Abstract: A demodulated audio signal for detection of engine-knock is digitalized to produce a sequence of digital signals corresponding to the various cylinders of the engine in turn. These digital signals are recursively filtered by applying a factor k to a newer value and its complement (1-k) to an older value from the same cylinder or a value obtained therefrom by partial filtering and the sum of the two factor-modified terms is formed to provide a reference value with which the most recent digital signal is compared, to produce an indication of engine-knock when the latter exceeds a reference value by a predetermined amount. The factor k is varied in accordance with acceleration of engine speed or with the level of signals from the vibration sensor that exists before the application of automatic gain control prior to integration.