The two-sided, DC-centered FFT provides a method for displaying a spectrum with both positive and negative frequencies. Most introductory textbooks that discuss the Fourier transform and its properties present a table of two-sided Fourier transform pairs. You can use the frequency shifting property of the Fourier transform to obtain a two-sided, DC-centered representation. In a two-sided, DC-centered FFT, the DC component is in the middle of the buffer.