CA1140692A - Adaptive electronic hybrid circuit - Google Patents

Abstract

Abstract An electronic hybrid circuit for matching a two-wire telephone circuit to a four-wire telephone circuit is described. In one embodiment, the circuit includes a variable or adaptive impedance circuit and a first impedance element for supplying a reception signal on the four-wire circuit to a terminal of the two-wire circuit. A second impedance element is connected to supply the recep-tion signal on the four-wire side through the adaptive impedance circuit and an adder is connected to add, in reverse phase, the reception signal appearing at the junction between the second impedance element and the adaptive impedance circuit to a signal provided from the terminal of the two-wire circuit. The resultant output from the adder is a transmission signal on the four-wire side which is fed back to a control terminal of the adaptive impedance circuit where-by the impedance of the adaptive impedance circuit varies depending on the characteristics of the two-wire circuit.

Description

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This application is a divisional of copending Canadian patent applica-tion serial No. 325,507 ~hich was filed on April 12, 1979 in the name of Takashi ~raseki.
This invention relates to an adaptîve electronic hybrid circuit for use in a two-wire to four-wire connection in a telephone transmission line.
A commercial telephone transmission line generally consists of a two-wire circuit in the vicinity ~f each subscribe~ and a four-wire circuit for long-distance link, with hybrid circuits used for connecting them.
A conventional hybrid circuit consists of a transformer as shown in Figure 4 of the article titled "The Effects of Time Delay and Echoes on Telephone Conversations" by J.W. Emling et alO published in The Bell System Technical Journal, ~November issue 1963), pp. ~869-2891 ~Reference l)o On the other hand an electronic hybrid circuit has been proposed, in which an active element is used instead of a transformer. For details of such a circuit reference is made to ~igure 5 of the paper titled !'PSS~l~ A Digital Local Switching System ~ith Remote Line Switches" by CoGo Svala published in National Telecommunication Conference 1977, pp. 39:5-1 - 5-7 ~Reference 2).
These hybrid circuits, however, require the terminal impedance or the balance impedance well matched with the impedance of the two~wire circuit.
~0 Inadequate matching between these impedances, as is well known, results in such phenomena as singing, oscillation or echoes, each of which disturbs telephone conversationO Therefore, particular care should be taken in matching the termi-nal impedanceO ~lowever, because the impedance of the two-wire circuit varies with its length and the terminal impedance at its other end, the impedances of two-wire circuits differ from one subscriber circuit to another. ~ccordingly, terl~nation with a fixed impedance may result in considerable mismatching.

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An object of the present invention is thereore to provide an adaptive electronic hybrid circuit capable of achieving sufficient impedance matching for all the two-wire and four-wire circuitsO
In accordance with the present invention, there is provided an adaptive electronic hybrid circuit comprising: an adaptive impedance circuit for connec-ting a two-wire circuit and a four-~ire circuit; a first impedance element for supplying a reception signal on the four-wire side to a terminal of the two-wire circuit; a second impedance element connected to one side of the adaptive impedance circuit the other side of which is connected to a terminal on the four-wire side for supplying the reception signal on the four-wire side through the adaptive impedance circuit; and an adder which adds, in a reverse phase, the reception signal appearing at ~he junction between the second impedance element ancl the one side of the adaptive impedance circuit to a signal given from the terminal of the two~wire circuit to provide at an output of the adder on the four-wire side a transmission signal~ the output of the adder being connected to a control terminal of the adaptive impedance circuit whereby the impedance of the adaptive impedance circuit adaptively varies depending on the characteristics of the two-wire circuitO
The invention will now be described in greater detail with reference 70 to the acco~npanying drawings in which:
Figure 1 illustrates an example of an electronic hybrid circuit according to the prior art;
Figure 2 illust~ates another example of an electronic hybrid ~2~

