A mono resonant filter with four modes and a contour that sweeps the cutoff every time you play a note. Put it after an oscillator, a drum loop or a guitar take and play it from a keyboard to make the tone move.
The idea
In a subtractive synth, nothing changes the sound as much per degree of rotation as the filter. Filter gives you the classic four shapes, plus an ADSR contour that opens and closes the cutoff in time with your playing. The contour is triggered by notes, so the audio passes through continuously while MIDI decides when each sweep starts and ends.
