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I'm trying to write a simple script that should plot the spectrum in scilab, to test it I use a sinus function with 440hz so that I get my dirac in this position , my problem is that it doesn't work and I don't understand why? here's the code :

Fs = 8000;
f = 440;
t= 0:1/Fs:1;
y = sin(2*%pi*f*t);

nf = 1024; // number of point in the DFT
Y = fft(y)
f = Fs/2  * linspace(0,1,nf/2+1);
clf();
plot(f,abs(Y(1:nf/2+1)));

and this is what I get : image

any idea why I get this ?

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Your fft vector Y has the same length as your input signal y, because you just specified nf without letting fft() know what your desired FFT-length is. This is why your peak does not appear at 440 Hz. Your vector Y does not correspond to the frequencies in f. It's just a matter of correct scaling of the x-axis.

EDIT: I do not know scilab, so I don't know how to pass the desired FFT-length to fft(). If you can't do that, you just need to make your time domain signal have the desired FFT-length.

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  • $\begingroup$ where does the 4000 come from? and what about the magnitude ? $\endgroup$ – Engine Mar 4 '14 at 11:02
  • $\begingroup$ Please edit your question to make clear what it is that is unexpected for you. I thought you were talking about the location of the peak, which is not at 440 Hz. This is what I tried to explain in my answer. $\endgroup$ – Matt L. Mar 4 '14 at 11:05
  • $\begingroup$ indeed ,but I though you could help me with that one ! $\endgroup$ – Engine Mar 4 '14 at 11:07
  • $\begingroup$ Sure, but what do you mean? The 4000 on the x-axis or on the y-axis? And what are your expectations concerning the magnitude (and why)? $\endgroup$ – Matt L. Mar 4 '14 at 11:12
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    $\begingroup$ No, the FFT computes the discrete Fourier transform (DFT), and in this case you get an impulse with height $N/2$ (where $N$ is the FFT length). However, you only get a discrete impulse in the frequency domain if your signal frequency lies on the grid of FFT frequencies (i.e. $N$ is an integer multiple of the period of the time signal). Hope this helps. $\endgroup$ – Matt L. Mar 4 '14 at 12:05

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