GB2109209A - Improvements in or relating to interference controllers and detectors for use therein - Google Patents

Abstract

An echo canceller with improved near-end speech detection consists of a transversal filter (12) which synthesizes from an input signal (x(k)) on a receive signal path (12) an estimate of an echo replica signal (@(k)). A combiner (20) subtracts the echo replica estimate signal from the actual return echo signal (y(k)) on transmit signal path (14) to generate a first echo cancelled signal e(k). The improved near-end speech detector (24) comprises a combiner (28) and a transversal filter (26) which operate to generate a second echo cancelled signal (ed(k)) which is employed by a near-end speech detector (22) which compares the echo cancelled signal to past and present values of the input signal x(k) to ascertain the presence of near-end speech and inhibit the echo canceller. <IMAGE>

Description

SPECIFICATION Improvements in or relating to interference controllers and detectors for use therein This invention relates to interference controllers and detectors for use therein.

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 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, which is 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, rou nd-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 cancellor, typically synthesizes a replica signal of the impulse response 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.Since the impulse response of an echo path may be rather long, accurate synthesis of the replica signal by a transversal filter may require many taps and associated multipliers, an arrangement which is complex and costly.

Also, speech from the near-end customer is generally troublesome for an echo canceller since the canceller will assume that such speech is the echo signal and attempt to model the speech, driving its estimate of the echo-path impulse response away from the true echo-path impulse response. Near-end speech detectors are presently applied to overcome this problem, inhibiting the echo-path impulse response modelling function of the echo canceller when the detector deems that near-end speech is present. One echo canceller arrangement which includes an active near-end speech detector is disclosed in U.S. patent 4,129,753 in which the near-end speech detector includes a memory and other active circuitry, where the memory stores the magnitudes of a number of past echo signal.In operation, the detector compares the magnitude of a current signal on the transmit signal path to the past echo signals stored in the memory and provides an inhibit signal to the echo-path modelling process when the comparison deems that the current signal is near-end speech and not an echo signal.

Although this arrangement overcomes the problem associated with other prior arrangements of falsely indicating near-end speech when the signal is in actuality a large magnitude echo signal, the near-end speech detector employed is quite extensive in design since in addition to requiring active circuitry, the accuracy of the arrangement is directly proportional to the size of the memory employed (i.e., the number of past echo magnitudes retained).

According to one aspect of this invention there is provided an interference controller for substantially cancelling an interference signal of an actual return signal propagating along a transmit signal path of a near-end transmitter/receiver, wherein in operation, an interference replica signal is subtracted from the return signal to produce an interference cancelled signal, the interference replica signal is produced in response to the interference cancelled signal and an input signal on a receive path of the transmitter/ receiver, a further interference replica signal is subtracted from the return signal to produce a further interference cancelled signal, the further interference replica signal is produced in response to the further interference cancelled signal and the input signal, and adjustment of the production of the interference replica signal is inhibited in dependence upon a predetermined relationship between the input signal and the further interference cancelled signal.

According to another aspect of this invention a detector for inhibiting adjustment of an interference controller in the presence of a near-end signal on a transmit signal path of a near-end transmitter/receiver, includes means for subtracting from an actual return signal propagating along the transmit signal path a detector interference replica signal to generate a detector interference cancelled signal, processing means for receiving the detector interference cancelled signal and an input signal propagating along a receive signal path of the transmitter/receiver, for storing a first plurality of M increasingly delayed sample values of the input signal, and for so combining the plurality of M values and the detector interference cancelled signal to generate the detector interference replica signal and control means for receiving the detector interference cancelled signal and the input signal, for storing a plurality of increasingly delayed sample values of the input signal, and for providing a signal for inhibiting adjustment of the interference controller in accordance with a predetermined relation between the detector interference cancelled signal and the plurality of increasingly delayed sample values.

In one embodiment the invention provides an echo canceller with improved near-end speech detector performance, and includes a near-end speech detector which avoids the high false detector rate of near-end speech associated with prior art arrangements.

