Open frequency = 800Hz
Close frequency = 2kHz
When the circuit input is a 1.25ms period square-wave the signal passes only through the Multiple-feedback Bandpass filter above with f0 = 800Hz.
If instead the input is a 500us period square-wave the signal passes only through the other BP filter with f0 = 2kHz.
The filters are very selective Q=20 and Q=25 respectively so any noise is cut off.
If the passed signal exceeds the thresold=6.51V of the comparators we change the state of the flip-flop at the end.
if the output of the comparator above is high
If the output of the comparator above is high for a moment the flip-flop is high else the output of the comparator below is high the flip-flop is low
