filter.dsp

// Figure 3.18: 1-pole TPT lowpass filter with resolved zero-delay feedback

g = hslider("Parameter g", 0., 0., 1., 0.01);

process(x) = out with {
  G = g/(1+g); // gain term

  /* the signal that enters the recursive structure (after multiplier)
     as a function of z1 the output of the delay box */
  xIn(memZ1) = (x-memZ1)*G;

  out = z1'+xIn(z1')
        /* the top summer (+) that produces output
           it's (z1') because the value is taken after the delay applied */
  letrec {
    /* expression which compute next value in the delay box according to
       the previous one */
    'z1 =
      xIn(z1) /* left branch which enters bottom summer (+) */ +
      (xIn(z1)+z1) /* right branch which enters bottom summer (+) */
    ;
  };
};