US2313815A - Telephone system - Google Patents

Description

-' 1943- E. P. FAIRBAIR N' 2,3 1

TELEPHOfIE SYSTEM Filed Oct. 24, .1940

. INVENTOR 1-72 ATTORNEY Patented Mar. 16, 1943 TELEPHONE sYs'rEM Eric Playfair Fairbairn, Coventry, England, assignor to The General Electric Company Limited, London, England, a British company Application October 24, 1940-, Serial No. 362,521v In Great Britain October 31, 1939 2 Claims.

The present invention relates to telephone systems, and has for its object the provision of such a system having improved transmission characteristics.

If a normal telephone connection as set up through one or more exchanges is considered, it will be seen that few conditions for best speech transmission are fulfilled. For example, the impe'dances of the subscribers instrument and line,

' and of the exchange speech bridges and junction lines if included in the connection, are usually widely different from one another. Again, it is often necessary to connect low impedance subscribers lines with either normal or high impedance lines or terminations during different calls.

,LfiClllCY from bad matching, echoes from reflection at junction points and possibly phase dis- The former condition causes loss of efplacement, owing to the inductive or capacitative nature of the reactances of the various components in circuit. The latter prevents adequate and complete matching of subscribers instru- Tments to the lines under all conditions, this resulting normally in an unnecessary amount of residual side tone.

According to the invention, a telephone system is provided in which all elements of a connection, that is the lines, trunk lines with feed bridges, and any other exchange equipment ina connection are faced with their iterative impedance, each substation circuit forming the closing impedance of a subscribers line which is therefore characteristic impedance of that line,

which is the same as the characteristic impedance of a connection as a whole. Tothis end, we provlde each telephone instrument with an efficient transformer, preferably with a closed or partially closed iron circuit, and a linebalancing network so that the combination provides the said characteristic impedance to the line. In such an instrument side tone can, if necessary, be substantially eliminated.

A correcting network is also provided for each subscriber's line, this being adjustable so that a vsingle type of network may be manufactured and then fitted on any type of line. For overhead lines these networks preferably comprise resist acteristic desirableover"the frequency band employed for speech transmission. Alternatively distributed loading of known type may be employed.

It is also arranged that the speech bridge or like transducer employed in any selector or cord circuit in the exchange acts as a filter element having the necessary characteristic impedance over the frequency band found necessary in practice. This may be accompl'shedieither by providing filter components i dition to those normally found in a speechbridge, or by adjusting the components to 'such values'that they themselves form the filter.

Junction impedances are'brought to the correct value by the use of suitable termination transformers, phase correcting networks'or the like, and by the employmentlof repeaters or amplifiers at suitable: points. f

Owing to the non-reflective characteristics of an exchange network of this'type speech quality is appreciably improved, In addition, as each subscribers instrument always works into the same impedance, it'is possible to provide a matching networkwhich'balances'the external line conbe eliminated over' the "chosenw'frequency band; I Similarly, 'bothway loudspeaklngtelephones may ditions exactly,:so that side tone can if necessary be provided between subscribers ifa'coustic coupling betweeneach subscribers loudspeaker and microphone is "reduced-to a suitably'low value. 'It should be noted'that since the terminal impedance of a junction of, anyappreciable length is substantially independent vofthat length, it would bepossibleto connecta telephone system gaccordingto the invention with an adjacent un- "corrected system viajunction lines without loss of any of the facilities; mentioned.

Reference should now-be made to the accompanying drawing" which shows particular embodiments of the inventionfbut it will be ap preciated by those 'skilled'in the art that it is not necessarily limited to the examples shown. In the drawing, Figure 1 is an explanatory diagram of subscribers instrument circuit, Figure 2 is a complete schematic of subscriber's instrument circuit, Figure 3 showsa typical speech bridge including relaysandFigu're 4 is Figure 3 redrawn as a filter ele nt';

' Referring to Fig g land. 2,.it will be seen that these consist essentially fhybrid type transformer Thaving tidings T T2, T3 and-T4 The transformer is connec ed to line wires-and line balance each of impedance Z, to a microphone G having impedance ZI, and to a receiver having an impedance-Z2; Assuming that Ti and in portions of the circuit as necessary.

T2 have an equal number of turns, Zl is made equal to Z/2. By adjusting the turns of T3 and T4, the impedance Z2 can be accurately matched. If the number of turns on winding TI is n, then the number of turns on coil T3 must be For speech voltages-developed across Z! a signal is passed out on to the line, the power being equally divided between the balance and the line, i. e. a 3 db. transmission loss and no voltage is developed across Z2. This is independent of frequency. For reception, assumingthat the line balance is exact and the transformer perfect, the power is divided equally between the receiver and transmitter.

The present invention uses the above method of design in the following manner. The hybrid coil may have a complete, nearly complete or open iron circuit. Both the line and balance are given a 600 ohms resistive impedance. Z2 is replaced by' a telephone receiver. If the receiver does not present a constant resistive load, then a simple correcting network can be used in conjunction with it. Zl can be replaced by a high resistance microphone, either of impedance placed directly in place of Zl, or of different resistance. In the latter case an additional induction coil or transformer is required to be used in conjunction with the microphone for matching purposes.

