Centralised supervisory and alarm system for remote subscriber units - uses three signal transmission system to detect and verify correct alarm condition

摘要

The supervisory and alarm system controls from a central post a number of remote installations via signals transmitted over telephone wires. Interruption of a signal during a predetermined time period actuates an alaram. The time also transmits during this period a succession of irregular signals which are designed to counteract fraudulent recorded signal transmission. In the central supervisory unit there is a section for each subscriber on which is mounted a differential transformer. Through this transformer to the subscriber circuits are applied signals from the incoming line. The transformer also introduces into the circuits the succession of irregular signals emanating from a central source. The differential transformer ensures during processing of the output signals that the effect of irregular signals from the central source are minimised. The subscriber circuits in the central unit have an amplifier with automatic gain control amplifying the frequency of the irregular signals.