US2214258A - Telephone substation apparatus - Google Patents

Description

Sept. 1940. H. c. PYE

TELEPHONE SUBSTATION APPARATUS Filed April 16, 1938 2 Sheets-Sheet. 2

INVENTOR.

HAROLD c. PYE fi MM ATTORNEYS.

Patented Sept. 10, 1940 rrso STATES TELEPHONE SUBSTATION APPARATUS ApplicationApril 16, 1938, Serial No. 202,392

12 Claims.

The present invention relates to telephone systems and more particularly to substation apparatus for use in a telephone system.

Various circuit arrangements have been used 5 in the past for preventing the audio frequency energy developed by the operation of a substation transmitter from being delivered to the receiver or sound reproducing device of the substation for reproduction. A circuit provided for this purpose is conventionally known as an antiside-tone circuit and is in effect an alternating current bridge having the sound reproducing device coupled to two arms of the bridge in a manner such that substantially zero voltage is developed across the input terminals of the sound reproducing device during operation of the transmitter. When a substation circuit of this character is involved in a communication connection to another substation of the system, one arm of the anti-side-tone bridge circuit is partially made up of the equivalent impedance, measured at the substation line terminals, of the series and shunt resistance and the shunt capacitance of the two lines, the impedance of the coupling elements included in the line connecting apparatus of the line terminating exchange and the impedances of the respective circuit elements provided or embodied in the distant substation. For satisfactory operation of the anti-side-tone substation circuit, it is necessary to provide therein a balancing network having a frequency impedance characteristic which is substantially the same that of the measured equivalent impedance throughout the operating audio frequency range. When regular subscribers lines of substantial length are involved in the communication connection, the predominant component impedances of the measured equivalent impedance are the series resistance and the shunt 4U capacitance of the two connecting lines. More articularly, in the case of a connection including two long subscribers lines, the components of the equivalent impedance measured at the line terminals of one substation and represented by the circuit elements at the distant substation and by the coupling elements embodied in the line connecting equipment of the line terminating exchange, are relatively small as compared to the series resistance and shunt capacitance of the two ccnnecting lines and, hence, may be neglected in designing a satisfactory balancing network. In this case, a-simple resistance-capacitance balancing network will provide a reasonabl' good balance of the anti-side-tone bridge over the major portion of the audio frequency range. In an arrangement of the type commonly found in private exchanges wherein the substation lines are short, however, the series resistance and shunt capacitance of the two substation lines involved in a communication 0011- 5 nection are relatively small and the effect of the coupling elements included in the line connecting equipment of the line terminating exchange and of the circuit elements provided at the distant substation are predominant, so that a simple resistance-capacitance network is, over the greater portion of the audio frequency range, of little effect in balancing an anti-side-tone substation circuit.

It is an object of the present invention to provide in an arrangement of the character described an improved anti-side-tone substation circuit so arranged that the circuit is substantially balanced at all frequencies within the operating range when included in a communication connection to a distant substation wherein the impedances of the coupling elements embodied in the line connecting equipment of the line terminating exchange and of the circuit elements provided at the distant substation are predominant in determining the equivalent impedance forming a part of one arm of the anti-side-tone bridge circuit.

It is another object of the invention to provide an improved and effective balancing network of simple arrangement for use in a substation circuit of the anti-side-tone type.

The invention is illustrated as being incorporated in a telephone system wherein a first substation circuit of the anti-side-tone type is provided with a pair of line terminals which are adapted to be connected by way of a telephone line, the line connecting equipment of an EX.- change and a. second line to another substation. The exchange is of the private automatic type such that the two substation lines involved in a communication connection including the first substation circuit are relatively short and the equivalent impedance, measured at the line terminals of the first substation, of impedance elements exter- 45 nal to or not directly included in the first substation circuit is substantially determined by the component impedances of the coupling elements embodied in the line connecting equipment of the exchange and the component impedances of the circuit elements included in the circuit of the distant substation. In accordance with the present invention, there is. provided in the anti-side-tone substation circuit a network for balancing the equivalent impedance, measured across the line 55 terminals of the circuit, which comprises a plurality of impedance elements having impedances individually corresponding to certain of the component impedances making up the measured equivalent impedance. The impedance values of the plurality of impedance elements are so proportioned relative to their corresponding component impedances and the elements of the network are so connected and arranged that the anti-sidetone circuit is substantially balanced at all frequencies within the operating frequency range. More particularly, there is provided in the balancing network, impedance elements which individually correspond to the coupling elements embodied in the line connecting equipment of the line terminating exchange and to the transmitter, receiver and inductance coil embodied in the distant substation circuit. Additional impedance elements are included in the balancing network which individually correspond to the lumped series and shunt resistances of the two connecting lines and to the lumped shunt capacitance of these lines. All of these elements are so proportioned and so connected and arranged that excellent anti-sidetone effect is realized at all frequencies within the audio range.

