US2257806A - Echo suppressing circuit - Google Patents

Description

Oct. 7, 1941. D. MITCHELL 2,257,806

ECHO SUPPRESSING CIRCUIT Filed NOV. 22, 1940 2 Sheets-Sheet 1 //v l ENTOR By D. M/ 7'CHE L L T0 WEST SUBS AITORNEV 0a. 7; 1941. D, MITCHELL I 2,257,806

ECHO SUPPRESSING CIRCUIT Filed Nov. 22, 1940 2 Sheets-Sheet 2 /A;VEN7'OR By 0.M/7'CHELLY ALE ENE V Patented Oct. 7,

2,257,806 nono SUPPRESSING CIRCUIT Doren Mitchell, Bound Brook, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 22, 1940, Serial No. 366,511

11 Claims.

The invention relates to two-way telephone systems and particularly to the switching circuits employed in such systems for suppressing echoes and preventing singing.

The type of echo suppressor most commonly employed in commercial two-way telephone systems operates in response to signal transmission in a transmission path for one direction to insert a loss in the transmission path for the opposite direction, which is of fixed value and sufiiciently large to reduce echoes in the latter path to an unobjectionable amount for the worst echo condition which might exist regardless of Whether the echo is strong or weak. In another type of echo suppressor the value of the loss inserted in the echo path by its operation is proportional to the magnitude of the talkers voice waves controlling the echo suppressor. The amount of echo from any telephone connection depends to a considerable extent on the return loss at each end of the telephone circuit. If the return loss is high, it is not necessary to put in as much loss to the echo as if the return loss were lower. An object of the present invention is to automatically adjust the loss inserted by an echo suppressor so that it is never greater than is necessary to properly suppress the echo, thereby providing improved operation particularly from the standpoint of break-ins.

This object is accomplished in accordance with the invention by a circuitarrangement operating under control of the voice currents to adjust the suppression loss in the echo path according to the amount of return loss, so that the totalloss to the echo, including the two-Wire return loss, is 'always the same.

In one embodiment this circuit arrangement comprises at each terminal of a four-wire telephone circuit, a receiving terminal type of echo suppressor with a delayed action transmitting disabler in combination with a difierential circuit which automatically compares the received speech waves with the echoes during the time in which the echo suppressor is operated, and adjusts the loss pad which is inserted in the echo path by operation of the echo suppressor to increase or decrease the suppression loss according to whether the echo is too strong or too weak relative to the received speech waves.

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

Fig. 1 shows a skeleton diagram of a four-wire telephone circuit with an echo suppressor in accordance with the invention applied to both terminals; and r Fig. 2 shows schematically in more detail the echo suppressor circuits at one terminal of the four-wire telephone circuit of Fig. 1.

In the skeleton diagram of Fig. 1, each of the single lines represents a two-wire transmission path. Contacting arrow-heads at a point in a line indicate that the transmission path is made at that point, and separated arrowheads at a point in a line indicate that the transmission path is broken or disabled at that point. An arrow pointed from a box towards a make point in a transmission path indicates that the transmission path will be disabled at that point in response to the output currents of the transmission device represented by the box, and an arrow pointing from a box to a break point in a transmission path indicates that the transmission path will be enabled at that point in response to the output current of the transmission device represented by the box. Each box containing an armature, connected by an arrow to a point on a variable loss device indicated for convenience as a potentiometer resistance in a transmission path, and two oppositely pointed arrowheads. pointing to the armature, respectively connected to different contacts of a differential relay, represents a'two-way relay stepping device operating to adjust the potentiometer resistance in one direction or the other depending on the direction of operation of the differential relay.

The functions of the other transmission apparatus in the diagram, represented by boxes, are indicated by explanatory legends.

