Listing 2. Code for a simple filter
          
          void simple_filter(double *x, double *y, int N,
          double a, double b)
          {
              double y0 = 0, y1 = 0;
              int i = 0;
          
              for(; i < N; i++)
              {
                 y[i] = a*y0 + b*y1 + x[i];
                 y1 = y0;
                 y0 = y[i];
              }
          }


