Patent GB-624923-A (GB)
Title: Improvements in or relating to signal transmission systems
IPC: H04B3/06
Filed: 1947-06-20
Abstract: 624,923. Automatic volume control. WESTERN ELECTRIC CO., Inc. June 20, 1947, No. 16342. Convention date, Dec. 5, 1942. [Class 40 (iv)] In a transmission system of the type in which the average total power transmitted is maintained constant by repeaters irrespective of variations in the signal power, different kinds of regulators are employed which differ in the rate at which they respond to corresponding changes in the control effect applied to them, and these kinds of regulators are spaced so that the effects of transients are reduced. Fig. 1 is a schematic diagram of a one-way multiplex system. Terminal circuits 1 translate the signals of the channels which together with pilot waves f<SP>1</SP>, f<SP>2</SP>  and f<SP>3</SP> are fed via amplifier 2 to line. The amplifier, which supplies a pilot wave f<SP>4</SP> varying automatically in intensity. so as to maintain the total power output constant, may be of the type disclosed in Specification 537,893. Along the line 3 are spaced repeaters of two types 4 and 5. Each repeater 4 is of the constant total output type and may be of the form shown in Specification 506,791, or as shown in Fig. 2. In either case it is a stabilized negative feedback amplifier having in the feedback path a current-dependent thermistor 7 which controls the gain. The Fig. 2 repeater has in the input circuit an impedance network to provide fixed equalisation and a feedback coupling impedance 11. The output stage uses an output transformer arranged as in Specification 505,492, [Group XL], to afford a negative feedback connection 14. The rate of response of the repeater to signal level changes depends on thermal time constants of thermistor 7 so that if sudden changes in level, such as one caused by ringing current on one or more channels, occur transients may be developed. Thermistors 7 are sluggish in action, having time constants of the order of 45 seconds for heating and 130 seconds for cooling, and such transients are therefore limited in rate of change, if not in amplitude. The repeaters 5 (of the type shown in Fig. 4 of Specification 537,893) are regulated by pilot waves, are fewer in number than repeaters 4, and are spaced approximately 150 miles apart on cables, or 100 miles on overhead lines, so as to divide the line into sections having repeaters 4 only. Each repeater 5, Fig. 3, comprises a negative feedback amplifier whose gain is controlled by variable networks 20, 21, 22, in the feedback circuit under the control of thermistors 23, 24, 25 heated by pilot currents f<SP>1</SP> f<SP>2</SP>, f<SP>3</SP> respectively. Network 20 controls flat gain and as thermistor 23 responds to changes in f<SP>1</SP> level much faster than does 7 to changes in total level, having a time constant of about 0.2 seconds, it substantially suppresses transients developed in the preceding section of the line. Networks 21 and 22 control changes in the slope and curvature respectively of the gain-frequency characteristic, thermistors 24 and 25 being comparatively sluggish (60 secs.) to prevent interaction between them and thermistor 23.