PURPOSE: To give a hysteresis characteristic to a digital comparator and to stabilize its comparing operation, by supplying an input signal to one input terminal of the comparator then delaying the output of the comparator by n bits and selecting two kinds of thresholds from the delayed output and supplying the result to the other input terminal of the comparator.

CONSTITUTION: A digital input signal 1 is inputted into one side of a level comparator 101 and the output 2 of the comparator 101 is delayed by n bits by means of an n-bit shift register 102. The signal delayed by the register 102 is inputted into a 2-1 selector 103 and two kinds of thresholds, high level H and low level L, are selected, and then, the output 3 of the selector 103 is inputted into the other side of the comparator 101. A hysteresis characteristic is provided to the comparator 101. The comparator 101 discriminates the height of the level of a multiplexed data sequence and, when the level of the input signal 1 is gradually increased or decreased and the signal 1 contains a sound component, stabilizes its comparing operation.