Question on Freqanalysis_tfr

[Paco Díaz]

Thank you Jan, it's great to have your questions answered so quickly.

jan-mathijs schoffelen escribió:
> Dear Paco,
>
> This looks like a bug to me.
> One way of normalizing fft'ed data, is to obtain a so-called spectral
> density, i.e. the amount of power per frequency bin. Because you have
> data.fsample/2 (=Nyquist frequency) frequency bins, this normalization
> would be:
>
> 2*(abs(cTmp).^2)/data.fsample.
>
> In the code it seems abs(cTmp)^2 is normalized with the square root of
> 2./data.fsample.
>
> Thanks for finding this; I will fix it and the fixed version will be
> available in the downloadable toolbox as of tomorrow. However,
> probably a good reason why this has gone unnoticed that long, is that
> freqanalysis_tfr is a very old piece of code (and very slow), and the
> exact same functionality should be covered by freqanalysis_wltconvol
> (but this guy is much faster). Even better, freqanalysis_mtmconvol
> operates in a very similar way (and you can even get it to mimick a
> classical waveletanalysis, given the proper settings), but here you
> have much more flexibility in defining your time-frequency resolution
> by playing with 'multitapers'.
> To give you a look in the kitchen: freqanalysis_tfr operates by
> applying a convolution in the time domain, between the time domain
> data and the wavelet. Convolution in the time domain is equivalent to
> multiplication in the frequency domain.
> Freqanalysis_mtmconvol/wltconvol operate by performing this
> multiplication in the frequency domain, rather than the slow convolution.
>
> Yours,
>
> Jan-Mathijs
>
> On Feb 16, 2009, at 2:58 PM, Paco Diaz wrote:
>
>> Dear fieldtrip users,
>>
>>   While reading carefully what the function freqanalysis_tfr do, I have
>> found something that seems very extrange to me. I think that the general
>> procedure for the wavelet transformation is clear but I can't get why
>> do you
>> multiply by 2, and divide by the sampling rate, the absolute value of
>> the
>> convolution.
>>   I have still used the function with very good results, but I would
>> like to
>> understand that step in order to sleep well.
>>
>> Here is a piece of the code:
>>
>>   for k=1:size(dat,1)
>>     for j=1:length(cfg.foi)
>>       cTmp = conv(dat(k,:),M{j});
>>       cTmp = (2*abs(cTmp)/data.fsample).^2;
>>       cTmp =
>> cTmp(ceil(length(M{j})/2):length(cTmp)-floor(length(M{j})/2));
>>       cTmp = cTmp(:,1:cfg.downsample:end);
>>       if strcmp(cfg.keeptrials, 'yes')
>>         freq.powspctrm(i,k,j,:) = cTmp';
>>       else
>>         freq.powspctrm(k,j,:) = squeeze(freq.powspctrm(k,j,:)) +
>> cTmp';  %
>> compute the running sum
>>       end
>>     end
>>   end
>>
>>
>> Thank you very much in advance, Paco.
>>
>> ----------------------------------
>> 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/fcdonders/fieldtrip.
>
> ----------------------------------
> 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/fcdonders/fieldtrip.
>

--
---------------------------------------------------------
 Francisco Javier Díaz Santaella
 Department of Psychiatry and Clinical Psychobiology
 University of Barcelona
 P. Vall d'Hebron 171 * 08035 Barcelona * Spain
 Telf.: +34 93 3125035 * Cell Phone: +34 678 89 47 57
 email: pacodiaz at ub.edu
 http://www.ub.edu/brainlab
---------------------------------------------------------

----------------------------------
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/fcdonders/fieldtrip.