| 摘要 |
<p>The radio communication system links a number of persons carrying a transmitter and a common centre. The transmitter provides a basic call - back to the centre notifying a need for assistance or a particular operation to be carried out. All transmitters work at the same frequency, the identity of each being labelled by breaking the basic carrier frequency into pulse trains, the number of pulses in each such train providing the identification. The transmitters, which can be carried in a pocket, are manually switched on or off. The central receiver is clock based and uses a counter and comparator to process the incoming transmissions and provide an alarm activation.</p> |
