//! fcd = cut-off (1=sampling rate)
//! ord = Filter order
//! ripple = passband ripple in dB
void chebyshev2_iir(iir_coeff& filt, float_type fcd, float_type stopband = 40.0) {
const float_type ten = 10.0;
auto order = filt.getOrder();
float_type delta = pow(ten, -stopband/20.0);
float_type epi = delta/sqrt(1 - delta*delta);
float_type wca = (filt.get_type()==filter_type::high) ? tan(M_PI * (0.5-fcd)) : tan(M_PI*fcd);
chebyshev2_s(filt, wca, epi, order);
filt.bilinear();
if (filt.get_type()==filter_type::bandpass || filt.get_type()==filter_type::bandstop) {
filt.make_band(filt.get_center());
} else {
filt.convert_to_ab();
}
if (filt.get_type()==filter_type::bandpass) filt.set_bandpass_gain();
}
//! Calculate poles (chebyshev)
void chebyshev2_s(iir_coeff& filt, float_type wp, float_type epi, size_t order) {
auto l = 1;
size_t n2 = (order + 1) / 2;
float_type x = 1 / epi;
float_type lambda = pow(x*(1.0 + sqrt(1.0 + epi*epi)),1.0/order);
float_type sm = 0.5*((1.0/lambda) - lambda);
float_type cm = 0.5*((1.0/lambda) + lambda);
for (size_t j = 0; j < n2; j++) {
float_type arg = M_PI * (2*l-1) / ((float_type)(2*order));
std::complex<float_type> p = std::complex<float_type>(sm * sin(arg), cm * cos(arg));
if (filt.get_type()==filter_type::low) {
// Get regular chebyshev pole first
// then transform for inverse chebyshev
filt.set_pole((-wp*p/norm(p)), n2-1-j);
// inverse chebyshev zero
filt.set_zero(std::complex<float_type>(0,wp/cos(arg)),n2-1-j);
} else {
filt.set_pole(-p/wp, n2-1-j);
filt.set_zero(std::complex<float_type>(0,cos(arg)/wp),n2-1-j);
}
l++;
}
}