Further features of the invention pertain to the particular arrangement of the circuit elements whereby the above objects are attained.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawings in which Figure 1 illustrates an automatic telephone system having incorporated therein a substation circuit connected and arranged in accordance with the present invention; Fig. 2 illustrates the circuit of an established communication connection between the two substations shown in Fig. 1; Fig, 2a illustrates the equivalent circuit formed by combining certain of the impedances shown in Fig. 2; and Fig. 3 illustrates the structural arrangement of one of the circuit elements shown in Figs. 1 and 2.

Referring now more particularly to Fig. l of the drawings, the system there illustrated comprises a plurality of telephone lines, two of which are indicated at H] and H terminating in the line connecting or line switching equipment of a private automatic exchange 121. The line switching equipment provided in the exchange l2 for setting up connections between the various lines of the systems may be of any desired impulse. responsive type and, since the particular character and arrangement of this equipment forms no part in the present invention, the exchange has been only diagrammatically illustrated. It will be understood, however, that this switching equipment includes the usual non-numerical switches such for example, as line switches, and the usual numerical switches of the impulse responsive type which are conventionally known as selector and connector switches.

The end of each of the telephone lines remote from the switching equipment provided in the exchange I2 terminates in a substation of the automatic type, that illustrated in association with the line H3 being indicated at A and that terminating the line ll being illustrated in detail in Fig. 1 and being indicated generally at B.

Referring now more particularly to the equipment provided at the substation B, this substation comprises a ringing or signal device I! con nected across the line conductors M and l 5 of the line H in series with a condenser l8; a transmitter H), a receiver 20, an impulsing device 2! and a' hook or cradle switch 22. The impulsing device 2| is of the conventional dial operated type and comprises a pair of impulsing springs 23, and three shunt springs 24, 25 and 26. The enumerated springs 24, 25 and 25 are normally restrained to the open circuit position as indicated in the drawings and are so arranged that the dial mechanism closes the same during operation' of the impulsing device. Also included in the substation equipment is an auxiliary sound reproducing device in the form of a loud speaker 21. This device is provided with input terminalswhich are arranged to be connected across one winding 28 of a transformer 29 by means of a key operated switch 30 having two sets of contact springs and a common actuating member or key indicated by the dotted line 3! The key operated switch 38 is of the locking type in that when once operated to its off-normal position it remains in this position until it is manually restored to the normal position. The transformer 29 further comprises two windings 32 and 33 coupled to the winding 28 and arranged to be connected across the line terminals l5 and N5 of the substation B in series with a balancing network indicated generally at 34. As indicated, the transmitter I9 is connected across one winding 35 of an impedance matching transformer 36 in series with a condenser 3 The series-parallel circuit comprising the transformer winding 35, the condenser 31 and the transmitter 9, is, in turn, connected in series with a second winding 38 of the transformer 36 between the junction point of the two windings 32 and 33 and the substation input terminal I 5.

Referring now to the operation of the system and assuming, for'example, that a call is initiated at the substation A and routed by way of the automatic switching equipment provided in the exchange i2 and the line H to the substation B at a time when the last-mentioned substation is idle, when the line H is seized by one of the connector switches provided in the exchange i2 and tests idle, ringing current is projected overthe line in the usual manner to actuate the ringing device l'l, thereby to give an audible indication that the substation B is being called. The call may be answered in either of two ways, i. e., by removing the receiver 20 from its supporting hook or cradle, or by actuating the answer switch 30 to its off-normal position. If the call is answered in the usual manner by lifting the receiver 20 from its supporting hook or cradle, the springs 39 and Gil of the hook switch 22 are closed to connect the receiver 20 across the winding 28 of the transformer 29 and to connect a direct current bridge between the conductors l3 and I4 of the line H. Specifically, this bridge extends from the terminal !5 by way of the transmitter [9, the transformer windings 33 and 33, the impulsing springs 23 and the cradle or hook switch springs 39 to the opposite substation line terminal l6. From an inspection of the drawings, it will be apparent that the receiver 20 is connected across the transformer winding 28 by way of the hook or cradle switch springs 40 and the springs 4| and 42 of the switch 38. The completion of the above traced bridging path across the conductors I3 and M of the line H causes the operated connector switch in the exchange l2 to respond to inter- .rupt the ringing operation and complete a talking connection between the respective calling and called substations A and B.

