[sdiy] PM vs FM was: Re: Buchla 295 10-band comb filter topology

[Ian Fritz]

OK, I've figured out what the FM/PM disagreement most likely stems from.

On Fig 8 of the patent the DX7 output is given by the expression

Sin{kwt + f(w_m t)}

 From fundamental theory, this expression is THE SAME for FM and PM. 
The difference between the two is in the function f().

For FM the function f is proportional to  delta(w)/w_m, where delta(w) 
is the maximum frequency deviation and w_m is the frequency of the 
modulating signal.

For PM f is proportional to just delta(w) itself.

Once again, these are textbook results: standard and well known. Their 
derivation can be found in any good text on radio theory.

I remember looking at the PM/FM question some time ago and being stopped 
because Yamaha never seemed to say explicitely what the definition of f 
is. Now I think it is given by the algo I outlined in my previous post. 
{NB k must have a different meaning here, and of course it must be time 
dependent or there would be no need for the accumulator.}

I'm enjoying this discussion, too, and would love to hear more comments 
from folks who have studied the theory.

Ian


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