| 摘要 |
A subscriber line control circuit for controlling a reversing switch to couple a subscriber station either to an AC signalling voltage generator or to a DC line voltage supply, depending on whether the subscriber station is on-hook or off-hook. The control circuit includes a capacitor connected between the subscriber line and the reversing switch and which by means of a rectifier is charged to a DC charge voltage by alternate half cycles of the AC signalling voltage. During the other half cycles the DC charge voltage superimposes on the AC signalling voltage and so is available for actuating an automatic switching arrangement by which the subscriber station is selected for coupling to the reversing switch. When the subscriber station goes off-hook the capacitor discharges over the subscriber line, resulting in a change in voltage or current which is detected by a detection circuit. The detection circuit then supplies a control signal to the reversing switch to cause it to change-over to the DC line voltage supply. Such control circuit thereby dispenses with the need for an auxiliary DC source for actuating the automatic switching arrangement.
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