As indicated above, the call as extended to the substation B may also be answered by operating the switch St to its off-normal position, without removing the receiver 20 from its supporting hook or cradle. In this case, the loud speaker 21 is connected across the winding 28 of the transformer 29 in lieu of the receiver 25). More particularly, the input terminals of the loud speaker 21 are connected across the winding 28 through closure of the springs 42 and it in response to the operation of the switch 3% to its oil-normal position. In this case, the bridging path across the conductors i3 and H!- of the line i l extend by way of the springs it, which are closed in response to the operation of the switch 36, rather than by way of the springs 39 of the hook or cradle switch 22.

With either the receiver 20 or the loud speaker 2? operatively included in the substation circuit, talking battery is supplied to the substation circuit over the line H through the windings of the back bridge or called party relay embodied in the operated connector switch forming a part of the switch train by way of which the connection extends. The path of the signal currents incoming to the substation may be partially traced as extending by way of the line conductor it, the balancing network 3 1, the two transformer windings 32 and 33, the impulsing springs 23 and the hook switch springs 39 or the key switch springs 64 to the opposite line conductor l4. Portions of the signal currents are, of course, by-passed through the branch circuit comprising the windings of the transformer 35. Alternating signal currents flowing through the windings 32 and 33 cause corresponding induced voltages to be developed in the winding 23 which are impressed on the receiver 20 or the input terminals of the loud speaker 21, depending upon which of the two translating devices is being used. Such voltages are reproduced by either of the two translating devices 2c and 21 in a wellknown manner. When sound waves impinge upon the diaphragm of the transmitter i9, undulating audio frequency currents corresponding to the sound waves are set up which cause corresponding voltages to be developed between the junction point of the two windings 32 and 33, and the line conductor l3. Such voltages cause currents to flow through the two windings 32 and 33 in opposite directions, the path of the current through the winding 32 extending from the junction point between the two windings 32 and 33 by way of this winding, the balancing network 3d, the condenser 31 and the two transformer windings 35 and 38 back to the junction point between the windings 32 and 33. The path of audio currents flowing through the winding 33 extends from the line terminal by way of the condenser 3?, the two windings 35 and 3%, the winding 33, the impulsing springs 23, the hook switch spring 39 or the key switch spring 46, the line conductor Hi and the loop circuit extending to the substation A back to the line terminal I 5. By virtue of the improved arrangement of the balancing network 34, the audio frequency current traversing the winding 32 during operation of the transmitter i9 is substantially equal in magnitude and opposite in phase to the current traversing the winding 33 at all frequencies within the operating frequency range. The winding 28 is substantially equally coupled to the two windings 32 and 33 and,

hence, substantially no induced voltage is developed in the winding 28 during operation of the transmitter it. Thus, the well-known anti-sidetone effect is obtained.

