US1695813A - Electric-current transmission - Google Patents

Description

Dec. 1'8, 1928.

' R. c. MATHES ELECTRIC CURRENT TRANSMISSION Filed oct. 9, 1925 4"sheets'sheet 1" Dec. 18, 1928` I 1,695,813

R. c. MATHEs v ELECTRIC CURRENT TRANSMISSION Filed Oc?. 9, 1923 4 Sheets-Sheet 2 ffm fLMMMM/m f Vvvvwvwfm A l F/gffl MMM/wm, \/\/\/\/vvvvwm l /nvemr Habe/7* I Ala/bets Dec. 18, 1928.

l R. c. MATHEs ELEGTHIC CURRENT TRANSMISSION Filed Oct. 9, 1923 4 Sheets-Sheet 5 ffl? Dec. 18, 1928. A 1,695,813

Y R. c. MA1-HESv ELECTRIC CURRENT vTRANSMISSION Filed oct. 9, 192s 4 sheets-sheet 4 9,5' Amiliferd Rectfer )naw/L' fn a e if ffy@ //0 v //a I //2 TH 'Mlm 'I'I'Ir .///IT //"r To Speech Amplifier v /nvemar Rabe/f6 Maf/M5 Patented Dec. I8, 1928.

UNITEDN STATES APATENT OFFICE.

ROBERT C. MATHES, OF WYOMING, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, IN CORPOBATED, OF NEW YORK, N. Y., `.A CORPORATION OF NEW YORK.

Application led October 9, 1923. Serial No. $67,437.

i its function, as for instance, by causing un-` due loss in articulation because of the fallure of the contacts of the switchingl means to condition a signaling path for efficient transmission in the desired direction in time'to transmit `properly the 'initial part .of the message incident upon'the path.v It has beenl suggested that such a tendency may be overcome by the proper use of a so-called delay circuit, consisting, for example, of a low-pass lter type of structure comprising' recurrent sections each having a series arm containing lumped inductance and a shunt arm contain- `ing lumped capacity. For instance, it has been proposed to insert such a circuit, which has the property of .appreciably delaying the propagation of signaling currents therethrough, in the part of the signaling path between the point inthe path at which the switching means responsive to transmission derives energy from the path and the'point in the path at Which the switching means conditions the path for eificient transmission. In accordance with this invention, in order to diminish the requisite delay time of delay circuits in suchnses, the switching means is caused to accomplish its function immediately upon the openin of a contact, the opening of the contact eing accomplished in response to signaling currents and without delay due to other mechanical movement. The reason Why this procedure-results in vdiminution of the requisite delay time of the delay circuit is that the time required for a relay to break, or open its contact, is far less than the time consumed in the travel of the armature vto a make, or closing contact.

It has been discovered that Where highy speed switching means, or other quick operatino means, responsive to transmission is used for controlling arepeating circuit in order that when the repeating circuit is in use'it may give good transmission in one and only one directionat a time, the operation of the controlling means may be deleteriously effected by transient energy which. may emerge from the line connected to the output of the yrepeater after the input to that line has ceased. In general, when the input transmission to the line ceases lsome energy will return from it and will enter the output of the repeater. There are various factors which may cause energy sent out by a repeater to reenter it. For example the energy may be reflected from terminal apparatus; or may be stored in the line and fed. backfinto the repeater, or stored in terminal apparatus and fed back into the line and repeater, upon cessation of input from the repeater to the line; or may be sent back into the line and repeater byy repeaters such for instance as ordinary, or standard, 22- type repeaters to which the line may be connected. An eXtreme case is that of a repeatered line with excessive echocurrents. In the case of a voice controlled telephone repeater, for instance, as far as tlierepeater and its controlling apparatus are concerned this transient returned energy acts in` the Same Way Vas do voice currents coming from the listener..y Consequently, if the energy of to the repeater from the line into which they were last transmitted, and may cause the repeater to aga'become operative in the d1- rection in whiclit first transmitted and transmit some of the transient energy to the line from which it was originally returned. Thus,

these transient returned currents is sufficient,

of its direction of transmission. Even if the repeater be not set intosustained singing, there may be sufficient lreflection of energy to cause it to reverse its direction one o'r more times. The effects of such reversals would de end upon the number of the reversals, tllfe type of the repeater and other factors.

I n accordance withthe invention the repeater controlling means is made to have at the end of each period of uninterrupted transmission, a relaxation time, or period of insensitivity7 to energ entering the output side of the repeater, t isrelaxation time being;r made suitable for protecting the repeater from deleterious elects of transientkickback or return of energy from the connected' lines; -for it has been discovered that, in general, it is possible to provide a sufiicient vrelaxation time without causingundue loss in articulation. because of interference with the ability of the transmission receiving station to break in or take control of the repeater vwith suiicient prom tness upon cessation of transmission from t 1e repeater into the line connected to that station.

vIt has heretofore been proposed, for example in John Mills Patent No. 1,434,790, November 7, 1922, to suppress echo currents in long two-wire circuits involving bilateral repeaters, that is, repeaters in condition atv some one time to transmit in either direction, for instance, the ordinary or standard 22- type repeaters, by `emp1oying meanswhich respond to transmission in one direction in Ithe systemin such manner as to cut oif transmission in the otherdirection. In accordance with the present invention, a repeater which is never in condition, at any one time to transmit in more than one direction, an

which responds to transmission in one direction to condition the repeater for`tra'nsmitting in only that direction, may be connected in a long two-wire circuit, either alone or in tandem with bilateral repeaters, tosupf press echoes as welll as to amplify the trans` mission in either direction; and where it is desired to reduce the number of possible echo paths or break up echoes other than those which would traverse the whole length of the circuit, a plurality of echo suppressing repeaters may be connected in the circuit, one at each point .at which it is desired to stop the passage of echo currents. The nature and effects of echo currents have been explained by A. B. Clark in his article on Telephone transmission over long cables, Journal of the American Institute of Electri-cal Engineers,

, January 1923, page 1, and reference may be had to that article for a better understanding of the phenomena of echo effects.

