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

[Donald Tillman]

Uhhh, not exactly.

The "k" value is the tuning of the operator with respect to the note being played.  So it's 1 by default.  It's not time varying.

The phase modulation happens in Fig 8.  The carrier phase value is kwt.  The modulator phase value is f(wt), which is the enveloped signal from another operator with its own k value.  They are added, which would be the phase modulation, and then sent to the sine lookup table.

(By the way, I'm loving this discussion.)

  -- Don
--
Donald Tillman, Palo Alto, California
https://www.till.com

> On Nov 27, 2022, at 11:15 PM, Ian Fritz via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> With a careful reading of the Yamaha patent it is easy to understand how the FM signal of the DX7 is generated. The algo is quite simple:
> 
> 1) A keypress produces a carrier wave with a frequency w (omega)
> 
> 2) This frequency is multiplied (modulated) by a time-varying signal called a scale factor k. The resulting product is the instantaneous frequency of the system.
> 
> 3) The instantaneous frequency values thus calculated are fed to an accumulator, which produces the time-integrated value of the instantaneous frequency. This is the phase of the output signal.
> 
> This is a calculation of a standard frequency-modulated signal. This is obviously FM, simply because the frequency is what is modulated (not the phase).
> 
> This algo is EXACTLY analagous to the analog circuitry we use for making FM modules:
> 
> 1) A carrier frequency is generated via the V/Oct input and the usual exponential control current generator
> 
> 2) This current is multiplied by the value of the signal fed to the Lin FM input
> 
> 3) This product is fed to the integrater which is part of any VCO to produce the FM signal
> 
> Looks to me that Chowning and Yamaha are correct in saying the DX7 is an FM synth.
> 
> Ian
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