DE2427102C3 - Echo canceller - Google Patents

Description

a) the value (^ wf J of the sum of the squares

all output signals (W ₁ ) of the branching device that are recorded in cycles are determined,

b) a correction factor (q) is formed by dividing the residual echo signal (<?,) produced by means of the available echo path model by the sum value f ^ vvf J determined according to the previous method step,

c) the coefficients (c} ²⁾ ) of the second echo path model are each calculated by summing the respective coefficient (c {") of the first echo path model with the product (q ■ w,) of the correction factor (q) formed according to the previous method step with the associated Output signal (w,) of the branching device fixed,

d) a second residual echo signal (e ₂ ) (internally) is determined by means of the second echo path model,

e) the selection device (K) checks whether the absolute value of the second residual echo signal (e ₂ ) falls below a specified threshold value (/) and, if this condition (Ie ₂ I </) is fulfilled, causes the second echo path model to become the new first echo path model; If the aforementioned condition is not met (\ e ₂ > f) , the previous first echo path model becomes the new first echo path model, either directly or modified,

the new first echo path model established according to the preceding method step is used to generate the new artificial echo signal and consequently to generate the outgoing signal (s) of the second transmission direction.

2. Echo canceller according to claim I, characterized in that the coefficients (£} · ") of the modified first echo path model to assume values, each between the associated Value of the first and the associated value of the second Echopfadniodell lie.

3. Echo path model according to claim 2, characterized in that the selection device (K controls the coefficients (r | ³¹ ) in such a way that sii the closer to the coefficients (cf ^] ) of the first: echo path model, the greater the second residual echo signal ( \ e ₂ \) is.

The invention relates to an echo compensator for a first and a second transmission direction-having transmission system in which the signals of the first transmission direction, in particular at the end of the four-wire transmission system, are partially reflected and as echo signals appear in the signal of the second transmission direction. This echo canceller is equipped with a device for generating an artificial echo signal, the two of which are based on the transmission properties of the echo path approximating echo path models, a branching device common to the echo path models with a number the number of output signals corresponding to the adjustable coefficients of each of the two echo path models having, with means for subtracting the artificial echo signal from the signals of the second transmission direction and with a selection device for selecting the one for the generation of the outgoing signal of the second transmission direction used echo path model.

An echo canceller of the aforementioned type is known from DT-OS 22 24403. This well-known Echo canceller contains a comparator, which the residual echo signals generated by means of the two echo path models level with each other and cause the echo path model to approximate better to the actual echo path is used to generate the outgoing speech signals.

The object of the invention is to specify an echo canceller that has a higher setting speed works as known arrangements with comparable effort.

This object is given by the application of the characterizing part of claim 1 Measures resolved.

The application of the measures according to the invention includes a two-step process that with comparably little effort leads to such a high setting speed that the residual echo signal is eliminated except for a quantization noise even with any frequency rejection in the echo path.

The invention is explained in more detail below with reference to the drawing. The drawing shows in detail in

F i g. 1 the basic mode of operation of an echo canceller and its inclusion in a transmission system, which may contain a non-synchronous carrier frequency system,

2 shows a diagram and the associated arrangement for determining the preliminary residual echo signal e and the correction factor q,

F i g. 3 shows a scheme and the associated arrangement for determining the final residual echo signal e.

F i g. 1 shows, as a detail of a long-distance telephone connection, a fork junction 5 equipped with a line replica, a four-wire path with a runtime connected to the fork junction 5 with a first, incoming transmission direction 1-2-4 and a second, outgoing transmission direction 7-8-11 as well as an also to the fork junction 5 connected two-wire path to a subscriber 6. The signal χ on the incoming transmission direction is here as a rule, especially at the end of the four-wire system, partially reflected because of the imperfect line simulation of the fork 5 and appears as an echo signal y in the outgoing transmission path. The echo canceller contains a quadrupole 9 connected across the four-wire path, which generates the artificial echo signal y from the signal χ of the incoming transmission direction 1 by automatically adapting itself to the transmission properties of the echo path 2-4-5-7-8. The artificial echo signal y is added to the signal y + n of the outgoing transmission direction 8 via the subtraction device 10 in the echo suppression function, so that in the ideal case (y - y) the residual echo signal e at the output of the subtraction device 10 is only that of the nearby subscriber 6 contains resulting signal η .

Many four-wire links do not require their own echo cancellers or echo blocks because of their short transit time. Echo cancellers, so that only long-distance connections, such as. B. Submarine cable or satellite links, are equipped with these devices at their terminals. Between these connections, which are heavily affected by runtime, and the fork transitions to the two-wire connections but can then still use other four-wire transmission systems such. B. a non-synchronous one Carrier frequency system 3 must be switched on, as indicated by dashed lines in FIG. 1.

