Fig. 2

Time delay of the six-pole 0.03-Hz low-pass Butterworth filter (dashed blue line), time delay of the two-pole 0.002-Hz high-pass Butterworth filter (dotted blue line), sum of the above two time delays (solid blue line), and amplitude response of the two filters (solid red line). These Butterworth filters were used in V17 and K19. Vertical dashed lines and horizontal arrows indicate the pass band of 0.002–0.03 Hz