SPT Filter

Switching Pole Topology 3320-based multimode VCF

The CFM SPT Filter is a 4 Pole Voltage Controlled Filter, based on the infamous CEM3320 Chip. SPT stands for Switching Pole Topology, referring to the ability of the filter's individual poles to be switched between High Pass and Low Pass Topology. This provides a multitude of different filter responses with each pole individually switchable on the fly. In addition to the Pole Topology switches it is possible to offset the pole’s center frequency. This changes the poles capacitor, adjusting the character of the filter and damping the resonance, the same way certain Japanese Synthesizer manufacturers have done in the past - only now this is in the control of the user and available across all poles. Additionally, circuitry is included to push the feedback loop to the extremities of what the chip can handle while remaining objectively musical.

The combination of these irregular controls gives the filter a unique character and makes it somewhat temperamental - small changes in settings can vary from almost imperceivable to drastic. It is recommended that you get to know your filter personally rather than approach it with expectations.

http://www.cfmodular.com/SPT_FILTER