US2274392A - Transmission control in two-way signaling systems - Google Patents

Description

Feb. 24, 1942. L. G. ABRAHAM 2,274,392

TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEMS Filed April 18, 1941 Fla.

E CHO SUPPRESSOR /E$ T A f S/ ,73 AMP DET s 7 19 I2 I I3 (I! AMP oar a A AMP 4I m/vawroe L. G. ABRAHAM ATTORNEY Patented Feb. 24, 1942 TRANSMISSION CONTROL IN TWO-WAY SIGNALING SYSTEMS Leonard. G. Abraham, Madison, N.. .L, assignor to Bell Telephone Laboratories, Incorporated, New York; N. Y., a corporation" of New York ApplicationAprilxIS, 1941*, SeriaLNo. 389,160

6- Claims.

The: invention. relates to twoeway signal transmission systems and particularly: to circuits for controlling" transmissiom in such systems.

Proper operation of long two-way telephone systemshas required the use of voice-operated.

switching devices, commonly known as echo suppressors, to directionally control signaltransmission in: the oppositely. directed repeating paths of the system while suppressing echoes and preventing singing. Such echo suppressors may comprise, for example, an amplifier-rectifier-relay circuit connected to-each repeating path,.operating in response'tos'ignal transmission-in that path todisable the oppositelydirect'ed' repeating pathto suppress the signal echoes;

Such echo. suppressorsoften introduce detrimental effects on transmission, such as an increase in the time required for a listening subscribertobreal in. on a talking subscriber, and

partial or complete signal transmission lockout When-both subscribers at the two ends-of the system start talking atab'out' the same time. There are many calls in present-day systems which do not require the operation of echa suppressors to suppress echoes but which causethe echo'suppressors, which are present because they willbe needed on other calls involving the same circuit, to function, and there is atv present no means available for distinguishing adequately whichca-ll's do need-echo supressors;

It is an object of the invention to'red'uce the efi ects of echo'es in" atwo way telephone system while" preventing or reducing the above-mentioned detrimental effects;

This object is attainedin accordance with the invention by circuits operating automaticallyto remove an echo suppressor from a system or to efiectively' disabl'euit when it is not needed to suppress echoes. In. oneembodiment, these circuits compare the voice. currentsreceived from a twoway line on.one side-of afour-wire telephone circuit withthe. echoes .sent back in. the other. directionfrom the two-wayline on. the other side of the, four-wire circuit, and if and when the echoes returned are weaker than the direct speech currents byanamountpredetermined as satisfactory from an echo standpoint, remove or disable the echo suppressor and restoreit when the echoes become relatively stronger than the limiting amount.

Th invention will be better understood from thefollowing: detaileddescription when read in conjunction with the accompanying drawing, Figs. 1 andZ of which show schematically a portion of a two-way telephone system embodying diflerent modifications of the invention.

Fig. 1 shows a four-wire terminating circuit at the east terminal of a two-way telephone system. It comprises a; westto-east incoming two-wire pathtEA including the one-way amplifying device Ala-and an: outgoing east-to-west two-wire-circuit WA including the one-way amplifying device Aw; coupled by th usual three-winding or twocoil hybrid transformer H in conjugate relation with each:ot'her and in energy-transmitting relation with the two-way telephone circuit TCwhich may lead to an east subscribers telephone set (not shown); The circuits EA and WA may extend to the west to a similar four-Wire terminating circuit at the west terminal of the system directly' or through an intermediate radio or two-way wire. link (not shown);

