Re: Strange behaviour of FFT

[paulcsouls]

This has to do with 'windowing'. The answer you want is what you would get if 
you
had infinite sinewaves but yours are cut off by a rectangular window. In order 
to get
a better response you should pass the data through a windowing filter( von hann,
hamming, etc.).

Paul C

On 19 Jul 2004 at 20:33, Jörg Kampmann wrote:

>
> I am using the attached little program in order to test the limits etc.
> of "fft(a,n)"
>
> I generate a superposition of sinewaves of 0.1 s duration and apply fft
> on it in order to get the power spectrum of the sinewaves.
>
> Strange things seem to happen, perhaps somebody is able to help me or
> clarify matters:
>
> when you run the Simulation02 you discover (with 10 Superpositions as
> indicated in variable "SuperPos") that a 200, 400, 600 etc. Hz you get
> nice power spectra peaks as expected. However at values in between I get
> strange effects at the footing area of the power spectra peaks, which
> you can easily observe.
>
> I assume that stems from the way I construct the variable "ySinewave"
> and "tone".
>
> I would like to get some practical hints on how to best use fft. The
> tutorial/documentation does not say much about it ...
>
> Thanks in advance
> Joerg
>
>
>




-------------------------------------------------------------
Octave is freely available under the terms of the GNU GPL.

Octave's home on the web:  http://www.octave.org
How to fund new projects:  http://www.octave.org/funding.html
Subscription information:  http://www.octave.org/archive.html
-------------------------------------------------------------