Wow quite some nonsense here!

Both PCM and PPM are FSK modulated signals (so not really 'FM'). Basically it transmits on TWO frequencies, one a little bit below the frequency you picked (the '0') and one a little above it (a '1').

In PPM your stick position is transmitted as the LENGTH of a pulse, so a low pulse of 1ms (milli seconds) means the stick full in one corner and a pulse of 2ms means the stick in the other corner. This does NOT give you infinite precision though, as switching on or off is never instantaneous, because radio waves are waves - you have only a limited number of waves per millisecond, and you need usually a few 'waves' to detect that something is transmitted. I read that the actual resolution of PPM is LESS than PCM in most practical cases.

In PCM your transmitter position is send as a 10 bit binary code. Midstick is 512 = 100000000, low end is 0 and high end is 1023=111111111. Both a 0 and 1 have fixed transmission time of 150 microseconds so PCM can transmit about 13 data bits in the time that PPM sends one servo pulse. However PCM is usually somewhat slower than PPM because alot more bits are sent. Most gory details are in a publication on my website:
http://graphics.tudelft.nl/~wouter