t = 0:0.001:0.6;  
％產(chǎn)生原始信號
x = sin(2*pi*50*t)+sin(2*pi*120*t);
％對原始信號加入0平均的隨機(jī)噪音干擾
y = x + 2*randn(size(t))
％繪制干擾后的信號頻譜圖，見圖9-28所示
plot(1000*t(1:50),y(1:50))
title('Signal Corrupted with Zero.Mean Random Noise')
xlabel('time (milliseconds)')
％運(yùn)行512點(diǎn)得快速傅立葉變換
Y = fft(y,512)
Pyy = Y.* conj(Y) / 512
%獲得開始的257個數(shù)據(jù)點(diǎn)，其余的225個點(diǎn)不用
f = 1000*(0:256)/512
%繪制頻譜圖，見圖9-29示
plot(f,Pyy(1:257))
title('Frequency content of y')
xlabel('frequency (Hz)')