PPM = Pulse "Position" Modulation and not "proportional", just to add something.

Just to know, of course there are advantages of AM-Amplitude modulation, otherwise there would be no AM radios anymore.
Considering that no satelites are available to everyone, AM is the way to modulate low frequency carriers, that means long wave lengh that reach much farther than high frequency carriers. FM is applied over relatively high frequencies (HF and up = 30Mhz and up) with good performance.
You can listen to AM radios from everywhere in the planet, without good quality and reliability, but works.
For RC and anything that needs to be reliable, Didital modulation of course is the better option, noise rejection, multiple digital channels and features.
The future however is CDMA (code division multiple access) that already works for a long time for the military and lately for Celular networks and hopefully will come to RC one day. This process will allow all pilots to transmitt at the same frequency, side by side, and have their planes just under control. On CDMA spread spectrum, each user have a unique "digital" code that makes the miracle. No cristals or frequency boads anymore!

Note.: Currently, the CDMA cellular phone networks allows up to 64 user to talk at the same frequency, at the same time.

Nilo