Re: channel question for programming Olimex EEG-SMT

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]
>> 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
>> ----------------------------------------------------------------------------
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