| 摘要 |
PROBLEM TO BE SOLVED: To prevent a failure from occurring by eliminating the collision of DC voltages applied from a dwelling-unit device, while using the same transmission line for individual calls and broadcasting. SOLUTION: Both trunk lines L1<SB>1</SB>, L1<SB>2</SB>of two systems are connected to a trunk line L1<SB>0</SB>at the side of a trunk line control unit 3 in a line switch 10 at broadcasting, thus transmitting a sound signal for broadcasting to all dwelling unit devices 2 that are branched into and connected to respective trunk lines L1<SB>1</SB>, L1<SB>2</SB>for output from a speaker 21b. In this case, since capacitors 103A, 103B for preventing DC voltages are inserted into signal lines Ld in the respective trunk lines L1<SB>1</SB>, L1<SB>2</SB>in the line switch 10, DC voltages applied between the lines of the signal line Ld by a trunk line power supply superposition circuit 200 of each dwelling-unit device 2<SB>11</SB>, 2<SB>21</SB>are blocked by the capacitors 103A, 103B for preventing DC voltages, thereby preventing a collision in the trunk lines L1<SB>1</SB>, L1<SB>2</SB>of the two systems. COPYRIGHT: (C)2009,JPO&INPIT
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