%p185，西電“信號處理”
%freqz_m函數可獲得濾波器的幅值響應（絕對和相對），相位響應及群遲延響應
%this function  is the modified version of freqz subroutine
%db is relative magnitude in db computed over 0 to pi radians
%mag is absolute magnitude computed over 0 to pi radians
%pha is phase response in radians over 0 to pi radians
%grd is group delay over 0 to pi radians
%w is 501 frequency samples between 0 and pi radians
%b is numerater polynomial of H(z) (for FIR:b=h)
%a is denominator polynomial of H(z)  (for FIR:a=[1])
function[db,mag,pha,grd,w]=freqz_m(b,a);
[H,w]=freqz(b,a,1000,'whole');
H=(H(1:501))';
w=(w(1:501))';
mag=abs(H);
db=20*log10((mag+eps)/max(mag));
pha=angle(H);
grd=grpdelay(b,a,w);