errors appear when doing FFT on data imported from text file

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?

Answer 1

[Wayne King]

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)

[modified covariance]

thx again for ur response. let me change fs to 1 now.

sorry to bother you again. how to make the adjustment since my signal lenght is odd.

Answer 2

[Wayne King]

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

[modified covariance]

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)??