PURPOSE: To perform the mixing of a large number of sound resources by interposing one cable by multiplexing an audio signal, a sound level control signal and a power supply voltage for amplifier driving on one transmission line.

CONSTITUTION: Digital data on each channel obtained from an A/D converter 2 is inputted to a parallel-serial converter 3, and is converted into serial data. The data is sent out to the transmission line 5 via a capacitor for DC blocking. While, the serial data is inputted to a serial-parallel converter 7 at a side to be controlled, and it is converted into the digital data at every channel, and furthermore, the digital data is converted to an analog voltage by a D/A converter 8. The control of a sound level is performed on the VCA 9 of a mixer by the analog voltage. Finally, the sound volume of each microphone MIC is controlled by the VCA 9, and it is taken out as a mixing audio signal 9S. The signal is sent out to the transmission line 5 via a band rejecting filter and the capacitor for DC blocking.