3.3.1. Hjorth Parameters

The Hjorth parameters [47] are obtained by applying signal processing techniques in the time domain, giving an insight into the statistical properties of the signal. The three Hjorth parameters are: activity, mobility and complexity (Equations (1)–(3)).

Activity gives a measure of the squared standard deviation of the amplitude of the signal x(t), indicating the surface of the power spectrum in the frequency domain. That is, the activity value is large if the higher frequency components are more common, and low otherwise. Activity corresponds to the variance of the signal.

Mobility represents the mean frequency or the proportion of standard deviation of the power spectrum. This is defined as the square root of the activity of the first derivative of the signal divided by the activity of the signal.

Complexity indicates how the shape of a signal is similar to a pure sine wave, and gives an estimation of the bandwidth of the signal. It is defined as the ratio between the mobility of the first derivative and the mobility of the signal.

To summarize, the three parameters can be referred as the average power, the average power of the normalized derivative and the average power of the normalized second derivative of the signal, respectively.