The Bubblesound Instruments Dual SeM20 is a two-channel VCF based around a mashup of the Oberheim SEM filter with the resonance section from the MS-20 for added character, as well as features found on the OSCar synthesizer. The signal path starts with a four-channel mixer with the option to route the inputs to filter 1, filter 2, or both. With the input knob at the center position, the input is at unity gain. Past that, the signal will begin to overdrive and distort. Each of the filters have two CV inputs with attenuators. There is a switch for either single or dual mode. In dual mode the top frequency knob and CV 1.1/1.2 control both filters. A filter 2 offset knob is available in dual mode and acts like the spread feature in the OSCar which spreads out the cutoff frequencies of the two filters. The notch balance knobs sweep between highpass and lowpass filtering. The many inputs and outputs make patching complex feedback loops easy and can result in unexpected behaviors. The SeM20 can really scream when pushed to its limits or can offer smooth Oberheim-style filtering.