US2355958A - Control of two-way signal transmission systems - Google Patents

Description

K. H. DAVIS Aug. 15, 1944.

CONTROL OF TWO-WAY SIGNAL' TRANSMISSION SYSTEMS Filed Sept. 17, 1942 2 Sheets-Sheet 2 VINVENTOR' K ht DAV/S ATTORNFV Patented Aug. 15, 1944 CONTROL OF TWO-WAY SIGNAL TRANSMISSION SYSTEMS Kingsbury H. BernardsvillflN. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y; ia'c'orporation of New York r Application September 17, 1942; Serial No. 458,658 g I 4 Claims. 1(c1,17s; -44)i The invention relates to two-way signal transmission systems and particularly to the signal-' controlled switching circuits used with such systems to directionally control signal transmission therein while suppressing echoes and preventing singing.

Such signal-controlled switching circuits, commonly called echo suppressors, usually employ two switching branches respectively connectedto each of two oppositely directed one-way signal transmission paths at a terminal'or intermediate point in the two-way system, each including a rectifie'r for rectifying a portion of the signals diverted into the switching branch and electromagnetic relays or other switching devices op erated by therectifled signals to suitablycontrol, the transmission efficiency of the signal transmission paths and todisable the other switching' branch so as to provide the desired direc tional control of signal transmission while suppressing echoes and preventing singing.

The copending patent application of K. H.

Davis and A. C. Norwine, Serial No. 421,704, filed December'3,'1941, which issued as Patent No.

2,306,689 on December 29, 1942, discloses such an' echo suppressor applied to a four-wire telephone circuit, modified so as to enable one subscriber by talking loudly to break in on another who is talking and has obtained directional control of'the system, while the latter continues totalk and'regardless of how loudly the latter party is" talking. This is accomplished by the use or relaftively insensitive auxiliary switching" controls, properly located with=-respect to the echo suppressor controls, operating on an amplitude basis to disable the echo suppressor controls of the talking party when the breaking party talkswith the required loudness.

An object of the invention is to improve break in echo suppressors ofthe above-described type.

A more specificobject is to provide echo, sup-(.1.

pression and to enable break-in in systemsof the above-described type with economyof apparatus.

These objects'are attained in accordance with the invention'by arrangements for automatically transforming the echo suppressin equipment as! sociated with each side of a four-wire circuitinto break-in equipment during periods in which it is not needed for erforming itsecho suppressing f functionl Fig.1 showsschematically a four-wire toll te1e-' N2, respectively.

phone circuit equipped with an echo suppressor of the amplitude breakin type embodying one modification of the invention; and

Fig. 2 shows schematically in more detail an :alternative echo suppressor of the amplitude break-in type in accordance with the invention which may be used in the system of Fig. 1.

InlFign'l, the four-wiretele'phone circuit in-.

cludes a'one-way,- two-wire transmission path .EA' including the amplifier As, normally conditioned totransmit in the direction from west to eastbetween the two-way line WL and the twowayline EL; and a one-way, two-wire transmission path .WA including theone-way amplifier Aw, normally. conditioned totransmit. in the dienergy transmitting relation with the two-way lineJWL or EL by the hybrid coil Hw and associated line balancing network. Nw, or hybrid COil-HE and associated line .balancing network Connected across the path EA at an intermediate point I is the inputof a switching circuit 2, operating as a combined echo suppressor and break-in, control circuit, including the one-way amplifier 3 =followedby the'wave rectifier (detector) 4 and the electormagnetic switching relays, 5 to 8 adaptedto be operatively-energized in parallel by the rectifiedsignal output of the rectifier 4., Similarly, connected across the path WAtatan intermediate point 9 is the inputof a switchingcircuit 10, operating as a combined echo suppressorand break-in control circuit, including the one-wayamplifier ll followed by the wave rectifier (detector) "l2 and the electromagnetic switching relaysu|3 to IE adapted to be operatively energized in parallel by the rectified signal output of the rectifier l2.

