A digital data processor which receives an N-bit input signal from a data source and converts the N-bit input signal into an M-bit output signal, the M-bit being larger than the N-bit. The digital data processor includes: an weighted addition circuit which is operable to perform weighted addition on at least the input signal and a signal being time-shifted with respect to the input signal and output as a weighted added input signal; an arithmetic shift circuit which is operable to perform an arithmetic rightward shift operation on the weighted added input signal for a predetermined number of shifts and output as a processed input signal; a bit extension circuit which is operable to attach a predetermined bits to an LSD side of the input signal to generate an intermediate signal of M bits; and an addition circuit which is operable to perform addition of the intermediate signal and the processed input signal so as to generate the M-bit output signal.