US2392496A - Transmission control in two-way signaling systems - Google Patents

Description

Jan. 8, 1946. A, Q N RwmE 2,392,496

TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEM Filed Sept. 5, 1942 SSheets-Sheet 1 I F/a/ I J a, I EA 4J 1 f ES, I

VL/ V WL DC/ EL W r M WA/ Q, 2"

(c) (b) l I l l (a) DESE'NSlT/ZATION (IN DEC/EELS) INPUT TO ECHO SUPREJSOR (IN DEC/EELS) INVEN TOR A C. NORW/NE AT T ORNE V Jan. 8, 1946. A. c. NORWINE ,3

TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEM Filed Sept. 5, 1942 3 Sheets-Sheet 2 v INVENTOR A. C NORW/NE ATTORNEY Jan. 8, 1946. v A. c. NORWINE 9 TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEM Filed Sept. 5, 1942 5 Sheets-Sheet 3 FIG. 4 25 /'A 057. I [27 32..- 057. NE

A 7' TORNE V Patented Jan. 8, 1946 TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEMS Andrew C. Norwine, Short Hills, N. J., assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application September 5, 1942, Serial No. 457,393

6 Claims.

The invention relates to two-way signaling systems, such as two-way telephone systems, and particularly to the signal-controlled switching circuits employed with such systems to direction- "ally control signal transmission therein while suppressing echoesand preventing singing.

Such signal-controlled switching circuits, commonly called echo suppressors, usually comprise two switching branches respectively connected to each of the two oppositely directed one-way signal transmission paths at a'terminal or intermediate point of the two-way system, each including a rectifier for rectifying the portion of the signal waves diverted into the switching branch, and electromagnetic relays or other switching devices operated by the rectified signalsto suitably control the transmission efiiciencies of the signal transmission paths and to disable the other switching branch, so as to provide the desired directional control of signal transmission while suppressing echoes and preventing singing.

The prior patent application of K. H. Davis and A. C. Norwine, Serial No. 421,704, filed December 5, 1941, which issued as Patent No. 2,306,689 on December 29, 1942, discloses such an echo suppressor applied to a four-wire telephone circuit,

modified so as to enable a subscriber at one terminal of the system by talking loudly to break in on another who is talking and has obtained directional control of the circuit, without waiting for the latter to pause and regardless of how loudly the latter subscriber is talking. This is accomplished by the use of relatively insensitive auxiliary switching controls, properly located with respect to the echo suppressor controls, operating on an amplitude basis to disable the echo suppressor controls of the talking subscriber when the breaking subscriber talks with the required loudness. These auxiliary controls necessarily have to be relatively low in sensitivity to above-described type, required for break-in, without increasing the probability of false operation by echoes.

These objects are attained in accordance with one embodiment of the invention by adding means to desensitize the break-in controls connected to each one-way transmission path in resented by the box. An arrow through the rep-.

proportion to the loudness of the speech signals in the oppositely directed one-way transmission path, so that a "loud talker needs to talk only slightly louder than the echoes of a "weak" talker who has obtained directional control of the system, to break in immediately.

In other embodiments of the'invention echo suppression is obtained and break-in facilitated by providing limited desensitizations of each one-way signal transmission path and the echo suppressor control branch controlled from that path, or, alternatively, of each signal transmission path on either side of the point of connection of the input of the associated echo suppressor control branch, which are different functions of the amplitude of the signals transmitted over the oppositely directed one-way signal transmission path controlling the other echo suppressor control branch.

The various objects and features of they invention will be better understood from the following complete description when read in conjunction with the accompanying drawings in which:

Fig. 1 shows a functional schematic of a fourwire telephone circuit equipped with an echo suppressor of the amplitude break-in type embodying one modification of the invention;

Fig. 2 shows schematically one detailed circuit arrangement for accomplishing the functions diagrammatically illustrated inFig. 1;

Fig. 3 shows curves used for explaining the different types of operation which may be obtained with circuit arrangements such as illustrated in Figs. 1 and 2; and

Figs. 4 and 5 show schematically, and in part diagrammatically, other modifications of the invention. Y

In the functional diagram of Fig. 1, each single line represents a two-wire signal transmission path. Contacting arrowheads at a point in a transmission path indicate that the path is normally enabled at that point, and an arrow pointing from a box towards the contacting arrowheads indicates that the path will be disabled at that point by operation of the apparatus representation of a resistance at a point in a transmission path, pointing from a box indicates that the sensitivity of the path at that point will be adjusted by operation of the apparatus represented by the box.

The four-wire telephone circuit of Fig. 1 comprises a one-way, two-wire transmission path EA normally conditioned to transmit telephone sigls the direction from west to east between the two-way line WI.- and the two-way line EL,

' and a one-way, two-wire transmission path WA suppressor branch ESi operating in response to applied telephone signals from the path EA, when that path is operative, to disable the east-to-west transmission path WA at the intermediate point D. Similarly, connected across the path WA at a. point beyond the disabling point I) in that path is the input of a second echo suppressor branch ES: operating in response to applied signals from the path WA, when that path is operative, to disable the path EA at a point (1 in front of the point a to which the echo suppressor ESi is connected. Thus, the echo suppressor branch connected to the one-way path of the tour-wire circuit transmitting the signals of the subscriber at one terminal of the system, who is first to talk, operates to disable the oppositely directed one-way path and eflectively the associated echo suppressor branch so as to give exclusive directional control of the system to the first talker.

Connected across the transmission path EA at a point e infront of the disabling point at in that path is the input of a break-in control circuit 3K1, relatively insensitive compared to the echo suppressor branch ESi, operating in response to applied telephone signals from the path EA of sufilcient amplitude to overcome its desensitization to disable the echo suppressor branch ES; controlled from the path WA, at the point 1 in its output. Similary, connected across the transmission path WA at a point g in front of the disabling point D is the input of a break-in control circuit BKz, relatively insensitive compared to the echo suppressor branch ESz, operating in response to telephone signals from the path WA of sum-- cient amplitude to overcome its desensitization to disable the echo suppressor branch ES1 connected to the path EA at a point h in its output. As described in detail in the aforementioned Davis- Norwine patent application disclosing similar-in control circuits, after the subscriber at one terminal of the system who first starts to talk, has operated the echo suppressor branch associated with the one-way path EA or WA transmitting his signals, to seize directional control of the system in the manner described above, and while that subscriber continues to talk, a subscriber at the other terminal of the system by talking loudly enough to cause operation of the break-in control branch fed from the oppositely directed one-way path which transmits his signals, may disable the echo suppressor of the first subscriber to interrupt the directional control of the circuit by the latter so as to allow the signals of the second subscriber to get through to the first subscriber. If the first subscriber continues to talk with normal loudness, the second subscriber by operating his own echo suppressor branch may disable the oppositely directed transmission path to seize directional control of the system and cut off the first subscriber, but if the first subscriber also talks with sufficient loudness to operate his own break-in control branch to disable the echo suppressor branch of the other subscriber, both subscribers will be able to hear each otherbut there will be no echo suppression.

As noted above, a disadvantage of break-in control echo suppressors of the type just described is that the auxiliary break-in controls necessarily have to be relatively low in sensitivity to avoid false operation by echoes of the loudest possible speech. In accordance with the present invention the loudness ot the second talker required for break-in is made substantially smaller without increasing the probability of false operation by echoes, thus improving the naturalness of conversation. This is accomplished, as indicated diagrammatically in Fig. 1, by adding means for causing the break-in control branch controlled from each one-way transmission path to be desensitized in proportion to the amplitude of'the speech signals in the oppositely directed one-way transmission path. Thus a loud talker needs to talk'only slightly louder than the echoes of the wea talker who has obtained directional control or the system to break in immediately. This desensitization may be accomplished as indicated diagrammatically in Fig. 1, by adding controls DCl, DC: to adjust a variolosser VL1 or VLc in the'input of the break-in control branch BKi, BK: connected to each oneway transmission path 'in accordance with the amplitude of the rectified signalsin the output of the echo suppressor branch ES: or ES: controlled from the oppositely directed one-way path, or, alternatively, the desensitization controls may be controlled by an auxiliary detector connected to the oppositely directed one-way path (as illustrated in Fig. 2). The hang-over of each variolosser VLi, VLo should be made such that the desensitization of the break-in control connected to each path in response to each controlling signal in the other path will be maintained for a suflicient time interval to prevent false operation of the control by the echo of that signal.