An echo suppressor represented by the box ES, which may be of any of the Well-known types, is associated with the paths EA and WA at the east terminal, as indicated. In the particular type of echo suppressor diagrammatically illustrated, the incoming path EA is normally enabled due to the normal break in the short-circuiting connection across that path at the point I; provided by the normal open condition of the contacts of the unenergized switching relay RE, and the outgoing path WA is normally enabled due to the normal break in the short-circuiting connection across the latter path at the point 2, provided by the normal open condition of the contacts of the unenergized switching relay Rw. In the ordinary operation of the echo suppressor ES, a portion of the incoming telephone signals received over the path EA from a west subscriber, diverted into the transmitting switching control TS, which mayinclude an amplifierrectifler, not shown, will cause the operation of the relay Rw to close its switching contacts to short-circuit the path WA at the point 2, thereby efiectively disabling the latter path at that point. Thus, any echoes of the incoming westto-east signals appearing in the input of the path WA, dueto improper unbalance of the hybrid coil H and associated balancing network N, or any subsequently receivedeast-to-west signals received over the circuit TC, will be prevented from passing out over the path WA to the west station or from entering the receiving switching control RS to cause false operation of echo suppressor ES. Also, in the ordinary operation of echo suppressor ES, when the transmitting control TS is not operated'byincoming west-to-east signals, and east-to-west signals received over the circuit TC are impressed on the outgoing path WA th'rough hybrid coil H, a portion of these signals diverted into the receiving switching control RS, which also may include an amplifier-rectifier (not shown) will cause its operation to operate switching relay RE to short-circuit the incoming path EA at the point i, to suppress any subsequently receivced west-to-east signals or echoes. In a modified echo suppressor, the relay Rw under control of transmitting switching control TS would operate to close the contacts at point 2 to insert loss in the path WA, as shown, but RE under control of receiving switching control RS would operate to disable the transmitting switching control TS of the echo suppressor to avoid false operation, etc. In such a case, a similar echo suppressor device at the west terminal of the system (not shown) would be arranged to operate in similar manner to block the outgoing path EA in response to incoming signals in the incoming path WA. In the suppressor as shown on Fig. 1, no echo suppressor need be used at the distant end of the circuit. I

The control circuits for accomplishing the objects of the invention, illustrated in Fig. 1, include one control branch 3 having its input bridged across the incoming path EA at a point in front of amplifier As, and a control branch 4 having its input bridged across the input of the outgoing path WA at a point in front of amplifier Aw. The control branch 3 includes the amplifierdetector 5 and connected in parallel across its output the left-hand operating winding of differential relay 6 and the operating winding of relay l. The control branch 4 includes the amplifier-detector 8 and connected across its output the right-hand winding of the diiferential relay 6, poled to oppose the left-hand winding of that relay. The differential relay 6 and relay 1 control the operation of the relays 9 and I0 and the associated circuits in the manner to be described in the following complete description of the operation of the circuits of Fig. 1.

With no speech current being received over the incoming path EA and no speech currents being transmitted over the outgoing path WA, the circuits described above are in the condition shown in the drawing, with the differential relay 6 deenergized so that its armature is in the neutral position, the relays land 9 deenergized and the cuits TS and RS effective for echo-suppressor operation.

Now, let it be assumed that speech currents from a west subscriber are being received over the incoming transmission path EA at the east terminal of the system, and at that time no speech currents from an east subscriber associated with the circuit TC are present in the outgoing path WA at the east terminal. The incoming speech waves in path EA are amplified in the amplifier AE and, the echo suppressor relay RE being in its normally unenergized condition so that the path EA is unblocked at the point I, the amplified voice waves will pass over the output portion of the path EA and through the hybrid coil H to thetwo-way circuit TC over which they will be transmitted to the east subscriber. A portion of wests speech waves will be diverted into the transmitting control TS of echo suppressor ES and will operate relay Rw to block WA at the point 2.

Another portion of wests speech waves passing over the path EA will be diverted into the control branch 3 of the differential control circuit, will be amplified and detected by the amplifier-detector 5 therein, and will be supplied as energizing current to the operating winding of relay 1 to operate that relay, and to the lefthand operating winding of the differential relay 6 tending to operate that relay upwards. The echoes of wests signals produced in the outgoing path WA due to improper balance of the circuit TC by the balancing network N will be transmitted over that path through amplifier Aw towards the west terminal.

A portion of the echo currents in the input of amplifier Aw will be diverted into the control branch 4 of the differential control circuit, will be amplified and detected in amplifier-detector 8 and supplied as energizing current to the righthand winding of differential relay 6 tending to operate that relay downward. The sensitivity of amplifier-detector 5 in control branch 3 and that of amplifier-detector 8 in control branch 4 are relatively adjusted so that the difierential relay 6 operates downward to close its armature and lower contact when the echo in the path WA is greater than desired compared to the direct signal transmission in the path EA, and operates upwards to close its armature and upper contact when the echo in the path WA is weak enough compared to the direct transmission in the path EA to be negligible. When the differential relay 6 is operated downwards, the relay 9 remains released (as in its normal condition) and thus the relay l0 controlled thereby remains in the operated condition with its upper and lower contacts closed to maintain the transmitting switching control TS and the receiving control RS of echo suppressor ES operative so that the echo suppressor will continue to function to maintain the outgoing path WA disabled at the point 2 so as to suppress the echoes of wests signals at that point in the manner described above. However, when the differential relay 6 is operated upward indicating that the echo in the path WA is negligibly weak compared to the direct signal transmission in the path EA, relay 9 tends to operate from battery I I through ground applied through the upper contacts of differential relay 6, but, due to the effect of condenser l2 in shunt with its winding and the resistance I3 in series therewith of properly selected values, relay 9 is slow to operate, requiring either a continuous closure of the upper contacts of differential relay 6 for a given time interval, or a considerable number of shorter closures of these upper contacts to charge up the condenser [2 before relay 9 operates. This is for the purpose of insuring that the return loss over the frequency band of interest is adequate, as a fast operating relay 9 might be improperly operated by a high return loss in one frequency range when a sound contained mostly those frequencies.

If at any time during this process, the differential relay 5 releases and stays neutral, nothing happens except that in time the charge on condenser I! would drain away through relay 9. However, if the differential relay 6 opcrates downwards while relay '1 is operated, condenser l2 will be shorted by a circuit extending through the made contacts of relay 1 and the made lower contacts of differential relay 6, and the charging process must begin over again before relay 9 can be operated. When relay 9 does operate, removing ground from the normally enand. RS forecho suppressor. ES and thus disabling the suppressor so that it is efiectively re- 'moved from the system. Relay 9 will lock up through its operated front contact and ground. Relay Swill then remain operated to hold relay l deenergized and. thus. the echo suppressor ES inefiective untilv such time as. the differential relay 6 again becomes operated downward in response to a greater echo in the path WA than desired compared to the direct transmission, and relay 1 is operated. simultaneously in response to the speech waves receivedfrom the path: EA. When. this happens, the winding. of relay 9- will be shortcircuited through the contacts of relay 1 and the lower contacts of. relay 6 and relay 9 will. therefore release to return relay I0: to. the normal energized condition whereupon: the echo suppressor ES again becomes operative.

The purpose of relay 1 is to prevent the near.-

end subscriber at the local toll terminal from restoring the echo suppressor to the circuit when he talks. For example, if the contacts of relay 1 were permanently closed when the subscriber at the near-end talks, differential relay 6 would be operated downward causing relay 8 to be. released and relay [8- operated, thereby restoring the echo suppressor ES to the circuit while there is no need for it,v With relay'l released, however, as it'would be with no speech coming in from the distant end of the four-Wire circuit, the. operation of the differential relay d downward will not cause the release of. relay 9 with the result that. the echo suppressor ES will remain inoperative.

