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Subject: Re: Most Recent Discover Magazine pp66-67 WaterFall Display
From: Mike Price mprice@........
Date: Sun, 08 Jul 2007 16:06:31 -0700

A 3.2 GHz Pentium 4 with a vectorized FFT implementation using SSE2 
(special extended 128-bit MMX instruction set of the Pentium) can 
execute a 1024 point FFT in well under 1mS. A 4096 FFT can be executed 
in under 6mS. This would blow the doors off any ca. 1972 implementation 
even if done entirely in HW.

The number of points needed in an FFT is driven by the desired frequency 
resolution (bin width = sample_frequency / number_of_points). You may be 
able to get away with fewer number of points if you don't need fine 
frequency resolution.

The FFT produces a complex number: a + bi. Think of it as a vector. You 
get magnitude and phase as follows:
mag = sqrt( a^2 + b^2 )
phase = atan ( b / a )

Mike Price

Geoffrey wrote:
> Wonderful and thanks;
> It has been a few years since I looked
> and I never found anything like that.
> It (FFT) was classified Secret in the US Navy in the 1970s
> but after I saw my "secret" machine sold to foreign
> countries in a commissiond officers magazine
> (My dad was a retired officer) I no longer accepted the navys 
> classification.
> for that machine.
> I now realize they want to keep it a secret
> only till they market the machine wherever.
> The secrecy is more for economic (manufacturing)
> than military reasons. We were probably only
> testing the device for Diagnostic Retreival Company.
> A general purpose computer like mine that works in a serial fashion
> can not match the speed of that parallel computing machine.

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