function exp4()
%add noise w(n) to xb(n), resulting in y(n) = xb(n) + w(n)

n=1:64;
a1=0.1; f1=0.0625;
a2=1.0; f2=0.0625;
a3=0.1; f2=0.4375;
x1=exp(-a1*(n-1)).*sin(2*pi*f1*(n-1));
x2=exp(-a2*(n-1)).*sin(2*pi*f2*(n-1));
x3=exp(-a1*(n-1)).*sin(2*pi*f2*(n-1));
w=randn(1,64);%noise
y1=x1+w;
y2=x2+w;
y3=x3+w;
close all;
subplot(4,2,1);stem(w);title('w');
subplot(4,2,2);stem(abs(myfft(w)));title('w的幅度譜');
subplot(4,2,3);stem(y1);title('y1(a=0.1 f=0.0625)');
subplot(4,2,4);stem(abs(myfft(y1)));title('y1的幅度譜');
subplot(4,2,5);stem(y2);title('y2(a=1.0 f=0.0625)');
subplot(4,2,6);stem(abs(myfft(y2)));title('y2的幅度譜');
subplot(4,2,7);stem(y3);title('y3(a=0.1 f=0.4375)');
subplot(4,2,8);stem(abs(myfft(y3)));title('y3的幅度譜');