Question on Discrete Cosine Transform FFT

Hello Team,

My Matlab code calculate a vector signal. When I plot the signal, it looks like the attached signal1.

After I apply fft, the signal becomes like signal2. It does look correct. Am I right?

Thanks

[Bruno Luong]

Uh yes. But this seems to be DFT (from FFT) not cosine transform

[Matlab Student]

Yea, I have used fft:

horFourier = fft(hProfile); plot(horFourier);

[Bruno Luong]

Why mention "Discrete Cosine Transform" in the subject? DCT is not DFT.

[Matlab Student]

What should I do to have a periodic DFT signal?

This is the image vector

and this is the results after DFT is applied.

[Matlab Student]

In my case, I did not get the same. What should I do?

[Bruno Luong]

It seems the graph is

abs(fft(data))

because fft returns complex data when when you plot it without ABS, you get the points in complex plane, and not amplitude vs frequency.

Take a look at the documents of functions PLOT, FFT, FFTSHIFT

[Matlab Student]

Thank you Bruno, that was really helpful. Also thanks for the documents you have advised.