DE3702316C1 - Method and circuit arrangements for adaptive echo cancellation in terminals for duplex transmission - Google Patents

Abstract

In electrical communications, echo signals occur at the transition from a four-wire to a two-wire line via a hybrid circuit owing to faults in transmission paths. Attempts are made to cancel these echo signals using an adaptive echo canceller. The coefficients are calculated using a variable adjustment step factor. If a relatively long echo is intended to be completely cancelled with an echo canceller of this type containing a single transverse filter, this filter must have an economically unviable number of elements. Furthermore, a large number of elements allows only a small adjustment step factor, which restricts the ability to achieve a high convergence speed. The object is to achieve a high convergence speed at low cost, even in the case of a long echo. A second, non-adaptive filter (RF) is connected downstream of the adaptive transverse filter (Fig. 1). The invention is suitable for use in the transmission modules of the integrated services digital telecommunications network (ISDN). <IMAGE>

Description

The invention relates to methods and circuit arrangements for adaptive echo cancellation in end devices for duplex transmission according to the preambles of claims 1 and 2 or 3 and 4.

Such methods or such circuit arrangements are out the European patent application 01 01 938 known.

Such methods and circuit arrangements are in the electrical communication technology in the transition from one Four-wire to two-wire line via a hybrid circuit used. As a result of imperfections in the transmission paths echo signals occur.

The echo signals are tried using an adaptive echo canceller to compensate. It consists of a transversal filter and a subtractor. The broadcast signal which the Four-wire input is fed to the hybrid circuit also supplied to the data input of the transversal filter. This delivers an echo cancellation signal at its output, which is fed to the minus input of the subtractor. The The subtractor's plus input is with the four-wire output connected to the hybrid, d. that is, he’s going to do that with him Echo signal superimposed received signal supplied. The coefficient calculation of the transversal filter happens with variable  Adjustment step factor. This makes a high one Convergence speed reached. Should with such Echo canceller completely deleted a relatively long echo become, this filter no longer has to be an economical one have a portable number of links. Under "links" the delay elements, the associated multipliers and other associated assemblies, e.g. B. the coefficient calculation, Roger that. A large number of links leaves furthermore only a small adjustment step factor to what the Achieving high convergence speed limits puts.

The invention is based on the object, less complex Method or less complex circuit arrangements of the above type, even if proportionately long echo signals achieved a high convergence speed becomes.

This task is characterized by the characteristics of Claims 1 to 4 solved. The sub-claim 5 gives an advantageous Advanced training.

From Figures 4 and 6 and the associated texts on the Pages 302 to 309 of the German magazine "Frequency", Volume 36 (1982), Issue 11, it is known per se, the transversal filter to be equipped with fewer links than for the complete one Echo cancellation are necessary. This will only be a part of the echo deleted. For the deletion of what is left The transversal filter has a second echo downstream, which is designed as a recursive filter is. This recursive filter also works adaptively. However, this is associated with a great deal of effort because the facilities necessary for the adaptation and because of measures to adapt to the known instability of recursive To prevent filters.

The invention is illustrated in FIGS. 1. . . 6 exemplary embodiments shown, their assignment is given in the following table:

Figs. 2 and 6 deal with the details, wherein Fig. 2 applies to all embodiments.

Figs. 1 and 4 show the circuit arrangements concerned itself again, while their functions with reference to FIGS. 3 and 5 will be described. In Figs. 1 and 4, only the features necessary for understanding of the invention have been located. Have been omitted e.g. B. the usual in such echo canceller analog-digital and digital-analog converter and other filters. On the other hand, such assemblies are shown that do not belong to the echo canceller, but whose representation facilitates understanding.

They are:

S a transmitter for the transmission signal a G a hybrid circuit Ü a two-wire transmission path from the station A shown here to the station B not shown E a receiver for digital signals

Embodiment 1 according to FIG. 1 is first described. It means:

A signal a transmitted by the transmitter S , that is to say the transmission signal, reaches the station B via the hybrid circuit G and the transmission path Ü . Conversely, a signal b sent by station B reaches receiver E via transmission path U , hybrid circuit G and subtractor Sb . The signal b is therefore the received signal for the station A considered here. Because of imperfections in the hybrid circuit G , the transmission path U and the facilities in station B , part of the transmission signal a is superimposed on the reception signal b as an echo signal e ; A sum signal b + e thus reaches the plus input + of the subtractor Sb .

