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Re: Problems with FFT and IFFT
From: |
Steve C. Thompson |
Subject: |
Re: Problems with FFT and IFFT |
Date: |
Tue, 28 Feb 2006 11:56:25 -0800 |
User-agent: |
Mutt/1.5.9i |
On 28 Feb 06 14:27PM, John W. Eaton wrote:
> On 28-Feb-2006, Sascha Berkenkamp wrote:
>
> | Must I set all values smaller than eps to zeros?
>
> You can do that if you think it is appropriate for your application.
I've had experience with this precision issue too. Here's an
example:
A1 = rand (1, 5) + j * rand (1, 5);
A = [0, A1, conj(fliplr (A1))];
a = ifft (A);
isreal (a)
A1 = rand (1, 50) + j * rand (1, 50);
A = [0, A1, conj(fliplr (A1))];
a = ifft (A);
isreal (a)
The output of the IFFT should be real-valued since the input
vector is complex-conjugate symmetric. On my system [uname -a:
Linux ... 2.6.12-10-386 ... i686 GNU/Linux; octave --version:
GNU Octave, version 2.1.71 (i486-pc-linux-gnu)], I get:
octave:1> A1 = rand (1, 5) + j * rand (1, 5);
octave:2> A = [0, A1, conj(fliplr (A1))];
octave:3> a = ifft (A);
octave:4> isreal (a)
ans = 1
octave:5>
octave:5> A1 = rand (1, 50) + j * rand (1, 50);
octave:6> A = [0, A1, conj(fliplr (A1))];
octave:7> a = ifft (A);
octave:8> isreal (a)
ans = 0
So for the longer vector, the precision issue creeps in. In this
particular case, I just follow the IFFT with a `real ()'.
Steve
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- Re: Problems with FFT and IFFT, (continued)
- Re: Problems with FFT and IFFT, Keith Goodman, 2006/02/28
- Re: Problems with FFT and IFFT, Keith Goodman, 2006/02/28
- Re: Problems with FFT and IFFT, John W. Eaton, 2006/02/28
- Re: Problems with FFT and IFFT, Dmitri A. Sergatskov, 2006/02/28
- Re: Problems with FFT and IFFT, Sascha Berkenkamp, 2006/02/28
- Re: Problems with FFT and IFFT, John W. Eaton, 2006/02/28
- Re: Problems with FFT and IFFT, Joe Koski, 2006/02/28
- Re: Problems with FFT and IFFT, Quentin Spencer, 2006/02/28
- Re: Problems with FFT and IFFT, Joe Koski, 2006/02/28
- Re: Problems with FFT and IFFT, Bill Denney, 2006/02/28
- Re: Problems with FFT and IFFT,
Steve C. Thompson <=
- RE: Problems with FFT and IFFT, Frank Palazzolo, 2006/02/28
Re: Problems with FFT and IFFT, Joe Koski, 2006/02/28
Re: Problems with FFT and IFFT, Dmitri A. Sergatskov, 2006/02/28
RE: Problems with FFT and IFFT, David Bateman, 2006/02/28