Patent GB-255221-A (GB)
Title: Volume control arrangements for signalling systems
IPC: H04B1/64
Filed: 1925-07-06
Abstract: 255,221. Western Electric Co., Ltd., (Western Electric Co., Inc.). July 6, 1925. Addition to 246,274. Systems for transmitting music; repeating and amplifying systems.- The method described in the parent Specification of distorting signals with the object of ensuring a substantially - constant amplitude for all components during transmission, and thus preserving a favourable ratio of signal-strength to line-noise, or to atmospherics in the case of radiated signals, is carried out by various automatic means. As shown in Fig. 1 a shunt circuit 20 diverts some of the signal currents into an amplifier-detector D so that rectified currents flow in the solenoid coil 17 of a strength proportional to the initial intensity of the signals. These, in turn, automatically regulate the setting of the input potentiometer 11, so that the output currents from the amplifier 10 are varied in reverse sense to the input. A delay network or filter F meanwhile retards the passage of the main signal currents in order to enable the correct setting of the potentiometer to take place before they reach the amplifier 10. For restoring the signal currents to their original amplitude-ratio at the distant end of the transmission line, an auxiliary circuit L<1> is fed with current from a generator G. The strength of the rectified output from a detector D<1> is controlled by the setting of a resistance 16, operated simultaneously but in the reverse sense to potentiometer resistance 11. The current so regulated controls, in turn, the setting of the input potentiometer 13 to the terminal tube 12, and thus restores the signal components to their original form. The nature of the transformation caused by the arrangement of Fig. 1 is illustrated in Fig. 7, where normal sound-volumes represented by the portion N P Q are dropped in transmission to the values represented by the curve N<1> p<1> Q<1>, whilst the lower range S, T, U is similarly raised to S<1> T<1> U<1>. In the system shown in Fig. 2, a direct distortion or change of wave-form is imparted to the input energy which results in the transmitted signal components being converted into a smaller or compressed range of amplitudes, this being shown in the curve of Fig. 8, as compared with the curve M N P Q R S T U of Fig. 7. Here the input signals are applied across a pair of parallel two-electrode tubes 72, 73 and a, high resistance 71. Owing to the complex impedance of the system, the ratio between the input voltage e and the output current i is as shown by the curve of Fig. 3. Although thus distorted, the transmitted currents retain all the details of the impressed signals, which are finally restored to their original form, at the distant end, by being passed through a similar combination of valves 76, 77 and a resistance. 78, the latter in this case having a value low in comparison with the effective resistance of the valves. The relation between the input voltage e and the output current e of the restoring combination is shown in Fig. 4. The same level-shifting effect may also be secured by using two three-electrode valves mounted in push-pull relation, and having a heavy grid bias and a low resistance shunting the output circuit. Or a push-rail arrangement of three electrode valves may be used as the variable impedance arm of a wheatstone bridge, comprising a hybrid coil and a constant balancing impedance, the losses in the hybrid coil then being high for lowlevel inputs and vice versa. A similar restoring combination at the distant end is arranged so that the valves are grid-biased in opposite polarity to those at the input end. For this purpose a rectified control current, proportional to the original signals, is carried either by a separate pilot channel or is superposed on the same transmission line at a frequency outside the normal voice range.