PPM is "regular" FM...frequency modulation. PCM is a totaly different type of modulation and you need a different transmitter to make that happen.

The nice part about that is every PCM transmitter I know of can be switched to transmit in PPM, so all of your regular FM receivers can still be used. Most PCM radios have a several model memory.

The main benefit with PCM are the failsafe settings...if you lose control of your model the RX will move the servos to a preset position (set by the user) or hold them in the last position sent by the transmitter. Each channel is set individually (i.e. throttle to idle if signal is lost).

Someone else can probably give you a more scientific explanation.