If one initially thinks the second filter RF away and the output of the last multiplier M _n directly connected to the summer Su , an echo canceller constructed from a single adaptive transversal filter AF results. Its function is as follows: The transmission signal a is fed to the multipliers M ₀ to M _n , specifically to the multiplier M ₀, the other via the delay chain built up from the delay elements V ₁ to V _n , all delay elements having the same delay time T. The immediate or delayed transmission signal is multiplied in the multipliers by the associated coefficients k ₀ to k _n . The multiplier output signals y ₀ to y _n thus formed are summed in the summer Su to the echo cancellation signal ê , which is fed to the minus input of the subtractor Sb . Thus, the echo canceling signal ê of the echo signal superimposed with the received signal b + e is subtracted, and there arises at the output of the subtractor Sb b the compensated reception signal + he, being referred with it the residual error. The compensated received signal b + er is fed to the receiver E and, via the multiplier M _{a, to} the coefficient calculator KR . This determines the coefficients k ₀ to k _n such that in the part of the echo to be canceled by the transversal filter due to the number of its elements, the echo cancellation signal ê is as equal as possible to the echo signal e . Then in the desired manner, the residual error is he, and the transversal Minimum located in the adapted state.

A variable adjustment step factor v is fed to the multiplier M _a . This ensures that the coefficients k ₀, k ₁ to k _n are calculated depending on this adjustment step factor. It is obtained as follows: the echo cancellation signal ê and the compensated received signal b + it are fed to the multiplier M _p , which forms the product therefrom and outputs it as an output value p at its output. From this, the mean value m is obtained in the low-pass filter TP , which is supplied to the “characteristic curve generation” assembly KE .

The "characteristic curve generation" module KE forms the adjustment step factor v from the mean value m as follows:

v = c _B · | m | at c _B · | m | ≧ v _min

respectively.

v = v _min at c _B · | m | < v _min

The value of the mean m ensures that the adjustment step factor v never becomes negative, which would result in a rapid divergence of the echo canceller. The minimum adjustment step factor v _min and the evaluation factor c _B are fixed values. By raising the adjustment step factor v to the minimum adjustment step factor v _min it is achieved that an adaptation takes place even if all the coefficients are at zero and there is still a residual error.

The low-pass filter, as shown in FIG. 2, can be implemented as a recursive filter. It means:

V _T a delay element 10 an adder 11 and 12 multipliers d an integration constant

P and m are the lines as shown in FIG. 1 p for the output value and designates the mean value m.

The highest possible convergence speed is achieved if the minimum adjustment step factor v _min , the integration constant d and the evaluation factor c _B are selected so that the following conditions are met:

d · c _B ≦ v _min

and

The values of c _B , d and especially v _min depend on the respective application. With v _min = 2 ^-19 as an example, the combination with d = 2 ^-13 and c _B = 2 ^{-12 is} particularly advantageous.

As shown in FIG. 1, the second filter RF is connected downstream of the adaptive transversal filter AF . The transversal filter AF is therefore the first filter. This is followed by the second filter RF in such a way that its adder Ad is looped into the connection from the output of the last multiplier M _{n of} the transversal filter AF to the summer Su . The second filter RF is a first degree recursive filter since it has only one delay element and one multiplier. It is not operated adaptively, but its multiplier M _R is operated with a fixed decay factor c _k stored in the memory Sp , which is less than one. It assigns the transfer function

on. It is the z- transform of the discrete-time impulse response of this filter.

The transversal filter AF and the recursive filter RF are constructed as a discrete-time system. The multipliers M ₀ to M _n only output multiplier output values at their outputs at discrete times i * T , ie each of the multiplier output signals y ₀ to y _n is obtained by a sequence of multiplier output values y _{0, i} , y _{1 , i} etc. to y _{n, i} . The adder Ad of the recursive filter RF also only outputs an output value at these discrete times. This and the multiplier output values are summed in the summer Su at the time i · T to the echo cancellation value ê _i . The sequence of the echo cancellation values ê _i represents the echo cancellation signal ê . If the echo cancellation values ê _{i are maintained} between the discrete times i · T , the echo cancellation signal ê is stepped. An echo signal e thus only experiences the desired compensation at these discrete times i · T. The receiver E evaluates the compensated received signal b + er only at these times. It is therefore sufficient to strive for equality of the echo signal and echo cancellation signal only at these discrete points in time.

In the expression i · T , T denotes the period of a cycle with which the time-discrete system is operated. This period is equal to the delay time T of the delay elements V ₁ to V _n and V _R.

