onepole - the filter has a pole at Z = -a1 , in the Z plane (and a zero at Z = 0). Notice that the one-pole exhibits either a lowpass or a highpass frequency response, like the one-zero. The lowpass character occurs when the pole is near the point Z = 1, which happens when a_1 approaches -1. Conversely, the highpass nature occurs when a_1 is positive.