When the conversation ensuing over the connection between the two substations A and B is terminated and the connection is released, the apparatus provided at the substation B may be restored to normal by returning the receiver 20 to its supporting hook or cradle in the event it is being used as the sound translating device, or by restoring the switch 30 to normal in the event the loud speaker 27 is being used as the receiving means. If the receiver 20 is used and is restored to its supporting cradle or hook, the closed loop circuit extending to the operated switching equipment in the automatic exchange I2 is interrupted at the springs 39. Similarly, if the loud speaker 2! is used as the sound reproducing device, the loop circuit extending to the operated automatic switching equipment in the exchange i2 is interrupted at the springs 44 when the switch 36 is restored to normal. The manner in which the operated switch train in the automatic exchange is released of course depends upon the nature of the switching equipment. For example, this equipment may be designed so that the switch train utilized in an established connection is released when the answering party at the called substation hangs up. Alternatively this equipment may be designed so that the release of an operated switch train is under the control of the last party to hang up. In order to initiate a call at the substation B intended for one of the other automatic substations such, for example, as the substation A, the calling party first closes a loop circuit extending to the switching equipment provided in the automatic exchange 12 by either removing the receiver 26 from its supporting cradle or hook to operate the switch 22 to its oif-normal position, thereby to close the springs 39; or operating the switch 36 to its off-normal position to close the springs Ml. The bridging path between the line conductors l3 and i4 established in response to either the operation of the switch 30 or the switch 22, is traced previously, and in either case the impulsing springs 23 are serially included in this path.- When the bridging path between the line conductors it? and M is completed, the associated ,o-r assigned non-numerical switch in the exchange I2 is caused to operate to extend the connection to one of the impulse-responsive numerical switches in the usual manner. During operation of the impulsing device 2| the impulsing springs 23 are alternately opened and closed, thereby to transmit impulses by way of the line i l to the automatic switching equipment in the exchange Hi. This equipment responds to the impulses in the usual manner to cause the connection to be routed by way thereof to the called line iii. During each operation of the impulsing device H, or more particularly, during each digit dialed at the substation B, the shunt springs 2 25 and 255 are moved to the closed circuit position. At the springs 25 and 26, the line conductor it is connected directly to the upper impulsing spring so that all circuit elements provided at the substation B are excluded from the impulsing circuit. At the springs 24- and 25 an obvious path is completed for short-circuiting the winding 23 by way of which energy is delivered to the loud speaker 21 and to the receiver 20, thus preventing microphonic noises frombeing reproduced by the one of these two translating devices which is operatively included in the substation circuit. When the operation of the impulsing device 2! is ended, the springs 24, 25 and 25 are moved to the open circuit position in the usual manner, to interrupt the above-traced short-circuiting path across the winding 28 and to condition the substation circuit for voice frequency current transmission and reception. In this connection it will be apparent that if the selected called line if! is tested by the automatic switching equipment in the exchange l2 and found busy, busy tone is returned over the calling loop circuit to traverse the windings 32 and 33 of the transformer 29, whereby a busy tone signal is generated by the one of the two translating devices 20 and 21 which is operatively included in the substation circuit. If, on the other hand, the called line is idle and ringing current is projected thereover to signal the called substation in the usual manner, ringback tone current is conducted back over the calling line and through the windings 32 and 33, so that a ring-back tone signal is produced by either the receiver 2! or the loud speaker 21. When the called subscriber answers, the desired conversational connection is completed in the usual manner. The paths traversed by the signaling currents incoming and outgoing from the substation B on a call originated at this substation are exactly the same as those traced above for a call incoming to the substation B.

The improved arrangement of the balancing network 33, whereby the anti-side-tone circuit described above, is substantially balanced at all frequencies within the operating frequency range, may best be explained by reference to Fig. 2 wherein the circuit elements involved in an established communication connection between the substations A and B are shown. In the circuit of Fig. 2 the component network comprising the two windings 35 and 38 of the transformer 36, the condenser ill and the transmitter I9, is resolved into an equivalent series circuit comprising an inductance element 45 in series with a variable resistor 45. This equivalent circuit is accurate for frequencies lying within the major portion of the voice frequency range. The series resistance of the line H is represented by the two resistors ll and 48. The shunt capacitances between the two conductors I3 and M of the line H, and including the capacitance of the condenser IB, are lumped together and shown as a condenser 39. The coupling elements embodied in the connector switch for conventionally separating the two talking circuits leading respectively to the two substations are shown as condensers 5i! and 5 l The series resistance included in the two sides of the called line In is represented by the two resistors 52 and 53, and the shunt capacitances between the conductors of the line Hi, including the capacitance of the condenser connected in series with the ringer or signal device at the called substation A, are indicated by the condenser 56. The impedances of the circuit elements provided at the called substation A are indicated by the impedance elements 55, 5'6, 5? and Za, respectively. Of the last enumerated elements, the Variable resistor 55 represents the resistance of the transmitter included in the substation circuit, the condenser 56 corresponds to the condenser 37 shown in Fig. 1, and the transformer 51 is the induction coil corresponding to the transformer or coil 28 shown in Fig. 1.. The impedance of the receiver provided at the called substation A, which impedance includes resistive and inductive components, each variable with frequency, is indicated at Za. Since the system is of the common battery type, talking battery is supplied to each of the substation loop circuits through the windings of certain of the relays provided in the connector switch of the operated switch train. More particularly, talking battery is supplied to the loop extended to the called substation A over a path including the windings of a back bridge or battery-reversing relay embodied in the connector switch, and talking battery is supplied to the loop extending to the substation B through the windings of the line relay conventionally embodied in the operated connector switch. Thus, the audio frequency transmission path is shunted on each side of the coupling condensers 50 and 5| by a series circuit including resistance and inductance and the exchange battery. The circuit to the right of the two coupling elements includes an inductance 58 and a resistance 59, and the-circuit to the left of the coupling elements includes an inductance 6G and a resistance 6|.

