Fuente: WIPO "rice" 1,212,972. Burglar alarms. G. RICE. 31 Jan., 1968 [2 Nov., 1966], No. 49130/66. Heading G4N. In an alarm system, a pair of record reproduction devices are arranged to reproduce identical recordings in synchronism, one of the reproducers being arranged to be reponsive to an unwanted condition, a comparator 18 being arranged to operate an alarm 21 if the outputs of the two reproducers is appreciably different. In the system shown, for protection of a safe at a bank, a double tape recorder 11 records random binary signals on duplicate tapes, one of which is attached to a reproducer contained in the safe whilst the other is taken in a sealed container to the police station where it is attached to a further reproducer 17. When the bank is to be vacated, an initiation signal, e.g. from a time switch, is arranged to start both reproducers in synchronism. The reproducer in the safe is arranged to be interrupted by an attempt to open the safe or by cutting line 19 whereupon comparator 18 operates an alarm 21. A delay device may be included so that the comparator only operates on selected parts of the signals from the two reproducers. The tapes may be recorded at high speed from a feedback shift register with a noise-operated gate and may be reproduced slowly over three days. Alternatively, the tapes could be replayed in steps over a period of some years. The interruption of the reproducer in the safe can be arranged also to stop reproducer 17 so that both devices may be re-operated in synchronism when the system is restored.