Re: Problem with using fft and ifft functions

[Sergei Steshenko]

--- On Tue, 7/21/09, babelproofreader <address@hidden> wrote:

> From: babelproofreader <address@hidden>
> Subject: Problem with using fft and ifft functions
> To: address@hidden
> Date: Tuesday, July 21, 2009, 7:37 AM
> 

[snip]

> when I use the functions fft and then ifft something seems
> to go wrong, but
> in my ignorance I can't figure out how to fix it, or even
> what is going
> wrong.

[snip]

Did I delete anything which describes what exactly went wrong ?
Or which describes why you think it went wrong ?

...

I haven't looked closely in your code, but a couple of quick tips:

1) 'octave' uses full complex FFT opposed, to say, typically used
half-complex ones;

2) because of the above direct FFT -> filter application -> inverse FFT
sequence produces ("slightly") complex results due to rounding errors.



So in 'octave' I typically do the following:

1) forcefully make second half of the spectrum to be complex conjugate of
the first (I mean the case of real, not complex, input signal);

2) apply filter to the first half of the spectrum;

3) again forcefully make second half of the spectrum to be complex 
conjugate of the first;

4) perform inverse FFT;

4a) do a sanity check to make sure complex parts of the filtered signal
are small;

5) discard complex parts of the filtered signal - since mathematically
they should be zeros.


Regards,
  Sergei.