[R] fft question

[Raphael Schneider]

Hi,

On Thursday 11 March 2004 16:04, Frank Gerrit Zoellner wrote:
> I am using the fft() function the base package to transform some 1d signal.
> If I use this standar fucntion I get a very huge first fourier coeficient.
In electronics, the first coefficient is the dc-component of the signal. If 
you remove the mean from the signal, the first coefficient should be zero.

> I think this dues to the handling of the borders of the signal.
> Usually in fft especially in image processing the signal is simulated to be
> continuous by adding the signal several times periodically.
If you doing a signal analysis of real signals with FFT, you normaly apply a  
window function to the data (e.g. Hamming window) so the begin and end of the 
signal is smoothly going down to zero. The reason doing this is that the 
Fourier transformation "thinks" it analyses a signal with infinit length. So 
the Fourier transformation "adds" the signal infinitely at the end of the 
signal. If the start and end of the signal is not the same value, then you 
have signal with a dis-continuity (the step from the end of the signal to the 
begin of the "added" signal). which adds a lot of noise in your frequency 
components.

Raphael
www.librasch.org