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cir.;lit accordillg to thc prlor ar~;
Figure ~ is a diasr.lm sllowill~ one erllhodimcnt of the prcsent invcn-tion;
Flgure 4 is a diag~vm sho~ing ?nother embodiment of this inven-tion;
Figure S illustrates a first examplo of an adaptive impedance circuit for use in this invention;
Figure 6 is a circuit diagram illustrating in further detail the embodiment of the invention shown in Figure 3.
Figure 7 is a graph representing a function ~1;
Figure 8 is a graph representing a function~2;
Figure 9 illustrates a second example of an adaptive impedance eircuit for u~e in this invention;
Figure lO is a eircuit diagram which illustrates in further detail tlle example of Figure 9;
Figure ll illustrates a third example of an adaptive impedance eircuit for use in this invention; and ~ igure 12 illustrates a ourth example of an adaptive impedance eircuit for use in this invention.
~ Throughout the entire drawing, like reference numerals represent like structural elements.
Referring to ~igure 1, an electronic hybrid circuit according to the prior art is given a reception signal vr from an input terminal 5 on the ~our-~ire side. Tlle signal vr passes threugll an ampliier 30 and nn impedallco element 10 an~l is supplied to the t~o-wire path from a term-millal l. I'e~erellce numerals 2, ~ and ~ denote gro-ul-lcd terminals. Anotller signllv fed from tl~e othcr end of tlle tl~o-wire circuit passes an addcr 20 alld is transmitted from an output terminal 3 on the four-wire sicle. Because ~, .

~LQ6~2 the rece~tion sirnal v is multipli-~d i~ a eoe.~ficient o~ in tne adder 2n anu ~d~e~ .o .he sign~l v o~^ the t~o-wi~^ circuiL, ir the i~pedance of the two-wire circuit ~atches the i~?edance element 10, only the signal fed from the other en~ of the two-wire circuit emerges as the output v5 of the adder 20. However, since impedance matching tends to be inadequate as stated above, a conâiderable part of the reception signal v on the four-wire side m~y lea~ out to the terminal 3.
Fioure 2 illustrates another electronic hybrid circuit according to the prior art corresponding to ~igure 5 of Reference 2 (see above). A
reception signal v on the four-wire side at a terminal 5 passes through an amplifier 30, and an impedance 41 to a terminal 1 to be supplied to the t~o-wire CilCUit, and at the same time is fed to an adder 20. The reception sigllal vr on the four-wire side is also passed througll an impedance 42 to the adcler ~0 whicll adds, in a reverse phase, these two input signals. Therefore, if the impedance of the tl~o-wire circuit is equal to a balance impedance 10, the reception signal v ~rill not appear at an output terminal 3 of the adder 20, but only the signal fe~ to the other end of the two-wire circuit appears at the output terminal 3 of the adder 20. In a usual two-wire circuit, how-ever, because ir,lpedallce varies at stated abovo, the balance is disturbed, and

2~ as a result, the four-~ire reception signal leaks out to the terminal 3.
Besides, in gelleral the t~o-wire circuit carries a high D.C. voltage for po~er supply or a large-amplitude A.C. voltage for ringing tone signals, ancl accorclil-gly, lligllly voltage-resistant el~ments are needed. To avoid tl~ese clisad~ tages, the circuit structure of Fi~ure 2 is more frequently employecl tlnall tlrat of Figure 1.
~ l~e~erring to Figure 3, a first embodiment of the present inven-`tion is sho-in. Tlle circuit is similar to that of ~igure 1 but an adaptive impecl:~nce circuit 50 replaces impedance 10. Ihe signal VS is ed back through .
,.
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circuit 50 via .erminal 9. As described :Yit}l refcrence to ~igure 1, if th~ imp~aancc of tlle t-~o-l~ire circuit r,latches a terminal impedanca circuit LO, no signal appears at a ~crmina~ 3 on the fo;lr-wire side in the presence of a four-~ire reception signal vr alone.
If a signal v emerges at the terminal 3, however, the signal v5 is fed bac'~ through the adaptive impedance circuit 50, so tnat an adap-tive operation may ta~e place to achieve better matching by modifying the impedance of the circuit 50 so as to minimi~e the po~er or amplitude of the signal v . If a signal is present at the other end of the two-wire circuit, no proper modification can be done and consequently the adaptive operation is stopped. It is thus possible to achieve matching bett~een the impedance of the t~o-~ire circuit and the terminal impedsnce by the use of all tlle signals ~ ich flow on the wire circuits and include the speech signal without using any additional signals such as signals for measuring thc impedance of the t~o-~ire circuit.
Since termination with the adaptive impedance, as is the case with this embodiment, results in decreased reflection on the t~o-l~ire cir-cuit, the gain of a bilateral repcater used for amplification on the t~.~o-wirc wirc circuit can be greater than that in the prior art hybrid circuits.
Rcferring to Figure 4, anot~er embodiment of this invention in-cludes an adaptive impedancc circuit 50 for performing an adaptive opera-tion to ma~e thc four-~ire transmission signal v5 smaller. Tllis circuit is similar to that shol~n in Figure 2 but an adaptive impedance circuit 50 rcl)1;lccs impcdallcc 10. Thc signal v5 is fod bacl~ througll circuit 50 via tcrmill;~
Referrillg to Figure 5, thc adaptivc impedance circuit for use il~ the cml)odimcllts of Figurcs 3 and 4 includes an impodancc elemont 51 collnectcd to a terminal 7 corerspondillg to terminal 7 of Figure 3 or Figure - , .

