moogvcf.js

moogvcf.js

function MoogVCF(){
  var y1, y2, y3, y4, oldx, oldy1, oldy2, oldy3;

  y1 = y2 = y3 = y4 = oldx = oldy1 = oldy2 = oldy3 = 0;

  var p, k, t1, t2, r, x;

  return function(cutoff, res, input){
    cutoff = 2 * cutoff / sampleRate;
    p = cutoff * (1.8 - (0.8 * cutoff));
    k = 2 * Math.sin(cutoff * Math.PI * 0.5) - 1;
    t1 = (1 - p) * 1.386249;
    t2 = 12 + t1 * t1;
    r = res * (t2 + 6 * t1) / (t2 - 6 * t1);

    x = input - r * y4;

    // four cascaded one-pole filters (bilinear transform)
    y1 =  x * p + oldx  * p - k * y1;
    y2 = y1 * p + oldy1 * p - k * y2;
    y3 = y2 * p + oldy2 * p - k * y3;
    y4 = y3 * p + oldy3 * p - k * y4;

    // clipper band limited sigmoid
    y4 -= (y4 * y4 * y4) / 6;

    oldx = x; oldy1 = y1; oldy2 = y2; oldy3 = y3;

    return y4;
  };
}