PURPOSE: To reduce the effect due to the fluctuation of an input signal and noise by rectifying a positive phase and/or inverse phase bipolar input signal, storing a voltage proportional to the peak value, dividing the stored voltage to a half of the peak value and using the result as a reference voltage for identifying a code.

CONSTITUTION: A bipolar signal 1 inputted to the primary winding 20p of a transformer 20 is converted into a positive phase input signal (a) and an inverting input signal (b) through a secondary winding 20s. The positive portion of the positive phase signal (a) or of the inverse phase signal (b) is charged in the capacitor 26 of a storage section 11 and its charging voltage (c) is a smoothed and nearly constant voltage proportional to the peak value of the bipolar input signal. The voltage (c) is divided by variable resistors 27, 28 of a reference voltage generating section 12 to generate a reference voltage (d) for identifying a half of the peak value of the standard bipolar input signal. Thus, comparators 29, 30 compare the positive phase signal (a), the inverse phase signal (b) with the reference voltage (d) respectively and output signals (e), (f) whose codes are identified are generated and the signals are synthesized by an OR circuit 31 constituting a synthesis section 15 and a unipolar signal 6 with a prescribed width is obtained.