PURPOSE: To prevent error in code inversion produced by overflow or underflow by using an overflow detection signal to replace a forecast error respectively into a prescribed value within a level range possible for positive or negative transmission if the forecast error causes a positive or a negative overflow error.

CONSTITUTION: A full adder 8 obtains a difference between an input A and a forecast error B and a full adder 9 inputs the input A and a code of the forecast value B. If the result of operation overflows in case of 4-bit (≥8), when it is regarded as 8-bit operation, the output is G00001*** (* can be either "1" or "0"), and if the result underflows (≤-9), the result is 11110***. When neither underflow nor overflow takes place (7∼-8), the result is 00000*** when positive and 11111*** when negative. Thus, the overflow and underflow are detected by observing the MSB (most significant bit) of the output of the full adder 8 and the LSB (least significant bit) of the output of the full adder 9.