| 摘要 |
A security system is disclosed in which a security control unit (12) transmits a coded signal component (S1) at a first frequency (F1) to a remote transponder unit (14). The security signal has two components (S1, S2) which are transmitted at different frequencies (F1, F2) and the frequency of transmission of the second signal component (F2) is encoded in the first signal component (S1). The remote unit (14) is arranged to decode the frequency (F2) of the second signal component (S2) from the first signal component (S1) and therefrom to tune itself to receive the second signal component (S2).
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