The echo cancellation value ê _i can be thought of as the sum of an echo cancellation value of the first type ê _i ' and an echo cancellation value of the second type ê _i '' . The first type is the one that would be generated without the recursive filter, that is, when the output of the last multiplier M _n is directly connected to the summer Su . The sequence of the echo cancellation values of the first type is referred to as the echo cancellation signal of the first type ê ' . The echo cancellation value of the second type is that which additionally occurs after the recursive filter has been inserted. The sequence of the echo cancellation values of the second type is referred to as the echo cancellation signal of the second type ê '' .

A single echo cancellation value of the first type ê _i ' occurring at time i · T is the sum of all multiplier output values y _{0, i} , y _{1, i} etc. to y _{n, i} occurring at the same time. The echo cancellation value of the second type occurring at the same time is denoted by ê _i '' . The sum of both results in the echo cancellation value ê _i occurring at this point in time.

The sequence of the echo cancellation values of the second type is generated by the recursive filter in that each of the output values of the recursive filter is supplied not only to the summer Su but also to the multiplier M _R , multiplied there by the decay factor c _k , in the delay element V _R by one clock period T is delayed and then fed to the adder Ad . There, the multiplier output value present at the other input at that time is added, and so the next output value of the adder Ad is created , which is then multiplied again by the decay factor and delayed by a clock period, etc.

Since each output value of the adder Ad has a summand which is defined as contributing to an echo cancellation value of the first type, namely the multiplier output value occurring at the respective time, is therefore a single echo cancellation value of the second type ê _j occurring at the discrete time j · T '' The sum of one or more times delayed by the period T and multiplied each time by the fixed decay factor c _k multiplier output values y _{n, i} , where:

ê _j ′ ′ = y _{n, j -1} · c _k + y _{n, j -2} · c _k ² + y _{n, j -3} · c _k ³ +. . .

Here y _{n, j -1} , y _{n, j -2} , y _{n, j -3} etc. are those at the discrete times ( j -1) · T , ( j -2) · T , ( j -3) · T etc. occurring multiplier output values y _{n, i of} the last multiplier M _n .

Further details are explained with reference to FIG. 3. The time t is plotted on the abscissa and the normalized amplitudes of the echo cancellation values ê _i , the echo signal e and the residual error er are plotted on the ordinate. Discrete times i · T for i = ν -1 to i = ν +8 are plotted on the abscissa. By the solid curve a detail of the echo signal e and with the dashed curve a detail of the residual error is it shown. Echo cancellation values ê _i for i = ν -1 to i = ν +8 are entered with bars. The representation of the residual error as a continuous curve does not mean that a signal with such a course must actually be present. Corresponding samples can occur instead at discrete times.

This figure and the following text apply to the special case that at the time i · T = 0 a Dirac burst was sent as the transmission signal a and a reception signal is missing.

Furthermore, since this Dirac surge is also at the input of the first delay element V ₁ of the transversal filter and the delay time T of this and the other delay elements is chosen equal to the clock period T , it follows that the number n is equal to the number ν . It follows that of the echo cancellation values ê _i those with i = 0 to i = ν are generated by the transversal filter AF , and they correspond to the multiplier output values as follows:

ê ₀ = y _0.0 ; ê ₁ = y _1,1 etc. to ê _ν = y _{ν, ν}

So this is the sequence of the echo cancellation values of the first type ê _i ′ .

The echo cancellation value ê _ν is still considered to be generated by the transversal filter because the associated multiplier output value y _{ν, ν} passes through the adder Ad without delay, ie the echo cancellation value ê _ν would also be generated if the recursive filter was not present and the output of the last multiplier M _n would be connected directly to the summer Su .

The sequence of the echo cancellation values ê _i with i < ν , that is to say the echo cancellation values of the second type ê _i '', are generated by the recursive filter RF from the last multiplier output signal y _n by each output value of the adder in the multiplier M _R having the fixed decay factor c _k multiplied, delayed by a clock period T in the delay element V _R and fed back to the adder and thus appears as the new output value of the adder. The output values obtained in this way arrive at the adder Su and appear as the sequence of the echo cancellation values of the second type ê _i ′ ′ . The following applies to these echo cancellation values:

ê _{i +1} = c _k · ê _i

Based on this relationship, starting from ê _ν = 0.1, the echo cancellation values ê _{ν +1} to ê _{ν +8 were} determined, whereby the decay factor was chosen to be exaggeratedly small with c _k = 0.6 for the sake of clarity. In reality, you choose it from the range between 0.75 and 0.95.

