Rob Hordijk OSC HRM

The OSC HRM (Harmonic Oscillator) is a 5U format module designed by Rob Hordijk. It features controls to adjust frequency, smoothly blend between a range of waveforms, adjust modulation, and includes a built in VCA.

Tuning
The OSC HRM can track 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 16000 Hz and the scale is perfectly tuned in the middle six octaves up to a pitch of 4000 Hz. With external CV the pitch range is 18 octaves. When playing pitches higher than 4000 Hz the 1V/Oct scale starts to break down, due to the necessary internal band-limiting in the harmonic generators.

Later versions feature a toggle switch allowing selection between manual tuning (up), a stable preset frequency (centre), and -1 octave (down). The module also features a CHAIN OUT function that simply passes or mults the 1V-OCT IN signal for use chaining multiple oscillators together.

Harmonics
While an intuitive understanding of the modules functions might suggest a mixer or blend between odd and even harmonics, the results are actually produced by a kind of linear frequency modulation (FM).

FM is normally done with two sine waves (such as the Yamaha DX series) but the OSC HRM uses a bi-quad sine/cosine oscillator at its core and through recursion or feedback a harmonic series of overtones are generated. There are two feedback paths using Chebyshev function, one that produces all harmonics and one that produces only odd harmonics.

By gradually opening the knobs that control the amount of feedback more and more harmonics are generated. When only the all harmonics path is used the waveform output 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 negligible amount of de-tuneing (less than 1 cent), though when modulating at audio rates an asymmetry in the modulating waveform can cause stronger de-tuning.

VCA
The OSC HRM also includes a built in VCA. The final output signal can be taken from a point just before the VCA (FULL OUT) as well as the output of the VCA, allowing for parallel processing of modulated and un-modulated amplitude.