Path: news.mathworks.com!newsfeed-00.mathworks.com!newsfeed2.dallas1.level3.net!news.level3.com!postnews.google.com!g12g2000prg.googlegroups.com!not-for-mail
From:  julius <juliusk@gmail.com>
Newsgroups: comp.dsp,comp.soft-sys.matlab
Subject: Re: how to use IFFT to reconstruct signal in a specific region t in [a, b]?
Date: Thu, 09 Aug 2007 21:34:02 -0000
Organization: http://groups.google.com
Lines: 43
Message-ID: <1186695242.168366.34680@g12g2000prg.googlegroups.com>
References: <f9e4pe$klc$1@news.Stanford.EDU>
NNTP-Posting-Host: 163.188.89.213
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
X-Trace: posting.google.com 1186695243 6340 127.0.0.1 (9 Aug 2007 21:34:03 GMT)
X-Complaints-To: groups-abuse@google.com
NNTP-Posting-Date: Thu, 9 Aug 2007 21:34:03 +0000 (UTC)
In-Reply-To: <1186691862.793100.90160@x35g2000prf.googlegroups.com>
User-Agent: G2/1.0
X-HTTP-UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.6) Gecko/20070725 Firefox/2.0.0.6,gzip(gfe),gzip(gfe)
Complaints-To: groups-abuse@google.com
Injection-Info: g12g2000prg.googlegroups.com; posting-host=163.188.89.213;
Xref: news.mathworks.com comp.dsp:226885 comp.soft-sys.matlab:423278



On Aug 9, 3:37 pm, "Ron N." <rhnlo...@yahoo.com> wrote:
> On Aug 9, 5:43 am, julius <juli...@gmail.com> wrote:
>
>
>
> > On Aug 8, 11:19 pm, "Luna Moon" <lunamoonm...@gmail.com> wrote:
>
> > > Hi all,
>
> > > Let's say by doing IFFT on F(v), which is the spectrum of signal f(t), I was
> > > able to reconstruct f(t), for t in [0, T].
>
> > > Now I want to ask is there a way to do another IFFT to reconstruct the
> > > specific part f(t) for t in [T, 2T], without any waste of previous
> > > calculations?
>
> > > Basically, I want to ask, if it is possible to use IFFT to reconstruct to
> > > any slot t in [a, b] in the time domain for signal f(t)?
>
> > > Thanks a lot!
>
> > You can use the Discrete Fourier Transform (DFT) to do it.
> > By definition, the FFT is restricted to the "Fast" version of
> > the DFT.
>
> > By the way, did you realize that relating a signal via the DFT
> > or FFT implicitly assume periodicity in both time and frequency?
> > I can't understand your notation, but if my guess is correct you
> > will find that x[n] is periodic in N.  In your notation somehow
> > you are using continuous time t, which is incorrect.
>
> Doesn't an ordinary infinitely periodic and bandlimited
> continuous function have a finite discrete spectrum F(w),
> from which it is possible to completely reconstruct
> f(t) in continuous time?  (and approached by several
> methods).

I know that, but the author specifically said "iFFT".  Either
the person is wrong in saying "iFFT" instead of "Fourier
series" or in using "t" versus "n".  Unless there is a "fast"
Fourier series computation in continuous-time that has
been invented ...