US1829806A - Transmission control - Google Patents

Description

Nov. 3, 1931.

R. c. MATHEs TRAN'SMISS'ION CONTROL 3 'Sheets-Sheet l Filed June ll. 1929 5 Sheets-Sheet 3 Sudam, S w H R. C. MATHES "TRANSMISS ION CONTROL Filed June 1l Nov. 3, 1931.

Patented ov. 3, 1931 e UNITED ySTATES .PATENT OFFICE ROBET O. MATHES, l' WYOMING, NEW

JERSEY, .ASSIGNOR TO BELL TELEPHONE' LABORATORIES, INCOPORAT'ED, OF NEW YOBI, N. Y.; A COBPORATIIIONKOF NIW YORK 'Application mea :une n,

This v invention relates to electric wave transmission systems, particularly to twowa'y electric communication systems.

The invention inits broad aspect-relates I6' to the controlling of transmission in a signal transmission system by control lenergy transmitted over the system under control of signals, and in advance of the signals, to opery ate circuit-control means at one or vmore 10 points in the system.

An object of the invention is improve the operationofa signal transmlssion system including circuit-control means operating under control 'of the signals transmlttedlover the system. l

The invention is especially applicable. to a system over which two-way transmlsslon of signaling energy is required withln the same frequency range, and mcludmg as a link therein a long section of two-way cable of high attenuation, such as a dee sea submarine cable. Signal-controlled circuit-control ap aratus,"for example, so-called slngm and ec o Suppressors, are usually assoclate with the oppositely directed ampllfymg paths at each terminal4 of the cable 1n such a system to insure that the stem 1s operative to.- transmit in only one dlrection at a tune, so as to effectively suppress echoes, singing or both. This is usually accomplished by mak ing the circuit-control apparatus at each ter- -m1nal responsive to 4transmission 1n one direction to'make the circuits thereat operative foi-transmission in that direction and to ef-.

fectively disable the .circuits `for transmission in the opposite direction.

There are two main characterlstics of such a system, the combination of which mayY introducev diiiculties in the application of such .signal-controlled, circuit-control apparatus thereto. One of these is the increased time of transmission over the cable due to its length and loading, and the other is the high att'enuation of the cable. Theincreased time 02E-transmission over the cable atly increases the chance that two subscri ers at .the ends of the system may .both start talking within the time interval.

equal to the transmission time of the cable, lresulting ,1n each subscriber 'by'means of his frmnsuissroN cominci.

1929.v serial No. 370,035.

,of a'v sustained alternating current wave,

called the courier, under the control of the speech waves, Afrom either terminal over the cable or an\ auxiliary circuit ahead of the speech waves to condltion the circuits for repeating in the direction of speech wave transmission. A method of vthis general nature is adapted for use in a cable system such as described above, but must be considerably modi- 4fied because of characteristics peculiar to the ystem, such as the very high attenuation and e limited, useful frequency range andload capacity of the cable to make it practicable for commercial use from-the standpoint of quality and economy.

To operate csuch a s stem it is necessary to. have information at oth ends of the cable 80 as to when talk is going in at either end, and this information may have to be'positively received through talk going out from the other end. The direction of energy ow is of little value in giving such information as the weak speech energy incoming from the cable at one terminal may bel completely lost among the reiected components of the outgoing speech. This information is preferably carried in accordance with the invention by auxiliary vwaves otherthan the speech waves. The difficulty associated with applying such auxiliary waves to the cable simultaneously with the speech waves whose presence they are to indicate is that they will use up part of the load carrying capacity of the transmitting amplifier and, more important still, of the loady capacity of the cable, if interference through intermodulation is to be avoided.

The above mentioned dificulties in a system ma* of the ltype described have been substantially minimized` in accordance with the invention by a modification of the courier method of control. In the method of the invention, the signal energy generated at each terminal of the cable or incoming from a land line connected thereto 'is made to control the sending out over the transmission circuit ahead of thesignals, of a short train of alternating current control Waves, which will be referred to hereafter as the courier, of frequency different vfor each terminal and outside `the speech frequency range to operate circuit-y control means to establish initial control in proper time relation of the transmitting and receiving circuits at each terminal of the sys- .tem and of any intermediate repeater circuits, this control being maintained by the speech energy when it arrives at the controlling points. The rst courier sent out will determine whichterminaI or talker gets exclusive control of the system.

' One type ofterminal circuit utilizing the general method of control of the invention may be used in connection with a system h aving characteristics such that it is feasible therein to select the incoming courier in the presence of simultaneous .outgoing speech energy, such yas a system employing a short cable or one of lesser overall attenuation. Another type of circuit utilizing this general method of `control may be used with a system having characteristics such that it is not feasible therein to select the courier in the presence of simultaneous outgoing Aspeech energy, such as a system employing a very long cable or one of very high overall attenuation.

My copending application Serial No.

)370,034,4iled J une 11, 1929, is specifically directed to circuits of the former type.

This ap lication is specifically directed to circuits ofp the latter type, these circuits being arranged softhat the courier is sent out far enough Aahead of the speech so that initial control is established-for one terminal or the other before speech energy is actually applied to the cable, thus preventing any interference with the courier by out oing i stood from the followi speech energy or loss of any part o the speech. A

rlhe exact nature and the obects and advantages of the invention will be tter underdetailed description thereof, when read 1n connection with the accompanyingdrawings, in which Fig.-

l--A shows ay schematic dia am illustratingthe principle -ofthe invention a igs. 1 and-2 show circuit diagrams of diferent types of cable terminal circuits embodying the 1nvention;"and

Fig. 3 snows a clrcuit diagram of an in termediate repeater' station embodying' the invention, which repeater station may beA used to couple two sections of cable, having mission time of the cable.

rasato@ lterminal stations such as Asltown .in Fig. 2 or 3.

For convenience the invention will be described as applied to systems for the twoway transmission of speech waves, but it is apparentthat the principles thereof apply equally Well to systems for the transmission of signal Waves in general comprising-frequencies within or outside the speech frequency range, such asy waves representing' speech, entertainment programs comprising music as `well as speech, etc.