The invention enables the provision of a near-end speech detector which is able to distinguish near-end speech from large magnitude echo signals by incorporating a second echo canceller into the near-end speech detector to form a correlation type near-end speech detector ahead of the active echo canceller. The echo canceller included in the near-end speech detector is located in front of a prior art speech detector arrangement and receives as an input the signal propagating along the transmit signal path. The near-end speech detector modifies this signal which is subsequently applied as an input to the prior art speech detector.

The invention will now be described by way of example with reference to the accompanying drawings, in which like references denote like parts and in which: Figure lisa block diagram of an exemplary prior art echo cancelling arrangement including a prior art near-end speech detector; Figure 2 is a block diagram of an exemplary echo canceller including an improved near-end speech detector embodying the invention; and Figures 3 and 4, which form Figure 5, illustrates an exemplary detailed embodiment of the echo cancelling arrangement of Figure 2.

Referring now to 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. Hybrid network 16 may generally include a balancing network (not shown) for impedance matching purposes. Ideally, all signals propagating on receive signal path 12 are only passed on by hybrid 16 to near-end transmitter/receiver 10, and signals from transmitter/receiver 10 are only passed by hybrid 16 onto transmit signal path 14.However, since impedance mismatches, cannot be prevented in the actual signal paths 12 and 14 connected to hybrid 16, a portion of the signal energy on receive path 12 appears on transmit path 14 and is returned to the far-end transmitter/receiver (not shown) as an echo in the absence of some sort of echo suppression or cancellation. Accordingly, echo cancelling apparatus, as illustrated in Figure 1, may be employed to eliminate the echo signal appearing on transmit signal path 14, which apparatus includes a near-end speech detector.

As shown in Figure 1, the echo canceller includes a transversal filter 18, a combiner 20 and a near-end speech detector 22. Transversal filter 18, which is also referred to in the art as a tapped delay line, receives and stores sequential samples of the signal x(k) approaching near-end transmitter/receiver 10 on receive signal path 12 and processes these samples to form as an output an estimate of the echo signal on transmit signal path 14 denoted echo replica signal y(k).

Combiner 20, which is disposed in transmit signal path 14, subtracts echo replica signal y(k) produced by transversal filter 18 from the actual return echo signal y(k) propagating along transmit signal path 14 to form a difference signal defined as an echo cancelled signal e(k). Echo cancelled signal e(k) is thereafter propagated along the remainder of transmit signal path 14to be received by a far-end transmitter/receiver (not shown). This same echo cancelled signal is fed back to transversal filter 18 to supply transversal filter 18 with the information necessary to improve the estimate of the echo signal, echo replica signal y(k), produced by transversal filter 18. Under ideal conditions, y(k) would be able to model y(k) exactly, echo cancelled signal e(k) would be equal to zero, and the far-end transmitter/receiver would receive no echo signal.

However, in actual practice some residual echo cancelled signal will always be returned to the far-end.

Near-end speech that may be present on transit signal path 14 will typically be stronger than the echo and is unwanted noise as far as transversal filter 18 is concerned. If transversal filter 18 were to continue operating during the presence of near-end speech, echo replica signal y(k) would diverge from the actual return echo signal y(k) and, instead, attempt to model the near-end speech signal. To alleviate this problem, near-end speech detector 22 is included to inhibit the continued undating operation of transversal filter 18 in the presence of near-end speech. As shown, near-end speech detector 22 is responsive to both the input signal x(k) and the actual return echo signal y(k), and by employing an algorithm relating these two signals, will inhibit transversal filter 18 when it deems that near-end speech is present.

One exemplary speech detection algorithm known in the art that may be implemented by near-end speech detector 22 is y(k) > 1/2max {x(k),x(k-1),..., x(k-(N-1))}, (1) where in accordance with equation (1), near-end speech detector 22 would include a transversal filter similar to transversal filter 18 for generating the sequence (x(k), ... x(k-(N - 1 ))} and a comparator for separately comparing the signal y(k) to each of the stored values of the input signal and generating an inhibit signal when y(k) is greater than one-half the value of the maximum of the stored values of the sequence.When the inequality of equation (1) is satisfied, near-end speech detector 22 sends an "i 1inhibit" signal to transversal filter 18, where transversal filter 18 will suspend operation until the inequality of equation (1) is no longer satisfied, and near-end speech detector 22 will transmit an "enable" signal to transversal filter 18 to reactivate the updating process of transversal filter 18 when the inequality of equation (1) is no longer satisfied. The factor of 1/2 in equation (1) is based on the assumption that there will be at least a 6 dB loss of signal through hybrid 16 from receive signal path 12 to transmit signal path 14. A problem arises in this arrangement, however, when the initial return echo signal y(k) is large, since near-end speech detector 22 may falsely inhibit the operation of transversal filter 18.

