Olimex Support Forum

Others => Biofeedback - EEG, ECG, EKG => Topic started by: Kris on October 24, 2012, 11:16:14 am

Title: Noise only on the first channel of the SMT?
Post by: Kris on October 24, 2012, 11:16:14 am
Hello everyone, I get a lot of noise (50hz I think) on the channel 1, and ONLY on the first channel (it's not related to the electrodes, when I permute the 2 channels I get the same result).

I tried to plug the SMT on a laptop, disconnected every electrical device around . The only way I found to get rid of that noise and get a crisp wave is touching with my finger the ground part of the USB plug of the SMT.

(http://imageshack.us/a/img546/2632/68426159.jpg)


What is the cause? CheapUSB cable? bad connection somewhere? faulty SMT?

Thanks
Title: Re: Noise only on the first channel of the SMT?
Post by: LubOlimex on October 24, 2012, 01:44:25 pm
Hello Kris,

This is the usual reading of the device when the DRL is not attached properly or not connecting properly. Make sure the DRL connects to a proper place on your body as far from the other electrodes as possible. Make sure the DRL is a passive electrode. Sometimes (especially if using gel) cleaning the electrodes with alcohol can improve the readings.

Additionally, it is possible that the DRL cable has malfunctioned.

In the original design there wasn't DRL electrode and users were supposed to touch the GND part to get a clear graph (like you did with the GND of the USB).

Best regards,
Lub/OLIMEX
Title: Re: Noise only on the first channel of the SMT?
Post by: Kris on October 24, 2012, 03:59:31 pm
Thanks for your quick support  :D

I tried to clean the electrodes, tried different electrodes (olimex and DIY electrode, passive of course), different places on the body, gel, no gel and I get the same result:

(http://imageshack.us/a/img690/696/brain2p.jpg)

but why only on one channel? that's odd


Title: Re: Noise only on the first channel of the SMT?
Post by: LubOlimex on October 24, 2012, 04:33:53 pm
Hey Kris,

What kind of electrodes you use for the two channels? Active ones or passive ones, Olimex-made or made by others?

May I also ask you to make an EKG reading instead of EEG this time and again post the pictures? First take a screenshot of the readings of channel one then a screenshot of the readings of channel two. Place the electrodes on both hands (in front of the elbow area) and the DRL on your right leg.

Regards,
Lub/OLIMEX
Title: Re: Noise only on the first channel of the SMT?
Post by: Kris on October 24, 2012, 06:44:37 pm
I've re-done an EEG reading with a set of active electrodes and I don't get that noise with them, this means that the other set of passive electrodes is the cause, sorry  :-X.

Could you help me finding what's wrong with my passive electrodes ? (the Olimex active electrodes are nice but not easily usable on every location on the scalp).

They are made of an AgCl electrode, a shielded coaxial cable and a 3.5 stereo jack

electrode side: "hot" wire is connected to the electrode (not in contact with the shield)
jack side: "hot" wire is connected to the middle ring of the jack and the shield to the ground (largest pin), the tip (5v?) is not used


thanks again for your quick support  :)
Title: Re: Noise only on the first channel of the SMT?
Post by: LubOlimex on October 25, 2012, 10:46:57 am
Hey Chris,

I contacted a couple of colleagues on the subject but unfortunately we can't advise you on electrodes we are not familiar with. There might be a lot of reasons for those cables to malfunction.

An obvious problem would be a bad contact with the connector or popped shielding/grounding around the connector.

Regards,
Lub/OLIMEX
Title: Re: Noise only on the first channel of the SMT?
Post by: Kris on October 25, 2012, 12:40:55 pm
Quote from: LubOlimex on October 25, 2012, 10:46:57 am
Hey Chris,

I contacted a couple of colleagues on the subject but unfortunately we can't advise you on electrodes we are not familiar with. There might be a lot of reasons for those cables to malfunction.

An obvious problem would be a bad contact with the connector or popped shielding/grounding around the connector.

Regards,
Lub/OLIMEX



Ok, I understand, everything seems to be alright though (checked with a multimeter), I don't know...
Can you tell me if my connections are correct (ground,middle ring, tip)?

Is there any potential danger to keep on touching the USB ground?


Thanks.
Title: Re: Noise only on the first channel of the SMT?
Post by: LubOlimex on October 25, 2012, 02:00:00 pm
Chris,

QuoteOk, I understand, everything seems to be alright though (checked with a multimeter), I don't know...
Can you tell me if my connections are correct (ground,middle ring, tip)?


Here is a schematic excerpt how to configure the cable connector: http://i.imgur.com/4o9mi.jpg (http://i.imgur.com/4o9mi.jpg)

QuoteIs there any potential danger to keep on touching the USB ground?


