Patent GB-972135-A (GB)
Title: Improvements in or relating to communication systems
IPC: H04B1/46
Filed: 1961-09-05
Abstract: 972,135. Communication systems. GENERAL ELECTRIC CO. Ltd., T. W. CHANDLER, and E. SMITH. Nov. 1, 1962 [Sept. 5, 1961], No. 31877/61. Heading H4L. In a two-way communication system the transmitted output of each station contains a characteristic modulation component, the absence of which in one direction causes the receiving station to interrupt the characteristic modulation of its output signal, thus indicating the existence of a fault and operating means to switch over to stand-by receivers and/or transmitters at both stations. The apparatus at each station comprises two transmitters Tx1, Tx2 tuned to the same frequency, modulated by unit 3 and supplying identical R.F. signals over leads 4 and 5, one or other of which is selected by contacts of relays L, M, N to feed transmitter aerial 6. It also contains two receivers Rx1, Rx2, fed by receiver aerial 9, one or other being connected to filter unit 12 by relay contacts COA1 and COA2. During normal operation a characteristic pilot signal supplied by oscillator 13 is injected into the modulating signal applied to the transmitters. At each station " A " and " B " (only one is shown), the filter unit 12 is arranged to select from the signal received by Rx1 or Rx2 the characteristic tone injected into the modulation signal at the other station and apply it to a receiver 14 to operate a relay F. If a fault should develop in receiver Rx1 resulting in the disappearance of its output, relay F is released, operating a relay A and hence releasing normally operated relay COA to effect the switch-over to receiver Rx2, whereupon relay F is again operated before a slow to release relay B, deenergized by the release of F has had time to fall off. Relay A also causes lamp 16 to be lit indicating a fault and locks over contacts B3, A2 and a push-button switch 15 so that switch 15 may be broken on repair of Rx1 to switch back to the original conditions. If a fault occurs in transmitter Tx1 at station " A " such that the output signal is no longer modulated, this sequence of events will occur at station " B " but since switching over to Rx2 does not reestablish the characteristic tone, the delayed release of relay B occurs. This energizes lamp 17 and breaks the holding circuit of A, extinguishing lamp 16 and reconnecting receiver Rx1. The release of B has the further results of connecting aerial 6 to transmitter Tx2, energizing lamp 19, and, by breaking circuit of relay E interrupts the injection of characteristic tone. At station " A " the cessation of this tone in the output of " B " causes the failed transmitter Tx1 to be replaced by Tx2. At each station delayed release of relay C then re-operates B which re-injects the characteristic tone and extinguishes lamp 17, the fault being indicated by lamps 19. If the fault at station " A " had been in the failure of tone generator 13 or modulating unit 3, the same sequence occurs up to point where receiver 14 in " B " responded to the return of tone. In this case relay B is not re-operated and fault is indicated by lamp 17 at " B." Finally, the carrier signals on leads 4, 5 are monitored over rectifiers 24, 25 at. each station and transmitter switch over is effected directly on the failure of either carrier by relays A<SP>1</SP>, B<SP>1</SP>.