This raises the question as to What happens when the. west subscriber at the distant end of the four-wire circuit and the" local subscriber associated with the circuit TC start talking at about the same time. In this case, the voice currents of the east subscriber would maintain control of the difierential relay 6- with the result that the echo. suppressor ES will. be. maintained operative to disable the incoming path EA until thusrequiringconsiderable overlap in simultane- 1 ous speech before the echo-suppressor restored. If this were not sufficient, it might be desirable to provide another relay operating from the out.- put of the receiving amplifier-detector 8 with such low sensitivity that itnever would be operated inresponse. to echoes but would operate in response to loud. near-end speech, and arranged for the release of the latter relay to closea break in wire M connecting the armature of relay 6 to the Winding of relay 9. Then Withloud. speech from the near end,.the echo suppressor ES would not be restored by double talking, since the open contact on the added relay would prevent relay from being released.

. In the above discussion, it will be understood that when the echo suppressor is stated to be operative and later inoperative, this might mean present on the circuit and later actually removed from the circuit and available for use on other circuits. Also, the two conditions might refer to difierent echo suppressor settings, say high and low zero level sensitivitiesxor two different values of suppression loss. A modification within the scope of the invention would be to provide two or more control circuits such as illustrated inv Fig. 1. associated with the incoming and outgoing paths. EA and. WA in similar manner as the control circuit illustrated, which operate to reduce the sensitivity of the echo suppressor ES in two or more steps, to obtain as little echo suppressor operation as possible.

Other modifications of the control circuit of Fig. 1 might be desirable in the interest of economy. For example, relay '1 might well be controlled fromthe amplifier-detector in the transmitting echo suppressor control TS instead of from the auxiliary amplifier-detector 5 as illustrated, and. the two amplifier- detectors 5 and 8 might be partly or wholly made up of the amplifier-detectors normally provided in the transmitting andv receiving switching controls TS and RS of the echosuppressor ES. In addition, the differential relay 6 might comprise a differential thermistor Wheatstone bridge l6 and an associated relay 22, such as illustrated in Fig. 2, or other differential. means.

In: thesystemof Fig. 2, the diiierential bridge It includes the thermistors l1 and I8. in two arms of the bridge and the equal resistors l9 and 20 in the other two arms of. the bridge. A source. of current 2!, which may be. a. direct current battery, is connected across one diagonal of the bridge. and the operating winding of. the relay 22. is connected across 'the other bridge diagonal. The heater of the thermistor If! in one arm of the bridge. is connected to the. output of. the amplifier-detector 5 in the transmitting control branch 3. across. a shunt condenser pro videdto suppress the alternating component of the detector output current from the heater element, and the heater element of the-thermistor,

l8- in the opposing arm of the bridge is. con.- nected across another condenser provided for a similar purpose to the output of the amplifierdetector 8a in the receiving control branch 5. The relays. E, 9i and it, and: their associated resistance and condenser elements are arranged inthe same manner as the similarly numbered elemerits-in the system of Fig; 1 and provide. the same functions.

When voice currents from the path EA are impressed on the amplifier-detector 5 of control circuit t, and; its output supplies heating current to. the heater of thermistor ll, which is relatively. greater than that applied to the heater of thermistor ill by the amplifier-detector 8 in the receiving. control. branch 4 in response to the signal echoes in that branch, the resistance value of thermistor ll becomes comparatively smaller than the: resistance value of thermistor 8 with the result that the Wheatstone bridge it. formed by the thermistor elements 1! and i8 and. the equal. resistors l9 and 23, is unbalanced, and the unbalance current of the bridge flows through the operating'winding of relay 22. Relay 22.is normally biased by the current supplied to its biasin winding from battery 23 so that the unbalance current flowing in the operating winding of the relay tends to operate its armature upwards towards the grounded upper contact. When this unbalance current is greater than a predetermined amount indicating that the signal echoes in the path WA are negligibly small relative to the direct signal transmission in the path EA, the relay 22 will be operated to the grounded upper contact causing relay 9 to be operated and thus relay H] to be released to disable the echo suppressor ES in the same manner as described for the system of Fig. 1.

