Patent GB-1073401-A (GB)
Title: Improvements in or relating to telephone apparatus
Inventor: LAX LADISLAV
IPC: H04M9/10
Filed: 1963-04-11
Abstract: 1,073,401. Loud-speaking telephones, gating circuits. PYE Ltd. July 2, 1964 (April 11, 1963], No. 14489/63. Headings H3T and H4R. A loud-speaking telephone comprises a microphone 1, Fig. 1, with its associated transmission channel including amplifiers 2 and 3 and a variable attenuator 10, and a loud-speaker, with its associated receive channel including amplifiers 18 and 19 and a variable attenuator 12 in which the attenuation of the attenuators is differentially varied by a switching circuit 11, 13, which is responsive to signals derived from both the receive and transmit paths and in which the switching device is more sensitive to signals from one path than it is to signals from the other path. The control signal channel from the microphone to the switching circuit includes a muting circuit 4 which, when there is a signal in the receive path, is controlled by the received signal via amplifiers 9 and 5 to reduce the effect of signals from the microphone on the switching circuit 11, 13. Control signal muting circuit 4, Fig. 1 and Fig. 2 comprises a bridge circuit R1-R4 and C1-C4, in which the control signal from the microphone to the switching circuit passes from terminal A to terminal B of the bridge while a signal from the loud-speaker channel is applied to terminal C. Signals on terminal C are rectified in diodes D3 and D4 and switch on transistor TR1 which causes diodes D1 and D2 to conduct and shunt to earth the signal path from terminal A to terminal B. When no signal is applied to terminal C transistor TR1 is cut off, diodes D1 and D2 are cut off and the signal passes virtually unattenuated from terminal A to terminal B. Switching circuit, Fig. 3.-Signals from the microphone amplifier 2, Fig. 1, are applied to winding Y of transformer T2, Fig. 3, of the variable attenuator 10. The transformer output is taken from winding Z and the transmission factor depends on the loading imposed by the third winding and hence on the degree of conduction of diodes D11 and D12. Similarly the received signal for the loud-speaker passes through transformer T1 via windings W and X and the transmission factor will depend on the degree of conduction of diodes D9 and D10. The degree of conduction of the diodes in the transmit and receive paths is inversely varied by the emitter coupled long-tailed pair of transistors TR4 and TR5. Signals on the base of transistor TR4 are derived from signals in the transmit path applied to terminal D which tend to make the voltage on capacitor C11 go more positive while signals in the receive path applied to terminal E tend to make the voltage on C11 more negative. Thus if signals in. the transmit path exceed those in the receive path transistors TR3 and TR4 tend to be cut off, making the potential at points P and Q more negative than the potential at point R and thus tending to increase the attenuation in transformer T1 in the receive path and reduce the attenuation in transformer T2 in the transmit path. Similarly signals in the receive path exceeding those in the transmit path cause the reverse condition making transistor TR4 conduct, lowering the attenuation in TI and increasing the attenuation in T2. In addition, signals in the transmit path are applied to terminal F and signals in the receive path to terminal G and these signals act in a similar fashion to those applied to terminals D and E to alter the conduction of the diodes in the attenuator circuits via transistors TR6 and TR5. In the circuit 15 which applies the transmit path signal to the control circuit a variable resistor R28 is provided which may be adjusted to vary the relative sensitivity of the control circuit to the signals from the two paths.