Path: news.mathworks.com!newsfeed-00.mathworks.com!nlpi057.nbdc.sbc.com!prodigy.net!border1.nntp.dca.giganews.com!nntp.giganews.com!postnews.google.com!z16g2000prd.googlegroups.com!not-for-mail
From: Greg <heath@alumni.brown.edu>
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 12:30:27 -0700 (PDT)
Organization: http://groups.google.com
Lines: 64
Message-ID: <3605952f-7b12-40e2-a651-863b88f9ae65@z16g2000prd.googlegroups.com>
References: <8c3ffec9-42f0-4ba1-ac3c-f826d56dd963@y7g2000yqa.googlegroups.com> 
	<c751a7c4-04ea-4891-82fd-e538fad25826@c19g2000prh.googlegroups.com>
NNTP-Posting-Host: 69.141.163.135
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
X-Trace: posting.google.com 1244835028 9711 127.0.0.1 (12 Jun 2009 19:30:28 GMT)
X-Complaints-To: groups-abuse@google.com
NNTP-Posting-Date: Fri, 12 Jun 2009 19:30:28 +0000 (UTC)
Complaints-To: groups-abuse@google.com
Injection-Info: z16g2000prd.googlegroups.com; posting-host=69.141.163.135; 
	posting-account=mUealwkAAACvQrLWvunjg50tRAnsNtJR
User-Agent: G2/1.0
X-HTTP-UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 
	2.0.50727),gzip(gfe),gzip(gfe)
Bytes: 3563
Xref: news.mathworks.com comp.soft-sys.matlab:546994 comp.dsp:258695 sci.math.num-analysis:108373


On Jun 12, 3:55 am, dbd <d...@ieee.org> wrote:
> 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?

I want all of the experts and nonexperts who read
comp.dsp to understand that

1.Using MATLABs dual input option of ifft will not
return a reasonable interpolation of the time
function obtained when using the single input version.
2. This has nothing to do with Nyquist components.
3. I can't think of any scenario where the dual input
version is of any use at all.
4. I think this is serious enough to issue a WARNING.

Hope this helps.

Greg