       Re: Fourier frequencies

• To: mathgroup at smc.vnet.net
• Subject: [mg43428] Re: [mg43401] Fourier frequencies
• From: Sseziwa Mukasa <mukasa at jeol.com>
• Date: Wed, 17 Sep 2003 07:58:44 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```On Friday, August 29, 2003, at 07:16 AM, akpovo wrote:

> Hello to all,
> How possible is it to compute a fourier transform of a data set {t,x}
> in
> order to have the fourier coefficient and the corresponding frequency?
> Tahnks
>
>

The Fourier Transform is computed as normal

Fourier[x]

Traditionally the 0 frequency is placed in the center of the spectrum,
after an FFT though it is at the left so we rotate the data:

RotateRight[Fourier[x],Quotient[Length[x],2]

The frequencies are determined from the time with the following function

freq[t_]:=With[{tau=t[]-t[],n=Length[t]},Table[-tau/2+i/(n
tau),{i,0,n-1}]

You can the use ListPlot to get the spectrum, assuming the data is in
the list x and the times in the list t

ListPlot[Transpose[{freq[t],RotateRight[Fourier[x],Quotient[Length[x],2]
]}],PlotJoined->True]

Regards,

Ssezi

```

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