h=fir1(48,0.3);
N=300;p=0.36;
u=randn(1,N)*sqrt(p);
x=u(1:N);
y=fftfilt(h,x);
subplot(221);plot(x);grid;
xlabel('t/Td'); 
ylabel('x(t)');
title('白噪聲輸入樣本');
subplot(222);plot(y);grid;
xlabel('t/Td'); 
ylabel('y(t)');
title('白噪聲濾波后輸出樣本');
subplot(223);
[Px,fx]=periodogram(x,window,N,N);
plot(fx,10*log10(abs(Px)));grid;
ylabel('幅度（dB）');
title('白噪聲輸入功率譜');
subplot(224);
[Py,fy]=periodogram(y,window,N,N); 
plot(fy,10*log10(abs(Py)));grid;
ylabel('幅度（dB）');
title('白噪聲濾波后輸出功率譜');