Abstract: A line circuit is disclosed for controlling communications over a communication pair between a switching machine and a key telephone station. The line circuit includes a dual sensitivity shunt detector arrangement (103, 104, 105) for improved detection of dc transients and ac ringing voltage signals appearing across the pair. A current diverter circuit (104) switches the sensitivity of the shunt detector arrangement depending on the operating state of the key telephone station.

Abstract: There is disclosed a line circuit for establishing a hold condition on a communication line, extending between a switching machine and a key telephone station, during an open switching interval (OSI) when no current flows on the communication line. The control signal required to enable the hold circuit is arranged to be time dependent. For a fixed time interval, which bridges the OSI period, after the telephone station answers a ringing line a hold condition is enabled (402) exclusively by a control signal on the A lead from the station. This time dependent hold enable arrangement permits a hold capability during an OSI condition. After this time interval elapses (403) a hold condition is enabled by (404) a joint signal condition on the A lead and a current continuity condition on the communication line.

Abstract: A modulated RF carrier signal distributing arrangement, utilizing existing line circuits, is disclosed for providing entertainment signals to key telephone lines in the active noncommunicating (hold) state. The disclosed system utilizes an existing option connector on the line circuit for connection of a music receiver circuit. A centrally located transmitter circuit frequency modulates its own RF carrier with an externally supplied music signal and is wired to the existing common dc power to distribute the modulated RF signals to the receiver via the option connector on each line circuit. The receiver circuitry demodulates the RF signal and couples the resulting music signal directly across the hold bridge leads of the associated line.