frequency resolution of the power spectrum

[Thomas Hartmann]

hi martin,
the spectral resolution of a fft is determined by the length of the data
used to calculate it. e.g. a length of 1 second would give a
frequency-resolution of 1Hz, 2 seconds of data would result in a
resolution of 0.5Hz and so on....

what you actually missed is that you dont want to calculate one fft on
your 5-minutes trial but rather several ffts on (overlapping) windows of
your 5 minutes. to do this, refer to the mail by nathan weisz "re:
preprocessing without trigger...". he shows some code there to cut long
trials into several short ones. calculate the freqanalysis on these and
you will get reasonable results.

greez,
thomas

Martin Bleichner schrieb:
> Hi there,
>
> this might be a stupid question. I probably miss something obvious.
>
> I want to compute the power spectrum over a single trial (around 5
> minutes).
>
> using this piece of code
> cfg        = [];
> cfg.method = 'mtmfft';
> cfg.output = 'pow';
> cfg.foilim = [1 100];
> cfg.taper  = 'hanning';
> freq       = freqanalysis(cfg, data);
>
> as a result I get
>  freq: [1x62534 double]
> powspctrm: [96x62534 double]
>
> that is a frequency resolution of 0.0032Hz!? What determines the
> frequency resolution here? What can I do to make it less fine grained?
>
> Thanks
> Martin
>
>
> ----------------------------------
>
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> the FieldTrip toolbox, to share experiences and to discuss new ideas
> for MEG and EEG analysis.
>
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>
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>


--
Dipl. Psych. Thomas Hartmann

OBOB-Lab
University of Konstanz
Department of Psychology
P.O. Box D25
78457 Konstanz
Germany

Tel.: +49 (0)7531 88 4612
Fax: +49 (0)7531-88 4601
Email: thomas.hartmann at uni-konstanz.de
Homepage: http://www.uni-konstanz.de/obob

"I am a brain, Watson. The rest of me is a mere appendix. " (Arthur Conan Doyle)

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.