[FieldTrip] Emotiv and Fieldtrip - filter in the streamer/buffer?

Boris Reuderink b.reuderink at donders.ru.nl
Fri Sep 28 13:14:37 CEST 2012

Dear Ricardo,

No problem, and thank you for your elaborate response :). When I look at your screenshot, the right subplot seems to show a rounding issue; all the values are indeed integers, while on the left a more continuous signal is displayed. So, I would say that the difference between the signals is at least a rounding issue, and I assume that the FieldTrip signal is the more precise one. This might explain the difference in variance.

For me this raises the question where this rounding comes from. You say that both OpenVibe and Emotiv's Testbench have these integer valued samples? Could this rounding to integers happen at some other stage (e.g. after importing in MATLAB?).



twitter.com/#!/breuderink | github.com/breuderink | borisreuderink.nl

On 28 sep. 2012, at 11:39, Ricardo Jorge <landpt at gmail.com> wrote:

> Dear Boris,
> Sorry for the late response, I tried to reply as earliest as possible.
> Thank for the answer, by the way.
> I don't think it's a scaling issue: it shouldn't affect the variance
> that much and Power spectral seem to be different from both ways.
> Absolutely that I can give you additional information; I am starting
> with explaining almost everything I've done:
> 1st: I picked up the function ft_realtime_signalviewer in
> "fieldtrip-20120904\realtime\example" and adapted it slightly so I
> could view it in realtime but at the same time, I could see the
> previous 20 seconds. Besides that, I disabled the
> ft_preproc_baselinecorrect(data.trial{1}) step in that function and
> added some steps that allowed me to save the whole record into a file
> at the same time (is there any function in fieldtrip that does that at
> the same time as realtime?). I'm not sure if I can send files through
> this discussion list: tell me if you want to try it. Just for
> visualization issues, I multiplied the signal in the plot by a factor,
> so it could be similar to Testbench visually, but it doesn't matter
> because I'm not changing any data, just the plot.
> 2nd: I opened Testbench and OpenVibe. In order to compare the results,
> I saved them into EDF an opened it in matlab. Testbench has an option
> to set an high-pass filter, but it does not alter the recording, only
> the visualization in real time.
> 3rd: I did put the emotiv in the table so it could be at the rest.
> 4th: I started all 3 of them. They're not be all exactly at the same
> time because I can't press it all at once (but almost) and so I'll
> record during a reasonable time so I can get enough samples.
> Results:
> 1. There is a weird thing in the testbench/openvibe data, all values
> in AF3 channel (for instance) are integer numbers, whereas Fieldtrip
> are not:
> Fieldtrip:
> 1.0e+03 *
>   4.030540530271679   4.030428591191215   4.030553469018260
> 4.030684982830449   4.030580735490210   4.030495361159973
> Testbench:
> 4029        4030        4031        4031        4032        4032
> They're almost the same and it isn't a problem since they didnt start
> exactly at the same time.
> However, analyzing their mean/variance, we have:
> Fieldtrip:
> Mean:  4.030526515189773e+03
> Variance: 0.011920664040352
> Offline (Testbench):
> Mean: 4.030526118470382e+03
> Variance:1.641380154961390
> Their mean are great but the same cannot be said to their variance: in
> fact it seems that it spreads way too much while offline. Let's see
> their min and max through a sample:
> Fieldtrip: [4.027692307692308e+03 ;  4.032307692307693e+03]
> Testbench: [4025 ; 4037]
> I've put a plot in my dropbox folder if you want to check it:
> https://dl.dropbox.com/u/846911/comparision.jpg
> I've checked if the signalviewer has any filter, but I can't find any,
> so I cannot understand exactly what could be happening.
> Their FFT is similar but not exactly the same as well.
> What do you think it could be happening?
> Thanks in advance,
> Best Regards
> Ricardo Jorge
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