Echo cancelling arrangement

In an echo canceller, an estimated echo signal is produced by an adaptive filter from a receive path signal and is subtracted from an incoming transmit path signal to produce an outgoing transmit path signal. Averages of signal levels on the receive path and the incoming and outgoing transmit paths are produced by exponential averaging. Adaptation of the adaptive filter is inhibited after initial convergence. Adaptation is enabled again in response to an echo attenuation parameter being less than a first threshold value and a variation of an echo attenuation parameter being less than a second threshold value, this situation representing a detected echo path change. The detection of an echo path change is inhibited in the presence of a near end signal. The echo attenuation parameters can be either or both of the ERLE (echo return loss enhancement) due to the subtraction of the estimated echo signal or the combined attenuation of the ERLE plus the ERL (echo return loss) of the hybrid circuit, these parameters being determined from averaged signal levels on the signal paths.

This invention relates to echo cancelling arrangements, and is particularly 
concerned with the detection of echo path changes in an echo cancelling 
arrangement. 
BACKGROUND OF THE INVENTION 
As is well known, four-wire and two-wire communications paths are commonly 
coupled by a hybrid circuit. Due to imperfect balancing of the hybrid 
circuit, a component of a signal incoming on the four-wire receive path, 
referred to as an echo signal, is inevitably but undesirably coupled to 
the four-wire transmit path, with an attenuation which is referred to as 
the echo return loss, or ERL. 
In order to cancel the echo signal, it is well known to provide an echo 
canceller which comprises an adaptive filter and a subtraction unit. The 
adaptive filter is supplied with the signal incoming on the four-wire 
receive path and produces an estimated echo signal, which the subtraction 
unit subtracts from the signal incoming on the four-wire transmit path to 
produce an outgoing four-wire transmit path signal which includes a 
residual echo signal. The adaptive filter is adapted in dependence upon 
the residual echo signal in a manner which seeks to reduce the residual 
echo signal to zero, i.e. in such a manner that the estimated echo signal 
corresponds exactly to the actual echo signal coupled via the hybrid 
circuit. 
Two echo attenuation parameters which are associated with an echo 
cancelling arrangement are the echo return loss enhancement or ERLE, and 
the combined attenuation Acom. The ERLE is the degree to which the echo 
canceller suppresses the echo signal, i.e. the ratio of the echo signal to 
the residual echo signal. The combined attenuation is the sum of the ERL 
and the ERLE, and thus is the total attenuation of a received signal from 
the receive path to the outgoing transmit path. 
As is also well known, it is necessary to inhibit adaptation of the 
adaptive filter whenever there is a so-called near end signal present, 
such a signal being coupled from the two-wire path to the four-wire 
transmit path, because such a signal constitutes noise as far as the 
convergence of the echo canceller is concerned. It is therefore common to 
provide a so-called double-talk detector which detects the presence of a 
near-end signal, or simultaneously-occurring signals in both directions of 
transmission, adaptation of the adaptive filter being inhibited in 
response to such detection. 
A double-talk detector typically monitors the average signal levels on the 
four-wire receive path and on the four-wire transmit path prior to the 
subtraction unit, and determines a double-talk condition, i.e. that a near 
end signal is present, if the latter average signal level exceeds the 
former average signal level reduced by the ERL, which is typically assumed 
to be a fixed value of 6dB. 
A problem with known echo cancelling arrangements is that double-talk 
detection is difficult and not instantaneous. Consequently, there are 
periods of time during which adaptation of the adaptive filter occurs even 
in the presence of double-talk. As a result, the adaptive filter 
coefficients initially diverge from their desired, or converged, values, 
are then frozen during continued double-talk when this has been detected, 
and re-converge when the near end signal is no longer present. This 
variation in the adaptive filter response has the disadvantage of 
producing audible and undesired echo bursts at the start and end of 
double-talk situations. 
In order to improve echo canceller performance, Horna U.S. Pat. No. 
4,360,712 issued Nov. 23, 1982 and entitled "Double Talk Detector For Echo 
Cancellers" describes an arrangement using three double-talk detectors in 
combination with an echo canceller having an adaptive filter and a center 
clipper. Two of the double-talk detectors are used in the presence of 
double-talk to selectively freeze the adaptive filter correction loop and 
to disable the center clipper, and the third double-talk detector is used 
to detect the initial adaptive period of the echo canceller. This patent 
does not address the problem and disadvantage described above. 
In addition, Takatori U.S. Pat. No. 5,153,875 issued Oct. 6, 1992 and 
entitled "Adaptive Balancing Network" describes an arrangement in which 
the coefficients of an adaptive filter of an echo canceller are frozen for 
the remainder of a connection following an initial adaptation period. Such 
an arrangement does not permit any re-convergence of the echo canceller in 
response to an echo path change which may occur during the connection, for 
example in response to a change in the hook state of an extension 
telephone. 
An object of this invention is to provide a method of detecting echo path 
changes, and an echo cancelling arrangement, in which the above 
disadvantages of known echo cancelling arrangements are avoided or 
reduced. 
SUMMARY OF THE INVENTION 
According to one aspect of this invention there is provided a method of 
detecting echo path changes in an echo cancelling arrangement in which an 
estimated echo signal is produced from a receive path signal and is 
subtracted from an incoming transmit path signal to produce an outgoing 
transmit path signal, comprising the steps of: monitoring signal levels on 
said paths; determining from the monitored signal levels when a first echo 
attenuation parameter of the echo cancelling arrangement is less than a 
first threshold value; determining from the monitored signal levels when a 
variation of a second echo attenuation parameter of the echo cancelling 
arrangement is less than a second threshold value; and detecting that an 
echo path has changed in response to determinations that the first echo 
attenuation parameter is less than the first threshold value and that the 
variation of the second echo attenuation parameter is less than the second 
threshold value. 
Preferably at least one of the first and second echo attenuation parameters 
comprises combined attenuation from the receive path to the outgoing 
transmit path and is determined from the monitored signal levels on the 
receive path and the outgoing transmit path. Instead, or in addition, at 
least one of the first and second echo attenuation parameters can comprise 
ERLE (echo return loss enhancement) due to the subtraction of the 
estimated echo signal and is determined from the monitored signal levels 
on the incoming and outgoing transmit paths. Thus the first and second 
echo attenuation parameters can be the same or different, and either of 
them can be the combined attenuation Acom or the ERLE. 
Conveniently, the step of determining from the monitored signal levels when 
a variation of the second echo attenuation parameter is less than a second 
threshold value comprises forming a ratio of a maximum value to a minimum 
value of the second echo attenuation parameter. The step of monitoring 
signal levels on said paths preferably includes averaging signal levels on 
the receive path and the incoming and outgoing transmit paths, desirably 
using exponential averaging. 
In order to avoid any erroneous detection that an echo path has changed, in 
the presence of a strong near end signal which can result in a small 
variation of the second attenuation parameter, the method preferably 
further comprises the step of inhibiting detection that an echo path has 
changed in response to a difference between the monitored signal levels on 
the receive and incoming transmit paths being less than a predetermined 
amount, for example 6dB corresponding to a minimum value of ERL. 
According to another aspect of this invention there is provided a method of 
controlling an echo cancelling arrangement in which an estimated echo 
signal is produced by an adaptive filter from a signal on a receive path 
and is subtracted from a signal on an incoming transmit path to produce a 
signal on an outgoing transmit path, comprising the steps of: monitoring 
convergence of the echo canceller; inhibiting adaptation of the adaptive 
filter in response to convergence of the echo canceller to a predetermined 
extent; and subsequently enabling adaptation of the adaptive filter in 
response to a first echo attenuation parameter of the echo cancelling 
arrangement being less than a first threshold value and a variation of a 
second echo attenuation parameter of the echo cancelling arrangement being 
less than a second threshold value. 
Thus, in accordance with this invention, variation in either the ERLE or 
the combined attenuation Acom, which is the sum of ERL and ERLE, is used 
together with the measured combined attenuation Acom or ERLE to determine 
when there is an echo path change, as distinct from a double-talk 
situation. As double-talk situations occur far more commonly than echo 
path changes, and adaptation of the adaptive filter in response to these 
gives rise to the undesired echo bursts discussed in relation to the prior 
art, adaptation of the adaptive filter is inhibited except for initial 
convergence of the echo canceller and in response to detection of an echo 
path change. The convergence of the echo canceller can also be monitored 
from either the ERLE or the combined attenuation Acorn. 
