Harmonic Circles II & 
Poly Circles

Phase Modulation vs. Frequency Modulation - a preliminary clarification

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.

Nevertheless both techniques have been included here. The waves of this module can go to very low frequencies and in this case the two techniques begin to differ significantly -  FM performed by a sub-acoustic 'modulator' on an audible 'carrier' will produce a distinctive pitch vibrato, while PhM will produce a "phase sweeping" effect instead.

The basics of this module

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

Keep in mind, that many of the undertone harmonics will lie below the range of audibility, so they will act not so much as FM operators on their 'carriers', but rather as LFO-s and produce pitch or amplitude vibratos.

The details of this module

•  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 Sum 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.