% [db,mag,pha,grd,w] = freqz_m(b,a);
%　db = Relative magnitude in dB computed over 0 to pi radians
% mag = absolute magnitude computed over 0 to pi radians
% pha = Phase response in radians over 0 to pi radians
% grd = Group delay over 0 to pi radians
%　 w = 501 frequency samples between 0 to pi radians
%　 b = numerator polynomial of H(z)　 (for FIR: b=h)
%　 a = denominator polynomial of H(z) (for FIR: a=[1])