FM Frequency modulation.........the tops of the waves are realtively constant, the wavelengths change

PCM.......using frequency modulation, you can essentially send a square wave signal.....square wave signals are great at xmitting digital code.........therefore each pulse can have a uniqie binary code preceeding the pulse......the code identifies your xmitter............the reciever ignores all signals that do not have the proper numerical digital pre-code attached to each pulse. B52 go-no go technology.

PPM....I don't know what that is