Three Sisters is a link filter that performs various tone shaping using three filters.
It consists of three filters, each with input and output. There is also an ALL input that inputs all three filters, and an ALL output that mixes and outputs the output of all filters. Useful for self-patching such as feeding back output to input. Depending on the modulation, you can create a formant sound like a talking voice, or use it as a triple sine wave / Ping sound source.
The three are separate filters, but their cutoff frequencies move in tandem. The filter settings vary greatly depending on whether the switch is "Crossover" or "Formant".
CrossoverThen, Low is a low-pass filter, HIGH is a high-pass filter, and CENTER is a variable-bandwidth band-pass filter that passes between them. FREQ controls the center frequency, and SPAN controls the difference between the LOW and HIGH frequencies.
FormantThen, Low, Center and High are all bandpass. REQ controls the center frequency, and SPAN controls the difference between the LOW and HIGH frequencies.
When "Quality" is turned to the right, it becomes a resonance control, and controls the degree of emphasis of the cutoff frequency of each filter determined by FREQ and SPAN. When fully turned, it self-oscillates and can take out sine waves of different frequencies from LOW to HIGH. Turning it to the left produces an effect not found in ordinary filters, which weakens the area around the cutoff frequency.