> The items after N/2 are the "negative frequencies" It is not the case for real FFTs, the negative frequency components are not stored in the returned array. Time domain data of length N are represented by N Fourier components with N/2 positive and N/2 negative frequencies. The negative components are "mirror image" of the positives, just complex- conjugate, as Adam said. In many cases only the positive frequency components are used in analyses. If the input vector with N elements are complex-valued, it is 2N long, and the FFTed data contains N Fourier components. Since each of the components is complex and represented by two values (real, imag), the length required to store the FFTed data are 2N, (N/2)*2 for positive frequencies and (N/2)*2 for negatives. If the input vector is of N real, we have only N length field (many of C FFT routines store the results in-plane) and thus only the positive frequency components of N/2 complex values are stored in the results.
on 2006-06-03 07:56