APPARATUS FOR REDUCING THE RISK OF UNDESIRABLE PARAMETERDRIFT OF AN ADAPTIVE FILTER USED FOR ECHO CANCELLATION

Abstract: Apparatus for reducing the risk of undesirable drifting of the parameters, so-called bursting, of an adaptive echo filter (15), which is coupled between a receiving branch (17) and a transmitting branch (16) included in a four-wire loop. A difference signal (E) which contains no echo from the receiving branch is generated in the transmitting branch in a known manner. The apparatus includes an adaptive filter (25) which is coupled between the transmitting branch and the receiving branch and which functions to generate a compensation signal from the difference signal (E) when transmitting narrowband signals from a near-end. The compensating signal is subtracted from a signal which appears in the receiving branch (17) and the filter is updated in a manner to minimize the correlation between the difference signal (E) and the input signal (X') of the echo filter. A relatively small filter (25) is sufficient to essentially eliminate this correlation when transmitting signals which, at maximum, consist of some few single sinusoidal tones.

Description: APPARATUS FOR REDUCING THE RISK OF UNDESIRABLE PARAMETERDRIFT O AN ADAPTIVE FILTER USED FOR ECHO CANCELLATION TECHNICAL FIELD The present invention relates to apparatus for reducing the ris of undesirable drifting of the parameters of an adaptive filte which is used for echo cancellation and which is coupled betwee a receiving branch and a transmitting branch of a four-wire loop wherein a difference signal is generated in the transmittin branch by subtracting a compensation signal from the filter fro a signal which is delivered to the transmitting branch via two/four-wire hybrid. TECHNICAL FIELD In telephony systems in which adaptive echo cancellation i effected with the aid of an adaptive filter in a two/four-wir hybrid in a four-wire loop, so-called bursting may occur unde certain operating conditions. This phenomenon occurs because th four-wire loop includes a feedback from the output of the adaptiv filter to its input, via the far-end of the loop when damping i low at the far-end. When transmitting narrowband signals from th near-end of the loop, a correlation is therefore found between th filter input signal and the difference signal formed on the outpu side of the filter, even when the settings of the filter parame ters are good, i.e. even when the filter has converged. Thi correlation can cause the filter parameters to drift in a manne which will result in the self-oscillation of the four-wire loop. When bursting occurs, the loop will self-oscillate over shor periods of time, while effectively cancelling echoes over longe intermediateperiods. The riskof self-oscillation isparticularl manifest when transmitting periodic signals, such as sinusoida tones, since a tone is strongly correlated with itself, even afte a delay. Tones are transmitted, for instance, from modems, facsimile apparatus and in conjunction with DTMF-signalling (Dual Tone Multi Frequency) . The bursting phenomenon and a method of reducing the risk o bursting are found described in: W.A. Sethares, CR. Johnson, Jr., C.E. Rohrs: "Bursting i Adaptive Hybrids"; IEEE Transactions on Communications, Vol. 37 No. 8, Aug. 1989, pp. 791-799; and Z. Ding, CR. Johnson, Jr., W.A. Sethares: "FREQUENCY-DEPENDEN BURSTING IN ADAPTIVE ECHO CANCELLATION AND ITS PREVENTION USIN DOUBLE-TALKDETECTORS", International Journal ofAdaptiveContro and Signal Processing, Vol. 4, pp. 219-236. The proposed method for reducing the risk of bursting uses a so called doubletalkdetector. A detector of this kind is intended t detect when signalling from a near-end subscriber is in progress, updating of the echo ilter parameters being interruptedwhen suc signalling is detected. It is difficult, however, to construct reliable doubletalk detector. It has been found that even a com paratively weak signal from a near-end subscriber is able t greatly influence the setting of the echo filter and lead to self- oscillation. Furthermore, there is a risk that self-oscillation which has commenced for some reason or other can be detected as doubletalk. This would result in interruption of the updating of the filter parameters, and-the self-oscillation would therefore persist, which is naturally unacceptable. DISCLOSURE OF THE INVENTION The object of the present invention is to provide apparatus which will reduce the risk of undesirable drifting of the parameters of an adaptive filter that is used in the aforesaid manner. Ac¬ cordingly, the inventive apparatus includes an adaptive filter which receives the difference signal in thetransmission branch as an input signal, and which, at least in conjunctionwithtransmit- ting narrowband signals from the near-end of the loop, forms a compensation signal which is subtracted froma signal that appears in the receiving branch. The filter is updated so as to minimize the correlation between the difference signal in the transmitting branch and the input signal of the adaptive echo filter. The filter used in the apparatus may be relatively small, while still considerably reducing the risk of undesirable drifting of the ech filter parameters. A relatively small filter is namely sufficien to reduce the correlation considerably when transmitting narrow band signals comprised, for instance, of one or more single tones whereas when transmitting broadband signals, such as speec signals for instance, the correlation is low already at th beginning, due to the delay that occurs in the echo path via th far-end of the loop. The invention is characterized by the features set forth in th following Claims. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in more detail with reference to the drawings, in which Figure 1 illustrates an arran¬ gement which includes a four-wire loop having a filter for adaptive echo cancellation; Figure 2 illustrates an arrangement of the kind shown in Figure 1 which is supplemented with apparatus in accordance with the invention; and Figure 3 illustrates a part of the arrangement shown in Figure 2 which also includes means for further improving the effect of the inventive apparatus. BEST MODES OF CARRYING OUT THE INVENTION Figure 1 illustrates an arrangement which includes a conventional four-wire loop having a digital filter for adaptive echo cancel¬ lation. The arrangement also includes a two-wire conductor 11 at the near-end of the loop and a further two-wire conductor 12 at the far-end of the loop. Each of the cables is connected to a respec¬ tive two/four-wire hybrid 13 and 14 respectively. A digital filter 15 is connected between a transmitter branch 16 and a receiver branch 17 at the hybrid 13. A compensation signal is generated by a filter input signal X. This compensation signal constitutes an echo evaluation and is subtracted in a subtraction device 18 from a signal which is delivered to the transmission branch from the near-end hybrid 13, therewith to form a difference signal E in the transmission branch. The filter parameters are updated in accordance with some known method in a manner to minimize th correlation between the filter input signal X and the differenc signal E. Those devices by means of which the filter is updated ar illustrated symbolically with an arrow 19. A certain delay is always found in each transmission direction The delays in both directions are represented in the Figure by tw respective delay devices 20 and 21. As will be evident from the aforegoing, the difference signal may be correlated with the input signal X of the echo filter whe damping in the far-end of the loop, i.e. in the hybrid 14, is poor. This can occur even when the filter has converged, by which i meant that the settings of the filter parameters are good. This correlation gives rise to undesirable drifting of the filte parameters, which may result in so-called bursting. When tone signalling, the self-oscillation that occurs with bursting will cause new tones to be generated, which are liable to result in erroneous information transmission. Figure 2 illustrates the arrangement according to Figure 1 supplemented with an example of apparatus according to the invention. Those devices which are included in the Figure 1 illus¬ tration have been identified in the Figure 2 illustration by the same reference signs as those used earlier. The supplementary apparatus includes an adaptive digital filter 25 which uses a so-called predictor and the input of which is connected to the transmission branch 16 downstream of the subtraction device 18 and which thus receives the difference signal E as an input signal. The output of the filter is connected to a minus input of a subtraction device 26 in the receiving branch 17. A difference signal X' is generated in the subtraction device 26 in the receiving branch by a signal which is received from the hybrid 14 and by a compensation signal formed in the filter. The arrow 27 symbolizes means for updating the filter parameters. The updating means 27 are updated in accordance with a kno method, in a manner to minimize the correlation between the inpu signal E of the predictor filter 25 and the difference signal X' The size of the filter is therewith conveniently adapted to enabl signals which are comprised of a limited number of sinusoida tones of unknown phase and amplitude to be essentially cancelle out. For instance, in order to enable a signal which consists o two tones to be cancelled out, the filter requires four filte parameters, i.e. a fourth order filter is required. Such a filte enables, among other things, double tones transmitted in con junction with so-called DTMF-signalling to be cancelled out. filter having only two parameters, i.e. a second order filter will suffice to cancel out single tones. It will be noted that the input signal E of the filter 25, e.g. on or two sinusoidal tones in accordancewith the aforegoing, derive from a near-end signal from the two-wire conductor 11, since an echo from signals which are received from the far-end 12 will b cancelled out with the aid of the echo filter 15 and the subtrac tion device 18. It will also be noted that the predictor filter 25 is connecte relatively close to the echo filter 15, i.e. relatively close t the near-end hybrid 13. It will be seen from the Figure that th delay devices 20 and 21, which represent the delays in the tw transmission directions, are located between the filter 25 and th far-end hybrid 14. It is not possible to cancel out broadban signals, such as speech signals, with a filter which is coupled i this way, since a large and unknown delay is induced in the four wire loop, via the far-end hybrid. When transmitting broadban signals from the near-end, the value of the filter parameters i zero, meaning that the filter will then disconnect itself. However, as will be evident from the aforegoing, broadband signal need not be cancelled out, since such signals are comparativel uncorrelated with themselves after a delay. It can be mentioned b way of example, however that a speech signal which has returned a an echo has low correlation with the "continued" speech signal transmitted a moment later. An echo canceller may be connected very close