One format converter connected to a mixer and another format converter connected to a head amplifier are interconnected via a single control/audio communication line. Each of the format converters includes a communication interface, control signal interface, audio signal interface, and a control section. The control section converts control data and audio data into data compliant with the EtherSound (registered trademark) that allows the control data and audio data to be transmitted over a single physical cable, and then it transmits, over the physical cable, the converted data from the communication interface to the other format converter.