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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