Referring now to Figure 2, a transversal filter 18 receives and stores samples of the input signal x(k) and synthesizes therefrom an echo replica signal y(k). Combiner 20 subsequently subtracts this signal from the actual return echo signal y(k) to form an echo cancelled signal e(k) which is thereafter propagated along the remainder of transmit signal path 14to a far-end transmitter/receiver (not shown). As indicated in Figure 2, a near-end speech detector 24 includes a transversal filter 26 and a combiner 28 in addition to prior art near-end speech detector 22.Transversal filter 26 performs in a similar manner as above-described transversal filter 18, receiving and storing sequential samples of the input signal x(k) and synthesizing therefrom a speech detector echo replica signal yd(k). Combiner 28, which is not contained in, and hence will not corrupt the signal propagating along transmit signal path 14, subtracts this speech detector replica signal 9d(k) from the actual return echo signal y(k) on transmit signal path 14 which leaks through hybrid 16 to form a speech detector echo cancelled signal ed(k). Speech detector echo cancelled signal ed(k) is subsequently fed back to transversal filter 26 to supply transversal filter 26 with the information necessary to improve the estimate thereformed of the echo path.Also, speech detector echo cancelled signal ed(k) produced by combiner 28 is applied as an input to prior art near-end speech detector 22, which is also responsive to input signal x(k). Prior art near-end speech detector 22 will recognize near-end speech and inhibit transversal filter 18 in accordance with the equation ed(k) > 1/2max (x(k),x(k-l), ..., x(k-(N-1))}. (2) In particular, when speech detector echo cancelled signal ed(k) becomes greater than one-half of the largest individual delayed value of the input signal stored in prior art near-end speech detector 22, near-end speech is deemed to exist and transversal filter 18 is inhibited from continued updating for the entire length of time that the inequality of equation (2) is satisfied.

As can be seen by comparing equations (1) and (2), the present invention differs from the prior art by employing an echo cancelled signal ed(k), instead of the actual return echo signal y(k), in conjunction with prior art near-end speech detector 22. Therefore, near-end speech detector 24, by employing elk), is less likely to mistake a large magnitude echo signal for near-end speech and incorrectly inhibit transversal filter 18 than a prior art near-end speech detector employing the signal y(k).

An exemplary detailed embodiment of the present invention is illustrated in Figures 3 and 4, which form Figure 5, and show the tapped delay line structure of both transversal filter 18 and near-end speech detector transversal filter 26. As shown in Figure 3, transversal filter 18 includes a plurality of (N-I) delay elements denoted 301-30N--lr N multipliers 321 -32N, N tap weight generators 341-341\1, and an accumulator 36.The signal travelling along receive path 12 is introduced into transversal filter 13 and prnpagates through the (N-l) delay elements 30i 30N-1 to form, in association with the present input signal value x(k), an N-length sequence comprising the values {x(k), x(k-1), ..., ,x(k-(N -1 ))) , as shown in Figure 3.

