Through-Zero FM

[Michael B. Irwin]

Hi Martin,
Thanks for your reply. 10 years ago I built a monophonic 2-op "FM"
synthesizer using an 8051 microcontroller running at 18 MHz ( scaled
8-bits with 19 KHz sample rate ). The algorithm was a linear
through-zero sine wave oscillator with 2 inputs: 1 for freq derived from
key pressed, and the other was the modulating sine input. The carrier
sine and the modulator sine both had  amplitude envelopes. The
carrier:modulator freq ratio stayed constant across the keyboard. The
modulator amplitude was scaled according to which note was being played,
resulting in a constant modulation index ( constant #semitones
deviation) as you went from one end of the keyboard to the other ( this
would make the scheme PM, as you say ). In order to keep the timbre from
varying as the same note was repeatedly pressed it was necessary to
reset both oscillators at the start of the note so the initial phases
were the same each time. The sound was same as a Yamaha synth, but
grittier, and only 2-op. Without resetting the phases the sound varied
slightly each time the note was played, but not that much. This leads me
to think that this could be done well enough in analog to be useful.
Use  linear through-zero VCO's with an exponential keyboard voltage-vary
the gains to set the c/m ratio-the modulator sine is dynamically varied
in amplitude with a VCA whose gain  in turn is linearly scaled by the
keyboard C.V. to effect phase modulation (or a drifty approximation of
it). i would be interested to know if anyone has tried this.