.In theswitchingscircuit 2, relay 8 to be referred to hereinafter as the west break-in relay, is relatively insensitive compared to the other switching relays 5 to l in that circuit, as indi catedzby the series resistances IT in the energizing circuit forthe former relayi only. Also, theenergizingcircuit for the relay 8i is normally disabled by an open circuit at thepoint l8 so that the relay is normally not'in condition-to be operated bythe rectified signal output of rectifi81::4,,Wh81eaS the energizing circuits for the relays '5.to larenormally operative so that the latter relays maybe immediately operated bya be'transmitted back over that path. I

rectified signal output from the rectifier 4. Similarly, in the switching circuit In the relay Hi, to be referred to hereinafter as the east breakin relay, is relatively insensitive compared to the other relays l3 to l5 in that switching circuit, as indicated by the resistances IS in the energizing circuit of the jormer relay only, Also, the I energizingtcircuitgfor the break in relay I6 is normally disabled by a normal open circuit at the oint so that normally the relay is not in condition to be operated by signal output from" the rectifier l2, whereas the energlzingrcircuits for the relays l3 to IE are normally operative so that these relays are in condition to be immediately operated by the signal output the rectifier I2.

Relay 5 in the switching circuit to' b e referred to hereinafter as the west echo suppressor relay i when operated, connects a short-circuiting connection across the path WA at a point 2| beyond the point Soto which the input of the switching circuitiifi is connected, to disable thatpath. Similarly, the relay l3 in switchingcircuit llLtobe referredlto hereinafter as the east echo suppressor .relay when operated, closes a short-cir the;energizing circuits for the relays l3 to l5 ,aofthe eastswitching circuit It), so as to prevent cuiting connection' acrosszthe path EA at the. point 22beyond the point] to-whichthe input a of; theiswitching-circuit 2 is connected, to disable-the latterpath. .The one-.way-amplifier As is connected in the path EA between the pointsl and: 22 to prevent the disabling of that path from affecting the input of the switchingcircuit 2. -Simi1arly,"ithe one-wayamplifier Aw is con nected inthe path'WAbetween' the/points 9 and" 2| to preventfthetdisabling of-the path WA from affecting the-input to the switching circuit-I0.

In the following description of operation of the system of; Fig. 1, it will be convenient to speak of a loud talkerfi meaning one who. can

can' operate. a 'breakin control and a 'weak' talker, :meaning' one who cannot. Since we are to deal with circuits at the terminals 'ofwhich portions of signals are returned asechoes, it is as'sume'dthat aloud talker in order to break-in must operate a control at least insensitive enoughsothatit will not operate on the echo of the other talkers speech energy Let it be assumed that a West telephonesubscriber :associated with the line:WL starts to talk, and at that'timeithe ea'stsubscriber associated" with theilinexELdsi silent; Thespeech wavesjof the; westcsubscriber received 4 over the line WL are impressed by the hybrid coil Hw=on the path. 'EA :of the four-wire circuit and, that I path-.lbeinglin its normally operative condition with-the short-circuiting connection at the point 22 -open,= will be transmittedthereOVer amplified byv-the-amplifier AE to' 'the' ea's't'terminal of'the' four-wire circuit, where the'rnain'portion will be transmitted through the'hybrid coil HE to the" line-EL leading to the east subscriber, and'an other portion will be reflected 'as echoes into the oppositely'directed" one-way'path WA and will Alportionof Wests speech signals willbe' diverted at thepoint I in path EA- into the switch- 4 ing circuit Land will be amplified by amplifier 3 and rectifiedtby the'wave rectifier 4. The rectified signals inthe output of rectifier 4wil1 cause the immediate; operation of: the. switching relays 5 32. lnbll i. regardless of their amplitude, will not-be able to;:operateithebreak-iri:relay 8 be- 319.5? of t ;nQ fm l iin perative energizing cir.-

