1476261 Alarm systems ABOYNE PTY Ltd 16 June 1975 [18 June 1974] 25631/75 Headings G4H and G4N An information transmission system comprises: (a) a transmitter arranged to transmit in two modes viz. a monitoring mode wherein monitoring signals of duration #t spaced apart by time t (greater than #t) are employed to modulate a carrier, and an alarm mode in which the carrier is modulated by alarm signals having a greater repetition rate than the monitoring signals, (b) a receiver which detects a change in the mode, and also if no signal with the duration 8t is detected within each period of successive predetermined (fault detecting) periods of time xt (greater than t). As disclosed, the monitoring and alarm signals are identical, each being a 600-microsecond multi-bit message in diphase form frequency-modulated on to the (RF) carrier, the monitoring mode involving repeating this message once per minute indefinitely and the alarm mode involving repeating it once per 800 microseconds (i.e. with 200 microsecond gaps) twenty times. A multi-storey building has a number of transmitters and a receiver, on each floor, communicating by radio, the receivers being coupled by cable to a control unit (with display) for the building which is connected by transmission line to a fire control authority station. A transmitter switches from monitoring mode to alarm mode on detecting fire (temperature or smoke). The message specifies the building, floor and sensor identities and includes a parity bit and extra bits. At a receiver, the received message is demodulated with signal duration and interval checks and fed to a shift register with a parity check. The message is rejected if it does not contain the correct building and floor identities and extra bits, and have correct parity. For monitoring, the sensor identity is decoded to set a respective sensor presence flip-flop. Every ten minutes these flip-flops are reset, and those which were not set in the previous ten minutes set respective sensor fault flip-flops to light respective fault lamps. The transmitters are battery-driven; fault may be flat battery. In the alarm mode, the sensor identity is compared with that in the previous message, equality setting a fire flip-flop respective to the sensor identity (as decoded), each fire flip-flop controlling a respective fire lamp (the two messages compared must occur within a predetermined period less than t). Ranges of values for the various intervals and durations are given, those above being merely preferred. Application to security and pollution level detection is mentioned.
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