The invention also extends to an echo cancelling arrangement for coupling 
to a four-wire communications path which is coupled via a hybrid circuit 
to a two-wire communications path, for cancelling an echo signal coupled 
via the hybrid circuit from a receive path of the four-wire path to a 
transmit path of the four-wire path, the arrangement comprising: an 
adaptive filter having an input coupled to the receive path and an output 
for an estimated echo signal; a subtraction unit in the transmit path for 
subtracting the estimated echo signal from a signal on the transmit path; 
and apparatus for determining variation of a ratio, of signal levels on 
the receive path or the transmit path before the subtraction unit to 
signal levels on the transmit path after the subtraction unit, and for 
comparing the variation with a threshold value for use in controlling 
adaptation of the adaptive filter. 
The arrangement preferably includes: averaging apparatus for producing 
averages Ra, Sa, and Ta of signal levels on the receive path and on the 
transmit path before and after the subtraction unit, respectively; 
apparatus for determining at least one of ratios Sa/Ta and Ra/Ta; and 
apparatus for inhibiting adaptation of the adaptive filter in response to 
one of said ratios being greater than a predetermined amount; wherein the 
apparatus for determining and for comparing the variation is arranged for 
enabling adaptation of the adaptive filter in response to one of said 
ratios being less than a first threshold value and the variation of one of 
said ratios being less than a second threshold value. The apparatus for 
determining variation conveniently comprises apparatus for forming a ratio 
of a maximum value to a minimum value of one of said ratios for comparison 
with the second threshold value. 
The arrangement preferably further includes a detector for inhibiting 
adaptation of the adaptive filter in response to a difference between the 
signal levels on the receive path and on the transmit path before the 
subtraction unit being less than a predetermined amount. 
Preferably the apparatus for determining variation of a ratio is arranged 
to determine variation of a ratio of signal levels on the receive path to 
signal levels on the transmit path after the subtraction unit.

DETAILED DESCRIPTION 
Referring to FIG. 1, a four-wire communications path, comprising a receive 
path 10 and a transmit path 12, is coupled to a bidirectional two-wire 
communications path 14 via a hybrid circuit 16. An echo cancelling 
arrangement includes an adaptive filter 18, an adaptation control unit 19, 
and a subtraction unit 20. A far end signal, typically a voice signal, 
incoming on the receive path 10 as a signal R is supplied to the hybrid 
circuit 16, to be coupled to the two-wire path 14, and to an input of the 
adaptive filter 18. Near end signals incoming via the two-wire path 14, 
and an echo signal which is undesirably coupled via the hybrid circuit and 
corresponds to the signal R attenuated by the echo return loss ERL, are 
coupled from the hybrid circuit 16 as a signal S via an incoming part of 
the four-wire transmit path to a positive input of the subtraction unit 
20. A negative, or subtraction, input of the subtraction unit is supplied 
with an output of the adaptive filter 18, and an output of the subtraction 
unit 20 is coupled as a signal T to an outgoing part of the four-wire 
transmit path for transmission to the far end of the four-wire path. When 
enabled by the control unit 19, the adaptive filter 18 is adapted, i.e. 
filter coefficients therein are updated, in dependence upon a residual 
echo signal which is fed back to the adaptive filter from the output of 
the subtraction unit 20, in a manner which is intended to maximize the 
echo return loss enhancement ERLE provided by the echo cancelling 
arrangement Maximizing the ERLE also maximizes the combined attenuation 
Acom of a far end signal from the receive path 10 to the transmit path 12 
due to the ERL and the ERLE. 
As explained in the introduction, adaptation of the adaptive filter 18 must 
be avoided when there is a near end signal present, because such a signal 
constitutes noise as far as the convergence of the echo canceller is 
concerned. Usually a double-talk detector is used to detect a near end 
signal and to inhibit adaptation of the adaptive filter if the average 
level of the signal S on the incoming part of the four-wire transmit path 
exceeds the average level of the signal R on the four-wire receive path 
minus the ERL. Thus it is usual for adaptation of the adaptive filter 10 
to be enabled except when a near end signal is detected by the double-talk 
detector, so that the echo canceller can re-converge in response to any 
echo path changes which may occur. 
