Importance

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Abstract

Line security apparatus for a direct-wire alarm system utilizes a transmitter at the remote premises to impose on the direct current a small amplitude carrier frequency which is on/off modulated by a clocked, continuous progression of pseudo-random digital bits. A receiver, tuned for the carrier frequency, is utilized at the monitoring station to recover the progression of digital bits, if received at the monitoring station. A microprocessor at the monitoring station produces an identical and initially synchronous sequence of digital bits, compares the produced sequence to the progression of bits received, and effects a temporary alarm signal indicating that the line security is disrupted if the bits do not correspond. Because the lack of correspondence may be due to deviations of the transmitter and microprocessor clocks, the microprocessor seeks correspondence by shifting the produced sequence and, if successful, continues the comparison as shifted. If the shifting does not result in correspondence, an alarm is signalled.

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First Claim