RE: SPCM or PPM
Sorry to burst someone's bubble, RedFox, but here's how it works:
PCM, XPCM, ZPCM, PPM, they're all transmitted using Frequency Modulation, FM.
It just depends on the method in which the information is used to modulate the frequency.
PPM, Pulse Positiion Modulation, is plain old FM as we all like to call it. The data is used to shift the frequency of the RF signal coming from the transmitter.
PCM, ZPCM, XPCM, are all Pulse Code Modulation techniques, different from one another, but still just digital information being used to modulate the RF from the transmitter.