The present invention is based on an echo canceller as it is, for. B. in the article "An adaptive echo cancellor", by MM Sondhi, is described, which is printed in "The Bell System Technical Journal", March 1967, pages 497-511. In the Sondhi echo canceller, the signal χ of the incoming direction is fed to a branching device which, in the arrangement according to FIG. 1, is a delay line and in the arrangement according to FIG. 3 is designed as a series of filters connected in parallel. In any case, the branching network has a number of N outputs, which correspond to systems with mutually linearly independent impulse responses, and the corresponding output signals χ are each passed through one of the setting elements M ₂₁ ... M _2n and then added to the artificial echo ζ, which is designated by y in the present invention. The Koffizienten gj ... g _N, designated in the present invention with c ... c _N c with or shortly, characterize in every moment, the transmission characteristics of the artificial echo signal y generating quadripole (transversal filter) and have at occurring frequency offset be continuously readjusted in the echo path. Since the control speed in the known echo cancellers has to be tightly limited in order to ensure convergence, these known eochocompensators can only determine the frequency offset very imperfectly. 2 and 3, the echo canceller according to the invention is described in more detail below, which (like the previously known echo canceller according to DT-OS 2224403) works digitally and accordingly an analog-digital converter 12 for the signal χ and an analog-digital converter 12 for the signal y + η Has converter 13 and a digital-to-analog converter 14 for the residual echo signal e leaving the echo canceller.

If the 4 kHz band-limited signal χ or x (i) the incoming direction of the four-wire path is sampled at 8 kHz, so the sample values

XIi- (II) T) = W ₁ , / = 1.2 ... N, T = \ 25 * ^

N = number of outputs of the branching network, the echo signal y {t) in the form

yU) =

represent.

The coefficients c _t reflect the transmission properties of the echo path 2-4-5-7-8 , in which the fork 5 and, under certain circumstances, the non-synchronous carrier frequency system 3 are contained. If the frequency offset disappears, the coefficients c, are constant, otherwise they are time-dependent.

If the echo canceller is equipped with a transversal filter, the artificial echo has the shape

y (t) = Σ

i = 1

wherein the coefficients c characterize the setting of the quadrupole 9 (FIG. 1), designed as a transversal filter, of the echo canceller. The equations (1), (2) give the expression for the resleecho signal e or e (t) leaving the echo canceller

e (t) = y (t) - y (t) =

1 = 1

In order to achieve the smallest possible residual echo signal, the aim of the known adjustment method is to match the coefficients c of the model contained in the transversal filter 9 to the coefficients c of the actual echo path. It can now be shown that for an arbitrary Sat2 of sample values \ v, (i = 1 ... N) and of coefficients c, a coefficient set c can be found in a simple manner, which does not necessarily meet the condition c, = c, is sufficient, but in the absence of interfering signals (n = 0), the residual echo signal e (t disappears. If there is no frequency offset, the coefficients c converge to the coefficients c with continuous application of the method according to the invention, which is only included Occurrence of a signal η from the nearby participant (cannot be used immediately, since in this case the residual echo signal e (t) does not disappear. On the other hand, a residual echo signal that does not disappear is, on the other hand, an indication of the presence of a counterpart

Speech signal η and, according to the invention, triggers a specific step to be described later. Let c | "be the setting of the echo canceller at the previous sampling time f - T, where T represents the sampling period. With this set of coefficients corresponding to a first echo path model, a residual echo signal results at time t according to equation (3)

MO = Σ ta-ei

ei ¹

(4)

i = 1

With this first, preliminary residual echo signal e, new coefficients cf corresponding to a second echo path model can be determined according to the relationship

C ₁ W ₁ , / = \ --- N (5) output signals u>_; the branching device is determined

c) the preliminary residual echo signal e,

e, (i) = j / (t) - j) ^(ll (i)

is formed and stored internally;

d) a correction factor q is obtained by dividing the provisional residual echo signal C _{1 produced} by means of the available echo path model by the sum value that has already been determined

Σ w?

ι = 1

15th

educated:

0 = - ■;

i = 1

calculate a new residual echo signal

<: J ^{Z) of} the second echo path Si d ji

- CT) W.

produce. Inserting (5) into (6)

i = 1

1 "<

-Σ

i = 1

/ = 1

(C-Cj ¹ V, --—- Σ ^ < ⁷ >

i = 1

i = 1

shows that the residual echo signal e ₂ disappears.

