I thought this topic might flush the academics out of the woodwork.
Both PCM and PPM are FSK modulated signals (so not really 'FM'). Basically it transmits on TWO frequencies, one a little bit below the frequency you picked (the '0') and one a little above it (a '1').
So it transmits on two frequencies. Would you say then that it is using different frequencies to "modulate" the input signal. Sorry but where I come from that is commonly called frequency modulation. Yes frequency shift keying is a specialist form of FM but it is still FM. To say that it is not FM would be like saying someone is not human because they are dutch!
In PPM your stick position is transmitted as the LENGTH of a pulse, so a low pulse of 1ms (milli seconds) means the stick full in one corner and a pulse of 2ms means the stick in the other corner. This does NOT give you infinite precision though, as switching on or off is never instantaneous, because radio waves are waves - you have only a limited number of waves per millisecond, and you need usually a few 'waves' to detect that something is transmitted. I read that the actual resolution of PPM is LESS than PCM in most practical cases.
I disagree here. Ok so you say we need a few "waves" to detect a signal. How many are we talking here?? 10?? 20?? Lets say 20. So now in a 72MHz radio we are getting 72 million waves per second. Thats 72000 in one millisecond, our worst case scenario. So to compare it to PCM it would be like having a 3600 bit PCM receiver. Seems a lot more acurate than 1024.
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In the long run however the precision on both systems is more than what is needed. Assuming full servo deflection is 60 degrees then even 512PCM has descrete servo positions 0.117 degrees apart. This gives a maximum error of 0.058 degrees. Even if the servo mechanics are that precise, the remaining helicopter mechanics would not be affected by such a small change (unless of course your servo horns are in the region of 30cm long). And if you manufactured a helicopter with links this precise I am sure the aerodynamics would not allow such a small change to be seen in flight preformance. If it were you would have a very uncontrollable heli in your hands.
As I said before, the main advantage with PCM is it's error detection abilities. A good analogy of that is having a construction worker receiving verbal commands as opposed to verbal+written. The the verbal only scenario (PPM) the command (signal) might not have been heard correctly over the construction noise (poor signal to noise ratio) and the worker might do the wrong thing. On the second scenario (PCM) the command is heard and then the written command is received (redundant error detection bits) and if they match then the command is performed else it is ignored).