EEG-SMT signal out of scale

Started by ekinox777, December 13, 2013, 03:27:03 pm

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ekinox777

Hello,

Few days ago I have received the EE-SMT device with 4 active and 5 passive electrodes.
I am on Linux so I installed OpenVibe.
I tested the device and electrodes on different spots and I believe the amplification is too high so often the signal is out of scale, even with the highest possible scale value.

Here I have some examples in which I used 2 active electrodes for signal and a passive one for DRL:

1. 1 channel, minus electrode on chest, plus on temple, DRL in hand. From picture you can see that vertical scale is set to 2000 (actually device truncates the signal at 1024 I guess)

 


2. 1 channel, electrodes symmetrical on temples, DRL in hand - eyes movements to the left:




3. 1 channel, electrodes symmetrical on temples, DRL in hand - eyes movements to the right:



I tried with electrodes on parietal spots, and the signal usually is in 1000 limits, but quite often goes out of scale. The same results with channel 2.

Is this normal behaviour?
Is anything I can do to bring the signal inside the limits of the scale?
I also tried it on a Windows machine, with ElectricGuru and the behaviour is the same. In documentation of EEG-SMT is recommended to set the limits between 256 and 762 (for the scale) but with these settings signal is mostly out of scale. With settings 0 - 1000 I obtain figures similar that from openVibe. So I suppose the "normal" values are half of what I get.

Any help will be very appreciated.

Thanks!


ekinox777

Dear Moderator,
Please review my post.
I need to know what to do with the EEG-SMT board. It's almost useless in actual condition.

Thank you!

LubOlimex

Hi ekinox777,

Excuse me for the delay.

It looks like the EEG-SMT works properly (judging on the readings of left-right looking), however it looks like the signal gets cut. I believe it might be because of the software.

The devices are calibrated with ElecGuru over here before shipping. I recommend you to test with that program. It is available at the EEG-SMT's web page. If it works as expected with ElecGuru, then probably the problem is software related and you need to find out how to adjust the software settings.

If the problem persists it is possible the device wasn't calibrated properly or the gain level is too high. The gain level can be adjusted by a small trimmer inside the box (that can be rotated with bare hands). The calibration is done by connection of the V_CAL and GND_CAL to the 4 electrode jack-connectors of channels 1 and 2 (V_CAL provides a square wave calibration signal 250µVpp +/-10%, 0.1 .. 100Hz).

What happens if you use channel 2 instead of channel 1?

Best regards,
Lub/OLIMEX
Technical support and documentation manager at Olimex

ekinox777

Thank you for the reply!

If ElecGuru is the same as ElectricGuru then please read the last part of my initial post (anyway I'll copy/paste here):

I also tried it on a Windows machine, with ElectricGuru and the behaviour is the same. In documentation of EEG-SMT is recommended to set the limits between 256 and 762 (for the scale) but with these settings signal is mostly out of scale. With settings 0 - 1000 I obtain figures similar to that from openVibe. So I suppose the "normal" values are half of what I get.

So I'm actually inclined to think that it's just a calibration issue.
The channel 2 behaves in the same way.

I'm not afraid to try to adjust the gain level or even with the calibration if it is ok from the point of view of warranty. If I open the box wouldn't I lose the warranty?

Best regards,
Mihai




LubOlimex

Hey Mihai,

As long as you are careful and you don't solder anything extra, you are good to go. We are usually pretty good at detecting user attempts to repair things with the soldering iron :)

Seriously though, removing the case might be a bit tricky around the USBs and audio connectors be careful not to snap them.

Best regards,
Lub/OLIMEX
Technical support and documentation manager at Olimex