Patent GB-633428-A (GB)
Title: Improvements in or relating to automatic level control arrangements in carrier current communication systems
IPC: H04B3/10
Filed: 1947-02-26
Abstract: 633,428. Automatic volume control. STANDARD TELEPHONES & CABLES, Ltd., JACOBSEN, B. B., ROCHE, A. H., and BALY, W. F. Feb. 26, 1947, No. 5590. [Class 40 (iv)] In a communication system comprising a plurality of multichannel carrier current circuits in which automatic adjustment of amplifier gain and equalization is effected by pilot currents, the pilot currents A and B transmitted over one of the circuits are utilized for the control of a plurality of said circuits and when transmitted over more than one circuit, e.g. 40, 41 and 42, Fig. 2, suitable coupling permits continuous control of all the circuits in the event of the failure of pilot currents in one circuit. The level of pilot A, generated in source 73, Fig. 5, of the type described in Specification 536,454, [Group XL], is maintained constant on the conductors 82 by the action of control oscillators 90 and 91 on thermistors 76 and 77 in the circuit of amplifiers 74 and 75, the control oscillators being themselves controlled by the pilot current on conductors 82 via band-pass filters 83 and 84, amplifiers 85 and 86, and rectifiers 87 and 88, according to which develops the highest voltage; a fault on any part of the pilot circuit or the pilot control circuit causes the control to be taken over by the alternative path. The regulated pilot currents are connected by lines 16, Fig. 1, to points between the transmitting circuits and the transmitting amplifier at station 1, and at the repeater station 4 a proportion of the pilot current A taken from the output side of repeaters 17, 18 or 19 via conductors 23 operates the gain control device 22, which supplies alternating current to conductors 24 for the heating of thermistors in the attenuating and equalizing pads 50-54, Fig. 2, the value of which depends on the control voltage derived from that pilot current producing the predominant voltage at the rectifier networks 61, 62 and 63, the high voltage of the one network blocking the other two. The attenuation equalizers 50-54 could alternatively be placed in the feed-back paths of the amplifiers 45-49 or the thermistors in them could be heated directly by the rectifier output. At station 2, Fig. 1, the receivers 25-29 are preceded by amplifiers 30-34 each with an individual gain control device 35-39 shown in Fig. 6. The attenuation equalizer 96 is controlled by pilot A through band-pass filter 98, amplifier 99, and control circuit 100, which may include a rectifier and a control oscillator as described above, and the attenuator 97 by the pilot B through a similar circuit 101, 102, 103. In a further embodiment, Fig. 7, the attenuation equalizers 108 and 109 have their own control oscillators 118 and 119 controlled by pilot currents through rectifier networks 114 and 115 as described above, but the rectifier outputs are shunted by resistances 120 and 121 each with the positive end earthed and the connections to the control oscillators are through. rectifiers 116 and 117. Rectifiers 123 and 122, which are normally blocked and are connected to taps on the resistances, provide cross-control should' one of the pilot circuits fail. Specifications 469,067, [Group XL], 545,866 and- 546,217 are referred to.