comp.lang.idl-pvwave archive: archive

Home » Public Forums » archive » Inverse FFT

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Re: Inverse FFT [message #33287 is a reply to message #33221]

Mon, 16 December 2002 15:56

jeyadev
Messages: 78
Registered: February 1995

Member

In article <3DFE131D.6000005@noemail.now>,
R.G. Stockwell <sorry@noemail.now> wrote:
> Colin Ault wrote:
>
>> Hi,
>>
>> I hope someone help me with a problem I am having with the FFT
>> function.
>>
>> 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.

This is quite all right. Consider the problem of the continuity in one
dimension

dq/dt + K dE/dx = 0

where q(x,t) and E(x,t) are functions of both variables of 'x' and 't'
and the derivatives are partials. Also the driving field E(x,t) is a
linear function of q(x,t) in the sense that

E(x,t) = Integral[q(x,t)P(x), x]

The solution is obtained exactly as you say. Taking the Fourier
Transform of the entire equation respect to 'x' separates the problem in
'k' space and each 'k' component q(k,t) propagages with a time constant
that depends on 'k', i.e. you get something like

dq(k,t)/dt + q(k,t)/a(k) = 0

where a(k) is the 'k' dependent time constant.

To solve this, you proceed exactly as you did. Just make sure that
you put the right flag for the reverse transform.

>> I am not having much luck at the moment, so any suggestions on this
>> problem will be gratefully received.

One thing that you should check is whether you are packing the
arrays for the transform in the correct fashion. One of the most
common mistakes is the treatment of the negative frequencies. (I
myself have been guilty of messing them up!) As you are in IDL,
it is easy to check and see if they are as expected.

Good luck.
--

Surendar Jeyadev jeyadev@wrc.xerox.bounceback.com

Remove 'bounceback' for email address

Report message to a moderator

[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

Previous Topic:	What I Want for Christmas.
Next Topic:	Re: IDLDE refresh in Windows XP (IDL 5.2.1)

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Sat Oct 11 01:58:38 PDT 2025

Total time taken to generate the page: 0.71828 seconds