Harmonic Circles II

Harmonic Circles II

is a complex oscillator primarily based on the phase modulation of sines, but also adding ring modulation and frequency modulation to it.

Phase Modulation vs. Frequency Modulation

Phase Modulation synthesis is very close to Frequency Modulation (FM) synthesis. As long as the carrier as well as the modulator are sines, PhM modulation and FM modulation often sound completely the same. So it went quite unnoticed, that many devices labeled as FM synthesizers in fact implemented Phase Modulation - a practice that had already started with the synthesizer that introduced this technique: the famous Yamaha DX7.

But there is a case in which PhM and FM really differ and that is, when the modulator wave has a very low frequency - FM will produce a pitch vibrato in this case, while PhM will produce a "phase sweeping" effect.

The basics

Harmonic Circles consists of four sine-oscillators, named "A", "B", "C" and "D". Each of those is tuned to a harmonic of the frequency that is received via the CV In jack. All harmonics can be overtones or undertones. Furthermore the four harmonics modulate each other clockwise: "A" modulates "B", "B" modulates "C", "C" modulates "D" and "D" again modulates "A".

Many of the undertone harmonics will lie below the range of audibility. So when using them exclusively, the module will become a complex low-frequency oscillator, capable of producing polyrhythmic undulations.

The details

• A push on the orange-colored buttons will cycle through the available types of modulation for each oscillator: phase modulation ("PhM"), frequency modulation ("FM"), ring modulation combined with low-pass filtering ("RiM") or a simple mix with the previous oscillator ("ADD").
• The intensity of the modulation (in "PhM" and "FM" mode), the frequency of the filter (in "RiM" mode) as well as the balance of the mix (in "ADD" mode) can be controlled by the adjacent Mod Amt knobs in conjunction with the CV input jacks attached to it. Attenuators nearby regulate the influence of the input jacks.
• Oscillators B and D can self-modulate as well. Here the modulation type will always be phase modulation, whose intensity is controlled by the Feedback B and Feedback D knobs to the extreme left and right of the module.
• The output send to the Out jack is obtained by summing the "A" and the "C" oscillator, the mix being set by the A + C Balance knob in the middle of the whole structure.

PS: You may notice that a little guardian inside the module prevents you from setting all four oscillators to frequency or ring modulation at the same time - such a combination wouldn't produce any sound.