Abstract: Synchronizing means is provided for synchronizing with a periodically repeating reference signal the change of level of an input signal which is capable of assuming either one of a first and second level. A bistable device is enabled periodically by the reference signal to respond to the input signal and assume either one of its two stable states in correspondence with the first or second level of input signal and produce an output signal which has either one of two different levels according to the stable state assumed. Means is provided to respond to the output signal when it has a value intermediate the two levels to generate a stop signal which indicates that the bistable device is in a metastable condition and which is used to interrupt the reference signal at a predetermined point in its periodicity before the bistable device is next enabled and until the bistable device assumes a stable condition.