When a positive or a negative overflow error condition is encountered, the present invention substitutes the most positive or most negative value for the erroneous sample. The sign bit of a potentially erroneous value is inverted to form the MSB of the substitute value, and this value is, in turn, inverted and expanded to form the n-l LSB's of the substitute value. If an overflow error condition is detected, the erroneous value is replaced by the substitute value.