matlab and NR (C) FFT routines

revyakin_at_yahoo.com
Date: 03/29/05

  • Next message: Poul Freeman: "Re: 32bit/64bit cpu's and significant figures?" 
    Date: 28 Mar 2005 18:16:20 -0800

    Hi all,

    I did search on this subject in the group, but it's still not working.

    Say, I want to do an fft of this array (actually, copied from a 8-year
    old post).

    [1+2*i 2+3*i 3+4*i 4+5*i]

    When I do fft(data) in Matlab, I get:
    10.0000+14.0000i -4.0000 -2.0000-2.0000i 0-4.0000i.

    When I do the NR's four1() routine, i do it like this
    float data[8] = {1,2,2,3,3,4,4,5};
    four1(data-1, 4, -1);
    ..taking into account the -1 shift in indexing and the -1 for forward
    fft convention. I get:
    -107374176.000000
    10.000000
    14.000000
    -2.000000
    0.000000
    -4.000000
    -4.000000
    0.000000

    Some values (actually belonging to different complex ##) are shifted,
    the one element appears lost, and there is a nonsense number
    -107374176.000000. I have a limited training in math analysis, so I am
    confused here.

  • Next message: Poul Freeman: "Re: 32bit/64bit cpu's and significant figures?"

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