The portion of the network shown in Fig. 2, external to the substation B and shown to the right of the line terminals l5 and I6 of the substation B, may be replaced by the equivalent circuit shown in Fig. 2a without substantially altering the impedance-frequency characteristic of this portion of the circuit as determined by measurement between the line terminals l5 and 16. In the equivalent circuit of Fig. 2a, the series resistances of the two lines l and I I, as represented by the resistors 48, 52 and 53, are lumped together and shown as a resistor 62. Similarly, the series capacitances of the circuit, namely, the capacitances of the two coupling condensers B and 5|, are lumped together and indicated by the condenser 63. The shunt line capacitances, as represented by the two condensers 49 and 5 3 in Fig. 2, are combined to form a single equivalent condenser 64 in Fig. 2a, and the shunt resistances between the two sides of the audio frequency current transmission path are combined and represented by the equivalent resistance 65.

As indicated in the introductory portion of the specification, in a telephone system of the usual type wherein the subscribers lines are rather long, the series resistance of the lines and the lumped shunt capacitance between the conductors of the lines are the impedances which predominantly determine the character of the equivalent impedance external to a substation and measured at the substation terminals, over the major portion of the voice frequency range. For example, if the two lines I l and l 0 were long lines, the equivalent impedance of the portion of the network external to the substation B and measured across the terminals 15 and 56 would be predominantly determined by the resistor 62 and the condenser 64 as shown in the equivalent circuit of Fig. 2a, over the major portion of the operating frequency range. In fact, with such lines these two components of the equivalent impedance would so greatly predominate that the impedances of the coupling elements represented by the condenser 63 and the impedances of the circuit elements provided in the substation A and indicated at 55, 56, 51 and Za can be neglected in devising a satisfactory balancing network.

In the particular arrangement illustrated, however, wherein the subscribers or substation lines are short, the series resistance of the lines as represented by the equivalent resistor 62 and the shunt capacitance as represented by the con denser 6 3 are of relatively small efi'ect as .compared to the effect of the coupling capacitance 63 and the impedances of the circuit elements provided at the substation A in determining the frequency-impedance characteristic of the equivalent impedance of the network external to the substation B and connected between the terminals l5 and i6. Hence, to provide a simple resistance capacitance balancing network of the usual type in the circuit of the substation B would mean that the anti-side-tone circuit could only be efiectively balanced for frequencies lying within a very small portion of the operating frequency range. To obviate this difiiculty and to secure a more perfect balance of the anti-sidetone circuit when used in a system utilizing short lines, the improved coupling network 34 is provided.

In brief, the network 33 comprises a plurality of impedance elements 56, El, 68, 69, '10, H, 12 and Zb which individually correspond to the impedance elements 62, $3, 64, 65, 55, 56, 5'3 and Za, in the order named, shown in the equivalent circuit of the Fig. 2a. More particularly, and considering Fig. 2, it will be observed that the criterion for maintaining the audio frequency currents flowing through the winding 32 equal in magnitude and opposite in phase to the corresponding audio frequency currents flowing through the winding 33 during operation of the transmitter or when audio frequency voltages are impressed across the resistor i6, is that the phase angle and magnitude of the equivalent impedance of the network 3 3 shall respectively be equal to the phase angle and magnitude of the equivalent impedance of the equivalent circuit shown in Fig. 2a, as measured between the terminals l5 and H6, at all frequencies within the operating frequency range. To this end, the resistance of the resistor 66 is made equal to the lumped series resistances of the two lines connecting the substations A and B as represented by the equivalent resistor 62; the resistance of the resistor 69 is made equal to the equivalent resistor 65; the capacitance of the condenser 68 is made equal to the equivalent capacitance E4; the capacitance of the condenser 67 is made equal to the combined capacitance 63 of the two coupling elements and Bi; the resistor Til is made equal to the average value of the transmitter resistance as represented by the resistor the induction coil 12 is a duplicate of the induction coil 57; the condenser H is chosen of a capacitance value equal to that of the condenser 56; and an artificial impedance element Zb is provided, having a frequency-impedance characteristic which is a substantial duplicate of that of the impedance Za representing the receiver at the substation A.

