Integration
Integration is the operation that integrates a signal according to time.
The processing steps applied to the signal when using Integration are as follows:
Remove DC: the average value (overall mean value) of the time signal is removed from the signal by subtraction.
A non-zero mean can lead to incorrect high magnitude values during the integration step.
Up-sample 4x: the time signal is resampled to four times its original sampling frequency.
Integration algorithms are known to create artifacts at high frequencies. By up-sampling the signal before integration, you ensure that the artifacts will be created outside the frequency band of interest.
Integrate: the time signal is then integrated using the cumulative trapezoidal method.
This method appears to be the most appropriate when dealing with acceleration and velocity time data.
Down-sample x1/4: the current time signal is resampled to the sampling frequency of the original signal.
To avoid aliasing, the resampling function applies a low-pass filter before the resampling itself. This ensures that the potential high-frequency artifacts are removed from the signal.
High-pass filter 20 Hz: a 6th order high-pass Butterworth filter with a 20 Hz cutoff frequency is applied.
This filtering step aims to remove potential constant components introduced by the integration step (the integration method can introduce a low frequency trend evolving all along the signal, potentially hiding the real phenomenon in which you are interested).