Fig. 1 of the accompanying drawings is a circuit diagram of a repeater embodying various features of the invention; Figs. 2, 4 and 5`are curves for facilitating explanation of the operation of the circuit of Figs. 1 and 3; Fig. 3 shows a modification offa part of the circuit of Fig. 1; Figs. 6 and 7 show two modifications of another part of the circuit of Fig. 1; Fig. 8 is a circuit diagram of a repeater embodying various features of the invention and employing only one speech ampliie'r and delay circuit; Fig. 9 shows a modication of a part of the circuit of Fig. 8; and

Fig. 10 shows schematically how repeaters such, for example, as thatshown in Fig'. 1 or Fig. 9, can be employed for su pressing echoes in long two-wire circuits, w ich may have bilateral repeatersr connected therein.

In Fig. 1 two lines 11 and 11 forming, for instance, part of a two-wire telephone transmission'system, are interconnected by a repeater circuit comprising an east repeater 13 for transmitting from west to east and a f west repeater 13 for transmittingfrom east to west, as indicated by the arrows. The repeater circuit also comprises the usual lthree-winding transformers or hybrid coils 15 land 15', and comprises resistances 17 and 17 in place of the impedance networks usually employed in a 22-type `repeater for balancing the impedances of the lines between which the repeater is connected. Preferably, resistances 17 and 17 approximately balance the impedances of lines 11 and 11' respectively; but, for reasons which will be made apparent hereinafter, close balancing is not required. If desired, transformers 19 and 19 ma be interposed between lines 11 and 11 an their hybrid coils, respectively, as shown.

' The output circuit of'repeater 13 includes the primary winding of a transformer 20.

`lThe secondary winding of transformer 20' is connected to one limb of a hybrid coil 21,

another limb of which is connected to a rel sistance 22 for balancing the output impedance of the repeater 13. The-series winding of the hybrid coil 21 is connected to feed into aodelay circuit 25, described hereinafter, the delay circuit in turn feeding the series Wintiing f the hybrid coil 15', that is the winding conjugate `to the bridge circuit of the hybrid coil. If desired, a transformer 26 may be interposed between the delay circuit 25 and the series winding of hybrid coil 15', as shown.l

The output circuit of repeater `13includes theV primary Winding of a transformer 20. The secondary winding of transformer 20 is connected to one limb of a hybrid coil-\21,

anotherlimb of which is connected t0 a resistance 22 for balancing the output imped: ance ofthe repeater 13. Theseries Winding f the hybrid coil21 is connected to feed into a delay circuit 25 similarto the dela circuit 25, the delay circuit 25 in turn lfee ing into the series winding of the hybrid coil 15. If

desired, a transformer 26 may be interposed between the delay circuit 25 and the series winding of the hybrid coil,15,asshown.

The output of the delay circuit is nerf -mally short circuited through a lead 30, the

armature'and back contact of a relay 31, a lead 32, the armature and back contact of a two-Windin g relay 33, and a lead 34.

The output of the delay circuit 25" is norerably comprising tandem connected electron tube amplifiers 42 and 43 feeding a high pass filter 44 havinga cut-o5 frequency of the order of 500 cycles per second. l'.lhe filtermay be, for example, of the type shown in Fig. 6 of G. A. Campbell Patent No. 1,227,114, May

' 22, 1917. The filter 44 feeds a rectifier 45,

preferably of the three electrode electron tube type, through a lead 46, the normally closed contact of a relay 50', a lead 51, the normally 0.1 microfarad is preferably. connected across closed contact of a relay 52', a lead 53, and, ifdesired, a transformer as 'shown at 54. A condenser 55 having. a' capacity of the order of 4 the output circuit of the rectifier, this output circuit including the Winding of the relay 31 and also the Windingof a relay 50.

The bridge circuit of the hybrid coil 21'.

preferably comprises the primary Winding of a transformer 40", the secondary Winding of Which feeds an amplifierand filter 41 'correv sponding to thel amplifier and filtler '41.- -The amplifier and .filter 41 feeds a rectifier 45', corresponding to the rectifier 45, ,through a lead 46', the normally closed contact of the relay 50, a lead 51', the normallyclosed contact of a relay 52, a lead 53', and, ifdesireda transformer as shown at 54". The rectifief- 454includes a condenser -(not shown) corresponding to the condenser 55, connected across its output circuit, and this output circuit comprises the winding ofthe relay 31' and also the Winding of the relay 50.

The right hand winding of the relay 52 and the left handwinding of' the relay 33 'are the operating windings ofth'ose relays, respectively, anda-re energized to operate those relays Whenever relay 31 closes .its front con-v tact, the energizing circuit extending from a grounded battery 60 through the left hand winding, or operating winding, ofthe relay 33, the operating Winding of the relay 52, the front cont-act of the relay 31, the head 30', and' a lead 61, to ground.

The left hand winding of the relay 52 is a holding winding, and is energized by alocal battery 62 whenever-'the front contact of relay 33 is in the,y closed condition.

`propagation of si nals.

' if desired, the right hand winding of the relay 33 `may be short circuited by a switch 63,

to retard the release of they armature of the relay, and a condenser with means for varying its capacity may be connected across the-operating Winding of the relay, as vshown at 64, to retardthe release of the armature ofthe rela yzo 52', the front conta-ct of the relay 31', the lead v 30 and a lead 6l', to ground. v

Theright hand Winding of the relay 52 is a holding Winding, and is energized `by a local battery 62 Whenever the front contact of relay 3 3' is in the closed condition.vv

If desired,the `left hand Winding of the relay 33' may be short circuited by afswitch 63',

to retard-the release of the relay armature, .and a condenser may be connected across the operating Winding of therelay, as shown at 64', to retard the release of the relay armature.

The delay circuit 25 is shown, by way of example, asa low pass filter type of structure, such as that of Fig. 7 of G. A. Campbell Patent No. 1,227,114, May 22, 1917, Electrical receiving, translating, v-or repeating circuit, comprising recurrent sections each having a series arm containing lumped inductance and a shunt arm containing lumped capacity. However, thedelay circuits are herepemployed as means to delay the propagation of .signalf ing' currentstherethrough, for reasons which W-ill be made apparent hereinafter, and they need not necessarily have filtering properties, but may be circuits or means of any type suitable forfproducing the desired delay in the The delay .circuit 25", lil". the delay circuit 25, is intended to represent any suitable means that may be employed for. introducing a desired delay in the propagation of signals therethrough.

To reduce the effect of line noise upon the operationof the repeater, the grid potential ofthe rectifier45 should preferably be set so that a definite minimum alternating current input tothe rectifier Will be required before the relay 31 or the relay 50 gets sufficient recv tified input to be operated; for if, instead, it be attempted to protect relay 31 or relay 50 against'line noises below agiven minimum by biasing the relay, as for instance by a biasing current in an auxiliary Winding'.(not shown) on the relay, then, noise energy being amplified in substantially the same ratio as speech energy, substantial gain in the speed of operation ofi-the relay is not readily obtained by increasing the driving or operating energy, since the biasing current must be correspondingly increased. Where the desired margin against noise is secured by choking ofi' the noise ener before it reaches the relay circuit, the Spee of operation,v of the -relay can be made high by applying a high value of driving or operating energy thereto.

Were the filter 44 and the condenser 55 omitted from the circuit, currents. ofvfrequenare similar to those of the amplifier and filterv cies in the neighborhood of the mechanical resonance frequencies of the tongues of relays 31 and 50 would tend to cause chattering of these tongues when they should be held in their operatedpositions by the relay windings, and, further, louT frequency noise currents, for instance, cross-ringing, Morse thump, and power interference,l which are likely to be greater in magnitude than the voice currents themselves, might deleteriously affect the operation of the'relays. The ,filter .44 prevents such disturbing currents from the rectified current to the relays.

The characteristics and functioning ofthe amplifier and filter 41.and of the rectifier 45 41 and of the rectifier 45, respectively.

The operation ofthe system in repeatingI from West to east will `now be described? and its operation in repeating from east to west,

which is A description.

Part of the transmission from line 11 passes to the input of rep-eater 13 and part is lost in the short circuit extending from similar, will be apparent from this the secondary winding of transformer 26 through leads 30 and 34 and the back contacts of relays 31` and 33. A. part will also be lost in resistance 17. Part of the output of repeater 13 is transmitted through liybrid coil 21 and amplifier and filter 41, to rectifier 45, which operates relays 31 and 50. The operation ofrelay31 places a gap in the short circuit which extended across the primary winding of transformer l26 through leads 30 and 34 and the back contacts of relays 31 and 33. Thus, the operation of relay 31V enables transmission to pass onto vhybrid coilv 15 and thence to line 11 At the Sametime relay 50 has place la gap in the output circuit lof the amplifienand filter 41', which will prevent the amplified transmission coming into the inputmof repeater 13 `from operating relay 31 to render the repeating path west operative by opening the above traced short circuit across the primary winding of transformer 26', or from' operating relay 50 to open the output circuit of the amplifier and filter 41.

' By providing the delay circuit`25, the above tracedsliot circuit across the primary winding of transformcr26`may bei opened before the transmission from re eater 13 reaches it,

thus avoiding any loss o the beginning of'a by the delay circuit, since the delay circuit insuresbthat relay 50 will begin to operate an appreciable time before any possible start-v ing of the operation of relays 31 and 50.

Upon cessation of transmission from line 11, or when transmission falls below a predetermined `energy level, relays 31 and 50 release. The value of this predetermined level is dependent upon the energy level of line noise or external disturbances to which the-system is subject, and upon what man gin or degree of protection'against such disturbancesit is desired to afford the system. In practice,.there is a tendency for the release o'f relay 31 to cut ofi the transmission of the last syllableof words ending weakly, such as those with the final syllables ly and ring, Relay -33 acts to reduce this terminal clipping in'the'following manner.- As has been indicated' above,- when relayv 31 closes its front contact in response to .transmission from line 11, it causes relay 33 to'placea gap. in theabove traced short circuit across the rimary winding of transformer 26, this gap eing in series with the gap produced at the back contact of relay 31. Therefore, when the energy level of the transmission from linel 11 falls to such a value that relay 31 releases, opening the operating? circuit of re- `lay 33 at the front contactcof the relay 31 andclosing the gap at the back contact of the relay31, the short circuit across the p rimary winding of transformer '26 remains open at the back eontact ofl relay 33 until that relay, which is slow operatin the condenser 64 (and the ri ht i and or short circuited winding of the re ay, if switch 63 is closed), releases its armature and closes' its back contact.

Another advantage which follows from the use of relay 33 is that,l if two of the syllables coming from line 1-1 are separated by an interval shorter, than'fth'e time requiredfor relay 33 to close"it's'back4 contact, the second syllable will find therepeater circuit already operated--temporarily biased in its favor-and due to' -hence there would be no tendency toward initial clipping of this syllable.

As indicated above, when relay 31 closesits front contact in response to transmission frontline 11, it causes relay 52 to place a gap in the output circuit of the amplifier and filter 41', this gap being in series with the gap produced at the back Contactv ofrelay 50. riherefore, when transmission from llne 11 ceases, or when the energy level of that vtransmission fa-lls to such a value that the relays 5() and 31 release, the relay 31 opening the circuit of the operating winding of relay 52 at the front contact of the relay 31 and the relay closing the gap at the back contact of the relay 50, the output circuit of the amplifier and filter 41 remains open at the back contact of relay 52 until that relay releases and closes its back contact. The release of relay 5,2. is delayed by its left hand winding or holding Winding, the local circuit of which remains closed until the armature ofthe slow reltase relay 33 has moved awayvfrom the front contact of the relay 33. The speeds of release relays 33 and 52 are so adjusted that, at the contact of relay 52, the output circuit of the amplifier and filter 41 remains open for a short interval of time after the last transmission'from line 11 which caused relays 31and 50 to remain operated has passed through the delay circuit 25 and onto line 11. vDuring this interval the repeater circuit'is not responsive to energy incoming from line 11. The provision of this time interval, or relaxation time, is to insure that energy transmitted from hybrid coil 15 into line 11 and returning from line 11 to hybrid coil v15 either in amplified orunamplified form, cannot reverse the repeater, nor cause the repeater to sing between Vthe lines 11 and 11 due to sustained repetition of such reversals in the manner referred to above.

Various factors must'be considered in determining what isa proper time interval to be employed as a relaxation time. If the interval is too long, it will unduly interfere with the ability of the listener to'break in on l the speaker, or it may even happen that the first part of a quick reply will be cut off. If

the interval is too short, the energy level of ^the transient energy returning to the repeater from the line will, at the end of the period, be'

sufficiently high to cause reversal of the repeater. The energy level of the'returning transient energy at any given instant will depend upon the magnitude of the last repeated energy and the condition of the line into which it Was repeated. For any given energy level of the returned transient at the end of the relaxation period, the likelihood of reversal of the repeater by the transient will depend upon how long, after the period of insensitivity, the means controlling the repeater switching means requires to attain a condition in which the subsequent operation of the Switching means is assured. With a circuit of the type shown in Fig. l, a swltching time of less than .003 second has readily been attained. By the switching time is meant the total time required for ordinary voice transmission to switch the repeater from the normal 01' neutral condition to the condition for repeating in one direction or the other. The likelihood of the occurrence of repeated reversals or singing will further ,depend upon this switching time, or the total time taken, after the period of insensitivity, to switch the repeater vand also upon the condition of the line into which the repeater feeds when first reversed by transient energy, and the magnitude of the energy so fed into that line. Itis entirely possible that even though single reversals from reflected energy would not take place with the transmission of ordinary speech, a large impulse such as would result from the operation of the subscribers switchhook might set the system 0E into repeated reversals or singing.

Extensive experiments have shown that in a circuit not including any bilateral repeaters, that is, repeaters' conditioned to repeat in opposite directions simultaneously, a relaxation period of the order of .02 or .03 second'is, under ordinary conditions, suiiicient to prevent returned transient energy from causin reversal or singing of the repeater. Shoul any bilateral repeaters be included in the eircuit this relaxation time would have to be increased to a value of the order/of 'double this value, the exact requirement as to minimum permissible relaxatlon time depending upon the individual case.

The shortest pause of a speaker which will permit the listener-to gain suiicient control of the repeater to insure that the repeater will be reversed, is longer than the relaxation time of the repeater, or inA other words, is ylonger than the period of insensitivity of the repeater controlling means to energy entering the output side of the repeater upon the cessation of the energy output of the repeater tothe line. Consequently,`serious i interference with the carrying on of conversation can be avoided only if the minimum permissible relaxation time ofv thel repeater is less than the longest silent interval that, in ordinary speech, occurs as often as it is required that the listener be able to break in on the speaker. It has been discovered by extensive experiment, that speech, silent intervals (or intervals when the energy level of the speech is so low that peater do occur as frequently as it is ref quired, for satisfactory conversation, that the listener be able to breakin upon the. talker. It has been determined that satisfactory ease in ordinary ,120

the effect of the speech in preventing the lisof break can be secured if the relaxation time of the repeater is of the order of .05 second. y Since a relaxation time of the order of .02 or .03 second is sufficient to prevent transient return energy' from causing reversal or singing-of the repeater in a circuit involving no bilateral repeaters, and since satisfactory ease of break can be secured with a relaxation time as great as .05 second, a relaxation time of ab0utv.03 second may be employed as prac4 ticable for such a circuit.

The back contact of relay 33 should close slightly before that of relay 52, so that the repeater path east will be rendered inoperative vbefore the repeater path west can be rendered operative and there will therefore be 'no ossibility f singing of the repeater due to oth of these paths being operative at the saine time. Since these two paths are never operative at the same time, the resistances 17 and 17 need not accurately balance the impedancesof lines 11 and 11',- respectively,'in order to prevent the repeater from singing. Neither is it necessary that such 4balancing be accurate in order to insure that energy from line 11 will divide between the other three impedances connected to hybrid coil 15 in such manner that a satisfactory proportion of the energy will enter the repeater 13. It may be here noted that if resistance 17 accurately balances line 11 the short circuit across the lower winding of transformer 26 by means of relays 31 and 33 will not affect the input from line 11 to the repeater 13. If the balance is not accurate this short circuit will produce some effect on the input to the repeater, but still none of importance.

It has been found that reduction of interference between the two branches of the cir! cuit of Fig. 1 is attained by locating the relay breaks in the amplifier rectifier systems on the grounded sides of those systems, as shown at the contacts of relays 50, 52 and 52 inv Fig. 1, instead of on the ungrounded sides of those systems.

In Fig. 2 the full line curve is an effective resistance-frequency curve, and the dotted curve is an impedance-frequency curve, typical of a delay filter such as 25 with its output short cireuited as is normally the 'case in the circuit of Fig. 1. The vertical arrows indicate, approximately, natural frequencies of the short circuited delay circuit, and these frequencies give steady state impedance minima and thereforewould give maxima of steady state input current for constant amplitude of voltage applied to the input of the delay circuit; but-,as explained hereinafter in connection with Figs. 4 and 5, when a voltage of anyone of these. frequencies is suddenly applied the current which f'lows into the delay circuit rises .to its final or steady state value slowly, in the manner indicated in Fig. 4. Therefore, if the input of the delay circuit and the primary1 *winding of the transformer 40 were serially connected to each other directly across the secondary winding of transformer 20, and the hybrid coil 21 were omitted from circuit, then, for.

any oneof these frequencies, the input to transformer 40, and hence the rectified current applied' to the relays 31 and 50, would,

rise-to its final value slowly, and consequently the relays would be slow operatin for any of these frequencies, since the sensitivity of the aiv relays must be kept low enough to insure' that they will not be operated by the steady A state value of relay currents-due to noise currents of these frequencies. For any'frequency midway between frequencies indicated by adjacent vertical arrows in Fig.v 2, the final .value of the current flowing into the short circuited delay circuit would be small., but the current would rise rapidlyto a value even exceeding the final value, in the manner indicated in Fig. 5. y If the input of the short circuited circuit and the primary winding of the transformer '40 were connected i n pan allel directly across the secondary winding of transformer 20, the hybrid coil 21 beingv 2 the .current in the transformer 40 would p rise slowly to a lhighsteady state value.

