Phase Shifter
16 Stage Analog Phase Shifter
The Pittsburgh Modular Phase Shifter is a complex, 16 stage, analog effect module designed to expand on the classic swirls, rich swooshes, and doppler effects associated with the phaser.
The Phase Shifter has a single 16 stage audio signal path to create a very rich, full sound. Phased audio outputs are tapped from stages 7, 8, 15, and 16. Stages 8 and 16 each have a mix output that allow the phased signal to be mixed with an attenuvertable dry signal.
Four CV inputs, an onboard LFO, and multiple CV routing options allow for maximum flexibility and extremely deep modulation options. The CV inputs are split into 2 groups. The first group of CV inputs controls either all 16 stages or just stages 1-8. The second group of CV input controls stages 9-16 and is switchable on or off. All CV signals routed to stages 9-16 can be inverted. Resonance can be positive or negative and the module will self oscillate in either direction.
To modulate the phase shifter circuit, the module includes a wide range low frequency oscillator. The LFO range varies from 1 minute per cycle to well into audio range. The triangle wave of the LFO is hardwired to modulate the frequency of all 16 channels of the Phase Shifter. External modulation sources are handled using 2 sets of CV inputs. Each set includes an invertible, uni-polar CV input that accepts 0v to 5v modulation signals and a bi-polar CV input which accepts -5v to +5v signals. All of the modulation CV inputs can be used simultaneously and can be mixed with the internal LFO to create very complex frequency modulations. Stages 1-8 and 9-16 can be modulated together or independently using the available switches and CV inputs.
HISTORY AND THEORY BEHIND THE PHASE SHIFTER
The origin of the phase shifter as an electronic effect starts with the studio tape technique of flanging. The technique involved playing identical copies of audio through two different tape machines simultaneously and pressing on the flange of one machine to slow down the tape. This action shifted the sound slightly out of sync to the first tape machine and created a moving notch filter effect.
Electronically, the effect is created using a series of all-pass filters. Each all-pass filter stage shifts the phase of the audio signal by 90°. Chaining a series of all-pass filters together creates a larger amount of shift and a richer sounding effect. Mixing the dry, unshifted signal with the wet, shifted signal produces a notch filter effect. The notch filter effect is caused by the interaction of the 2 waves. Some frequencies are canceled out and some are reinforced creating peaks and troughs in the frequency spectrum. For every 2 stages (180° of shift) another peak and trough is created. More notches in a notch filter the deeper the effect. The real magic of the phase shifter becomes apparent when the center frequency of each stage is modulated using an triangle wave or other modulation source to produce the signature sweeping sound
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