Patent GB-1078945-A (GB)
Title: Improvements in or relating to communication systems for the transmission of signals through a transmission cable
IPC: H04B3/10
Filed: 1965-02-09
Abstract: 1,078,945. Automatic volume control systems; magnetic amplifiers. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. Feb. 9, 1965 [Feb. 12, 1964], No. 5532/65. Headings H3B and H4R. In a communication system including one or more repeater stations and a final station which have automatic level controlling circuits, the level controlling circuits of the repeater stations are adjusted by means of a level controlling signal derived from a pilot signal received at the final station and the control is such that the level at the controlled repeaters is caused to deviate from the nominal value in the direction opposite to the deviation detected at the final station. The level control circuit at the final station is arranged to bring the pilot signal substantially to the nominal value. Fig. 3 shows a preferred embodiment of a communication system employing repeater amplifiers 3, 4, 5 ... 27 on a line between stations 2 and 28. The gains of amplifiers 3, 7, 11 ... 23 are controlled in common by the current through a line 36 which, via current converters 41 varies the amplifier feedback resistors 35. The gains are controlled in accordance with the level of the pilot signal received at station 28 via a selective amplified 37, rectifier 38, comparator 39, D.C. amplifier 40, and maximum/minimum relay switch 43 which controls the operation of a motor 42. Motor 42 adjusts a capacitive divider 46 on the output of oscillator 45 to adjust the A.C. signal applied to the rectifier 47 and hence the D.C. signal fed via amplifier 48 and D.C. converters 41 to the gain controlling circuits of the repeater amplifiers. The output of amplifier 40 is also fed to a gain controlling circuit of the amplifier in the final station to provide the final adjustment of signal level to the nominal value. Fig. 5 shows in detail the D.C. converter 41 which amplifies the D.C. control signal in line 36 to the level required to operate the level controlling resistance, e.g. thermistor or incandescent lamp, 35. The converter comprises a magnetic amplifier in which the control current on line 36 flowing in the coils 64, 65, 66, and 67 saturates the cores associated with these windings, reducing the impedance presented by coils 62 and 63 in the circuit between oscillator 60 and rectifier 74. The current from the oscillator 60 flowing in the coils 62 and 63 tends, on the peaks of the waveform, to drive the core associated with one or other of these coils out of saturation and thus to increase the impedance of the respective coil. The circuit therefore limits the waveform fed to the rectifier 74 at positive and negative values dependent on the value of the current flowing in the line 36. Line 36 couples each core twice, i.e. on both outward and return paths, so that longitudinal currents induced in the line, e.g. due to the proximity of electric traction equipment or due to lightning, flows in opposite directions in the coils associated with each core and so has no effect on the control circuit. Inductors 68 ... 71 are provided to decouple the line 36 from h.f. currents induced in the line at the repeater, and the oscillator 60 and repeater amplifier 58 are powered in conventional manner from D.C. in the centre conductor of the coaxial cable 1.