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

Ricardo Jorge landpt at gmail.com
Fri Sep 28 11:39:58 CEST 2012

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.


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:

 1.0e+03 *

   4.030540530271679   4.030428591191215   4.030553469018260
4.030684982830449   4.030580735490210   4.030495361159973

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:


Mean:  4.030526515189773e+03
Variance: 0.011920664040352

Offline (Testbench):

Mean: 4.030526118470382e+03

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:

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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