Pineapple standards converter.

[ppppenguin]

I think intercarrier modulation is possible on +ve systems, so long as it's done right. Needs a good linear upconverter to take the baseband video plus 3.5MHz sound carrier up to RF. Intercarrier demodulation is not possible, for the reason you have stated.

I remember the Pineapple converter. It's a very potent RFI generator - there's no ground plane and lots of fast digits. Needs to be in a metal box with filtered connections.