| 摘要 |
1311253 Automatic exchange systems COMPAGNIE INDUSTRIELLE DES TELECOMMUNICATIONS CIT-ALCATEL 17 May 1971 [15 May 1970] 15224/71 Headings H4K and H4L A multi-channel heterodyne frequency-modulation telecommunications transceiver is adapted to scan for an idle noise free channel and if no such channel exists to repeat the scan for a free noise laden channel. In the receiver section shown a stepping selector 12, 13 steps each time it is pulsed to select a heterodyning frequency corresponding to a channel. For each step the intermediate frequency is amplified at 4 and combined at 5 with a test frequency from a local oscillator 22 supplied at a first intensity via a gate 23. The intensity of the combined signal is limited at 6, the extent to which the test frequency is clipped varying with level of sound on the incoming channel. The clipped test frequency is filtered out at 33 and its intensity compared at 35 with a reference level S and if this reference level is exceeded a storage element 38 is set terminating the scan and selecting that channel. The level of the test frequency passed by gate 23 is such that the comparator 35 will only produce an output terminating the scan when an idle noise free channel is scanned. The stepping is controlled by a clock 41 which cycles a counter 44 controlling the logic, stepping the selector 13 once per cycle and after all N channels have been scanned switching a bi-stable 52 to its other state via a divide by N circuit 51. Switching of the bi-stable 52 closes a gate 23 and opens a gate 24 which passes the test freqeuncy at a second intensity level. The channels are scanned a second time using this test frequency intensity which is such as to be still attenuated below the reference level S by the sound level on any busy channel but will not be attenuated below the reference by a noise laden idle channel. Accordingly the scanner will stop during the second scan on the first idle channel. |
