Fourier Transform - signal

Question

viet le 2016-12-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/317669-fourier-transform-signal

评论： Image Analyst 2016-12-21

采纳的回答： Walter Roberson

tremor_analysis.txt

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Hello everyone,

I am doing relates to Fourier Transform form signal (as attached) to find the frequency of the signal. But my result is not correct (just I think because the result is so weird), I don't know where am I wrong. Plz help me fix this code.

Thank for your time

t1=0:359;
t=t1';
 x = load('tremor_analysis.txt');
plot(t,x)
y = fft(x);
m = abs(y);
p = angle(y);
f = (0:length(y)-1)*359/length(y);
subplot(2,1,1)
plot(f,m)
title('Frequency')

This is my result:

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Answer 1

Walter Roberson 2016-12-21

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https://ww2.mathworks.cn/matlabcentral/answers/317669-fourier-transform-signal#answer_248014

Remember that the first entry of the output, y(1), will store mean(x)*length(x), which is sum(x). Whenever your data is not centered around 0 you need to expect a peak at y(1).

You can either subtract off the mean(x) before doing the fft() or you can zero out y(1) before plotting.

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Walter Roberson 2016-12-21

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y = fft(x - mean(x));

Image Analyst 2016-12-21

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Not sure what you're unclear about. He showed you one way, by subtracting the mean like he originally said. Or alternatively zeroing out y(1) like this: