From: John Woodgate
Subject: Re: SwCADIII
Date: Sun, 5 Jan 2003 13:31:31 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Sun, 5 Jan 2003 15:46:24 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Don Pearce
wrote (in ) about
'SwCADIII', on Sun, 5 Jan 2003:
>Just tried it. I generated a random noise signal, did an FFT to get
>frequency domain, then an inverse FFT to return to time domain. I got
>all my original numbers back. It would seem that the noise density in
>the frequency domain is not perfectly smooth, and the small
>irregularities are sufficient to generate a unique and identifiable
>time domain signal.
There are a number of ways in which that could be a very misleading
positive result. The noise may come from a 'seeded' random sequence,
which is not truly random. The noise density isn't smooth because your
sample duration isn't very long compared with the reciprocal of the
lowest frequency present in the spectrum. Note that if your spectrum
goes down to d.c., NO duration is ever long enough!
I think there are some other possible reasons, but my brane begins to
'urt when I think of them.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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