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Dear all matlab users,

I wrote this code

l = 128;

t = 0:l-1;

fs = 32;

f = 32/128 * 17; % f=4.25 Hz

x = sin(2*pi*f/fs*t);

l1 = 128;

l2 = 64 ;

x1 = fft(x,l1)/l1;

x2 = fft(x,l2)/l2 ;

fres1 = fs/l1;

fres2 = fs/l2;

figure(2);

plot([0:length(x1) - 1]*fres1,abs(x1), 'b');

hold('on');

plot([0:length(x2) - 1]*fres2,abs(x2), 'r');

hold('off');

What is the relation between x1 and x2 ?

Thanks in advanced.

Arturo Moncada-Torres
on 17 May 2011

OK, I suggest doing a quick plot:

figure(1);

plot(x);

figure(2);

hold('on');

plot(abs(x1), 'b');

plot(abs(x2), 'r');

hold('off');

You can see that even though it appears that x1 and x2 are different, they are actually the same. This is logical, since you are obtaining the FFT of the same signal. The only difference is that since in x1 you use 128 points and in x2 you use 64 points, the FFT of x1 has a better resolution in the frequency domain (look here for a detailed explanation).

The division between 128 and 64 is just to normalize them and make a fair comparison (try plotting without the division and you will see what I mean).

Arturo Moncada-Torres
on 18 May 2011

Maybe you mean the Dirichlet conditions? These are necessary for a function to have a Fourier Transform. Check the following links, they might be useful (specially the last one):

http://cnx.org/content/m10089/latest/ http://www.roymech.co.uk/Useful_Tables/Maths/fourier/Maths_Fourier_transforms.html http://www.math.utk.edu/~freire/m400su06/Fourier%20series%20and%20transforms-history.pdf

Regarding the relation between x1 and x2, I still support that in essence they are the same, since it is the Fourier Transform of the same signal. Maybe what you are looking is to prove that x1 and x2 both fulfill the Dirichlet conditions...?

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