A differentiation circuit outputs a normalized time variable signal for each phase. This time variable signal is multiplied by a gain parameter by a multiplier, thus reproducing an AC amplitude level lost in normalization. The reproduced output is added to an amplitude parameter by an adder, thus reproducing a DC amplitude level lost in the normalization. A CPU reads out the parameters of the differentiation circuit and adder from an ROM, and writes them into a parameter memory. When phase-change data is detected, the content of the parameter memory is rewritten, thereby forcibly changing the phase.