Theschematic diagram of Fig. l--A illustrating the principle of the yinvention will be first described. In the system shown in that ligure the speech wave l controlsthe sending out'of a short train of control Waves 2, which will be' referred to hereafter as the courier ahead of the speech Wave, over the cable 3 from one terminal -thereof tol the other. As

indicated in the figure, the courier 2 precedes the speech Wave 1in transmission over the cable 3 by 'a time interval T.' In the manner described in detail in connection with the circuits of the other figures, the transmitted courier 2 may be'utilized to establish initial control of switching circuits at both terminals of the cable and at intermediate repeater points to, condition the apparatus thereat for efficient transmission in the direction of the speech waves only, which control will be maintained by the speech wave 1 on its arrival at the control points. Thetime interval T is substantially equal to the full transmission time of the cable 3, so that the initial control of the switching circuits at the far terminal is established for one terminal or the other before any speech energy actually is applied to the cable.

Figs. 1 and 2 show alternative types of terminal circuits for a two-way cable system employing the general principle outlined in connection with Fig. 1-A to prevent transmission lockouts when two subscribers at op osite ends of the system both start talking within the time interval e ual to the transince the terminal stations at both ends ofthe cable system may be substantially identical in construction, except for the values of the frequencies of the courier currents transmitted from each stationand the range of some of the filters employed, itfhas been deemed sufficient to ilvlustrate the terminal station at one end ofthe system'only. the east stat-ion,-and in the following description the manner of the operation of the system will be clear from reference to the single vstation shown.

The terminal station of Fig. 1 comprises ',a transmitting circuit TA and a receiving circuit RA are ,connected to the east end of the fier 5,. The receiving circuit RA comprlses reading ,from west lto east, the band pass filter 6, the band pass filter 7 ,and the one-way receiving amplifier 8. Connected to the transmitting circuit TA between the circuit TL v and the input of delay circuit 4 is a control circuit 9 comprising the current-controlled relay-controlling device 10, and the fast-operating relay 11 and the slow-operating relay 12 connected in parallel to the output of the device 10. The normally open switch 13 connected to a generator 14 of alternating currents of frequency f2 which may be any frequency outside the speech frequency range, is adapted to be closed by operation of the krelay 11 to connect the generator 14 across the transmitting circuit TA in the output of the transmitting amplifier 5 therein. The

normally closed switch 15 also connected to generator 14 is adapted to be opened by oprelay eration of the .relay 12 to disconnect the generator 14 from the transmitting circuit TA.

Connected to the transmitting circuit TA' between the output of the delay circuit 4 and the input of amplifier 5 is a second control circuit 16 comprising the current-controlled relay controlling-device 17 andthe 18 connected to the output of the devlce 17. .y l

Connected tothe receiving circuit RA between the band pass filters 6 and 7 therein' is"a third control circuit 19 comprising the current-controlled, relay controlling device 20- and the relays 2l to 23 connected in par allel to the output of the device 20.

A normally closed switch 24 in the inputcircuitigof control device 20 in control circuit 19 is adapted to be opened by the operation of relay 18. A normally closed switch 25 in the transmitting circuit TA between the point of connection of control circuit 16 .thereto and theoutput of delay circuit `4, and a normally closed 4 switch 26 in transmitting circuitTA betweenthe connection of control circuit 9 thereto and the circuit TL arel adapted to be opened by operation of relays "21 andj 22, respectively. A normally open v ,-,sw1tch27 in receiving circuit RA between the output of 8 therein and the circuit TL is adapted to be closed by the oper-I ation of relay 23.

It is seen that the terminal circuit which has just been described is of the form which is known in the art' as semi-biased, that is, the circuit is normally operative vfor'transmission in one direction (in this case from east to west) andv transmission in the opposite direction depends upon a switching operation.

The control devices 10, 17 and 20 may be vacuum tube 'amplifier-rectifier devices of the type well known in the art or any other devices vvhicli will respond to alternating current impressed on their inputs to control the operation of a relay or relays in the output thereof.

l The one-way delay circuit 4 may be of any type forproducing a delay of the required amount, which will be specified below, in alternating currents translnitted therethrough, as for example, a low pass filter, such as disclosed in the U. S. Patents -to Campbell 1,227,113 and 1,227,114, issued Mayv being described. .The frequency f1 may be Aany frequency outside the speech frequency range and differing from the frequency f2.-

The filter at the west terminal station correspondlng to the filter 6 at the east terminal station 1s designed to pass frequencies Within the speech frequency range and the courier frequency f2.

The one-way amplifiers 5 and 8, the mechamcal relays r11, 12, 18 and 21 to 23 and the generators utilized at the east and west terminal stations, respectively, for generating courier frequencies f2 and f1' may be of any of the types Well known in the art.`

As the system operates in similar manner when telephonie transmission is set up first in the direction from east tovwest as when telephonic transmission is set up first in the opposite direction, the operationof the system for the former case need onl be described. It will be assumed that te ephonic currents for transmission from east to west are first set up at the east terminal .station (the station shown in Fig. l), and that not more than T seconds later` where T is the total transmission time of the cable TC between the east and west terminal stations,

.telephonie currents are set up at the west The `westwardly directed telephonie currents'from the sourceTL are impressed directly'upon the transmitting circuit TA. A

' small portion of the impressed currents are diverted' into control circuit 9 and operate the control device -therein which causes th'e windings of'relays 11 andy 12 to be energized. The relay 11 will quickly operate to close the switch 13 so as to connect the source 140i courier frequency f2 across the transmittingl circuit'. TA.. Slow-operating relay 12r will operate `an instant later to open the switch 15 so asfto disconnect'the generator 14 trom-thetransmitting circuit TA.' Thiswill cause a short .impulse'of current of frequency. f2, which we will refer tohereafter as the courier fg, tobe transmitted over the transmitting circuit TA. The transmitted impulse may be made .as shortyas is desired or as is necessary by the design of relays 11 and 12..

The relays 11 and 12 are so designed that they will remain operated as long as speech waves from the source TL are being su plied to the control device 10. They are designed so that they will release only aftera complete pause in a conversation and not for the usual pause between words or syllables.

The courier f2 will be impressed upon thev l l cable. TC by the hybrid coil I-I1 andtransmittedl over the. cable to the west terminal station, where it will establish initial control of the switching circuits thereat in a manner which will be described later.

