double *openAFE::dcof_bwlp( int n, double fcf ) {
int k; // loop variables
double theta; // M_PI * fcf / 2.0
double st; // sine of theta
double ct; // cosine of theta
double parg; // pole angle
double sparg; // sine of the pole angle
double cparg; // cosine of the pole angle
double a; // workspace variable
double *rcof; // binomial coefficients
double *dcof; // dk coefficients
rcof = (double *)calloc( 2 * n, sizeof(double) );
if( rcof == NULL ) return( NULL );
theta = M_PI * fcf;
st = sin(theta);
ct = cos(theta);
for( k = 0; k < n; ++k )
{
parg = M_PI * (double)(2*k+1)/(double)(2*n);
sparg = sin(parg);
cparg = cos(parg);
a = 1.0 + st*sparg;
rcof[2*k] = -ct/a;
rcof[2*k+1] = -st*cparg/a;
}
dcof = binomial_mult( n, rcof );
free( rcof );
dcof[1] = dcof[0];
dcof[0] = 1.0;
for( k = 3; k <= n; ++k )
dcof[k] = dcof[2*k-2];
return( dcof );
}
void design_butter_lowpass_filter(
unsigned int order,
float wc,
float *b,
float *a)
{
int i, n;
double fcd, fca;
double *p;
double *d;
double pmr, pmi;
double ppr, ppi;
double ppmr, ppmi;
double d1, d2;
n = order;
fcd = M_PI * wc;
fca = tan( fcd / 2.0 );
p = (double *)calloc( 2 * n, sizeof(double) );
/* Calculate analog Butterworth coefficients */
for( i = 0; i < n; ++i )
{
p[2*i] = -fca * sin(M_PI*((double)i+0.5)/(double)n);
p[2*i+1] = fca * cos(M_PI*((double)i+0.5)/(double)n);
}
ppmr = 1.0;
ppmi = 0.0;
for( i = 0; i < n; ++i ) {
pmr = p[2*i] - 1.0;
pmi = p[2*i+1];
ppr = p[2*i] + 1.0;
ppi = p[2*i+1];
d1 = pmr * pmr + pmi * pmi;
p[2*i] = (ppr * pmr + ppi * pmi) / d1;
p[2*i+1] = (ppi * pmr - ppr * pmi) / d1;
d1 = ppmr;
d2 = ppmi;
ppmr = -( d1 * pmr - d2 * pmi );
ppmi = -( d1 * pmi + d2 * pmr );
}
ppmr = pow( fca, (double)n ) / ppmr; /* scaling factor */
d = binomial_mult( n, p );
b[0] = (float) ppmr;
d1 = (double)n;
d2 = 1.0;
for( i = 1; i <= n; ++i ) {
b[i] = (float) (ppmr*d1/d2);
d1 *= (double)(n - i);
d2 *= (double)(i + 1);
}
a[0] = 1.0f;
for( i = 0; i < n; ++i )
a[i+1] = (float) (d[2*i]);
free( p );
free( d );
}