A unique voltage controlled filter circuit, designed by Nyle Steiner circa 1975. Steiner was designing synth circuits for his company Steiner-Parker, and he wanted something different from the transistor ladder VCF designs typically used at the time, plus he wanted to avoid having to pay royalties to Moog since their patent on the transistor ladder circuit was still in effect at the time. Also, he wanted a multimode design.
Steiner settled on a circuit based on a Sallen-Key topology to produce a multimode, two-pole filter. An unusual characteristic of the filter is that instead of having multiple outputs for the different passband responses, it has low pass, bandpass, and high pass inputs. The desired response is selected by routing the signal to be filtered to the proper input. Multiple signals can be routed to the different inputs, and the filter will apply the desired response to each and mix the results, with some intermodulation occurring within the circuitry. Oddly, none of the Steiner-Parker products made use of the capability to route multiple signals into the inputs, but when the circuit started to appear in modular synths in the 1990s, the individual inputs were exposed.
The circuit now appears in module designs from a number of modular synth manufacturers. Some add an all pass input, to take advantage of the filter's phase shifting capability.