I have one PCM RX and it is installed in my GP Christen Eagle II. This is the only plane I own running a gas-ignition engine, a Fuji BT-50SA. It was not my original intent to run PCM just because of the engine, though I understand that it probably does prevent ignition noise from interfering with my controls.

I was convinced with the PCM RX after running a simple test. I turned on my Futaba 9C, set it to PCM encoding, and did a range check with the airplane. All was great -- range was awesome. I then took out my Hitec Eclipse 7 (older version without the PCM capability), selected the same channel on the Spectrun module, and turned it on. Side-by-side, both in close proximity of the RX and no interference from the Hitec TX.

I did the reverse with another airplane that contained a PPM RX. I set the Hitec Eclipse to the proper channel and turned it own and range checked the airplane. Range with this combo was also acceptional. I then turned on the Futaba 9C, set to PCM encoding, and it immediately caused the airplane's controls to go haywire.

Since the PCM is based on error-controlled receipt of data packets it can easily ignore errant signals. NOTE: One method used by many digital systems for error control is the use of check digits. A data slice is composed of a number of on-off conditions. A check digit is an extra on-off condition that is set by the TX either on or off depending on the ratio of on-off conditions in the rest of the data slice. If you have interference in the data slice there is a possibility the ratio of on-off conditions will no longer agree with the check digit -- the RX can then declare that data slice as bad and ignore its info. Expand this concept across numerous data-slices and you get a full error-control system. Each manufacturer utilizes a unique protocol (how they use the check digit(s)) and that is why their PCM RXs are not compatible with other brand TXs runnning PCM encoding.

NOTE: This is the same concept used in modern-day highspeed digital communications, like the signal between two computers accross a network or telephone wires. It is also the same concept used in Digital Cell Phones.