US1922627A - Two-way radio communication system - Google Patents

Description

Aug. 15, 1933. R. c MATHES TWO-WAY RADIO COMMUNICATION SYSTEM Filed Nov. 1, 1924 ANZ;

J T h M H M w I .I J T m .k w 2% H N M PM W W W Z 0 fi T "owl? FFL W. m R a u N2 3 H1 5 M M A A .I!!. 1.5L

Patented Aug. 15, 1933 TWO-WAY RADIO COMMUNICATION SYSTEM Robert C. Mathes, Wyoming, N. J., assignor to Western Electric Company, Incorporated, New York, N. Y., a Corporation of New York Application November 1, 1924. Serial No. 747,164

"9 Claims.

This invention relates to two-way radio communication systems and especially to four-wire systems constituted by the combination of a land line and a radio link.

One object of the invention is to provide improved methods of and apparatus for operating four-wire circuits. The methods of this invention make possible the operation of such systems with a substantial avoidance of echo and singing effects and with markedly greater simplicity and lower cost, than results from the use of means heretofore known.

Another object of the invention is to adapt the principles governing the eflicient operation of a four-wire land line to the operation of a four-wire circuit comprising the combination of a land line and a radiolink, and to realize therein the advantages usually accruing to four-wire operation.

A four-wire circuit, as defined for present purposes, is a two-way circuit in the operation oi which the transmission in one direction for the greater portion of the distance between two points geographically remote from each other is carried on over a diii crent path from transmission in the other direction. Such a circuit is distinguished from a two-wire circuit the operation of which is characterized by the use of the same path for transmission in both directions for the greater portion of the distance between the two points.

In the operation of four-wire and two-wire circuits it is desirable that the circuit be operative for transmission in only one direction at a time. The purpose in sodoing is to avoid singa ing and echo effects.

The nature of singing and echo phenomena is well understood as they exist in fourwire and twc-wire circuits and will not require exposition 40 here, reference being made to a paper by A; B.

Clark entitled Telephone transmission over long cables (Journal of the A. I. E. E., January 1923) whereinthe subject of echo phenomena is treated. A difference in. the conditions aiiecting these phenomena in tour-wire and two-wire circuits, and which largely determines the measures required for their prevention, relates to the dinerence in the time required for the propogation of the singing or echo disturbances'through their round-trip paths, on account of the diiference in the types and lengths of suchpaths. (In the case of two-wire circuits the practical case is assumed in which singing and echo currents circulate in the four=-wire local circuit at repeater points between two adjacent line sections.)

are illustrated in this application two forms of gression of the initial signal impulse around the singing or echo path being suflicient to insure the completion of the switching operation by the time the echo currents are incident on the receiver.

A similar operation can be performed at a repeater station in a two-wire circuit by inserting a delay circuit" in each branch of the local four-wire circuit, ahead (in the direction of transmission) of the points whence the relay circuits are derived.

These delay circuits are defined sufficiently by a statement of their function. They must delay the transmission by the required amount and in doing so must not impress on the transmitted currents any other eiiect, although particular designs of delay circuit may, for example, be used to give a delay which is different for the different frequencies in a band and therefore to compensate for an equal and opposite delay which is already present in the transmissions and which is an inherent characteristic of transmission circuits generally. Such a delay circuit is disclosed and claimed in Patent 1,633,6l5,granted June 28, 1927. Band pass filters having a suflicient number of sections (as many'as 100 have been required in known instances) may be used. There mechanical (or acoustical) delay circuits in which electrical energy is transformed into mechanical energy and retransformed into electrical energy, the delay occurring during the mechanical phase.

In an alternative operation of a two-wire cir cuit the two branches may normally be in an inoperative condition and signaling currents may be used to actuate switching means to make its own branch operative. Under this operation the delay circuits are positioned as before but their use is required for a difierent reason, that is, to prevent clipping of the first part of the signal transmission while that branch of the circuit is being conditioned for transmission.

The system in which the present invention has its most effective embodiment is a four-wire circuit of a unique type. In accordance with one practical application for which it has been suggested it may be assumed for convenience of delink and the land line.

scription that it comprises a radio link connecting Rocky Point, Long Island and London, England and a low frequency land line connecting Rocky Point and New York city. The radio link obviously does not interpose any substantial delay to signal transmission. The land line interposes some delay, but on account of its relatively short length, not enough to simulate the function of the usual four-wire land circuit as above described. though of the four-wire type, approximates, so far as impedance characteristics are concerned, the type of four-wire circuit used at local repeater circuits of a two-wire circuit. However, the presence of the relatively short land line portion, to the extent that it may be made use ful, differentiates the circuit from both fourwire and two-wire circuits of known type and from any combination of them.