As described hereinabove in association with Figures 1 and 2, echo cancelled signal e(k) is fed back to transversal filter 18 to assist in updating echo replica signal y(k) so as to improve the modelling of the actual return echo signal y(k). Echo cancelled signal e(k) is not, however, suitable by itself for improving echo replica signal y(k). Accordingly, echo cancelled signal e(k) is passed through the plurality of N tap weight generators 341 -34N which function individually to form a plurality of N tap weight values ho(k) to hN-l(k)r where these values may be employed in connection with the plurality of N sampled and delayed input signal values x(k) to x(k-(N- )) to form echo replica signal y(k) after their addition in accumulator 36.As illustrated in detail in association with tap weight generator 341, an exemplary tap weight value ho(k) is formed from echo cancelled signal e(k) by multiplying echo cancelled signal e(k) with its associated input signal sample value x(k) in a multiplier 35a, and the resultant composite signal is averaged in an integrating network 371 to produce tap weight value ho(k), whose polarity and magnitude indicate the appropriate correction necessary based on the input signal sample value x(k).Each of these tap weight values ho(k)-hN1(k) produced by generators 341-34N, respectively, is subsequently multiplied with its associated delayed input signal value X(k)--XN-l(k) in its associated multiplier 321-32N. More particularly, tap weight value ho(k) is multiplied with inputsamplex(k) via multiplier 321, h1(k) with x(k-1) via multiplier 322, and soon, with tap weight value hN--l(k) multiplied with input sample x(k-(N-1)) via multiplier 32N The N outputs of multipliers 3232N are subsequently applied as separate inputs to accumulator 36 which sums the N products ho(k)x(k) to hN1 (k)x(k-(N- 1)) to produce the echo replica signal y(k) as the output of transversal filter 18. As stated hereinabove in association with Figure 2, echo replica signal y(k) is subtracted from the actual return echo signal y(k) in combiner 20 to form subsequently updated values of echo cancelled signal e(k) as the output of the echo canceller which is thereafter transmitted to the far-end.

A detailed embodiment of near-end speech detector 24 is included in Figure 4 in which transversal filter 26 comprises similar components as transversal filter 18, including a plurality of (M-1) delay elements 401-40,~1, M multipliers 42i 42M M tap weight generators 441 -44M (where only tap weight generator 441, including a multiplier 411 and an integrating network431, is illustrated in detail in Figure4to avoid complicating the illustration), and an accumulator 46.In operation, the input signal x(k) propagating along receive signal path 12 is introduced to transversal filter 26 and propagates through delay elements 401-40M-1 to form, in association with the present value of the input signal x(k), an M-length sequence (x(k),x,k-l), ..., x(k-(M- 1 ))}.Tap weight generators 441-44M, in response to speech detector echo cancelled signal ed(k) produced by near-end speech detector combiner 28 and through the same process as described hereinabove in association with tap weight generator 341, produce a plurality of M speech detector tap weight values jo(k) to jM-1 (k). Each speech detector tap weight value is subsequently mu Itiplied with its associated delayed value of the input signal in its associated multiplier 42i 42M More particularly, speech detector tap weightvalueJO(k) is multiplied with input sample value x(k) via multiplier42" jl(k) with x(k-1) via multiplier 422 and so on, with speech detector tap weight value iM-l(k) being multiplied with delayed input value x(k-(M-1)) via multiplier 42M The M outputs of multipliers 42i 42M are subsequently applied as separate inputs to accumulator 46 which sums the M products jo(k)x(k) to IM-1 (k)x(k-(M- 1)) to produce the output signal of transversal filter 26 corresponding to near-end speech detector echo replica signal y'(k). As discussed hereinbefore in association with Figure 2, speech detector echo replica signal y(k)is subtracted from the actual return echo signal y(k) in combiner 28 to form the signal applied as an input to prior art near-end speech detector 22, i.e.

speeched detector echo cancelled signal ed(k).

As discussed hereinabove in association with Figure 2, near-end speech detector echo cancelled signal ed(k) is fed back to transversal filter 26 and applied as an input to prior art near-end speech detector 22. In association with transversal filter 26, speech detector echo cancelled signal ed(k) is applied as an input to each tap weight generator441-44M, allowing tap weight values jo(k)-jN1-l(k) to be continuously updated, thereby allowing speech detector echo replica signal y(k) to continuously converge toward the actual return echo signal y(k). As discussed hereinabove in association with Figure 2, speech detector echo cancelled signal ed(k), as an input to prior art near-end speech detector 22 and in association with equation (2), allows near-end speech detector 24 better to distinguish between large magnitude echo and near-end speech.

Therefore, near-end speech detector 24, by achieving a lower false detection rate of near-end speech than prior art arrangements, will less often incorrectly inhibit the operation of transversal filter 18.

