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problem in for loop for plotting

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Mary
Mary on 3 Apr 2017
Commented: Mary on 3 Apr 2017
hi, I have an EEG data and its matrix dimention is: 1068*4 now I want to undrestand which column relate to which brain wave, so I use Fourier transform and I write this code:
if true
load 'sig_x.dat'
fs = 500; % Hz
[N, M] = size(sig_x);
n = N/2;
f = (0:n-1)/n * fs/2;
for j = 1:M;
subplot(M, 1, j)
ff1 = fft(sig_x(:, j));
eegfft = abs(ff1);
figure(3)
plot(f, eegfft(1:n))
ylabel(['Wave ' num2str(j)])
end
but I don't get the spectrum of the first column. is there anything wrong whit this code ? or is there something I missed?

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Accepted Answer

Joseph Cheng
Joseph Cheng on 3 Apr 2017
Edited: Joseph Cheng on 3 Apr 2017
do you mean you're missing the first spectrum because you put the figure after the first instance of subplot? you should move figure(3) before the for loop. with this you create the subplot in whatever is the active figure/axes for the first iteration, jump to figure(3) to then plot the first iteration fft. however with that you overwrite that first iteration with the subsequent subplots.
if you're commenting that there is no frequency response maybe taking the log10(eeft(1:n)) would help you see the smaller frequency responses.

  2 Comments

Joseph Cheng
Joseph Cheng on 3 Apr 2017
a good sequence of events for axes creation is
  1. create/define figure
  2. instantiate subplots if desired
  3. plot/surf/imagesc/etc
as you'll create the container for the visual display in order, otherwise items 2 and 3 will be to whatever is considered the active figure.
Mary
Mary on 3 Apr 2017
many thanks for your answer.

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