wp=0.2*pi;wr=0.3*pi;Ap=1;Ar=15;
T=1;Omegap=(2/T)*tan(wp/2);Omegar=(2/T)*tan(wr/2);

%function [b,a]=afd_butt(Omegap,Omegar,Ap,Ar);
if Omegap<=0
    error('通帶邊緣必須大于0')
end
if (Omegar<=Omegap)
    error('阻帶邊緣必須大于通帶邊緣必須')
end
N=ceil((log10((10^(0.1*Ap)-1)/(10^(0.1*Ar)-1)))/(2*log10(Omegap/Omegar)));
fprintf('\n***巴特沃斯模擬低通濾波器階數=%2.0f\n',N)
OmegaC=Omegap/((10^(0.1*Ap)-1)^(1/(2*N)))

%function [b,a]=u_buttap(N,OmegaC);
[z,p,k]=buttap(N);
p=p*OmegaC;
k=k*OmegaC^N;
B=real(poly(z));
b0=k;
b=k*B;
a=real(poly(p));