Decoding = demodulating digital audio, PPM = pulse position modulation, FM/FM or spread spectrum is very complicated, but easy with the correct LSI = large scale integration chip.
If I remember correctly, there are 4 methods of decoding FM = frequency modulation: ratio detector, slope detection, quadrature something and I forgot.
AM amplitude modulation, a 1N34 diode. Single side band, same as AM except you have to reinsert the carrier frequency before detection with the 1N34 or one of several other methods. My guess is even very advanced ET = extraterestials can't decode some of our transmissions, and we can't decode ET transmissions without the correct LSI which are only manufactured on ET's home world. Reverse engineering is futile. ET likely has some modulation methods, we have never dreamed of. I'm sure my list above is seriously incomplete. Neil