INTERCOM SYSTEM

PROBLEM TO BE SOLVED: To prevent an entrance slave unit from making clicking noise at a start of conversation between the entrance slave unit and a master unit or at an end of conversation. SOLUTION: The master unit 2 of the intercom switches dc voltage of a transmission path L between a standby time and a conversation time. The entrance slave unit 1 detects the dc voltage of the transmission path L. It turns off a slave unit power supply stabilization circuit 11 and a slave unit conversation circuit 14 when it detects the dc voltage for the standby time of the transmission path L and turns on the slave unit power supply stabilization circuit 11 and the slave unit conversation circuit 14 when it detects the dc voltage for the conversation time. The master unit 2 is provided with a mute signal output circuit 26 which outputs a mute signal before switching from standby to the conversation or from standby to conversation. The entrance slave unit 1 is provided with a mute signal detecting circuit 13 which mutes the slave unit conversation circuit 14 of the entrance slave unit 1 at the time of switching from standby to conversation or from conversation to standby, according to the mute signal from the master unit 2 for preventing the master slave unit 1 from making clicking noise. COPYRIGHT: (C)2005,JPO&NCIPI