Simple high pass and low pass filters with frequency cutoff

hlp.py

/*
    simple high pass and low pass filters with frequency cutoff.
 */
var tau = Math.PI*2;
var v = 0;

lp = LP();
hp = HP();

function dsp(t) {
  return lp(hp(noise(), 5000+4000*Math.cos(tau*t*0.1)), 5000+4000*Math.sin(tau*t*0.1)) * 0.5;
}

function noise() {
  return Math.random();
}

function LP() {
  var dout=0, din=0;
  return function (x, cutoff) {
    var rc = 1.0/(cutoff*2*tau);
    var dt = 1.0/sampleRate;
    var alpha = dt/(rc+dt);
    dout = dout + alpha*(x - dout);
    din  = x;
    return dout;
  };
}

function HP() {
  var dout=0, din=0;
  return function (x, cutoff) {
    var rc = 1.0/(cutoff*2*tau);
    var dt = 1.0/sampleRate;
    var alpha = rc/(rc+dt);
    dout = alpha*(dout + x - din);
    din  = x;
    return dout;
  }
}