The data in a PCM signal is digital, ones and zeros, like a CD. The data in PPM is analogue, it is the length of time the signal is on. They are data types, nothing to do with radio, they can be sent by electrical signal, light flashing, smoke signals, sound signals and so on.

As said in the post above, for our purposes both are sent on normal FM radio, there is no difference there. PPM is often called FM but that is false, both PPM and PCM are sent on exactly the same FM radio.

All servos are PPM, so PCM Rx have to convert the incoming signal back into analogue PPM before they can send it to the servos.

PCM's big advantage used to be that it alone could do error detection (though not error correction) and then go into failsafe. However the modern PPM Rx from a few firms such as Multiplex can also do error detection and have fully programmable failsafe.

PCM can only be used with that brand's Tx and Rx, you can't mix brands, whereas with PPM you can mix brands including PPM failsafe.

PCM Rx are more expensive than PPM or PPM failsafe Rx.

PCM is only available on computer Tx, PPM failsafe is available to any Tx even a basic 4 ch non-computer Tx.

At the extremes, if interference is switching rapidly on and off PCM can get a bit more of its signal through uncorrupted compared to PPM due to the way it tends to split the whole data set into smaller packets whereas PPM has to send the whole data set in one packet.

FMA claims that its Digital Signature Recognition technology on PPM with failsafe is even better than PCM at getting the signal through interference but I have not been able to find anyone who has bought one and tried it for real.

H.