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From: Ken Smith
Subject: Re: fft in hc11
Date: Mon, 14 Oct 2002 15:10:58 +0000 (UTC)
Organization: a2i network
NNTP-Posting-Date: Mon, 14 Oct 2002 15:10:58 +0000 (UTC)
In article <3DA67B43.C5E3AD2F@earthlink.net>,
Robert Baer wrote:
>Ken Smith wrote:
>> 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.
>> email@example.com 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!
Yes a Million point FFT is "harder" for the following reasons:
Every pass through the outer loop of the FFT loses you about 1 LSB of the
mantissa of the numbers involved. If you do a 10 result DFT you can use
a shorter floating point format. Remember this was a microcontroller not
a DSP he was using.
Doing a 10 answer DFT takes on the order of 10*N operations. Doing a
million point FFT takes on the order N*log(N) operations. Long before you
get to the million point version, the 10 anwer version beats the FFT.
If you do an FFT you need to keep all of the numbers in memory. For the
10 answer DFT you just need to keep 10 answers and a few other values in
memory. You can process the values as they go by.
firstname.lastname@example.org forging knowledge
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