pulse code modulation uses the same fm signal [ppm] and most pcm transmitters can be programmed to use either pcm or ppm[regular fm] so you can buy the less expensive recievers.the pcm rx will go into fail safe if it does not recieve the proper signal or just ignore any improperly coded signals till it gets the correct one that is why they can reject interference that fm cannot.i prefer to use pcm on my scale and multi engine planes,i use am or fm on my sport planes.