%例程4－25    基于頻率抽樣方法2實現Ⅲ型FIR數字帶通濾波器

clear all;            
N=41;
T1=0.1095;
alpha= (N-1)/2;
l=0:N-1;
wl= (2*pi/N)*l;
Hrs=[zeros(1,6),T1,ones(1,7),T1,zeros(1,11),T1,ones(1,7),T1,zeros(1,6)];                                       %理想振幅采樣響應
Hdr=[0,0,1,1,0,0];
wdl=[0,0.3,0.3,0.7,0.7,1];
k1=0:floor((N-1)/2);
k2=floor((N-1)/2)+1:N-1;
angH=[pi/2-alpha*(2*pi)/N*(k1+0.5),-pi/2+alpha*(2*pi)/N*(N-k2-0.5)];                                            %相位約束條件
Hdk=Hrs.*exp(j*angH);                        %構成Hd(k)
h1=ifft(Hdk,N);
n=0:1:N-1;
h=real(h1.*exp(j*pi*n/N));                   %實際單位沖激響應
[db,mag,pha,w]=freqz_m2(h,[1]);
[Hr,ww,a,L]=hr_type3(h);                     %實際振幅響應
subplot(221)
plot(wl/pi+1/N,Hrs,'.',wdl,Hdr)
title('頻率樣本Hd(k) :N=41')
axis([0 1 -0.1 1.2])
subplot(222)
stem(l,h)
title('實際單位脈沖響應h(n)')
subplot(223)
plot(ww/pi,Hr,wl/pi+1/N,Hrs,'.')
title('實際振幅湘響應H(w)')
axis([0 1 -0.1 1.2])
subplot(224)
plot(w/pi,db)
title('幅度響應(dB)')
axis([0 1 -80 10])