Rob Hordijk Phaser Filter

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The 2U wide Rob Hordijk Phaser Filter module combines 5 allpass poles with three lowpass poles in one module. The five allpass poles are configured as a phaser with positive feedback resonance control, creating two resonant peaks when opened. Then a crossfade knob fades between the input and the output of the phaser and this crossfade mix is the input signal into the lowpass filter. At the end of the allpass chain before the crossfader is a phaser monitor output. The lowpass section has a cutoff slope of -18dB/Oct and its own resonance control.
The Phaser Filter flowchart
Both the phaser section and the lowpass section have a 1V/Oct control law. The phaser has one modulation input and if it is not connected the audio input signal is used as the modulation signal. This allows for dynamic waveshaping of the input signal on the time axis without detuning to signal.

The filter section has two modulation inputs, if the first is not connected the filter audio input signal is used to modulate the filter cutoff, allowing for even more dynamic waveshaping just like in the phaser section. If the second modulation input is not used it uses a signal from halfway the lowpass poles to self-modulate, thus producing all-harmonic distortion on the resonance peak when the resonance is set fairly high.

When the 1V/Oct input jack for the filter is not used it inherits the signal from the phaser section 1V/Oct input jack.

Both phaser and filter can sweep over a range of roughly 18 octaves and can be modulated up to really high audio rates. In this last case FM-type and ring-modulator-type effects occur, but with much more timbral control than traditional ring-modulators. E.g. when the outputs of two OSC HRM modules, set to sinewave output and tuned in some interval, are mixed and routed into the filter just slight amounts of the internal modulation on either the phaser or the filter will start to produce ‘undertones’ and ‘overtones’ that are sum and difference frequencies of the interval. This exemplifies the idea behind the Phaser Filter architecture, to not only take material away like a normal filter does but to also produce new material not present in the input signal and combine the both to create a vast range of possible timbres.

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