[PDF] difference between fft and fftshift in matlab

What is fftshift in MATLAB?

In the book "Computational Fourier Optics, A Matlab Tutorial" by David Voelz, it is written that a call to fftshift is needed before a call to fft or ifft, but in the MATLAB documentation of fftshift it's only written that this command rearranges the outputs of fft, fft2, and fftn by moving the zero-frequency component to the center of the array.

What computations MATLAB does to produce the FFT output?

What computations MATLAB does to produce the FFT output is irrelevant. The output of the FFT is given by the definition of the DFT, which has frequencies k=0..N-1. There are no "negative frequencies" in this output. The DFT is periodic, meaning that the value at k=0 is identical to the value at k=N, and at k=-N+1.

How do I Shift a signal back to FFT or iFFT?

If you want to shift the signal back to the unordered form to compute fft or ifft (which you essentially should), you should use ifftshift. Its not shifting ifft. Its opposite of fftshift. apply fftshift to the result to get back the output same as signal form. directly apply fft or ifft.

Does fftshift affect the calculation of fast Fourier transform?

The simple answer is call to fftshift not needed before a call to fft. fftshift does not influence the calculation of Fast fourier transform. fftshift rearranges values inside a matrix. For eg: