US1998071A - Transmission control circuits in two-way signaling systems - Google Patents

Description

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April 16, 1935. B. G, BJORNSON TRANSMISSION CONTROL CIRCUITS IN TWO-WAY SIGNALING SYSTEMS Filed Feb. 3, 1932 4 Sheets-Sheet 1 /NVE/VTOR B B.G.BJRNSON y A TTORNE V 'April 16, i935. B. G. BJoRNsoN "1,998,071

TRANSMISSION CONTROL CIRCUITSy IN TWOWAY SIGNALING SYSTEMSv Filed Feb. z, 1952. 4 sheets-sheet 2 JMW/gwn ATTORNEY pml I6, 1.9354 B. G. BJQRNSON TRANSMISSION CONTROL CIRCUITS IN TWO-WAY SIGNALING SYSTEMS Filed Feb. 5, 1932 4 Sheets-Sheet A3 /VVE/VTOR B.G.BJONSON BV ATTOR/VE Y B; G. BJoRNsoN `TRANSMIVSSION CONTROL 4CIRCUITS IN TWO-WAY SIGNALING SYSTEMS Apri; 1e, v193.5.

Filed' Feb. 5, 1932 /N VEA/TOR @y B. G. @JURA/.SON

VsmlAHHV ATTORNEY v @Nl UNITED STATES Partnr orties TRANSMISSION CONTROL CIRCUITS IN TWO-WAY SIGNALING SYSTEMS Bjorn G. Bjornson, Brooklyn, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 3, 1932, Serial No. 590,537

11 Claims.

This invention relates to two-way signaling systems and particularly to the signal-controlled means employed therewith for suppressing echoes and preventing singing.

An object of the invention is to improve the operation characteristics of long, two-way signaling systems employing signal-controlled apparatus for suppressing echoes and preventing singing.

To obtain proper operation of a long two-Way signaling system, including over at least a portion of its length a four-wire circuit having one path adapted for transmission in one direction,

, and another path adapted for transmission in the ample, at a midway point.

opposite direction, it has been found necessary in the past to employ signal-controlled apparatus for effectively disabling one ci the associated one-way paths while signal transmission is taking place over the other, so as to prevent echo currents or reected currents from being transmitted back to the transmitting end of the system and causing a disturbance or singing. The disabling apparatus usually comprises means, such as an amplier-rectier control circuit for diverting a portion of the signaling current from one path and utilizing it to control the open-circuiting or short-circuiting of the oppositely directed path or to control in proper manner the operatic-n characteristics of an amplifier in the oppositely directed path. In certain circuits of the prior art, these signal-controlled devices or echo Suppressors as they are commonly called, one of which is associated with each two-wire path, are located at some intermediate station, for eX- In other circuits oi the prior art, the tivo units of the echo suppressor are located at or near the ends of the four- Wire circuit. The latter arrangement, the sccalled terminal echo suppressor, may be either of the sender or the receiver type. In the sender type of terminal echo suppressor, the suppressor units located near each terminal comprise a suppressor responsive to signals in the outgoing two-wire circuit to disable the incoming two-wire circuit, and means responsive to signals in the incoming circuit to disable the suppressor. In the receiver type of terminal echo suppressor, the suppressor units located near each terminal of the four-wire circuit, comprise a suppressor responsive to signals in the incoming two-wire circuit to disable the outgoing two-wire circuit, and means responsive to signals in the outgoing two-wire circuit to disable the suppressor.

The invention relates to improvements in twoway signaling systems, for example, two-way telephone systems, employing echo Suppressors of the above mentioned types, the improvements residing inthe provision of means for facilitating break-ins by a listening party after one party has obtained exclusive control of the system. This is accomplished by the provision of means under control of the rst talkers signaling currents operative a given time after he has Obtained control of the system, for rearranging the control circuits of the system so as to bias the system from the standpoint of break-ins in favor of the listener. While the system is so biased, the listener can, on perceiving a pause by the first talker, immediately break in, gain control of the system for his own signaling currents and shut out the i-lrst talker.

The nature of the circuits of the invention and their advantages will be clear from the iollowing detailed description thereof when read in connection with the accompanying drawings:

Fig. l of which shows schematically a four-wire toll telephone circuit employing terminal echo Suppressors of the receiver type and embodying the invention;

Fig. 2 of which shows schematically a four-wire toll telephone circuit employing terminal echo Suppressors of the sender type, and embodying the invention;

Fig. 3 of which shows schematically a fourwire toll telephone circuit employing echo suppressors of the mid-point type and embodying the invention; and

Fig. 4 of which shows schematically a fourwire toll telephone circuit employing the cornbination of receiver terminal with mid-point echo Suppressors and embodying the invention.

For convenience the invention will be described as applied to systems for the two-way transmission of speech waves, but it is to be understood that the principles of the invention apply equally well to systems for the two-way transmission of signal waves in generalof frequencies within or outside the speech frequency range, such as waves representing speech, entertainment programs comprising music as well as speech, etc.

The four-wire toll telephone circuit of Fig, 1 comprises a one-way transmission path EA including the one-way amplifying devices I and 2 for repeating telephonic currents in the direction from west to east between the west twoway telephone circuit L1 and the east two-wayV telephone circuit L2, and the one-way transmission path WA including the one-way amplifying devices 3 and i for repeating telephonie currents in the direction from east to west between the east two-way telephone circuit L2 and the west two-way telephone circuit L1. In yell known manner, the oppositely directed one-way transmission paths EA and WA may be connected in substantially conjugate relation with each other and in energy transmitting relation with the telephone circuits L1 and L2, respectivel by the hybrid coil transformers H1 and I-I2, and the associated balancing networks Ni and N2.

A receiver type terminal echo suppressor is located near the west end of the fourwire circuit, which will be referred to hereinafter' as station A, and another receiver type terminal echo suppressor is located near the east terminal of the four-wire circuit, which will be referred to hereinafter as the station B.

The terminal echo suppressor located at station A includes a control circuit 5 having its input bridged across the output oi the path WA, and comprising a relay-controlling device t, which may be a vacuum tube amplier-rectier device such as is well known in the art, and the windings of mechanical relays l, S and 9 connected in parallel to the output of the control device I5. The mechanical relay Q is made to operate in response to operation oi the control device 5 a denite interval of time, :Z1 seconds, after the relays 'I and 8 have operated, for example, by the insertion of a delay circuit if: oi any suitable type between the output of the device 6 and the winding of mechanical relay 5 designed to delay the transmission ci energizing current to the winding of relay 9 for a time inter-- val of di.

The terminal echo suppressor at station A also includes a control circuit l I having,r its input connected across the path EA in the output of the amplifying device therein, and comprising a relay-controlling device I2, similar to the device 6, and the windings of mechanical relays I3 and I4 connected in parallel to the output of the device I2. The mechanical relay I 4 is made to operate in response to operation ci the control device I2 a denite time interval T after the device I2 has operated, for example, by the insertion of electrical delay circuit I5, similar to the delay circuit I0, between the output oi device I2 and the winding of mechanical relay I4, designed to delay the transmission of energizing current to the winding of relay i4 for the interval of time T, where T is the transmission time between station A and station B over the transmission path EA.

In the input of the path EA at station A between the input of the amplifying device i and the hybrid coil H1 are the normally closed switch contacts IG which are adapted to be opened by operation of the mechanical relay 'l when its winding is energized by operation of the control device 6 in control circuit 5. In the control circuit II between the output of the delay circuit I5 therein and the winding of mechanical relay I4, are the normally closed switch contacts adapted to be opened by the relay 3 when its winding is energized by operation oi control de vice 6. In the output of the control device I2 in control circuit II between the point of connection of delay circuit I5 thereto and the winding of mechanical relay I3 are the normally open switch contacts I8 adapted to be closed by the mechanical relay 9 when its winding is energized by operation of the control device E.

In the path WA at a point therein at station A between the point of connection thereto of the control circuit 5 and the output of the amplifying device 4 are the normally closed switch contacts I 9 adapted to be opened by relay I3 when its winding is energized in response to operation o the control device I2 in control circuit 5 I. In the output of the control circuit 5 between the output of the control device 6 therein and the windings of mechanical relays 'I, and .i, are the normally closed switch contacts 23 adapted to be opened by relay I4 when its winding is energized in response to operation of control device i2 in control circuit II.

The terminal echo suppressor at station B includes the control circuit 2i having its input bridged across the output of the path EA, and comprising a relay-controlling device 22, similar to the devices and i2 at station A, and the windings of mechanical relays 23, 24 and 25 connected in parallel to the output of the device 22. Relay 25 is made to operate after the relays 23 and 24 in response to operation oi the device 22 by a time interval of di, for example, by the insertion of a delay circuit 26 between the output of the control device 22 and the winding of relay 25 to delay the transmission of energizing current to the winding of relay 25 for the interval di.

The terminal echo suppressor at station B also includes a. control circuit 27 having its input connected across the input of the path WA in the output of the amplifying device 3 therein, and comprising a relay controlling device Zii, similar to the device 22, and the windings oi mechanical relays 29 and 33 connected in parallel to the output of the device 23. The mechanical relay SI2 is made to operate after operation of the control device 28 by a time interval of preferably T, for example, by the insertion of the delay circuit 3| in the control circuit 2'1 between the winding or" relay 3S and the output of the control device 28, to delay the transmission of energizing current to relay 30 from the output of the control device 23 for the time interval T.

In the input of the path WA at station B are the normally closed switch contacts 22 adapted to be opened by relay 23 when its winding is energized in response to operation of control device '.2 in control circuit 2|. In the control circuit 2'I between the output of the delay circuit 3i and the Winding of relay 3i) are the normally closed switch contacts 33 adapted to be opened by relay 24 when its winding is energized by operation of the control device 22. In the output or" the control circuit 2'5 between the point of connection thereto of delay circuit SI and the winding of mechanical relay 29 are the normally open switch contacts 34 adapted to be closed by relay 25 when its winding is energized in response to operation of the control device E2 in the control circuit 2 I. In the output of the control circuit 2i between the output of the control device Z2 therein and the windings of the mechanical relays 23, 24 and 25 are the normally closed switch contacts 35 adapted to be opened by relay 35i when its winding is energized in response to operation of the control device 28 in control circuit 2l.

The operation of the system of Fig. Il will now be described. It will be assumed that the circuit is inactive so that the switching contacts con trolled by the echo suppressor circuits at both stations A and B are in their normal condition as indicated in Fig. l. Then, let it be assumed that a west subscriber associated with the twodyay circuit L1 starts to talk and that thereafter', but within a time interval of T, an east subscrice associated with the circuit L2 also starts to talk.

The west subscribers speech current received over the circuit L1 at station A will be impressed by hybrid coil H1 upon the input of the west to east transmission path EA over which it will be transmitted after ampliiication by the amplifying device l at station A to the distant station B where it will be amplified again by the amplifying device 2. The main portion of the west subscribers speech current in the output of the amplifying device 2 will be transmitted over the output of the path EA to the hybrid coil H2 which will impress it upon the circuit L2 over which it will be transmitted to the listening east subscriber.

Meanwhile, a small portion of the west sub scribers speech current at station A is diverted in the control circuit l and causes the operation of the control device l?- therein. The operation of control device i2, however, will not cause enerM gizing currents to be supplied to the winding of the mechanical relay i3 because of the normally open switch contacts i3 in the input of that relay. The operation of control device l2 will cause energizing currents to be supplied to the winding of mechanical relay Ill, but only after a delay of T due to the action or the delay circuit l5. This means that the relay it will operate substantially at the same time as the west subscribers speech currents arrive at the station B over the path EA, to open the normally closed switch contacts 20 in the output of control circuit 5 to disable the inputs of mechanical relays ll, t and connected to the output of the control device ii.

Thev portion of the west subscribers speech currents diverted into control circuit 2l at station B will cause the operation of control device 22. The operation of control device 22 will cause energizing currents to be supplied directly to the windings of relays 23 and 24 and through delay circuit 26 to the winding of relay 25. Relay 23 will operate immediately to open switch contacts 32 in the input of the path WA rendering that path inoperative thereafter to transmit to the circuit L1 at station A or to the input of control circuits 2'! and 5. Relay 24 will operate simultaneously with relay 23, to open the switch contacts 33 in control circuit 21 thereby disabling the input of relay 3%. The west subscriber has now obtained exclusive control of the switching circuits at both station A and station B.

lf the east subscriber associated with circuit Lz starts to talk immediately after the west subscriber does so, a portion of the formers speech currents may pass the point in path WA where switch contacts 32 are located before these contacts have been opened in response to the west subscribers speech currents, and will cause operation of the control device 23 in control circuit 2? at station B and of control device E in the control circuit at station A. A portion of the east subscribers speech currents will also be transmitted over the output of the path WA to circuit Li and will be heard by the west subscriber associated therewith. The operation of control device will not cause false operation of relays l, 8 and Si at station A because the input of these relays has been previously disabled by the opening of switch contacts 2E! in response to the west subscribers speech currents.

The operation of the control device 28 will notJ cause energizing currents to be supplied to the winding of mechanical relay 29 because the switch contacts St in its input are still open. rEhe operation of control device 29, however, will cause energizing current to be supplied tc the winding of mechanical relay 30 but with the' delay of T caused by the delay circuit 3l in the output of the device 28. The transmission of energizing current to the winding of relay 3Q will be delayed for the time interval T by delay circuit 3i and will therefore not be eiiective in causing operation of that relay because its input will have been previously disabled by the opening of switch contacts 33 in response to the west subscribers speech currents. Then, the west subscribenhas obtained complete control of the switching circuits at both stations.

Substantially d1 seconds after the west subscriber has obtained control of the switching circuits in the manner described, the energizing currents being transmitted through the delay circuit 25 in the output of control circuit 2i cause the winding of relay 25 to be energized. Relay 25 then operates to close and maintain closed the normally open switch contacts 3d in the output of control circuit 2l, thereby biasing the fourwire circuit as regards break-ins in favor of the east subscriber. Now, if the east subscriber breaks in during a pause by the west subscriber sufficiently long to enable the relay 23 and the relay 2li to release momentarily, his (the east subscribers) speech currents diverted into control circuit at station A, will cause operation of the control circuit 2S therein to immediately energize the winding of relay 29 causing the normally closed switch contacts 35 in the output of the path EA to be opened to disable that path, and a time interval T later tov energize the windof relay 3d causing the normally closed switch contacts 35 in the output of control. circuit 2| to be opened to disable that circuit as regards subsequent operation under contro-l of the west subscribers speech currents. A portion of the east subscribers amplified speech currents transmitted over the path WA during this pause will be diverted into control circuit 5 at stationA and will cause operation of the control device 6 therein to energize the windings of mechanical relays l and 5 in the output thereof. These relays will immediately operate respectively to disable path EA by opening switch contacts l5 therein and to disable the input of relay lliby opening switch contacts il', thus giving complete control of the switching circuits to the east subscriber.

The operation of control device ii after a delay of an additional interval oi d; caused by delay circuit iii will cause the winding of relay 9 to be energized and that relay will operate to close normally open switch contacts i8 in the output of control circuit il, so as to bias the four-wire circuit as regards subsequent brealdns in favor of the west subscriber.

As indicated in Fig. l, the relay 9 in control circuit 5 at station A and the corresponding relay 25 in control circuit 2l at station B should have a small hang-over, h1, in their operation, and the relay i3 in control circuit i i at station A and the relay 2S in control circuit 2l at station B should have a small hang-over, hz, in their operation. These hang-over times should be such as to kee relays i3 and 29 operated to maintain the switch contacts i9 in the path WA and the switch contacts 36 in the path EA open, respectively, until the dissipation of the energy which might have passed by the controlling suppressor before it came into operation. These hang-overs may be F obtained by proper mechanical design of the relays to make them slowreleasing, or by utiiiz ing in connection t erewith auxiliary hang-over circuits, for example, such as disclosed in the patent to J. Herman, 1,798,202, issued March 3l, 1931. The echo suppressor relay 'l at station A and the echo suppressor relay 23 at station B should also have suicient hangover in their operation to hold them operated for a slight time aiter the supply of speech currents to the control devices 6 and 22, respectively, ceases to prevent premature closing of the switch contacts l5 and 32, respectively.

Fig. 2 shows a circuit arrangement for facilitating bres-.loins by a listening party after one party has obtained exclusive control in a fourwire toll telephone circuit employing sender terminal echo Suppressors.

The echo suppressor circuits at station A in the system of 2 includes a control circuit 3l' having its input connected across the path EA at station A, and comprising a relay controlling device which may be a vacuum tube amplifier-rectifier circuit, the winding of a mechanical relay il connected to the output or" the control device 33 over a circuit 4l, and the windings of mechanical relays l2 and 43 connected to be operated in parallel from the output oi the control device 33 over a circuit 4@ including the delay circuits fl5 and 5S. Connected around the portion ci control circuit @Il includingr the delay circuit d5 of relay 43 is a by-pass circuit The normally closed switch contacts d8 in the path WA at station A are adapted to be opened to open-circuit the output ol the path WA by means of relay in response tc energization ci its winding by the operation of control device 33. The normally closed switch contacts le in the path WA at station A are also adapted to be opened to open-circuit the output of path WA by relay i? when its winding is energized by operation of the control device 38. The normally closed switch contacts 5B in by-pass circuit 4l are adapted to be opened to disable that circuit by relay 43 when its winding is energized by operation of the control device 38.

Bridged across the output of the path WA between the point of connection therein of switch contacts 45 and 9 and the hybrid coil H1 is the input of a control circuit 5| comprising the control device 52, similar to the device 38, and the windings of mechanical delays 53, 54, 55 and 56 connected in parallel with the output of the control device 52. The operation of relay 55 is delayed with respect to that of relays 53, 54 `and 55 in response to the operation of control device 52 for a small time interval di by a delay circuit of suitable design connected between the winding of relay 55 and the output of the control device 52.

The normally closed contacts 58 in the input of the control circuit 3'! are adapted to be opened to disable that control circuit by relay 53 when its winding is energized in response to operation ci the control device 52. The normally closed switch contacts 5S in the output of the control circuit 114 are adapted to be opened to disable the input of relay 42, by relay 55 when its winding is energized in response to operation of the control device 52. The normally closed switch contacts 55 in circuit 44 between delay circuit 45 and delay circuit 48 are adapted to be opened to disable the circuit 44, and therefore relays 42 and 43, by relay 54 when its winding is energized in response to operation of the control device 52. The normally open switch contacts 6l in the circuit 4I connecting the output of the control device 38 to the winding of relay 4D, are adapted to be closed to render operative the circuit 4l, by relay 56 when its winding is energized by operation of the control device 52.

The echo suppressor circuits at station B in the system of Fig. 2 include a control circuit 62 having its input connected across the input of the path WA at station B, and comprising the relay controlling device 63, similar to the devices 52 and 33 at station A, the winding of a mechanical relay 64 connected to the output of the device 53 by a circuit 65, and the windings of mechanical relays 66 and 6l connected to be operated in parallel from the output oi the control device 53 over a circuit 68 comprising the delay circuits 69 and 75.

Connected around the portion of the circuit 68 including the delay circuit 'lo and relay 61 is the by-pass circuit ll.

rIhe normally closed switch contacts l?. in the output of the path EA are adapted to be opened to open-circuit that path by relay Eid when its winding is energized in response to operation of the control device 63. The normally closed switch contacts 'i3 in the output of the path EA are adapted to be opened also to open-circuit that path by relay 56 when its winding is energized in response to operation of the control device 53. The normally closed switch contacts 'M in the by-pass circuit 'H are adapted to be opened to disable that circuit by relay 6l when its winding is energized in response to operation oi the control device 63.

Connected across the output of the path EA between the switch contacts 'l2 and 'I3 therein and the hybrid coil H2 is the input of a control circuit 'l5 comprising the relay-controlling device i6, similar to the device 63, and the windings of mechanical relays l1, 18, 18 and connected in parallel across the output of the control device l'. The relay 8i) is made to operate a small time interval, d1, after relays ll, '.'8 and 19 have operated in response to operation of the control device 16 by means of a dela-y circuit 8i of suitable design connected between the winding in response to operation of the control device '56.

The normal closed switch contacts 82 in the input of the control circuit 62 are adapted to be opened to disable that circuit by operation of relay TI when its winding is energized in response to operation of the control device 76. The normally closed switch contacts 83 in the output of the circuit S8 are adapted to be opened to disable relay 66, by relay 'I9 when its winding is energized in response to operation of control device 16. The normally closed switch contacts 84 in the circuit 68 between delay circuits G9 and 'ID are adapted to be opened to disable that circuit, by relay I8 when its winding is energized in response to operation of control device l5. The normally open switch contacts 85 in the circuit 65 are adapted to be closed to render operative the circuit 65 connecting the winding of relay 64 to the output of the control device 63, by relay 80 when its winding is energized in response tol operation of the control device 16.

The delay circuits 45 and 46 in the circuit at station A, and the delay circuits 65 and 'lil in the circuit 68 at station B are each designed to produce a delay T in the transmission of currents therethrough, Where 'I' is the transmission time between the stations A and B over one of the one-way transmission paths EA or WA.

The operation of the system of Fig. 2 will now be described. rIt Will be assumed that the west subscriber associated with circuit L1 terminating at station A starts to talk before the east subscriber associated with circuit L2 terminating at station B starts to talk. The west subscribers speech currents will be impressed by the hybrid coil H1 upon the input or" the path EA and transmitted thereover towards station B. A portion of the 'west subscribers speech currents will be diverted into the input of the control circuit 3l causing the operation of the control device 38 therein.

The main portion of the west subscribers speech currents will be transmitted out over the path EA to the distant station B being amplied in transit by the amplifying devicesY in that path. At station B, the main portion of the west subscribers amplified speech currents will be transmitted over the path EA to the hybrid coil H2 which will impress it upon the circuit L2 over which it will be transmitted to the listening east subscriber'.

A small portion of the west subscribers'speech currents at station B will be diverted into the control circuit 'l causing the operation of the control device lt therein. The operation of the control device iii will cause energizing current to be supplied to the windings of mechanical relays ll to dt. Relay "all will operate immediately to open the normally closed switch contact-s "2 in the input of the control circuit S2 making that circuit thereafter inoperative to transmit speech currents from the east subscriber thereafter received from the path WA. Relay i3 will operate simultaneously with relay Tl to open the normally closed switch contacts Sd in the output of the delay circuit ESQ in circuit G3 thereby preventing any currents which may be stored in the delay circuit SS before the switch contacts S2 in the input of control circuit E2 have opened from being transmitted to the delay circuit lli or to the circuit "l, The relay i9 will operate simultaneously with relays 'il and 'i3 to open the normally closed switch contacts 33 in the input of the relay 5S' thereby preventing false operation thereafter of that relay by speech currents which may have been stored in the delay circuit it at the time relay 'il operates to disable the input of the control circuit 62. Meanwhile at station A the operation of the control device t8 under control of the west subscribers speech currents has caused energizing currents to be transmitted through delay circuit i5 in circuit which will be divided between the 1oy-pass circuit and the input of the delay circuit it in circuit dfi. The portion transmitted through the by-pass circuit di will cause the winding ofY relay to be energized, and the relay l2 will then operate immediately to disable the path WA at station A by opening the normally closed switch contacts t9 in that path. Because of the delay of T introduced by delay circuit 55, the relay @i2 will operate to, disable the path WA by opening switch contacts i9 therein substantially at the same time as the relays 'El' to EQ have operated at station E to disable the control circuits thereat which normally would be responsive to the east subscribers speech currents. The west subscribers speech currents now have obtained complete control of the switching circuits at both terminal stations. Y

if the east subscriber associated with 'the cir-` cuit Lz starts to talk immediately after the west subscriber associated with the circuit L1, it will be apparent that with the circuit in the operated condition described, the east subscriber will not be able to get his speech currents through to the west subscriber or to seize control of the switching circuits away from the west subscriber because the switch contacts in their path will have been previously opened in response to the west subscribers speech currents before the east subscribers currents have reached the switching points.

Now, the portion ofthe energizing currents transmitted to the input of delay circuit in the circuit di will be delayed in transmission therethrough another interval4I of T after which it will be supplied to relays 42 and d3. Relay 43 will then o-'ierate to Vopen the normally closed switch contacts te in the by-pass circuit 41 thus cutting off the supply of current to the relay-42 over that circuit. However, relay 42' will not release because it will be maintained energized by the currents from the output of delaycircuit d6 transmitted through unilateral device il to the winding of that relay.

Substantially di seconds after relays 'i1 to 'i9 have operated at station B, relay 86, the energization of the winding of which has been delayed or that interval of time due to the action of delay circuit ti, will operate to close the normally open switch contacts S5 in the circuit 65 at station B rendering that circuit operative thereafter to supply energizing current to the winding of relay @il immediately on operation of the control device 63 in the path 62. Then, the east subscriber on perceiving a pause in the continuous transmission of the west subscribers speech currents sufficiently long to cause relays 'il and 2 at stations B and A, respectively, to release, may break in and seize exclusive control of the switching circuits at both terminal stations during the pause in the following manner.

IThe east subscribers speech currents will be impressed upon the input of the path WA by hybrid coil H2. The main portion will be transmitted thereover to the distant station A, and a small portion will be diverted into control circuit t2, the switch contacts 82 therein being in the closed condition during the pause and will cause the operation of the control device t3. The opration of the control device 63 will cause energizing currents to be transmitted both to the circuit 68 and, the switch contacts 85 still being in the closed condition due to the hangover of h1 in the operation of relay 8@ controlled by the west subscribers speech currents, also to the circuit S5. The energizing current impressed upon the circuit will be immediately transmitted thereover to energize the winding of control relay 64 which will'immediately operate to open the normally closed switch contacts 'l2 in the output of the path EA making that path thereafter inoperative to transmit the west subscribers speech currents to the circuit L2. The energizingy currents transmitted to the circuit 68 from the output of control device dit will be delayed in that circuit in transmission through delay circuit 69 for an interval or" T after which they will be divided between the by-pass circuit 1I and the delay circuit l. vThe portion of the currents transmitted over the by-pass circuit Tl will cause energizing currents to be transmitted to the winding of relay 66 causing that relay to operate immediately to put an additional open circuit in the output of the path WA by opening normally closed switch contacts '13. The currents transmitted to delay circuit will be delayed therein for a time interval of T and subsequently will cause operation of relay 67 tov disable the by-pass circuit 'll and through uni-lateral device 58. will maintain thel relay 5S operated for the additional time interval T in a manner similar to that explained above for the corresponding apparatus at station A. n

Meanwhile, the east subscribers speech currents which have been transmitted over the path WA will arrive at station A. The main portion of these currents will be transmitted from the output of the path WA to the hybrid coil H1 which will impress it on circuit L1 over which it will be transmitted to the listening west subscriber. A portion of the east subscribers speech currents in the path WA at station A will be diverted into the control circuit 5i and cause the operation of the control device 52 therein to supply energizing currents to the windings of relays 53 to 56 ccnnected to its output. Relays 53, 54 and 55 will operate simultaneously to open switch contacts 58 in the input or" control circuit 31, switch contacts 5t the input of control relay 42 and switch contacts SS in circuit 45 to seize control of the switching circuits at the station A for the east subscriber. Because of the delay of T introduced by delay circuit S9 in the circuit 68 at station B, it will be seen that the control relay 66 at that station operates to seize control of the switching circuits thereat for the east subscriber substantially at the same time as the relays 53, 54 and 55 at station A operate to seize control of the switching circuits thereat for the east subscriber.

A period of time di after relays 53 to 55 at station A have operated, the energizing currents from the output of control device 52, which have been delayed in delay circuit 5l, will cause the Winding of relay 56 to be energized so as to cause operation of that relay to close normally open switch contacts 5l in the circuit 56 biasing the system as regards subsequent operation of the switching control in favor o the west subscriber.

Proper operation or" the system of Fig. 2 requires that the relays 54 and 55 controlled by the control device 52 at station A, and the corresponding relays 'I8 and lil controlled by the control device-16 at station B have a hang-over in their operation of substantially T, where T is the transmission time between station A and station B over one one-Way path therebetween. As indicated in the drawings, the relay 5S controlled by the control device 52 at station A and the corresponding relay 8S controlled by the control device 16 at station B should be designed to have a smaller hang-over in operation of h1, and the relay 45 controlled by the control device 38 at station A` and the corresponding relay 64 controlled by the control device 63 at station B should have a smaller hang-over of h2 in their operation. The hang-over times h1 and h2 should be sufllciently long to keep relay 40 and relay 64 operated until the dissipation of the energy which may have passed by the input of the controlling suppressor before it came into operation. The suppressor relay 42 at station A and the suppressor relay 66 at station B also should be designed to have suicient hang-over times in their operation to prevent premature closing of the switch contacts 4S in path WA and switch contacts 13 in the path EA with cessation of supply of speech currents to the control `device 38 and the control device B3, respectively. Thehang-overs in the several relays may be obtained by suitable mechanical design of these relays to make them slowreleasing to the required degree or by employing in connection, therewith suitable auxiliary hangover circuits. The relay 53 controlled by the control device 52 at station A and the relay 'il controlled by the control device 'l5 at station B are preferably designed to have little or no over in their operation so that they will release substantially instantaneously with cessation in the supply of voice currents to the controllingdevices.

Fig. 3 shows a circuit arrangement for facilitating break-ins by a listening party after another party has obtained exclusive control in a four-wire toll telephone circuit employing echo Suppressors of the mid-point type.

As indicated in Fig. 3the echo suppressor circuit located at some point intermediate the two terminals of the four-wire circuits, which Will be referred to hereinafter as station M, includes a control circuit 86 having its input Yfmnnected across the path EA at station M, and comprising a relay-controlling device 81, which may be a vacuum tube amplier-rectier circuit wel] chown in the art.

A mechanical relay 8E is connected to the output of the control device 8l by a circuit 3:9. A mechanical relay 9B is connected to the output of the control device 8'! by a circuit 9G. A niechanical relay S2 is connected to the output of control device 8l' by a circuit 93 including a delay circuit 34. Two mechanical relays e5 and are connected across the circuit S3 in the output or" the delay circuit Si! by a circuit 9'! including a delay circuit 98.

Connected across the path WA at station M is the input of a control circuit 9S comprising a relay-controlling device lo@ similar to the device 8l. A mechanical relay lill is connected to the output of the device H30 by the circuit m2. A mechanical relay 193 is connected to the output of the device Il by the circuit m4. A mechanical relay |85 is connected to the output of the device 100 by the circuit |06 including the delay circuit It?. The two mechanical relays Hi8 and H353 are connected in parallel across the circuit |55 in the output f the delay circuit I6? by a circuit l!) including the delay circuit Ill.

The normally closed switch contacts H2 in the path WA at station M are adapted to be opened to disable that path by relay 88 when its winding is energized by operation of control device l. The normally closed switch contacts i i3 in the output of circuit H15 are adapted to be opened to disable the input of relay 1&5 by relay 9) when its winding is energized by operation of the control device 8l. The normally cle-sed switch contacts l I4 in the path WA are adapted to be opened to disable that path by relay 32 when its Winding is energized by operation oi control device 31. The normally closed switch contacts l l5 in circuit E39 are adapted to be opened to disable the input of relay 8S by relay 95 when its Winding is energized by operation of the control device 81. The normally open switch contacts H5 in circuit 104 are adapted to be closed to render the input of relay 33 operative by relay 9E when its winding is energized by operation of control device 8l.

The normally closed switch contacts lil in the path EA at station M are adapted to be opened to disable that path by relay icl when its winding is energized by operation of control device |00. The normally closed switch contacts H8 in the output of circuit 93 are adapted to be opened to disable the input of relay 92 by relay I B3 when its Winding is energized by operation of the control device I 0D. The normally closed switch contacts ils in the path EA at station M are adapted to be opened to disable that path by relay iii when its winding is energized by operation of control device itil. The normally open contacts l2@ in circuit Si are adapted to be closed to render operative the input oi relay QS by relay when its winding is energized by operation oi the control device Idil. The normally closed switch contacts l2! in circuit E92 are adapted to be opened to disable the input of relay iti by relay it when its winding is enk ergized by operation of the control device MIIB.

The operation of the system oi' Fig. 3 will now be described. It will be assumed that the west subscriber associated with the circuit L1 starts to talk irst.

The west subscribers speech currents will be impressed upon the input of the path EA by hybrid coil H1 at station A and after being amplied by the amplifier at that station will be transmitted over the path EA towards the distant station B. The main portion or" the west subscribers speech currents, after amplification by the various amplifiers in its path, will be transmitted out over the output of the path EA to the hybrid coil i-lz which will impress it on the circuit L2 over which it will be transmitted to the listening east subscriber.

At the intermediate station M a small portion of the west subscribers speech currents is diverted into the control circuit 36 and will cause the operation of the control device 3l therein. The operation of the control device 8l will supply energizing current to the relay 83 over the circuit S9 and will also supply energizing current to relay Q2 over the circuit t3 through the delay circuit gli, and will supply energizing current to the. windings oi relays d5 and 95 over circuit 93 through delay circuit Sii and over circuit di through delay circuit 9,3.

Relay 38 will immediately operate whereas the operation of relay Si? will be delayed for a time interval of T where T is the transmission time between the station A and station B over the path EA, and relays and 9S will operate after a delay of T-l-di, where di is any desired time interval.

The operation of relay St will open the normally closed switch contacts H2 in the path WA at station `M rendering that path inoperative thereafter to transmit the east subscribers speech currents to the west subscriber associated with the circuit Li or to the control circuit 99 at station M.

After an additional time interval T provided by the delay circuit Sie in the circuit 93, relay 92 will operate to further disable the path WA at station lvl by opening the normally closed switch contacts iid therein. An interval of time d1 seconds later required for the energizing currents to be transmitted through delay circuit 98, the relays @E and il?) will operate simultaneously. The operation or relay 25 will open normally closed switch contacts i l5 in circuit 859 thereby disabling the input of relay which will release after a hang-over interval or h2 causing contacts H2 to return to their normal closed condition. Relay @t will simultaneously operate to close normally open switch contacts l lb in the circuit 64, making that circuit operative thereafter to transmit energizing currents from the control device 50 to the winding of relay ifwhen the control device i353 is operated in response to currents impressed upon the input of control circuit 99.

From the above description, it is apparent that if the east subscriber associated with the circuit Lz terminating at station B starts to talk after the west subscriber but within the time interval of T, the former will get his speech currents started within the time T through to the west subscriber associated with the circuit L1 over the path WA. In other words, the west subscriber can hear what the east subscriber says before the end of the time interval T. However the east` subscribers speech current transmitted over the path WA within the time interval T, although it will cause operation of the control device itil in control circuit at station M, will not be able immediately to seize control of the switching circuits at station M away from thewest subscriber because of the open switch contacts in the input of relay 93, and the delay circuits lill and Hl in circuits IFBB and HB, respectively, providing a delay of T and oi T-l-di in the transmission of energizing current to relays H35, IES and |69, respectively.

However, at the end oi the interval T+d1. relay Sii has operated to render circuit |94 operative and relay Q5 has operated to disable the circuit Se leading to the winding of relay 8, and the echo suppressor circuits are now biased in favor of the listening east subscriber associated with the circuit Lz terminating at station B, and he (the east subscriber) on perceiving a pause in the west subscribers speech transmission may break in and seize control of the switching circuits away from the west subscriber in the following manner.

The relays 95 and 9E have a small hang-over of 7u seconds during which they will remain operated after the west subscriber stops talking. Now, if the east subscriber says something when he observes west pause, his (easts) speech currents will be impressed upon the path WA by hybrid coil H2 at Station B and will be transmitted over that path to station M arriving thereat at the time at which relay 92 has released during this same pause to return the switch contacts l lli in path WA to their normally closed condition.

A portion of the east subscribers speech currents, thereiore, will be diverted into control circuit Se causing the operation oi control device ISS therein. The operation of the device ISG, the circuit llill being still in the operative condition in which the switch contacts l I6 are closed because of the hang-over of h1 in the operation oi relay Sii, will energize the winding oiy relay I causing that relay to open the normally closed switch contacts H8 in circuit 93 to prevent the west subscriber from regaining control of relay 92 at the end of the pause in his conversation.

At the end of the time interval T after control device lili! has been operated by the east subscribers speech currents, the energizing currents which have been delayed in transmission through delay circuit lul, will be transmitted to the winding of relay i535 causing the operation oi that relay to open normally closed switch contacts l i9 in the path WA at station M, rendering that path inoperative thereafter to transmit the west subscribers speech currents to the control circuit 86 or to the east subscriber associated with circuit L2 over the output of the path EA. The east subscriber has now obtained exclusive control of the switching circuits of the system.

At the end of an additional interval of timey of d1 the energizing currents, which have been delayed in transmission through delay circuit Hl, causethe operation of relays 108 and |09 to render operative the circuit 9| by closing switch contacts |2|| therein, and to render inoperative circuit |32 by opening normally closed switch contacts |2| therein. The system is now biased as regards break-ins in favor of the west subscriber and, on perceiving a pause by the east subscriber sufficient to cause relays |35 and mi to release momentarily, he may now break in and seize control of the system away from the east subscriber in a manner similar to that de scribed in connection with the converse condition.

The relays |68 and |39 controlled by the control device |38 have a small hang-over of h1, equivalent to that of relays and 98 controlled by control device 8l and provided for a similar purpose. Relay |0| controlled by the control device |63 and relay 83 controlled by the control device 31 each have a small hang-over in their operation of h2, r1"his hang-over is sufficient to insure that these relays will not release during the pause in the speech currents of the subscriber who has controlled their operation, to unblock the path over which the currents of the subscriber attempting to break in are being transmitted at the time these currents arrive at the blocking point in that path. These hang-over times may be obtained in well known manner by design oi the several relays to make them slow-releasing to the required degree or by en-- ploying in connection with these clays auxiliary hang-over circuits.

Fig. 4 shows a circuit arrangement for facilitating break-ins by a listening party when one party has obtained control in a fourwire toll telephone circuit employing the receiver type oi echo Suppressors at the two terminal stations A and B and mid-point type Suppressors at an intermediate station M.

The suppressor circuits at station A include a control circuit |2| having its input connected across the output of the path WA at station A, and comprising a relay-controlling device |22, such as a vacuum tube amplier-rectier circuit, and the windings of mechanical relays |23 and |24 connected in parallel to the output of the device |22. A delay circuit |25 for introducing a delay of d1 seconds in the transmission of energizing currents to relay |23 when control device |22 is operated, is inserted between the output oi the devic-e |22 and the winding of relay |24, to insure that relay |24 operates a denite time after relay |23. The echo suppressor circuits also include the control circuit |26 having its input connected across the input of the path EA at station A, and comprising a relay-controlling device |27, similar to the device |22, and the winding of a mechanical relay |28 connected to the output of the device |21. The normally closed switch contacts |29 in the input of the path EA between the point oi connection of control circuit |25 thereto and a hybrid coil H1, are adapted to be opened by relay |23 when its winding is energized by operation of the control device |22. The normally open switch contacts |33 in the output oi control circuit |25 are adapted to be closed to render operative the input of relay |23, by relay |24 when its winding is energized in response to operation of the control device |22. The normally closed switch contacts |3| in the output of the path WA at station A east of the point of connection of control circuit |2| to the path WA, are adapted to be opened to disable the path WA, by

relay |28 when its winding is energized in response to operation of control device |127.

The echo suppressor circuits at station B include a control circuit |32 having its input connected across the output of the path EA at station B, and comprising a relay-controlling device |33 similar to the devices |22 and |27, and the windings of mechanical relays |34 and |33 connected in parallel to the output of the device ifi-5. A delay circuit |33 connected between the output of the control device |33 and the winding of relay |35 is provided to produce a time delay oi d1 in the operation of relay |35 with respect to the operation of relay |34 in response to the operation of control device |33.

The echo suppressor circuits at station B also include a control circuit |31 having its input connected across the input of the path WA at station B, and comprising a relay-controlling device |38, similar to the device |33, and the winding of a mechanical relay |39 connected to the output or" the device |38.

The normally closed switch contacts |40 in the path WA between the point of connection thereto of control. circuit |31 and the hybrid coil H2, are adapted to be openedto disable the input of the path WA, by relay |34 when its winding is energized by operation of the control device |33. The normally open switch contacts |4| in the output of the control circuit |37 are adapted to be closed to render operative the input of relay |39, by relay |35 when its winding is energized by operation of control device |33. The normally ciosed switch contacts |42 in the path EA at station B to the west of the point of connection of the control circuit |32 to that path, are adapt-ed to be opened to disable the output of the path EA, by relay |39 when its winding is energized in response to operation of the control device |38.

The echo suppressor circuits at station M include a control circuit |43 having its input connected across the path WA at station M, and c mprising a relay-controlling device |44, similar to the device |33, and the windings of mechanical relays |45 and |45 connected in parallel to the output of the device |44. A delay circuit |47 providing a delay of T in the transmission of current therethrough, where T is the transmission time between station A and station B over the path EA or the path WA, is inserted between the winding of relay |48 and the output of control device MA, to delay the operation of relay |46 with respect to that of |45 by the time interval T in respense to operation of control device |34.

The normally closed switch contacts |48 in the path EA at station M are adapted to be opened to disable that path, by relay |45 when its winding is energized in response to operation or" control device |44. The normally closed switch contacts |49 in the control circuit |43 are adapted to be opened to disable the input of relay |'|5, by relay |46 when its Winding is energized in response to operation of the control device |44.

