Re: channel question for programming Olimex EEG-SMT
Daniel Baker <[email protected]>
| Newsgroups | gmane.comp.science.openeeg.general |
|---|---|
| Message-ID | <[email protected]> |
Hi Paddy, No, I hadn't seen this. Thanks for the heads up! - Daniel On 01/02/2013 10:27, Paddy Duncan wrote: > > Hi Again Daniel, > > I forgot to mention, have you tried the Olimex EEG forum? > > https://www.olimex.com/forum/index.php?board=25.0 > > Cheers > > Paddy > > *From:*Daniel Baker [mailto:[email protected]] > *Sent:* 01 February 2013 09:23 > *To:* [email protected] > *Subject:* Re: [Openeeg-list] channel question for programming Olimex > EEG-SMT > > Hi Arnold, > > That's very interesting. I wonder if my device has a loose connection > that causes crosstalk between the channels. That would explain the > extra activity. I guess it's not necessarily a problem, if the data I > get out of the first two channels are OK, perhaps I can ignore the > other ones. > > - Daniel > > > On 31/01/2013 19:04, nocom wrote: > > Hi Daniel, > > I am trying to understand my recently received EEG-SMT as well. My > results are somewhat different than yours. From the six data > channels numbers 0 and 1 yield data, while 2, 3 and 5 yield 6 and > channel 4 holds one or other high constant (something like 265436) > which changes each time when you start to sample. I assumed that > ch- is a reference for ch+. The Olimex is able to output to six > channels (look at the schema's) but comes with only 2 to keep > costs down. > > I am somewhat alarmed that our two devices differ that much, maybe > one of them is somewhat defective? > > Arnold > > On 31-Jan-13 17:13, Daniel Baker wrote: > > Hi Paddy, > > Thanks for your response. If this is the case, then for a > two-channel device, four of the outputs should be silent, but > they are clearly not. I've written a blog post with some > example data, code, and details of exactly what I've done: > > http://bakerdh.wordpress.com/2013/01/31/a-first-look-at-the-olimex-eeg-smt/ > > I've tried unplugging some of the electrodes to see what > effect this has on the output. When I remove electrodes CH2+ > and CH2- I get activity only on the first two channels, and it > looks a little odd (the second channel never goes above 511 > (out of 1023)). When I plug only those ones in and remove CH1+ > and CH1- then the first channel goes silent, but the other > five are active. > > I guess if you're right and the first two channels are the > differences between the electrodes, then maybe the other four > outputs are just interference. Still, it's puzzling that they > go silent when CH2± are unplugged. > > - Daniel > > > > On 31/01/2013 15:17, Paddy Duncan wrote: > > Hi Daniel, > > As I understood it, starting at byte 5, the 2-byte 10-bit numbers are the > > data for each channel ie the difference between the 2 channel electrodes. > > There is no data representing a single electrode, as the channels are > > differential. The six outputs are for the six channels that the protocol > > supports. > > Hope this helps. > > Paddy > > > > Below is the relevant section of the P2 firmware file: > > ////////// Packet Format Version 2 //////////// > > > > // 17-byte packets are transmitted from the ModularEEG at 256Hz, > > // using 1 start bit, 8 data bits, 1 stop bit, no parity, 57600 bits per > > second. > > > > // Minimial transmission speed is 256Hz * sizeof(modeeg_packet) * 10 = 43520 > > bps. > > > > struct modeeg_packet > > { > > uint8_t sync0; // = 0xa5 > > uint8_t sync1; // = 0x5a > > uint8_t version; // = 2 > > uint8_t count; // packet counter. Increases by 1 > > each packet. > > uint16_t data[6]; // 10-bit sample (= 0 - 1023) in big > > endian (Motorola) format. > > uint8_t switches; // State of PD5 to PD2, in bits 3 to > > 0. > > }; > > > > ////////////////////////////////////////////////////////////// > > > > -----Original Message----- > > From: Daniel Baker [mailto:[email protected]] > > Sent: 30 January 2013 20:26 > > To:[email protected] <mailto:[email protected]> > > Subject: [Openeeg-list] channel question for programming Olimex EEG-SMT > > > > Dear all, > > > > Last week I ordered and received an Olimex EEG-SMT