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Re: FFT wierdness in WAVE [message #6080]

Thu, 28 March 1996 00:00

Jackel
Messages: 30
Registered: April 1993

Member

In article <4jbl6k$2dp@news.tamu.edu> howie@skeefum.tamu.edu (Matt Howard) writes:
> Subject: FFT wierdness in WAVE

> Can anyone here enlighten me on why the plots produced with the
> following commands are so different?
[Text deleted]
>> plot,abs(fft(findgen(8395),-1)) - works!
>> plot,abs(fft(findgen(8396),-1)) - fails! Peaks where none should be.

A couple of versions ago, the IDL bug-fix notes mentioned a problem with
FFT's when the length was not factorable into small primes. For example,
of the two numbers in your example, the "fails" case does not factor well:

IDL> print,prime_factors(8396)
2 2 2099

but the "works" case does (relatively speaking).

IDL> print,prime_factors(8395)
5 23 73

(Note that this is also the reason why the first case takes much longer to
compute.)

Since PV-Wave got their source from IDL quite a while ago, it seems
likely that you're seeing the same problems. You can either hope that it gets
fixed in the next version, or switch to IDL :)

Brian Jackel
University of Western Ontario

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[Message index]

		Re: FFT wierdness in WAVE By: Jackel on Thu, 28 March 1996 00:00
		Re: FFT wierdness in WAVE By: Sergei Senin on Thu, 28 March 1996 00:00
		Re: FFT wierdness in WAVE By: Sergei Senin on Thu, 28 March 1996 00:00
		Re: FFT wierdness in WAVE By: Sergei Senin on Thu, 28 March 1996 00:00

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