FR2446519A1 - Telephone line signalling system - normally applies high frequency signal to line which decreases on actuation and is fed to loudspeaker and preselected station - Google Patents

Abstract

The signalling system for a subscriber telephone line comprises a power supply unit (3) connected across the subscriber line wires (1, 2) and supplying an oscillator (5). The oscillator is coupled through a normally closed switch (10) to a series capacitor (11) and an inductance (12) circuit connected to one of the line wires (1), an also through a second normally open switch (9) to a loudspeaker (8). The oscillator (5) normally applies a hf signal of e.g. 9 KHz to the line through the first switch. The two switches are gauged with a third switch (14) and controlled by a control switch (13). Under normal circumstances, the 9 KHz is detected by a receiver on the line. If the subscriber actuates the control switch, the switch (10) opens and the switch (14) closes to lower the oscillator frequency which is fed to the loudspeaker. Simultaneously the receiver detects the absence of the 9 KHz signal and automatically notifies a preselected line terminal.