| 摘要 |
System which comprises a central post PC in communication with the fixed posts PF1,...PFi,...PFn by means of a transmission line and where said fixed posts PF1,...PFi,...PFn communicate via radio with the movable posts PM1,...PMi,...PMm. The central post PC generates a free channel tone FCT to indicate that it is ready to transmit or to receive information from the fixed posts PF1,...PFi,...PFn and from the movable posts PM1,...PMi,...PMm and it comprises means for modulating MM the free channel tone FCT with a digital data signal and, using this, it transmits more information. The bandwidth of the free channel tone FCT, once modulated, is less than or equal to that defined by the tolerance of the free channel tone FCT. The fixed posts PF1,...PFi,...PFn comprise means for modulating MM and means for demodulating MD the free channel tone FCT and the movable posts PM1,...PMi,...PMm include means for demodulating MD the free channel tone FCT. <IMAGE>
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