FIR Software Implementations

In matlab, an efficient FIR filter is implemented by calling

        outputsignal = filter(B,1,inputsignal);

where

$$\texttt{B} = [b_0, b_1, \ldots, b_M].$$

It is relatively efficient because filter is a built-in function (compiled C code in most matlab implementations). However, for FIR filters longer than a hundred or so taps, FFT convolution should be used for maximum speed. In Octave and the Matlab Signal Processing Toolbox, fftfilt implements FIR filters using FFT convolution (say ``help fftfilt'').

Figure 5.6 lists a second-order FIR filter implementation in the C programming language.

Appendix I lists a C++ class for general FIR filters, observing programming conventions used in the Synthesis Tool Kit (STK) [15].

Figure 5.6: C code for implementing a length 3 FIR filter.

 

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

  typedef struct _fir3Vars {
      pp outputAout;
      pp inputAinp;
      word b0;
      word b1;
      word b2;
      word s1;
      word s2;
  } fir3Vars;

  void fir3(fir3Vars *a)
  {
      int i;
      word input;
      for (i=0; i<NTICK; i++) {
          input = a->inputAinp[i];
          a->outputAout[i] = a->b0 * input  
                +  a->b1 * a->s1  +  a->b2 * a->s2;
          a->s2 = a->s1;
          a->s1 = input;
      }
  }