a high - pass filter which provides a digital word output indicative of a high - pass filter function of the quantity of logic &# 34 ; 1 &# 34 ; inputs to the counter . the output of a counter is low - pass filtered and applied in conjunction with a bias signal to a second counter which counts clock inputs from a value determined by the count input from the low - pass filter to a predetermined limit and outputs a feedback pulse to be used by the first counter in opposition to a second bias string of pulses where the second bias string of pulses and the feedback signal have substantially the same frequency of pulse rate under steady state conditions of the filter .