Various types of operation which may be obtained with the desensitizing arrangements of the invention as shown in Fig. 1 (or the alternative arrangements of Fig. 2) depending on the adjustment of the desensitizing gaincontrols are illustrated in the curves of Fig. 3, in which the desensitization (in decibels) of a break-in control branch controlled from one side of the four-wire circuit is plotted against the input (in decibels) to the echo suppressor branch controlled from the oppositely directed side of the four-wire circuit. Curve a shows limited range desensitization, that is, the break-in control operated from one side of the four-wire circuit is desensitized in direct proportion to the loudness of signal input to the echo suppressor branch operated from the other side. up to the knee of the curve; beyond that, the desensitization (and thus the break-in control sensitivity) is constant. With this adjustment, echo suppressor characteristics which may be identified by the code letters NBE are obtained. The first code letter N of the three means that the echo suppressor provides normal echo suppression if both talkers are weak or only one talker is speaking. The second letter B of the code means that if only one of the talkers is loud enough to operate his break-in control a normal break-in occurs and he gains directional control of the system. The third letter E in the code mean that if both talkers talk loudly enough to operate their break-in control both echo suppressors are lifted off so that both talkers hear each other, and double talking echoes occur. With this adjustment. the initial breaking sensitivity can be increased by the range of desensitization, if the desensitization controls are provided with sufficient hangover in operation by any suitable means to prethe echo of the weak talker to be able to break in immediately.

The range of input over which desensitization is changing determines the range for which the middle letter B" of the code will hold. That is, both talkers will have to talk louder than the input defining the knee of thecurve a to reach the condition referred to by the last code letter E. In the case of curve b, there is no range where this condition can be reached, so that we have an NB echo suppressor which is similar to the ordinary differential echo suppressor except that breaking will be faster because the hang-over of differential is applied to desensitize, not to dis-- able, the opposite side of the four-wire circuit. With the adjustment of curve c, the break-in controls are disabled for inputs higher than the knee of the curve, so that the suppressor is changed to what may be called the NBN type, that is, one that provides normal suppression for weak talkers or only one talker; if one of the talkers is loud enough to operate his break-in control.

a normal break-in occurs and he gains control of the echo suppressor; and when both talkers talk loudly enough to operate their break-in controls. the echo suppressor functions normally to give directional control to the first talker to speak.

To make the above-described types of break-in effective, it is necessary that the relative sensitivities of the various echo suppressor control and the relative normal sensitivities of the breakin controls be properly proportioned.

Fig. 2 shows in more detail the circuit; and apparatus which could be used to accomplish the echo suppressing, break-in control and desensitizing control functions more diagrammatically illustrated in Fig. 1. The four-Wire telephone c rcuit of Fig. 2 includes the one-way transmission circuit EA including the amplifier AE, normally conditioned to transmit in the direction from east to west, and the one-way transmission circuit WA including the one-way amplifier Aw, normally conditioned to transmit in the direction from east to west. The input of the circuit EA and the output of the circuit WA may be coupled at tl'e west terminal of the four-wire circuit by the usual hybrid coil and associated line balancing network (not shown) to a west two-way line leading to subscribers stations, and the output of the circuit EA and the input of the circuit WA may be coupled at the east terminal of the four-wire circuit by the usual hybrid coil and associated line balancing network (not shown) to an east two-way line leading to subscribers stations.

Connected across the circuit EA at an intermediate point I is the input of an echo suppressor branch circuit 2. to be referred to hereinafter as the west echo suppressor, including the wave detector 3 and the electromagnetic relay 4 adapted to be operated in response to the rectified voice signal output of the detector 3. Similarly, connected across the east-to-west circuit WA at an intermediate point 5 is the input of an echo suppressor branch circuit 6, to be referred to hereinafter as the east echo suppressor, including the A point 9 in front of the point 5 to which the input of the east echo suppressor Ii is connected, 'to disable that circuit, and simultaneously to close a short-circuiting connection across the input of the detector 1 in the echo suppressor 6 to disable iii) that echo suppressor. Similarly, the relay 8 of the east echo suppressor 6 operates to close a short-circuiting connection across the circuit EA at the point III in front of the point I to which the input of the west echo suppressor 2 is connected to disable that circuit, and simultaneously to close a short-circuiting connection across the input of detector 3 in the west echo suppressor 2 to disable that echo suppressor,

Connected across the west'-to-east transmission circuit EA at a point H in front of the disabling point I0 therein is the input of a third control circuit [2 relatively insensitive compared to the west echo suppressor 2, which will be referred to hereinafter as the west break-in control, including the wave detector I3 and electromagnetic relay l4 adapted to be operated in response to operation of the detector [3 by applied voice signals of sufilcient amplitude to overcome the desensitizing loss in its input, to put a disabling break in the short-circuiting connection across the circuit EA at the point it, and thus to effectively-disable the echo suppressing function of the east echo suppressor 6.-

Similarly, connected across the east-to-west circuit WA at a point i5 in front of the disabling ,pointS is the input of a control circuit l6 relatively insensitive as compared to the west echo suppressor 6, which will be referred to hereinafter as the east break-in control, including the wave detector ll and the electromagnetic relay l8 adapted to be operated by operation of the detector in response to applied voice signals,

in front of the disabling point I0 is the input of a fifth control circuit 20, to be referred'to hereinafter as the east break-in control desensitizer, including the wave detector 2 I, which may be of the electron discharge tube type as indicated, and

a plurality of marginal electromagnetic relays R1, RN of different sensitivities ranging from a high sensitivity for relay R1 to a low sensitivity for the relay RN, connected in series across the output of the detector 2|. The highest sensitivity relay R1 is adapted to be operated by the rectified signal output of the detector 2| in response to the amplitude of voice signals applied to the input of the detector 2| which would be produced by the weakest direct voice signal (not echoes) passing over the circuit EA; the lowest sensitivity relay RN is adapted to be operated by the rectified voice signal output of the detector 2| in response to an amplitude of the voice signals applied to the input of the detector 2| which would be produced by the loudest received voice signals passing over the path EA; and the other relays, R2, R3 are adapted to be respectively operated by different rectified signal outputs of the detector 2| in response to amplitudes of the voice signal inputs to'that detector produced by voice signals of different intermediate stagesof loudness passing over the circuit EA. Therelays R1, R2, R3 Ru when operated respectively connect the losses L1, L2, L3 LN across the input ofthe detector H in the west break-in control IS. The value of the loss L1 is selected such that when it is connected across. the input of the west break-incontrol 16 it will reduce the sensitivity of that control by an amount sufficient to sert the losses L11, L12, L13

Just prevent false operation 01' that control by echoes applied from the path WA of the signals causing operation of the relay R1. Similarly, the value of each of the other losses L2. L: Ln is selected so that when each in turn is connected across the input of the break-in control IS the total loss in the input of that control will reduce its sensitivity by an amount just sufficient to prevent false operation of the control by echoes of the signals of the amplitude causing the operation of the respective relays which are operated. Also, each of the relays R1, R2, R3 Rn is provided with the same hang-over in operation, which is sufilcient to maintain the loss which it controls connected in the input of the west breakin control I6 for just suflicient time to prevent false operation of that control by the echo of the signal causing the operation'of the relay. The number of relays used in the chain R1 Rs will depend on the range of speech volumes for which desensitization is required and the decibel loss inserted per relay. 1

Similarly, connected across the circuit WA at a point 22- in front of the disabling point 9 is the input of a sixth control circuit 23, to be referred to hereinafter as the west break-in control desensitizer, including a wave detector 24, like detector 2i, and a plurality of marginal relays R11, R12, R13 Rm of different sensitivities covering the same range as the relays R1 RN in the west break-in control desensitizer 20 andwith hang-overs computed in the sameway, connected in series across the output of the wave detector 24 and operating in response to applied voice signals from the circuit WA of difierent amplitudes over the desired range of control, respectively in- Lin, equivalent to the losses L1, L2, L3 LN in the west break-in control desensitizer 20, across the input of the detector l3 of the west break-in control l2, to prevent false operation of the west break-in control l2 by the echoes of the signals controlling the operated relays.

The circuit arrangement of Fig. 2 operates as follows:

Let it be assumed that a west telephone subscriber at the west terminal of the four-wire circuit starts to talk, and at that time the east subscriber at the east terminal of the four-wire circuit is silent. Wests speech waves will pass over the circuit EA to the east subscriber being amplified en route by the amplifier An. A portion of Wests signals will be diverted from the circuit EA at the point ll into the input of the west break-in control l2, which, atthe time, with no speech. waves being received from the east subscriber over the circuit WA, will be in its most sensitive condition so that at least a portion of Wests speech signals may be'of high enough amplitude to momentarily cause operation of the break-in detector l3 and thus of relay M to put a break in the short-circuiting connection across the circuit EA at the point ID. This will have no effect on the directional control of transmission because that short-circuit-ing connection is broken at another point with the east echo suppressor unoperated.

Another portion of Wests speech signals will be diverted from the circuit EA at the point l9 into the east break-in control desensitizer 20 and will cause operation of the detector 2| therein to operatively energize one or more of the chain of relays R1 Rn in its output, depending on the loudness of Wests speech, to cause one or more of the losses L1 LN to be inserted into the input of'the detector ll of the east break-in control Hi to just sufliciently desensitize that'control so as to prevent its false operation by applied echoes from the circuit WA or Wests signals causing the operation of the operated relays in thechain R1 RN.

Another portion of Wests speech waves will be diverted from the circuit EA at the point I into the west echo suppressor 2 and will cause the operation of the detector 3 and thus of the electromagnetic rela 4 to close the short-circuiting com nection across the circuit WA at the point 9 to disable that circuit, and to close a second short- Now, if the east subscriber desired to break in on West, the former will raise hi voice so that at least a portion of his speech signals transmitted over the circuit WA-and diverted into the east break-in control 1.6 will be of sufiicient amplitude to overcome the desensitization of that control produced by West by operation of the east breakin control desensitizer 20 in a manner which has been described. As the desensitization of the east break-in control 16 is only that which will just prevent false operation by the echoes of Wests speech signals, it will be apparent that East need only talk slightly louder than the echoes of Wests speech signals to accomplish this. If East does so, his speech waves will cause operation of the detector I1 and thus of the electromagnetic relay While West continues to talk, however, his voice signals will maintain the relay 4 in the west echo suppressor 2 operated to hold the east echo suppressor 6 disabled in its input. Thus, both transmission circuits WA and EA will be operative to respectively transmit the signals of the west and east subscribers to the other subscriber, but there will be no suppression of echoes.

Another portion of Easts speech waves will be diverted at the point 22 into the west break-in control desensitizer 23 and will cause the operation of the detector 24 and thus of one or more of.the relays R11 Rmin its output, depending upon the amplitude of East's signals, to connect one Or more of the losses L11 Lm into the input of the west break-in control l2. The losses thus inserted into that control will desensitize that control just sufliciently to prevent its false operation by echoes from the pathEA of East's signals which produce the operation of the operated relays of the chain R11 Rm.

Now, if West also raises his voice sufiiciently to overcome the desensitization produced in Wests break-in control I2 his voice signals will cause operation of the detector l3 and thus of relay M to put a break in the short-circuiting connection across the circuit EA- at the point ill, but this will be inefiective to cause any change in the operation of the circuit for that short-circuiting connection is already opened because of the non-operation of relay 8 of the east echo suppressor 6 held disabled in its input by the west echo suppressor 2.

This condition of the circuits will continue until one of the subscribers stops talking so as to release his echo suppressor and his break-in control. This will allow the other talker by oper ating his suppressor to block the echo path and disable the first subscriber's echo suppressor, to seize directional control of transmission in the system, and by continuing to operate the desensitization control for the first subscriber's break-in control making it possible for the latter to interrupt by talking just slightly louder than the first subscriber's echoes. V

An echo suppressor of the proportional losser type, for example, such as shown in A. B. Clark Patent No. 1,821,928, issued September 8, 1931, can be made to suppress echoes well by inserting losses in the echo path which increase several I decibels for each decibel increase in signal in the oppositely directed path, or it can be made to permit the signal in one path, it loud enough. to interrupt the signals in the other path if the inserted losses increase but slowly with increases in signal amplitude. In such an .echo suppressor, the requirement of good echo suppression is incompatible with the requirement that break-in be easy.

