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RE: Problems with FFT and IFFT


From: David Bateman
Subject: RE: Problems with FFT and IFFT
Date: Tue, 28 Feb 2006 23:17:14 +0100
User-agent: Mozilla Thunderbird 0.8 (X11/20040923)

Franck,

Check liboctave/oct-fftw.cc and you'll see that octave already uses fftw_execute_dft_r2c for the real to complex FFT. The issue is that as we don't know that the data is complex-conjugate symmetric then we can assume that the reverse function fftw_execute_dft_c2r function. We might check if the data is complex-conjugate (ie something like "any(a(2:end)-conj(a(end:-1:2)))") but this is an additional cost and it seemed to me that this would be a fairly rare case, and so why pay a penalty for complex-to-complex fft's to allow the c2r functions to be used ensure that no minuscule imaginary part.

Regards
David

--
David Bateman                                address@hidden
Motorola Labs - Paris +33 1 69 35 48 04 (Ph) Parc Les Algorithmes, Commune de St Aubin +33 6 72 01 06 33 (Mob) 91193 Gif-Sur-Yvette FRANCE +33 1 69 35 77 01 (Fax) The information contained in this communication has been classified as: [x] General Business Information [ ] Motorola Internal Use Only [ ] Motorola Confidential Proprietary



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