On the other hand, when the current through the heater for thermistor I8 is relatively greater than that through the heater of thermistor ll indicating that the amount of signal echo in the path WA is too great with respect to the direct signal transmission in the path EA, the Wheatstone bridge IE will be unbalanced in the opposite direction and the unbalance current will flow in the opposite direction in the operating winding of relay 22, operating the armature of that relay to its lower contact. Then the winding of relay 9 will be shorted through the made lower contact of relay 2| and the made contact of relay 1 which has been operated by the output currents of amplifier-detector 5. Relay 9 will therefore release to reenergize the winding of relay I0 and the latter relay will reoperate to restore the echo suppressorES to the circuit in the manner which has been previously described in connection with Fig. l.

Other modifications of 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 signaling system, a four-wire circuit for transmitting signals in opposite directions, a two-way signal transmission medium coupled to one terminal of said fourwire circuit, an echo suppressor device responsive to signal transmission in the side of said four-wire circuit transmitting to said medium to insert an echo suppression loss in the other side of said four-wire circuit and means controlled by the transmission in both sides of said four-Wire circuit for effectively disabling said echo suppressor device when the level of signal echoes in said other side of said four-wire circuit is less than a predetermined value and for rendering said echo suppressor device operative again when the level of the signal echoes in said other side of said four-wire circuit is above the predetermined value.

2. The combination of claim 1, in which the last-mentioned means comprising a differential circuit for continuously comparing the signals in said one side with the echoes of said signals in said other side of said four-wire circuit, means controlled by said differential circuit operating only when the ratio of the echo level in said other side to the signal level in said one side of said four-wire circuit is within predetermined limits considered tolerable, to disable said echo suppressor device.

3. The combination of claim 1, in which the last-mentioned means comprises differential means for continuously comparing the level of the signals in said one side of said four-wire circuit at a point in front of the input to said echo suppressor device with the level of the echoes du to said signals at a point in said other side of said four-wire circuit in front of the suppression point, and means controlled by said differential means and operating only when the ratio of the echo level to the signal level is within predetermined tolerable limits, to disable said echo suppressor device.

4. In combination in a two-way signal transmission system, a two-way line, a four-wire terminating circuit operating to transmit signals to, and to receive signals from said line, one switching circuit connected to the transmitting side of said four-wire circuit, responsive to signal transmission therein to disable the receiving side of said circuit to suppress echoes therein, another switching circuit connected to said receiving side of said four-wire circuit at a point beyond the disabling point therein, operating when said receiving side is operative, in response to signal transmission therein to disable the transmitting side at a point beyond the point of connection of said one switching circuit thereto, a differential circuit continuously operating to compare the level of signals in said transmitting side at a point in front of the disabling point therein with the level of the signal echoes in said receiving side of said four-wire circuit at a point in front of the disabling point therein, relay means controlled by said differential circuit operating only when the echo level is relatively less with respect to the signal level than a predetermined value considered as tolerable, to disable both switching circuits, and means to prevent while signal transmission in said transmitting side continues, the two switching circuits from being restored to normal in response to signals subsequently received by said four-wire circuit from said two-way line.

5. The system of claim 4, in which said differential circuit comprises a differential relay hav ing two opposing windings respectively energized by transmission in the two sides of said four-wire circuit, operating in one direction when the signal echo in said other side compared to the direct signal transmission in said one side of said fourwire circuit is greater than considered tolerable, and in the opposite direction when the echo is sufiiciently weak compared to the direct signal transmission to be within the tolerable limits, and a relay circuit operating only when said differential relay operates in said opposite direction, to disable both said switching circuits.

6. The system of claim 4, in which said difierential circuit comprises a Wheatstone bridge circuit including two opposing arms the resistances of which are respectively varied in accordance with the level of the transmission in th respective sides of said four-wire circuit, and said relay means is responsive only to an unbalance condition of said bridge corresponding to a ratio of the transmission levels in the two sides of said fourwire circuit obtained when the signal echo in said other side of said circuit is within the tolerable limits, to disable the two switching circuits.

LEONARD G. ABRAHAM.

Images