PURPOSE: To prevent the increment in a distortion of an output waveform by discriminating the overflow of a prescribed number of bits of digital data, and on the other hand, adding the fixed offset data to a data part exceeding a prescribed value.

CONSTITUTION: When the number of bits operating really of an A/D converter 1 is less than the prescribed number of bits, by an overflow detection circuit 3, a signal switching circuit 5 is turned (ON), and the circuit 5' is turned (OFF), and the data adding no offset data are outputted. When the number of bits is more than the prescribed number of bits, the signal switching circuit 5 is turned (OFF), and the circuit 5' is turned (ON), and the data adding the data of 16 bits (LSB-) among the output data of the A/D converter 1 to the offset data by an adder 7 are outputted. Thus, since the data inputted to an encoder 8 are the prescribed bits (16 bits) or below, the conventional encoder is usable The output data of the encoder 8 are sound-recorded on a digital audio sound recording/reproducing equipment 9.