u c lli connec on across. the path WA: at the ;R- Will Qperateto close the short-cirsubsequentoperation of those relays by operati'on'of thewave rectifier I2 in response to later jsignals' offthe east subscriber in the path WA, while'thewest subscriber continues to talk. Relayl will-simultaneously operate to close the open circuit at the point 20 inthe energizing circuit for'the east break-in relay I6 in the east switching; ,c ircuit I0, soas to enable the latter relay tobethereafter operated by any rectified signals in the outputof the wave rectifier l2 ofsufficient amplitude .to overcome ;the desensitization provided by-the resistances IS in itsenergizing circuit. The west subscriber now has obtainedexelusive directional control of the echo suppressor circuitsand has conditioned those circuits so as to enable break inbythe. eastsubscriber.

Now, it While Westis still talking, East desires,

todnterrirpt him, the latter will start to talk and-,ra ise his voiceso that at least a portion of his speech signals will have an amplitude sufficientto overcome the desensitization of the east break-in relay l5 provided 'by theresistances H! in its energizing circuit. 'lhen the portion of Easts higheramplitude, speech Waves diverted into the switching ,circu itglfl from the path WA willl be amplified by amplifier H and rectified by rectifierv -lzand will cause the operation of the east break-in-relay 6,4120 open the energizing circuit for ,the westecho suppressorrelay 5 only.

The consequent release of the west echo suppressoifvrelay 5 to break thefshort-circuiting connection across the path WA at the point 2| willenable that path so as to allow E'asts speech waves 'to.pass over, the output of that path through hybrid coil Hw to line WL and over that lineto thewe st subscriber.

If the westsubscriber continues to talk, the rectifier}! iniswitcliing, circuit 2 will be maintaine" operatedby; his speech currents to hold relays Band I operated tomaintain'the energizingcircuit fortheeast break-in relay l6 operativej and theenergizing circuits for the other relays ;I3,-,IQ- and l5 in theeastswitching circuit l0 disabled, The path EA; therefore, will be also maintained unblocked so that both subscribers will'hear eachrother, but there will be no echo v suppression and both subscribers will hear echoes.

This condition 'will continue until the loud talker (East) lowers "his voice sufiicie'ntly to cause the east break-in relay 1 6 to releaseand thus allow the westecho suppressor relay 5 tob'again operated by West't block the path WA attne'poi t 2 l giving "directional controlof the circuit: again tq' 'th'efjlatter, or until the weak talki-(Wstif ops" t l ng. in; the latter case. the wa s 5 and'l'ofthe switching circuit 2 will relaseand ewire e s l3 l4 a ite b pe e b East to iY reQ Qnalo cont l Qtt e circui tQ-him, dtocondi n th west, breakr n e ay. 8 .for 1 subse'quentjoperation by;West-,if the latter again I starts talking with sufiicie'nt: loudness to 'over-.

come the desensitization of that relay provided bythe resistances l1. i

Now, ifwhile East is-talking loudly, West also raises his voice .to a loudness suflicient to over-.

come the desensitization of the :west break-in relay 8, there willbe no change in theoperation since, because the energizing circuit for the relay EA and WA will remain. enabled and both: talkers willhear each other and echoes during the period of double talking. v n