Fig. e shows a proportional losser type of echo suppressor in accordance with the invention which meets both break-in and. echo suppressing requirements. Referring to Fig. i, the four-wire telephone circuit consists of one one-wamtwowire transmission path EA including the one-way amplifiers An, normally conditioned for transmitting voice signals from west to east, and a second one-way, two-wire transmission path WA including the amplifiers Aw for transmitting voice signals in the direction "from east to west. These Paths are associated at their terminals with a west two-way line WL and an east two-way line EL bythe usual hybrid coil and associated line balancing network.

Connected across the transmission path EA at an intermediate point 25 is the input of'a combined echo suppressor and break-in control circuit it including the wave detector 2'! and connected in parallel across the output of the latter, the variable loss devices 28 and 29, the loss values of which are adjusted in accordance with the amplitude of the signal output of the detector 21, as shown diagrammatically. Similarly, connected across the transmission path WA at an intermediate point 30 is the input of acombined echo suppressor and break-in control circuit 3| including the wave detector 32 and the variable loss devices 33 and 34 connected in parallel across the output of the latter, the 1058 values of which are adjusted in accordance with the amplitude of the signal output of the detector 32, as 11- lustrated diagrammatically. The variable loss -device 23 controlled from the output 01' detector 21 so that its loss determines the sensitivity of that circuit.

The normal loss values 01' the variable loss do vices 2B and 38 in the paths WA and EA, respectively, are made very low so that the normal transmission efilciencies or those paths are high. Also, the normal loss values'of the variable loss devices 34 and 259 in the inputs of the control cincuits 2G and 3!, respectively, are also made very low being only large enough to prevent false operation'oi. the detectors in those circuits in response to ,line noise from the associated signal transmission paths.

Now, let it be supposed that a west telephone subscriber associated with the line WL starts to talk first. Because of the normal low loss condition of the variable loss device 33 in the path EA the west subscriber's speech waves will be transmitted over that path to the line EL leading to the east subscriber with maximum transmission efficiency being amplified by the amplifiers As: on route. A portion of West's speech waves will be diverted from the path EA into the control circuit 26 and, the variable loss device 34 therein being in its normal low loss condition, with no spee h signals of the east subscriber being passed over the path WA, will quickly cause the operation of the detector 2! to produce an increase in loss of the variable loss device 28 in. the path WA, to reduce transmitted echoes of West's speech waves reflected into that path at the east terminal of the four-wire circuit. The operation of detector 21 will also cause an increase in the loss of the variable loss device 29 in the input of control circuit 3i to desensitize that circuit, so as to prevent its subsequent operation by applied speech waves oi the east subscriber of normal loudness subsequently received from the line EL to reverse directional control of the circuit. Similarly, if the east subscriber associated with the line EL starts to talk first, the variable loss device 28 being in its normal low loss condition, his speech waves will be transmitted over the path WA to the line WL leading to the west subscriberwith maximum efliciency. The portion of West's speech waves diverted into the control circuit 3!, the variable loss device 28 therein being in its low loss condition, will quickly oper ate the detector 32 which in turn will cause the loss of the variable loss device 33 in thepath EA to be increased so as to reduce the transmitted echoes of East's speech waves reflected from the west terminal of the four-wire circuit into the path EA. The operation of detector 32 will also cause the loss of the variable loss device as in the control circuit 26 to be increased thereby desensitizing that circuit so as to prevent its false operation by subsequently applied speech waves of the west subscriber of normal loudness subsequently received over the line WL and the path EA;

