SVF [State Variable Filter]

A state variable filter (SVF) is a common design for synth filters. This design lends itself to allowing low pass, high pass, and bandpass all being available simultaneously. Another side effect is that they are not prone to oscillating at high feedback (resonance) settings, although some have certainly figured out how to make this happen. The Oberheim SEM (Synthesizer Expander Module) filter is perhaps the most famous state variable design. I have been told this design introduces a lot of phase shift on output, which can add coloration.