Similarly, if the east subscriberstarts talking first, he will obtaindirectional control of the circuit by operating switching circuit [0:130 disable the path EA at the point .22; to disable the energizing circuits for theJrelays-S, .6 and I of the west switching circuit 2, and to render the ener- .To make the above-describedtype of break-in Y effective, it isnecessarysthat the various echo suppressor and break in. controls have their rela- The tive sensitivities properly proportioned. values of the resistances l1 and I9 in the energizing circuits for the west break-in relay 8 and the east break-inrelay l6, respectively,:should beselected so that when the energizing circuits iorthese relays are operative, the break-in control wi11 be made sufficiently insensitive so that each break-in relay will not be operated in response to the maximum amount of signal echo which may occur in the associated signal trans mission path. -Also, the several switching relays should be provided with sufficient 'hang-oversin operation to prevent [their premature release to cause clipping or allow echoes. i Fig. 2 shows aportioniof. a four-wi native type of amplitude break-in echo suppressor in accordance with the invention. Asshown, the hybrid transformer H1: .is inserted inthe 'west to-east path EA of'the four-wire'circuitat an intermediate point corresponding to the pointl of Fig. l, anda hybrid transformer H2 isconnected in the east-to-west path wA at an inter normally closed back contacts of the unoperated east echo suppressor relay.'24,;and the hybrid transformer H2 comprises two line windings con nected in the path. .WA anda third winding 25} 212); gizing circuit for the westbreak-in relay 8 operative to enable West to break-in subsequently bytalking loudly enough to overcome the desensitization of that relay provided by the resistances I QEQCathOdGOiI'CUlt of the direct current amplifier 32;

break-in control circuit 27, to be referred to hereinafteras the west switching circuit, including in 7 order: the vacuum tube amplifier 28; the two-v elem'entvacuum tube detector 29; the west breakin .master relay 30 controlling the operation of th .1west-;break-in' relay 3| and its associatedhang-over circuit; the direct current vacuum tubefamplifier 32; the west master switching re-i layj 33;. and, the west echo suppressor relay 26.

with its associated hang-over circuit. .Similarly,

connected across the path WA at a point be tween the. line windings of thehybrid. trans former. HaiS the input of the combined echo sup-H pressor and break-in control circuit 34, to be rergferredto hereinafter as the east switching .cir-

cuit, including in order: the vacuumtube arn'pliav fier.35'; the two-element vacuum tube detector- 36;.the east break-in master relay 31 controlling the operation of the east break-in relay 38 and rent vacuum tube amplifier 32, the east master ;its associated hang-over circuit; the direct-curswitching relay 39 having itsoperating winding connected in series with the operating winding of the west master switching relay 33 in the plateand the eastzecho suppressor relay 24 :The west two-element vacuum tube detector Y 29 and the east two-element vacuum tube demgtectorBG, which may as shown, be containedin e telephone' circuit like that of "Fig-l equipped withanalterinductively coupled 'theretox'which *is' normally" short-circuited through the normally closed back contacts of thelunoperatedwest echo suppressor relay 26. In the normalconditiori'ofthe'hybrid transformer H1 With-its thirdlwinding 23 short missionover the pathWA.

. condition of theh brid transformer Hz with its third winding 25 short-circuitedthe latterv trans y 70' former is unbalanced so as to permitsignal trans:

Connected; across the path EA. at a point be- 15 tween. the. line windings of hybrid transformer 5 it P 1 1491 293?! Qmbined echosuppressor a w a single tube, are oppositely poled with respect to; the control grid-cathode circuit of the direct current amplifier 32. Eachis also biased in the non-conducting direction by batteries 56 and 51;

r srespectively; The .west master switching relay" 33 is, more sensitive than the east master switch ing relay 39 so" that the former is held operated by the normal platecurrent (current with no voltage-,a'pplied to its control grid from either 4oi-;detector) of the direct current amplifier 32 flow:

ing through its operating winding and the oper ating winding of the east master switching relay 39, this normal plate current not being sufficient to operate the latter relay the armature-of which is. maintained on the back switching contact. ""This normal plate current is determined by the difference. in the selected voltages of batteries and 58. w

The other elements of theecho suppressor cirdescription. of operation.

cuitsof. Figo2 will be referred to in the following is, speaking, and at that time the subscriber at thepeastterminal of the four-wire telephone circuit.,is silent. Because of the normal unbalanced condition 'ofzthe hybrid transformer Hi, the west subscribersspeech waves will pass freely over the path. EA .with the amplification provided by amplifier As andxother amplifiers (not shown) togthe line leading to the east subscriber; and at the {east terminal of the four-wire circuit a ertiomofj, these waves will be reflected as echo-cur rentsinto the oppositely directed transmission" at W ;A, portion of verted from the path EA' at a point between the line windings of the hybrid transformer Hr into Wests speech waves will be dithe West switching circuit 21 and will be amplified insensitive'so that it will not be operated by'the I pyithe amplifier 2a andrectifiedfby the detector;

rectified signals of normal amplitude passing through its operating winding from the output of detector-29. The rectified signal. current passes through the winding of the latter relay, the.

