Re: Very large FFT


HI

Yes it is clear to me that this is the same Twiddle array that I previous
calculated that for increasing the speed of the FFT calc, but why is he
making it then again ?

It looks like
- FFT with twiddle
-Twiddle again (???)
- FFT with twiddle
-final answer

I noticed the same in a PhD thesis on radio astronomy in the appendix C
where the author made an extra Twiddle step. In the referenced
presentation from Dillon Engineering there was also this step.

What is the purpose of it? Is the 1D FFT calculated as 2D matrix really
that much different to the image processing 2D FFT ?

R






AdriaanB wrote:
(snip)

He seems to follow the general thing that was also mentioned here of
splitting it in a row * column and then doing FFTs row wise, then
transpose and repeat the rows again (in effect on the columns) and
then
transposing it again. New question is: Why is the extra twiddle factor
multiplication needed halfway through the algorithm?

There is no extra twiddle factor, it is the same one that
comes on any step of the FFT. It just looks extra because of
the way you are separating it.

I wondered some time ago about an FFT of a whole CD, so maybe
44100*60*70 = 185220000 samples (per channel).

-- glen





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Relevant Pages

  • Re: FFT twiddle calculation and evaluation
    ... I am aware that for a DIF radix-2 FFT the total number of butterflies is ... Also the last stage involves multiplication by a twiddle ... Lyons, R. "Program Aids Analysis of FFT Algorithms", ...
    (comp.dsp)
  • Re: Very large FFT
    ... It is clear that when the recipe says you add salt and two eggs it is best ... a Image processing 2D FFT, the difference is exactly the extra Twiddle ... presentation from Dillon Engineering there was also this step. ...
    (comp.dsp)
  • Re: Phase/twiddle factors calculations
    ... I am aware that for a DIF radix-2 FFT the total number of butterflies is ... the last stage involves multiplication by a twiddle factor of 1. ... Lyons, R. "Program Aids Analysis of FFT Algorithms", ...
    (comp.dsp)
  • Re: Pure ANSI-C DIF FFT code
    ... Thanks for your reply, Ofcourse my FFT size is fixed, So do you think ... that all the thing hat I need is just a twiddle factor table? ...
    (comp.dsp)
  • Re: Very large FFT
    ... calculated that for increasing the speed of the FFT calc, ... FFT with twiddle ... where the author made an extra Twiddle step. ...
    (comp.dsp)