[Eeglablist] Power Spectra - Window Type

[Marco Montalto]

Dear Arno,

What you are saying about FFT method is only applicable to continuous data though. My data is segmented into 2-second epochs. So I am presuming that when I use FFT on this data the entire epoch length is being used for the FFT (the amount of overlap, if any, in this case remains a mystery to me).

If I treat the epoched data as continuous ('continuous', 'on') to extract shorter epochs for the FFT, then I can play around with the overlap. And this causes some slight differences in the power values obtained. This seems impossible to do if 'continuous' is left 'off'.

So really my question is: if 'continuous' is left 'off' (which is the default value), (1) is the entire (in my case) 2-second epoch being used as window size for the FFT, (2) what window type, if any, is being used for the FFT, (3) can I somehow change the amount of overlap?

Regards,
Marco

On 02 Feb 2013, at 22:20, Arnaud Delorme <arno at ucsd.edu> wrote:

> Dear Marco,
> 
> the default window size when using the FFT method is 1 second with 50% overlap. You may play with 'nfft' and 'epochrecur' parameters to change these default values. 'psd' which is a different option from 'fft' which uses the pwelch Matlab function. Both methods are equivalent although the scale of the signal might be different. 'fft' is faster though.
> Best,
> 
> Arno
> 
> On 1 Feb 2013, at 08:03, Makoto Miyakoshi wrote:
> 
>> Dear Marco,
>> 
>> This is from std_spec(). Looks like the entire data are first Hamming
>> windowed and then fft with window length of g.nfft.
>> 
>> Makoto
>> 
>> else % fft mode
>>   if oritrials == 1 || strcmpi(g.continuous, 'on')
>>       X = bsxfun(@times, X, hamming(size(X,2))');
>>   end;
>>   if all([ EEG.trials ] == 1) && ~isempty(boundaries),
>> disp('Warning: fft does not take into account boundaries in continuous
>> data'); end;
>>   tmp   = fft(X, g.nfft, 2);
>>   f     = linspace(0, EEG(1).srate/2, floor(size(tmp,2)/2));
>>   f     = f(2:end); % remove DC (match the output of PSD)
>>   tmp   = tmp(:,2:floor(size(tmp,2)/2),:);
>>   X     = tmp.*conj(tmp);
>>   if strcmpi(g.logtrials, 'on'),  X = 10*log10(X); end;
>>   if strcmpi(g.savetrials, 'off'), X = mean(X,3); end;
>>   if strcmpi(g.logtrials, 'off'),  X = 10*log10(X); end;
>> end;
>> 
>> 
>> 
>> 
>> 2013/2/1 Marco Montalto <mmon0006 at um.edu.mt>:
>>> Dear Makoto,
>>> 
>>> Hamming window seems to be used when precomputing power spectra using specmode "psd" as it calls the spectopo function, which in turn calls the pwelch function. So it still remains unclear what type of window is used when specmode is set as "fft". Any suggestions?
>>> 
>>> Regards,
>>> Marco
>>> 
>>> 
>>> On 31 Jan 2013, at 20:12, Makoto Miyakoshi <mmiyakoshi at ucsd.edu> wrote:
>>> 
>>>> no worries. good luck.
>>>> 
>>>> Makoto
>>>> 
>>>> 2013/1/31 Marco Montalto <montaltomarco at onvol.net>:
>>>>> Thanks Makoto for your help once again! You've been extremely kind!
>>>>> 
>>>>> Best regards,
>>>>> Marco
>>>>> 
>>>>> On 31 Jan 2013, at 19:56, Makoto Miyakoshi <mmiyakoshi at ucsd.edu> wrote:
>>>>> 
>>>>>> Dear Marco,
>>>>>> 
>>>>>> Looks like this is it.
>>>>>> 
>>>>>> pwelch: Power Spectral Density estimate via Welch's method.
>>>>>> Pxx = pwelch(X) returns the Power Spectral Density (PSD) estimate,
>>>>>> Pxx, of a discrete-time signal vector X using Welch's averaged,
>>>>>> modified periodogram method.  By default, X is divided into eight
>>>>>> sections with 50% overlap, each section is windowed with a Hamming
>>>>>> window and eight modified periodograms are computed and averaged.
>>>>>> 
>>>>>> Makoto
>>>>>> 
>>>>>> 2013/1/30 Marco Montalto <montaltomarco at onvol.net>:
>>>>>>> Dear List,
>>>>>>> 
>>>>>>> What type of window is used to compute power spectra, using fft, in STUDY design?
>>>>>>> 
>>>>>>> Thanks and regards,
>>>>>>> Marco
>>>>>>> _______________________________________________
>>>>>>> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
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>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> --
>>>>>> Makoto Miyakoshi
>>>>>> JSPS Postdoctral Fellow for Research Abroad
>>>>>> Swartz Center for Computational Neuroscience
>>>>>> Institute for Neural Computation, University of California San Diego
>>>>>> 
>>>>> 
>>>> 
>>>> 
>>>> 
>>>> --
>>>> Makoto Miyakoshi
>>>> JSPS Postdoctral Fellow for Research Abroad
>>>> Swartz Center for Computational Neuroscience
>>>> Institute for Neural Computation, University of California San Diego
>>>> 
>>> 
>> 
>> 
>> 
>> -- 
>> Makoto Miyakoshi
>> JSPS Postdoctral Fellow for Research Abroad
>> Swartz Center for Computational Neuroscience
>> Institute for Neural Computation, University of California San Diego
>> 
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> 
>