Temporal alignment of asynchronously sampled biomedical signals

Temporal alignment of signals obtained using different acquisition systems is often complicated by asynchronous sampling. In the current study we aligned 3-lead ECGs recorded in a phonocardiogram setup with 12-lead Holter ECGs. Two signals with common morphology (lead II from both devices, recorded synchronously with closely placed electrodes) were resampled to similar sample rates. An initial alignment was obtained by cross-correlation analysis. Next, instantaneous delays were estimated using cross-correlation analysis of the two signals in a running window of four seconds duration. The first derivative of the instantaneous delays is related to the variation in sample rate between signals. Consequently, a smoothed version of this derivative was used for local resampling of one of the signals before final alignment. Visual inspection confirmed that all recordings were synchronized by the alignment procedure. The mean and standard deviation of the delay between R-peaks in the synchronized ECGs were 0.3±4.5 ms. We present a fully automated method for alignment of signals sampled asynchronously with drifting clocks.