PCM is of FM... Both PCM and PPM use FM (frequency modulation) as the medium to transmit a signal or carrier wave. The difference is the codes schemes employed PCM (Pulse Code Modulation) or PPM (Pulse Position Modulation).

For the few bucks difference the PCM RX is worth owning IMO. If you need a PPM RX they are reasonably priced and there is always the new IPD stuff on the market, which is very cost effective.

PCM is able to work through more noise than PPM and when PCM is hit KNOWN parameters or results occur in the event of interference. With PPM anything can and will happen with no absolutes. Each has its merit IMO.

Obviously there is a lot more to all this than I describe, PCM uses a micro-processor in the RX to determine if the signal received is valid and acts pro-actively to ignore bad frames or engages failsafe or hold modes. PCM RX's must be used exclusively with the OEM TX, or is brand specific. Most modern TX's have the ability to transmit PPM and or PCM. The only difference is weather or not a PCM or PPM RX is utilized.

There is also the new intelligent pulse decoding RX's on the market known as IPD or DSR and others. This code scheme works with all PPM TX's and offers similar functionality to PCM, this maybe the wave of the future.

Do a search here on RCU, you'll find this topic comes up regularly.