Technically it’s not really true FM synthesis, as what we are doing is really done with phase modulation (As it was in the Yamaha FM series of Synthesizers.
Frequency Modulation, used as a form of synthesis, was discovered experimentally by John Chowning, and was popularized initially by the Yamaha DX-7 synthesizer.
The idea is to use the output of one oscillator to modulate the frequency of another. The result is a rich, complex sound containing many, many more harmonics than is possible with subtractive synthesis.
The Yamaha DX-7 used a variation on direct frequency modulation called phase modulation. The result is the same, but with phase modulation it’s easier to control the pitch and depth of modulation independently. (Phase modulation should not to be confused with phase distortion.)
Recreating FM synthesis with SynthEdit
The first oscillator is referred to as the modulator, and the second oscillator the a carrier. Rename the modules to make this clear as you progress. I have just used structure groups to illustrate this. This is a very basic form of FM synthesizer. The scope is there so you can see as well as hear what is going on when you increase the amount of modulation. Both oscillators should be sine wave, as the results with other waveforms can become rather unpredictable. Likewise it’s a good idea for both oscillators to receive a Sync pulse from the trigger output of the MIDI to CV2 module.
Dynamic Modulation.
As things stand this is a very basic design, and any tonal variation has to be done manually, which isn’t always very convenient, or quite what we want. However there’s quite a simple solution to this:
We can add another ADSR module with the usual Trigger/Gate connections to control the amount of Modulation reaching the carrier oscillator, so now we can dynamically change the modulation level every time a note is played.
Adding a feedback loop.
Yes you can have feedback in SynthEdit with one caveat: this is due to the way SynthEdit works it introduces a delay one sample long, so although we can have feedback it won’t behave quite like the feedback in a hardware FM synth (sorry, but there’s no way round this) . We just need a special module for this, the Feedback – Volts module which feeds the output of the Carrier oscillator back to the Modulator oscillator’s PM input via a Level Adj module. You’ll find you only need quite moderate amounts of feedback, as too much can cause some quite “unmusical” results. However despite these points it’s worth adding the feedback loop as it does increase the sonic possibilities quite drastically.
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