel 29 and you have moved the channel.
This is CABL.com posting #168063. Tiny Link: cabl.co/mRSR