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From: dbd <dbd@ieee.org>
Newsgroups: comp.soft-sys.matlab,comp.dsp,sci.math.num-analysis
Subject: Re: WARNING: Is there a use for ifft(X,M)?
Date: Fri, 12 Jun 2009 00:55:59 -0700 (PDT)
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On Jun 11, 9:21 pm, Greg Heath <he...@alumni.brown.edu> wrote:
> 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

I can find no use for mayonnaise, but I don't post a warning to
comp.dsp about it.

Since you have included comp.dsp in this post and I've read the basis
of your concern, why do you think those in comp.dsp need to be
concerned about maintaining non-zero Nyquist components when
interpolating via spectrum zero fill and ifft? Why are you?

How does this help?

Dale B Dalrymple