In the method of operation of such a circuit as provided by this invention, the switching means are located at the junction of the radio The west branch (the branch used for westward transmission) is normally operative and is rendered inoperative by eastward signal transmission, a delay circuit b ing inserted to insure that the switch which short circuits the west branch isactuated before the return current is incident on the part of that branch where the short circuit occurs. The east branch is normally inoperative and is made operative by the eastward transmission at the same time that the west branch is made inoperative, and by the same relay control circuit. The

delay circuit accordingly functions both as above described and also for preventing the clipping of the eastward signals. The westward transmis sion is concerned in the switching function only to the extent that it, through a similar relay control circuit, open-circuits, the other relay control circuit so as to prevent current that passes through the balance circuits connecting thefourwire with the terminal two-wire circuit from energizing that control circuit to reorganize the circuit as awhole for transmission. The land line introduces sufficient delay to enable a timely performance of this function although it might not introduce sufiicient delay to enable the received currents to render the transmitting branch inoperative for transmission by closing a short circuit, if it were attempted to operate the circuit as in conventional four-wire practice, without the use of elaborate circuit means to'insure quick action. 1

A radio circuit of this type is subject tofurther difiiculties of operation, as compared with an all wire circuit, which makes it desirable to maintain a breakin the roundtrip singing path as is here done by keeping the eastward branch inoperative. This'is due to the phenomena of fading whereby the loss in the radio part of the circuit is subject to great variations. If both branches are maintained operative at the best gain for a certain condition of the intervening space, a diminution of the transmitting loss would result in the circuit singing. I

The above and other objects and features of the invention will become apparent by a reading of the following detail description in connection with the accompanying drawing.

Fig. 1 of which illustrates a preferred embodiment of the invention, as applied to a four-wire circuit of the type described above and 7 Figs. 2 and 3 illustrate forms of mechanical Accordingly, the circuit, al

later.

Ali/I5 and AMe.

delay circuits that may be used in the system of Fig. 1.

Fig. 4 illustrates a form of an electrical delay circuit.

Referring to the Fig. 1 L1 and L2 indicate low frequency lines con ected, for example, to central telephone exchanges. ln'the particular circuit that will be assumed for convenience, L1 is connected to a central station in New York city and L2 to a central station in London. The

circuits between these lines are adapted to transmit signals in both directions over individual paths,'partly by wire and partly by radio. The radio link comprises the portions between antennae AN1 and ANZ for the east branch of the four-wire circuit and between antennae ANs and AM for the west branch of the four-wire circuit. Antennae Arh and AN3 in the practical embodiment assumed are located at or near Rocky Point, Long Island, and antennae ANz and AN4 are located at London, England. The circuits at the east station connecting the antennae with the low frequency line comprise, in the receiving portion, the receiving circuit RC1 and amplifier AM1, the corresponding circuits for the transmitting portion comprising transmitter T1 and amplifier AMz. These transmitting and receiving portions of the circuit are joined to the low frequency line L2 by means of triple Winding transformer or hybrid coil arrangement H1 which cooperates with line L2 and with balancing network N1 in a well-known manner to insure conjugacy between the transmitting and receiving portions. The amplifiers AM1 and Alvis may be used or not as desired. Expediency will justify their use in the usual practical case. The transmitted T1 and receiver RC1 embody combinations of con ventional elements for respectively generating and transmitting a signal modulated high frequency wave and for demodulating an incoming signal modulated high frequency Wave to reproduce the signal. The antenna ANZ and AN; are geographically or geometrically related to each other in such manner that local singing at that station is prevented. This effect may be aided by the use of different frequencies for the two directions of transmission and the frequency selection at the antennae made possible thereby.

The circuits at the west station between the antenna AN1 and ANs and the low frequency line L1 comprise a low frequency transmission path of several miles in length. The transmitting and receiving portions of this path are ,.conjugately related to low frequency line L1 by flow therethrough or it may be of the mechanical type illustrated in Figs. 2 and 3 and described The criteria for the eiiicient accomplishment of this function will be described later. The transmission circuit for receiving at this station may similarly comprise the amplifiers Antennae AN1 and AN3 are so related to each other, by any of the means referred to in the description of the east station circuits, as to prevent singing at the west station. For example, one or both of these antennae may be made directive in such a direction as to exclude reaction on or from the other antenna. As a practical matter it will also be found expedient to position these antennae amile or more apart. Amplifiers AM4 and AMs may conveniently be located at their respective antennae. Amplifiers AMs and AM5 may together constitute a four-wire repeating station located intermediate the antenna: and the low frequency line L1.

