GB1261521A - Improvements in or relating to telecommunication systems - Google Patents

Abstract

1,261,521. Substation amplifier circuits. POST OFFICE. 5 Feb., 1969 [5 Feb., 1968], No. 5750/68. Headings H4K and H4R. Telephone substation has a receive amplifier 6, Fig. 1, to feed incoming signals to a telephone receiver 7, the gain of which is controlled by the output from an auxiliary microphone 8 arranged to pick up the ambient noise in the vicinity of the substation, the receive amplifier gain being increased as the noise level increases. As shown, send-receive switching is employed controlled by the speech input to microphone 2 and the threshold level at which the station switches from receive to send is also controlled by the output of the auxiliary microphone 8, to increase the threshold level with increase in ambient noise. The auxiliary microphone 8 is arranged to respond mainly to the noise rather than the speech signals while the signal from the normal microphone 2 is filtered at 14 to make the speech detector 15, which operates the send receive gates 4 and 16, more responsive to speech than ambient noise. Detailed circuit of the apparatus is shown in Fig. 2 in which speech signals outgoing from microphone 2, are amplified by transistors TR1 and TR2 and fed via the send gate comprising series resistors R43 and R45 and shunt variable impedances formed by transistors TR13 and TR14, to the send amplifier TR16 which feeds the line. Incoming signals from the line feed via resistor R48 and the receive amplifier TR7 and TR8 to the receiver 7, the receive gate being formed by the transistor TR15 which can be switched to low impedance to shunt the receiver 7. Signals picked up by the auxiliary microphone 8 are amplified by transistors TR3 and TR4 and rectified by transistors TR4 and TR5 to control the impedances of the groups of silicon diodes D1 to D4 and D5 to D8. The group of diodes D1 to D4 forms a shunt impedance on the negative feedback path in the receive amplifier to increase the receive gain as the noise level increases, while the diodes D5 to D8 forms a shunt impedance in an attenuator including the resistors R40 and R42 which feeds the microphone 2 amplifier output to the speech detector circuit including transistors TR9, 10, 11 and 12. With no signal applied to TR9 in the speech detector transistors TR10, 11 and 12 are non-conducting, making transistor TR15 of the receive gate non-conducting so that received signals are fed to the receiver 7. Transistors TR13 and 14 are however made conducting so as to prevent signals being fed to the outgoing amplifier TR16 via resistors R43 and 45. With speech signals at sufficient level applied to transistor TR9 of the detector, transistors TR10, 11 and 12 saturate to reverse the conditions of the send and receive gates to inhibit reception and allow speech transmission.