The time range in which the echo cancellation values ê _i generated by the transversal filter AF , ie the echo cancellation values of the first type ê _i ' , occur is referred to as the compensation window of the transversal filter. In this case it extends from t = 0 to t = ν · T.

It is also assumed that the transversal filter AF is in the adapted state, ie, within the compensation window, the echo cancellation values ê _{i are} equal to the respective values of the echo signal e at the respective discrete times i · T. It follows that the residual error is zero within the compensation window of the transversal filter AF .

The echo signal e is so long that it cannot be completely compensated for by the transversal filter AF because of the small number of its elements. This means that the compensation window comprises only a part of the echo signal. Outside the compensation window, ie at t ≧ ( ν +1) · T , the echo signal e is compensated by the echo cancellation values of the second type ê _i '' . This compensation is only exceptionally complete, ie, the residual error it is only exceptionally zero, namely when the echo signal e coincide by chance with the echo canceller values of the second kind. As a rule, as shown in broken lines in FIG. 3, a residual error deviating from zero will result. It is no longer shown that in the range t <( ν +8) · T the echo signal e and the echo cancellation values ê _i (for i < ν +8) decay to zero and that the residual error thus also decays to zero.

Since, according to the invention, the echo cancellation signal of the second type ê '' is obtained from the last multiplier output signal y _{n of} the transversal filter AF , whenever the decay factor lies within the range specified above, there is only a slow and moderate increase in the transmission quality, which does not yet impair the transmission quality Residual error. Nevertheless, it is advantageous to choose a "favorable" decay factor, ie one in which the residual error is as small as possible.

For the determination and storage of such a favorable decay factor c _k , one of the possibilities is to determine one from examinations of the transmission paths in question and to set this permanently in the memory Sp during manufacture of the device. The other possibility is to make the memory Sp adjustable. Thus, the maintenance personnel can set a favorable decay factor during commissioning or later by setting the decay factor for as long as. B. changed in steps until the eye diagram shows the largest possible opening or the smallest possible bit error number on a bit error measuring device.

If the echo cancellation signal of the second type ê '' to be more precisely adapted to the echo signals that result from different transmission paths, a method or an arrangement according to claims 2 and 4 is cheaper. The corresponding exemplary embodiment 2 is described with reference to FIGS. 4 and 5, in which only the differences from the exemplary embodiment 1 are referred to below.

Instead of the memory for the decay factor, a decay factor computer AR is provided, to which the two last coefficients k _{n -1} and k _{n of} the transversal filter AF are applied. From this, he calculates the variable decay factor c _{v based} on the relationship

This relationship applies under the generally applicable assumption that the last delay element V _{n of} the transversal filter AF and the delay element V _{R of} the recursive filter RF 'have the same delay times. The multiplier M _{R is} operated with the decay factor c _v obtained in this way. The transfer function then reads accordingly:

Since the coefficients k _{n -1} and k _{n can} change, the decay factor can also change. Therefore, when specifying the regularity with which a specific echo cancellation value of the second type ê _j ′ 'is formed, a distinction must be made as to whether the respective multiplier output value of the adder Ad is first multiplied by the decay factor and then delayed, or whether the sequence is reversed . In the first case, that is to say with a circuit arrangement according to FIG. 4, the following applies:

ê _j ′ ′ = y _{n, j -1} · c _{v, j} + y _{n, j -2} · c _{v, j -1} · c _{v, j -2} + y _{n, j -3} · c _{v, j - 1} · c _{v, j -2} · c _{v, j -3} +. . .

In the second case, that is, when the adder Ad is followed by the delay element V _R and this is followed by the multiplier M _R :

ê _j ′ ′ = y _{n, j -1} · c _{v, j} + y _{n, j -2} · c _{v, j} · c _{v, j -1} + y _{n, j -3} · c _{v, j} · c _{v , j -1} · c _{v, j -2} +. . .

Here, y _{n -1} etc. again mean the multiplier output values as in introductory example 2.

With c _{v, j} ; c _{v, j -1} ; c _{v, j -2} ; c _{v, j -3} etc. are the decay factors occurring at the discrete times j · T , ( j -1) · T , ( j -2) · T , ( j -3) · T etc.

The effect of the above measure is explained with reference to FIG. 5. It agrees with FIG. 3 in the following details:

• a) In the amplitudes of the two echo cancellation values ê _{ν -1} and ê _n ; the following applies: ê _{ν -1} = 0.125 and ê _ν = 0.1;
• b) in the curve of the echo signal e .

