GB2109208A

GB2109208A - Improvements in or relating to interference cancellers

Info

Publication number: GB2109208A
Application number: GB08229957A
Authority: GB
Inventors: Dietrich Anselm Alsberg
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1981-10-23
Filing date: 1982-10-20
Publication date: 1983-05-25
Also published as: AU8965582A; SE8205841L; FR2515453A1; NL8204100A; KR840002178A; JPS5881345A; DE3238550A1; SE8205841D0

Abstract

An echo canceller arrangement performs a two-stage cancellation while employing only a single echo replica generating means (18). Initially the generating means (18) generates a preliminary echo replica signal (@1(k)) in response to the application by switching means (24) of the resulting preliminary echo cancelled signal (e1(k)) to the echo replica generating means. When the controller (26) deems that sufficient cancellation has taken place ie the preliminary echo cancelled signal is reduced to a level below that at which it might wrongly be identified as near end speech by detector 30, the switching means (24) is activated to provide a final echo cancelled signal (e2(k)) to the echo replica generating means to form the second echo replica signal y2(k), the final value of the preliminary echo replica signal @1(k) being stored in RAM (28). <IMAGE>

Description

SPECIFICATION Improvements in or relating to interference cancellers This invention relates to interference cancellers.

Echoes commonly occur in a communication system when electrical signals on a receive signal path meet an imperfectly matched impedance at a hybrid junction and are partially reflected back to the distant source over a transmit signal path. As a result, the reflected signal, or echo, is heard at the far end of the transmit path some time after the original signal has been transmitted. As the distance between the talking and the listening parties is increased, the echo takes longer to reach the talking party, and as a result, the echo becomes, at least subjectively, more annoying to the talking party.

An attempt is therefore generally made to control echoes. One echo controlling arrangement, disclosed in U.S. Patent 4,005,277, includes a speech signal operated device known as an echo suppressor. Typically, echo suppression involves some form of selective attenuation, performed in response to voice levels in the transmission path, so that the echo that would otherwise be returned to the talker is suppressed. An arrangement such as this is usually satisfactory for terrestrial communication paths in which the echo delay on the round-trip propagation time between the source of the signal and the return of the echo is not long. In communication paths via satellite links, however, the transmission delays are much longer and the echo is more disturbing and may even disrupt conversation by chopping the return signal during intervals where both parties are talking, i.e., double-talking.

On the other hand, rather than interrupt the outgoing path, another echo controlling arrangement, known as an echo canceller, typically synthesizes a replica signal of the echo signal and algebraically subtracts the estimate from the outgoing signal to obtain an echo cancelled signal. Most conventional echo cancellers, as for example the arrangement disclosed in U.S. Patent 3,499,999 synthesize the replica by using a tapped delay line with adjustable multipliers in an adaptive feedforward arrangement also called a transversal filter.

The multipliers are automatically adjusted by a control signal derived from the difference between the echo and the replica signal.

Ideally, the hybrid included in the 4-wire/2-wire interconnection between the signal paths and the transmitter/receiver would allow, at most, 6dB of the received signal to leak onto the transmit signal path. In practice, however, it has been found that a hybrid may be misconnected, or lacking its associated balancing network, thereby resulting in a network imbalance where a substantial portion of the received signal will leak through to the transmit signal path. Often, prior art echo cancellers will mistake this large echo signal for near-end speech and, therefore, recognizes this signal as near-end speech and hence, not cancel the echo signal.

According to this invention an interference canceller for substantially cancelling an interference signal of a first signal propagating along a transmit signal path of a near-end transmitter/receiver includes first means for subtracting an interference replica signal from the first signal to produce an interference cancelled signal, replica generating means responsive to an input signal on a receive signal path of the transmitter/receiver and the interference cancelled signal for producing the interference replica signal, a detector for inhibiting adjustment of the replica generating means when a signal from the transmit signal path is greater than a predetermined value, second means for subtracting a preliminary interference replica signal from the actual return interference signal from the transmitter/receiver to produce the first signal, and switching means for applying the first signal to the replica generating means when interference cancellation in the first signal is less than required whereby the replica generating means produces the preliminary interference replica signal, and for applying, instead, the interference cancelled signal to the replica generating means when interference cancellation in the first signal is at least as required whereby the replica generating means produces the interference replica signal.

