function [b,a] = afd_elip(Wp,Ws,Rp,As);
% 橢圓型模擬低通濾波器設計
% --------------------------------------
% [b,a] = afd_elip(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

ep = sqrt(10^(Rp/10)-1);
A = 10^(As/20);
OmegaC = Wp;
k = Wp/Ws;
k1 = ep/sqrt(A*A-1);
capk = ellipke([k.^2 1-k.^2]); % Version 4.0 code
capk1 = ellipke([(k1 .^2) 1-(k1 .^2)]); % Version 4.0 code
N = ceil(capk(1)*capk1(2)/(capk(2)*capk1(1)));
fprintf('\n*** 橢圓濾波器階次 = %2.0f \n',N)
[b,a]=u_elipap(N,Rp,As,OmegaC);