The echo cancellation values ê _{ν -1} and ê _ν have the same values for a Dirac burst as the transmission signal as the coefficients k _{n -1} and k _n . It can thus be shown on the basis of the echo cancellation values ê _{ν -1} and ê _ν that in this example the decay factor calculator AR calculates a decay factor c _v of 0.8. As described in exemplary embodiment 1, this decay factor and the echo cancellation value ê _ν result in the echo cancellation values of the recursive filter.

The inventive calculation of the decay factor results in a better match of the echo cancellation values of the second type with the actual course of the echo signal e compared to embodiment 1.

Based onFig. 6 becomes an embodiment of the decay factor calculator AR described according to claim 5. It has two entrancesE 1 andE 2nd on which the Coefficientsk _{n -1} respectively.k _n  a first or a second AveragerMW 1 respectively.MW 2nd be fed out of it the mean values _{n -1} respectively. _n  form. These averages will be a first input of a multiplierMU or the Plus input of a differential elementDG fed. The exit of the multiplierMU is with the minus input of the Differential elementDG connected. Its exit is with a first input of a comparatorVG connected. A second Receipt of this comparatorVG becomes a thresholdSch  fed. The output of the comparator is with a first one Input of an AND circuit1 connected. On the second Entrance is one barCL. The output of this AND circuit 1 is with the control input of a decay factor memory AFS connected. In it are in a cyclic memory different decay factors from the one in question Value range saved. Here it is the values 0.75; 0.8; 0.9; 0.95. Depending on the position of the cyclical memory one of these decay factors appearsc _v  at the exit of the decay factor memoryAFS and therefore also on the second Input of the multiplierMU. About the exitA the decay factor calculator AR reaches this decay factorc _v  also, like in theFig. 4 is shown at the input of the multiplier M _R .

In the multiplierMU becomes the mean _{n -1} with the respective Decay factorc _v  multiplied. In the differential elementDG  is the product obtained and the mean _n  a differencedf won which in the comparatorVG with the ThresholdSch is compared. Is the differencedf smaller than the thresholdSch, takes the output of the comparator  VG the value logically "0" and none arrive Clock pulses of the clockCL via the AND circuit1 to the Control input of the decay factor memoryAFS. The cyclical Speicher maintains its position.

However, if the difference df is greater than the threshold value Sch , the output of the comparator VG assumes the value logic "1", the AND circuit 1 switches the clock CL through to the control input of the decay factor memory AFS . With each clock pulse, the cyclic memory is advanced by one position, and the next stored decay factor appears at the output of the decay factor memory AFS . This switching continues until the difference df is less than the threshold value Sch and the AND circuit 1 blocks the clock CL . The last appearing decay factor then appears continuously.

Due to the adaptation process of the adaptive transversal filter AF, the coefficients k _{n -1} and k _n fluctuate around their mean values in the adapted state. These fluctuations are suppressed by the mean value formers MW 1 and MW 2 and therefore do not influence the decay factor.

Another advantage of this decay factor calculator is that one according to claim 2 or 4 for determining the decay factor required and can only be carried out with great effort Division is avoided.

When using the circuit arrangements according to the invention, the number of elements of the adaptive transversal filter is chosen so large that after the end of its compensation window the amplitude of the echo signal e decreases, as shown in FIGS. 3 and 5. That is, the last coefficient k _n is less than the penultimate coefficient k _{n -1} , the division according to claim 2 or 4 leads to a decay factor which is less than 1, and the echo cancellation signal of the second type ê '' sounds more desirable Deny to zero asymptotically. However, if, contrary to expectations and perhaps only briefly, the last coefficient is greater than the penultimate one and the division gives a "decay factor" greater than one, the execution of the decay factor calculator according to claim 5 also ensures the desired decay of the echo cancellation signal of the second type in this case to zero because the decay factor memory AFS only stores decay factors that are less than one and therefore only those can reach the multiplier M _R.

The invention is well suited for use on the subscriber side and exchange-side transmission modules of the future so-called integrated service digital telecommunications network, also known by the abbreviation "ISDN". In this new network should be the existing from the conventional telephone network Subscriber cables are used to lay out to avoid new cables. The transmission paths obtained in this way however, have relatively long echo signals. By application this invention achieves satisfactory echo cancellation with economic effort, so that these transmission routes for the digital signal transmission provided in the new network become usable with high bit rate.

