Improvements relating to communication systems

• Main Authors: PELLS BRIAN ALBERT, FLEETWOOD CLIFFORD HENRY JAMES, HITCH ERIC GIBBON
• Format: Patent
• Language: English
• Published: 21.05.1964
• Subjects: ELECTRIC COMMUNICATION TECHNIQUE; ELECTRICITY; TELEPHONIC COMMUNICATION

958,576. Direct call systems. HER MAJESTY'S POSTMASTER GENERAL, and ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Feb. 24, 1961 [Feb. 26, 1960], No. 6867/60. Heading H4K. In a communication system in which a master station supplies direct current to extensions to power bi-directional amplifiers at the extensions and controls the direction of amplification at the extensions in accordance with the polarity of the direct current it supplies, at least one further extension is included in the system the bi-directional amplifier of which is powered by a local source while its direction of amplification is governed from the master station by putting earth or battery to the line. The master station can call the further extension, shown to the right of Fig. 1, but cannot control the direction of the amplifier at the extension until permitted to do so by operation of a control switch with contacts K11 and K21 at the extension. To call the extension the master station energizes relay SW over rectifier Rf which causes earth to be applied symmetrically to the wires L1, L2, to the extension where relay SC operates to connect power to the extension amplifier from the local source over terminals S. The extension-calling relay AC at the master station is connected in the circuit which energizes SC at the extension but if the resistance of the winding of SC is high AC does not operate. Relay AC may be allowed to operate, however, since the value of the resistor R1 it bridges across SW is so high as not to effect the operation of SW. The master station can now talk to the extension but has no control over the direction of the extension amplifier until the extension party operates his control switch to throw contacts K11 and K21. Contacts K21 connect power to the amplifier without depending on relay SC. Contacts K11 connect relay AS in series with AC so that AS operates in response to release of SW as the master station reverses polarity on its line L1a, L1b. Since relay AS acts to reverse the connections of the extension amplifier, polarity reversal at the master station has a talk/receive function. To call the master station from the extension the contacts K11 and K21 are operated by means of the control switch at the extension to energize the extension amplifier and to energize relay AS at the extension and AC at the master station. Introduction of resistor R1 into the master station loop acts to signal a call and, as pointed out above, the value of R1 is such as not to affect operation of SW. The double wound relay AC may be replaced by a single wound one connected to the centre tap of the directly connected windings Sx1 and Sx2. A two-way amplifier with a D.C. through path may be incorporated in the line L1, L2. The system is otherwise the same as that described in Specification 871,583 except that not all extension amplifiers are powered from the master station.