Therefore, in the case of either the series f -orthe parallel connectionof the vtransformer 40 and the short circuiteddelay circuit, -tlie speed of operation of both relays 31 and 50 would be diderent forfdifferen't frequencies of voltage applied to hybrid coil 15 by line f 11, and therefore would be different for different syllables coming into the repeater from line 11.

When the primary Winding of transformer.

40 and the input of the delay circuit 25 are connected to the secondary winding of transformer 20 through the hybrid. coil 21 instead of directly, so that the impedance of the delay circuit is in conjugate relation to the primary winding of transformer 40 or circuit which supplies energy to the relays 31 and 50, the impedance of the delay circuit will not'affect the amount of the current which flows to the relays at any frequency. 1t has no effect at any frequency,y it can have no transient effect. Therefore, by `thus .employing the hybrid coil 21, the speed of operation of the relays can be equalized for different frequencies, and yet the energy ,for cont-rolling the relays can be picked' off from the main transmission circuit` at the output side of the speech amplifier 13, so that is generally stepped up to as high a voltage as practicable, with the danger of driving the grid of the first tube decidedly'positive. In such cases a degredation of speed being transmitted may occur, due to the introduction of harmonics. By employing a balanced coil', asl that at 21 in Fig. 1, such harmful reaction can be prevented. As the amplifierrectifier is in conjugate relation to the outgoing circuit carrying the main transmission, any harmonics generated in the amplifier rectifier circuit are balanced out from that out? going circuit.

In Fig. 1 the repeater 13, the hybrid co1] 21 and the delay-circuit 25, are shown as con1-.

prised in a block 66, and in Fig. 3 is shown a block 66n which may be substituted for the block 66 in Fig. 1. In the block 66a no hybrid coil corresponding to the hydrid coil 21 of Fig. l is employed. The delay circuit 25 is connectedto the repeater 13 through the transformer 20, and also through,I a transformer 67. A transformer 40, the secondary winding of which feeds the amplifierrectifier 41-45 (not shown in Fig. 3) just as the secondary winding of transformer 40 of Fig. 1 feeds that amplifier-rectifier, has a primary winding 68 connected acrossthe circuitbetween transformers 20 and 67 and has a primary Winding 69 connected across a resistance 70 which is serially connected in that circuit. The steady state impedance of the short circuited delay circuit passes through wide variations (see Fig. 2) as the frequency' changes, and correspondingly its transient impedance, and at each of the low impedance points in the range of applied frequencies that is, at each frequency such as these indicated by the vertical arrows in Figs. 2) the current entering the dela'y circuit, and consequent-ly the voltage across winding 69, Varies in the general 'fashion indicated in Fig. 4, whereas when a voltage of any frequency midway between those indicated by adjacent vertical arrows in Fig. 2 is suddenly applied the current entering thedelay circuit,I and therefore the voltage across Winding 69, varies in the general fashion indicated in Fig. 5. However, when the current entering the delay circuit varies as in Fig. 4,' the voltage across the delay circuit, and therefore aero-ss winding 68, will vary in the generalfashion indicated in Fig. 5, wherein the time scale .is the same as in Fig. 4; and when the current .y

entering the delay circuit varies as in Fig. 5. the voltage across the delay circuit, and therefore across winding 68, will vary in the general fashion in'dicated in Fig. 4. Consequently, by combining the fluxes produced by windings .68 and 69 in transformer 40, the voltage wave applied to the amplifier-rectifier L1:1--415 by the secondary winding of transformer 40 can be made to be vof approximately the same form as that applied to hybrid coil 15 byline 11.'

In Fig. 1 there is shown a block 73 comprising the .transformer 26,'hybrid coil 15',

resistance 17 and transformer 19. In Fig.

v6 there is shown .ablock 7 3al whichanay be substituted for the block 73 in Fig. 1. Leads 711', 75', 76, 61, 34 and 30 in Fig. 6 are the same as correspondingly designated leads in Fig. 1. Leads 76 and 61, which are'the output leads from the. delay circuit, may be short circuited by the circuit through leads 30 and 34 and the back contacts of relays 31 and 33 (not shown in Fig. 6), if a short section of artificial line, indicated as 80 in Fig. 6, be inserted between the short circuited and the junction of the input of repeater 13 with line 11'; for the artificial line 8() will prevent the short circuit from also short circuiting the input of repeater 13 with respect to line 11. c

The portion of the circuit of Fig. 1 which is included in the block 73 may also be replaced by a circuit of the form shown in the block 7 3b in Fig. 7. In the latter figure, the

transformer 26 feeds into line 11 through a balanced bridge 85, instead of through a hylbrid coil as in Fig. 1. One side of the bridge comprises a resistance 88 and a secondary winding 89 of a transformer which has its primary winding fed from the transformer 26; and the other side of the bridge comprises a resistance 90, preferably equal to resist-A,V

ance 88, the circuit which extendsr through lead 30, thence through the back contacts of relays 33 and 31 (not shown in Fig. 7), and thence through lead-34, and a winding 91,

preferably having the same number of turnsv as winding 89. Normallv the bridge is in the balanced condition and'currents fed into it from transformer 26 produce no voltage across line 11'; b ut whenever the last mentioned side of the bridge is open at the back.