The four-wire telephone circuit of Fig. 1 comprises a west-to-east one-way transmission path EA and an east-to-west one-way transmission path WA coupled at the west terminal by the hybrid transformer HW and associated balancing network NW to a two-way line or circuit LW leading to a west subscriber's telephone circuit (not shown) and coupled at the east terminal by the hybrid transformer HE and associated balancing network NE to atwo-way telephone line or circuit leading to an east subscriber's telephone set (not shown). The path EA includes at. the west terminal a normally short-circuited potentiometer resistance P1 and the amplifier A1, and at the east terminal the amplifier A2. The path WA includes at the east terminal the normally short-circuited resistance P2 and the amplifier A3, and at the west terminal the amplifier A4. t

The transmitting switching control circuit TSw a receivin portion RSE.

of the echo suppressor at the west terminal includes the amplifier A5 having its input bridged across the outgoing portion of the path EA on the output side of amplifier A1. The output of the amplifier A5 connects through the band-pass filter F1 and the wave detector D1 to two parallel control branches, one comprising the upper operating winding 101 of the differential relay DRl, and the other, in order, the delay circuit DCr, the hang-over network HN1 and the switching control 61. V

The receiving switching control RSwof the echo suppressor at the west terminal includes the amplifier A6 having its input bridged across the ,in-

coming portion of the path. WA in front ofar'nplifier A4. The output of amplifier AG connects through the band-pass filter F2 and -th e wave detector D2 to the lower operating winding w; of the differential relay DRI, poled to oppose the operating windingwi of that relay. Theooutput of amplifier A5 also connects through, w ave 'detector D2 and the hang-over network HNz in parallel to the four (switching controls 111, hi, c1 and 11. The differential'relay DRi controls the two-way stepping switch SS1 which controls the adjustment of the variable loss ,P1..

Similarly, at the east terminal the echo suppressor comprises a transmittingportion TSE and The transmitting portion TSE comprises in order an amplifier A7 having its input bridgedacross the outgoing path WA on the output side of amplifier'As, a band-pass filter F3 and the wave detector D3. The output of detector D3 connects to the lower operating winding. we of the differential relay DB2, .and

through delay circuit D02 and hang-over network HNs to the switching control e2. The receiving echo suppressor portion RISE includes in order, the amplifier As having itsin'put connected across the incoming portion of the path EAin front of amplifier A2, the bandfilter F4 and the detector D4; The output of detector D4 is connected in parallel to the upper operating windinw ws of'diffe'rjential relay DB2, poledjto oppose winding, 104, and through the hangover network HNi in parallel .to the fourswitching controls 112, baczand The differential realy DB2 controls the two- 'way stepping switch SS2 which controls the adjustment of variable loss R2.

The function. of the differential relays, the

various switching controls and. the other apparatus of Fig. 1 will be brought out ininore detail in the following complete description of operation. V

Let is be assumed that the westsubscriber associated with the line LW shortsspeaking. His

speech wave Will dividein hybrid coil Hw and the useful part will pass upward into the west-to-ea'st transmission path EA, and will be transmitted over that path through amplifiers A1, A2, hybrid transformer HE and linefLE to the east subscriber. The loss control P1 will not be effective to insert any loss in the transmitted Waves because control. a. being in its normally unop-;

erated conditio'nQPr is short-circuited. I v I A portion of wests speech waves at the west terminal will be diverted into TSw and pass through bridged amplifier am band-pass filter F1 and will divide between the winding wi of differential relay DR; and the inputof delay circuit The delay circuit DC 1 will delay the trans-, mission of the speech waves for a time T, whereT is substantially equal to the transmission time wire circuit. Thus, at the "same instant that eter P2 is controlled by th amplifier As, band-pass filter F4 and detector D4 to energize the upper winding we of differential relay DB2, and through hang-over network l-IN;

"will. cause .the operation of switching controls (12, b2, 02 and d2. Control a2 operates to insert the part of the potentiometer resistance P2, de-

termined by the setting of its variable arm, into the outgoing portion of the path WA at the east termihalto attenuate the echoes of wests speech currents returned from the east subscribers line "llEjthrb lgh hybrid cpiin he e ti ior the 'variabl] arrnioif gbgm p n' switch SS2 ,thecperation of which is cont, olle'd'by differential 1 over the path EA between terminals of thefour- 'scriber.

