Abstract: A digital circuit discriminates between valid transitions in the level of a two level logic signal in which the signal remains at its new level for a predetermined time period, and those transitions in the level of the logic signal which may be caused by noise. Upon the transition of the logic signal from one level to another, a timer is actuated. If the logic signal remains in its new level until the timer expires, the transition is considered valid and an output signal indicative thereof is produced. If the logic signal reverts to its initial level before the timer expires, the transition is considered invalid, the timer is reset and no output signal is produced. A discrete output is produced when a valid transition occurs between either of the two separate logic signal levels.