PURPOSE: To reset a forced sound circuit at emergent broadcast with a detection signal, by detecting a DC voltage change by rotating a knob of a variable resistor slightly counterclockwise.

CONSTITUTION: In normal broadcast, an output 11 of a register 4 turns on an analog switch 9 and an analog switch 6 is turned off, a voice signal is controlled to a suitable volume by a variable resistor 12 and broadcasted from a speaker 10. Further, in the broadcast of forced sound volume, the output 11 of the register 4 turns off of the switch 9 and turns on the switch 6, then the voice signal inputted from an input terminal 1 is broadcasted via the analog switch 6 in the maximum sound volume. In turning counterclockwise the variable resistor by about 10°, the reduction in DC voltage is detected at a detection circuit 15, the register 4 is reset with a reset signal 16 from the detection circuit 15, then the analog switch 6 is turned off and the analog switch 9 is turned on, and the sound volume is controlled to a suitable value.