In one embodiment of the present invention, an echo cancelling process is broken into two stages which provide a first preliminary cancellation and a second final cancellation, respectively. A near-end speech detector is responsive to the preliminary echo cancelled signal produced by the first stage, instead of the actual return echo signal as in the prior art arrangements. Thus, the near-end speech detector is less likely to mistake a large echo signal for near-end speech.

The invention makes judicious use of the fact that since only one echo canceller is adapting at any given time, many components of a two-stage echo canceller may be shared. Specifically, a single transversal filter and accumulator is capable of performing the adaptation of the echo cancelled signal associated with either stage of the echo canceller.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 illustrates a two-stage echo cancelling arrangement embodying the invention; and Figure 2 illustrates an alternative embodiment of a two-stage echo cancelling arrangement.

The following description and accompanying illustrations describe the invention in terms of an echo canceller arrangement disposed at the 2-wire/4-wire interconnection of a telephone circuit. Generally, however, the invention may be used to control any sort of analog or digital interference signal appearing in, for example, radar, satellite, sonar, radio, television, systems etc. Therefore, it will be understood that the description which follows hereinafter is exemplary only and for the purposes of exposition and not for the purposes of limitation since the present invention is applicable in any situation wherein an interfering signal is present.

In the ideal, an echo canceller could be constructed which uses only the correlation properties between an incoming signal x(t) originating at the far end and the echo return signal y(t) to control the adaptation process of an included transversal filter, where the correlation signal +(t) may be defined by

When using a correlation detector, the time required for the canceller to give a prescribed degree of cancellation will be determined by the incoming signal's bandwidth or bit rate, the resulting maximum sample rate of the incoming signal, the relative level of noise, and near-end speech. When using the correlation process, the adaptation time becomes excessive from a telephone user's point of view in the presence of high levels of noise and typical levels of near-end speech.Conventional prior art echo cancellers, therefore, use a near-end speech detector to inhibit the adaptation process when near-end speech is present.

However, these near-end speech detectors cannot distinguish high level echo signals from near-end speech, and hence, prior art echo cancellers will often function incorrectly in the presence of high level echo signals.

The present invention seeks to solve this problem by dividing the cancellation process into two stages. An exemplary arrangement which illustrates the interference-cancelling properties of the present invention, as it applies in the specific example of an echo canceller, is illustrated in Figure 1. A single transmission terminal is basically illustrated for interconnecting a near-end transmitter/receiver 10 with a receive signal path 12 and a transmit signal path 14 by way of a hybrid network 16. A balancing network 17 is connected to hybrid network 16 in order to match, as nearly as possible, the impedance of transmitter/receiver 10.

As illustrated in Figure 1, an echo canceller includes an echo replica generator 18 disposed between receive signal path 12 and transmit signal path 14, a first combiner 20, a second combiner 22, a switching means 24, and a controller 26. Echo replica generator 18 is responsive to the input signal x(k) approaching near-end transmitter/receiver 10 on receive signal path 12 and processes samples of input signal x(k) in accordance with equation (1 ) to form, as an output, an estimate of the echo signal that exists on transmit signal path 14, where this estimate is defined as preliminary echo replica signal ,(k). Echo replica generator 18, which in digital form may comprise a plurality of delay elements, multipliers, and integrators forming a transversal filter in association with an accumulator, is thereby capable of generating the impulse response of the signal applied as an input thereto and is described in greater detail in copending application No. (U.S.

application no. 314490).

From the output of echo replica generating means 18, preliminary echo replica signal y1(k) is subsequently applied as an input to switching means 24, which is set in the position shown in Figure 1 at the initiation of the process. Therefore, preliminary echo replica signal Y1 (k) is applied via switching means 24 as a first input to first combiner 20, where the other input to first combiner 20 is the actual return echo signal y(k) appearing at the output of hybrid 16. First combiner 20 subsequently subtracts preliminary echo replica signal y1 (k) from the actual return echo signal y(k) to form as an output signal the difference thereof, denoted preliminary echo cancelled signal e1(k).

Preliminary echo cancelled signal e1(k) propagates along transmit signal path 14 and is applied as an input to both second combiner 22 and switching means 24. Switching means 24, set in the position shown in Figure 1, functions to apply preliminary echo cancelled signal e1(k) as an input, along with input signal x(k), to echo replica generating means 18. In accordance with the operation of echo replica generator 18, preliminary echo cancelled signal e1(k) functions to adapt input signal x(k) to form an improved value for preliminary echo replica signal y1(k), where this adaptation process is described in greater detail in the above-cited copending application.

