The 2U wide Rob Hordijk Harmonic Oscillator module (OSC HRM) is used to create pitched waveforms with dynamically controlled timbres. Pitch control law is 1V/Oct and the module uses a platinum element for temperature stabilization of the pitch curve. Maximum frequency range is from 0.5 Hz to 16.000 Hz and the scale is perfectly tuned in the middle six octaves up to a pitch of 4.000 Hz. When playing pitches higher as 4000 Hz the 1V/Oct scale starts to break down, due to the necessary internal bandlimiting in the harmonic generators.
The OSC HRM flowchart.
The module uses a biquad sine/cosine oscillator at its core and through a process of recursion harmonic series of overtones are generated. There are two recursion paths, one that produces all harmonics and one that produces only the odd harmonics. By gradually opening the knobs that control the amount of recursion more and more harmonics are generated. When only the all harmonics path is used the waveform morphs smoothly from a sinewave to a waveform that closely resembles and sounds like a sawtooth or an inverted sawtooth. Opening only the odd harmonics knob will smoothly morph from a sinewave to a squarewave. When opening both knobs effects like pulse wave modulation are possible. Building up these harmonic series is under full voltage control and can be modulated from slow LFO speeds to fast audio rates to create FM timbres. When the waveforms are modulated there is a negligable amount of detune (less than 1 cent), though when modulating at audio rates an asymmetry in the modulating waveform can cause detune effects on deep modulations.
There is an additional VCA incorporated in the module. The final output signal can be taken from a point just before the VCA and at the output of the VCA. This enables the module to be easily used in a situation where one wants to modulate another module by an audio rate signal and have the modulation depth under voltage control using e.g. a LFO waveform, an envelope voltage signal or a play controller that produces a control voltage, while still having the full output level signal available on the full output to serve different purposes.
The waveforms have an exceptionally warm sound and when dynamically modulated have a deep spatial and organic character. With only one OSC HRM and one DUAL ENV module you can already have a voice with dynamic timbral and volume control that can do e.g. a pretty solid bass line.