One way to create more complex waveforms (and therefore, harmonics) than traditionally supplied by oscillators is to use one oscillator to modulate the frequency of another. Below I’ll describe how to set up an “FM” (frequency modulation) patch, and include a movie from my Learn Modular Synthesis course that demonstrates the “exponential” flavor of FM.
In general, there are two approaches:
- exponential frequency modulation, where the “modulator” oscillator is connected to a 1 volt/octave exponential input on the “carrier” oscillator
- linear frequency modulation, where the modulator is connected to a special FM input on the oscillator (some oscillators have a switch that can convert a modulation input between exponential and linear)
It’s a good idea to have at least an attenuator – and better yet, a VCA connected to an envelope generator or other modulation source – between the modulating oscillator’s output and the CV input on the carrier. This allows you to play with the “index” or depth of modulation; in general, deeper modulation results in more complex harmonics. You can tune the oscillators to whatever interval you like, but start with unison than branch out to octaves, fifths, and other intervals. Then route the carrier’s output straight to a final VCA. After you get a handle on the basic patch, try running the carrier through an additional filter, waveshaper, or other sound-modifying modules.
FM patches tend to produce very complex harmonic spectra, so it’s customary to start out with relatively low-harmonic waveforms such as triangles or sines for both your modulator and carrier, and then move onto more complex waveforms once you’ve got a good grasp of the patch and if you desire noisier, more complex sounds. Tuning is critical; when the oscillators are out of tune with each other, the result is even more clangorous than simple beating between two detuned oscillators (which can be great for percussion sounds, and annoying for melodic applications). This shows the result – both in the waveform, and harmonic spectra – of one triangle modulating the frequency of another:
Some synth manufacturers make “complex” VCO modules that contain two oscillators pre-configured as an FM pair, with convenient controls for the index of modulation as well as optional waveshaping. However, thanks to the nature of modular synths, you can patch together almost any two VCOs to create an FM pair. Here’s a typical exponential FM patch (you can ignore the white patch cables going to the Disting and Braids):
(As usual, I’m using the color scheme where white cables carry pitch control voltages, blue carry other modulation control voltages such as envelopes, red carry gates, and yellow carry audio. The black cables are coming from my controller keyboard.)
The movie below shows how to create an exponential FM patch using the two oscillators in a Roland 512 module. Initially I run the output of the carrier oscillator through one half of a Roland 530 Dual VCA; later I patch the modulating oscillator through the second VCA to dynamically control the index of modulation:
An advantage of exponential FM is that you can patch it on virtually any modular synthesizer, as the VCOs tend to be exponential by nature. The downside is that the perceived pitch of the carrier oscillator’s output can change whenever you alter the index of modulation or even the waveform of the modulating oscillator. Once you’ve tuned the oscillators, the resulting sound can be played chromatically; just be careful about enveloping the modulator’s wave into the carrier, because the perceived pitch may change during the envelope (which is actually cool for percussion).
Linear FM – especially when your carrier oscillator has “through zero” capabilities (meaning the output waveform reverses rather than flat-lines when the aggregate pitch being asked of it goes below 0 Hz) – subjectively sounds “sweeter” to my ears than exponential FM, making it better suited for tonal sounds. A big bonus is that the perceived pitch remains stable while you change or envelope the index of modulation. I particularly like the combination of the a Mutable Instruments Braids in “FOLD” mode – which outputs either a sine or triangle through a wavefolder (and this can be voltage controlled itself) – fed through a VCA and then into Input Y on an Expert Sleepers Disting in mode 4C: VCO with Linear FM; the Disting is perfectly comfortable with both negative voltages and negative frequencies. I take output A, which is a very pure sine wave before it is modulated.
Hopefully that gives you a start to exploring FM sounds with your modular. As you get comfortable with this patch, you can create more complex modulator/carrier relationships, such as cascading three oscillators instead of two (which in essence has the effect of dynamically creating more complex modulation waveforms to feed the final carrier oscillator), or patching up feedback or cross-modulation from the carrier back to the modulating oscillator.
There is a dedicated Training page that has more information about my Learn Modular Synthesis course, including a URL you can use for
10 30 days of free access to it and the other courses available on LinkedIn/Lynda.com. My course also has movies on linear FM, AM, and waveshaping, and other related techniques. Enjoy!