relayIlRz, which in turn. depends on the relative degreejlof,energization of, its two windings w: and i014. WindiI .S. 'ws'and jwrare energized respectively bythe incomin'gnvoice currentin EA Ithrough ,the receivingfsuppressor 'RSE, and by the outgoingor echo currentsin WA transmitted intothe transmitting suppressor TSm The differential relay'DRz cannot cause any adjustment unless the receiving, suppressor RS'Eis operated .to causefthe operation 'o'fcontrol dz to, connect battery 'B2 to the relay armature, The ,irelay DB2 may either operate its armaturejtothe right or lef CQ ltact or sta'y in its, neutral position. It

will operate, to the right contact iif the incoming speech waves supp-lied to the upper winding w; of

the relay areutoo-'stro"ng. ;in relation to the echo currentsuppliedtofthe lower ,r'elay winding 1.04,

and to the leftlcontact if the echo is 'to'ostrong with relation 'tothe incoming speech waves The armatlite of the relay DRz will remain in "the neutral. position if the echois correct with regard totheincomingwave, Operation of relay DRz to the right will cause theloss in Pzfto decrease,

as indicated by shortiria'dut, more or "less of the loss as the case may be "and operation in the left direction will cause the loss provided by P2 in the path WA toincrease. I

: Controls b2 .andcz operate in 'responsejto the incoming speech. waves at the east terminal, to

disable jthe transmitting suppressor TSE at the V input andoutpht jot delay circuit 'DCz, which has a 'delay of T, respectively. This ,is necess'ary to prevent operation of control (e2 by echoes and also to 'givefc'ontrol to -the west. subscriber .durin'g double talking provided ,he spoke first.

Thus,.let it be, assumedthat, the west subscriber started to talk slightly, before [the leastfsub- Theispee'ch waves of, both would get started over the respective e ast.-towest 'and west-to-each speech transmission Q path without undergoing any attenuation by 'the potentiometen re'sistarice at the, near terminahand the divertd'speech energ or west jand'e'ast at the west and east terminals will be started through N delay circuit, DCI and D02, respectively. However, since 'wests'tarted ,talking f rst; hisjspe ch wave wouldvarriveiatvthe eastend of the fourwire Lcirc'iiit ,beioreueastfs speech impulse gets .through'the 'dflay circuit .DQz, and also before theses; jsub'scriberswavefgets'ftothe west end of the four wirecircuit. Controls bz and at -fier A1 a part of wests outgoing waves will be the east terminal would then operate to prevent operationof control e: by east, and would also wipe off any speech impulse started through delay circuit DC2 by east Control e1 at the west end of the circuit would also be operated by west to disableRSw, thus preventing operation of any of the controls at the west terminal by easts speech when it arrivesat the west terminal. A small spurt of easts speech would, however, get through to the west subscriber Without attenuation, and if both subscribers continue talking later, later speech from east would arrive at the west terminal attenuated by whatever value of loss which will be inserted in the westto-eastf path WA by potentiometer resistance P2.

It is apparent that breaking would be made appreciably easier by the arrangement of Fig. 1 than'in the usual echo suppressor which always puts in a fixed amount of loss which is large enough to suppress echoes under the most severe conditions. Thus, when the return losses are good, the suppression loss provided by the variable loss potentiometer devices P1 and P2 might be quite small, and the action of the circuit would not be Widely different from that of a circuit with no echo suppressor at all. Only when the return losses are quite poor would the loss have to be as large as it is at all times with most present echo suppressors.

Continued double talking would tend to operate both differential relays DB1 and DR2 in the direction to increase the suppression loss at both ends of the circuit. Normally this loss would only have to be adjusted once during a call, however, and thus it is believed that the loss adjustment could be made moderately slow, say, to require two or three seconds to get over its total range. In this case, with the double talking which ordinarily occurs in such circuits, the change in the suppression loss would ordinarily not be very large during double talking and, at any rate, the loss would be adjusted back to its regular value a short time after the double talking had ceased.

Fig. 2 shows the detailed circuit arrangement which could be used at each terminal of the system of Fig. 2 to accomplish the functions described above for Fig. l. The west terminal of the circuit is illustrated and the corresponding transmission apparatus circuits are designated with similar identification characteristics. The circuit details will be brought out in the following detailed description of operation of the circuit of Fig. 2.

