Patent GB-879836-A (GB)
Title: Improvements in or relating to loudspeaking telephones
Inventor: CLARKE JOHN PHILIP
IPC: H04M1/60
Filed: 1958-10-28
Abstract: 879,836. Two-way Transmission. GENERAL ELECTRIC CO. Ltd., and CLARKE, J. P. Oct. 28, 1959. [Oct. 28, 1958], No. 34487/58. Class 40 (4) When speech incoming over a receive amplifier to a loud-speaker is a predetermined amount in excess of speech outgoing from a microphone over a transmit amplifier a control circuit closes a gate in the transmit path and when outgoing speech is a predetermined amount in excess of incoming speech the control circuit closes a gate in the receive path, the control circuit opening the gate in one path only after the gate in the other path has been closed. The amplifiers are preferably transistor amplifiers. The transmit amplifier shown in Fig. 1 receives signals from a microphone on terminals 1 and 2 and between stages 6 and 7 and coupling transformers 9 and 10 has a gate circuit 11 with shunt rectifiers 20, 21, the impedance of which is switched by control voltage on terminals 22, 23. Winding 13 on transformer 9 monitors transmitted speech and over terminals 15, 16 drives transistor 85 in the control circuit of Fig. 3, the output of 85 being rectified to supply a positive potential over resistor 93. The output of the transmit amplifier is taken from terminals 3, 4 of transformer 26 and is applied to the hybrid 42 shown on the left-hand side of Fig. 2 which illustrates the receive amplifier. Speech incoming from line is applied to terminals 33, 34, of the hybrid and is taken off over resistors 45, 46, a switch 47 cutting 45 out whenever it is necessary to cope with low-level signals. Between stages 37 and 38 and coupling#transformers 48 and 49 a gate 50 is provided with shunt rectifiers 59 and 60 the impedance of which is controlled by voltage on terminals 62 and 22. Output is taken from a push-pull stage to a loud-speaker on terminals 35 and 36. A winding 52 on transformer 48 monitors incoming speech and over terminals 54, 55 drives transistor 75 in the control circuit of Fig. 3, the output of 75 being rectified to supply a negative potential over resistor 83. In the control circuit the potentials over resistors 83 and 93 indicate speech in each channel and are combined differentially to control a normally cut-off transistor 95. Control potentials supplied to gates 11 and 50 over terminals 22, 23 and 62 are such that terminal 23 is held at - 3.4 volts and terminal 62 is held at - 3 volts, while terminal 22 common to both gates is normally at about - 6 volts but may be driven to earth potential when transistor 95 is made to conduct. Transistor 95 is normally cut-off so that gate 11 in the transmit path is held open but gate 50 in the receive path is held closed. So long as transmitted speech provides a dominant positive voltage, transistor 95 will remain cut-off. As soon as received speech provides a negative voltage to the base of transistor 95 the potential of terminal 22 falls towards earth, shutting gate 11 in the transmit path when it reaches - 3.4 volts, and opening gate 50 in the receive path when it passes - 3 volts. It is difficult for incoming speech to break in on outgoing speech as gate 50 short-circuits the transformer 48 when the gate is closed and coupling to the monitoring winding 52 is reduced. This also protects the circuit against retroaction should the hybrid be unbalanced. Break in by transmitted speech is made easier by placing resistors 18 and 19 between rectifiers 20 and 21 and the transformer 9 which is, therefore, never wholly short-circuited. Variable resistors 32, in the transmit amplifier, and 74, in the receive amplifier, provide volume control. A capacitor 65 in the receive amplifier is switched in or out to give a two-tone control.