6~2 4 and a control circuit 5 'naving a suitable transmission characteristic K
whicll is a~plied to the voltaGe difference between tlle two ends of the im-pcdance element 51. Terminais ~ and 9 corres~ond to t~rminals ~ and 9 c-^
Figures 3 and ~. The signal v is applied to control circuit 53 and the signal vr is applied to an adder circuit 52 to w}lich an output from the control circuit 5~ is also fed. The output of adder 52 is connected to impedance 51.
If tlle input impedance of the control circuit 53 is greater thall the output impedance of same, the impedance Z between the terminal 7 and tlle ground is represented by the following equation:
Z = Zi (1 ~ K) .......................... ~1) In this instance, only the terminal 7, i.e., the impedance ele-ment 51, is colmected to the two-wire circuit. A signal representing mis-matching (the four-wire transmission signal v5 in Figure 3 or Figure 4) is given from a termin~l 9 to a control circuit 53 so that the circuit 53 modi-fies itself to minimize the signal v . The algorithm to minimize the power of the signal VS is obtained in the following manner.
In the circuit of Figure 3, when no signal from the other end of tlle two-l~irc circuit appears, Z ~ ZL
Vs = v - vr/2 = 2 i .. . ....~ 2) wllerc ZL represents the iml~edance of the tl~o-l~ire circuit. Assuming ZL = Zi (1 -I L), from Equations (1) and (2), is derived K - L
Vs 2 Zi i .................................... ~3) Zi i ill Ecluation ~3) corrcsponds to the potential differellce bet~.~een thc t~o cnds of the impcd;lllce clement 51 in ~igure 5.
It is assumcd that tilc transmission function K can ~c repre-selltcd by a product obtainc(l by multipl~ing a plurality of function .

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~Fo~ Fl, ..... , F~ 1) crossing one anotller at ri~l~t angles by weights C~O~ k~ i.e-, ~-1 j_0 j j ................................. ~43 wllere ~ is an adequate value to bring tlle imyeda~lce of the two-wire circuit elose to tha terminal impedailce.
Iiere, partial differentiation of ~v5)~ with kj~j=0, 1, ... N-l) to make ~V5)2 small gives ak ~vs) = 2v5 . Fj Zi i ......................... ..(5) It is thus found tllat the followillg relationship has eventually to be aellieved:
kj = kj - g.vs.Fj.Zi.i ............. ,,. ......... ..~6) ~ere is a value sufficiently smaller than 1 and Fj . Zi i represents the result of having a signal Zi . i go througll the function F.
Equation ~6) can be transformed into:
j j l~v5) ~2(Fj Zi i) ............. , ~7) w;lere ~-`1 and '~2 are non-decreasing functions and delta ~ is a positive number suffieiently smaller than 1.
Reforring to ~igure 6 whicll illustrates in further detail the first embodilment based on Equatioll ~7), the potential difference betweell ~o tllc two ends of the impeclanee element 51 is obtained by an adder circuit Vill~ all opcr~tional amplifior 191, whicll is comlected to at-tenuators 167, lGS alltl 169 tllrou~ll Eullctional circuits 160, 161 and 162. The outputs of tlle attellllators lG7, lGS and lG9 are fed to 111 opcrational al.~plificr 192 `all~t;l n~ er o~ resistors 173, 17~ and 175. Tlle sign.ll vr provitled at ter-mil~ and the out~ut of the ol)erat~ona amplitier 192 are a(lded in an ;9;~

