       Re: Discrete Fourier Transform

• To: mathgroup at smc.vnet.net
• Subject: [mg43856] Re: Discrete Fourier Transform
• From: "Robert Nowak" <robert.nowak at ims.co.at>
• Date: Thu, 9 Oct 2003 01:54:29 -0400 (EDT)
• References: <bm0juq\$prl\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```hi Catherine,

as you perhaps know a time series signal can contain more than one
frequency.
because of that you have to specfy which frequncy you are interested in.
maybe you are interested in the most dominat (maximum amplitude) frequency.
in this case perform f=Abs[Fourier[series]];
ListPlot[f]; and look for the position of the maximum in the plot.
Position[f,Max[f]] will give you this position without the need to perform
the plot.

assuming that your series is evenly sampled with a total duration time t you
get the frequencies as follows:
position 1 gives the DC content of your signal
position 2 gives the frequency 1/t
position 3 gives the frequency 2/t
position 4 gives the frequency 3/t

position n gives the frequency (n-1)/t

hope this short introduction to spectral analysis helps

robert

"Katie Neish" <cdneish at interchange.ubc.ca> wrote in message
news:bm0juq\$prl\$1 at smc.vnet.net...
> Hello everyone,
>
> I am attempting to perform a discrete Fourier transform on a time series
> using Mathematica 4.0.  (I'd like to determine the period of the signal.)
> Unfortunately, I noticed that I cannot control the frequency resolution of
> this transform.  I also have no idea what units the frequency is in.
>
> Do you know of any way to get around the problem?
>
> Thanks so much.
>
> Catherine Neish
>
>

```

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