[Tutor] fourier transform

Christian Meesters meesters at uni-mainz.de
Sat Jul 30 14:05:40 CEST 2005

Jeff Peery wrote:
> Hello, I have a signal that I want to do a fourier transform on. I 
> tried using FFT.fft(data).real but I don't understand the output.  
> what is output from this function, and why doesn't it require 
> amplitude and time as inputs?

Hi Jeff,

As Danny wrote, your input are real numbers and your output will be 
complex numbers. I don't want to point you to the numarray 
documentation once more, but you might to check out this: 
http://mathworld.wolfram.com/FourierSeries.html as for why you get 
complex numbers back. (To get only the real value part try real_fft() 
in numarrayl.)
In general in numerical computation your input is simply a series of 
real numbers - of equal spacing in the dimension you want to consider. 
Which dimension you choose (amplitudes and time/phase or amplitudes and 
spacial frequencies and so on) is up to you. It depends on your 
physical or mathematical question. IMHO the best introduction you can 
get on Fourier Transforms & programming can be found in Numerical 
Recipies (here the "Numerical Recipies in C"-link 
http://www.library.cornell.edu/nr/bookcpdf.html - check chapter 12).

Have fun with FFTs!


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