Biquad Software Implementations

In matlab, an efficient biquad section is implemented by calling

        outputsignal = filter(B,A,inputsignal);

where

$$\begin{eqnarray*} \texttt{B} &=& [g, g\beta_1, g\beta_2],\\ \texttt{A} &=& [1, a_1, a_2]. \end{eqnarray*}$$

A complete C++ class implementing a biquad filter section is included in the free, open-source Synthesis Tool Kit (STK) [15]. (See the BiQuad STK class.)

Figure B.10 lists an example biquad implementation in the C programming language.

Figure B.10: C code implementing a biquad filter section.

 

  typedef double *pp;  // pointer to array of length NTICK
  typedef word double; // signal and coefficient data type

  typedef struct _biquadVars {
      pp output;
      pp input;
      word s2;
      word s1;
      word gain;
      word a2;
      word a1;
      word b2;
      word b1;
  } biquadVars;

  void biquad(biquadVars *a)
  {
      int i;
      dbl A;
      word s0;
      for (i=0; i<NTICK; i++) {
          A = a->gain * a->input[i];
          A -= a->a1 * a->s1;
          A -= a->a2 * a->s2;
          s0 = A;
          A += a->b1 * a->s1;
          a->output[i] = a->b2 * a->s2 + A;
          a->s2 = a->s1;
          a->s1 = s0;
      }
  }