The invention relates to a public address system, a control unit of the public address system and a monitoring device of a public address system. A bi-directional communication with modulated data between a control device and a monitoring device and/or an end of line monitoring device is monitoring the presence and status of the loudspeaker line and loudspeakers constantly. The communication is frequency multiplexed upon the existing audio wire from the audio amplifier to the loudspeakers. In combination with the bi-directional data communication the power supply for the monitoring devices and/or the end of line monitoring devices is extracted from audio frequencies, in fact preferably a 20 kHz pilot tone generated by the audio amplifier. Filtering prevents interference between audio signals and/or communication signals and/or the power supply.
