US2301530A - Telephone system - Google Patents

Description

Nov. 10, 942- E. P. FAIRBAIRN I 2,301,530

TELEPHONE SYSTEM Filed Oct. 7, 1940 2 Sheets-Sheet 1 [0 I F157. 1 D

TELEPHONE IA/l 111+ A [III INVENTOR ERC PLAYFMR FMRBAIRN ATTORNEY Nov. 10, 1942. FAlRBAlRN 2,301,530

TELEPHONE SYSTEM Filed Oct. 7, 1940 2 Sheets-Sheet 2 INVENTOR ERC PLAYFAIR FAIRBMRN ATTORNEY Patented Nov. 10, 1942 TELEPHONE SYSTEM Eric Playfair Fairbairn, Coventry, England, as- Signor to The General Electric Company Limited, London, England, a British company Application October 7, 1940, Serial No. 360,099 In Great Britain October 31, 1939 3 Ciaims.

The present invention relates to loud-speaking telephone systems, and has for its object the provision of means for enabling both way loud-speaking facilities to be provided by the elimination of certain electrical difiiculties previously encountered.

The invention is more particularly applicable to loud-speaking systems employed in connection with private automatic exchanges. in these exchanges the line resistances and impedances are all very similar, as also are the impedance characteristics of the subscribers instruments.

In order to give loud-speaking facilities on any instrument employing only a two-Wire interconnecting channel, it is necessary to provide a microphone and amplifier the output of which is applied to the line wires in such a way that substantially none of this output is fed back into the local; loud-speaker circuit. This is normally effected by passing the output of the microphone amplifier through a balanced transformer in which equal proportions of the output are fed to the line and. to a balance network, this latter being arranged to simulate as closely as possible the impedance characteristics of the line, exchange transmission bridge, and subscribers termination as seen from the loud-speaking termination. Connections to the local loud speaker are then derived from equipotential points in the transformer windings.

Such a system operates correctly provided that the line and instrument impedance found on any call made from the lou -speaking instrument is substantially constant. If, however, an attempt is made to interconnect two such loud-speaking terminations it has hitherto been found that the variation in impedance between such a loud speaking termination and an ordinary subscribers instrument has been too great to permit more than a relatively small amount of overall amplification of speech, since if more than this critical value of gain were attempted, an oscillation due to out-of-balance conditions would be obtained.

According to the invention, such out-of-balance conditions are prevented in a system of the type described, by arranging a compensating network in the line circuit of each loud-speaking equipment, this network causing the characteristic impedance of the loud-speaking equipment to resemble closely that of the normal subscribers instrument termination.

The loud-speaking instrument is then provided with a line balance which can be made to approximate very closely to the impedance characteristic found on any call, whether to a normal subscriber or to another loud-speaking installation, this greater accuracy of balance permitting a higher electrical gain and much greater stability than was previously the case.

In loud-speaking instruments of the type described, it is desirable to arrange that speech may be transmitted and received within a fraction of a second of the subscriber pressing the reply or calling button or switch. Even with directly heated valves, such a condition is not easily met where alternating current derived from a transformer or direct current through a choke or barretter is employed for filament heating. it is therefore arranged according to a feature of the invention that the filaments of the valves of a -loud-speaking installation are heated directly upon switching-on of the installation by a current considerably higher than that of their normal running value, the magnitude of this current being reduced to normal directly an adequate filament temperature is reached.

When alternating current is employed, a suitable arrangement consists of a relay connected across the valve high-tension supply. The filaments of all valves are initially heated by alternating current considerably in excess of their normal; value, and as soon as high tension is available, operation of the relay either reduces the alternating current to a normal value or changes over the filament supply to a direct current source, supplying current of the correct figure.

If D. C. power mains are employed, the relay is connected across the valve filaments, which in turn are connected in series with a barretter. ihis latter is initially cy-passed by an ordinary metal filament lamp. Both this lamp and the valve filaments, when cold, are of relatively low resistance, so that immediately on switching on the valves, the valve filaments substantially short-circuit the relay. The valve filaments are heated rapidly in series with the metal filament lamp, and when they reach their working temperature sufiicient current is diverted by their increased resistance through the relay to operate it. This action disconnects the metal filament lamp and allows normal current through the barretter to keep the filaments at their correct temperature.

The subscribers telephone instrument is provided with a signalling lamp and as the instrument is placed close to the subscriber, and is handled by him, it is arranged according to a further feature of my invention, that no direct connec tion from the said instrument is made at any time to the supply mains, if used. For this purpose, transformers are inserted at suitable points to isolate the microphone, loud-speaker and hand micro-telephone, and it is arranged that the signalling lamp is lit either from low voltage A. C. when the supply is of this nature, or from alternating current derived from a valve oscillator when D. C. mains only are available. In each case the transformer winding supplying the lamp is suitably insulated and/or earthed.

