Patent GB-779172-A (GB)
Title: Improvements in and relating to telephone systems and the like
IPC: H04M9/08
Filed: 1955-02-15
Abstract: 779,172. Two-way transmission. SVENSKA RELAFABRIKEN AKTIEBOLAG. Feb. 15, 1955 [Feb. 24, 1954], No. 4551/55. Class 40 (4). In a two-way amplifier in which a voltage derived from speech currents control separate amplifying units for each direction of transmission in such amanner that their gains are varied in opposite respects from an intermediate initial value according to the direction of speech, the combined gain of the two amplifying units is arranged to reduce when speech currents are received so that the loop gain over a telephone system incorporating the amplifier can at no time exceed unity. A two-station system incorporating the amplifier is shown in Fig. 1, each station comprising a microphone 1, 2 and a loud-speaker 3, 4. Identical amplifying units are provided for each direction of transmission. Speech currents from microphone 1 are applied over an input transformer 5 to the left-hand autobiased section of a double triode 9, and after amplification are received by control grid 25 of the hexode section of a hexode-triode 27. This valve provides regulated variable gain in the amplifying unit and is preset by potential on grid 35 to give moderate initial gain in the absence of speech. The output stage of the amplifying unit comprises valve 55 and transformer 63 arranged to provide negative feedback. A voltage obtained from the output stage controls the gain of the hexode portion of 27. This voltage is applied over resistor 71 and capacitor 73 to the right-hand portion of the double triode 12 in the return amplifying unit, and after amplification is available at the anode 90 of one section of a double diode 94 and at the cathode 92 of the other section. Both diodes are biased against conduction by D.C. potentials obtained from tappings on a potentiometer R1-R6 which also supplies the necessary bias potentials to other valves. The diode comprising anode 90 requires a smaller control voltage to conduct (1 volt) than the diode comprising cathode 92 (2 volts). When the former diode conducts, the change of potential at anode 90 causes the gain of triode-hexode 28 in the return amplifying unit to reduce, whereas conduction of the other diode causes a change of potential at cathode 92 which increases the gain of triode-hexode 27. Due to the different conductive thresholds of the diodes, the reduction in gain in the return amplifying unit exceeds the increase in gain in the direction of speech. In the case of speech current increasing in amplitude from zero, the reduction in gain commences before any increase in gain occurs in other amplifying unit. Filters comprising resistors 101-106 and capacitors 111- 116 are incorporated in the circuits between the diodes and the triode sections of the triodehexodes. Capacitors 83-88 are also provided to delay restoration of the amplifier units to their initial condition when the speech currents terminate. In a modification, the control voltage is derived from a stage before the variable gain hexode-triode 27, in which case the circuits indicated by dotted lines in Fig. 1 are used and disconnections to the grids 75, 76 are effected where indicated by crosses.