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Re: Inverse FFT [message #33309 is a reply to message #33221]

Mon, 16 December 2002 10:01

Streun Andreas
Messages: 4
Registered: October 2000

Junior Member

Colin Ault wrote:
> I have a signal f_t, which I then take the FFT of to produce its
> corresponding spectral components. I then want to manually compute its
> inverse FT, rather than using the IDL FFT( .../inverse) function.
>
> The reason for this is that I want each spectral component to
> propogate at different velocitys over a time period t. Hence, when the
> signal is recombined t seconds later, the signal *should* look
> different.
>

Hi Colin,

I'm not sure if I understood the problem, but I don't see the reason
why you can't use the standard FFT: You transform to frequency
domain, apply your function to the spectrum, for example
a filter, and transform back to time domain. Of course, you have
to work on the complex frequencies, not on the absolute values
(i.e. power spectrum) in order not to loose phase informations
for backtransformation.
Attached find a code fragment, where I wanted to see how a
time signal looks, if I consider only significant frequencies
(i.e.with absolute values above some threshold). Maybe that's
related to your problem.

Best regards,
Andreas

; get profile of image
p =total(roi,1)

; and show it
plot, p

; make fft
f =fft(p,1)

; get absolute value of [complex] fft
fa=abs(f)

; define filter threshold relative to maximum
filter=0.1

; filter fft by deleting all frequencies lower than threshold
ff=f
ff[where(fa lt filter*max(fa))]=0.0

; inverse fft to make filtered profile
pf=fft(ff,-1)

; show filtered profile
oplot, pf, color=250

; show absolute fft, but only the first channels
plot, fa[0:100]

; show filtered fft too
ffa=abs(ff)
oplot, ffa[0:100], color=250

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[Message index]

		Inverse FFT By: aultc on Mon, 16 December 2002 08:33
		Re: Inverse FFT By: R.G. Stockwell on Tue, 17 December 2002 07:02
		Re: Inverse FFT By: aultc on Wed, 18 December 2002 01:19
		Re: Inverse FFT By: R.G. Stockwell on Tue, 17 December 2002 06:42
		Re: Inverse FFT By: aultc on Tue, 17 December 2002 03:10
		Re: Inverse FFT By: aultc on Tue, 17 December 2002 03:06
		Re: Inverse FFT By: jeyadev on Mon, 16 December 2002 15:56
		Re: Inverse FFT By: Streun Andreas on Mon, 16 December 2002 10:01
		Re: Inverse FFT By: R.G. Stockwell on Mon, 16 December 2002 09:53

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