Software and hardware methods for phase shift cancellation in ECT recordings

Analog filtering of ECG signals to remove low-frequency drift and prevent aliasing errors is usually mandatory before more complex digital processing of the signal is possible. Phase shifts introduced by analog filters result in distortion of the ECG signal and artifactual ST segment abnormalities. Present filter designs cannot provide linear-phase, rapid-rolloff, high attenuation in the stop band and zero ripple in the passband without compromise of either the phase or magnitude response. A software solution is proposed for phase distortion at low frequencies, by approximating the analog high-pass filter with a first-order digital filter and passing the digitized ECG signal through it with time reversal. A ninth-order analog low-pass antialiasing filter is introduced, with nearly ideal characteristics of constant group delay and 60-db/octave rolloff at the corner frequency.<>