Rob Hordijk OSC HRM

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Panel design for a 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 also includes a built in VCA.


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 25kHz 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.[1] 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.[2]

Later versions of the OSC HRM feature a toggle switch allowing selection between manual tuning (up), middle C or 261 Hz (centre), and C-1 octave or 130 Hz (down). In the middle position (TUNE) the PITCH knob is disconnected to avoid accidental detuning, though the FINE knob still works. This preset tuning makes use with external control such a 1V per octave MIDI input more stable.

The Fine knob course is about two half notes up and two half notes down. It is limited for the reason that it should be able to precisely set the detune between two oscillators To make a detune of e.g. a fifth it is more interesting to use the quantizer from the NodeProcs, as this module can add the correct voltage to e.g. a 1V/Oct signal for transposing a particular note interval, up to five octaves up or down and optionally quantized to a chosen scale.

The module also features a CHAIN OUT output which functions as a mult simply passing the 1V-OCT IN signal for ease when chaining multiple oscillators together.


The OSC HRM flowchart.

The ODD HRM control produces an output that morphs between a sine (left) and square (right) wave at the output. The ALL HRM control produces an output that morphs between an inverted saw (left) to the output of the ODD HRM control (centre) to a regular saw (right).

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.[2]


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.

It can be used simply to control the level, like other VCAs, or patching the VCA out to one of the harmonic modulation inputs makes the module's waveform modulate itself under control of the VCA[1]


The SUB OUT provides a pulse wave taken from the ODD HRM path, one octave below the PITCH frequency. The output level is controlled by the VCA LVL and VCA MOD knobs and input signals. The curve of the SUB OUT is a bit louder than the VCA out at a lower level and a bit less loud at full level.


This page uses Creative Commons Licensed content from Mod Wiggler Wiki:Rob Hordijk Designs (View authors).

  1. ^ a b The Harmonic Oscillator (OSC HRM) by Benoit Faivre (2018)
  2. ^ a b Rob Hordijk

External links