adder ~2 so th;lt thc sum o~ the a~der 52 ma~f be sup?lied to the impedance element 51. ;ltllough in Figure 5 t'le wei,llting coelficient o~ tha si~,nal passing .hrough tlne ~ulc~ional circuit 160 is constant, the weigtlting coefficients l~j for tlle signal passing through the functional circuits 161 and 162 are controlled. The weighting coefficients kj are obtained by nonlinear circuits 163 and 164 giving -~2, another nonlinear circuit 181 giving ~1~ multiplicrs 165 and 166 calculating the products thereof and integrators 170 and 171.
~en the functions Yl and ~2 can be represented by the graphs OI
Figures 7 and 8, resyectively, the multipliers only have to multiply dif-ferent cor.l.binations of ~1, 0 and -1, simplifying their circuit structures.
~eference letter A in Figure 7 represents a value smaller by a certain diference tllan tlle root-mean-square value of the four-wire reception signal vr or some other value related thereto, which is obtained by a mean value circuit 180 ~Figure 6). l~len the introduction of the value A has resulted in the approach of the impedance Z to the impedance of the two-wire circuit to some extent and in the diminution of the signal vSJ the modification of the weigllting coefficient ~i is suspended. Also, if the level of the signal v5 has surpassed that of tlle sign~l vr, a signal from ~0 the other end of the two wire circuit, is produced with the result that a com~arator circuit 182 is given an instruction to ma~e the output of the nonlincar circuit 181 zero so as to suspend the modifying proccdure.
The functional circuits 160 through lG2 of Figure 6 may be inte-gr.ltors, diffcrcntiators, or transversal filters usillg delay ci.rcuits. In tilC l.ast installce, a numbcr o~ dclay circuits can be comlected in cascade, witll tl~cir outputs corresl)ondill~T to thc outi~uts of the functional circuits 16() tllro-l~ll 16~. ThOU'TII it is assulllcd that the wei(Tllting coefficient for thc ~wlctional circuit 160 in l~igure 6 is ~nown in advance, if the LC~69~

prc~e-erTT.inatioll o tlle ~leigllting coefficie~ is difficult, the ~ttenuator 167 should be controlled to ~ary said coefficient.
The structure of Figure 6 ~al;es possihle the simplirication and integratioll of circuit com~osition, and conse(luently in a substantial cost reduc~io.l.
.~ltllougll description of ~igure 6 refers to the embodiment of Figure ~ based on Equatioll ~7), the same effect can be expected from the use of the em~odiment of Fig~e 4.
Referring to Figure 3, anotller adaptive impedance circuit for use in tllis inventioll includcs an impedance circuit 51 connected to terminal 7, and a control circuit 53 having inputs from terminals 7, 8 and 9 at which sigllals v, v and v5~ respectively, are applied. An output of circuit 53 is apl)licd to addcr 52 to whicll vr is àlso applied. The output of adder 52 is connected to impedance 51. The circuit of Figure 9 has the follo~ing im-pedallce 7 betl~een termilu.ll 7 and the ground:
2 = 2i/(1 - K) ........................ (~) From v5 = v - vr/2 is derived Z - Z
v = L ~v - vr) If thc impedallce of tlle two-wire path can achieve tlle optimum appro~ tion at ZL = Zi / ~1 - L), Vs = ~v - vr) .................................... ~9) 2~1-L) If L is sufficiontly smallcr than 1, a control circuit 53 can ~e simil~rly structure to wllat is illustrated in Figurcs ~ and 5, e~ccpt that tlle sul~tr;lctioll in Equation (7) for weightillT cocfficient modific.ltioll should bc rcplacc(l I)y all additioll.
If L is not negli~Til~ly small ns compared ~ith 1, from v = v-v /2 -- ~3 --.

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is ~erive~

L - K
v = -`~ .................... ~10) s 2~1- k~
Ir the amper~ge Oî the CUI rent whicn flows wilen VS is fed to a terminai 7 o~ the adaptive impedance circuit 50 of Figure 9 is represented l~ 15, v5 = i5.Z~ K) ............... ,.. ,.......... .(11) Hence, Equation ~10) can be transformed into s i V -----------........................ .~12) ~here iS.Zi represents the voltage a~ both ends of the impedance Zi For the algorithm to minimi~e is.Zi, as is the case ~rith the first embodiment, a mo~i~ied al~oritl~n of j kj g iS Zi-Fj.v ......................... ~13) is obtained in a manner similar to that for obtaining Equation (6), and further to simplify aritllmetic operations, this equation. li~e Equation (7), c~n be transformed into kj l(i5-zi) ~2(Fj-v) ----------- ~1~) ~ igure 10 illustrates in detail the adaptive impedance circuit o~ urc 9 bascd on Equatioll tl:l). In Fi~urc 10, attenuators 16~ and 169 ~re controlle~, as is obvious from Equation ~14), by the use of signals v and ~o iS.Zi. Tlle principal part of circ~lit of Fi~urc 10 is composed of a loop comprising .-n impc(lancc element 51 and operational amplifiers 190 througll 1~. I`llc sign:ll v tfrom a tcrmillal 7) passes an operational amplifier 290 all~ tllcn f~mctional circuits 261 and 262, and is giVCII to nonlinear circuits 16~ ;In~l lG~ ! sigllal v5 givell from a -terminal 9 is fe~ to an impedance clcmcnt 250. ,\ loopcolnprisirlg tlle impedallce element 250 and o~erational a~ if icr:, 292 tllroug!l 29~ has tho samo imllc(lallcc as tlle loop involving the _ 10 _ -~ . .

