Patent GB-535286-A (GB)
Title: Control of transmission in two-way telephone systems
IPC: H04B3/20
Filed: 1940-03-12
Abstract: 535,286. Two-way transmission. STANDARD TELEPHONES & CABLES, Ltd. (Western Electric Co., Inc.). March 12, 1940, No. 4603. [Class 40 (iv]) In an amplifying system, e.g. for giving loudspeaking telephone communication between two or more conference rooms, the receiving amplifier gives volume-range expansion and is associated with means for disabling the transmitting path. In a symmetrical arrangement, Fig. 1, speech currents from a microphone MA after amplification at A1 are further amplified in a volume-limiting amplifier VL1 having the characteristic shown and pass over the upper pair LE of a four-wire line to a volume-range expander RE1 having the characteristic shown, a portion being tapped off at the input side for the control of the expander and of an echosuppressor ES1 which disables the other path at 1 and also reduces the gain of the volumelimiting amplifier VL2 in that path. Both echo-suppressors may be at the same station, one disabling the other. In a modification, (Fig. 2, not shown), the transmitting path at station A includes an amplifier which combines the volume-limiting and range-expanding functions of VL1 and RE1. Currents at the input side of this amplifier control an echosuppressor which disables the other path, the other echo-suppressor and the control path for the range-expanding receiving amplifier. In a further symmetrical arrangement, Fig. 3, in which a two-wire transmission line is used, the transmitting paths are normally disabled at 21, 22 and both paths at each station include echo-suppressors each of which disables the other and each of which reduces or keeps low the gain of the amplifier VL or RE of the other path. Volume-limiting amplifier, Fig. 4. Normal signals arriving at terminals 46, 47 are attenuated slightly by a network 33 comprising series rectifiers, through which pass the anode current of a control pentode 45, and shunt resistances on their way to the input circuit of a pentode amplifier 34 the output circuit of which is coupled to the line by a hybrid coil 42. When the output signals reach a pre-determined level a portion taken from the left-hand side of the hybrid breaks down a neon tube 44 and biasses the suppressor grid of the pentode 45 so that its anode current is reduced and the attenuation of network 33 is increased. The condenser 54 ensures that the limiting action is not changed between syllables. The loss of network 33 is also increased when the leads 56, 57 are connected together by the echo-suppressor (not shown). Combined volume-range expanding and limiting amplifier, Fig. 5 The limiting effect is produced in the same way as in the amplifier shown in Fig. 4. Volume-range expansion is given by varying the.bias on the control valve 63 by rectified signals selected by an adjustable current 67, 73, 74, 75 broadly tuned to 900 cycles. The expansion effect is adjustable by means of variable resistances PI, P2 ganged to a switch S which in its extreme position short circuits the biassing arrangement. The expansion effect extends only over a.limited range determined by a 1 megohm resistance 69 and a rectifier 70 which limit the maximum positive bias applicable to the control grid of tube 63. The rectifier also delays the response of the device to sudden high amplitude signals. A further portion of the input signals is amplified at 78 and, provided the echo-suppressor of the other path is not operative, rectified at 83 to operate relay 86 which disables the other echosuppressor circuit and energizes 124, 91 which disable the receiving path at 10 and put the expander in the other path into its low gain condition to delay its response when the echosuppressing relay 124 is released. The above arrangement is for the transmitting path at station A of the unsymmetrical system of Fig. 2 (not shown). With the omission of the limiter circuit it is suitable for the receiving path expander RE3 and for the expanders RE1, RE2 of Fig. 3. With the further omission of the device for shunting condenser 58 under control of the echo-suppressor of the other path and of the suppressor-disabling circuits 84, 85 and 88, 89, it serves for the expanders RE1, RE2 of Fig. 1. Volume-limiting amplifier with anti-singing as well as echo-suppressing arrangements, Fig. 6. The volume-limiting arrangements resemble those shown in Fig. 4. A portion of the input to terminals 107, 108 is diverted via amplifiers 102, 103 to a rectifier 104, provided the echosuppressor of the other path is not operative, and operates 106 which disables the other echo-suppressor, energizes 116 to remove the anti-singing shunt at 109 and energizes 117 to disable the other transmission path.