13 and the lines 11 and 11, and also- elements 15, 15', 17, 17', 31, 31', 5o and 50 in Fig. 8

are the same as in Fig. 1. Relay 152 in Fig. 8 has its operating winding in series with the front contact of relay 31 and its contact in series with the contact of relay 50, just as has the relay 52 in Figfl, and'is a slow release relay although the circuit of Fig. '8

has not been shown as rovided withl any relay corresponding to t e relay 33 of Fig.'

1. Similarly, relay 152 in Fig. 8 has its operating winding in series With the front contact of relay 31 and its contact in series ,with the-contact of relay 50', justas has the'relay 52 in'Fig. 1, and is a slov'v7 release relay although the circuit of Fig. 8 has not been shown as provided with any relay corresponding to the relay.33 of Fig. 1. vAmpliier-rectitiers 95 and 95 in Fig. 8 correspond to the ainpliier-rectiiers 41-45 and 414.5, respectively, of Fig. 1, but may, if

desired introduce greater ampliication, to a make up for the fact that the input to am- J pliier-rectiiers 95 and 95 is obtained directly from the hybrid coils and 15', re-

i spectively, Without amplification.

A balanced bridge 9G, which can be balanced .quite closely, enables transmission from either line 11 or 11 to be supplied to the input of repeater 13 without get-tlng over to the other line.

The operation of the circuit in transmitting from West to east will now be described,

and this will make apparent the operation-in transmitting from east to .West, which is similar.

The output of the repeater 13 is normally short circuited at the contact of-relay 31 and also at the contact of relay 31"; but, as

pointed out in connection with Fig. 1, the short clrcuiting of the serles Wlndlngs of the hybrid coils 15 and 15 does not mate-` rially interfere ivith the supply of energy from lines 11v and 11 to the bridge circuits of the hybrid coils 15 and 15 respectively.

ySpeech coming from line 11 Will start through the'bridge 96, repeater13 and delay to open the back contacts of relays 31 and 50,

there will be no danger ofimproperly operating relays 31 and 50 or setting up a local singing circuit. The closing of the front cont-act of 'relay 31 causes the operation tof relay 152 through a local energizing circuit.

When transmission from line 11 ceases, re'd lays 31, 50 and 152 releases, but the latter relay maintains its contact open for a prede.- termined time to protect the system against .transient kick-back of energy from line 11.

The time 'of closing of this relay 152 is made sufficiently great to give the system a. relaX ation time of a value such as is specified above for the system of Fig. 1. The relay 31 be a close balance'vfor the impedance of line 11.

If desired rela-ys corresponding` to the relays 33 and 33 of Fig. 1 may, of course, be

incorporated in Fig. 8, to delay closure of the short circuit-s `controlled by relays 31 and 31 respectively and thus reduce terminal clipping of Words, and to delay release of rclays 50 and 50 respectively. y

Instead of employing the hybrid coils 15 and 15 in Fig. 8 to prevent the short circuits across the output of the repeater 13 from materially interfering with the supply of energy from lines 11 and 11 to the input of the repeater, artificial lines (not shown in Fig. 8) may be employed. after the fashion in which, as pointed out above, the artiiclal line 80 of Fig. 6 may be employed instead of the hybrid coil 15 in Fig. 1.

Fig. 9 shows a circuit which may be substituted for the balanced bridge 96 in Fig. 8, to connect theinput of the repeater 13 to the bridge points of the hybrid coils 15 and 15. This circuit comprises an amplifier 110, preferably of the electron discharge type, fed from the bridge points of the hybridy coil 15 (not shown in Fig. 9) through the back contacts of relays 50 and 152 (not shown in Fig. 9) and leads 111 and 112, and in turn feeding into the speech amplifier 13 (not shown in Fig. 9) and the circuit also comprises an amplifier fed from the bridge points of the hybrid coil 15 (not shown in Fig. 9) through the back contacts of relays 50 and 152 (not shown in Fig. 9) and leads 111 and 112 and in turn feeding into the speech amplifier 13 (not shown in Fig. 9). Since the amplifiers 110and 110 are unidirectionally transmitting devices, they allow energy to be supplied from either line, 11 or 11', to the input of repeater 13, but prevent that energy from gettlng over to the other line Without passing through the repeater 13,

' the delay circuit 25 and the series Winding of the hybrid coil 15 or 15 connected to that other line.

Fig. 1() shows a long two-wire circuit, for' instance a telephone transmission line, repeatered with one or more bilateral repeaters 210, for exam le ordinary or standard 22-type repeaters suc as that shownin Fi s. 10- or 11 of G. A. Campbell Patent No. 1,22 ,114,'May 22, 1917. One or more unilateral, voice oper- ,ated repeaters 215, which maybe, vfor example, a repeater of the type shown in Fig. 1 or of the type shown in Fig. 8. are -also con nected `in the line 210. The dottedlines at each side of the repeaters are to indicate that the repeaters may be electrically or geographically remote from each other and from a subscribers station 215 atthe West end ofthe time adjusted, as pointed out in connection with the circuit of Fig. 1, to prevent energy i returned or fed back into the repeater from the line from reversing the point of the repeater. Therefore, the repeater will suppress echo currents, mitting in one direction, must have its pointing reversed before it is capable of transmitting in the other direction. vAs explained in connection with the circuit of Fig. 1, the relaxation time of the repeater should be made suiiciently great to take care of the longest period of appreciable'returned energy or the longest echo of objectionably high volume or energy level, but vshould not be sufficiently long to cause undue interference with the ease with which a listener can break in upon a speaker.

'Operating airelay from energyof a current having high and low frequency components by separating the high frequency components from the low frequency components, rectifying the high frequencycom'ponents alone, removing current fluctuations of said high frequency from the rectified current, and

. applying the resultant current to the relay, is

claimed in R. V. L. Hartley Patent 1,588,186, .Tune 8, 1926, assigned to the assignee of this application. I

Rendering a voice controlled repeater operative to transmitin the desired direction immediately -upon' the openings of switch contact, the opening of the contact being accomplished in response to voice currents and without delay due to other mechanical movement, is claimed in W. S. Gorton application Serial No. 658,514, filed August 21, 1923, as-

signed to assignee of this application.

