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From: "Chen Sagiv" <chensagivron@gmail.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: Does Matlab do FFT correctly ?
Date: Sat, 3 May 2008 04:39:03 +0000 (UTC)
Organization: Sagiv Enterprises
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Hi again,

I am not here to blame Matlab for anything, but to 
understand better what Matlab is doing. 

I think that a short white paper would do the work. If you 
know one, I will be happy to get a pointer. If not, I will 
try to find the time to write something up. 

In any case, I am very happy with the fruitful discussion 
that took place here, and I certainly learnt quite a lot. 

Best and thanks for your efforts. 

Chen 

"Steven G. Johnson" <stevenj@alum.mit.edu> wrote in message 
<3d638b38-6307-4a73-b305-
b1eda3ebbbbd@34g2000hsh.googlegroups.com>...
> On May 2, 6:09 pm, "Chen Sagiv" <chensagiv...@gmail.com> 
wrote:
> > Nevertheless, I will have to argue that this point is 
not
> > clear at all. Most people refer only to the abs of the 
FT
> > anyway, and this problem will simply go un-noticed. Only
> > when phase is of interest, you really look at this 
subtle
> > point.
> 
> Oh, absolutely.  Fourier transforms are subtle to begin 
with, and then
> when you add discretization etc. on top of that it is a 
tricky
> subject.  Lots of students find it difficult, and digital 
signal
> processing and DFTs are usually a relatively advanced 
subject in an
> university curriculum.  And it's easy to find people 
online who are
> confused about this topic.
> 
> However, it's not Matlab's fault, and I don't think 
there's much that
> they can do to eliminate the subtleties of this subject.
> 
> (If you think they should shift the origin of the FFT to 
the center of
> the array for you, then think again.  First, doing so 
would make them
> incompatible with the universal convention of textbooks, 
papers, etc.
> on this subject.  Second, it's not a trivial matter 
to "center" the
> origin, since the location of the "center" pixel is 
ambiguous for even
> N; this makes the ifftshift function more tricky to use 
properly than
> it may seem.  Third, it makes the DFT formula more 
complicated-
> looking.  Fourth, the location of the origin is really 
the least of
> the subtleties of the DFT.)
> 
> Regards,
> Steven G. Johnson