The main portion of the telephonic cur- .rents impressed by the line TL upon the transmittingI circuit TA will be .passed through the one-way delay circuit 4 .being delayed therein for a time interval of T seconds, where. T is the totalftransmission time of the cable T C betweenthe east and west terminal stations. The telephone currents passed by the delay circuit 4f' will be divided between thecontrol circuit 16 and the input.

of the transmitting am 4liiier 5. `The main portion of the passed te ephone currents will be amplified by the transmitting amplier 5 and impressed by the hybrid coil 1 on th'e cable TC following the courier f2 in transmission thereover by the time interval T.

i' A small ortionof the telephonie currents passed by t e delay circuit 't willbe diverted into control path 16 and will operate the control device- 17 therein, which in turn will energize. the relay 18. The control device 17 should not operate. unless control device 1G.

has been' previously operated by the Vtelephonie currents. The v relative sensitivities of control devices 10 and 17 ,so that the latter will be operated in response to a wave transmitted vover vthe path TA only when those waves have at the point, in TA Where the con-` trol circuit 9 is connected .thereto 'an' amplitude level sufficiently highto cause operation of the device 10 in control: circuit 9. An.- other-way of accomplishing this would be to insert normally open contacts inthe input of control device 17 and to utilize an additional relay controlled b the operation of control device 10 to close t ese contacts when the device 10 is operated.

Control devices-10 andY 17 are both actuated by the telephonie currents,but with a time difference in operation of T seconds.

' The operation of relay 18 will therefore open normally closed switch 24 in the input of con- `trol device 20 in control circuit 19 substantially T seconds after the courier f2 is transmitted out over the cable TC. The switch 24.will remain open as long as speech currents are being vsubstantially continuously.

transmitted from the east terminal, and will revent courier f1 transmitted ,over the cable TG from the West terminal station in response to the West to east telephonie currents, after'the courier f2 is sent out from the east station, from reaching the input of the control device 20 which controls relays 21 to 23. The relays 21 to 23 cannot be operated, therevfore, by courier f1 or the following speech waves, respectively, to disable `the .transymitting path TA by o'pening switches 25 and 26-therein and to make operative the receivv ing path RA by closing switch 27. In the manner described the westwardly directed.

telephone currents at the east terminal have obtained complete .control of the transmit- .ting and receiving circuits at the east teratthe east terminal, but followed by a prime mark.

The courier f2, which 'is transmitted over the cable TC in advance of thewestwardly directed speech waves by T seconds, on arrival at the west terminal station is impressed 0n the receiving circuit RA thereat by hybrid 'coil Hi. `The received courier f2 will be passed by the band ass filter 6 and then,

because the band passy lt-er 7 in the in'ain rel in, thus causing the winding'oi relays 21 to 23. to be energized. .y

,Relay-23 willoperate to close the. normal ly open switch/"27' in receiving path RA7 andthus make' that path operative for trans- `n1ission of the east to west speech waves from cable TC to thecircuit TL .15; Relays 21 and 22.' will simultaneously operate to interrupt 13 i iis .ceiving path RA will not v'ass the frequency. 120 f2, it will bediverted into t e control path 19" and initially operate control device 2Q there- 35 of relay 52.

45 operation.

' 10 of filter 35 therein is a third control circuit 49 comprising th current-controlled relay controlling-device 0, and the relays 51 to 53 connected in parallel to the output of con'- trol device 50.

Connected between the input of dela circuit 31 and the output of the transmitting amplier 33 in transmitting circuit TA is a byvpass circuit 54 com rising the yfilter 55 and the one-Way ampli er 56.

4 A'normally closed switch 57 located in the receiving circuit 'RA between the output of bandpass filter 34 and the point of connection of control circuit 49 to receiving circuit RA is adapted to be opened by the operation of relay 48. A normally closed switch 58 located in transmitting circuit TA .between the hybridv coil l-I2 andthe point of connection of control circuit 37 to transmitting circuit TA is adapted to be opened by 'operation of relay 51. Another normally closed switch v59 connected in transmitting circuit TA between the hybrid coil H1 and the outputs `of'transmitting amplifiers 33 and 56' is adapted to be opened by operation A normally closed switch 60 located in control path'46 in the input of control device 47 therein is adapted to be openedv by operation of relay 53.

The terminal-,circuit which has just been 40'. described is also of the semi-biased type, but

in this case the circuit is normally operative for` transmission in 4thevdirection from west to east, and vtransmission in the direction from east to westvdepends upon a switching The control lthe circuit of Fig. 1,.

The delay circuits 30 and The delay circuit 30 is designed to provide a delay of t seconds in .the transmission of the alternating rcurrents ytransmitted therel through where tis very small in comparison 55 to the totalltransmission time T of the cable TC between the eastand west terminal sta'- tions. The delaycircuit'31 is designed to introduce a delay of T seconds in the transmission of alternating currents passed there- '69 through.

The filters 32, 34, and 55 are'similar to the filters in the circuit of Fig. 1. The fil- . ters 32 and 35 are designed to pass frequencies within the speech frequency range and *J to suppress courier frequencies fland f2.'

devices 38, 47 and 50 are Asim-i ilar to the" control devices 10, 17 land 20 in l 31 are similar to, 50, the Idelay circuit 4 in the circuit of Fig. 1.

The filter 55 is designed to pass the courier frequency f2 generated by the generator 43' and to suppress other` frequencies. I ilter 34 is'designed to pass the courier frequency f1 generated at the west terminal'station and frequencies within' the speech frequency range, yandpto suppress the courier frequency f2. Amplifiers 33, 36 and 56 and the relays 39 to 41, 48 and 51 to` 53may be of any of the types well known in the prior art.

It" will be assumed that telephonie currents for transmission from east to west are set up first at the east terminal station, and not more than T seconds .later where T is the total transmission time of the cable, telephonic currents are set up at the West terminal, station for transmission from west to east.

The westwardly directed telephonie cur- .rents'from the source TL are impressed upon the input of transmitting circuit TA through hybrid coil H2. A portion of these impressed currents will be diverted into control circuit 37 and operate control device 38 therein causing the windings of relays .39, and 41 vto be energized. Relay`39 will operate to close normally open switch in the transmitting circuit TA so as to make that circuit operative for transmission of the speech .waves between source TL and the cable TC.

Fast-operating relay 40'operates to close switch 42, thus connecting the genera/tor 43 of frer ency f2 across the transmission circuit T An instant later slow-operating rclay 41 operates to open normally closed switch 44, thus disconnecting the generator 43 from transmitting circuit TA. The momentary connection of generator 43 to transmitting circuit TA will cause a short impulse of current of frequency f2, which will be designated hereafter as courier f2, to be rtlxw'iansmitted over the transmitting circuitv A portion of this courier f2 will be diverted into by-pass circuit 54, passed by the filter therein, amplified by amplifier 56,im pressed by hybrid coil Hl'on cable TC and directly transmitted thereover to the West terminal station, where it will be effective to establish initial control of the circuit thereat in a manner which will be described later.