Claims (9)

1. An interference controller for substantially cancelling an interference signal of an actual return signal propagating along a transmit signal path of a near-end transmitter/receiver, wherein, in operation, an interference replica signal is subtracted from the return signal to produce an interference cancelled signal, the interference replica signal is produced in response to the interference cancelled signal and an input signal on a receive path of the transmitter/receiver, a further interference replica signal is subtracted from the return signal to produce a further interference cancelled signal, the further interference replica signal is produced in response to the further interference cancelled signal and the input signal, and adjustment of the production ofthe interference replica signal is inhibited in dependence upon a predetermined relationship between the input signal and the further interference cancelled signal.

2. A controller as claimed in claim 1, including means for subtracting the interference replica signal from the return signal to generate the interference cancelled signal, first processing means for receiving the interference cancelled signal and the input signal for storing a plurality of N increasingly delayed sample values of the input signal, and for so combining the plurality of values with the interference cancelled signal to generate the interference replica signal, and a detector including means for subtracting the further interference replica signal from the return signal to generate the further interferance cancelled signal, second processing means for receiving the further interference cancelled signal and the input signal, for storing a plurality of M increasingly delayed sample values of the input signal, and for so combining the plurality of M values and the further interference cancelled signal to generate the further interference replica signal, and control means for receiving the further interference cancelled signal and the input signal, for storing a plurality of increasingly delayed sample values of the input signal, and for locking the processing means in accordance with a predetermined relation between the further interference cancelled signal and the plurality of increasingly delayed sample values.

3. A controller as claimed in claim 2 wherein the second processing means includes a plurality of (M-1 ) delay elements forming a transversal filter which is responsive to the input signal for generating the plurality of M increasingly delayed sample values of the input signal, a plurality of M tap weight generators, each being responsive to the further interference cancelled signal for generating a respective one of a plurality of M tap weight values, a plurality of M multipliers, each being responsive to a respective one of the M increasingly delayed sample values and a respective one of the M tap weight values, for generating respective products of those values, and means for summing the M products to generate the further interference replica signal.

4. A controller as claimed in claim 2 or 3 wherein the control means serve to lock the first processing means when the further interference cancelled signal is greater than one-half of a maximum sample value of the plurality of increasingly delayed sample values of the input signal.

5. A detector for inhibiting adjustment of an interference controller in the presence of a near-end signal on a transmit signal path of a near-end transmitter/receiver, including means for subtracting from an actual return signal propagating along the transmit signal path a detector interference replica signal to generate a detector interference cancelled signal, processing means for receiving the detector interference cancelled signal and an input signal propagating along a receive signal path of the transmitter/receiver, for storing a first plurality of M increasingly delayed sample values of the input signal, and for so combining the plurality of M values and the detector interference cancelled signal to generate the detector interference replica signal and control means for receiving the detector interference cancelled signal and the input signal, for storing a plurality of increasingly delayed sample values of the input signal, and for providing a signal for inhibiting adjustment of the interference controller in accordance with a predetermined relation between the detector interference cancelled signal and the plurality of increasingly delayed sample values.

6. A detector as claimed in claim 5 wherein the processing means includes a plurality of (M- 1 ) delay elements forming a transversal filter which is responsive to the input signal for generating the plurality of M increasingly delayed sample values of the input signal, a plurality of M tap weight generators each being responsive to the detector interference cancelled signal for generating a respective one of a plurality of M tap weight values, a plurality of M multipliers, each being responsive to a respective one of the plurality of M increasingly delayed sample values and a respective one of the plurality of M tap weight values, for generating respective products of those values, and means for summing the M products to generate the interference replica signal.

7. A detector as claimed in claim 5 or 6 wherein the control means serves to provide a signal to inhibit adjustment of the interference controller when the detector interference cancelled signal is greater than one-half of a maximum sample value of the plurality of the increasingly delayed sample values of the input signal.

8. An interference controller substantially as herein described with reference to Figure 2 or to Figures 3, 4 and 5 of the accompanying drawings.

9. A detector substantially as herein described with reference to Figure 2 or 4 of the accompanying drawings.