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%x(n)的心电脉冲函数 
x=[-4,-2,0,-4,-6,-4,-2,-4,-6,-6,-4,-4,-6,-6,-2,6,12,8,0,-16,-38,-60,-84,-90,-66,-32,-4,-2,-4,8,12,12,10,6,6,6,4,0,0,0,0,0,-2,-4,0,0,0,-2,-2,0,0,-2,-2,-2,-2,0]; 
subplot(2,2,1); 
n=0:55; 
stem(n,x,'.'); 
xlabel('n'); 
ylabel('x(n)'); 
title('x(n)的心电脉冲函数'); 
%通过滤波器H1(z)后的y1(n)函数 
A=0.09036; 
b1=[A,2*A,A]; 
a1=[1,-1.2686,0.7051]; 
h1=filter(b1,a1,x); 
[H1,w]=freqz(b1,a1,100); 
%通过滤波器H1(z)、H2(z)后的y2(n)函数 
b2=[A,2*A,A]; 
a2=[1,-1.0106,0.3583]; 
h2=filter(b2,a2,h1); 
[H2,w]=freqz(b2,a2,100); 
%通过滤波器H1(z)、H2(z)、H3(z)后的y3(n)函数 
b3=[A,2*A,A]; 
a3=[1,-0.9044,0.2155]; 
h3=filter(b3,a3,h2); 
[H3,w]=freqz(b3,a3,100); 
subplot(2,2,2); 
stem(n,h3,'.'); 
xlabel('n'); 
ylabel('y(n)'); 
title('通过滤波器H1(z)、H2(z)、H3(z)后的y3(n)函数'); 
%通过滤波器H1(z)、H2(z)、H3(z)后的对数频率响应20log[Ha3(ejw)]函数 
subplot(2,2,3); 
H4=H1.*(H2); 
H=H4.*(H3); 
mag=abs(H); 
db=20*log10((mag+eps)/max(mag)); 
plot(w/pi,db); 
xlabel('w/п'); 
ylabel('20log[Ha3(ejw)]'); 
title('通过滤波器H1(z)、H2(z)、H3(z)后的对数频率响应20log[Ha3(ejw)]函数'); 
grid;