Hafid

fs=8000;
t=0:1/fs:1;
x=sin(2*pi*500*t);
x=x+sin(2*pi*2100*t);

bBP = [0.0201 0.0000 -0.0402 0.0000 0.0201 ]
aBP = [1.0000 -2.1192 2.6952 -1.6924 0.6414 ]
y=filter(bBP,aBP,x);

Ak1=abs(fft(x))/length(x);
k1=0:1:length(x)-1;
f1=k1*fs/length(x);

Ak2=abs(fft(y))/length(y);
k2=0:1:length(y)-1;
f2=k2*fs/length(y);

figure(1);
subplot(2,1,1)
plot(f1,Ak1)
subplot(2,1,2)
plot(f2,Ak2)
figure(2);
freqz(bBP,aBP,512,fs);