Fundamental waveform data transferred through a serial interface is stored in a register. A fundamental waveform generation circuit uses the data to generate a fundamental waveform. A selector is capable of providing fundamental waveform data stored in a fixed memory to the register to replace the fundamental waveform data provided through the serial interface. An initial counter provides a low level signal to an exclusive OR gate until its count value reaches a first predetermined value. A direction control signal M/I is provided to the exclusive OR gate from an external source and the direction control signal M/I at high or low level inverts a direction control signal to be provided to a direction selector.