The circuitin Figure 2 indicates one embodiment of the invention so far as it relates to subscribers' instruments. In. this figure, M is a microphone corresponding to Zl and G in Figure 1. As the impedance of the microphone is different from ZI, an auto-transformer AT is employed to change its effect on the circuit to the required value. A receiver R and the network C2, C3 and R3 correspond to the impedance Z2,

both in magnitude and phase angle. The other portions of the circuit such as Cl are means em-. ployed to control the direct current derived from the exchange to energise the microphone and for signalling purposes. Hook-switch and dial contacts can be provided if required and inserted If the microphone talking resistance with the required value of feed current is RI, then a step-up from RI to 2/2 (i. e. 300 ohms) is required. This is' achieved by. means of auto-transformer AT in known manner, the ratio of the turns'on windings ATI and ATZ being in the ratio This transformer can be made with a complete, nearly complete or open iron circuit, The battery feed is localised to the transmitter circuit by means of the blocking condenser Cl. A high resistance microphone of 200 or 300 ohms can be employed, requiring'only a feed current of 30 milli-amperes cr'20 milli amperes respectively, the speech output of which can be' arranged to over-compensate for the transmission loss referred to above. The lower feed currents simplify the hybrid coil design.

If the circuit is ideally balanced, there will be no side-tone or voltage developed in the receiver during transmission. In practice, however, some side tone is desirable, the actual amount depending to some extent on the extraneous noise. The amount of side-tone can be controlled by upsetting the balance of the hybrid coil by means of connection of a resistance in shunt of winding TI of the transformer. The circuit also includes one form of correcting network for a receiver, comprising a resistance R3 and the condensers C2 and C3. This, together with the receiver, can be made to present to the hybrid coil a 400 ohms impedance sensibly resistive and independent of frequency.

Referring now to Figures 3 and 4, Figure 3 shows a speech bridge of a type normally found in cord circuits or selectors. The speech bridge consists of two condensers KI and K2, and two relays SI and S2. Hitherto, no attempt has been made to design this combination as a filter,

one of the reasons being the very low value of the inductance-resistance ratio (or Q) of the relay coils. It is found that by employing a laminated core construction and suitably choosing the number of turns and the condenser capacities, a speech bridge can be provided having the characteristics of a filter of the type shown in Figure 4. For example, if each relay has a phase angle of degrees and the combined inductance of its coils is .476 henry, then each of the condensers KI and K2 should have a value of 1.32 micro-farads in order to provide a highpass filter having a cut-off frequency of 200 cycles, the network having a'characteristic impedance of 600 ohms. The battery connections to the relays are similar tothose shown in Figure 3. As however the relays cannot be made perfect inductances, some slight variation of these values may be necessary to secure best results in practice. Such adjustments will be obvious to those skilled in the art.

It is preferable to provide, inaddition to the inductances shown, a current stabilizing device such as a barretter in series with the line current circuit as this enables the constants of the various current-carrying inductances to be more easily obtained and maintained.

Where both long and short lines are to be connected in series e. g. junction lines and subscribers lines, it is preferable that the characteristic impedance of each line should be the same as that of every other line to which it is connected. This can be assured by the provision of matching networks or loading of known type.

The telephone circuit shown in Figure 2 can, if necessary, be modified so as to provide loud speaking facilitiesin fact, the system as a whole is eminently suitable for use in connection with such loud-speaking devices. For this purpose, the michrophone M and auto-transformer AT are removed and the output stage of a microphone-amplifier is substituted. Similarly, the receiver R and its input matching circuit are removed and replaced by the input stage of a loud-speaker amplifier. If necessary, these two amplifiers can be made into one, and suitable switching is then employed so as to enable the amplifier to perform its dual functions.

It may occasionally be necessary to tap off small quantities of speech current, as for example to an operators or monitors circuit. Such branch circuits are preferably made of such a high impedance, as for example by including a valve amplifier, that the characteristic impedance of the connection as a whole is substantially unaffected. r

I claim:

1. In a telephone system, subscribers substae tion circuits, lines connecting said circuits to the exchange, a plurality of connecting circuits in the exchange, battery feeding bridges in each connecting circuit, means including said connecting circuits for connecting one line to another, a microphone in a bridge circuit at each substation circuit together with a balancing network on one side of the bridge to balance the impedance of the connecting means or line connected to the other side thereof, a receiver circuit connected in anti side tone relation to said microphone, a corrective network connected in said receiver circuit to equalize the impedance thereof over the desired frequency range, the impedance of said elements in the substation circuit adjusted so as to match the characteristic impedance of the subscribers line when viewed from the line, the impedance of the feeding bridges of each connecting circuit adjusted to present the same characteristic impedance as the subscribers line as a whole, whereby each line and each connecting circuit of a connection faces its iterative impedance when the same are connected together.

2. In a telephone system, a subscribers line, a plurality of connecting circuits each having a feeding bridge, each feeding bridge comprising an impedance adjusting filter adjusted to provide the same characteristic impedance to each circuit, a sub-station circuit connected to each line, a transformer in the substation circuit, a microphone in bridge of the line at each substation, an impedance bridge at each substation having an impedance equal to the characteristic impedance of theline to which it is connected, the impedance of the transformer, the microphone bridge and the impedance bridge of each substtion being adjusted so that the substation circuit has a characteristic impedance matching the characteristic impedance of the line when viewed from the line, each line and connecting circuit in the system having the same characteristic impedance.

ERIC PLAYFAIR FAIRBAIRN.

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