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[[File:Hordijk osc.png|thumb|200px|Panel design for a Rob Hordijk OSC HRM]]The '''OSC HRM''' (Harmonic Oscillator) is a 5U format module designed by [[Rob Hordijk Design|Rob Hordijk]]. It features controls to adjust frequency, smoothly blend between a range of waveforms, adjust modulation, and also includes a built in [[Voltage controlled amplifier|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
Later versions of the OSC HRM feature a toggle switch allowing selection between manual tuning (up),
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.
=== Harmonics ===
[[File:Hordijk osc fc.png|thumb|right|200px|The OSC HRM flowchart.]]
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.
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=== 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.
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<ref name=":0" />
=== SUB OUT ===
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.
== References ==
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