' -Let it be assumed that the west subscriber connected to the line LW speaks. His speech waves will divide in hybrid coil Hw, partbeing dissipated in the output of amplifier A4 and the useful part passing into the input of the westto-east path E'A. Wests speech currents will first pass through the loss pad L1, the function of which is to effectively remove from the twowire line the efiects of the varying impedance of potentiometer resistance P1, and then will pass through the two repeating coils T1 and T2 and amplifier A1 out over the path EA to the last terminal of the four-wire circuit. If the receiving suppressor relay RHO1, corresponding to the control (11 in the system of Fig. 1, is not operated, the potentiometer P1, the resistance setting of which is controlled by the two-way stepping switch SS1, comprising the two stepping magnets f1 and f2, will be shorted out and there will be no suppression loss applied to wests outgoing speech Waves, At the output of amplidiverted into the transmitting portion TSwof the echo suppressor at the terminal and will be amplified by bridging amplifier A5 and passed ing 201 of the difierential relay DR1 and, if strong enough, will operate relay TM1, and will produce a pull on the relay DR1 tending to move its armature to the upper relay contact. Let it be assumed that relay TM1 is operated. This removes the ground from condenser 02 through resistance R2 and then allows relay DM1, normally operated by current from battery B3 through resistance R3, to start to release. The relay DM1 and the associated circuit perform the function of the delay circuit D01 in the circuit of Fig. 1. After the desired time interval, T which is equal to the one-way transmission time over the four-wire circuit between terminals, and may be controlled by proper proportioning of the values of condenser C2 and resistance R3, relay DM1 will release, thereby operating relay THO1, corresponding to control 61 in the system of Fig. 1, almost immediately due to the discharge of condenser C3 through R1 which is comparatively small. Relay THO1 will be operated by current from battery B4 transmitted through its winding and resistances R5 andRi. Operation of relay THO1 immediately disables. the receiving detector D2 in its input, thus preventing its operation by echoes from the distant end of the circuit. (With this arrangement, there will always be some echoes, and these echoes may be relatively strong in the case of a very strong local talker.) Relay THO'1 is given sufiicient hang-over by the combination of C3 and R5, corresponding to hang-over network HN1 in the system of Fig. 1, so that it holds the receiving detector D2 disabled for a time sufiiciently long to take care of all the delayed echoes.

The differential relay DR1 will probably be operated very frequently in the up direction by the local talker, but this will have no effect on the stepping of potentiometer P1, since relay RHO1 will not be operated at such times, and thus there is no ground on the armature of the difierential relay DR1.

Let it be assumed now that wests outgoing speech has ceased, and incoming speech waves from the eastsubscriber are present in the path WA. A portion of these waves will be diverted into receiving portion RSw of the echo suppressor and will be amplified by A6, filtered by F2 and detected by detector D2 which is now operative due to the release of relay THO1. The pulsating direct current in the output of D2 will be smoothed by condenser C4 and will tend to cause a pull in the down direction on relay DR1 and windings and resistances R6 and B1 from battery B4.

The operation oi'relay RI-IO1 immediately'inserts loss in the path 'E-A by removing ground from the arm of potentiometer P1 thereby removing the short from the potentiometer. When the armature of RHO'i reaches its lower contact, it'will also put ground on the armature of the diflerential relay DB1 and thus allow adjustment by the stepping magnets f1 and f2 under control of differential relay DB1. Operation of relay RHOz immediately discharges condenser C2 through resistance Rs. Thus, even though the release of relay DM1 has beeninitiated by'a previous operation of relay TM1, this action will be completely annulled by operation'of relay BHOz.

The differential relay DB1 will now'be affected by incoming waves'through winding wz and echo Waves attenuated by the loss provided by the variable resistance of potentiometer P1 through winding 101. If the 'echo wave amplitude is too high compared to that'of the incoming speech waves, the settingof the differential relay DB1 will be such that its armature will operate in the up direction. This will cause the potentiometer to step in the clockwise direction thereby increasing the series resistance in the path EA and thus increasing the loss in that path. When the loss is sufficiently increased by successive operations, the currents in w1'and wz will become equal and there will be no further adjustment since the armature of DB1 will then remain in the center (neutral) position during' incoming speech waves.