The portion of the courier f2 not bypassed th rough circuit 54 will pass through delay circuit 31 being delayed therein for the time T. and, as filter- 32 is designed not to pass the frequency f2, will vbe diverted into control path 46 to operate control device 47, causing the winding of relay 48 to be energized. The lrelay 48 will then, operate to open the normally closed switch 57 in receiving circuit RA; Due to the delay produced inthe diverted courier by delay circuit 31, the switch 57 will be open to disable path RA substantially 'l seconds after the courier f: has been lfill CII mageos cuits at the stationby unbalanced received enorgy orby energy stored in the delay circuit 4 prior` to the operation of control device 20. The westwardly directed speech wave which follows the courier wave in transmis-- sion over the cable TC by the time interval T seconds, on its arrival at the west terminal station will also be impressed by the hybrid coil H1 on the receiving circuit RA and passed by the band pass filter 6', which, as stated above, is designed to pass the speech frequency bandas well as the frequency f2. A portion of the speech wave passed by the filter G', will be diverted in the output thereof into control circuit 19 and will be impressed on the input of control device 20 so as to maintain that device in the operated condition initiated by the preceding courier f2. The relays 21 to 23 will therefore be maintained operated as long as speech is being received over the cable TC from the east terminal station. and thus will maintain the receiving circuit RAloperative and the transmitting circuit TA disabled for that length of time.

The main portion of the speech currentsin the output of the band pass filter 6 in receiving circuit RA will be supplied to the band pass filter 7 therein, passed by that filter, amplified bythe receiving amplifier 8 and transmitted from the output thereof to the circuitI TL.

lt is readily seen that because of thev particular relation oi' the delay produced in transmitted waves by delay circuits i and a at the east and west terminals, respectively, to the transmission time of the cable TC, that whichever terminal first 'sends out its courier impulse will gain exclusive control of both sets ot terminal apparatus. Because the speech energy from cach station is not impressed upon the cable TC until after the initial operation ot the switching apparatus at the tar terminal by the irst courier to be transmitted, the incoming courier need not be selected in 'the presence of simultaneously outgoing speech energy and there will be no loss of speech because ot interference therewith.'

" of control devices l() and 17 will need to be very accurately maintained ifkthe different rates of speech energyin the case of dierent -ivords are not to introduce deviations in the relative operating times of the two control devices. i

Fig. 2 shows a modification of the circuits of the terminal station of F ig. 1 in which the device controlling the disabling of the receiving circuits in responseto energy to be transmitted, corresponding to the control device 17 of F ig. 1, is operated under all conditions very positively and in a deinite time relation with respect to the time in Xwhich the courier is sent out over the cable. This is accomplished by causing this control device to be initially operated by a local courier impulse suitably -delayed with respect to the courier impulse sent out over the cable instead o'f by the delayed speech impulseonly.

In the terminal station of Fig. 2 (which is the east station), the output of the transmiting circuit TA and the input of the receiving circuit RA are associated in energy transmitting relation with the cable TC and in conjugate relation with each other, as the system of Fig. 2, by a hybrid coil H1 and its associated balancing network N1, and the input of the transmitting circuit TA and the output of the receiving circuit RA are coupled in conjugate relation with each other, and in energy transmitting relation with the circuit TL through the hybrid coil H2 and its associated balancing network N2. The transmitting circuit TA comprises, connected between the hybrid coil H2 and hybrid coil H1 reading from eastl to west, the one-way delay circuit 30,the one-way delay circuit 31, band pass filter 32 and the transmitting amplier- 38. The receiving circuit RA comprises, connected between the hybrid coil H1 and the hybrid coil H2 reading 'from west'to east, the band pass lter, the band pass ilter 35 and the receiving ainplier 36.

Connected to the transmitting circuit TA between the hybrid coil H2 and the input o delay circuit 30 is a control circuit 37 comprising the current-controlled relay controlling device 38, and the relay 39, quick-operating relay a0 and slow-operating relay al connected in parallel to the output of device 38. The normally open switch i2 connected to a say ico

source e3 of alternating current of a ref quency ,'fz, which frequency is outside the speech frequency range, is adaptedto. be closed by operation cit the relay 40 to conneet the source 113 across thetransmitting circuit TA at a point between the output of delay circuit 30 therein and the input of delay circuit 31 therein. A normally closed switch 44 connected to the circuit TA, and source i2-l is adapted to be opened by operation of relay t1 to disconnect the source 43 from the transmitting circuit TA. A normally open switch` l5 located in the transmitting circuit TA between the output of delay circuit 3() and the point of connection ot the source 43 to the transmitting circuit TA is ,adapted to be closed by operation of relay 39.

igeaaeae f transmitted out'over the cable TC, and therefore will prevent currents which arel sent out over the cable TC from the west terminal station after courier f2 has been transmitted thereover from the east terminal station from reaching the input of control device in control path 49 and causing its operation, and therefore the operation of relays 51 to 53 to disable the transmitting path TA.

The main portion of the westwardly directed speech currents are delayed in transmission over the transmitting circuit TA for zf-fT seconds 'due to the action of delay circuits 30 andr'Sl therein. I The small delay of t seconds produced by delay circuit 30 insures that the switch 45 has been closed to make operative the transmitting path TA and that control device 47 in control path 46 has been operated by the preceding courier before the speech currents arrive thereat.

The portion of the delayed speech currents in the output of delay circuit 31 which is diverted into control circuit 46 and impressed on the input of control device 47 serves to maintain that device in the operated condition initiated by the preceding courier f2. Relay 48 controlled by controldevice 47 will therefore be maintained operated as long as speech currents are being transmitted into control circuit 46. I

The main portion of the speech currents in the output of delay circuit 31 will be passed by the band pass lilter 32 in transmitting cirlcuit TA, amplified by the transmitting amplifier 33 therein, impressed by means of hybrid coil H1 upon the cable TC and transmitted thereover to the West terminal station T-l-t seconds behind the precedingcourier f2.

Thev manner in which complete control of the transmitting andy receiving circuits at'the west terminal station is obtained for the east terminal station will be vclear from the following description by reference to the receiving circuits at the east terminal station. The apparatus at the west terminal station corresponding to that at the east terminal station will be designated with the same characters as the latter but followed by a prime mark.