2-megohm resistance 40 and the input. potentiometer 4| for the direct current amplifier 32 in such direction as to apply a negative bias to the control grid of that amplifier thus reducing' the plate current of the latter. The reduction; in

the plate current of the directcur'rent' amplifier 32 :flowing through the operating winding of the west master relay 33' and the operating winding of the east master relay 39 miseries-will cause the-former more sensitive relay 33-:to release to 42.- The-make of the back switching contact ofrelay 33 will cause the condenser of the resistance-condenser hang-over arrangement 43; normally charged by current; from thel battery M through the hang-over winding of Vthelrelay 26,

to be discharged to ground, so that when West ceases talking the release of the west echo"suppressor relay 26 will be delayed for a desired.

hang-over interval determined by the'tim'e' rer quired for that condenser to be charged iup'to'v the potential of battery 44 throug'h'thehangoverwind-ing of relay zfi when relay 33 returns: 1

t -its normal operated condition, the current through the hang-over windin energizing'thati winding to hold the relay operated.

-When the west echo suppressor relay. 26 oper-' ates it will break its back switching contact: and

make its front switching contact. The break of the back switching contact will open the normal short-circuit across the winding 25 of the hybrid transformer H2 in the path WA causing. that hybrid transformer to be balanced and thus efiectively insert a disabling loss into the path WA which will suppress the echoes of .Wests speech waves when they arrive at that point in the path.

The make of the front switching contact of relay 26 will cause the negative battery 35 to be connected to the upper terminal of the 'potentiom-r eter through the 2-megohm resistance 46. As

an equivalent negative battery 41 is' connected to the lower terminal of that potentiometer, this will effectively isolate. the output of the east de-: l

tector 35 from the input of the direct current amplifier 32 while retaining an operative connection of the east break-in master relay 3'! in series with battery 45 across the output of the east detector sfithereby conditioning the circuit NQW, if, while West is still. talking, theeast forbreak-in by the east subscriber.

subscriber desires to interrupt him,rthe latter start talkin and will raise his voice sufficiently so that the portion of "his speech waves diverted from the east to west path WA at thehybrid amplified by: the amplifier 35.- and rectified by the, detector 36 therein, will be of sufficientamplitude to cause the operation of the east break-in master relay 3] in the output of that-detector. The operation of the latter relay to H close its switching contactswill cause the operating windingto'i the east break-in relay 38 to be supplied with energizing current from the battery 49,;and

will cause the condenser oi the associated hangcoil H2 into the east switchingjcircuit 34 and overcircui't 501 normally charged from battery" 49, to be discharged to ground. The consequent.

operation of the east break-in relay 38 will conmet the negative battery 5| to one side of the winding 25 of the hybrid' c'oil H2. As anequivalent negative battery 45 is. connected to the other side of the winding 25, that winding isthus effectively short'ecircuited to unbalance the a hybrid coil H2 in the path WA allowing Easts speech waves to be transmitted out over that path to the west subscriber.

