errors appear when doing FFT on data imported from text file
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trying to determine the frequency estimate after doing FFT on data imported from a text file.
load spot_num.txt
ssn = spot_num(:,3);
ssn = ssn-mean(ssn);
x=ssn;
xdft = fft(x);
xdft = xdft(1:length(x)/2+1);
freq = 0:Fs/length(x):Fs/2;
[maxval,idx] = max(abs(xdft));
freq(idx)
it returns:
Error in ==> Untitled3 at 7
freq = 0:Fs/length(x):Fs/2;
so whats wrong with that?
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Wayne King
2012-10-24
编辑：Wayne King
2012-10-24
Your sampling interval is 1 month, so your DFT bins will be in cycles/month. So you can just set Fs = 1, but then keep in mind that the frequencies will be in cycles per month, so your highest frequency of 1/2 is 1 cycle/2 months.
Also, the code I had given you earlier was assuming you have an even number of samples. If your signal length is odd, you have to make a small adjustment.
Fs = 1;
xdft = fft(x);
xdft = xdft(1:length(x)/2+1);
freq = 0:Fs/length(x):Fs/2;
[maxval,idx] = max(abs(xdft));
freq(idx)
1 个评论
modified covariance
2012-10-24
编辑：modified covariance
2012-10-24
更多回答（1 个）
Wayne King
2012-10-24
0 个投票
What error message does it produce?
What is the value of Fs? Nothing in the code you've shown us gives us the value of Fs.
Fs should be the reciprocal of the time interval between each sample in spot_num
1 个评论
modified covariance
2012-10-24
编辑：modified covariance
2012-10-24
thanks for ur help to tell the problem lis in Fs. the spot_num.txt file contains the data as follows with thrid column extracted onto matlab.
thrid column is SSN/monthly averages. i guess Fs is therefore 1/(24*31*60*60)??
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2012-10-24
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