In order that the nature of the invention may be more fully understood, reference should be made to the accompanying drawings which show particular embodiments of my invention.

Fig. 1 shows a telephone line terminating in a substation including loud speaking telephone equipment.

Fig. 2 shows one arrangement for supplying power to the valves of the amplifier.

Fig, 3 shows another arrangement for controlling the power supply to a valve.

Fig. 4 shows a method of supplying a low voltage to the filament of a low voltage lamp when the main supply is of high potential direct current.

In Figure 1, line wires LI and L2 are connected to a compensating network comprising an aircored inductive impedance ACI, an attenuating network consisting of four non-inductive resistances RI, R2, R3 and R4, a condenser Cl which isolates the line current, and a hybrid coil I-IC having line windings LWI and LWZ, line balance windings LBI and LE2 and microphone input winding IW, which is connected to a microphone M by means of conductors IA and IB. A holding coil H maintains a loop across the line wires for supervisory purposes. Connected to the line balance coils is a line balance network having linear impedance elements arranged to simulate the impedance/frequency characteristic of the circuit terminated across terminals LI and L2. The line balance network comprises an air-cored inductance AC2, non-inductive resistances R5, R6 and R7, condenser C2 and an iron-cored inductance LA. Conductors IC and ID are connected to the input terminals of a loud-speaker amplifier (shown diagrammatically), a non-inductive resistance R9 forming a low resistance terminating impedance in case the amplifier input impedance is high, and the output of the amplifier is shown connected to a loud speaker LS.

In Figure 2, an iron-cored transformer T having a primary winding P connected by conductors MA and MB to a supply of alternating current has three secondary windings S2, S3 and S4. A supply of direct current which is provided by means of secondary winding S3 and a rectifier valve VA is connected to the high-tension supply terminals of an amplifier such as used in the telephone circuit of Fig. 1 via conductors HT+ :and HT. The filament of the rectifying valve VA is heated by means of an alternating current provided by the secondary winding S2. A similar supply of alternating current is provided by means of the secondary winding S4 to filaments of the amplifier valves of such amplifier over conductors FA and FE. The potential across the filaments of the rectifier and amplifier valves when supplied by the whole of the windings: S2 and S4 is in excess of the required normal working potential in order that the filaments shall be heated quickly. When the rectifier valve is functioning correctly, the potential across the, conductors HT+ and HT- to the amplifier reaches a value which causes relay A to operate whereupon contacts Al reduce the number of turns in the portion of winding S2 and contacts A2 reduce the number of working turns in winding S4 so that the potentials to the valve filaments are reduced to the values required for normal working.

Referring now to Figure 3, another method for reducing filament current to an amplifier is shown wherein a source of high-potential direct current connected to the conductors DC+ and DC' heats the filament of a thermionic valve VB of the amplifier in series with a barretter BR and a metal filament lamp PL. The resistance of the valv filament when cold is low enough to substantially short-circuit the relay B. When the power supply is switched on, the valve filament heats to its normal working temperature very rapidly whereupon the resistance of the filament increases and relay B operates. Contacts Bl disconnect the lamp so that the amount of current passing through the valve filament is reduced to the amount required for normal working and is limited by the barretter.

In Figure 4, a local oscillating valve circuit consisting of a thermionic valve VO, condensers PC and G0, a resistance R3 and an iron-cored output transformer T having windings T I, T2 and T3, receives a direct-current high tension voltage supply over the two conductors and A low voltage alternating current is induced into winding T3 and fed to an indicating lamp over conductors FA and FE.

I claim:

1. In a telephone system, a telephone line having loud speaking equipment including a loud speaker and a microphone associated therewith, a hybrid coil connecting said equipment to said line, a compensating network connected between the coil and the line circuit operative to cause the characteristic impedance of the line and the loud speaking equipment to resemble closely the impedance of a line having a regular substation telephone, and a line balancing network associated with said loudspeaking equipment through said coil to balance the line impedance.

2. In a telephone system, a line terminating in a substation having loud speaking equipment in cluding an amplifier, a hybrid coil connecting said equipment to the line and having a balanced network which balances the impedance of the line, and a compensating network between the coil and the line for bringing the impedance of the line having the loud speaking equipment approximately equal to the impedance of a line having a standard transmitter and receiver thereon.

3. In a telephone system, a line having loudspeaking equipment including a loud speaker, a microphone and an amplifier connected thereto, a compensating network to cause the line to resemble a regular substation telephone line and a network to balance the line impedance, wherein the amplifier has filament type valves therein, and means for supplying an initial high current to said valves and means for cutting down the current thereto after an interval.

ERIC PLAYFAIR FAIRBAIRN.

Images