In EEG-based wireless BCI systems, additional signal conditioning is essential to enable the
transmission of precise neurophysiological signals. Many noise sources are present such as
physiological interferences and power line noise. Physiological interferences are the other
biopotential signals such as electromyogram (EMG), electrocardiogram (ECG), and electrooculogram
(EOG). They have relatively lager amplitudes around 50uV and up to 20-30mV
while the amplitude of EEG signals is much smaller on the scale of roughly 10~100uV. Thus,
the EEG signals are easily buried by these physiological signals unavoidably. In the case that
the BCI system is connected to a desktop which operate with the electric power outlet, we
also have to consider the power line noise as well. The power line noise contaminates the
desired EEG signals in the range of 50 or 60Hz. Furthermore, the users of portable wireless
BCI systems are usually in an active state making free motions and postures, whereas the
users of wired BCI system are asked to stay in a motionless state, while their EEG signals are
monitored. Therefore, the measured EEG signals of wireless BCI systems are also subject to
heavy motion and vibration artifacts.
To avoid interference from the various noise components and recognize the user’s intention
correctly, the system must be designed carefully. Figure 2 is the general block diagram of a
typical wireless BCI system. In EEG acquisition block of the system, there are two main
parts, namely, the analog front end circuit and the digital system.