The isolation the utput of the east detector- 36 .fromthe input of the direct current ampl g t fier 32 maintained by operation of the westecho' suppressor relay 26 in response to'Wests' speech waves in the manner which has been described; I will preventEast'from causing the operation of the east master switchin relay 39 in the output of that amplifier, while West continues to talk; 'I'husthe 'east echo suppressor relay 24 for that condition will-be maintained in its normally unoperated condition to hold former Hlin the path EA unbalanced to allow the hybrid trans Wests continuing speech waves to be transmitted over that path to the east subscriber. Thus, both subscribers. will: hear each other during the pe riod of-double talking, but there will be no echo suppression and both parties will hear echoes; This condition will continue. until the loud talker (East) lowers his voice sufiiciently to cause the" release of the east break-in master relay 3'! and thus finally the release of the east break-in re' lay 38'to remove the negative battery 5| from i one side of the winding '25'0f hybrid coil Hz to 1 remove the e'fiectiveshort circuit from that wind-- ing. This will allow West to regain directional control of the, circuit by continued operation of the west,echo suppressor relay 26 to hold its back contact open. @Therelease of the east breakin relay38, of course, will be delayed for a given hang-overt interval required for battery 49 to charge up the condenser of cult .59 through the winding of relay 38 after the east master relay 35*releases, thus maintaining the break-ineffective for normal pauses transmission of Easts speech waves.

in the Ifwhile East is talking loudly, the weak talker (West) stops talking so that his relay 33 releases causing the release of the west echo suppressor relay 26 after theex-piration of its hang-over (completionwof, charge'of the condenser of hangover circuit 43, from battery 44 through the hang over winding), the connection of the negative battery 45 to the upper terminal of the potentiometer 4.! "for direct current amplifier 32 will i be broken.

Thus the isolation of that amplifier" from the-output of the .east detector 36 will 5 cease.

Then the rectified'speech signals of East in, the output of the detector 36 will apply through i the; windi of .relay3l,- resistance 46 and poten 'tiometer 4|, a positive.

biasto the control gridof the direct current amplifierfltube 32 to increase the plate current of that amplifier suincientlyso that thejeast -master relay 39 in its out- I putwill be operated and the west'master relay; in its normal operated 33 will be maintainedcondition. The consequent shiftingof'the armature of relay 3-9. from thebaek to the front contact will causetheoperating-winding of the eastecho suppressor-relay I 2 1te -"Joe energized fro m' f battery 52 andfIwill-causeth' condenser ofthecondenser-resistance hang-over'-arrangemerit 55' normallyv charged from battery '53 through 'the hang-over winding of relay 24 to be'discharged i to, ground; through the made" front contact of tact will 'remove the normal'fsl'iort circuit from the winding 23 of the hybridl coil'Hi inthe path L EA to ,unbala ric'e .that hyb d t ransformer' soas to put" an echo suppressing oss into that? path.

ruse; the" make. of theiro'nt; fcontactfoffthe ref lay 124 will connect negative battery 54 through the 2-megohm' resistance An s the ub l. termii Y nal ofv the input potentiometer 41" for'the direct current amplifier 36". As "an equivalent negative battery 41 is already connected to the lower terminal of that potentiometer, this will cause the input of the direct current amplifier 32 to be effectively isolated from the west detector 29,

while maintaining an operative connection of the west break-in master relay 30 in series with battery 54 across the output of that detector. Therefore, the relay 30 is in condition to be operated in response to rectified signals of the west subscriber of sufiicient level (greater than normal level) to enable break-in by West in a manner similar to that which has been described in the case of the east subscriber.

When East ceases talkin the east echo suppressor relay 24 will be maintained operated to block the path EA for a desired hang-over time interval determined by the time required for the condenser of the hang-over network 55 to be charged up from battery 53 through the hangover winding of the relay.

Now, if while the west or east subscriber is talking loudly, the other subscriber east or west also starts to talk loudly, there will be no change in operation from the case where one subscriber is talking weakly and the other loudly, because, in that case both master break-in relays 30 and 31 will be operated to cause the operation of the west break-in relay 3| and the east break-in relay 38, respectively, to unbalance the respective hybrid transformers H1 and Hz to render both paths EA and WA operative.

