One of the modular synthesis subjects that I get asked about the most is the different types of frequency modulation, and what effects that have in the real world when you build a patch. In the video below, I demonstrate their differences – including their strengths and weaknesses, and how they affect your patch.

The term “frequency modulation” itself can have multiple similar but different meanings. In general, anything that changes the frequency of your sound sources – including the keyboard or sequencer pitch control voltage, as well as LFOs for vibrato, envelopes for pitch bends, etc. – can be called frequency modulation. 

For the sake of simplicity, I use the term “frequency modulation” or “FM” as shorthand for when I really mean “frequency rate modulation”: what happens when you modulate any parameter on a module at audio-rate frequencies (in other words, frequency rate modulation). You can “FM” a filter’s cutoff frequency, a crossfader’s mix between two sound sources, and so on. In this article, FM refers to using one audio-rate source (the “modulator”) to modulate the pitch of another audio-rate sound source (the “carrier”) – as in this example from the book PATCH & TWEAK:

When one VCO is patched to the pitch input of a second VCO, that second VCO’s pitch speeds up and slows down in response to the first VCO. See how the triangle wave of the carrier VCO is compressed or expanded in response to the sawtooth of the modulator VCO being positive or negative. The resulting harmonic spectrum is unlike either original waveform.

The following movie demonstrates the different types of FM, and how to tame them. For more details, read on below...