FM vs PM

danial stocks diode at
Mon Aug 7 11:03:41 CEST 2000

>But: Yamaha operators do PM, this sounds different.
>We had this dicussion some time ago, it was concluded that using a
>differentiator before the FM input would theoretically give
>PM, but it is of course not feasible.

The difference between PM[phase mod] & FM is that PM is not only 
proportional to amplitude, but frequency of the modulating source as well.. 
to create FM from a PM modulator you need a 6db/oct LPF at the modulator 
i/p. Conversely, to create a PM from FM modulator, a 6db/oct HPF is used. It 
should be possible to use a simple RC network so long as the modulating i/p 
is high-z. Of course, the DC offset will create differing effects between a 
'synthetic PM' and a real PM.

>I remember that I posted a proposal to use an unmodifyed
>osc as index into a table (really a function network)
>in order to produce tri waves, another control voltage (the PM input)
>is added to that. This way the index will move arround in PM fashion.
>The table has of course to contain a couple of tri cycles in
>order to get a reasonable output.
>But: frequency accuracy is really important doing this kind
>of modulation (be it FM or PM). It is very hard to have this
>kind of accuracy with analog osc.

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