If desired, the impedance Zb may be of the form shown in Fig. 3. In brief, the element shown in Fig. 3 comprises two L-shaped core members 13 and i l arranged in the manner illustrated so that two short air gaps l5 and '16 are provided between the oppositely disposed pole faces thereof. Wound respectively on the core members '13 and M are two coils Ti and i8 which are connected in series-aiding phase relation. The two terminals of the windings ill and F3 are connected across the terminals of the winding of the transformer 12 shown as entering the box identified at Zb. Specifically, the core members '13 and M with the windings TI and 18 respectively wound thereon, may comprise standard receiver parts. The air gaps l5 and T6 are preferably adjusted so that each has a length of approximately .025 inch.

By proportioning the impedance values of the elements included in the network 3a in the manner described, and by connecting these elements in a manner identical with the mode of connecting the elements of the equivalent circuit of Fig. 2a, the equivalent impedance of the balancing network 34 is caused to have substantially the same frequency-impedance characteristic as the frequency-impedance characteristic of the equivalent circuit shown in Fig. 2a, and hence, an entirely adequate balance of the anti-side-tone substation circuit is achieved for all frequencies within the operating frequency range. Obviously, an exact balance cannot be obtained for all possible communication connections for the reason that the impedance characteristics of different circuit elements embodied in diiferent substations vary as'do also the impedance characteristics of diiferent lines. It is possible, however, by proportioning the impedance values of the elements embodied in the network 3% so that a substantially perfect balance is obtained when the substation B is connected through to a called line having average impedance characteristics, both with respect to the line itself and the circuit elements of the substation connected thereto, to obtain a sufficiently accurate balance of the anti-side-tone circuit, when connected to other lines, to prevent any substantial portion of the audio frequency energy developed by operation of the transmitter it from being reproduced by the receiver 28 or loud speaker 21 provided at this substation.

While it will be understood that the circuit specifications may vary according to the requirements of a particular system, the following specification of circuit constants for the network it are satisfactory when the network is included in a substation forming a part of a private auto matic telephone system wherein the substation lines are short.

Resistance 66 50 ohms Resistance 69 400 ohms Resistance "Ill ohms Condenser 6i 1 microfarad Condenser E8 .04 microfarad Condenser H 1 microfarad. Impedance Zb 39 microhenries,

85 ohms D. C. resistance While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a telephone system including a first substation having a pair of line terminals adapted to be connected by way of a telephone line to another substation, and wherein the equivalent irnpedance, measured at said terminals, of impedances external to said first substation, is substantially determined by the impedances of the circuit elements included in said other substation; an anti-side-tone circuit included in said first substation and comprising a network for balancing the equivalent impedance across said terminals, said network comprising a plurality of impedance elements having impedances individually corresponding to certain of the component impedances making up said equivalent impedance, and said plurality of impedances being so proportioned relative to their corresponding component impedances and being so connected and arranged that said anti-side-tone circuit is substantially balanced at all frequencies within the operating frequency range.

2. In a telephone system including a first substation having a pair of line terminals adapted to be connected by Way of a telephone line to another substation, and wherein the equivalent impedance, measured at said terminals, of impedances external to said first substation, is substantially determined by the impedances of the circuit elements included in said other substation; an anti-side-tone circuit included in said first substation and comprising a network for balancing the equivalent impedance across said terminals, said network comprising a plurality of impedance elements having impedances individually corresponding to certain of the component impedances making up said equivalent impedance, and said plurality of impedances being so proportioned relative to thelr corresponding component impedances and being so connected and arranged that the frequency-impedance characteristic of said network is substantially similar to the frequency-impedance characteristic of said equivalent impedance.