In this network, a basic connection must be within a prescribed Time can be activated. The invention enables the high speed of convergence necessary for this possible large adjustment step factor. Since the first adaptively operated filter a second, not adaptively operated Filter is connected, the first filter needs not have as many links as would be necessary in order to use this first filter alone to produce an echo signal  to delete its entire length. So the first filter can executed with a relatively small number of links will. The lower the number of limbs of an adaptive Transversal filter is selected, the larger the Adjustment step factor can be selected without instabilities must be feared. So at the beginning of the adaptation to Achieve a high rate of convergence with one large adjustment step factor can be worked. With increasing Adaptation then becomes automatic with a small adjustment step factor worked on the other hand also required to achieve high accuracy of adaptation.

Claims (8)

1. Method for echo cancellation in terminals for duplex transmission with the following features:

• a) the transmission signal (a) of a data transmitter (S) is gradually delayed;
• b) multiplier output signals ( y ₀, y ₁... y _n ) are obtained by multiplying the transmission signal (a) and the delayed transmission signals by coefficients ( k ₀, k ₁ to k _n );
• c) the multiplier output signals ( y ₀, y ₁... y _n ) form an echo cancellation signal of the first type (ê ') ;
• d1) the last multiplier output signal ( y _n ) is represented by a sequence of multiplier output values ( y _{n, i} );
• d2) these multiplier output values ( y _{n, i} ) occur at discrete times (i · T) ;
• d3) a sequence of echo cancellation values of the second type (ê _i ′ ′) is obtained from the sequence of the multiplier output values ( y _{n, i} ), a certain echo cancellation value of the second type (ê _j ′) occurring at the discrete time j · T ′) Is formed as follows: ê _j ′ ′ = y _{n, j -1} · c _k + y _{n, j -2} · c _k ² + y _{n, j -3} · c _k ³ +. . where y _{n, j -1} , y _{n, j -2} , y _{n, j -3} etc. of the multiplication output values ( y _{n, i} ) mean those at the discrete times ( j -1) · T , ( j -2) · T , ( j -3) · T etc. occur,
  c _k : a fixed decay factor which is chosen to be less than one;
• d4) the sequence of the echo cancellation values of the second type (ê _i ′ ′) represents an echo cancellation signal of the second type (ê ′ ′) ;
• e) a compensated received signal (b + er) which still has a residual error (er) is obtained by subtracting from the received signal (b + e) superimposed with an echo signal:
  • e1) the echo cancellation signal of the first type (ê ′) ,
  • e2) the echo cancellation signal of the second kind (ê ′ ′) ;
• f) the coefficients ( k ₀, k ₁ to k _n ) are determined from the compensated received signal (b + er) in such a way that the residual error (er) becomes minimal;
• g) method step f) is carried out with a variable adjustment step factor (v) ;
• h) the variable adjustment step factor (v) is obtained as follows:
  • h1) the echo cancellation signals of the first and second types (ê ′ and ê ′ ′) are summed to form an echo cancellation signal (ê) ;
  • h2) an output value (p) is formed from the echo cancellation signal (ê) and the compensated received signal (b + er) by multiplication;
  • h3) an average value (m) is formed from the initial value (p) ;
  • h4) the adjustment step factor (v) is obtained from the mean (m) as follows: v = c _B · | m | at c _B · | m | ≧ v _min or v = v _min at c _B · | m | < v _min here, v _{min is} a fixed minimum adjustment step factor and c _{B is} a fixed evaluation factor;

the features a, b, c, d1, d2, e, e1, f, g, h, h2, h3, h4 form the Generic term; the remaining features form the characteristic part. 2. Method for echo solution in terminals for duplex transmission with the following features:

