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From: Greg Heath <heath@alumni.brown.edu>
Newsgroups: comp.soft-sys.matlab,comp.dsp,sci.math.num-analysis
Subject: WARNING: Is there a use for ifft(X,M)?
Date: Thu, 11 Jun 2009 21:21:00 -0700 (PDT)
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This is a followup to the thread "Why is fft(x,M) useful?

http://groups.google.com/group/comp.soft-sys.matlab/
msg/2bacfcce9454065e

As long as the N components of the time sampled row
vector function x are presented in chronological order,
the use of fft(x,M) as shorthand for the zeropadding
operation fft([x,zeros(1,M-N)]) is relatively
straightforward.

In contrast, it has been show in recent posts, e.g.,

http://groups.google.com/group/comp.soft-sys.matlab/
msg/a76d7837e3d84bcf?hl=en

that when zero padding in the spectral domain, it is
essential to add the zeros so that both the real and
imaginary parts of the result are conjugate symmetric.
When N is even, this results in the insertion of the
zeros within a two part decomposition of the Nyquist
component. However, workarounds are available when
the zeros are placed on either side of the Nyquist
component.

The purpose of this thread is to warn potential users
that none of the three techniques in the above reference
involves appending the zeros at the end of the spectrum
obtained from fft(x).

Therefore, I can find no use for ifft(X,M).

Hope this helps.

Greg