The provision against local singing at the two stations insures that the system as a whole func tions as a unitary four-wire line and radio circuit.

The west branch of the four-wire circuit is normally operative but is made inoperative by actuation of the switch 1 by relay R1. The cast branch of the four-wire circuit is normally inoperative by reason of the short circuit provided by switch 2 which may be actuated to opencircuit position by relay R2. For efficient operation of such a four-wire circuit it is desirable that only that branch be operative for transmission which is being employed at the moment. This insures that the round-trip path for singing or echo currents is at all times interrupted. The means for actuating the switches 1 and 2 so as to satisfy this requirement will now be described.

During transmission east, signal currents flow through amplifiers AM3 and AM4 and thence through delay circuit D to the radio transmitter. A relay control circuit 3 is derived from this branch at the input of the delay circuit. Signal currents flowing through this control circuit are amplified and rectified by device AMR and the resultant currents flow through and actuate relays R1 and R2. Relay R1 actuates switch 1 to circuit closing position and accordingly renders the west branch inoperative. Relay R2 actuates switch 2 to open circuit position and accordingly renders the east branch operative.

The delay circuit D insures that the actuation of switch 1 will have been completed before echo currents transmitted to and returned from the east station have progressed to the points of connection of the circuit containing switch 1. This delay circuit also insures that the switch 2 will have been actuated to render the east branch operative before the initial signal impulses reach the output of the delay circuit, so that clipping is prevented. Since it requires less time to open switch 2 than it does to close switch 1, the design of a delay circuit to satisfy the first condition would ordinarily be effective for satisfying the second condition. This obviously would not be true if the west branch were made inoperative by a circuit opening operation, as would be possible by a simple rearrangement of the circuits. However it has been found expedient in a practical case to use the circuit closing alternative disclosed. The amplifier rectifier AME may comprise any means for operating on the signal currents tov obtain a wave which can efficiently operate relays R1 and R2. In a practical case it would comprise a combination of amplifier and rectifier elements, preferably of the vacuum tube type. An example of an arrangement well adapted for the purpose is disclosed in Patent 1,588,186, granted June 8, 1926. The switches l and 2 are biased, by means not shown, so as to return respectively to their open and closed positions when not actuated by the relays R1 and R2.

During west transmission the detected signal currents will find an uninterrupted path to the low frequency line L1. Since the east branch is normally inoperative, the currents which pass through the hybrid coil H2 cannot result in singingor echo phenomena in the four-wire circuit as a whole. However, in order to insure that such currents cannot enter control ,circuit 3, which would reorganize the circuits for eastward transmission and accordingly produce a periodical vibration of the switches l and 2 with an attendantinterruption of the received signals and a tendency to singing and echo phenomena, a portion of the received signal currents interrupts this relay control circuit by opening the switch 4. This is accomplished by relay R3 in relay control circuit 5. Amplifier rectifier AMRl in this circuit has the same function as that of AMR in circuit 3. The delay in transmission over the land line from the output of amplifier AMG to the output of amplifier AM4 insures that switch 4 will be opened before these currents reach the output of amplifier AMl.

This delay would not ordinarily be sufiicient if it were attempted to utilize the received signal currents to actuate a circuit closing means to render the east branch inoperative or at least it would not be sufficient without the employment of more elaborate means than would be required for circuit opening. Therefore, if it were attempted to operatethis circuit, to prevent singing and echo phenomena, in the conventional manner of 22-type repeaters in a two-wire circuit, it would be necessary to insert a delay circuit in the west branch ahead of the points from which the relay control circuit 5 is derived. Therefore in accordance with this invention a situation, which in accord ance with the prior art would have to be met by the use of two delay circuits, is equally well met by the use of a single delay circuit, a'novel fea-- ture incident to the use of the single delay circuit being a biased type four-wire circuit, that is, a four-wire circuit in which one portion is normally operative and the other portion is normally inoperative. The basic principle of the invention is applicable also to a circuit having no inherent delay characteristics, as in a 22 type repeater, by an arrangement substantially as described but in which a delay circuit of relatively few sections replaces the low frequency Such an arrangement would be an land line. improvement over the conventional two delay circuit arrangement in that the delay accomplished by the second delay circuit would need to be very small, and the cost of' the delay circuit correspondingly very low, in comparison with the other delay circuit.

The general nature of delay circuits has been pointed out above. Specific types of such circuit adapted to be used in the system of Fig. 1 will 1 tiesforce, displacement, velocity, mass, stiffness Corresponding to each electrical and friction. system there may be conceived a mechanical system constituted by mechanical elements having analogous properties and combined to produce analogous results.

