Key result
Anti-aliasing filters delay signals up to ~46 ms, potentially deteriorating inter-signal phase shift accuracy.
For multichannel biosignal acquisition intended for inter-signal phase shift analysis, identical or very similar anti-aliasing filters with lower orders and higher corner frequencies, oversampling, and digital low pass filtering are recommended to minimize signal delay.
May compromise phase-shift accuracy in multichannel biosignals; leaves open standardized filter protocols for reliable timing.
The availability of microcomputer-based portable devices facilitates the high-volume multichannel biosignal acquisition and the analysis of their instantaneous oscillations and inter-signal temporal correlations. These new, non-invasively obtained parameters can have considerable prognostic or diagnostic roles. The present study investigates the inherent signal delay of the obligatory anti-aliasing filters. One cycle of each of the 8 electrocardiogram (ECG) and 4 photoplethysmogram signals from healthy volunteers or artificially synthesised series were passed through 100-80-60-40-20 Hz 2-4-6-8th order Bessel and Butterworth filters digitally synthesized by bilinear transformation, that resulted in a negligible error in signal delay compared to the mathematical model of the impulse- and step responses of the filters. The investigated filters have as diverse a signal delay as 2-46 ms depending on the filter parameters and the signal slew rate, which is difficult to predict in biological systems and thus difficult to compensate for. Its magnitude can be comparable to the examined phase shifts, deteriorating the accuracy of the measurement. As a conclusion, identical or very similar anti-aliasing filters with lower orders and higher corner frequencies, oversampling, and digital low pass filtering are recommended for biosignal acquisition intended for inter-signal phase shift analysis.
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Keresnyei et al. (2014) studied this question. Anti-aliasing filters (Bessel and Butterworth) was evaluated on Signal delay. Anti-aliasing filters introduced a diverse signal delay of 2-46 ms depending on filter parameters and signal slew rate, which can deteriorate the accuracy of inter-signal phase shift measurements.
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