Filtering

Filtering is a method to extract information from a time-series such as the EEG. When people are talking about alpha or SMR they most commonly refer to a specific filtered output such as 8-12 Hz (Alpha) or 12-15 Hz (SMR)

There are several types of filters, such as an FIR or IIR filter. For a technical overview of real-time filtering also see an overview by Brainquiry. More filtering information can also be found on the following link: Maxim

Filtering also changes the morphology of your signal, so this is very important for interpreting filtered physiological signals.On the right see a screenshot of an unfiltered ECG signal (in red) and the exact same segment filtered by an FIR filter (yellow), a Butterworth filter (light blue), a Chebyshev filter (dark blue) and an Elliptic Filter (purple). Note the big differences in morphology of the P-Q-R-S complex of the ECG. All ECG amplifiers perform their filtering of the signal in the hardware, so one could wonder whether there exist any Cardiologist who has ever seen a 'real' unfiltered ECG or only filtered ECG signals!