The echo suppressor circuits at station M include a control-circuit |53` having its input connected across thel path EA at station M, and cornprising a relay-controlling device |5|, similarV to the device |44, and the windings of mechanical relays |52 and |53 connected in parallel to the output of the device |5|.

A delay circuit |43 is connected between the winding |53 Vand the output of the control device |5| to provide'a delay of Tin theoperation of relay |53 with respect to that of relay |52 in response to the operation oi control device l5 l.

The normallyl closed switch contacts |55 in the path WA at station M are adapted to be opened to disable the path WA, by relay |52 when its winding is energized by operation of control device |5|. The normally closed switch contacts 55 in the output of control circuit |53 are adapted to be opened to disable the input of relay |52, by relay |53 when its winding is energized by operation of control device |5i.

The relays |45, m3, |52 and |53 in echo suppressor circuits at station M are designed to have a hang-over of T in their operation. The relay |24 in control circuit |2| at station A and the corresponding relay |35 in the control circuit |32 at station B are designed to have a smaller hangover in their operation of h1, soA as to maintain switch contacts |33 in control circuit |26 at station A and switch contacts Ml in controlcircuit |37 at station B, respectively, closed for a sufficient time interval after the control devices |22 and |33, respectively, release during a pause in the continuous transmission of the controlling speech currents of the talking subscriber to enable the subscriber trying to break in to seize control of the switchingA circuits during that pause. The relay |23 in control circuit |26 at station A and the relay |39 in control circuit |31 at station B are designed to have a hang-over in their operation of substantially T. These hang-over times may be provided by suitable mechanical design of the various relays to make them slowreleasing to the required degree, or by employing in connection with these relays suitable auX- iliary hang-over circuits.

The operation of the system of Fig. 4 will now be described. It will be assumed that the west subscriber associated with the circuit L1 terminating at station A first starts to talk. His speech currents will be impressed by the hybrid coil H1 upon the input of the path EA and transmitted thereover towards the distant station B.

The main portion of the west subscribers speech currents will be transmitted over the path EA being ampliiied en route by the various amplifying devices in that path, to the hybrid coil H2 at station B which will impress it upon the circuit L2 over which it will be transmitted to the listening east subscriber. A small portion of the west subscribers speech currents, however, will be diverted into the control circuit |25 at station A, causing the operation of the control device |21 therein. The operation of the control device |21, however, will not cause operation of the relay |28 in its output because of the normally closed switch contacts |36 in the input of that relay.

Another portion of the west subscribers speech currents will be diverted into the control circuit |53 at station M causing the operation of control 'device |5| therein. The operation of control device l5! will cause energizing currents to be immediately supplied to the windingof relayr |52 causing immediate operation of that relay to disable the path WA at station M by opening the normally closed contacts |55 in that path. Substantially T seconds later the winding of relay |53 will be energized. by the currents from the output or the control device |54 which have been delayed in delay circuit |54, causing the operation of that relay to open the normally closed switch contacts |53 in the input of relay |52. The relay 52, because of the hang-over in its operation, however, will not release for an additional interval or time T thereafter, at the end of which interval it will cause the switch contacts |55 to return to their normally closed condition rendering the path WA operative thereafter for transmitting current thereover.

Another portion of the west subscribers speech currents will be diverted into the control circuit |32 at station B causing the operation of control device |33 therein. The operation of control device !33 will immediately supply energizing currents to relay |34 causing its immediate operation to open-circuit the input of path WA at station B by opening the normally closed switch contacts |43 in that path.

The west subscribers speech currents have now obtained exclusive control of all the switching circuits in the system. p

Substantially d1 seconds after the west subscribers speech currents have obtained exclusive control of the system, the system is biased as regards break-ins in favor of the east subscriber. under control of the west subscribers speech currents in the following manner.

Substantially d1 seconds after relay |34 has operated in response to operation of control device |33, the energizing currents for relay |35 which have been delayed for that interval of time of delay circuit |36 are supplied to the winding of relay |35 causing its operation to close normally open switch contacts |45 in control circuit |31, thereafter rendering that circuit operative to transmit energizing currents to relay |39 from the output or control device |38 when that olevice is operated. Relay |35 because of the hangover in its operation will remain operated h1 seconds after the west subscriber has stopped talking.

Now, let us suppose that the east subscriber associated with the circuit L2 terminating at station B says something when he observes the west subscriber pause. The east subscribers speech currents will be impressed by the hybrid coil H2 on the input of the path WA and, the relay |36 having released during this pause by the west subscriber to temporarily return switch contacts |40 to their normally closed condition. easts speech currents will be transmitted over the path EA towards the west station A. A portion of the east speech currents will be diverted into control circuit |31 at station B causing the operation of control device |38 therein. Because of the hangover of 71,1 in the operation of relay |35 the switch contacts IM in the output of control circuit |31 are still in the closed conditions() that energizing currents are supplied to the winding of relay |39 which operates to open the normally closed switch contacts |42 in the output of the path EA at station B rendering that path thereafter inoperative to transmit to circuit Le or to the control circuit |32. Control circuit |32 having been thus disabled during the pause in the west subscribers speech currents, the west subscriber will not be able to regain control of control device |33 and the east subscriber has, therefore, gained exclusive control of the switching circuits at station B.

At intermediate station M a portion oi the east subscribers speech currents are diverted into control circuit |43' causing operation of the control device |44 therein. The operation of control device IM. will cause energizing currents to be immediately supplied to the winding of relay |4i5 causing its operation to disable the path WA by opening switch contacts |48 therein at station M.Y

The energization of the winding of relay |45 in response to the operation of control device Hill will be delayed after the operation of relay |45 for an additional interval T due to the delay circuit |41 in the input ci relay Mt at the end of which interval the relay M will operate to disab-le relay |45 by opening switch contacts |49.

The subscribers speech currents have been able to get through to the control. circuit |63 at station M because the switch contacts 55 in the path WA at that station, which had been previously opened in response to the operation of control device |5| by the west subscribers speech currents, have now returned to their normally closed condition because the input of relay |52 will have been disabled by the opening of switch contacts |53 by relay |53 in response to the operation of control device |51. The operation of relay iii?) in response to operation of control device |5| has been delayed for a time interval of T after relay 52 operated due to the delay introduced by delay circuit |54. Thus, it is seen that the switch contacts |55 are returned to their normally closed condition just before the east subscribers speech currents reach the abling point in path WA at station M.

The main portion of the east subscribers speech currents, the switch contacts |31 being in their normally closed condition because of the normally open switch contacts |33 in the input of relay |28, will be transmitted over the output of the path WA at station A to the hybrid coil H1 which will impress it on the circuit L1 over which it wil be transmitted to the west subscriber.

A portion of the east subscribers speech currents will be diverted from the path WA at sta-- tion A into control circuit |2| causing the operation of control device |22 therein. The operation of the control device |22 will iirst cause the winding of relay |23 to be energized which relay will then operate to open the normally closed switch contacts |29 in the input of path EA at station A rendering that path thereafter inoperative to transmit the west subscribers speech currents to the east subscriber or to the control circuits |26, |58 and |32. The east subscribers speech currents have now obtained exclusive control of the switching circuits of the whole system.

