Background

The Syncing Process

To sync two sources with each other, they need a simultaneously recorded signal with a characteristic signature at two timepoints in common. This could be the magnitude of the accelerometer for example, if multiple devices are shaken together.

Selecting common segments

The script can detect prominent shakes automatically with the ShakeExtractor. This is done by detecting the peaks above a certain threshold. These peaks are then merged into sequences of peaks that are within distance milliseconds of each other. Sequence candidates need to fulfill the following conditions:

• must have at least min_length peaks

• must be contained in start_window_length or end_window_length, respectively

The sequence with the highest weight, i.e., mean + median of the peaks in the sequence, is selected to represent the start- or end segment.

Calculation of the timeshift

To compensate offsets in the system time of different sources, the timeshift to synchronize the selected start segments with each other is calculated. For the automatic computation of the timeshift between two signals, the cross-correlation for each segment with the reference signal is calculated. The signals are shifted so that the correlation between the selected segments is maximized.

Adjusting the frequency

As no clock is perfect, an additional issue that arises when using multiple sensors is that of clocks with an offset in running speed. While clock speeds can drift over time, these influences are typically very small, and it can generally be assumed that the offset from the correct speed is constant for anything but long trials (Zhou, Hui, et al. “Frequency accuracy & stability dependencies of crystal oscillators.” Carleton University, Systems and Computer Engineering, Technical Report SCE-08-12 (2008)).

The result of these differences in running speed is that signals desynchronize over time. To compensate, a stretching factor is calculated, which brings the difference between the synchronization timeshifts for the start- and end segments to zero. After stretching the signal, the timeshift to remove the offset between signals is removed again, resulting in the final timeshift and stretch factor values.