February 06, 2018

The Harmonic Oscillator (OscHRM)

Let's start the description of the modules with this beautiful oscillator.

The Harmonic Oscillator (OscHRM) is a fully featured, wide range, voltage controlled oscillator with simultaneous dynamic control over pitch, timbre and amplitude.

Pitch control law is 1V/Oct and the module uses a platinum element for temperature stabilization of the pitch curve. The scale is perfectly tuned in the middle six octaves up to a pitch of 4.000 Hz. With external CV the pitch range is 18 octaves. When playing pitches higher as 4000 Hz the 1V/Oct scale starts to break down, due to the necessary internal band limiting in the harmonic generators.

The OscHRM uses a biquad sine/cosine oscillator at its core and through a process of linear FM feedback harmonic series of overtones are generated. By gradually opening the knobs that control the amount of recursion more and more harmonics are generated. This arrangement allows all sorts of filter-y effects without using a filter.

The first circuit adds odd harmonics morphing the sine wave into a square wave.

The second circuit adds all harmonics, modulating between a down-saw, through a sine, to an up-saw. The center position is basically whatever the first circuit is currently doing to the waveform. The waveforms have an exceptionally warm sound and when dynamically modulated have a deep spatial and organic character. The HRM MOD CV input has an attenuator. PWM can be created by turning up the ODD knob and changing ALL knob setting.

Dynamic wave shaping is available 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.

The OscHRM also includes a VCA with manual initial gain control and an attenuated CV input. This is useful for manipulating FM levels. Having a VCA at the end of the signal chain does not affect timbre, this arrangement does allow timbre changes, as well as ring modulation type sounds. 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. The velocity parameter can be controlled with MIDI. The advantage is that sometimes effects like distortion are depending on loudness. Direct oscillator output (Full out) is doubled for convenient patching to two destinations (i.e. to have two separate signal paths sourcing from one oscillator).

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.

There is a Sub Out to the OscHRM. This suboctave is a pulse one octave below the osc that inherits its pulsewidth from the EVEN and ODD settings and modulations. Also 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 as the VCA out at a lower level and a bit less loud at full level.

The switch has three positions. In the upward position the pitch knob can tune the osc between +/- 0.5Hz and 25kHz, so over the full range. In the middle position the pitch knob is disconnected and the osc tunes to middle C, so it can be easily played by a 1V/Oct MIDI CV without the possible chance of severely detuning the osc by accidentally twisting the pitch knob. The fine knob still works, of course. The third position tunes the osc an octave below middle C.

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 Chain Out jack is like a multiple when chaining two or more oscs to one 1V/Oct signal. It is handy when e.g a MIDI->CV box or analog keyboard has only one 1V/Oct jack, in which case a cable is connected between the first osc chain output and the second osc 1V/Oct input.

Here is the flowchart of the Harmonic Oscillator :

Check these videos for sounds and more explanations :

And this one explains very well how the harmonics are created in the Harmonic Oscillator.

Thanks to Todd Barton who provided the picture of the module.

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