Our Envelope Filter is based on DOD 440, which became famous after Johny Greenwood from Radiohead used it in their live shows.
The circuit uses Vtl5c4/2 optical chip that triggers the high-pass filter the more you hit the strings.
The controls are “Level” (duration of the effect) and “Range” (sensitivity). Note that if you set “Range” too high, the signal overdrives the optical chip and produces a distortion effect.
We modified the circuit by increasing input and output capacitors' values, so the effect has more low end than the original.