The present echo canceller arrangement departs from this usual arrangement 
in that, after initial convergence of the echo canceller, adaptation of 
the adaptive filter 18 is inhibited and is only enabled again by the 
control unit 19 in response to an echo path change being detected in the 
absence of a near end signal. This difference is based on a recognition 
that changes in the impulse response of the echo path are relatively rare 
compared with the frequency of double-talk situations, which typically 
occur for about 20% of the time during telephone calls. To this end, the 
control unit 19 is supplied with a value Ac and a signal EPC which are 
described below. 
Thus in the present echo canceller arrangement, when a call or telephone 
connection is initially established, adaptation of the adaptive filter 18 
is enabled by the control unit 19 until the echo canceller has converged 
to a desired extent in which the echoes are substantially eliminated. The 
control unit 19 then inhibits adaptation of the adaptive filter 18, i.e. 
the filter coefficients are frozen, unless and until a change in the echo 
path is detected as described below. This prevents any change in the 
filter coefficients, and resulting divergence and re-convergence of the 
echo canceller with consequent audible echo bursts or convergence noise as 
in the prior art, in response to the starting and ending of double-talk 
situations which may commonly occur. If a change in the echo path is 
detected, then adaptation of the adaptive filter 18 is again permitted 
until the echo canceller has again converged to the desired extent, when 
the filter coefficients are again frozen. 
As is well known, the extent to which the echo canceller has converged can 
be determined by monitoring either of the two echo attenuation parameters 
of the echo cancelling arrangement already referred to, namely the 
combined attenuation Acorn or the ERLE. In this preferred embodiment of 
the invention, the value Ac which is supplied to the control unit 19 
represents the combined attenuation Acorn as described below. On initial 
establishment of a call or telephone connection, the control unit 19 
enables adaptation of the adaptive filter 18 only until the value Ac is 
greater than a predetermined amount, corresponding to convergence of the 
echo canceller to a predetermined extent. 
A change in either the combined attenuation Acorn or the ERLE can be used 
to determine a degradation in echo cancellation; however, it is necessary 
to distinguish between a change corresponding to a degradation in echo 
cancellation due to a change in the echo path response, and a change due 
to double-talk situations. The remaining parts of the arrangement as shown 
in FIG. 1 serve to provide the signal EPC which reliably represents an 
echo path change, as distinct from a double-talk situation, and which is 
used to enable adaptation of the adaptive filter 18 after its coefficients 
have been frozen. In the preferred embodiment of the invention shown in 
FIG. 1, the signal EPC is produced in dependence upon variation in the 
ERLE when the combined attenuation Acorn is less than a threshold value 
and in the absence of a near end signal as detected by a conventional 
double-talk detector. 
As shown in FIG. 1, the levels of the signals R, S, and T are averaged by 
averagers 22, 24, and 26 respectively to produce average signal levels Ra, 
Sa, and Ta respectively. A calculator 28 calculates the value of the 
combined attenuation Acorn, which constitutes the value Ac, from the 
average signal levels Ra and Ta in accordance with the equation Ac=Ra/Ta. 
The calculated value Ac is supplied to the control unit 19 as described 
above, and is also supplied to a comparator 30 which compares the value Ac 
with a first threshold value At, and produces a control signal Cs in the 
event that Ac&lt;At. The threshold value At represents a value of combined 
attenuation Acorn which corresponds to non-convergence of the echo 
canceller, or to a double-talk situation. 
Another calculator 32 calculates the value Ea of the ERLE from the average 
signal levels Sa and Ta in accordance with the equation Ea=Sa/Ta. A 
further calculator 34 produces, as described further below, a value Ve 
which represents variation of the value Ea of the ERLE. A comparator 36 
compares this value Ve with a second threshold value Vt, and produces a 
control signal Vc in the event that Ve&lt;Vt. In the presence of both of the 
signals Cs and Vc, a counter 38 is incremented to produce the signal EPC 
in response to a predetermined maximum count being reached. 
A conventional near end signal or double-talk detector (NE/DT DET.) 39 is 
supplied with the average signal levels Ra and Sa and with a minimum value 
Em of the ERL, for example representing a minimum ERL value of 6 dB, and 
produces an output signal when Sa&gt;Ra-Em (or equivalently, when the 
difference Ra-Sa is less than the predetermined minimum value Em), and 
hence when there can be no doubt that a near end signal is present. This 
output signal is used to reset the count of the counter 38 to zero in the 
presence of a near end signal. The detector 39 and its output signal are 
provided to ensure that, in the presence of a strong near end signal when 
the calculated value Ea is very small so that its variation Ve is also 
small, a false incrementing of the counter 38 and consequent determination 
of an echo path change is avoided. A relatively slow operation of the 
detector 39 is not a problem, because it is used to reset the count of the 
counter 38 which is itself only incremented slowly. 
