to reduce disturbance and interference signals , and particularly recurring disturbance signals due to vibration or out - of - round conditions of rotating elements coupled to a speed transducer , a low - pass filter is connected to the speed transducer and , in addition , a high - pass filter which , however , is disconnected upon sensing that the speed of the wheel or of the vehicle is below a predetermined limit , or that the anti brake - lock system unit has responded . thus , disturbance signals , and particularly periodic disturbance signal which might cause oscillatory conditions to arise in the filter and transducer circuitry , are effectively suppressed . to further increase the reliability of response , the output from the filter circuits is connected to threshold circuits with a variable threshold level in which comparators compare a peak signal with a weighted , then existing signal , so that the comparison level threshold is continuously shifted based on previously occurring peaks , so that disturbance signals are reliably excluded from affecting signal processing within the anti brake - lock system unit .