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Re: idl and pv-wave [message #7919 is a reply to message #412]

Sat, 25 January 1997 00:00

Big Brother
Messages: 1
Registered: January 1997

Junior Member

Achim Hein wrote:
> Try the following:
> wave>test=findgen(4097)
> wave>print,max(test-fft(test,-1),1))

Shurely it should be max(test-fft(fft(test,-1),1)) :-)

> wave>(31.3898,0.350983)

fairly close, my HP gives
(29.4698,0.247685)

> In IDL you will get as result:
> IDL>test=findgen(4097)
> IDL>print,max(test-fft(test,-1),1))
> IDL>(0.000244141,1.39110e-05)
> If you try to evaluate this FFT with a 4096-length, the results in > > > both programs are quite equal.

test=findgen(4095)
print, max( test-fft(fft(test,-1), 1))
( 0.00219727, -0.000417931)

> There was a discussion some month ago with Sergei Senin...:
> In his opinion:

IMHO :-)

>> <There seems to be no mistake, but simply no check in the procedure > > <for the number of elements in the array being 2^X.

> But I think:
>> It seems so, but I think a Fourier-Transform-Algorithm has to
>> run for every array length

I do agree with you now - WAVE fft is weird.
I'm using it to plot amplitude and phase spectral densities, which
requires a certain precision, and to avoid this problem, I run a
"power-of-two check" routine and zero padding before doing fft.

Sergei Senin
UoP, UK
ss@ee.port.ac.uk
WAVE mailing list archive:
http://www.ee.port.ac.uk:80/~ss-www/WAVE/wave-maillist/maill ist.html

PS: Sorry about this "Big Brother" stuff - I'm tired of getting junk
e-mail. My real address is in the signature.

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[Message index]

		IDL and PV-Wave By: ritscher on Fri, 21 August 1992 06:39
		Re: idl and pv-wave By: Achim Hein on Thu, 23 January 1997 00:00
		Re: idl and pv-wave By: Achim Hein on Mon, 27 January 1997 00:00
		Re: idl and pv-wave By: Big Brother on Sat, 25 January 1997 00:00
		Re: idl and pv-wave By: brian.jackel on Fri, 24 January 1997 00:00
		Re: idl and pv-wave By: Achim Hein on Fri, 24 January 1997 00:00
		Re: idl and pv-wave By: decre on Thu, 23 January 1997 00:00

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