PURPOSE: To ensure an assured reproduction by recording the clock signals simultaneously at the sound recording via the PWM modulation.

CONSTITUTION: For example, the signals from pulse oscillation circuit (a) of 50K HZ are put into PWM modulation circuit (g) after processes including the integration, amplification and others. The audio signals are also put into circuit (g) and then put into MIX after modulation and division. On the other hand, the signals from circuit (a) are divided down to 1/4 through circuits (d) and (e) to be put into MIX and then to be mixed with the above mentioned modulation signals. These modulation signals underwent the 1/2 division, and thus their isolation is easy from the synchronous signals. As a result, an assured reproduction is secured to be applied effectively to the sound recording/reproduction of the normal recorder and the like.