The Verbos Complex Oscillator is an interesting interpretation of the classic Buchla “complex oscillator” pair where one modulates the other. It too has two oscillators with internal wiring and options so the modulation oscillator on the left can amplitude or frequency modulate the carrier oscillator on the right. The carrier oscillator is internally connected to a Harmonics section to its right that contains the classic wavefolding plus even versus odd and high versus low harmonic emphasis, all under voltage control. This result is presented at the Master output, along with other individual waveforms.

What makes the Verbos different than some other complex oscillator pairs is that it is optimized for below-audio-rate modulation:

• The modulation oscillator is tuned ~4 octaves lower than the carrier
• Its waveforms are unipolar (ranging from 0v between +8 and +10 volts)
• The internal frequency modulation bus is exponential rather than linear

As Mark Verbos explained, this in particular makes it easier to set up predicable vibrato that tracks across the keyboard, as well as good tremolo effects when the modulation bus is set to AM instead of FM. In this mode, the amplitude modulation is “two quadrant” at lower modulation amounts (unipolar, like modulating a normal VCA), but changes to bipolar four-quadrant ring modulation at higher modulation depths.

If you prefer linear frequency modulation, or to use a bipolar oscillator waveform to modulate either of the Verbos’ two internal oscillators, both oscillators feature separate linear FM (labeled “f.m.”) and exponential FM (labeled “c.v.”) inputs in addition to the normal, tracking 1 volt/octave inputs. In the movie below, I demonstrate the Verbos’ internal exponential frequency modulation, as well as using the VCO in the Moog Mother-32 to provide linear FM for the Verbos’ carrier oscillator (fed through an Intellijel uVCA to control the modulation amount):

This is one of five movies on the Verbos Complex Oscillator that is in my course Learning Modular Synthesis: Eurorack Expansion. The first one is an overview of the various sections of the Verbos CO and its waveform outputs; the second covers the Harmonics section; the third is the movie above; the fourth demonstrates amplitude modulation; the fifth shows off the soft sync function of the Verbos where the carrier oscillator can reset the modulation oscillator if it is close to completing a wave cycle. This wiring allows you to set up a cross-modulation loop where the left frequency or amplitude modulates the right, and the right re-syncs the left. All in all, it’s a deep implementation of the classic “complex oscillator” design and an interesting choice to consider adding to your core East Coast system.

By the way, I also demonstrated exponential and linear FM with a pair of “normal” oscillators in my original Learning Modular Synthesis course; if you’re interested, here’s a movie on patching exponential FM using the two halves of a Roland 512 dual (but not “complex”) VCO. If you want more background on creating sounds with frequency modulation, here’s a good article & compilation of video+audio examples by Ali Jamieson.