function [b,a] = afd_butt(Wp,Ws,Rp,As);
% 巴特沃斯型模擬低通濾波器設計
% -----------------------------------------
% [b,a] = afd_butt(Wp,Ws,Rp,As);
%  b = Ha(s) 分子的系數
%  a = Ha(s) 分母的系數
% Wp = 以弧度/秒為單位的通帶邊緣頻率; Wp > 0
% Ws = 以弧度/秒為單位的阻帶邊緣頻率; Ws > Wp > 0
% Rp = 通帶中的振幅波動的+dB數; (Rp > 0)
% As = 阻帶衰減的+dB數; (As > 0)
%
if Wp <= 0
        error('通帶邊緣必須大于 0')
end
if Ws <= Wp
        error('阻帶邊緣必須大于通帶邊緣')
end
if (Rp <= 0) | (As < 0)
        error('通帶波動或阻帶衰減必須大于0')
end

N = ceil((log10((10^(Rp/10)-1)/(10^(As/10)-1)))/(2*log10(Wp/Ws)));
fprintf('\n*** Butterworth Filter Order = %2.0f \n',N)
OmegaC = Wp/((10^(Rp/10)-1)^(1/(2*N)));
[b,a]=u_buttap(N,OmegaC);