FM or PCM?
RC FM is not true FM it's FSK. The transmitter is either transmitting at the carrier frequency or the shift frequency. The difference in timing between the shift in pulses is where the data is encoded. The PPM signal is encoded this way, using one frequency for 0 and the other for 1. True FM is designed for audio and micro fluctuations in the carrier frequency are where the encoded signal is stored. RC isn't variable, it's full on, or full off. Yes, to a lot of people this is considered splitting hairs, but it is how they actually function and it's best to refer to them that way. It's 'technically' FM because the frequency is shifted from one to another to get the modulation, but it's actually FSK which is a subset. True FM is a little harder to tune because it's easier to accidentally over modulate the signal because it's purely analog in nature. FSK is binary in nature, and because you're dealing with full on or full off it generates harmonics a lot easier. The further away from being a sine wave the more the harmonic generation, and FSK is square wave so it's definitely as far from a sine as you can get.