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> exa040801_fft.m: for example 4.8.1
> to test fft.m dn ifft.m
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clear all;
> 产生两个正弦加白噪声；
N=256;
f1=.1;f2=.2;fs=1;
a1=5;a2=3;
w=2*pi/fs;
x=a1*sin(w*f1*(0:N-1))+a2*sin(w*f2*(0:N-1))+randn(1,N);
> 应用FFT 求频谱；
subplot(3,1,1);
plot(x(1:N/4));
f=-0.5:1/N:0.5-1/N;
X=fft(x);
y=ifft(X);
subplot(3,1,2);
plot(f,fftshift(abs(X)));
subplot(3,1,3);
plot(real(x(1:N/4)));