FSK synthesis

via Ricko on electro-music.com
"I have been playing around for the last week using FSK (frequency shift keying) as a synthesis method. I don't think I have ever seen anyone write about this method in public, so here goes.

It is a way of getting good control over two separate formants with only two oscillators.

The configuration is simple: a slave (sine) oscillator synced to a master (square) oscillator. The output is from the slave. The base frequency is set from by the master oscillator, and the formant frequency is determined by the frequency of the slave oscillator, as you would image.

How the FSK set up is unusual is that you add to the Fin of the slave a little of the square wave (adjusted so that it the "space" is always 0V). So during the mark part of the square wave, the sync oscillator is a higher frequency than the space period: the effect is that you get an extra formant. The frequency of this 2nd, higher formant is set by the amount of the square modulation from the master (if the mark is 1V signal, the 2nd formant will be 18ve higher, etc). The relative levels of the formants to each other are determined by the pulse width of the master.

(Obviously, if the master frequency is higher than the formant frequencies, you don't get this formant effect.)

What is quite interesting is that if you want to have fairly fixed formants, you don't need to feed in the keyboard CV, and even if you want variable formants you can do it with a very simple oscillator without high-end (expensive) tempco because the base frequency oscillator stability is entirely determined by the master oscillator: fluctuations in the slave will show up as minor tone changes which may indeed be pleasant.

It would be easy to build an FSK waveshaper: just a syncable LFO driven to audio rates at a minimum.

The kinds of tones that are available tend to be woodwind-ish and sparse, and thin and buzzy at the low end, but interesting. Changing the square to another waveshape stops the FSK effect and is useless for the formant effect. Selecting richer waveforms than the sine obviously makes more harmonics which can disguise the formant effect but can be interesting in their own right.

(To do this digitally, you have to clip the square wave rather than use a band-limited version.)

I have found what is quite useful is to use the FSK in conjunction with a BP filter (signal from the master VCO) also controlled by the formant1 CV and output +/_ with the slave osc. That allows a richer band of harmonics, with exact formant either being emphased as if you had some super Q going on, or cancelled out."