Darryl, AM radio is the worst affected by RFI partly because it is an analogue signal, but at least as much because it is at a relatively low frequency (Medium Wave) - close to the even lower frequencies where most of the transmitted power of the RFI is concentrated. FM radio is both somewhat immune to random noise, and at a higher frequency (VHF). Conventional television is also FM-VHF, with a few FM-UHF channels at even higher frequencies. All of those don't see much RFI from low-power sources such as engine spark ignition systems - though a commutator motor attached to the same 240 Volt mains feed can mess them up considerably. Digital TV is even more noise-immune than FM, and is at even higher frequencies. So, the different frequency range is probably more important than the different modulation system, but both of them are important factors. However if you are in a weak signal area, it doesn't take much to mess you up. This is a key factor.
