PURPOSE: To prevent the occurrence of distortion at the time of bit rate changeover by changing the time constant of an integration circuit constituted at the output part of a D/A converter linked with a changeover switch.

CONSTITUTION: Transistors Q1 and Q2 are turned on when a bit rate changeover switch S1 is connected to the side of a bit rate changeover input D1, and is turned off when it is connected to the side of the bit rate changeover input D2. Microphone output MIC-OUT is made as a high cut filter by a resistor R2 and a condenser C3 in a state where the transistors Q1 and Q2 are turned off. A phenomenon where it is quickly distorted by the frequency of an input is prevented by obtaining the optimum frequency characteristics of the input before A/D conversion for each bit rate and frequency characteristics and granulated noise are balanced so as to set at an integral time constant optimum value after the D/A conversion.