INTERPFTW

Modified INTERPFT to do ALIASED sinc interpolation.

The INTERPFT function does sinc interpolation by taking an FFT, padding its end with zeros, and then taking an IFFT. Essentially, INTERPFT resamples the signal after low-pass filtering it.

Instead of putting all zero padding at the FFT *ends*, that padding can be split so that some are at the beginning of the FFT and some are at the end. The result is identical to resampling the signal after bandpass filtering it.

Hence, samples can be reconstructed from non-baseband aliasing windows.

So INTERPFTW is a generalization of INTERPFT.