Now, let it be assumed that'a new call comes up in which the return loss is considerably better than that on the one just previous. In this case the loss due to the potentiometer Piwill be quite high, and during in'comi'ngspeech the current in winding wz of relay DB1 will be considerably greater than in winding wi. This will cause operation of the differential relay DB1 in the down direction and willstart'a series of stepping operations in a counter-clockwise direction in potentiometer'Pi. Thiswill continue until the echoes at the input of amplifier A5 are increased sufficiently so that the'armature of differential relay DB1 once'more stays in the'neutral position.

As noted abova'doiible talking while it lasts will produce an eiiect on the differential relay DB1 which is similar to loud echoes'and thus will probably operate it in the up direction and increase the suppression loss. By making 'the stepping switch slow oper'ating byproper design,

however, this effect canbe made unobjectionable. Another way of 'making the ,effectunobjectionable would be to design thestepping switch so that it would not operate'at' all'for very brief pulses. Actually moststepping "switches orselectors are such that they do not operate on very brief pulses anyway.

Various modifications of the circuits which have been illustrated and de'scrib'edwhich are within the spirit and scope of the'invention will occur to persons skilled in the art.

What is claimed is:

1. In a two-way "signal transmission system including a two-way signal transmission path, one one-way'signal transmission path for transmitting to said two-way path'and a second oneway signal transmission path for transmitting from said two-way path, an echo suppressor operating "in response'toincoming signals insaid one one-way path to insert "anecho suppression loss in said second'one waypathj means to autotrolled 'by the cdrnparingmeans to automatically adjust the value "of said echo suppression loss *so that its magnitude is just sufiicient to'properly suppress said returned echoes.

;2. The system oi claim l,'in which said comparing means compares the level of the signals in lsaid one path with the level of the signal echoes said-second 'path' beyond the point at which said echo suppression loss in inserted "therein, said adjusting means is controlled by said comparing means to increase the value of said lsiuppr'es'sion loss to the required degree when the level of the 'echoes'transmitted through said suppression loss is toolarge with respect to the le ve'l of the received signals in said one path, to decrease the value of said suppression loss when the level of the echoes transmitted therethroughis too small with respect to the level of said received signals, and to make no adjust- 'ment of the value of said suppression loss when "the comparison of the signals and echoes by said comparing means indicates that the suppression loss is of the correct value.

3. In a two-waysignaltransmission system, a two-wire line, a f our-wire signal transmission circuit connected thereto including one one-way circuit for transmitting to said line and a second oppositely-directedone-way circuit for receiving from said "line, an echo suppressor including a portion connected to said one one-Way circuit responsive to incoming signals therein to introduce. an echo suppression loss in said second one-Way circuit, control means for automatically varying the value'of said echo suppression loss, during the period in'which said portion of said echo suppressor is operated by incoming signals,

so'that the value of saidloss is'increased when the ratio of the level of the signal echoes in said one one-way circuit with respect to the level of the incoming signalsin said second one-Way circuit is greater than a tolerable value and the 'value of said loss is decreased when the ratio is less than the tolerable value.

4. The'syst'em of claim 3'in which said echo suppressor includes. a secondportion connected to saidsecond one-way circuit at a point beyond said suppression loss, operating in response to outgoing signals in said second circuit but with a fdelay substantiallyequal to the overall transmis- "sion time oversaid second "one-way circuit between the terminals of said four-wire circuit, to

disable the first portion of? said echo suppressor.

5. In "a two-way signal transmission system including a two-way transmission circuit, one one w'ay'path for'transmitting from said circuit and a'second- "one way path'for transmitting to said "circuit, means responsiveto signal transmission'betwee'n said second path and said circult for inserting an echo suppression loss in said one path, auxiliary means responsive to one given ratio of the level of thesignal waves in said second path to' the level of the signal echoes returned from-said circuit to saidone one-way path to increasethe amount of s'aidsuppression loss, and

responsive to another given lower ratio of the level of the signal'waves in said second path to the'level of the echoes in'said one path to reduce the" amount of said echo suppression loss.

