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

[René Schmitz]

Hi Ian and all,

On 27.11.2022 00:30, Ian Fritz via Synth-diy wrote:
> Many years ago on this list I asked why a distinction was made between PM and FM, as they are equivalent, phase being proportional to frequency (at fixed frequency, of course).
>
> In response, Juergen Haible pointed out the difference only becomes relevant when considering what happens to the signal when you go up and down the keyboard. For FM the waveform stays the same. For PM it does not. This behavior can easily be seen in the standard textbook equations going back to early radio theory.
>
> The book by Chowning and Bristol, “FM theory and applications” deals extensively with the DX7. It is specially noted more than once that the DX7 waveform does not change with keyboard pitch. So from this fact and the book title I conclude the DX7 does indeed produce FM signals.
>
> So why is there this apparent disagreement? Does the DX7 waveform change with keyboard pitch or not? I’ve asked this question before but never got an answer. I do not have a DX7, but the waveforms of the plugin emulations I’ve seen are constant.
>
> I’ve looked at a Yamaha patent and seen how the computational engine works. It does, of course, calculate in terms of changes in phase. But that does not necessarily mean that what is calculated is a standard PM signal, does it?
>
> Ian
>
>

A PM waveform should stay the same too.

Provided you move both carrier and modulator in the same manner. If you 
turn off operator tracking on a DX7, or it's emulations then of course 
not...

Comparing output waveforms on one particular implementation, can not be 
considered definitive proof.

As there are too many other accidental influences . (Including operator 
error, firmware secret sauces, filters, EQs, bypass caps, distortion, 
what not).


Sonically, both PM and FM can cover pretty much the same terrain.

In a nutshell the difference between PM and FM is what happens when the 
modulating operator has a DC offset.

With plain FM you get a net de-tuning (and your note is off), with PM 
you only get an inaudible phase difference.

This is what makes PM the mode that is musically more useful.

(For FM you integrate over this offset, and that DC creates the rising 
phase that detunes things.)

When you do complex patches with several cascaded operators, any 
integration error gets worse, as now your frequency ratios are off.



Best,

  René


>
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