PCM provides 1024 degrees of resolution on a 60 degee plane, meaning that in a servo arm at 90 degrees that travels from 60 to 120 degrees has 1024 points to stop in between in a PCM system. Older 1st generation radios PCM radios had a resoultion of 512. 1024,512 is all binary Eg: 1,2,4,8,16,32,64,128,256,512,1024,2048 on and on. A PCM radio has 10 bits of word and a few extra for error checking Eg: 0000000001 , 0000000011, 0000000111, 1001001011. I hope I explained that good enough :-). FM radios are no different in a sense than AM radios. They vary the amplitude and magnitude of the signal to tell the servo where to be. Harmonics, RF interference and a number of other things can interfear with the signal. Since PCM sends out a parity (the extra 2 bits with the 10) the reciver will "Lock" the servo(S)vif the data signal does not match the parity bit, thus by providing a "Fail Safe"

Hope that helps



P.S. "Fail Safe":"Servo Memory" will crash your vehicle the same as and old AM radio, PCM is much more "Clear"