The portion of the courier' f2 transmitted over cable TC from the east terminal station, on arrival at the west terminal station will he impressed by hybrid coil H1 on the receiving path RA and passed by the band pass .is filter 34 therein, which filter is designed to pass frequencies within the speech-frequency range and the courier frequency f2 and to suppress the courier frequency f1 (which is the courier frequency generated atthe west stal tion). As filter 35 is designed not to pass the frequency-f2, the courier f2 inthe output of.

the band pass filter 34 is diverted into control circuit 49 and initially operates control device 50 causing the windings of relays 5l ,ci to 53' in the output thereof to be energized.

Relays 51 and 52 immediately operate to open normally closed switches 58 and 59',

respectively located in the input and output of the transmitting circuit TA making that circuit inoperative to transmit speech currents 0r the courier f1 generated by generator 43 to the cable TC. Relay 53 simultaneously operates to open the normally closed switch 60 in control circuit 46. The disabling of the transmitting circuit TA in the several places is for the purpose of preventing false operation of control device 47 by unbalanced received energy or energy which may be stored in delay circuits 30 and 31 prior to the operation of control device 50.

The speech wave from the east terminal station following the courier f2 over thecable TC by the interval of time T-i-t seconds on arrival at the west terminal station will also be impressed u on the receiving pathiRA by the hybrid coll H1 and passed by the band pass filter 34 in the input thereof. These speech Waves in the outputy of the band. pass filter 34 will be divided between the control circuit 49 and the main receiving path RA. rilheportion diverted into control circuit 49' control device 50 and serve to maintain that device in the operated condition initiated by the preceding courier frequency f2. The relays 51 to 53 1n the output of control device 50' will therefore be maintained in the operated condition `as long as speech waves from the east terminal station are being received by the receiving circuit RA from the cable TC.

IThe main portion of the westwardly directed speech waves in the output of the band pass filter 3 4 will be transmitted over the main receiving path RA', passed b the band pass filter 35 therein, amplified y the receiving amplifier 36 and impressed by the hybrid coil H2 upon the circuit TL.

In the manner which has been described,

complete control of the transmitting and re-v ceiving circuits 'at both east and west terminal stations has been obtained yfor the east terminal station. If the speech Waves to be 4transmitted had been first set up at the west terminal station, the latter station would have obtained control of the transmitting and rewill be impressed upon the input circuit of ceiving circuits at both terminals in a similar manner. v

Fig. 3 shows the circuits of an intermediate repeater station'which may be used for connecting two sections of the cable'TC, an east section TC2 leading to an east terminal stan tion auch as illustrated in Figl or2 and a west section TC1 leading to a similar west terminal station. The circuits of this intermediate repeater station are'arranged so that the terminal station which first sends out its courier gets complete control of the lintermediate repeater circuits as well as those of the two terminal circuits.

. The circuits ofthe intermediate repeater station comprise a. westwardly directed amplifying-path WA and aneastwardly directed amplifying path EAhaving a portionCA in common. The inputof the path WA and `the output ofthe vpath EA are coupled in 4energy transmitting relation-with the west end of the east cable section TC2 and in con- --jugate relation to each other by means of H3 and associat d .balancing network N3.

. Similarly, the input ofthe ypath EA and the output of the path WA are coupled in con- 1.fjugate relation to each other and in energy ..5

transmitting relation with the east end of the west cable section TG1 by thefhybrid coil H4 and its associated balancing network N4. The' .westwardl directed amplifying path WA comprises etween the hybrid coils H3,`

and H.L in order from east to west, the bandv pass filter. 60, the one-'way amplifier 61 and the transmitting amplifier 62, while the east- .wardly transmitting path EA comprises connected-b'etween the hybrid coils H4 and Hg in order from west to east the band pass filter 63, the one-Way amplifier 64 and the transemally'closed switch 71 controlledbyfthe re- 72 is located in the amplifying path EA the output of filter'63 andthe input between f of amplifier 64. l.

' The one-'way-amplifier 64 in the path EA vand the one-way amplifier 61 in the path WA are for the purpose of preventing transfer of speech currents between these paths at their junction with the common branch CA.. This may be prevented'byl the use of other means, `fo'r example, a balanced bridge at the junction of paths EA, WA and CA, suchas disclosedin Fig. 8 ofl my United States PatentrNo. 1,695,813, issued December 18, 1928.

A westwardly directed one-way by-pass circuit BW is connected between the output vof filter in amplifying path WA and point-s in the output of path -WA between the switch65 therein and the hybrid coil H4.

' Similarly, an eastwardly directed4 one-way by-pass circuit BE- is' connected between the 'output of filter 63 in the amplifying path EA l and points in the voutput of said path lie-- tween the switch 67 therein and the hybrid Q coil H35 The by-pass circuit BW comprises in order from east to west the band pass filter 65' 73, the band pass fiter 74, the one-way amthe three-winding transformer or hybrid coil W between the filters 7 Sand.

4 therein is a control circuit 79 comprising the delay circuit/80, the current controlled relay controlling device 81, and the relays 66,

72 and 83 connected in parallel to the output of the control device 81. Similarly connected to the by-pass circuit BE between filters 76 and 77 therein is the control circuit 84 comprising 'a delay circuit 85, the current controlled relay controllingdevice 86, and the V relays 68, and 87 connected in parallel to the output of control device 86. A normally closedu switch 88,J4 which vis controlledby the relay 87 is located' in the control circuit 79 between the'output of the delay circuit 80 and the input of control device 81therein. Similarly,.a normally closed switch 89 is located in the control circuit 84 between the output of delay circuit 8 5. and the input of the control device 86 therein. j

The control devices 81 and 86 are similar to the control devices' used at 4the terminal stations shown in Figs. 1 and 2.