3. In a telephone system including a first substation having a pair of line terminals adapted to be connected by way of a telephone line, the line connecting equipment of an exchange and a second line to another substation, and wherein the equivalent impedance, measured at said terminals, of impedances external to said first substation, is substantially determined by the impedances of the coupling elements embodied in said line connecting equipment and the impedances of the circuit elements included in said other substation; an anti-side-tone circuit included in said first substation and comprising a network for balancing the equivalent impedance across said terminals, said network comprising a plurality of impedance elements having impedances individually corresponding to certain of the component impedances making up said equivalent impedance, and said plurality of impedances being so proportioned relative to their corresponding component impedances and being so connected and arranged that said anti-side-tone circuit is substantially balanced at all frequencies within the operating frequency range.

4. In a telephone system including a first substation having a pair of line terminals adapted to be connected by way of a telephone line, the automatic switching equipment of an automatic exchange and a second line to another substation, and wherein the equivalent impedance, measured at said terminals, of impedances external to said first substation, is substantially determined by the impedances of the coupling elements embodied in said automatic switching equipment and the impedances of the circuit elements included in said other substation; an anti-side-tone circuit included in said first substation and comprising a network for balancing the equivalent impedance across said terminals, said network comprising a plurality of impedance elements having impedances individually corresponding to certain of the component impedances making up said equivalent impedance, and said plurality of impedances being so proportioned relative to their corresponding component impedances and being so connected and arranged that said anti-side-tone circuit is substantially balanced at all frequencies within the operating frequency range 5. In a telephone system including a first substation having a pair of line terminals adapted to be connected by way of a telephone line to another substation, and wherein the equivalent impedance, measured at said terminals, of impedances external to said first substation, is substantially determined by the impedances of the circuit elements included in said other substation. at least two of which are of the reactance type; an anti-side-tone circuit included in said first substation and comprising a network for balancing the equivalent impedance across said terminals, said network comprising a plurality of impedance elements having impedances individually corresponding to certain of the component impedances making up said equivalent impedance, at least two of said impedance elements being of the reactance type and individually cor- L responding to diiferent ones of said two circuit elements, and said plurality of impedances being so proportioned relative to their corresponding component impedances and being so connected and arranged that said anti-side-tone circuit is substantially balanced at all frequencies within the operating frequency range.

6. In a telephone system including a first substation having a pair of line terminals adapted to be connected by way of a telephone line, the line connecting equipment of an exchange and a second line to another substation, and wherein the equivalent impedance, measured at said terminals, of impedances external to said first substation, is substantially determined by the impedances of the coupling elements embodied in said line connecting equipment and the impedances of the circuit elements included in said other substation, at least two of which are of the reactance type; an anti-side-tone circuit included in said first substation and comprising a network for balancing the equivalent impedance across said terminals, said network comprising a plurality of impedance elements having impedances individually corresponding to certain of the component impedances making up said equivalent impedance, at least two of said impedance elements being of the reactance type and individually corresponding to different ones of said two circuit elements, and said plurality of impedances being so proportioned relative to their corresponding component impedances and being so connected and arranged that said anti-side-tone circuit is substantially balanced at all frequencies within the operating frequency range.

'7. In a telephone system including a first substation having a pair of line terminals adapted to be connected by way of a telephone line, the line connecting equipment of an exchange and a second line to another substation, and wherein the equivalent impedance, measured at said terminals, of impedances external to said first substation, is substantially determined by the reactance of the coupling elements embodied in said line connecting equipment, the resistive impedance of the transmitter at said other substation and the reactive impedance of the induction coil and receiver at said other substation; an anti-side-tone circuit included in said first substation and comprising a network for balancing the equivalent impedance across said terminals, reactive impedance elements included in said network and corresponding to said coupling elements, said induction coil and said receiver, and a resistive impedance element iniii) cluded in said network and corresponding to said transmitter, said elements of said network being so proportioned relative to the corresponding elements of said other substation and being so con nected and arranged that said anti-side-tone circuit is substantially balanced at all frequencies within the operating frequency range.