The variable loss devices 28 and 33 in the transmission paths WA and EA, respectively, would preferably be arranged so that the losses introduced by them in the respective transmission paths WA and EA in response to operation of the control circuit 26 and the control circuit its loss determines the sensitivity of the latter 7 3!, respectively, will be just large enoughso that circuit. Similarly, the variable loss device 33 controlled from the output of detector 32 is connected in the path EA beyond the point 25o! connection of the control circuit 26 so that its for each value of input signal applied to the controlling circuit the echo of that signal will be reduced to a commercial amount. On some commercial four-wire telephone circuits this may loss is effective in that path only, and the varia- 7o requi so large a loss for all inputs that we have ble loss device 34 is connected in the input of the control circuit 28 in front of the wave-desimply a .proportionally desensitized echo suppressor. However, if the loss introduced by the : variable loss device 28 or 33 in response to operation of the associated control circuit can be limitedand still suppress echoes, or if it is made "to follow the syllable variations of the speech signals and these variationsare delayed in application to match the delay oi echoes, a loud talker could be heard through it even before his speech signals 33 in the transmission path is reduced, the continuing break-in speech becomes gradually louder, as heard by the talker being interrupted. For this reason, it may be less annoying than abrupt break-in, such as that aiforded by relay type break-in arrangements.

The variable loss devices 34 and 23 in the control circuits 26 and 3|, respectively, may be arranged so that when they are adjusted by the control detector 32 or the control detector 21, respectively, their losses do not increase faster than the input to the opposite side of the circuit, for this amount 01' desensitization will protect against false operation on echoes.' Also, the losses inserted need not exceed the value which will protect against the highest amplitude echo. This assumes, of course, that there is eflective hang-over in the desensitizing control equal to the delay time of the worst echo to be protected against.

Just as in the case of the desensitization applied to amplitude break-in controls, it is important to designate the point at which hangover is applied. It is intended that the hangover of proportional sensitivity reduction should mean the maintenance during the hang-over period of a desensitizing loss sumcient to prevent false operation on echo. Two things are to be noted about this definition: first, that the desensitizing loss reduces the sensitivity of the oppo-.

sitely directed device rather than totally disabling it, and, second, that an input louder than the postulated echo can operate the reduced-sensitivity device immediately.

The theory underlying the design of echo suppressors of the amplitude break-in type is that loud echo might be permissible if one or both talkers on a circuit are talking loudly. In a somewhat similar way, it may be said that the divided function proportional losser echo suppressors of Figs. 4 and 5 are based upon the v theory that echo suppressing losses may be re-' duced during the. period when both talkers are x that of Fig'. 4 only in that the desensitizing variable loss device 34 for the control circuit 26 is placed in the path EA in front of the point to ,which the control circuit 23 is' connected, in

stead of in the control circuit 26, and the desensitizing variable loss device 29 for the control WA, respectively, may b arranged to insert an echo suppressing loss in these paths smaller than that which would be necessary in the circuit of Fig. 4, by the amount of the desensitizing loss for the associated control circuit.

The variable loss devices 28, 23, 33 and 34 in l the circuits of Figs. 4 and 5 may be or the relayresistance type used for desensitizing the breakin control circuit in the circuit of Fig. 2.

Various modificationsof the circuits illustrated and described which are within the spirit-and scope of the invention will be apparent to persons skilled in the art.

What is claimed is:

1. In combination in a two-way telephone transmission system, two oppositely directed oneway signal transmission paths, one switching device. connected to each path responsive to voice signal transmission therein, in the absence of prior voice signaltransmission in the other path, to insert an echo suppression loss in the other path, and to effectively disable said one switching device connected to said other path, a second relatively insensitive switching device connected to each path in front of the echo suppression point therein, operating in response to applied voice signals from the connected path of sumcient amplitude to overcome its desensitization, to rendersaid one switching device connected to the other path inefiective to disable the first path, so as to allow break-in, and means to desensitize said second switching device connected to each path in-proportion to the amplitude over a given range of amplitudes of the voice signals transmitted over th other path, to enable operation of the latter device by the lowest amplitude voice signals in the connected path consistent with preventing its false operation by echoes of the voice signals in said other path.