The averagers 22, 24, and 26 produce relatively short-term averages of the 
respective signal levels, representing the signal envelopes or signal 
powers of the respective signals. The combined attenuation Acorn value Ac 
calculated by the calculator 28 and the ERLE value Ea calculated by the 
calculator 32 are also relatively short-term averages, because they are 
ratios of the short-term averages Ra, Sa and Ta. The values Ac and Ea are 
dependent upon the level of any near end signal which may be present. In 
the presence of a near end signal at a sufficient level, these values Ac 
and Ea are relatively small because components of the near end signal are 
not subtracted by the subtraction unit 20. In the absence of a near end 
signal, or the presence of a near end signal at a very low level, the 
values Ac and Ea are relatively large (after initial convergence of the 
echo canceller) because most or all of the transmit path signal S is 
constituted by the echo signal. 
In the arrangement of FIG. 1, the comparator 30 produces the signal Cs when 
the value Ac is below the threshold value At, and hence at times when 
there is a relatively low combined attenuation Acorn and ERLE. This can be 
due either to an echo path change or, more commonly, to the presence of a 
near end signal such as in a double-talk situation. 
By way of example, all of the units 18 to 39 of the echo cancelling 
arrangement can be incorporated as functions of a digital signal 
processor. The signals R, S, and T can be sampled at a sampling frequency 
of 8 kHz, and the samples of each signal can be exponentially averaged by 
the respective averager 22, 24, or 26 all in a similar manner. For 
example, the averager 24 can operate in accordance with the equation: 
EQU Sa(n)=.alpha..vertline.S(n).vertline.+(1-.alpha.)Sa (n-1) 
where S(n) represents a sample at the current instant n, Sa(n) represents 
the average Sa for the current instant n, Sa(n-1) represents the average 
Sa for the previous instant n-1, and .alpha. is an exponential averaging 
constant. The value of .alpha. is not critical; for example 
.alpha.=2.sup.-7. The averager 26 can operate on the signal T in a similar 
manner with the same constant. The averager 22 can operate on the signal R 
in a similar manner also with the same constant, but in known manner may 
also take into account a fixed delay for the echo path (or for the loudest 
echo if there is more than one echo). Thus the averager 22 can more 
generally operate in accordance with the equation: 
EQU Ra(n-k)=.alpha..vertline.R(n-k)+(1-.alpha.)Ra(n-k-1) 
where k represents the delay, in sampling periods, of the (loudest) echo 
path . The value of k is determined using a known delay estimation 
technique, such as cross correlation of the signals R and S. 
The functions of the units 28 to 39 can be carried out at a much slower 
rate, for example at instants. m occurring once every 24 ms. 
As described above, the calculator 32 calculates the value Ea of the ERLE 
from the average signal levels Sa and Ta in accordance with the equation 
EQU Ea=Sa/Ta 
or, more precisely, in accordance with the equation: 
EQU Ea(m)=Sa(m)/Ta(m) 
where Ea(m), Sa(m), and Ta(m) are the values of Ea, Sa, and Ta respectively 
at the current instant m. The current value Ea(m) and a plurality p of 
immediately preceding and stored values Ea(m-1) to Ea(m-p) are used by the 
calculator 34 to determine the variation value Ve in accordance with the 
equations: 
EQU E max=max{Ea(m),Ea(m-1), . . . ,Ea(m-p)} 
EQU E min=min{Ea(m),Ea(m-1), . . . ,Ea(m-p)} 
EQU Ve=20log.sub.10 (E max/Emin) 
where max {. . .} represents the maximum of the values within the braces 
and min {. . . } represents the minimum of the values within the braces. 
Thus the value Ve is a measure of the variation of the value Ea of the ERLE 
over an interval of p 24 ms periods. The value of p is not critical; for 
example p is in a range from 5 to 10, corresponding to a period of 120 to 
240 ms. While other calculations could be performed to determine such a 
variation measure, for example a true statistical variance determination, 
a particularly accurate measurement is not required and the equations 
above allow for easy determination of a useful value Ve. As can be seen 
from the above equations, this is determined as the ratio, in decibels, of 
the maximum to the minimum value of Ea during the relevant interval. 
