PPM is not digital. It varies the length of a pulse, the length of that pulse dictates how far the servo moves. The length of the pulse can be affected by noise and seeing as PPM rxs don’t have any software to recover the signal they have no way off telling if this is the intended signal or not. Even if they did, how would they know if this new signal is correct or not as it can vary infinitely between the two extremes.

PCM is digital as it uses binary code like 011001110. This type of code can be recovered as it is either on or off, nothing else, so the intended signal is seen at the servo end. Or something along those lines anyway.

Angus