The application of a delay circuit to a. voice controlled repeater to delay transmission until the voice operated switching means has had time -to operate is claimed in H. D. Arnold Patent 1,565,302, December 15, 1925, Two-way repeater circuits, assigned to the assignee of this application.

The general principles herein disclosed may be embodied in many organizations widely different from those specifically illustrated and described without departing from the spirit of the invention defined in the apended claims. A

Although the invention has been set forth with especialreference to its application to the repeating of voice frequency currents, it is of course also applicable to the-transmission and repeating of currents of other frequencies, whether higher or lower, as for instance currents of the frequencies commonly used for radio `and carrier currents.

The expression unilateral means, as used in the claims, refers to means never in condition, at any one time to transmit in more since the repeater, after trans-.A

renderingy said i transmission 1. The methodI of' operating a currentti'ansmission device connectedl in a circuit and subJect to being rendered operative to transmit in a given direction 1n said circuit by transmission returning from said circuit to said device, which comprises preventing said returning transmission from so affecting said device.

2. In a signal transmission system, a .signal transmission path comprising a normally closed switch contact in that condition directly reducing the transmission eficiencyof the path, and means for translating received signaling variationsl into ment, and for directly mechanically employing the initial movement so produced, at its inception, to open -said switchcontact. v

3. In combination, a circuit comprising a current transmission path, means including a closed switch and decreasing the transmitting eiicienoy of said path while said switch is closed, means variations received over said path into mechanical movement, and for mechanically employing the initial movement so produced, at its inception, to open said switch, and thereby causing increase in the transmitting eiiciency of said path, and means for delaying transmission in said path until said switchl has opened. v

4. A circuit comprising two transmission paths, means, comprising a closed switch, shunting one of said paths and decreasing the transmitting eiiiciency of said path, means for opening said switch in response to transmission in said circuit, and means for responsive means insensitive for a predetermined time after said other path has ceased transmittin 5g Thel method of operating a current transmission path and a circuit controlling device having parts so movable with respect to each other as to increase the impedance of said device, which comprises causing alternating signaling currents to flow in said path, rectifying said alternating currents, causing said parts to so move iiiresponse to How of said rectified currents and withoutdelay to other mechanical motion, and increasing the transmission eiiiciency of said path immemechanical movefor translating the current sol diately upon the consequent increase of imow'of transmission in either direction in said circuit for rendering said device operative to transmit, but insensitive to flow of -transmission in one direction in said circuit for a predetermined time after said device rection substantially just suicient to pre- -vent return of transmission to said device from rendering said device operative to transmit.

7 A circuit, a two-way unilateral repeater connected therein, means at said repeater responsive to transmission of a given energy level in one direction in said circuit for rendering saidrepeater operative to transmit in said direction in less time than the time required for energy fed from said repeater to said circuit in the opposite direction to return to said repeater and there 'decrease to an energy level of the order of said first mentioned level, and means for preventing said returned energy from deleteriously affecting the operation of said transmisison responsive means. f

8. In -a transmission system, a transmission path, means forming normally substantially a short-circuit path across said transmission path, andmeans responsive to transmission at a partof said system other than said shortcircuit pathfor causing the opening'of said short-circuit path While permitting transmission over said transmission path.

9. In combination, two unidirectionally transmitting amplifiers each having an-input circuit and an output Qircuit,a closed trans-y mission circuit connecting the outputvclrcuit of one of said amplifiers to the input circuit of said other amplifier, a closed transmission circuit connecting the output circuit of said other amplifier to the input circuit of said one amplifier, and means preventingr local singing of said amplifiers, said means comprising a normally closed switch contact in one of said transmission circuits and depending upon the closed condition of said cqnt'act for preventing said singing. f

10. In combination, a circuit comprising al current transmission path, a device normally decreasing the transmitting 4efficiency of said path, said device having parts so ymovable as to increase its 1mpedance,'means so movlng said parts 1n response to transmission in vsaid circuit-and without delay due to other 'mechanical movement, and'means for delaying'iiow lof transmission to said path until said parts have thus'been moved.-

11. In combination, a circuit comprising a current path, a relay having an armature, said armature in the normal unenergzed condi tion of'said relay closing a short-circuit decreasing the transmitting efficiency of'said path, means for translating signaling variations into current variations for actuating said. relay to controlsaid armature to open -said short circuit, the opening of said short circuit increasing the transmitting eiiiciency of said patl1,and means for delaying passage of said transmission to said path until said switch/has opened.

12. A circuit comprising two oppositely directed unidirectional transmission paths, a switch having conta-cts'in one transmission path which when closed decreases the trans# mission efficiency of said one path, means for translating signaling variations received over 470 said one path into mechanical movement, and l for mechanically employing the initial movement so produced, at its inception, to open said switch contacts, and thereby increasing the transmitting efiiciency of said path upon the opening of said 'switch contacts, and means for rendering said transmission rcsponsive means insensitive for a predetermined time after the other of said paths has ceased transmitting.

13. In combination, a current transmission path, a circuit controlling device having its parts so movable relatively to each other as to increase the impedance of said device, means for rectifying speech variations received over said path, means for converting theenergy of the rectified speech variations into mechanical movement, and for mechanically employing the initial movement so produced, at its inception, for operating said 4movable parts to increase the impedance of said device, vand meansresponsive tov said increase of impedance for immediately` increasing the transmission efficiency of said path.

14. A transmission network comprising a repeating path, a switch in its closed condiception, to open said switch thereby increasingthe transmitting efficiency of said repeating path.

15. In combination, a circuit comprising a current transmission path, means operative in response to transmission in saidcircuit for increasing the transmission efficiency yof said. path, and means for maintaining said increased efliciency after said first mentioned means assumes its unoperated condition.

16. In combination a circuit comprising a current transmission path, means operativev in response to transmission in said circuit for rendering said path operative to transmit, and a relay controlled by said means for maintaining said increased efliciency after said first mentioned relay 'as returned to its unoperated condition.

