Fft

[TB]

Hello. I've downloaded a DLL for Fourier Transformations.

Declare Sub FFTSingle Lib "FFT.dll" Alias "fft_float" _
(ByVal NumSamples As Long, ByVal InverseTransform As Boolean, _
RealIn As Single, _
ImagIn As Single, _
RealOut As Single, _
ImagOut As Single)

:confused:

RealIn and ImagIn are the first elements of an array containing the information of the wave.

But what are those inputs Real and Imag for. I thought I do only ne one input array.. that contains the wave. why two arrays??

Thanks for any help.

[kedaman]

I'm not sure but theres a hint it's complex values with real and imaginary part

[TB]

But what for are 2 inputs if I only need 1 array with the wave I want to apply the FFT on??

[kedaman]

eh, a complex value is a value. I think you pass one element at a time, but not sure, where did you find the library?

[TB]

If found it on http://www.fullspectrum.com/deeth/.
The problem is that I have 2 input arrays and 2 output arrays... but isn't it logically that I should only need 1 input and 1 output array for the FFT??

[kedaman]

pass an array of 0'es as imaginary input and output, since you probably only work with real values.

[TB]

I have already tried that.. but then I get very different values at the Real and Imag output... When I put the same values in the Real and Imag input.. then I will get only one output... but this output has nothing to to with the frequences the wave contains.. I mean I won't get the Frequency Spectrum of the input...

[kedaman]

Hmm, I can't help you sorry.

[fbokker]

ok, here are some links that will help.


http://www.ulib.org/webRoot/Books/Numerical_Recipes/

there you can download very in-dept explanation of FFT. The books you can download there are written for fortran or c, but the explanation is the same, if you use VB.

possibly easier to understand is:
www.dspguide.com

sorry, never used FFT myself. last time i did frequency analysis i just used normal fourier-transform.