In electrical phenomena a time relay is a phase the second inductance element.

change of relatively higher order than occurs in simple reactive networks. Therefore, itis reasonable to expect an electrical delay circuit to consist of a reiterative series of such simple networks so as to repeat and continue the phase shifting effect of each unit through successive units to eventually obtain any desired delay. Obviously, in order to produce a periodic repetition of the effect of a single unit both magnetic and electrostatic storage elements, that is, both inductance and capacity, are required. Such an expedient carried out in a systematic manner to give a uniform sheet for a band of frequencies results in a structure that may be identical with conventional attenuation type filter structure,

except that it has, inthe practical case, many more sections. In fact, the delay principle is inherent in the operation of such filters and is necessary to the development of their theory of operation. 7

Fig. 4 illustrates a simple typeoifilter, a lowpass filter, which is adaptable to use as a delay circuit if provided with a sufficient number or" sections. The-theory of the operation of this and other types of filter is developed in U. S. patent to Campbell 1,227,113, granted May 22,1917. The physical principle involved is explained in an article by Campbell in the Bell System Technical Journal for November 1922 entitled Physical theory of the electric wave filter, wherein it is shown on pages 10 et seq. that transmission through the filter is attended by a progressive change of phase from section to section for each frequency component. If a certain amount of magnetic energy is stored in the first inductance element, it discharges into the first shunt condenser, building up a counter E. M. F. therein. A time which is determined by the time constant of one section, comprising the combination of one inductance and one capacity, is required for this operation. When the condenser has received its maximum charge, it discharges into This cycle is repeated for each section, the total delay accordingly being determined by a choice of the number of sections.

Fig. 2 illustrates a type of mechanical delay circuit which is the analogue of the electrical circuit of Fig. i, the mass of elements M and the stiffness of elements Sbeing analogous respectively to the inductance of the coils and the capacity of the condensers of Fig. i. The design data pertaining to electrical delay circuits as pointed out in the Campbell references above mentionedare equally applicable to the mechanical delay circuit of Fig. 2, having regard for the differences in units and conversionfactors. For ideal conditions simulating those existing in..a resistanceless electrical filter circuit made up of lumped inductances and capacities each of which is completely individual in its function, the massesmust be assumed to have zero dimensions, the stiffness elements, for example, springs, must be assumed to have no weight, pendant means P must be assumed to he perfect- 1y flexible and there must be no frictionanywhere in the system. Conversion from electrical to mechanical energy and back to electrical energy is accomplished respectively by the electromagnetic receiver device R and the microphone transmitter device T, which function as in accordance with similarly denominated devices in systems of telephone transmission The first mass element at the left is impulsed by the receiver diaphragm. In a manner entirely analogous to the operation of the end section of the network of Fig. 4, the energy is absorbed in actuating the mass into movement. After an; interval determined by the time constant of the first section comprising a mass and a stiffness element, this energy will have been stored in the mass and then expended in storing up an equivalent amount of energy in the first stiffness element. This stiffness element, the first spring, in turn discharges this energy into the second mass element, and soon throughout the transmission path. It is substantially immaterial so far as the physical principle is concerned whether the mecahnical energy transfers occur in a single plane including the several pendant means ,P or in planes transverse thereto. The number of sections, in a practical case, would be many times as great as that shown.

Fig. 3 illustrates an acoustical delay circuit. The receiver and transmitter elements are the same as in Fig. 2. Intermediate transmission is by means of acoustic waves directed and regulated by the sound conduit 0 as in accordance with conventional practice in the construction of the horn elements in acoustic transmitters and receivers. The delay in this type of transmitting medium is inherent in the material of the medium, although if the elem nts of this medium were reduced to molecular dii -ensions it would be obvious that the conditions inherent in the op eration of the arrangement or Fig. 2 are closely paralleled. In common in the two systems, transmission effected by wave motion, the velocity of propagation being determined by the mass (or density) and the stiffness or elasticity) characteristics of the medium. In the arrangement of fig. 3 the desired delay is regulated solely by vary ng the length of the path since the characteri tics of the medium are fixed. In the arrangement of Fig. this delay is correspondingly regulated both by the length of the path and the character of the medium, since the transmission characteristics of the medium are subject to adjustment.

, Having described the nature of the invention and its mode of operation the features believed to be novel are set forth in the appended claims.

What is claimed is:

1. A two-way signaling system comprising two paths adapted for transmission in opposite directions between two stations, means connecting said paths in one 'gy flow relation at each station, and automatically operative means including a single delay circuit, inserted in the transmission path corresponding to one direction, to the exclusion of a similar delay circuit in the other path or elsewhere, for preventing round-trip transmission of echo and singing currents.

