Well, newbies like me are here to learn :D
So, I'll answer my questions here, for others to save time:
When looking at the board docs for EEG-SMT it is clear:
CH1+ and CH1- electrode data are combined as well as the readings from CH2+ and CH2-.
The difference between CH1+ and CH1- is contained in data[0]
and the difference between CH2+ and CH2- data[1].
data[2] to data[5] contain some faint mirror image of data[1].
I also verified this by measuring, i.e. connect all five electrode according to the docs
and look at simple line plots of the raw data and the above will be obvious.
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References:
Quite a bit of the above is mentioned in some forum posts.
I added the connection to the firmware resp. modeeg-source code.
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For clarification, here the snippet from the comment in main.c (firmware v2)
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.
};
Many thanks for the hardware!