Patent GB-456116-A (GB)
Title: Electric signal transmission systems
IPC: H04B3/10
Filed: 1936-04-17
Abstract: 456,116. Two-way transmission. STANDARD TELEPHONES & CABLES, Ltd., 63, Aldwych, London.-(Assignees of Abraham, L. G., and Grenell, A. F.) April 17, 1936, No. 11033. Convention date, May 16, 1935. [Class 40 (iv)] In a two - way system, control currents are sent over the line periodically and when an operator plugs in from each terminal station to the other to adjust the gain in the receiving path at each station to correct for variations of line attenuation. As shown in Fig. 1, a control current from terminal station 12 adjusts potentiometers 20, 25 at an intermediate station 13 and terminal station 11 in accordance with the peak values of the current. Upon reception of the control current at terminal station 11, a return control current is sent to adjust potentiometers 32, 37 at stations 13, 12 in a similar manner. The control current sent when an operator plugs in is of different duration from the periodic control current and gives a calling signal at the distant terminal station in addition to adjusting the gain. The operations at a terminal station are described in detail below with reference to Figs. 2, 3. Transmission of control current, Figs. 2, 3. A shaft 49 carrying cams 1-5 is driven through one revolution by a motor 43 through a clutch 57, 54 which is engaged upon momentary operation of a magnet 48 and is maintained by a cam 58 on the shaft 49. Magnet 48 is controlled by relay B which, shortly after it energizes, is short-circuited by series relay C. Relays B, C are connected up by sleeve relay A when an operator plugs into jack 44 or by relay V which energizes every 30 minutes over cams 8, 9 continuouslv driven from the motor 43. Control current of frequency 800 is sent during the operative periods of relays D, E, the former being controlled over cam 1 with V back or cam 2 with V up and the latter over cam 3, so that the duration of the service control current is 0À5 sec. and that of the periodic control current is 0À7 sec. The operation of relays H, K, L, J removes volume control apparatus 63, 64 from the receiving and transmitting paths. Reception of control current. A starting circuit 41 effects the operation of relay MA only when pure control current is received. The anode circuit of the amplifier 74 includes two transformers 79, 82 shunted respectively by a rejector circuit 75 and an acceptor circuit 76, the outputs being rectified and applied in opposite senses to relay MA which is polarized. The potentiometer 37 is included in the receiving path between the hybrid transformer 38 and the amplifier 95, and a resistance 102 is adjusted so that this portion of the receiving path produces zero gain when the potentiometer is in the central position. A measuring circuit 42 fed from the receiving path includes two-impulse transformers 106, 109 having high permeability cores which are magnetized by biassing currents, so that they pass only peaks of control current which are sufficient to neutralize the biassing flux. The resulting pulses are fed through a rectifier 110 to marginal relays LO, RA which control the potentiometer 37, which operates to increase or reduce the gain until a condition is obtained in which one of relays LO, RA is operated and the other is released. The arm 101 is rotated by a shaft 122 which is driven one way or the other from a motor 123 through clutches 129, 130 controlled by magnets 127, 128. These magnets are under the control of LO, RA for a period of 0À4 sec. while relay P, which is connected up by MA, remains energized. At this time relays H, K, L, J are operative to cut out volume control apparatus 63, 64. Relay MA also brings about the operation of relay T which causes a shaft 151 carrying cams 6, 7 to be driven through one revolution in the same way as the shaft 49, and after a short interval, relays B, C are connected up over cam 6 to send a return control current of 0À5 sec. duration (V being back) in the manner already described. In the case of an incoming service control current of 0À5 sec. duration, relays MA, T are back when cam 7 closes its contact and relay U energizes over cam 4 and lights lamp 154. This operation is prevented in the case of a control current of 0À7 sec. duration since T remains up until after cam 7 has closed its contact. The receipt of a return control current effects only gain control, since relay C is held by relay A or V, thus preventing operation of the sending cams 1-5.