.17. In combination, a clrcuit comprising a current transmission path, a closed .switch decreasing the transmission efiiciency of said y-tlon reducing the transmitting efficiency of and means for maintaining said path at said v of the supply of signaling variations to said ath.

18. The method of reducing echo effects in a transmission circuit repeatered with bi? lateral repeaters, which comprises maintaining said system normally inoperative to repeat from either end of the circuit tothe other, and upon flow of transmission in either direction in said circuit conditioning said circuit for transinit-ting in that direction, and holding it conditioned in that direction for a predetermined time after said transmission has ceased.

19. A telephone transmission system comprising a two-way unilateral repeater and a bilateral repeater in tandem.

v20. A telephone transmission system coinprising a normally inoperative, transmission controlled, two-Way unilateral repeater and a bilateral repeater in tandem.

21. A current transmission system coinprising bilateral repeaters and a transmission controlled, echo-suppressing unilateral repeater in tandem, said repeaters being geographically remote from each other, and said unilateral repeater having a relaxation time atleast as great as the time required for the longest echo which said unilateral repeater is to suppress to pass from the repeater to -tlie reflecting point and back to the repeater. v

22. A circuit, an impedance for approximately balancing the impedance of said circuit, tWo unidirectional currentvpatlis, a hybrid coil connecting said paths to said circuit and said balancing impedance and in conjugate relation to each other, one of said paths transmitting toward said coil and the other path transmitting from said coil, -a switch short circuiting said coilwith respect to said one path, and means for openingsaid switch in response to flow of transmission in said one path. "f

23. A circuit, two transmission paths con-l nected to one end of said circuit, means substantially short circuiting one of said paths for preventing transfer of energy between said paths, means for transmitting -to said circuitenergy flowing towards said paths, and means for avoiding great loss of energy iii transmitting from said circuit to said otherl path.

24; In combination, a. line, two unidirectionally transmitting amplifiers therein, each having an input circuit and an output circuit, a short circuit across the output circuit of each amplifiery for preventing local singing of the amplifiers, and a hybrid coil at the output side of each amplifier for avoiding great loss of incoming energy between the line and the input circuit of the other amplifier.

25. In combination, a circuit comprising a,

current path including a normally balanced bridge decreasing the transmitting efficiency of said path, said bridge including a'switcli for controlling the balancing thereof, and

means responsiveto transmission in said circuit for controlling said sWitcli.

26. In combination, a line, two unidirectionally transmitting amplifiers therein,-

each having an input circuit and an output circuit, a short circuit across the output circuit of each amplifier for preventing local singing of the amplifiers, and an impedance line at the output side of each amplifier for avoiding great loss of' incoming energy between tlie line and the input circuituof the other amplifier. Y

.27. In combination, in a transmission circuit and incoming line, an outgoing line, an energy abstracting device tending to introduce harmonics into the circuit, a net work simulating' the impedance of the incoming line, and a bi-conjugate network associating all of said elements together so that the incoming line and its network are' conjugate, and the harmonic producing device and the outgoing line are conjugate.

28. In combination, a unidirectionally transmitting circuit, an impedance balancing net-Work therefor, a reactance, a voice current responsive switch, an operating circuit for said switch, and means so interconnecting said first circuit, 'said balancing network, said reactance and said operating circuit that said reactance and said operating circuit areA conjugate to each other. v 29. In combination, an amplifier, an impedance for balancing the impedance presented by the output side of said amplifi er, a

reactance, a voice operated switch, an operating circuit forsaid switch, and means so in-v terconnecting said output side of said amplifier, said balancing impedance,said reactance and said operating circuit that said reactance and said operating circuit are conjugate to eachother.

30. In combination, a switch, an operating circuit-therefor, a unidirectionally transmitting circuit comprising a filter and means for connectingthe input end of said operating circuit in conjugate relation to the input end of said filter. f

31.' In combination, a unidirectional circuit comprising an incoming line andan o utgoing line, and aI rectifiercircuit comprising branches connected in serial relation and 1n shunt relation, respectively, vto said first circuit.

cui for rendering said path'cperative, at

different times, to transmit in both directions, in said circuit, wherebytwO-way transmission may be had, vand means for preventing transmission pturning from said circuit l'to said path from operating said transmission responsive means.

33. A transmission system comprising in combination, a circuit, a plurality of transmission paths connected thereto, each including a closed switch contact in that condition directly reducing the transmission efficiency ofthat path, and means for translating sigmaling variations 'received over one of said paths into mechanica-l movement, and for directly mechanically employing the initial movement so roduced, at its inception, to open a selected) one of said contacts thus increasing the transmitting efficiency of the corresponding path.

34. In combination, a unidirectionally transmittingpath, two circuits for feeding signaling Waves into said path, and means connecting sai-d circuits to the input end of said path and in conjugate relation to each other.

3. In.- combination, a unidirectionally transmitting path, two circuits for feeding signaling Waves into said path, .and a balanced bridge circuit connecting said circuits to the input end of said path and in conjugate relation to eachother.

36. A system comprising a source of current having a relatively low frequency componentand a relatively high frequency component, a rectifier, a filter for suppressing said low frequency component connected bctween sal-d source and said rectifier, a trans- Alating device fed by said rectifier, and noninductive means connected tothe output circuit of said rectifier for smoothing out said 'rcctlfied current.

37. A system comprising a source ofcuri rent having a relatively low frequency component and a relatively high frequency com-`` tween said source and said rectifier, a normally closed direct current relay operative to open its contact in response to current from said rectifier, and means connected to the output circuit of said rectifier for smoothing out said rectified current, said means consisting solely of a condenser.

38. In combination, a transmission circuit, subject to noise currents, a direct current electromagnetic relay, an electron discharge rectifier fed from said circuit and feeding said relay, said rectifier comprising a control electrode and means maintaining said electrode at such a potential that said noise currents cannot be propagated through said rectifier.

In Witness whereof, I hereunto subscribe my name this 2nd da'y of October A. D., 1923.

ROBERT o. MATHES.

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