6. The system "of claim'5 in which the coupling between said two-way circuit and said one and said second path comprises a hybrid coil and an associated balancing network, which provides an approximate conjugate relation between said one and said second path, and said auxiliary means comprises a differential circuit for automatically comparing the signals in said second path with the signal echoes in said one path during the period in which said signal responsive means is operated, and means operating automatically to increase or decrease the value of said suppression loss according to whether the echoes are too strong or too weak relative to the signals in said path.

7. In a two-way telephone system including a two-way transmission circuit and incoming and outgoing one-way signal transmission paths connected in approximate conjugate relation with each other and in energy-transmitting relation with said two-way circuit, an echo suppressor comprising a receiving branch connected to said incoming one-way path, responsive to incoming telephone signals therein to insert an echo suppression loss in said outgoing one-way path, and a transmitting branch connected to said outgoing one-way path, responsive to outgoing telephone signals therein to disable said receiving branch, a difierential circuit controlled from said outgoing and incoming one-way paths for automatically comparing the signals in said incoming path with the echoes in said outgoing path beyond the point of insertion of said echo suppression loss therein, during a period in which said transmitting branch of said echo suppressor is operated, and means controlled by said difierential means for automatically adjusting the value of said suppression loss so that it is never greater than is necessary to properly suppress the echoes.

8. The system of claim 7 in which said diiferential means comprises a differential relay having two opposing windings, means for energizing one winding in proportion to the level of the signals in said incoming one-way path when said receiving branch of said echo suppressor is unoperated, and means for energizing the other winding in proportion to the level of the echoes transmitted through said suppression loss in said outgoing one-way path, said relay operating in one direction when the signal level exceeds the echo level by a given amount indicating too great a suppression of echoes by said loss, and operating in the opposite direction when the signal level exceeds the echo level by a given lower amount, indicating too little suppression of echoes in said loss, said means controlled by said diiferential means comprises a two-way stepping switch operating to decrease the value of said echo suppression loss in response to operation of said relay in said one direction until said relay releases, and to increase the value of said suppression loss in response to operation of said relay in said opposite direction until said relay releases.

9. The system of claim 7, in which the echo suppression loss inserted in said outgoing one-way path by operation of the receiving branch of said echo suppressor is a potentiometer resistance, said resistance being short-circuited when said receiving branch is unoperated so as to be ineffective in said outgoing one-way path, and said means for automatically adjusting the value of said suppression loss under control of said differential means comprising a stepping selector switch operating to adjust said potentiometer resistance in a direction to increase the insertion loss in said outgoing path when the comparison by said differential circuit indicates that the loss is smaller than that required to properly suppress the echoes, and operating to adjust said potentiometer resistance in a direction to reduce its insertion loss when the comparison by said differential circuit indicates that the loss is greater than is necessary to properly suppress the echoes.

10. In combination with a four-wire telephone circuit interconnecting two two-way telephone circuits, said four-Wire circuit comprising two oppositely directed one-Way transmission paths for respectively transmitting telephone signals in opposite directions between said two-way circuits, echo suppressor means at each terminal of said four-wire circuit comprising a voice-operated receiving switching branch connected to the incoming one-way path and responsive to incoming telephone signals therein to insert an echo suppression loss in the outgoing one-Way path at the terminal, a voice-operated transmitting switching branch connected to said outgoing oneway path and responsive to outgoing telephone signals, but with a delay substantially equal to the oneway transmission time between terminals of said four-wire circuit, to disable said receiving switching branch, a differential circuit operating automatically to compare the level of the incoming signals in said incoming path with that of the echoes of the signals beyond the point of insertion of said suppression loss in said outgoing path and means controlled by said differential circuit to increase the value of said echo suppression loss when the echoes are too strong and to decrease the value of said suppression loss when they are too weak with respect to the incoming signals.

11. The combination of claim 10, in which the delay in the operation of said transmitting switching branch is attained by the use of a delay circuit of the required value in that branch, said receiving switching branch when operated also disabling said transmitting switching branch on each side of said delay circuit and conditioning said diiferential circuit for operation.

DOREN MITCHELL.

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