[PDS]função freqz_m

function [db, mag, pha, w] = freqz_m(b, a)
% Versao modificada da funcao freqz
[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);