Substantiallyv di seconds after relay |23 has operated the energizing currents from the output of control device |22 which have been delayed in delay circuit |25 will be transmitted to the winding of relay l24 causing its operation to close normally open switch contacts |30 in control circuit |2E rendering operative the input of relay |28. The system is now biased as regards breakins in favor of the west subscriber. The west subscriber now, on perceiving a pause by the east subscriber, may break in and seize control of the system away from the east subscriber in a manner similar to that which has just been described for the converse condition.

From the above description, it may be noted that the mid-point echo suppressor at station M in the system of Fig. 4 functions only during operationa-l changes, i. e., when the circuit goes from inactive into active condition, and with the circuit active, when the control of the circuit is changed from one party to another. Therefore, the delay has only to be effective during this change, and may be provided for by slow-operating relays.

Although the invention has been described in connection with four-wire circuits employing continuous two-Wire paths for transmission in either direction between terminal stations, it is apparent that the principles of the invention apply equally well to circuits employing sections of two-wire lines for transmittinDA and receiving at each terminal station, respectively, and employing as an intermediate link a long two-way transmission medium, such as the air link of a two-way radio telephone circuit or the submarine cable link of a two-way submarine cable telephone circuit.

Although in the particular embodiments of the invention illustrated and described, mechanical relays have been shown for conditioning the transmission paths by operating movable switches to close or open-circuit the paths, the invention is not limited to circuits employing the particular conditioning means shown. For example, within the scope o the invention the transmission paths may be effectively disabled by short circuits or loss networ is inserted in the paths, controlled by relays. Stationary devices having no movable elements may be employed in place of the mechanical relay shown, for example, vacuum tube relays, such as disclosed in Crisson Patent 1,547,233 issued November l, .1.926 or in the article by C. A. Beer and C. T. Evans in the Institute of P. O. Electrical 'Engineers (London) Volume 20, 65 to 72, inclusive, published in 1920.

What is claimed is:

tem for transmitting signals in opposite direc between stations comprising means recponsiv.. to tlr gnals from one station for givexclusive directional control of the system Dreto in preference to later initiated signals f' ion, and means automatically operative w en id control has been. obtained by :oid s'gnals from one station to bias the 'tvs-tcm rego, luent directional control in favor ol' later Yted signals from said other station.

2. The system of claim 1 and in Which said means for biasing said system is responsive directly to the signals from said one station.

3. A system for transmitting signals in opposite directions between stations, comprising means responsive to the initiation of signals at one of said stations for preventing transmission of signals later initiated at the other station to said one station., while allowing transmission of the signals initiated at said one station to said other station, and means also responsive to 'the signals initiated at said one station for altering the circuit connections at said other station to bias the system as regards subsequent directional control of transmission in favor of later initiated signals at said other station.

4. The system of claim 3 and in which the last-mentioned means operates to bias the system in favor of later initiated signals from said other station after directional control of the system has been obtained by the earlier initiated signals from said one station.

5. In a system for transmitting signals in opposite directions between stations, control circuits for determining directional control of transmission therein, means responsive to signals initiated at one station for controlling said control circuits so as to permit transmission of said signals from said one station to another station, while preventing transmission of signals later initiated at said other station to said one station, and means automatically operative after directional control of the system has been obtained for the signals initiated at said one station, to rearrange said control circuits so as to permit later initiated signals from said other station to immediately obtain directional control of the system at a slight pause in the transmission of the signals from said one station.

6. In a system for transmitting signals in opposite directions between stations, control circuits for determining directional control of transmission therein, means responsive to signals initiated. at one station for controlling said control circuits so as to allow transmission of said signals from said one station to another station, while preventing transmission of signals later initiated at said other station to said one station and means for rearranging said control circuits after directional control of the system has been obtained by the signals initiated at said one station, to bias the system as regards subsequent directional control thereof in favor of signals later initiated at said other station.

7. A four-wire signaling circuit comprising normally operative, oppositely directed one-way transmission paths connecting the terminals of said circuit, an echo suppressor unit at or near each terminal comprising means connected to the incoming one-way path thereat and responsive to signals therein to disable the input of the outgoing path at said terminal, and means connected to the outgoing path and responsive to signals therein to disable the means for disabling said outgoing path, a normally disabled control circuit at each terminal which when operative is responsive to signals in the outgoing path thereat to disable the incoming path at the same terminal and means at each terminal responsive to the signals in the incoming path thereat for rendering the normally disabled control circuit at the same terminal operative a given time after the outgoing path is disabled in response to the incoming signals.

8. A four-wire signaling circuit comprising normally operative, oppositely directed one-Way paths connecting the terminals of the circuit, an echo suppressor unit at or near each terminal of the four-wire circuit, each unit comprising means connected to the outgoing one-way path and responsive to outgoing signals therein to disable the output of the incoming one-way path, and means connected to the incoming path and responsive to incoming signals therein to disable the means for disabling the incoming path, a normally disabled control circuit at each terminal which when operative is responsive to signals in the outgoing path thereat to disable the incoming path at the same terminal, and means at each terminal and responsive to signals in the incoming path thereat for rendering the normally disabled control circuit at the same terminal operative a given time after the incoming path-disabling means at said same ter- `minal has been disabled in response to the same signals.

9. A two-way signaling system comprising transmitting and receiving circuits at each terminal thereof, and an echo suppressor unit at or near each terminal comprising means responsive to signals in the receiving circuit thereat to disable the transmitting circuit at the same terminal, means responsive to signals in said transmitting circuit to disable the transmitting circuit disabling means, a normally disabled control circuit which when operative is responsive to signals in said transmitting circuit to disable said receiving circuit, and means responsive to signals in said receiving circuit a definite time after they have disabled said transmitting circuit, to render said control circuit operative.

10. The system of claim 9 and in which said means for disabling the transmitting circuit disabling means operates in response to signals in said transmitting circuit substantially a time T after the initiation of said signals therein, where T is the transmission time between the terminals of said system.

l1. A two-way signaling system comprising transmitting and receiving circuits at each terminal thereof, and an echo suppressor unit at or near each terminal of said system, each unit comprising means connected to the transmitting circuit and responsive to signals therein to disable the receiving circuit at a time T after the initiation of, and to maintain it disabled an additional time interval T after cessation of said signals in said transmitting circuit, where T equals the transmission time between the terminals of said system, means connected to the receiving circuit and responsive to signals therein to disable the receiving circuit disabling means, a normally disabled control circuit which when operative is responsive to signals in said transmitting circuit to disable the receiving circuit and to maintain it disabled for a deinite time after the controlling signals cease, means responsive to signals in the receiving circuit to render the normally disabled control circuit operative a given time after the receiving circuit disabling means is disabled in response to the signals, and means tc maintain said control circuit operative for a deiinite time after the signals in the receiving circuit cease.

BJORN G. BJORNSON.

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