The two-step process defined by equations (4) to (6) is now implemented using the following process steps:

a) the coefficients cf 'of the first echo path model and the artificial echo signal j) ^{ni formed therefrom}

i = 1

are with a clock t ,, z. B. corresponds to a cycle time of f = 125 JiS, determined and stored in a known manner, where N is the number of outputs of the branching network or ZaId of the samples w, of a shift register realizing the branching network;
b) the value

1 = 1

the sum of the squares of all the bars recorded

e) the coefficients

model are calculated by adding up the respective coefficient c | ^{u of} the first echo path model (6) with the product q · w of the one after the preceding one

Correction factor q formed in the method step with the associated output signal w, - of the branching device:

f) a second artificial echo signal is generated by means of the second echo path model

y ^a) = Σ3 ²¹ * /

i = 1

generated;

g) a second residual echo signal e _{2 is} determined internally by means of the second echo path model using the generated second artificial echo signal y ^(2):

45

h) the selection device K checks whether the absolute value of the second residual echo signal e ₂ falls below a specified threshold value /, ie whether no interference signal η, in particular from the nearby subscriber 6, occurs in the signal e _2; if this condition \ e ₂ \ <f is met , the selection device causes the second echo path model to become the new first echo path model; in the event of non-fulfillment ( ₂ each)> /) of the aforementioned condition, the previous first

Echo path model directly or modified to the new echo path model;

i) the new first echo path model determined in accordance with the preceding method step is used for generation of the new artificial echo signal and

used to generate the outgoing signal e of the second transmission direction 11

In a further development of the invention, the selection device K is designed such that the coefficients c ^{(3) of} the modified first echo path model

Assume values which are in each case between the associated value ei ^{11 of} the first and the associated value rj ^{21 of} the second echo path model. The selection device K controls the coefficients

rj ³ 'in such a way that the greater the second residual echo signal Ie ₂ I, the closer it is to the coefficients c) ¹ ' of the first echo path model, which can be expressed as follows: c [ ³⁾ = £ ·} + α q ■ W ₁ -, where is a modification factor between 0 and 1 (0 <<1). This modification factor is controlled cyclically in such a way that it is smaller, the larger the second residual echo signal | e ₂ | is.

Figures 2 and 3 show one with a transversal filter built-up digital echo canceller, which carries out method steps a) to i): In detail gives F i g. 2 process steps a) to d) again.

In the shift register S, the samples are W ₁ -. (Z = 1 ... N) and stored in the shift registers S2 and S3, the transversal filter coefficients cj "Which of the two memories S 2 or S 3 used for carrying out process step a) depends from the switch controlled by t _f . The function of t _f is explained further below. Using a multiplexing technique, the functions a) and b) are carried out by the multiplier M and the adder A2 and the intermediate results y ^a) or

stored in the memories S4 and SS. The multiplex clock r controls the respective inputs of the multiplier M and the adder A 2 so that in one switch position the product w, - · cf ' and in the other position the product wf is formed and in the corresponding memories S4 and S5 is accumulated. Subsequently, the first residual echo signal <? Generated by a subtracter A3 is stored in the memory S6 and the quotient is stored by a divider D.

40

educated. By multiplying with the multiplexed multiplier M of the signal e, with the quotient

E "?

the correction factor q arises.

In Fig. 3 that part of the echo canceller is shown which carries out method steps e) to i).

Using the multiplex clock I ₂ , the product q ■ W ₁ - is first formed, which is stored in the memory S7 and added to the respective coefficient c} ¹¹ . The coefficient £} ²⁾ arises at the output of the adder A 1. After switching the clock t ₂ , the second artificial echo signal j> ^{(2) is} formed via the adder A2 and the memory S4. In the absence of interfering signals (n = 0), the second residual echo signal e ₂ at the output of the memory S 4 should now disappear according to equation (7). Due to the quantization errors resulting from the digitization, the absolute value \ e ₂ ] of the second residual echo signal can only fall below a quantization value /. If this absolute value Ie 2 I is greater than /, the state of _{two-way talk is present, ie.} a duplex signal η is superimposed on the echo signal y. Since in this case the direct application of the algorithm according to equations (4) to (6) leads to an incorrect setting of the coefficients cj ²¹ and thus to an incorrect calculation of the echo signal y ⁱ²⁾ , the preliminary residual echo signal C ₁ and the original setting remain of the transversal filter coefficient cj ". For this purpose, the clock t _f , which is generated from the selection device K containing a comparator, controls the memories S2 and S3 and the input of the digital-to-analog converter 14 in such a way that, in the event of a contradiction, the provisional residual echo signal e _{t is used} for the final residual echo signal e and in the digital-to-analog converter 14 as the outgoing signal in the outgoing direction 11. For the next sampling time t + T , the content of the memory S3 is used to form the new preliminary artificial echo signal j? ⁽¹⁾ ( t + T) .

For this purpose 3 sheets of drawings 709 609/341

Claims (1)

Patent claims: 1. Echo canceller for a transmission system having a first and a second transmission direction, in which the signals of the first transmission direction, in particular at the end of the four-wire transmission system, are partially reflected and appear as echo signals in the signal of the second transmission direction, with a device for generating an artificial echo signal, which has two echo path models approximating the transmission properties of the echo path, a branching device common to the echo path models with a number of output signals corresponding to the number of adjustable coefficients of each of the two echo path models, with means for subtracting the artificial echo signal from the signals of the second transmission direction and with a selection device for selecting the echo path model used for generating the outgoing signal of the second transmission direction, characterized by means for performing the following ver Step-by-step steps after setting the coefficients (cf) of the first echo path model in cycles: