BP Track uses parallel pitch-tracking bandpass filters to produce… strange, unruly sounds. Alien moans, birds chirping. In many ways, it reminds me of the product of an old analog modular synthesizer.

**WARNING**: This patch uses feedback loops; there are some precautions placed to limit their effect, but I cannot guarantee they will be effective in every circumstance.

Instead of oscillators, though, the patch resynthesizes its input; the input can be thought of as both the source of pitch information and the exciter for the circuit.

There are three bandpass filters; their PITCH refers to their offset frequency from the incoming signal, so A1 would be one octave above the input, etc. The most pronounced resonance will be at the fundamental, so it is good to keep one filter tuned to this, but experiment, too.

Each filter (A, B, C) has a FEEDBACK path. The feedback paths… feed back. Squeals, oscillations, gurgles … the circuit can shut down if there is too much gain. Such is life.

There is also a CROSS FEEDBACK path, which feeds A into B and B into A.

The LEFT STOMPSWITCH turns on a mix between filters A, B, and C and filters A and B combined through a BIT MODULATOR to add distortion and harmonics.

Because the filters can get… wild, I hacked together a sort of LIMITER, using an envelope follower and a VCA. When the threshold is set low, it responds to softer sounds but more gently; as it is set higher, it responds to louder sounds more aggressively.

Also, the MIDDLE STOMPSWITCH will kill the feedback loops, if they start getting too out-of-hand (this is a momentary footswitch, because it can also be used as an effect in its own way).

There is a simple ATTACK DECAY envelope, triggered by the input. It can be applied to the input itself to control the amplitude of the “exciter,” — this condition is set by ENV BYPASS button — or it can be applied to filters B and C’s frequency with the FREQ ENV control, for pitch effects. The LINEAR button will change the FREQ ENV from exponential (off) to linear (on).

There is also a NOISE MOD, which modulates the filters B and C with digital noise (random module).

There is a FREQ GLIDE control, which determines how quickly the filters react to changes in pitch.

The sound exits through a plate reverb, to add atmosphere and space, with controls for REVERB DECAY and REVERB MIX.