Tom Fawcett posted this the other day in the Wildhare support forum so credit goes to him for the explanation. This is also about the best and most concise explanation of the pro vs. cons of PCM vs. PPM(FM).

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In cases where the radio link is less than perfect, PCM can cause as many problems as it solves.

A regular FM receiver just takes the signal coming in, splits it into 8 streams (for an 8 channel radio) and sends the individual streams out to the 8 different servo channels.

If there is noise or an invalid servo signal, that also gets passed straight out to the servo. This is why, with a PPM (FM) radio you see the surfaces jump around when noise creeps in, they are just following the signal which sometimes is not valid.

With PCM there is processing involved at both ends, Tx and Rx. In the Tx, the 8 (or whatever) signals are combined into a frame where each servo position is represented digitally by a binary number. This is where the "PCM 1024" comes from, they use 10 bits to represent the servo's position so there are 1024 possible positions.

The values for all the channels are then packed into a frame (now nominally 80 bits of data for 8 x 10 bit channels) with a header and trailer, and at the end of the fram a check sum is appended that is calculated as the bits are transmitted.

When the receiver gets the frame, its internal processor recalculates the checksum of the received frame and compares it to what was received. IF they are equal the new values for each servo are transformed into the signal which is sent to each the servo.

But if the checksums do not match, the received frame is discarded and the values from the most recent good frame are retained. This is what is called going into a "hold", the receiver "holds" the last known commanded position that was without errors.

It gets more complicated with fail safe, but the basic strory here is that PCM covers up small errors that will show up with PPM. If you are having problems, you may not see them with PCM resulting in a mistaken idea that your radio is working great when it's not.

If you EVER have an insufficient range check, do not fly, and I recommend installing a PPM/FM receiver (temporarily) to help diagnose the problem. It's sort of like turning on a light, suddenly you can see all the problems that were hidden by the PCM Rx.

Once you get an acceptable range check with a PPM/FM receiver and with the engine running, now you can go back to the PCM and recheck the range.

It's worth the $60 (Hitec Supreme 8 channel FM) to keep one of these around just for checking out you plane, even if you never fly with it.