Re: [R] Fourier Transform with irregularly spaced x

From: stephen sefick <>
Date: Mon, 03 Nov 2008 11:07:32 -0500

neat. I learn something everyday

On Mon, Nov 3, 2008 at 11:01 AM, Claudia Beleites <> wrote:
>> Try or
>> (do
>> RSiteSearch("Lomb periodogram") --
>> the Lomb periodogram does a discrete (although not fast) Fourier
>> transform of unevenly sampled (1D/time-series) data, accounting for
>> the sampling distribution of points (which will the bias the results
>> if you try to do a naive Fourier sum).
> Thanks Ben, that looks like a good start point.
> Stephen, my aim are neither spline nor linear approximation but something in
> the line of matlab's interpfft
> I do have the vibrational spectrum. Such spectra are frequently computed by ft
> from their (measured) interferograms. I.e. if you use an FT-spectrometer.
> However, the spectra can also be measured directly with a dispersive
> instrument. The difference between neighbouring frequencies of such spectra
> varies over the spectrum. E.g. I measure from 600 cm^-1 to 1800 cm^-1: at 600
> cm^-1 I have a data point spacing of 1.04 cm^-1, while at 1800 cm^-1 it is
> only 0.85 cm^-1. So doing a ft (like spec.pgram ()) only on the signal means
> that I do not use periodic functions (sin x), but something rather like sin
> (x^2) - the sinus changes its frequency. This does not help.
> The idea is to calculate the interferogram (space or time domain) taking into
> account this variation of delta nu. Then do a backtransform to evenly spaced
> frequencies.
> The next step will then be to do other interesting things like downsampling,
> denoising etc. using the interferogram.
> Thanks,
> Claudia
> --
> Claudia Beleites
> Dipartimento dei Materiali e delle Risorse Naturali
> Università degli Studi di Trieste
> Via Alfonso Valerio 6/a
> I-34127 Trieste
> phone: +39 (0 40) 5 58-34 47
> email:
> ______________________________________________
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> PLEASE do read the posting guide
> and provide commented, minimal, self-contained, reproducible code.

Stephen Sefick
Research Scientist
Southeastern Natural Sciences Academy

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annoying little problems of being mammals.

								-K. Mullis
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Received on Mon 03 Nov 2008 - 16:09:21 GMT

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