• a) the transmission signal (a) of a data transmitter (S) is gradually delayed;
• b) multiplier output signals ( y ₀, y ₁... y _n ) are obtained by multiplying the transmission signal (a) and the delayed transmission signals by coefficients ( k ₀, K ₁ to k _n );
• c) the multiplier output signals ( y ₀, y ₁... y _n ) form an echo cancellation signal of the first type (ê ') ;
• d1) the last multiplier output signal ( y _n ) is represented by a sequence of multiplier output values ( y _{n, i} );
• d2) these multiplier output values ( y _{n, i} ) occur at discrete times (i · T) ;
• d3) a sequence of echo cancellation values of the second type (ê _i ′ ′) is obtained from the sequence of the multiplier output values ( y _{n, i} ), a certain echo cancellation value of the second type (ê _j ′) occurring at the discrete time j · T ′) Is formed according to one of the following two relationships: ê _j ′ ′ = y _{n, j -1} · c _{v, j -1} + y _{n, j -2} · c _{v, j -1} · c _{v, j -2} + y _{n, j -3} · c _{v, j -1} · c _{v, j -2} · c _{v, j -3} +. . . ê _j ′ ′ = y _{n, j -1} · c _{v, j} + y _{n, j -2} · c _{v, j} · c _{v, j -1} + y _{n, j -3} · c _{v, j} · c _{v , j -1} · c _{v, j -2} +. . . mean: y _{n, j -1} ; y _{n, j -2} ; y _{n, j -3} , etc. of the multiplier output values ( y _{n, i} ) those occurring at the discrete times ( j -1) · T , ( j -2) · T , ( j -3) · T , etc. , c _{v, j} ; c _{v, j -1} ; c _{v, j -2} ; c _{v, j -3} etc. the decay factors at the discrete times j · T , ( j -1) · T , ( j -2) · T, etc .;
• d4) the variable decay factor c _v is obtained according to the following relationship: k _n is the last and k _{n -1} is the penultimate coefficient;
• d5) the sequence of the echo cancellation values of the second type (ê _i ′ ′) represents an echo cancellation signal of the second type (ê ′ ′) ;
• e) a compensated received signal (b + er) which still has a residual error (er) is obtained by subtracting from the received signal (b + e) superimposed with an echo signal:
  • e1) the echo cancellation signal of the first type (ê ′) ,
  • e2) the echo cancellation signal of the second kind (ê ′ ′) ;
• f) the coefficients ( k ₀, k ₁ to k _n ) are determined from the compensated received signal (b + er) in such a way that the residual error (er) becomes minimal;
• g) method step f) is carried out with a variable adjustment step factor (v) ;
• h) the variable adjustment step factor (v) is obtained as follows:
  • h1) the echo cancellation signals of the first and second types (ê ′ and ê ′ ′) are summed to form an echo cancellation signal (ê) ;
  • h2) an output value (p) is formed from the echo cancellation signal (ê) and the compensated received signal (b + er) by multiplication;
  • h3) an average value (m) is formed from the initial value (p) ;
  • h4) the adjustment step factor (v) is obtained from the mean (m) as follows: v = c _B · | m | at c _B · | m | ≧ v _min or v = v _min at c _B · | m | < v _min here, v _{min is} a fixed minimum adjustment step factor and c _{B is} a fixed evaluation factor;
  • the features a, b, c, d1, d2, e, e1, f, g, h, h2, h3, h4 form the generic term; the remaining features form the characteristic part.

3. Circuit arrangement for adaptive echo cancellation in terminals for duplex transmission with the following features:

• a) a first filter (AF) designed as a transversal filter with adaptive coefficient calculation is provided with the following structure:
  • a1) it has a delay chain consisting of several delay elements ( V ₁... V _n );
  • a2) a multiplier ( M ₀... M _n ) is connected to the input, to each tap and to the output of the delay chain;
  • a3) each multiplier ( M ₀... M _n ) is connected to a coefficient calculator (KR) ;
  • a4) the output of each multiplier ( M ₀... M _n ) is connected to an input of a summer (Su) ;
• b) the first filter (AF) is followed by a second filter (RF) ;
  • b1) the second filter (RF) is designed as a recursive filter;
  • b2) the second filter (RF) has the transfer function on;
  • b3) the second filter (RF) is connected downstream of the first filter in such a way that in the connection between the last multiplier ( M _n ) of the first filter (AF) and the summer (Su) the adder (Ad) of the second filter (RF) is inserted;
  • b4) the decay factor ( c _k ) has a fixed value less than one;
• c) a subtractor (Sb) is provided, the plus input of which is the input for the received signal (b + e) superimposed with an echo signal and the minus input of which is connected to the output of the summer (Su) ;
• d) the output of the subtractor (Sb) is the output for the compensated received signal (b + er) ;
• e) the output of the summer (Su) is connected to the first input of a multiplier ( M _p );
• f) the output of the subtractor (Sb) is connected to the second input of the multiplier ( M _p );
• g) the output of the multiplier ( M _p ) is connected to the input of a low pass (TP) ;
• h) the output of the low-pass filter (TP) is connected to the input of a "characteristic curve generation" module (KE) ;
• i) the module "generating characteristic curves" (KE) is designed in such a way that it outputs a mean value (m) from its output from the low-pass filter (TP) present at its input from the output value (p) of the multiplier ( M _p ) Adjustment step factor (v) forms as follows:
  • i1) the amount is formed from the mean (m) ;
  • i2) the amount is multiplied by a predetermined fixed evaluation factor ( c _B );
  • i3) the adjustment step factor (v) does not fall below a predetermined fixed minimum adjustment step factor ( v _min );
• k) the output of the "characteristic curve generation" module (KE) is connected to a multiplier ( M _a );
• l) the output of the subtractor (Sb) is connected to the multiplier ( M _a );
• m) the output of the multiplier ( M _a ) is connected to the coefficient calculator (KR) .

