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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: fft in hc11
Date: Fri, 11 Oct 2002 07:18:31 GMT
NNTP-Posting-Date: Fri, 11 Oct 2002 00:18:31 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
Ken Smith wrote:
> In article ,
> Albert Leng wrote:
> >After studying through the code "fft routine" from motorola, i really
> >have no idea what the code is about. Can you please give me some ideas on
> >how the code relates to the above 4 steps and how i can set the A/D for
> >the inputs?
> Since I'm not looking at the code I can only give a few guesses about what
> may be the problem.
> The reordering of the data can happen before or after the
> butterflies. The difference in the math for this is quite small.
> Does it look like the code is looking for the prime factors of N before it
> does much else. If so it may be the version that works for non-power of
> two lengths. Usually only 2, 3, 5 and 7 are allowed as factors in this
> Is there a bunch of code to do with the padding? If so this may be
> confusing you. If it pads to a power of two length with a constant plus
> cosine function, it is trying to reduce the artifacts caused by padding.
> Some versions of the FFT take two runs at the data. The simplest of these
> just finds the average of all of the values and subtracts it away before
> the main code starts to work. The FFT tends to lose log(N) bits in the
> process. If you don't want all of the bits to be junk, you sometimes have
> to do things to reduce the number of bits needed. Subtracting the average
> is the simplest of these. More complex versions strip out known
> Now for the bad news. Chances are the FFT is the worst way to do what you
> are trying. If the signal generator is not run from the same crystal as
> the sampling system, the frequency slipping spreads the harmonics out and
> makes them hard to measure exactly.
> If you know the input frequency exactly you know the harmonics. There is
> no need to make the DFT produce anything but the amplitudes at the
> frequencies you care about. Doing a 1 million point FFT is a lot harder
> than making 10 or 20 channels of DFT for 1 million points.
> firstname.lastname@example.org forging knowledge
Million point FFT harder???
As simple as 1000 point FFT; i work with multi-million point
algorithms, and faster than almost anybody else in the field.
On speed, once i saw an ad by TI on some "hardware" 1024 point FFT
that stated its speed, and my *software* had it beat!
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