box. I have successfully > > written code to read from the device using Matlab, directly over the virtual > > serial port interface. The data packets arrive in blocks of 17 bytes, and I > > have converted the six outputs (from bytes 5-16) specified in the packet > > protocol v2 into ten bit numbers. > > > > All six of the outputs produce a sensible looking trace, with mains > > artefacts at 50Hz as expected. What I don't understand is how these traces > > map onto the electrodes. There are four active electrodes and one ground > > electrode on the device. I initially thought that the mapping must be: > > > > Bytes 5&6: Channel 1+ > > Bytes 7&8: Channel 1- > > Bytes 9&10: Channel 2+ > > Bytes 11&12: Channel 2- > > Bytes 13&14: All of channel 1 (i.e. the difference between positive and > > negative) Bytes 15&16: All of channel 2 > > > > Presumably all referenced to the ground. However subtracting the first two > > traces does not produce the fifth trace (and so on). I'm at a loss to > > understand why else a device with four electrodes and two notional channels > > would produce six outputs. > > > > All help much appreciated. I intend to make some code publicly available > > once I'm happy with it. > > > > Many thanks, > > > > - Daniel Baker > > ---------------------------------------------------------------------------- > > -- > > Everyone hates slow websites. So do we. > > Make your web apps faster with AppDynamics Download AppDynamics Lite for > > free today: > > http://p.sf.net/sfu/appdyn_d2d_jan > > _______________________________________________ > > Openeeg-list mailing list > > [email protected] <mailto:[email protected]> > > https://lists.sourceforge.net/lists/listinfo/openeeg-list > > Go to the above address to change your > > subscription options, e.g unsubscribe. > > > > > > > ------------------------------------------------------------------------------ > > Everyone hates slow websites. So do we. > > Make your web apps faster with AppDynamics > > Download AppDynamics Lite for free today: > > http://p.sf.net/sfu/appdyn_d2d_jan > > > > > _______________________________________________ > > Openeeg-list mailing list > > [email protected] <mailto:[email protected]> > > https://lists.sourceforge.net/lists/listinfo/openeeg-list > > Go to the above address to change your > > subscription options, e.g unsubscribe. > > > > > > ------------------------------------------------------------------------------ > > Everyone hates slow websites. So do we. > > Make your web apps faster with AppDynamics > > Download AppDynamics Lite for free today: > > http://p.sf.net/sfu/appdyn_d2d_jan > > > > > _______________________________________________ > > Openeeg-list mailing list > > [email protected] <mailto:[email protected]> > > https://lists.sourceforge.net/lists/listinfo/openeeg-list > > Go to the above address to change your > > subscription options, e.g unsubscribe. > > > > > > ------------------------------------------------------------------------------ > > Everyone hates slow websites. So do we. > > Make your web apps faster with AppDynamics > > Download AppDynamics Lite for free today: > > http://p.sf.net/sfu/appdyn_d2d_jan > > > > > _______________________________________________ > > Openeeg-list mailing list > > [email protected] <mailto:[email protected]> > > https://lists.sourceforge.net/lists/listinfo/openeeg-list > > Go to the above address to change your > > subscription options, e.g unsubscribe. > > > > ------------------------------------------------------------------------------ > Everyone hates slow websites. So do we. > Make your web apps faster with AppDynamics > Download AppDynamics Lite for free today: > http://p.sf.net/sfu/appdyn_d2d_jan > > > _______________________________________________ > Openeeg-list mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/openeeg-list > Go to the above address to change your > subscription options, e.g unsubscribe. ------------------------------------------------------------------------------ Everyone hates slow websites. So do we. Make your web apps faster with AppDynamics Download AppDynamics Lite for free today: http://p.sf.net/sfu/appdyn_d2d_jan