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Re: Complications with variance using FFTs [message #40106]

Thu, 22 July 2004 10:06

olde_english33
Messages: 7
Registered: June 2004

Junior Member

Craig Markwardt <craigmnet@REMOVEcow.physics.wisc.edu> wrote in message news:<onbri943hd.fsf@cow.physics.wisc.edu>...
> olde_english33@hotmail.com (Eric) writes:
>>
>> From what I can gather from my program, the positive frequencies are
>> those from 1:15. Then the frequencies from 16:30 are the complex
>> conjugates of the frequencies from 15:1. Therefore, I thinkt that IDL
>> is already accounting for the complex conjugate in the negative
>> frequencies, unless I am missing something.
>
> I think you are missing that when you multiply the positive frequency
> components by a complex phase, then you must also multiply the
> negative frequency components by the complex conjugate, i.e. the
> negative of that phase. To preserve a real signal that is.
>
> That's another probing question, is the final result of your technique
> real or complex?
>
> Craig

Using my technique of making the phase symmetric produces results that
are real, or very nearly real considering machine eps. I also tried
it using the technique you suggested above, but was again
unsuccessful. For example, one of the returned numbers was (30.3417,
-3.79635e-15). So I do not think that is the problem, but still have
no idea what the problem is. Do you have any more suggestions on
things to check? My colleague believes that inserting a random phase
is throwing off the ability to INVERSE FFT the data. Do you know if
this could be a possibility?

Eric

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[Message index]

		Re: Complications with variance using FFTs By: olde_english33 on Thu, 22 July 2004 10:06
		Re: Complications with variance using FFTs By: Craig Markwardt on Wed, 21 July 2004 11:30
		Re: Complications with variance using FFTs By: olde_english33 on Wed, 21 July 2004 10:53
		Re: Complications with variance using FFTs By: Craig Markwardt on Tue, 20 July 2004 06:54
		Re: Complications with variance using FFTs By: olde_english33 on Mon, 19 July 2004 09:19
		Re: Complications with variance using FFTs By: Craig Markwardt on Fri, 16 July 2004 14:21

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