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

[Makoto Miyakoshi]

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
>
>
>
>
> _______________________________________________
> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
> To unsubscribe, send an empty email to
> eeglablist-unsubscribe at sccn.ucsd.edu
> For digest mode, send an email with the subject "set digest mime" to
> eeglablist-request at sccn.ucsd.edu
>