TwinPeak is a 12db voltage controlled filter designed by Rob Hordijk. The filter includes a “wave folding” circuit that allows change to its response-curve—from low pass to variable width band pass and in between. This response morphing capability is based on the “inverse-parallel” principle for low pass filters, where two filters with identical gain can be combined to create a band pass curve but with two separate peaks at the corners of the pass band. It is this same principle that forms the basis of the Twin Peak Resonator in Hordijk’s Blippoo Box as well as his own 5U-18db module.
This set up offers distinct advantages over a low pass/high pass-series configuration. E.G., in band pass mode, with the curve knob fully clockwise, the cutoff frequencies of the filters can be crossed and, regardless of which is tuned higher or lower, the filter output will be a band pass response. In either case, all frequencies below the cutoff of the lower tuned filter are suppressed or subtracted from the filter output. Drawing on this same functionality, we can also set the pass band to near zero bandwidth with both filters tuned in unison, regardless of the cutoff frequency.
The inverse parallel architecture of this filter allows for a wide array of nuances and gradations across the response spectrum. However, what truly sets this filter apart is its exceptional resonance—ideal for creating warm, percussive ringings and bell-like effects. The TwinPeak is specially designed to tilt momentarily into self-oscillation when pulsed waveforms (even gates, triggers or one-shots) are used as audio inputs. These pings have a longer duration at low frequencies, e.g.; when either peak is tuned to approximately 200hz, it will sustain for 4 to 5 seconds.
When the module is used in this way each complementary filter has a separate, tunable ping. In full band pass mode, where the TwinPeak is at highest resonance, each ping represents the two peaks on either end of the pass band. This dual-pinging action adds another interesting dimension to the filter’s sound as the pings can be tuned and detuned together, in ways that mimic basic two-string frequency dynamics and the primitive, “sensory beats” of Helmholtzian physics.
This is a unique and high quality filter that allows for many subtle possibilities and nuances. It works great with audio frequency signals, particularly at lower resonance settings, while higher resonance settings are ideal for creating pings and other percussive effects. This being said, the module’s two inputs and level/fade control allow one to mix between low frequency and audio rate signals. Going this route and modulating the filter’s resonance elicits outlandish cross-fertilizations, disclosing new, hybrid spaces that blur its functional dichotomy.