Preliminary echo cancelled signal e1(k), or alternatively preliminary echo replica signal y,(k) (as shown by a dotted line connection in Figure 1), is also applied as an input to controller 26, which is also responsive to input signal x(k). Controller 26 monitors the adaption process being performed by echo replica generator 18 by comparing either e1(k) or y1(k) to a predetermined value of x(k) and functions to alter the position of switching means 24 when either e1(k) or 91(k) exceed a predetermined threhold.That is, controller 26 functions in accordance with either the relation e1(k) > Ax(k) or y1(k) > Bx(k), where A or B are chosen so that preliminary echo cancelled signal e, (k) will comprise a sufficient cancellation of the actual return echo signal y(k) to allow a conventional near-end speech detector to function properly. It is to be understood that controller 26 may comprise any arrangement well-known in the art which is capable of performing threshold detection.

When the predetermined threshold is succeeded, controller 26 activates switching means 24, ending the first stage of cancellation by disconnecting preliminary echo cancelled signal e1(k) as an input to, and preliminary echo replica signal y1(k) as an output from, echo replica generating means 18. The final value of preliminary echo replica signal y1(k) is stored in a random access memory 28 and is continued to be applied as an input via switching means 24 to first combiner 20. Therefore, first combiner 20 will continue to produce preliminary echo cancelled signal e1 (k) which propagates along transmit signal path 14 towards second combiner 22.At the moment switching means 24 is activated to begin the second stage of cancellation, preliminary echo replica signal y1(k) is applied by echo replica generating means 18 via switching means 24 as a second input to second combiner 22. Second combiner 22 functions to subtract this echo replica signal from preliminary echo cancelled signal e1(k) to form final echo cancelled signal e2(k) which propagates along the remainder of transmit signal path 14 back to the far-end receiver/transmitter (not shown).

Final echo cancelled signal e2(k) is also applied via the alternate position of switching means 24 (shown by a dotted line in Figure 1), as the new input to echo replica generating means 18, replacing preliminary echo cancelled signal e(k). Echo replica generating means 18 thus adapts x(k) in relation to e2(k) to produce as an output final echo replica signal y2(k). Final echo replica signal y2(k) is subsequently applied via the alternate position of switching means 24 (shown by a dotted line in Figure 1), as the second input to second combiner 22 where it is subtracted from preliminary echo cancelled signal e1(k) to form an updated value for final echo cancelled signal e2(k).The above-described process then continues, with final echo cancelled signal e2(k) fed back to echo replica generating means 18 to continuously adapt final echo replica signal y2(k) for the duration of the presence of input signal x(k), or until the threshold requirement of controller 26 is no longer met, as will occur in the case of a slowly time-varying actual value of y(k). Specifically, if at any time during the adaptation of final echo cancelled signal e2(k) eithere1(k) becomes less than Ax(k) ory1(k) becomes less then Bx(k), controller 26 will reactivate switching means 24, thus reapplying preliminary echo cancelled signal e1 (k) as an input to echo replica generating means 18.As described hereinbefore, preliminary echo cancelled signal e1(k) will be adapted in association with the input signal x(k) until once again the predetermined threshold of controller 26 is exceeded and switching means 24 is activated so that final echo cancelled signal e2(k) is again applied as an input to echo replica generating means 18. Controller 26 will continue to monitor the value of either e(k) or y1(k) in the above-described manner and alter the input to and output from echo replica generating means 18 as necessary in accordance with the predetermined threshold value.

The absence of an input signal x(k), or alternatively, the presence of a near-end speech signal is detected by a near-end speech detector 30. The presence of near-end speech may be detected, for example, by comparing either preliminary echo replica or cancelled signal 31(k) or el(k) to either the input signal x(k) or the actual return echo signal y(k), using methods well-known in the art in present near-end speech detector arrangements and explained in greater detail in the above-cited copending application. For example, near-end speech detector30 may store N past values of the input signal x(k) and determine near-end speech exists when the present value of the preliminary echo cancelled signal is greater than one-half of the maximum value of the N stored values of the input signal.When near-end speech detector 30 does, in fact, recognize near-end speech, an inhibit signal is transmitted to echo replica generating means 18, thus disconnecting echo replica generating means 18 from transmit signal path 14, inhibiting any further change of the data stored in echo replica generating means 18, thereby allowing the near-end speech to travel unchanged to the far-end transmitter/receiver.