2. A two-way signaling system comprising two paths adapted for transmission in opposite directions between two stations, a circuit connected in energy flow relation with each of said paths at each station, means in one path to render it normally inoperative for transmission, means in the other path for rendering it normally operative for transmission, means whereby energy impressed on the first mentioned path renders the normally inoperative path operative for transmission and the normally operative path inoperative for transmission, and a single delay circuit, which is inserted the first mentioned path, to the ex clusion or a similar delay circuit in the other path or elsewhere, adapted to delay the roundtrip transmission of echo currents until said operative path has been made inoperative.

3. A two-way signaling system comprising two paths adapted for transmission in opposite directions between two stations, means connecting said paths in energy flow relation at each station, means in one path to render it normally inoperative for transmission, means in the other path for rendering it normally operative for transmission, means whereby energy impressed on the first mentioned path renders the normally inoperative path operative for transmission and the normally operative path inoperative for transmission, a delay circuit adapted to delay the round-trip transmission of echo and singing currents until said operative path has been made inoperative, and means whereby energy impressed on the second mentioned path renders the last mentioned means inoperative, whereby energy in the first mentioned path derived from the second mentioned path cannot cause an undesirable reorganization of circuits, said last two mentioned means being separated by a sufiicient length of transmission circuit including portions of both of said paths to delay the transmission of said derived energy, without the use of other delay means, until the last mentioned means has operated.

4. A four-wire radio signaling system comprising means normally rendering one branch operative for transmission, means normally rendering the other branch inoperative for transmission, signal responsive means for substantially simultaneously rendering said normally operative path inoperative and said normally inoperative path operative for transmission and a single delay circuit, said delay circuit being the only one used in the entire four-wire system, posititined in the normally inoperative branch for preventing round trip transmission of echo currents over the loop constituted by said fourwire circuit during the operation of said signal operated means.

5. The system of claim 4 in which the signal responsive means is responsive to signal transmission over the normally inoperative path.

6. In combination a two-wire circuit, a second two-wire circuit, a four-wire circuit between said circuits comprising a wire portion and a radio link, biased means normally rendering one branch of said four-wire circuit operative, biased means for normally rendering the other branch inoperative, a circuit in shunt to the normally inoperative branch, said. circuit containing relay means for actuating each of said biased means to substantially simultaneously render the said normally inoperative circuit operative and said normally operative circuit inoperative for transmission, the combination as a whole including only one delay circuit, said delay circuit being positioned in said normally inoperative branch between said shunt circuit and the biased means, said delay circuit being adapted to delay the transmission to each of said biased means until they have respectively been actuated by said relay means.

7. The system of claim 6 in which the wire portion is a low frequency transmission line and the radio link comprises in each branch means for transforming the impressed low frequency currents to a high frequency wave modulated by the low frequency currents, and for retransforming it back to the low frequency currents at the other end of the radio link.

8. In combination a two-wire low frequency circuit, a second two-wire low frequency circuit, a four-wire circuit between said circuits comprising in one branch in series in the given order, a low frequency transmission line, a delay circuit, a radio transmitter and a radio receiver, and in the other branch, symmetrically with the elements of the first branch, a second low frequency transmission line, a radio receiver, and a radio transmitter; a shunt circuit containing a switch normally biased to closed position across the output of the delay circuit, said delay circuit being in the normally inoperative path and also being the only one used in the entire four-wire circuit, a shunt circuit containing a switch normally biased to open position across the input end of the second low frequency transmission line, a relay control circuit in shunt to the first low frequency transmission line substantially at its output end, a relay control circuit in shunt to the second low frequency transmission line substantially at its input end, the first mentioned relay control circuit comprising relay means for actuating both the said switches to their respectively alternate positions and a switch normally biased to closed position, and the second mentioned relay control circuit containing relay means for actuating the switch in the other relay control circuit to open position.

9. In combination a two-wire circuit, a second two-wire circuit, a four-wire circuit between said circuits comprising a wire portion and a radio link, means normally rendering one branch of the four-wire circuit operative for transmission and the other branch normally inoperative for transmission, signal responsive control means for substantially simultaneously rendering said normally operative branch inoperative and the normally inoperative branch operative for transmission, a delay circuit in the normally inoperative branch .posterior to the connection of said control means thereto, said delay circuit being the only one used in the entire four-wire circuit, and a second control means for controlling the operation of the first control means, said second control means utilizing the delay inherent in the propagation of signals along said wire portion of the four-wire circuit.

ROBERT C. MATHES.

Images