Phase 90
Four all-pass stages sweeping two notches, and exactly one knob.
Notches, from cancellation
All-pass stages shift phase without touching level, and summing the shifted signal back against the dry one cancels wherever the two disagree. Four stages put two notches in the response, and the oscillator walks them up and down.
- Sweep low
- Sweep high
Drawn to illustrate the description above. Horizontal axis: frequency, low to high.
What the circuit does
Four all-pass filter stages shift phase without changing level, and the shifted signal is summed back with the dry one. Where the two cancel, a notch appears, and four stages give two of them. A low-frequency oscillator moves the stages, so the notches sweep.
What to listen for
A narrow, dry swirl rather than a liquid one, because there is no resonance feeding back into the sweep. One control means it is voiced rather than adjustable, which is a large part of why it is so recognisable.
Where it goes
The modulation slot
After the gain and before the delay, so the delay repeats a modulated signal rather than modulating an already repeated one. Reversed, the movement gets applied to the echoes as a block and the effect smears instead of swirling.
Controls
- Speed