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The present work proposes an embedded real time hardware-software platform for brain computer interfaces (BCI) based on steady state visual evoked potentials (SSVEP) and alpha rhythm. The complete implementation of the embedded system is described, including the electroencephalogram (EEG) amplifier, signal acquisition and processing stages, and practical implementation. The device is a 25 characters hybrid-BCI speller that uses 5 visual stimulus and visual alpha waves for control instructions. The system focuses on simplicity and portability, using only two electrodes, a simple EEG amplifier and an embedded computer. The speller does not require any training from the user and provides real time biofeedback to increase attention on stimuli. The platform is based on an embedded system with a real time operative system, Windows CE. Experimental results are presented in order to show the feasibility of the proposed system.

In this work, the signal modes necessary for the analysis of amplifiers for double differential and normal double differential electrodes are obtained for the first time. Superficial electromyographic (sEMG) electrodes may have multiple dry contacts with particular geometric arrangements in order to better pick up signals originated in motor units just below the electrode while rejecting other EMG signals. The amplifiers used for these electrodes are thus not differential and in order to evaluate their performance, the commonly used decomposition into common mode and differential mode is not applicable. Hence, a set of orthogonal signal modes were derived considering linear combinations of the input signals of a selection of spatial filter amplifiers. The first signal mode was associated with the desired output and the second to common mode electromagnetic interference, but there are other modes present in the amplifier that account for EMG crosstalk interference. The rejection coefficients associated with these modes were derived and the obtained laws were applied to a comparison of two double-differential electrode circuit designs.