[Eeglablist] using 'psd' or 'fft' in std_spec creates 60dB difference

[Delorme, Arnaud]

Please use this issue report on Github to continue the discussion

https://urldefense.com/v3/__https://github.com/sccn/eeglab/issues/172__;!!Mih3wA!XDtTiToWfaCeHYr3-3xe6kOHdjwPZuPt_qTuM4C9nPM9Wq_KNhlB89q3os8G7ltCTNmD5g$ 

Arno

> On Jun 16, 2020, at 3:06 PM, Makoto Miyakoshi <mmiyakoshi at ucsd.edu> wrote:
> 
> Dear Seyed,
> 
> That sounds familiar to me.
> If I remember correctly, output from the 'fft' option resembled raw output
> with uV^2 as a unit. If you convert it to dB, will it be similar to PSD
> plot curve? Use this conversion 10*log10(x) where x is the output
> from using the 'fft' option.
> 
> Makoto
> 
> On Tue, Jun 16, 2020 at 8:17 AM Seyed Yahya Shirazi <shirazi at ieee.org>
> wrote:
> 
>> Hi,
>> I posted this issue on EEGLAB GitHub a few days ago. Sorry for
>> cross-posting. I use "std_precomp" to create the power spectra of the ICA
>> components in a source cluster. I tried computing the spectra using `psd`
>> or
>> `fft` and results have almost 60 dB difference (it is not exactly dB, but
>> more 10Log(power), since it does not have a reference power). From
>> experience, and also the time-frequency data, I know that the results of
>> the
>> `fft` mode might be correct.
>> 
>> Also, the `fft` results appeared to be very noisy. Do you have any comments
>> for adjusting parameters to get smoother results?
>> 
>> A friend of mine at another university also observed this discrepancy using
>> a totally different dataset but a similar (yet independent) pipeline.
>> 
>> the full script for the computing spectra is as follows:
>> 
>> % PSD
>> [f_STUDY, f_ALLEEG] = std_precomp(f_STUDY, f_ALLEEG, 'components',
>> 'recompute' ,'on','spec','on',...
>>                'specparams', {'specmode', 'psd', 'logtrials', 'off',
>> 'freqrange',[3 80], 'freqfac', 10}, ...
>>                'recompute', 'on', 'savetrials', 'on');
>> 
>> % FFT
>> [f_STUDY, f_ALLEEG] = std_precomp(f_STUDY, f_ALLEEG, 'components',
>> 'recompute' ,'on','spec','on',...
>>                'specparams', {'specmode', 'fft', 'logtrials', 'off',
>> 'freqrange',[3 80]}, ...
>>                'recompute', 'on', 'savetrials', 'on');
>> 
>> 
>> BTW, changing logtrials and/or freqfac values do not seem to make any
>> change. I am using EEGLAB 2019.0 and Matalb R2018b.
>> 
>> The report together with an image of the difference between the two modes
>> of
>> computing spectra are available at
>> https://urldefense.com/v3/__https://github.com/sccn/eeglab/issues/172__;!!Mih3wA!SqkCLK1Bq7r6WLhrG5pqnP4M1UqCWdc0qyr8uA3E-vs-Gt0p5Mhqwtj2Sh9EGR1CT954Bw$
>> 
>> Thanks
>> Seyed
>> 
>> --
>> Seyed Yahya Shirazi
>> GRA, BRaIN Lab
>> University of Central Florida
>> 
>> 
>> 
>> 
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