Olimex Support Forum

Others => Biofeedback - EEG, ECG, EKG => Topic started by: lukasv on July 21, 2013, 07:03:48 pm

Title: EEG-SMT problem
Post by: lukasv on July 21, 2013, 07:03:48 pm

I've recently purchased your EEG-SMT device along with 4 active and 1 passive electrode. I've tried using it with the software BrainBay, and have received some questionable results.

This is how the signals look when I have the electrodes on my head:

It is basically just noise. The bottom two scopes should represent differential signals.
But then I've noticed something else:
I tried laying all of the electrodes on the table, and tapped a single one of them with my finger. The next 4 images show the responses I got:

Tapping ch1+ with a finger:

Then I tapped ch1- with a finger:

...and ch2+:

..and ch2-:

It strikes me really odd that channels 2-4 look practically the same, as well as the differential channels 5 and 6.
So my question is: Is this a hardware problem, or do you have any other idea what could be causing such behavior? Is this a known issue? Do you maybe know of a way how this could be fixed?

Best regards,
Title: Re: EEG-SMT problem
Post by: LubOlimex on July 23, 2013, 03:45:03 pm
Hey Luka,

EEG-SMT has just two channels. Each channel consists of two electrodes + and -.

To make a reading place the + and - of each channel symmetrically on your head. I like what you see in the second and third pictures.

When evaluating if there is a problem with channels first try with one set of electrodes (two) and plug it two channel one (+ and -) then test with the same electrodes in the other channel. Place the electrodes at your temples. Remember to plug and place the DRL also (or hold it firmly in hand). If the DRL is not properly connected it is possible to see a lot of noise. If everything is correct you should  see almost the same image from both measures.

Best regards,
Title: Re: EEG-SMT problem
Post by: lukasv on July 23, 2013, 05:22:14 pm
Hey Lub,

thank you for your answer. I've tried what you suggested and it looks OK.


What I thought was that, using the protocol p2, you get out individual data for each electrode. According to the p2 protocol, an EEG device packet consists of 17 bytes:

Byte 1:  Sync Value 0xa5
Byte 2:  Sync Value 0x5a
Byte 3:  Version
Byte 4:  Frame Number
Byte 5:  Channel 1 High Byte
Byte 6:  Channel 1 Low Byte
Byte 7:  Channel 2 High Byte
Byte 8:  Channel 2 Low Byte
Byte 9:  Channel 3 High Byte
Byte 10: Channel 3 Low Byte
Byte 11: Channel 4 High Byte
Byte 12: Channel 4 Low Byte
Byte 13: Channel 5 High Byte
Byte 14: Channel 5 Low Byte
Byte 15: Channel 6 High Byte
Byte 16: Channel 6 Low Byte
Byte 17: Button States (b1-b4)

It looked to me that you get individual data in channels 1-4 for electrodes 1-4, respectively, and differential data in channels 5 and 6. If the only "correct" data, that is differential channel data, is on channels 1 and 2(bytes 5,6,7 and 8 ), what are the other channels there for? Have bytes 9-16 no meaning at all and are they just to be discarded?

Best regards,
Title: Re: EEG-SMT problem
Post by: LubOlimex on July 25, 2013, 11:43:45 am
Hey Luka,

Glad you got the EEG-SMT going!

I recommend you to check how we have adapted the original firmware for EEG-SMT. The source is available here: https://www.olimex.com/Products/EEG/OpenEEG/EEG-SMT/resources/EEG-SMT_firmware.zip (https://www.olimex.com/Products/EEG/OpenEEG/EEG-SMT/resources/EEG-SMT_firmware.zip).

The other channels should have no meaning when the packat doesn't populate data for them.

Best regards,
Title: Re: EEG-SMT problem
Post by: lukasv on July 26, 2013, 03:56:13 pm
Ok, thank you very much for the information.

Best regards,