Variable Allpass Fbk is the phaser building block: a two-pole allpass filter whose notch frequency is swept up and down by an internal low-frequency oscillator, with a feedback path that sharpens the effect. Depth sets the depth of the allpass pole, Lower and Upper set the bottom of the sweep and its span, Rate (Hz) sets the sweep speed and Skew its shape, Width sets the notch bandwidth, Feedback (0 to 1) feeds the output back into the filter, and Output trims the level in dB. It runs in mono and stereo.
An exponential sweep wavetable, advanced at the chosen Rate, supplies a time-varying centre frequency; from it the two allpass coefficients are recomputed and the incoming sample is filtered, with the previous output added back through the Feedback gain. It processes one sample-frame per tick, in step with WaveWarp's sample-per-sample engine.