You are correct, but technically speaking, both PCM and PPM are encoding schemes, NOT modulation schemes. All of the PPM/PCM switchable radios that I know of use FM (frequency modulation). The old AM radios were also PPM encoding, and there is no reason that a PCM radio could not transmit in AM (not that anyone would want to!).

Think of it this way, if you use a DSC cord to connect the transmitter to the receiver, you will be sending the PPM or PCM pulse train to the receiver before it is modulated to RF (radio frequency). Without the DSC cable, the transmitter then modulates the pulse train to the RF frequency, and all modern radios that I know of use FM.

I have an older Ace Silver Seven radio, and I can use either AM or FM RF decks. Either way, the encoding scheme is the same -- PPM.

Having said all of that, it is generally understood that PPM stands for FM.

PPM uses pulse widths, or "pulse positions", to indicate the servo position information. Decoding this is simple, each pulse is separated and sent directly to the corresponding servo. One long pulse is used to indicate the end of a pulse frame, the next pulse will then be channel 1, the next is channel 2, etc.

PCM takes the channel information and converts it into codes, very similar to the way two computers communicate via serial ports, with error-checking. The information is in a proprietary binary format, and each channel position is represented by a number (10 bits, or 0 to 1023 for most radios). Several numbers (channels) will be sent together as a frame. Additional information is included at the end of each frame that is used to verify the frame was received correctly by the receiver (I don't know the exact scheme, whether it is checksum or CRC). This is how the failsafe feature works, by being able to differentiate between good and bad frames. PCM receivers must have a microprocessor to work, and this is one reason why they are more expensive than the regular PPM receivers.

To get back to the original question, according to Futaba's website, the 6X transmitter can do either PCM or PPM. You will need a receiver that matches whichever mode (PCM or PPM). If the corresponding PPM receiver is a dual-conversion type (designated by a DF in the model number) then you can move the crystal from the PPM receiver to the PCM receiver.