Another echo canceller arrangement is illustrated in Figure 2 in which controller 26 and near-end speech detector 30 are combined into a single detector/controller 32 which is capable of performing the same functions as the separate components contained in Figure 1. In particular, detector/controller 32 is responsive to the input signal x(k), preliminary echo cancelled signal e1(k), and in alternative operative modes, the actual return echo signal y(k) and preliminary echo replica signal y1(k). Functioning as a controller, detector/controller 32 compares preliminary echo cancelled signal e1(k) to input signal x(k) in the manner described hereinabove in association with Figure 1, and alters the position of switching means 24 in accordance with the relation e1(k) < Ax(k), where the threshold value A is chosen at the user's discretion.

Alternatively, detector/controller 32 may compare the input signal x(k) to preliminary echo replica signal y1(k) (shown by a dotted line connection in Figure 2) and activate switching means 24 in accordance with the threshold decision y1(k) < Bx(k). In another configuration, detector/controller 32 may function to determine the time derivative of preliminary echo cancelled signal e1(k) to form a rate of echo cancellation signal de1(k)/dt. Detector/controller 32 then compares echo cancellation signal de1(k)/dt to a predetermined constant C, and alters the position of switching means 24 in accordance with the relation del(k) < (2) dt The value of C is chosen such that when the rate of change of e1(k) drops below a certain value, sufficient preliminary echo cancellation is deemed to have taken place. In its capacity as a near-end speech detector, detector/controller 32 inhibits echo replica generating means 18 in accordance with any of the methods described hereinabove in association with Figure 1.

The present invention achieves a two-stage echo cancellation process while only using a single echo replica generating means, instead of using a separate echo replica generating means with each stage, a saving that allows the invention to be included on a single integrated circuit. Further, it is to be understood that the present invention is capable of being implemented in association with either digital or analog signals, where the only difference in construction is the arrangement internal to echo replica generating means.

Claims

1. An interference canceller for substantially cancelling an interference signal of a first signal propagating along a transmit signal path of a near-end transmitter/receiver, including first means for subtracting an interference replica signal from the first signal to produce an interference cancelled signal, replica generating means responsive to an input signal on a receive signal path of the transmitter/receiver and the interference cancelled signal for producing the interference replica signal, a detector for inhibiting adjustment of the replica generating means when a signal from the transmit signal path is greater than a predetermined value, second means for subtracting a preliminary interference replica signal from the actual return interference signal from the transmitter/receiver to produce the first signal, and switching means for applying the first signal to the replica generating means when interference cancellation in the first signal is less than required whereby the replica generating means produces the preliminary interference replica signal, and for applying, instead, the interference cancelled signal to the replica generating means when interference cancellation in the first signal is at least as required whereby the replica generating means produces the interference replica signal.

2. A canceller as claimed in claim 1, including storage means for storing a last-look value of the preliminary interference replica signal and for applying the last-look value to the second means when the interference cancelled signal is applied to the replica generating means.

3. A canceller as claimed in claim 2 wherein the storage means is a random access memory.

4. A canceller as claimed in claim 1,2 or 3 wherein the switching means is responsive to the input signal and the first signal for applying the first signal to the replica generating means when the first signal is less than a predetermined weighted value of the input signal, and for applying, instead, the interference cancelled signal to the replica generating means when the first signal is at least equal to the predetermined weighted value of the input signal.

5. A canceller as claimed in claim 1, 2 or 3 wherein the switching means is responsive to the input signal and the preliminary interference replica signal for applying the first signal to the replica generating means when the preliminary interference replica signal is less than a predetermined weighted value of the input signal, and for applying, instead, the interference cancelled signal to the replica generating means when the preliminary interference replica signal is at least equal to the predetermined weighted value of the input signal.

6. A canceller as claimed in claim 1,2 or 3 wherein the switching means is responsive to the first signal for determining the time derivative to form an interference cancelled signal rate, for applying the first signal to the replica generating means when the interference cancelled signal rate is greater than a predetermined value, and for applying, instead, the interference cancelled signal to the replica generating means when the interference cancelled signal rate is at most equal to the predetermined value.

7. A canceller as claimed in any preceding claim wherein the detector is responsive to the input signal and the preliminary interference replica signal.

8. A canceller as claimed in any one of claims 1 to 6 wherein the detector is responsive to the input signal and the first signal.

9. An interference canceller substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.