There is always danger :)

Best regards,
Lub/OLIMEX
Title: Re: Noise only on the first channel of the SMT?
Post by: Kris on October 25, 2012, 02:28:01 pm
Quote from: LubOlimex on October 25, 2012, 02:00:00 pm


There is always danger :)

Best regards,
Lub/OLIMEX



;D
Title: Re: Noise only on the first channel of the SMT?
Post by: Ruben on November 15, 2012, 05:11:19 am
Hi,

I´ve recived the EEG SMT a week ago. I have got the same problem that Kris, but I dont get a good signal in any channel. When i open a protocol (beta-theta, v.g.) and i push record the signal becomes in noise. I have got a passive electrode set, but i use conductive paste.
Could you help me??
Thanks
Title: Re: Noise only on the first channel of the SMT?
Post by: LubOlimex on November 15, 2012, 02:00:49 pm
Hey Ruben,

Did you check the manual over here: https://www.olimex.com/Products/EEG/OpenEEG/EEG-SMT/resources/EEG-SMT.pdf (https://www.olimex.com/Products/EEG/OpenEEG/EEG-SMT/resources/EEG-SMT.pdf)? What are the results with the simple electguru or the more advanced brainbay?

Best regards,
Lub/OLIMEX
Title: Re: Noise only on the first channel of the SMT?
Post by: Ruben on November 16, 2012, 02:03:44 am
Hey Lub,

Thank you for your answer. Yes, I did it, I checked the manual. I have got brainbay, not ElectGuru because it isn´t disponible. When I want record ModEEG smt data it only produces noise; I have tried a better electrodes´placement but it isn´t the problem, the signal is not clear and has a lot of noise.

thanks
Title: Re: Noise only on the first channel of the SMT?
Post by: LubOlimex on November 19, 2012, 01:17:40 pm
Hey again Ruben,

If you use only 1 channel it is advised to connect the free channel to V_REF. Please refer to this thread for more info: https://www.olimex.com/forum/index.php?topic=321.0 (https://www.olimex.com/forum/index.php?topic=321.0)

I have uploaded Electric Guru here:
https://www.olimex.com/Products/EEG/OpenEEG/EEG-SMT/resources/ElecGuru40.zip (https://www.olimex.com/Products/EEG/OpenEEG/EEG-SMT/resources/ElecGuru40.zip)

Could you please also take a few pictures of the measurements?

Best regards,
Lub/OLIMEX
Title: Re: Noise only on the first channel of the SMT?
Post by: Ruben on November 21, 2012, 08:32:06 pm
Hi Lub,

Thanks for your help.
I have some problems with my internet connection this week and i can´t work in a correct way. I have download ElectricGuru from your link, thank you; i´ve purchased a new usb cable too. I going to try this and then i´ll put some pictures here and say you what´s happens?.

Thank you very much for your help.

regards

from Spain

Ruben
Title: Re: Noise only on the first channel of the SMT?
Post by: Ruben on November 22, 2012, 09:43:06 am
Good morning Lub

I've tried with electricguru and with the new usb cable but  it's the same. I can't put here any pictures because i havent internet at home this week and i write you with my smartphone, but the drawings are similar Kris' pictures. When i take a elecguru's reading i found a lot of noise but when i put my finger in metalic usb plug in eeg-smt this produces a more correct signal. Do you know whats happening??
Thanks

regards
Title: Re: Noise only on the first channel of the SMT?
Post by: Kris on November 23, 2012, 12:20:20 pm
Hi Ruben, I can give some advice after spending some time with the hardware.

1. Use a portable PC on battery (NOT connected to anything)
2. Skin preping is paramount: use some nu-prep (or alcohol and a bit of water on the area) THEN use some paste to "glue" the electrodes to the skin (I'm using my DIY electrodes because the Olimex are just too big, your head quickly becomes goo-land and they don't even stay in place). THIS STEP IS CRITICAL for the final result
3. Disconnect as many electrical devices in the vicinity (and switch off lights if it's possible)
4. Travel around the room with all the hardware connected on your head  and litteraly find the best spot where you're not getting those damn 50Hz!


Good luck! It works but it's not that easy.
Title: Re: Noise only on the first channel of the SMT?
Post by: Ruben on November 25, 2012, 01:34:09 am
Hi Kris,

Thank you for your help. I´ve visited a engineer and he has tested with his equipment. He has found that there was problems with usb plug and the eeg-smt was recording noise from electric network. He has installed a special ground and the signal is now better.
I m going to follow your recommendations too and I´ll tell you later.

Thank you very much

regards

Rubén
Title: Re: Noise only on the first channel of the SMT?
Post by: LubOlimex on November 26, 2012, 08:14:51 am
Hello Kris and Ruben,

Thank you for the well-written suggestion list Kris, if you don't mind I might include it in the next revision of the manual/quickstart guide.

Ruben, please feel free to post the workaround for the noise! We would be interested in your solution and if we find it reliable we might include it in the next revision of the EEG-SMT.

Regards,
Lub/OLIMEX
Title: Re: Noise only on the first channel of the SMT?
Post by: Kris on November 26, 2012, 01:16:35 pm
Quote from: LubOlimex on November 26, 2012, 08:14:51 am
Hello Kris and Ruben,

Thank you for the well-written suggestion list Kris, if you don't mind I might include it in the next revision of the manual/quickstart guide.

Ruben, please feel free to post the workaround for the noise! We would be interested in your solution and if we find it reliable we might include it in the next revision of the EEG-SMT.

Regards,
Lub/OLIMEX


I don't know if my suggestion list works for anyone else but me but thanks anyway ;D

I'd be interested in hearing about that "ground mod" too

A wireless EEG-SMT powered with a pair of batteries would definitely fix this (and make life a lot easier  :) ) but it would be much more expensive I guess