Patent GB-430158-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04B3/20
Filed: 1933-11-14
Abstract: 430,158. Two-way transmission. RYALL, L. E., 52, Crescent Gardens, Eastcote, Ruislip, Middlesex. Nov. 14, 1933, No. 31753. [Class 40 (iv)] A device for automatically controlling the condition of signal transmission circuits in accordance with the requirements of signals, e.g. for controlling the operative direction of a two-way substation or line repeater or for determining which of two signal paths transmitting in the same direction shall be effectively associated with a third, has a first or main input derived from the operative path a second or break-in input derived from the input side of the inoperative path and a third or anti-echo input which renders the signals in the break-in input ineffective unless they are of a certain strength and so counteracts the tendency of echo &c., signals to break-in. The effect of any given signal in the anti-echo input persists only long enough for the corresponding echo signal to arrive in the break-in input and so does not prevent even a weak break-in signal from reversing the direction of the repeater when speech in the operative path has ceased. The arrangement shown in Fig. 2 (Comp.) is a twoway terminal repeater in a loudspeaking telephone system. Both transmission paths are normally blocked by networks 8, 11 of the kind described in Specification 413,383 which have a low or high attenuation in accordance with the strength and direction of a biassing current or voltage and which pass from one condition to the other somewhat abruptly. These networks together with similar networks 21, 16 included in the control paths are associated as shown in Fig. 4 (Comp.) with the anode circuits of control valves 7, 10 which as shown function also as amplifiers in the respective transmission paths. Biassing resistances 37, 38, 39 are provided and the whole arrangement is such that under normal conditions there is no potential difference between the points 34, 35 and networks 8, 11 are in the high-impedance and networks 16, 21 in the low-impedance state but that when, owing to the application of a bias-reducing voltage to the grid of valve 10, the voltage of point 35 is reduced the change in current through the networks maintains the normal condition of 21, 11 but causes 16 to assume the highimpedance and 8 the low-impedance condition. The biassing arrangements are such that these changes take place in succession, an inductance 36 ensuring that the control voltage builds up gradually. The converse of these changes takes place when a control voltage is applied to the grid of valve 7. The effective control voltage is the resultant of three factors, two of which depend on the signals in the operative path and the third on signals reaching the inoperative path. Speech currents from the microphone circuit 40 after amplification by valve 7 pass through network 21 to voltage-doubler rectifier 22 and so produce a voltage across resistance 44 and condensers 42 in opposition to the normal bias on the valve 10. As a result, network 8 in the transmitting path assumes the low attenuation state and network 16 in the main control path from the receiving side assumes the high attenuation state the latter change taking place first. Echo and unbalance currents arriving in the receiving path tend to produce, with the aid of a rectifier 45, a voltage such as to discharge condensers 42. This action is opposed however by a voltage produced by a voltage-doubler rectifier 46 across condensers 47 and resistance 48 due to signals derived from the transmission path over circuit 20. On the cessation of speech in the transmission path the circuit returns to normal after a hang-over time which is determined by the dimensions of condensers 42 and resistance 44 and which is long enough to bridge the gaps between words. The discharge time of condensers 47 through resistance 48 is only just longer than the time required by a particular signal to produce its echo effect. Consequently if speech currents arrive from the distant party at the end of a word by the local party they are able to produce a voltage for discharging condensers 42 to start the reversal of the condition of the repeater immediately after the echo due to that word has produced its effect and before the end of the normal hang-over period even if they are of low amplitude. Highamplitude received signals reverse the condition of the repeater on the occurrence of a sufficiently weak syllable in the transmitted speech. The arrangement is substantially symmetrical, controls being applied to valve 7 when speech is being received similar to those applied to valve 10 when it is being transmitted. Fig. 3 (Comp), (not shown), illustrates the application of identical control arrangements to a two-wire intermediate repeater. In this case however the valves 7 and 10 are not in the main transmission paths but amplify the control currents only. Arrangements described in the Provisional Specification Speech currents from circuit 41, 42 after amplification at 43 pass the normally low - attenuation net. work 44 and after rectification at 64 adjust the bias of valve 51 so as to cause a potential difference between the points 57, 58. The resulting current flow further reduces the attenuation of 44 and reduces the attenuation of the normally high-attenuation network 46 so that the speech currents can pass to line 47, 48. At the same time the network 50 is made to assume the high-attenuation state. At the cessation of speech, normal conditions are restored after a time determined by the dimensions of condenser 91 and the backward resistance of rectifier 64. The arrival of speech from line 47, 48 in a similar manner reduces the attenuation of network 50 and increases the attenuation of 44, 46, thus increasing the normal disablement of the transmitting path and improving the transmission efficiency of the normally operative receiving path. Normal conditions are restored on the cessation of speech after a time fixed by the dimensions of condenser 90 and the backward resistance of rectifier 66. If during the reception of speech, speech occurs in the transmitting path 41, 42 the flow of grid current in valve 43 tends to discharge condenser 90 so that on the occurrence of a pause or weak syllable in the received speech the local party is able to reverse the condition of the repeater without waiting for the normal hang-over period. Echo signals due to acoustic coupling between the loud-speaker and the microphone are prevented from producing this effect by a suitable choice of the ratios of transformers 65, 92. The arrangement shown does not give the distant party the corresponding power to break in because the control network 50 is on the input side of the amplifier 51. The network could however be arranged in the output circuit of 51. In the modified system shown in part in Fig. 2 (Prov.) when signals are being received a part of the signals is applied through transformer 65 to a voltage-doubler rectifier to charge condenser 90 and condenser 90 tends to be discharged by a local signal not only owing to the flow of grid current in 43 but also owing to a voltage produced by rectifying a portion of the local signal at 97. Resistance 98 prevents wrong operation by echo signals. In a further modification not shown in the drawings a condenser shunted by a resistance is inserted in series with rectifier 97 and resistance 98 and part of the main (received) signal is rectified and applied to this condenser, the voltage produced is such as to prevent rectification of echo signals by rectifier 97. The discharge time of the additional condenser is only slightly greater than the echo time.