Alignment of Multi-Sensored Data: Adjustment of Sampling Frequencies and Time Shifts

Objectives: For a more comprehensive clinical picture, measuring vital signs with multiple devices is advantageous due to the acquisition of complementary information (ECG, body movement, temperature and respiration) and the possible compensation of signal loss. Our aim is to find a robust way for the correction of sampling frequencies and the alignment of non-synchronized sensors.Methods: We used data from an experiment including five different devices, which simultaneously measured the activity of the heart and other vital parameters (Hexoskin Hx1 Smart Shirt, SOMNOtouch NIBP, Polar RS800 Multi, eMotion Faros 360°, NeXus-10 MKII). Our alignment procedure is based on pairwise comparisons of 300 consecutive heart beat intervals to the Hexoskin reference interval sequence, using minimization of the overall absolute sum of differences for alignment. Robust linear regression fits were used to adjust for general deviations in the sampling frequencies and for non-linear resampling in a sliding window.Results: Altering sampling frequencies were identified in Faros and Polar devices in the course of experimental measurements in all of 13 subjects. SOMNOtouch and NeXus had the lowest standard deviation across all subjects. In two identical Faros devices, the average sampling frequency was +0.0293% and +0.0175%.