PURPOSE: To always converge the output data to '0' when the input data have continuously '0' by controlling the digital output data in accordance with the code bit of this output data.

CONSTITUTION: When the code bit of the input data is negative, +1 is applied to the code bit by an adder 14. Thus the code bit is turned into positive state from the negative one as shown by (1) → (2) in a figure for the data supplied to a RAM 15. This positive code bit is omitted to '0'. As a result, the output data is always converged to '0' even in case the input data following the cut input data have '0' continuously and also the code bit of the output data on the RAM 15 is negative immediately before the data is cut.