to a near-en hybrid. For instance, the echo canceller may be mounted on th same circuit board as the hybrid, this arrangement normally bein referred to as an adaptive hybrid or an adaptive balance. relatively small and simple echo filter will suffice in cases suc as these, since a very small delay, atmost, will occurbetween th hybrid and the filter. Echo cancellers may also be mounted at much greater distance from the near-end hybrid than in th aforedescribed case, for instance in the case of long-way connections. However, the distance to the far-end hybrid is much greater than the distance to the near-end hybrid even in cases such as these. Connections which are established via a satellite are examples of such connections. When the echo canceller is distanced from the near-end hybrid, the echo filter used must be larger than would otherwise be required, because a larger delay occurs. It should be mentioned that the invention can be applied in both of these cases, i.e. irrespective of where the echo canceller is mounted. The Figure 3 embodiment includes the arrangement illustrated in Figure 2, only a part of said arrangement being shown, and means for further improving the effect of the inventive apparatus. These means include a doubletalk detector 29. The detector includes a level-comparingmeanswhichfunctionsto comparethe signal levels of the signal X 1 in the receiving branch 17 with the signal in the transmitting branch 16. Doubletalk may be defined, for instance, as existing when the signal level in the receiving branch is less than 6 dB stronger than the signal level in the transmitting branch. Naturally, doubletalk detection can be performed in other ways than that described above. Dependingonwhether doubletalk is detected or not, i.e. depending on whether or not signalling is in progress from a near-end subscriber, a first or a second output signal is produced on an output of the level comparing means 29. The output signal is delivered to ameans 30 which generates two control signals. These control signals are delivered to the echo filter 15 and the predictor filter 25 respectively, the purposes of which are to control the speeds at which the filters are updating. This control can be effected so as not to interrupt updating of the parameters of the echo filter 15 when doubletalk is detected, since such interruption would result in a risk of self-oscillation. Instead, updating of the echo filter 15 is retarded, so that the predictor filter 15 is updated and converges more quickly than the echo filter. In those cases when a near-end signal is not detected, the updating rates of said filters is, instead, controlled so that the echo filter will be updated more quickly than the predictor filter. Changes in the filter updating rates can be effected by suitably changing the step size-used in the updating process in accordance with the control signals. It will be understood that the invention is not restricted to the described and illustrated embodiments thereof and that modifica- tions can be made within the scope of the following Claims.

Claims: CLAIMS 1. Apparatus for reducingthe risk of undesirable drifting of the parameters of an adaptive filter (15) which is used for echo cancellation and which is coupled between a receiving branch (17) and a transmitting branch (16) of a four-wire loop, wherein a difference signal (E) is generated in the transmitting branch by subtracting (18) a compensation signal from the filter from a signal which is delivered to the transmitting branch via a two/four-wire hybrid (13), c h a r a c t e r i z e d in that the apparatus includes an adaptive filter (25) which is coupled between the transmitting branch (16) and the receivingbranch (17) and which is intended to generate a second compensation signal from said difference signal (E) ; means (26) for producing a second difference signal (X 1 ) in the receiving branch, by subtractingthe second compensation signal from a signal which is received from a far-end; andmeans (27) for updating the parameters of this latter filter (25) in a manner to minimize the correlation between the difference signal (E) in the transmitting branch and the second difference signal (X 1 ) • 2. Apparatus according to Claim 1, cha ra c t er i z ed in that the filter (25) which is intended to generate a second compensation signal has a size which is sufficient to ensure that said correlation will be essentially non-existent when a signal delivered to the transmitting branch from a near-end consists at maximum of a few single sinusoidal tones. 3. Apparatus according to Claim 2, c ha r a c t e r i z e d in that the filter (25) which is intended to generate a second compensation signal is a fourth order filter, whereby said correlation will be essentially non-existent when a signal delivered to the transmitting branch from a near-end consists at maximum of two sinusoidal tones. 4. Apparatus according to any one of Claims 1-3, characte ¬ r i z ed in that said apparatus further includes means (29) for detecting signalling from a near-end subscriber; and means (30) for generating control signals which function to control the updating speeds of said two adaptive filters (15, 25) in a manner such that the filter (15) used for echo cancellation will be updated more slowly than the filter (25) which is intended to generate a second compensation signal when signalling from a near- end subscriber is detected, and vice versa.

Inventors: ANDRE TORE MIKAEL, ANDRE, TORE, MIKAEL
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1992
Main classification: H03H21
IPC codes: H03H15, H03H17, H03H21, H04B3