imped.lnce eiement 51. Li'~cwise, each of structural clemenLs associated with ti~is loop ;~at, :lg ,,'le ir,Dedallce eler;lent ~S0 has the ~ame function as that .n the loop l}avin~T the ele;:lc3lt 51. .\ccordillgly, the potential differenc- be-tweell the tl~o ends of the ir,lpedance element 250 is i Zi The signal i Zi is fed to a nonlinear circuit 181, and modi.~ication is achieved in the same ~nner as in the irst erllbodiment.
r~eferrillg to Fi~ures 11 and 12, still another adaptive ir,~.pedance circui, ad.,ptablA for a balanced ~wo-wire circuit has terminals 7 and 7' connccted to the balanced two-~ire circuit. In these structural examples, the circuits of Figures 11 and 12 are identical to those of Figures 5 and 9 e~g~ e~cept for the additioll of inverter 52' and an impedance element 51' ~YiliCIl are serially connected between terminal 7' and the ou~put of adder 52.
Inlereas separate adaptive impedance circuits are illustrated in Figu~re 5 and 9, it is also conceivable to combine the two, i.e., to use a strUCtUl`e ill WiliCIl the voltages of the terminals 7 and S are applied to an arbitrary control-circuit and the voltacTes at the two ends of the impedance element Sl are applied to another arbitrary control circuit so that each control circuit can bc aclaptivcly r,~.odified.
` ~s hitllcrto described, the present invention mal~es it possi~le to inte<Trate circuits in a sir.lple structure and reali~e adaptive clectronic l~ybrid circuits Wit]l little reflcction from all the t~o-~Yire ,md four-l~ire circuits

Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An adaptive electronic hybrid circuit comprising: an adaptive impedance circuit for connecting a two-wire circuit and a four-wire circuit; a first impedance element for supplying a reception signal on the four-wire side to a terminal of the two-wire circuit; a second impedance element connected to one side of the adaptive impedance circuit the other side of which is connected to a terminal on the four-wire side for supplying the reception signal on the four-wire side through the adaptive impedance circuit; and an adder which adds, in a reverse phase, the reception signal appearing at the junction between the second impedance element and the one side of the adaptive impedance circuit to a signal given from the terminal of the two-wire circuit to provide at an output of the adder on the four-wire side a transmission signal, the output of the adder being connected to a control terminal of the adaptive impedance circuit whereby the impedance of the adaptive impedance circuit adaptively varies depending on the characteristics of the two-wire circuit.

2. An adaptive electronic hybrid circuit as claimed in claim 1 wherein said adaptive impedance circuit comprises first, second and third terminals corresponding, respectively, to the one side, other side and control terminal;
an impedance element having two ends one of which is connected to said first terminal; and a circuit which, using as its inputs the voltage of said first terminal and a current or voltage derived at the other end of said impedance element, drives said impedance element from the other end of said impedance element and modifies the internal characteristics on the basis of a signal from said third terminal.

3. An adaptive electronic hybrid circuit as claimed in claim 1 wherein said adaptive impedance circuit comprises first, second, and third terminals corresponding, respectively, to the one side, other side and control terminal;
an impedance element having two ends one of which is connected to said first terminal; and a circuit which, using as its inputs the voltages of said first and second terminals, drives said impedance element from the other end of said impedance element and modifies the internal characteristics on the basis of a signal from said third terminal.

4. An adaptive hybrid circuit as claimed in claim 2 or claim 3 further comprising a fourth terminal and a second impedance element having two ends one of which is connected to the fourth terminal and the other of which is connected to the other end of the first impedance element through inverting means whereby the first and second impedance elements are driven in mutually reversed phase.