The delay circuits and 85 located in coni trol paths 79 and 84, respectively, vare similar to the delay circuits .used at the termin-al stations shown in Figs. 1 and 2. Delay circuit 80 is designed, to produce a delay in the currents transmitted ltherethrough of substantially T1 seconds where'Ti is the total transmission time of the west cable section TG1 between tl'iefintermediat'e repeater station and the West terminal station. Similarly, delay circuit 85 is designed to introduce a delay in the currents transmitted therethrough of substantially T2 seconds, where 'll2 is the total transmission time of the east cable section TG2. Y l

For proper operation of the system it is only necessary that the total time of transmission from the east terminal station to the control device 81 at the intermediate repeater is substantially equal to the total time of' transmission from the west terminal station to the control device 86 at the intermediate repeater. lf the transmission time T1 of cable TG1 is greater than the transmission time T2 of cable T02, the delay time of delay cir-` cuit 85 may be 0 seconds, that is, delayfcircuit 85 may be eliminated, and the 'delay time of delay circuit 80 made 'T1-T2 seconds or -if T2 is great/er than T1 the delay time of delay circuit 85 may be made 'T2-*T1 seconds and the delay time of delay circuit 81 made zero, that is, the latter may be eliminated. Filter 60 in the amplifying path WA, filter 63 in the amplifying path EA, filters 73 and 74 in the by passcircuit BW and filters 76 vand 77 in the' bypass circuit Blf/are similar tothe filters used at the terminal .stations shown in Figs. 2- and .3. The ,lter 66 istieisa nsaaeoe signed to pass frequencies Within the speech lfrequencyfrange and the courier frequency f2 generated at the east terminal station and to suppress courier frequency f1 generated at the west terminal station. Similarly, filter 63 is designed to pass frequencies Within thev speech frequency range and courier frequen-y cy f1 and to suppress courier` -frequency f2. Filter 73 is designed to' pass frequencies Within the speech frequency range and the courier frequency and to suppress Waves of other frequencies, while ilter 7 4 is designed to pass the courier frequency f2 and to suppress lWaves of other frequencies. Filter 76 is designed to pass frequencies Within the speech lfrequency range and the courierfrequency f1 vthan T seconds later where T is the total transmission time over both sections of the cable TG1 and TG2 and therefore equals Tl-l-'lz seconds, speech Waves for transmission from east toA West are set up at the east terminal station. llt will be assumed that the transmission time' T1' of the cable section TG1 is `greaterthan that of the cable section TCL..

' cable section TC, T seconds in advance of the speech vvavesin the manner which has been described in connection with the terminal circuits of Figs. l and 2. The courier f1 on arrival at the intermediate repeater station will be impressed by hybridl coil H4 thereat upon the eastvvardly ldirected amplifying path EA and passed by the lter 63 therein. ln the output of the filter 63 the courier f1 will divide. The portion trans mitted over the path EA will be dissipated therein due to the normally open switch 67 in the output thereof. The other portion of courier f1 will be passed by the band pass filter 7 6 in the byc-pass circuit BE and will divide in the output thereof. A portion will be Vb a-nsmitted over the by-pass circuit BE being passed .by iilter 77 and amplified by amplifier 7 8 therein. rlhe amplihed courier f, will then be impressed on the east cable `section. TS2 by hybrid coil H8 and tran..n

mitted directly thereover to the east terminal station to initially operate the control appa-- ratus thereat in the manner previously de? scribedin connection with AFigs. 1 and 2.

The other portion of the courier f1 in the output of filter 7 6 will be diverted into the control circuit 84 and will be transmitted through delay circuit 85, which according to our assumptions will introduce zero delay, and then will initially operate control device 86, `which in turn `Will cause the windings of relays 68, 70 and 87 to'be energized. vRelay 70 `will operate to open the normally closed switch 69 in the inputof amplifying path WA to disable that path for the transmission of currents from the output of filter 60 to amplifiers 6l and 62. Relay 68 will operate to close the normally open switch 67 in the output of the amplifying path EA, making that path operative'for the transmission of currents in the west to east direction between cable sections TG1 and T02. Relay 87 will operate to open the normally closed switch '88 in the control circuit 7 9, making that circuit inoperative for the transmission of currents from the output of lter 73 in by-pass circuit BW to the input of control device 8l in control circuit 79.

Relays 68 and 76 should be relatively adi'usted so that switch 69 in amplifying circuit WA opens to disable that circuit before switch 67 in amplifying circuit EA closes to make circuit EA operative, thus avoiding a singing path. The relays 72 and 66 associated with control circuit 79 should be similarly relatively adjusted for a similar purpose.

lhe eastivardly directed speech currents following the courier f1 in transmission over the cable section TCLby the time interval of T seconds upon arrival at the intermediate repeater station will also be impressed by hybrid coil Hi upon the eastwardly directed amplifying circuit EA and passed by the filter 63 in the input'thereof. A portion of the speech currents in the output of filter 63 will be transmitted over the by-pas's circuit BE and passed by the filter 76 therein. As the lter`77 is designed. so as not to pass waves vvithin the speech frequency range, this p0rtion of the speech currents will be diverted into control circuit 8a and after passing through delay circuit 85 having a delay of zero seconds Will be impressed on the input `of control device 86 to maintain that device in the operated condition initiated by the preceding courier as long as the speech Waves are being continuously transmitted from cable section TG1 to cable section TG2 in the eastwardly direction.

The main portion of the speech waves in the output circuit of filter 63 will be transmitted over the eastwardly directed path EA, amplified in amplifiers 64 and 62 therein, and, as the normally open switch 67 in path EA is now in the closed condition due to the action of the preceding. courier frequency. f1,

' .and the control path 79 inthe manner which has just been pointed out will prevent the courier f2 from the east terminal station, which accordingto our assumption,I would not be'started over cable section' TG2 until after the'courier 1 from the west terminal station has starte out over the cable section TG1-.to the intermediate.- repeater station,

' pressed by from setting up a singing circuit, or being effective to operate control device 81 and thus to seize control of the intermediate repeater apparatus for the east terminal station..

The operation of the intermediate repeater station for the case when the speech waves for transmissionv from east to west are set up at the east terminal station before the speech Waves for transmission from west to east are set up at the West terminal station is similar to the o ration Whichhas just been described. In this case the courier f2 received over cable section TG2 Will be passed by lter in the inputof the amplifying path WA and lilter .7 3 in by-pass circuit BW. A portion thereof will be passed by filter 74, lampliied by amplifier 7,5, impressed on the west cable section' TG1 and transmitted thereover to initially operate the control of the switching apparatus at the West'terminal station in the manner which was described in .connection with Fi s. l andQ. The other portion of the courier g 2 inthe youtput of filter 73 in bypass circuit BW will be diverted into the con trol circuit 79 andafter being delayed therein for-Tf-TB seconds according to our assumptions, by the delay circuit 80 will operate control device 81 toinitially operate the relays 72, 83 and 66 so as to first open'the normally closed switches 71 and 89 in amplifying pat 1 EA and the-control circuit 8d, respectively, and then to close the normali open switch inampliying path EA. T e speech Waves transmitted from the West terminal station T seconds behind the courier f2" on arrival atl the intermediate repeaterstation will be imthe hybrid lcoil H3 upon the amplifying path WA and passed by the filter 60 1n the input thereof. A small portion .of the speech vWaves in the output of the filter 60 Will be passed by the filter 73 in the by-pass circuit BW and will be diverted into the control path 79 and lafter being delayed for -T1--T2 rseconds in the delay circuit 80 will be impressed on the input of control device 81 so as to maintain that device 1n the operated condition initiated by the preceding courier as long as speech waves are being continu-V ouslyl received for the cablef section TG2. The repeater circuits having been conditioned in the manner described, the main portion of the speech currents in the output of ilter 60 will be transmitted over the amplify ing path WA amplified vby amplifiers 61 and 62- therein and impressed. on cable section TG1 and transmitted thereover to the west terminal stati0n,"where a small portion thereof will be utilized to maintain the switching circuits thereat in the condition initiated by the receding courier and the main portion will e delivered to the receiving circuits in the manner described in ldetail in connection with Figs. 1 and 2..

