Path: news.mathworks.com!newsfeed-00.mathworks.com!newsfeed2.dallas1.level3.net!news.level3.com!postnews.google.com!b36g2000prf.googlegroups.com!not-for-mail
From: dbd <dbd@ieee.org>
Newsgroups: comp.soft-sys.matlab
Subject: Re: About FFT and Even Function
Date: Sun, 1 Nov 2009 09:37:16 -0800 (PST)
Organization: http://groups.google.com
Lines: 56
Message-ID: <e605aa13-a63b-4032-8b9d-039cdd5efa7a@b36g2000prf.googlegroups.com>
References: <hcj73a$b5$1@fred.mathworks.com> <AwaHm.1482$%U3.1017@newsfe21.iad> 
	<a220a52b-a008-4b16-acc7-0656a03275b3@w37g2000prg.googlegroups.com> 
	<d1jHm.3896$Mg.1515@newsfe01.iad>
NNTP-Posting-Host: 75.15.86.181
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
X-Trace: posting.google.com 1257097036 23069 127.0.0.1 (1 Nov 2009 17:37:16 GMT)
X-Complaints-To: groups-abuse@google.com
NNTP-Posting-Date: Sun, 1 Nov 2009 17:37:16 +0000 (UTC)
Complaints-To: groups-abuse@google.com
Injection-Info: b36g2000prf.googlegroups.com; posting-host=75.15.86.181; 
	posting-account=E_gaFgoAAACfAhF2MzvkivNAUVGQbrBP
User-Agent: G2/1.0
X-HTTP-UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.20) 
	Gecko/20081217 Firefox/2.0.0.20 (.NET CLR 3.5.30729),gzip(gfe),gzip(gfe)
Xref: news.mathworks.com comp.soft-sys.matlab:581634


On Nov 1, 8:52 am, "Nasser M. Abbasi" <n...@12000.org> wrote:
> "dbd" <d...@ieee.org> wrote in message
>
> news:a220a52b-a008-4b16-acc7-0656a03275b3@w37g2000prg.googlegroups.com...
>
>
>
> > On Oct 31, 11:11 pm, "Nasser M. Abbasi" <n...@12000.org> wrote:
> >> ...
> >> If the data sequence itself is real and even (i.e. x[n]=x[N-n]) where N
> >> is
> >> ...
> >> --Nasser
>
> > Let's try that:
>
> >>> % Nassar's formula, N = 6, x[n] = x[N-n] for n=1:N-1
> >>> fft([1 2 3 2 1 0])'
> > ans =
> >   9.0000
> >  -2.0000 + 3.4641i
> >        0
> >   1.0000
> >        0
> >  -2.0000 - 3.4641i
>
> Ofcourse, I said above n=1:N-1 which is clearly wrong (the number of points
> is N now N-1 points), I meant to write n=0:N-1 to make it N points. I made a
> typo starting it from 1 not 0.
>
> May be now you can try my formula? ;)  but watch for the index.

No, Matlab indices start at 1. I applied your formula to 1:N-1 because
it doesn't work for N, but I did fft a set of N values. (These are
samples of a triangular waveform.)

Anyway, did you mean something like this?

>> fft([0 1 2 3 2 1 0])'
ans =
   9.0000
  -4.5489 + 2.1906i
   0.1920 - 0.2408i
  -0.1431 + 0.6270i
  -0.1431 - 0.6270i
   0.1920 + 0.2408i
  -4.5489 - 2.1906i
>>

These are not real. The point is that symmetry is not the same as "fft-
evenness".

>
> --Nasser

Dale B. Dalrymple