GB1444828A - - Google Patents

Abstract

1444828 Loudspeaking telephones NORTHERN ELECTRIC CO Ltd 12 March 1974 [12 March 1973] 11001/74 Heading H4R In a loudspeaking telephone, Fig. 1 incorporating switching from transmit to receive, either manually or automatically in response to signal level, the gain in the transmit channel is controlled inversely in dependence on the ambient noise level picked up by the microphone so that the signal transmitted to line is more nearly a constant level. As described a transmit channel 10 and receive channel 20 are coupled to a line 2 by a hybrid network 3. The transmit and receive channel signals at hybrid 3 are rectified at 16 and 24 and compared in comparator 60 to provide an output signal which is high when the receive signal is greater than the transmit signal, and low when the receive signal is less than or equal to the transmit signal. In addition the signal in the transmit channel 10 at the input to the variable attenuator 13 is rectified at 14 and processed at 30 to provide a noise reference signal which is compared with the rectified transmit channel signal in comparator 40 to provide a transmit channel active signal in line 71. The outputs of comparators 40 and 60 are fed to the reference signal source 70 to provide a control signal for varying the loss in the attenuators 13 and 21 in the transmit and receive channels, inversely to main the loop gain approximately constant. The reference signal source 70 provides a control signal for the attenuators which has an upper limit which gives maximum receive channel gain and minimum transmit channel gain when the receive signal exceeds the transmit signal has an intermediate value in an idle position, and a value tending to a lower limit when the transmit channel signal exceeds the receive signal, the actual value of the control signal in the idle or transmit conditions being variable in dependence on the value of the noise reference signal current fed to the switch 45 from the circuit 30. The output of the transmit/receive comparator 60 is fed to a receive transition booster 62 to increase the signal level from rectifier 24 immediately after switching to receive so as to increase the sensitivity to weak received signals. The output of comparator 60 is also coupled via the transition delay circuit 67 to augment the noise reference signal fed to comparator 40 when the circuit is in the receive condition, and for a short time, e.g. 0À4 secs., after, so as to reduce the sensitivity to voice signals and prevent acoustic coupling from the receive to transmit channels effecting switching to transmit. The noise averaging circuit 30 which has a slow attack and fast release times is described with respect to Fig. 2 (not shown). The reference signal source 70 which provides different switching time constands depending on whether the switching is taking place to or from an idle condition or between active states is described with respect to Fig. 3 (not shown). The transition booster 62 and transition decay circuits are described relative Figs. 4 and 5 (not shown) and the signal rectifiers 14, 16 and 24 with respect to Fig. 6 (not shown).