From MODELNET

<I'm new to this hobby. Could someone please tell me what PPM and PCM stand 
for and how they work?>

PPM - Pulse Position Modulation
PCM - Pulse Code Modulation

These relate to the way that the control information is encoded into the radio 
signal from the transmitter. An 8-channel transmitter transmits a signal that 
consists of "frames" of eight servo instructions followed by a "reset" command 
called a synch pulse. The receiver waits for the synch pulse and then sends 
each instruction that follows to the 8 channels in turn. If it loses track, 
say due to a bit of interference or a weak signal, it waits for another synch 
pulse and then starts again.

The original RC systems used simple pulses as servo instructions, with the 
width of the pulse indicating the servo position (called Pulse Width 
Modulation or PWM). A typical system might use a 1 millisecond (ms) pulse for 
one end of the servo travel, a 2ms pulse for the other end and a 1.5ms pulse 
for center. The signal transmitted by the current PPM transmitters is 
actually the same as the PWM system, but as the receiver actually measures the 
length of time from the end of the synch pulse to the end of each servo pulse 
it is called Pulse *Position* Modulation (PPM).

A relatively recent development in R/C has been to use microprocessors to 
encode the servo information in a digital binary format called Pulse Code 
Modulation (PCM). Instead of simple variable length servo pulses the 
transmitter now sends eight separate groups of fixed length pulses which comprise a binary number representing the servo position. In addition, the Synch Pulse is now a Synch "Word" - another binary number - which some would suggest gives it slightly better interference rejection. A PCM receiver needs a 
Microprocessor, so it is larger and drinks more current. To offset this 
disadvantage the microprocessor can be used for other things. PCM transmitters 
can be programmed to periodically transmit "failsafe" settings that the 
receiver stores and uses if it loses the signal from the transmitter. The 
Microprocessor in the PCM transmitter can be used to provide a range of 
programmable functions and mixing systems which are probably only used 
seriously by 5% of those who own them.

Your implied question is which system is best, and this isn't an easy one to 
answer. For most modelers the PPM systems are more than adequate. They are 
cheaper, lighter and probably more reliable. I race FAI-F3D class pylon racers 
and I NEVER use a PCM receiver in these models because they don't like the 
high G forces generated during cornering (and inadvertent triggering of the 
failsafe system would be highly dangerous on a model that does 200mph and 
flies at only 10-15 feet). On the other hand large models benefit from the 
added safety of the failsafe system - indeed in the UK it is a legal 
requirement to have a "failsafe device" on any model weighing more than 7Kg).

Most PCM transmitters can be switched to either PCM or PPM, so you can always 
get a system like the Futaba Field-Force Super 7 with a PPM receiver until you 
actually *need* a PCM receiver. These computerized sets are expensive, so my 
advice to you would be to get a simple 6 channel PPM set for now and only get 
an expensive computer set later if you feel the need. The simple set will be 
half the price...

Peter D Rieden
(Surrey, UK)