8. In a telephone system including a first substation havin a pair of line terminals adapted to be connected by way of a telephone line, the line connecting equipment of an exchange and a second line to another substation, and wherein the equivalent impedance, measured at said terminals, of impedances external to said first substation, is substantially determined by the reactance of the coupling elements embodied in said line connecting equipment, the resistive impedance of the transmitter at said other substation and the reactive impedance of the induction coil and receiver at said other substation; an anti-side-tone circuit included in said first substation and comprising a network for balancing the equivalent impedance across said terminals, reactive impedance elements included in said network and corresponding to said coupling elements, said induction coil and said receiver, a resistive impedance element in said ne wcrk and corresponding to said transmitter, and additional impedance elements in said network individually corresponding to 'the lumped series resistances of said first and second lines and to the lumped shunt capacitance of said first and second lines, said elements of said network being so proportioned and being so connected and arranged that said anti-side-tone circuit is substantially balanced at all frequencies within the operating frequency range.

9. In a telephone system including a first substation having a pair of line terminals adapted to be connected by way of a telephone line, the line connecting equipment of an exchange and a second line to another substation, and wherein the equivalent impedance, measured at said terminals, of impedances external to said first substation, is substantially determined by the impedances of the coupling elements embodied in said line connecting equipment and the impedances of the circuit elements included in said other substation; an anti-side-tone circuit included in said first substation and comprising a network for balancing the equivalent impedance across said terminals, said network comprising impedance elements individually corresponding to said coupling elements and to the circuit elements at said other substation, and additional impedance elements in said network individually corresponding to the lumped series resistance of said first and second lines and to the lumped shunt capacitance of said first and second lines, said elements being so proportioned and being so connected and arranged that said anti-side-tone circuit is substantially balanced at all frequencies within the operating frequency range.

10. In a telephone system including a first substation having a pair of line terminals adapted to be connected by way of a telephone line, the line connecting equipment of an exchange and a second line to another substation, and wherein the equivalent impedance, measured at said terminals, of impedances external to said first substation, is substantially determined by the impedances of the coupling elements embodied in said line connecting equipment and the impedances of the circuit elements included in said other substation; an anti-side-tone circuit included in said first substation and comprising a network for balancing the equivalent impedance across said terminals, a transformer included in said anti-side-tone circuit and including a plurality of windings, a sound reproducing device coupled to one of said windings, and a transmitter included in said anti-side-tone circuit, said network comprising impedance elements individually corresponding to said coupling elements and to the circuit elements at said other substation, said elements being so proportioned and being so connected and arranged that said anti-side-tone circuit is substantially balanced at all frequencies within the operating frequency range, whereby substantially no audio frequency energy is delivered to said reproducing device during operation of said transmitter.

11. In a telephone system including a first substation having a pair of line terminals adapted to be connected by way of a telephone line, the line connecting equipment of an exchange and a second line to another substation, and wherein the equivalent impedance, measured at said terminals, of impedances external to said first substation, is substantially determined by the impedances of the coupling elements embodied in said line connecting equipment and the impedances of the circuit elements included in said other substation; an anti-side-tone circuit included in said first substation and comprising a network for balancing the equivalent impedance across said terminals, a transformer included in said anti-side-tone circuit and comprising a plurality of windings, a sound reproducing device coupled to one of said windings, a transmitter included in said anti-side-tone circuit, said network comprising impedance elements individually corresponding to said coupling elements and to the circuit elements at said other substation, and additional impedance elements in said network individually corresponding to the lumped series resistance and the lumped shunt capacitance of said first and second lines, said elements being so proportioned and being so connected and arranged that said anti-side-tone circuit is substantially balanced at all frequencies within the operating frequency range, whereby substantially no audio frequency energy is delivered to said reproducing device during operation of said transmitter.

12. A balancing network adapted for use in a telephone substation circuit of the anti-side-tone type whichcomprises, in combination, an artificial receiver having a substantially non-linear frequency-impedance characteristic simulating in form the frequency-impedance characteristic of an actual receiver, an induction coil, a resistor having a resistance substantially approximating the average effective resistance of a transmitter, and additional impedance elements corresponding to the reactive and resistive impedances of two connected telephone lines interconnecting said substation with a distant substation, said artificial receiver, induction coil, resistor and additional impedance elements being so related and the impedances thereof being so proportioned that the equivalent impedance thereof substantially matches the equivalent impedance of said lines and the elements provided at said distant substation at all frequencies within the operating frequency range.

HAROLD CHARLES PYE.

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