Patent GB-554911-A (GB)
Title: Two-way signal transmission system
IPC: H04B3/40
Filed: 1942-05-14
Abstract: 554,911. Measuring resistance. STANDARD Co., Inc.). May 14, 1942, No. 6548. [Class 37] Co., Inc.). May 14, 1942, No. 6548. [Class 37] [Also in Group XXXIX] In a system in which a two-wire line is coupled to a four-wire line with the aid of a hybrid coil or its equivalent and a balancing network, means are provided for adjusting a single network automatically until it balances the two-wire line as well as possible or for selecting automatically the most suitable of a number of networks in accordance with tests performed with speech currents arriving over the four-wire line. Systems in which a network is adjusted.-In one arrangement, Fig. 1, unbalance currents received from the far end of the four-wire line and finding their way into the transmitting branch are fed in turn to the heaters of equal thermistors forming arms of a Wheatstone bridge by a constantly operating change-over switch 7. A synchronously operated switch 2 short-circuiting a resistance 1 in series with the balancing resistance N when the right hand thermistor is connected up. If the balancing resistance is too small the balance will be better when the lefthand thermistor is connected up and so the righthand thermistor will receive more heat. The resulting unbalance of the bridge causes relay 13 to close one of its contacts to cause motor 15 to increase the balancing resistance. Similarly if the resistance is too large, the motor is driven to reduce it. Currents from the two-wire line affect both thermistors equally and are therefore without effect. In another arrangement, Fig. 2, the voltage produced by currents from the far end of the four-wire line on the line and network sides of the hybrid coil are compared by a relay MR which controls a pair of clutches to cause a constantly running motor M to increase or decrease the balancing resistance. Speech from the two-wire line operates DR to prevent any adjustment. DR is unaffected by speech from the fourwire line owing to the effect of a shunting resistance R1. System in which a network is selected, Fig. 3. -When the two-wire line TL is plugged-in, relay 100 energizes relay 101, to connect TL in one arm of a bridge circuit the adjacent arm of which is formed by a network N1, relay 102, to disconnect the network side of the hybrid coil, and relay 103, to apply the rectified unbalance voltage obtained from the bridge to a condenser 110, in the grid circuit of a tube 117. A circuit is also made for 106 which responds sluggishly and interacts with 111 until the selecting operation is complete. On the first operation of 106, 107 pulls up to release 103 and connect N2 to the bridge m place of N1. The unbalance voltage due to the new testing operation is thus applied to condenser 112 in the grid circuit of a tube 116 and since the operation of 107 also energized 119 to connect the respective windings of 118 to the anodes of the tubes, 118 now operates or fails to operate according as the balance afforded by N2 is better or worse than that afforded by N1. If 118 operates, it energizes 104 which is followed by 105 whereupon both relays fall back in turn. On the operation of 105, both condensers are discharged and 113 is operated to make a tentative selection of network N2 and locks up. On the relapse of 105 condenser 110 is charged in accordance with the unbalance voltage with N2 connected up and if or when next 106 is released, 108 is operated to connect N3 to the testing circuit instead of N2 and release 107. The unbalance voltage representing the unbalance of N3 is then applied to condenser 112 and 119 is re-operated to complete the comparing circuit. Assuming N2 is better than N3, 118 is not operated, 104, 105 remain de-energized and 110 continues to hold the charge representative of the degree of unbalance of N2. 112 however is discharged when 109 pulls up on the next energization of 106 to connect N4 to the bridge. If as a result of the new test N4 proves to be the best network 115 pulls up and locks on the operation of 105 and on the operation of 126, which takes place on the next relapse of 106, relays 101, 102 are released and the talking condition is established. If desired, a relay 127 may be provided which normally operates in series with the lower winding of 118. If, owing to good balance, the current in this winding is insufficient to operate 127, the relay 126 is operated at once to stop the testing operation. In a modification of this arrangement the hybrid coil itself is used in place of the bridge.