function [db,mag,pha,w] = freqs_m(b,a,wmax);
% s域頻率響應計算的改進版本- 
% ------------------------------------------------------------
% [db,mag,pha,w] = freqs_m(b,a,wmax);
%   db = [0 to wmax]區間內的相對振幅
%  mag = [0 to wmax]區間內的絕對振幅
%  pha = [0 to wmax] 區間內的相位響應
%    w = [0 to wmax]區間內的500個頻率樣本向量
%    b = Ha(s)的分子多項式系數
%    a = Ha(s)的分母多項式系數
% wmax = 希望獲得響應的頻率區間的最大頻率
%
w = [0:1:500]*wmax/500;
H = freqs(b,a,w);
mag = abs(H);
db = 20*log10((mag+eps)/max(mag));
pha = angle(H);