The generic term is formed by the features a, a1. . . a4, c. . . m; the remaining Characteristics form the characteristic part. 4. Circuit arrangement for adaptive echo cancellation in terminals for duplex transmission with the following features:

• a) a first filter (AF) designed as a transversal filter with adaptive coefficient calculation is provided with the following structure:
  • a1) it has a delay chain consisting of several delay elements ( V ₁... V _n );
  • a2) a multiplier ( M ₀... M _n ) is connected to the input, to each tap and to the output of the delay chain;
  • a3) each multiplier ( M ₀... M _n ) is connected to a coefficient calculator (KR) ;
  • a4) the output of each multiplier ( M ₀... M _n ) is connected to an input of a summer (Su) ;
• b) the first filter (AF) is followed by a second filter (RF ') ;
  • b1) the second filter (RF ′) is designed as a recursive filter;
  • b2) the second filter (RF ') has the transfer function on;
  • b3) the second filter (RF ') is connected downstream of the first filter such that in the connection between the last multiplier ( M _n ) of the first filter (AF) and the summer (Su) the adder (Ad) of the second filter (RF ') Is inserted;
  • b4) a decay factor calculator (AR) is provided, which is acted upon by the last two coefficients ( k _{n -1} , k _n ) of the first filter and from this calculates the decay factor ( c _v ) of the second filter, the relationship applies;
• c) a subtractor (Sb) is provided, the plus input of which is the input for the received signal (b + e) superimposed with an echo signal and the minus input of which is connected to the output of the summer (Su) ;
• d) the output of the subtractor (Sb) is the output for the compensated received signal (b + er) ;
• e) the output of the summer (Su) is connected to the first input of a multiplier ( M _p );
• f) the output of the subtractor (Sb) is connected to the second input of the multiplier ( M _p );
• g) the output of the multiplier ( M _p ) is connected to the input of a low pass (TP) ;
• h) the output of the low-pass filter (TP) is connected to the input of a "characteristic curve generation" module (KE) ;
• i) the module "generating characteristic curves" (KE) is designed in such a way that it outputs a mean value (m) from its output from the low-pass filter (TP) present at its input from the output value (p) of the multiplier ( M _p ) Adjustment step factor (v) forms as follows:
  • i1) the amount is formed from the mean (m) ;
  • i2) the amount is multiplied by a predetermined fixed evaluation factor ( c _B );
  • i3) the adjustment step factor (v) does not fall below a predetermined fixed minimum adjustment step factor ( v _min );
• k) the output of the "characteristic curve generation" module (KE) is connected to a multiplier ( M _a );
• l) the output of the subtractor (Sb) is connected to the multiplier ( M _a );
• m) the output of the multiplier ( M _a ) is connected to the coefficient calculator (KR) .

The generic term is formed by the features a, a1. . . a4, c. . . m; the remaining Characteristics form the characteristic part. 5. Circuit arrangement according to claim 4, characterized in that in the decay factor calculator (AR) the penultimate and last coefficient ( k _{n -1} and k _n ) a first or second averager ( MW 1 and MW 2 ), the outputs of which are supplied with a Input of a multiplier (MU) or connected to the plus input of a differential element (DG) , that the output of the multiplier (MU) is connected to the negative input of the differential element (DG) , that its output is connected to a first input of a Comparator (VG) is connected that its second input is supplied with a threshold value (SCH) and its output is connected to a first input of an AND circuit ( 1 ), that its second input is supplied with a clock (CL) and its output with a control input of a decay factor memory (AFS) is connected, that its output is connected to the second input of the multiplier (MU) and to the multiplier of the recursive filter ( M _R ), that the comp Eicher (VG) outputs a logical "yes" signal at its output if the difference (df) at the output of the differential element ( DG) is greater than the threshold value (Sch) that different decay factors are stored in the decay factor memory (AFS) these decay factors appear one after the other at the output of the decay factor memory (AFS) , as long as the clock (CL) is present at the control input, that the last decay factor appears continuously, as long as the clock (CL) is blocked by the AND circuit ( 1 ), and that the stored decay factors are less than one.