The disabling of the' amplifying path EA and the control path Sellin the manner which has just been described Will prevent the .courier' f1 and the succeeding speech Waves from the west terminal station from being edective to seize control of the intermediate repeater apparatus.

The relation of the delays produced by the delay circuits 8,0 and 85 in the control circuits I 79 and 84, respectively, at the intermediate repeater station, to the transmission times of the two cable sectionsv TG1 and TG2 insures that complete control of the intermediate repeater is given to that terminal'which first starts its courier on its way.

'It is to be understood that the invention is not limited to the particular circuits, and details ofthe particular terminal and intermediate repeater stations `illustrated and described as numerous modifications thereof may be made by persons skilled in the'art without departing from the spirit and scope of thel invention. For example, although the terminal stations which have been illustrated and described comprise either a normally operative' receiving circuit, a normally disabled transmitting circuit and switching means responsive to Waves to be transmitted to disable the receiving circuit and make operative the transmitting circuit, or a normally operative transmitting circuit, a normally disabled receiving circuit and switching means responsive to received waves to render the receiv.' ing circuit operative and to disable the transmitting circuit, itis apparent that the invention may be applied to other types of terminal circuits; for example, to terminal stations comprising normally disabled receiving and transmitting circuits and switching means responsive to transmission in either-direction to make the circuits operative tor that direction.

Although the intermediate repeater station which has been illustrated and described comprises normally inoperative amplifying paths for the two directions of transmission "having aportion in common, and switching circuits responslve to transmission 1n either- Lacasse direction to make the amplifying path in that direction operative, it is apparent that the in- -vention is not limited to thatJ type oi' interf mission have no portion in common, or in which one path is normally operative and the other path normally disabled and the switchin circuits are responsive to transmission in eit er direction to make the amplifying pa-.hs operative to repeat the transmission in that direction only. l l,

Although in the terminal circuits which have been illustrated and' described, the receiving circuit or the transmitting circuit in some cases has-been shown normally disabled,

or adapted to bedisabled, in a single place, because of the high gain .which may be used in terminal rircuits of this t e it may be desirable to have the 'transmitting and receiving circuits normally disabled or `adapted to l be disabled in several places.

Although 'n the particular embodiments of the invention which'have been illustrated and described mechanical relays have been employed for conditioning the transmission paths by operating movable switches to close or open circuit these paths, the invention is not limited to the particular means shown. For example within the scope of the invention the transmission paths may be effectively disabled by short circuits or loss networks inserted in the paths, controlled by current'controlled relays. Stationary devices having no movable elements may be employed in place of the mechanical re ays, for example, vacuum tnbefrelays, such as disclosed in Crisson lUnited States Patent l1,647.212. issued November l, 1927 or in the article by C.. A. Beer andG. T. Evans, in the institute of P. 0. Electrical Engineers '(London), vol. 20, parres to 72 inclusive, published in 1920. v

What is claimed is:

1. ln a signal transmission system comprising a signal transmission path over which the time of transmission is appreciable and in which two-Way transmission is secured by signal-controlled circuit-control means. the method ot avoiding mutual lockouts of transmission and consequent loss of part ot' the signal transmission. which consists in establishing control of the circuitlcontrol means by means of a short train of control Waves sent over the system. and delaying the application of signals to the transmission path until such control is established.l

2. The method of operatingv a signal-directed two-Way transmission systemincluding a signal transmission medium over which the time of transmission is appreciable, vvhich consists in transmitting a. short train of control waves over said medium under control of the signals to be transmitted, to condition the system for the transmission of said signals thereover, and delaying the application of the signals to the transmission medium by a time approximately equal to the overall transmission time of said transmission medium.

' 3. transmission system com rising a signal transmission path supplied' rom a source of signal Waves, control means `for determining the operation of said path, means under the control of said signal Waves for .transmitting to said control means and in advance of the signals an impulse of current'to initially operate said control means, means to delay the sending out of the signal Waves over said path at least until said control means has been initially operated by said impulse of current, and means for utilizing the delayed vsigii'lwave when it arrives at the control point to maintain said control means in its initially operated condition.

4. A s stem for the two-Way transmission ot signa ing currents including current-controlled means at av oint therein for conditioning the system or efficient operation in the direction of transmission, means'at another point in said system and controlled by the signaling currents when they arrive thereat for causing a momentary .impulse of.

control current to be transmitted over the system in the direction of the controlling signalingcurrents to initially actuate saidI current-controlled means, means for delaying the transmissionpof the signaling currents over said system .from said other point for an interval of time at least equal to the Wave transmission time of said system from said other point to the control point.I and means for utilizing the delayed signaling currents when they arrive at the control point to maintain said current-controlled means in its initially operated condition.