Because of the conjugate relation of the third Winding 23 of the hybrid transformer H1 to the input of the west switching circuit H and the conjugate relation of the third winding 25 of hybrid transformer Hz to the input of the east switching circuit 34, the disabling of the respective transmission paths EA and WA will not affect the inputs of the two switching circuits.

It will be apparent that because of the combination functions of certain of the controls of the system of Fig. 1, and of the system of Fig.2, that both echo suppressor and break-in may be obtained with the use of a smaller amount of appa ratus than would be required if these two functions were exercised by two entirely separate control circuits.

Various modifications in the circuits which have been illustrated and described and within the scope of the invention will occur to persons skilled in the art.

What is claimed is:

1. In a two-way telephone sy WO PP sitely directed one-way transmission paths for respectively transmitting over the system the voice signals of the subscribers at opposite ends of the system, one switching device connected to .each path having a plurality of switching branches with some transmission apparatus in common, certain of said switching branches operating in response to the voice signals of one subscriber in that path, in the absence of prior voice signals of the other subscriber in the other directed path, to disable said other ath 'andsaidcertain switchingbranches of the otherswitchinggdeviceconnectedthereto, so as to g eff directionalfcqntrol of signal transmission over t e system it the subscriber, wh first starts t talk, and. means; to facilitate interruption of the" a i g-'- a i r-ywi n her. s bs r comprising 3'1 normally disabled, relativelyinsensitive switching branch for each switchin device, which," when operated, disables ,the transmiss on' at disabling function. n y oflthe other s'witchi'rig device and means responsive to operation of said one switching device connected to one path by the voice signals of said subscriber who first starts to talk to immediately render operative the relatively insensitive switching branch of the other switching device connected to the other path so as to enable operation of the latter branch in response to subsequent voice signals of the other subscriber which are of sufficient loudness to overcome its desensitization.

2. In a two-way telephone system, two oppositely directed one-way transmission paths for respectively transmitting the oppositely directed voice signals, and a switching device connected to each path, consisting of a common signal detector operatively responsive to voice signal transmission in the connected path, and a plurality of switching branches operatively controlled by the signal output of said detector, certain of said switching branches operating in re- 0 while the signal transmission in the first path operation of the associated continues, in response to subsequently initiated voice signals in said other path, only of an amplitude level at least a given amount greater than normal level, to cause said other path to be enabled, and to disable all functions of said other switching device except its transmission path enabling function.

3. The system of claim 2, in which one of said plurality of switching branches of the switchin device connected to each pin, is relatively insensitive and is normally disabled, said one switching branch when operated in response to detector by applied voice signal transmission of suflicient amplitude level in the associated transmission path, disabling the tranmission path disabling function only of the other switching device, the transforming of the other switching device caused by operation of said one switching branch of the first switching device consisting in removing the normal disability from the relatively insensitive switching branch of said other switching device.

4. The'system of claim 1, in which said plurality of switching branches of the switching device connected to each path includes a common wave detector operatively responsive to the direct voice signals in the associated transmission path but unrepsonsive to the signal echoes of maximum amplitude produced therein, and individual relays in the several branches, the individual relays in said certain branches of the switching device connected to each path being responsive to the rectified voice signal output of the detector in that switching device, in the absence of a prior rectified signal output for 6 was the detector in the other switching device connected to theother path, to insert anecho suppression loss in said other path, and to effectively disable the individual relays .in said certain branches of 'saidjother switching device, the v relay in the other switching branch of each switching device being made efiectively relatively insensitive compared to the relays insaid certain branches thereof; so as to beinop'erative for a rectified signal output of the detector in that device produced byvoice signals of normal amplitude in the associatedtransmisslon path but to be operated in response to a rectified signal output for that detector produced by voice "signals ofian amplitude a given amount greater than normal amplitude in said associated path, "to effectively prevent the echo suppression loss "from being inserted in said associated transmission path by operation of the other switching device connected to the other transmission "path, or to effectively remove that loss if already inserted in said associated path.

KINGSBURY H. DAVIS.

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