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From: Greg <heath@alumni.brown.edu>
Newsgroups: comp.soft-sys.matlab
Subject: Re: WARNING: Is there a use for ifft(X,M)?
Date: Fri, 12 Jun 2009 20:44:49 -0700 (PDT)
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On Jun 12, 5:10 pm, "Matt " <x...@whatever.com> wrote:
> Anyway, I think the bottom line is this. Because of duality, it doesn't matter what domain your input data is in (spectral, temporal).

If the measurements are in the temporal domain for both
temporal and spectral interpolation, then the scenarios should
not be considered dual:

Spectra of the real and imaginary parts of complex temporal
data must have conjugate symmetry,

> However, if the input data array you are passing, be it to fft() or ifft(),  has been "folded" (i.e. shifted so that the sampling origin is at the beginning of the array via ifftshift, circshift or whatever...) then fft(x,M) or ifft(Y,M) are probably not going  to implement meaningful zero-padding for you.

The difference is that ifft operates on folded data and fft does not.
Therefore fft(x,M) is useful whereas ifft(X,M) is not.

Hope this helps.

Greg