5. A system for the two-Way transmission.

of telephonie currents comprising a two-Way line, stations lat various points along said line and comprising oppositely directed amplifying paths for said currents, currentcontrolled means at said stations responsive to transmission of waves'in one direction for conditioning the amplifying paths thereat for efiicient transmission in said one direction only, and responsive to transmission of waves 1n the opposite direction for conditioning said paths for efficient transmimion in said opposite direction only, means at one of said stations and controlled b the transmission of telephonie currents in t e amplifying path thereat for said one direction, for causing a momentary impulse of control current to be transmitted over the system in the direction of thel controlling vtelephonie currents to .initially operateI the current-controlled means at another of said stations, means to delay the transmission of saidtelephonic currents from said one station to said line for an interval of time at least equal to the transo' positely directed amplifying paths for sald lcurrents, means at each station responsive `mission time over said system from said one station'to said other station, and means for utilizin the delayed telephonie ,currents when tiey arrive lat said other station to maintain the current-controlled means thereat in the Condition initially produced by the preceding impulse of current. 6. A system for th two-way transmission of' telephonie-currents comprising a two-way line connecting stations each comprising opto transmission over the system in one direction for conditioning said paths for eiiicient operation in said onel direction only, and relsponsive to transmission over the system in the other direction for conditioning said paths for eiicient operation in vsaid other direction only, and means for insuring complete control ofthe-'conditioning means at both stations to the one of said stations at which telephonie current transmission )firstv starts, the last mentioned means comprising at each station' means controlled by the telephonic currents to be transmitted therefrom for sending out over said line an impulse of current to initially operate the conditioning means atthe other station, means for delaying the transmission of the telephonie currents to said line for an interval of time after.

said impulse has been sent out thereover which is substantially equal to the transmission time of said two-way line between said stations,means for utilizing the delayed telephonic currents as they arrive at said other station for maintaining the conditioning means thereat in their initially operated con dition, and means for preventing incoming currents from the other station, if started v'therefrom over said line later than the impulsey from the first station, from being et- ,tective to operatethe conditioning means at said first station until transmission of the telephonie currents `initiating the impulse, from said first station is interrupted for a time interval at least equal to the usual pause between sentences.

7. The system of claim 6, and in whichV the last mentioned means is controlled by the delayed telephonic currents at said first station.

S. ln a system for thetWo-way transmission of signaling waves, a two-way line having an overall transmission time of T seconds of current has been transmitted thereto, means responsive to currents recelved by said receiving circuit from said line for effectively disabling said transmitting circuit and making operative said receiving circuit, and means responsive to the delayed signaling Waves for disabling the last mentioned means for operation by said received currents until substantially continuous transmission of signaling Wavesfrom said source to saidl transmittingcirc'uit has been interrupted for an appreciable interval of time.

9. ln a system for the two-way transmission of signaling waves comprising a two- Way line having an overall transmisslon tlme of T ,seconds and a station comprising a `for impressing a portion of said impulse ofv control'current upon sald llne for transmlssion thereover, means for delaying transmission of said signaling Waves to said lme for transmission thereoverfor substantially T+t 'mounds after-said portion of control current has been transmitted thereto, Where t is very small in comparison to T, means re1 sponsive to currents received from said line by said receiving circ-uit for efi'ectively .disabling said transmitting circuit for the transmission of waves from said source to said line, means for utilizing the other portion ofk said control impulse for disabling said receiving circuit and said transmitting circuit, for o eration by said received currents, substantially Fll seconds after the first portion of said control impulse has been transmitted to said line and means responsive to the delayed 'signaling Waves for maintaining said receiving circuit and said transmitting circuit, disabling means disabled until transmission of signaling waves over said transmitting circuitY has been interrupted for an appreciable interval of time.

10. ln a system for the two-way transmission of signals, a repeater connecting a West section of a 'two-Way line having an overall transmission line of T1 secondsand an east section of 'a two-way line having an overall transmission time of T2 seconds, said repeater comprising an eastwardly directed one-Way amplifying path and a 'westwardly directed one-way amplifying path `for'said signals'` both paths being normally disabled in their outputs, an eastvvardly directedone-way circuit for lay-passing a small portion of the signals and Waves off certain characteristic frequencies outside the frequency'range of said signals received over said West Section of line around said repeater to said east section of line, a Westwardly directed one-Wa circuit for ley-passing a portion of the signals and other Waves of characteristic frequencies different from said certain frequencies and c outside the frequency range of said signals,

lill

around said repeater to said West section of line,` a control path connected to the east- Wardly directed by-pass circuit for diverting a portion of the Waves transmitted thereover and including means for delaying the diverted Waves therein`T2 seconds, another control path connected to the westwardly directed b -pass circuit, for divertin a portion of t. e Waves transmitted thereover and including means for delaying the diverted Waves therein for T1 seconds, means responsiveto the delayed, diverted Waves in the first mentioned control path for disabling said westwardly directed amplifying path in its input, and said other control path, and for making said eastwardly directed amplifying path operative, and means responsive to the delayed, diverted Waves in said other control path for disabling said eastwardly directed amplifying path in its input, and the lirst mentioned control path and for making operative the Westwardly directed amp fying ath. f p ll. In a signaling system in which exclusive control of a station for'transmitting or for receiving is seized through the agency .f of the lsignaling Waves entering the system,

the method comprising directively'seizing control of a transmitting and a receiving station iii-'response to signal Waves, and delaying transmission, between the seized. stations, of the signals until sach control has been established. y

12. ln a two-Way system for transmitting andreceiving signals over a transmission path of appreciableoverall transmission time, a transmitting and a receiving branch ata station, one of which is normally operative and the other normally inoperative, signal controlled means to maire operative the normally .inoperative branch and to maire inop.

erative the normali operative branch to conv dition the station or transmitting over said or receiving from. said path, means at .4

said station controlled by signals to be transmitted over said path for operating said signal controlled means to condition the station so that it can transmit signals to but cannot receive signals from path, means for deiaying` transmission of said signals to said forsubstantialiythe overal transmission time of the path, and means for delaying conditioning the station so that it cannot receive for a time substantially egaal, the overall transmission time of said path after conditionin of the circuit so it can tr. W,mit parte.a

y rebeive from said path by signals at said station to be transmitted over said path.

14. A system according to claim 12 in which said station is maintained in condition to receive from said ath but is disabled to transmit to said path' y signals received over said path.

15. In a two-Way system for transmitting and receiving signals over a. Wave transmission path of appreciable overall transmission time, a station having connected to said path a transmitting circuit and a receiving circuit one of which is normally operative and the other normally disabled, signal-controlled means to make operative the normally disabled circuit and to disable the normally op- Y erative circuit to condition the station for receiving from said path or transmitting to said path, means at said station controlled by Waves received over said path for operating said signal-controlled means to condition the station lso that it can receive waves from said path but cannot transmit waves thereto, and means for delaying transmission of signals tol said path for substantially the overall transmission time of said path.

16. A system according to claim 15 and in which said station is maintained in condition to receive waves from said path and disabled for transmitting signals to said path by the si als received thereover.

n Witness whereof, l hereunto subscribe my name this 10th da of J une 1929a nos er c. harnas.

station is maintained in condition

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