figure(1);
subplot(2,2,1);
N=64;n=0:N-1;t=0.01*n;q=n*2*pi/N;
x=2*sin(4*pi*t)+5*cos(8*pi*t);
plot(t,x)
title('原始信號(hào)')
subplot(2,2,3)
y=fft(x,N);
plot(q,abs(y))
%l=abs(y);
title('FFT N=64')
x1=2*sin(4*pi*t)+5*cos(8*pi*t)+2.5*randn(1,N);
subplot(2,2,2)
plot(x1)
title('加噪信號(hào)')
subplot(2,2,4)
y1=fft(x1,N);
plot(q,abs(y1))
title( '加噪FFT N=64')