2. In combination in a two-way telephone system including oppositely directed one-way transmission paths for the signals for opposite directions, one echo suppressor device connected to each path, responsive to voice signal transmission of the subscriber at one end of the system transmitted over that path, in the absence of priorvoice signal transmission of a subscriber at the other end of the system in the other path, to disable said other path and said one echo suppressor device connected thereto, a relatively insensitiv break-in control device connected to each path at a pointin front of the disabling point therein, responsive to applied voice signal transmission of suflicient amplitude, to efiectively disable the transmission path-disabling function of the echo suppressor devic controlled from the other path, .and means, to desensitize the break-in control device connected to each path in accordance with the amplitude level of .the voice signal transmission, over a given range of amplitudes, applied to the echo suppressor applied voice signals above normal level consist-- ent with preventing false operation thereof by applied echoes of the signals transmitted over said other path in the absence of direct voice signal transmission in the first path.

3. The system of claim 2 in which the ranges of densitization of said break-in control devices are selected so that if the voice signals of the talking subscribers at both ends of the system are weak or if the subscriber at one end only is talking, signal transmission over the system is efiected only in one direction with the echoes of that signal transmission in the opposite direction sup pressed; if the signals of one talking subscriber are loud and those of the other weak, the loud talker will obtain directional control with suppression of his signal echoes; and if both talkers are loud the signals of each will be transmitted to the other subscriber but there will be no echo suppression until one subscriber stops talking.

4. In combination in a two-way telephone system, two oppositely directed one-way transmission paths for respectively transmitting the oppositely directed signals of th subscribers at opposite terminals of the system, one switching device connected to each path, operating in response to voice signal transmission therein, in the absence of prior voice signal transmission in the other path, to insert an echo suppression loss in said other path and to desensitize said one switching device controlled therefrom, so as to give directional control of the system to the subscriber who first starts to talk, and means to facilitate breakin by th subscriber at one terminal of the systern when the subscriber at the other terminal is talking and has obtained directional control of the system, and to avoid undesirable operation characteristics otherwise obtained with loud talking by the subscribers, comprising means'to make the echo suppression loss inserted ineach transmission path by operation of the switching device connected to the other path, to vary in proportion to the amplitude of the voice signals in said other path controlling the operation of that switching device, and to make the desensitization of th switching device connected to each path. by operation of the other switching device connected to the other path to vary in accordance with another function of the amplitude of the signals in said other path controlling the operation of said other switching device, such as to allow operation of the first switching device by voice signals applied from the first path of the lowest amplitude consistent with preventing its false operation by applied echoes of the voice signals transmitted over said other path.

5. In combination with a two-way telephone transmission system including two two-way line sections terminating at widely separated points in said system and 'two oppositely directed oneway transmission paths for respectively repeating voice signals in opposite directions between said two-way line sections, two voice-operated switchin: devices respectively connected to a different one of said paths at points in said system relatively close together, means responsive to operation of the switching device connected to each path under control of voice signals transmitted over the path to which it is connected, in the absence of prior voice signal transmission in the opposite direction in the other path, to insert a variable loss in said other path beyond the point of connection thereto or the other switching device, which is at any time just sufficient to reduce in said other path the transmitted echoes of the controlling signals to a predetermined tolerable amount, and other means responsive to operation of the switching device connected to each path to provide a desensitization of the other switching device varying with the level of the voice signals in the first path controlling the first switching device, and such as to permit operation of said other switching device by applied voice signals from said other path, of the lowest amplitude level consistent with preventing its false operation by echoes of the signals transmitted over said first path, so as to facilitate break-in. I

6. In combination with a two-way telephone transmission system including two two-way line sections terminating at separated points in said system and two oppositely directed one-way transmission paths for respectively repeating voice si nals in opposite directions between, said two-way line sections, two voice-operated switching devices respectively connected to different ones of said two paths and means responsive to operation of either switching device under control of voice signals transmitted over the path to which it is connected, in the absence or prior voice signal transmission in the other path, to insert a variable loss in said other path on each side of the point of connection of the other switching device thereto, which losses are different functions of the amplitude level of the signals in the first path controlling the operation of the first switching device connected thereto, the sum of' the losses thus inserted in said other path being limited to that value which will reduce the transmitted s18- nal echoes therein to a predetermined tolerable amount, and the loss inserted in said other path in front of the point of connection of said other switching device thereto being limited to that value which will effectively produce just sufllcient desensitization of the latter device to prevent -its false operation by applied signal echoes.

ANDREW C. NORWDIE.

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