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From:  NZTideMan <mulgor@gmail.com>
Newsgroups: comp.soft-sys.matlab,sci.math.num-analysis,comp.dsp,sci.math,sci.physics
Subject: Re: How to zoom into a certain part of FFT?
Date: Wed, 27 Jun 2007 08:21:35 -0000
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On Jun 27, 7:14 pm, Oli Charlesworth <c...@olifilth.co.uk> wrote:
> On Jun 27, 4:06 am, "Vista" <a...@gmai.com> wrote:
>
> > Hi all,
>
> > Suppose I have a signal f(t), t is in [0, +infinity).
>
> > And I have its spectrum F(w).
>
> > Let's say I found out that its main spectrum has 99.9% in [-B, B].
>
> > So I truncate/extract out the portion of F(w), for w in [-B, B], and
> > discretized the interval into small grids with step size deltaB.
>
> > And I then do the inverse FFT on the above samples of F(w), let's call the
> > inverse FFT reconstruction f_hat.
>
> > Which part of f(t) does this inverse FFT f_hat represent?
>
> Just as sampling a non-bandlimited function in the time domain causes
> time-domain aliasing, the dual occurs in your scenario.  You are
> sampling a non-time-limited function in the frequency domain, which
> will cause frequency-domain aliasing.
>
> --
> Oli

It's very interesting.
We've all got different ideas about what Vista is trying to do.
The only thing we all agree on is that he/she is going about it the
wrong way.
Perhaps Vista is a troll, deliberately posing an inane question, then
sitting back to watch the fun.