The value Ve is used to enable a distinction to be made, between an echo 
path change and a double-talk situation, when the signal Cs is produced as 
a result of the value Ac being below the threshold value At as described 
above. In the event of an echo path change, the echo cancellation 
deteriorates but there is still a close correlation between the envelopes 
of the signals S and T. Consequently, there is a relatively small 
variation in the value Ea of the ERLE, and thus the variation value Ve is 
relatively small, typically being less than about 3 to 6 dB. Conversely, 
in the event of a double-talk situation there is little correlation 
between the signals S and T, so that the variation of the value Ea of the 
ERLE will be relatively large. Consequently, the variation value Ve is 
relatively large, typically being much more than about 3 to 6 dB. 
The threshold Vt is set for example to this value of about 3 to 6 dB, and 
the comparator 36 compares the variation value Ve with this threshold Vt 
and produces a variation control signal Vc in the event that Ve&lt;Vt, i.e. 
when the variation value Ve is consistent with an echo path change rather 
than a double-talk situation. 
The counter 38 is supplied with the signals Cs and Vc, and is incremented 
once every 24 ms period when both of these signals are present, and 
otherwise is reset to a zero count. In response to the counter 38 reaching 
the predetermined maximum count, for example a count of 10 representing a 
period of 240 ms during which both of the signals Cs and Vc are present to 
represent a change in the echo path, the counter produces the signal EPC. 
As explained above, the detector 39 serves to reset the count of the 
counter 39 in the presence of a strong near end signal, so that the signal 
EPC can not be produced in this situation. 
As already described above, in response to the signal EPC the control unit 
19 again enables adaptation of the adaptive filter 18, until the echo 
canceller has again converged, whereupon the adaptive filter coefficients 
are again frozen and monitoring for a subsequent echo path change is 
resumed as described above. 
Although as described above the value of the combined attenuation Acorn is 
used to constitute the signal Ac supplied to the control unit 19 and to 
the comparator 30, the value Ea of the ERLE could instead be used to 
constitute the signal Ac and/or could be supplied to the comparator 30 for 
comparison with an ERLE threshold value. In addition, or instead, although 
as described above variation of the value Ea of the ERLE is calculated in 
the calculator 34 and used to enable a distinction to be made between an 
echo path change and a double-talk situation, variation of the value of 
the combined attenuation Acorn could instead be calculated in a similar 
manner by the calculator 34 and used to enable this distinction to be 
made. Because the combined attenuation Acorn is equal to the sum of the 
ERL and the ERLE, there is a correlation between these two echo 
attenuation parameters, dependent upon the ERL and hence upon the 
characteristics of the hybrid circuit 16, which enables the parameters to 
be used to some extent interchangeably. 
FIG. 2 illustrates; in a flow chart with blocks 40 to 62, steps associated 
with operation of the echo cancelling arrangement. 
Referring to FIG. 2, on establishment of a connection at the block 40, 
adaptation of the adaptive filter 18 is enabled at the block 42 and this 
adaptation is continued in a loop back to the block 42 until either the 
connection is ended as determined at the block 44, in which case a branch 
is made to the end block 62, or the adaptive filter has converged to a 
predetermined desired extent as determined at the block 46, in which case 
the counter 38 is reset to zero and further adaptation of the adaptive 
filter is inhibited at the block 48. As described above, the convergence 
of the echo canceller is monitored by the control unit 19 by monitoring 
either of the echo attenuation parameters, namely the combined attenuation 
Acorn, as shown in FIG. 1, or the ERLE. 
From the block 48, there is a wait at the block 50 for the next update 
time, every 24 ms as described above, and unless the connection has ended 
as determined at the block 52 (in which case the process is again ended at 
the end block 62) it is determined at the block 53 whether a near end 
signal is present as detected by the NE/DT detector 39 as described above. 
If so, there is a loop back to the block 48, resetting the count of the 
counter 38, with adaptation of the adaptive filter remaining inhibited. 
Otherwise, a selected one of the two echo attenuation parameters is 
compared with the first threshold value at the block 54. As described 
above, this is a comparison of the combined attenuation value AC with the 
threshold value At in the comparator 30. If the selected echo attenuation 
parameter is not less than the first threshold, there is a loop back to 
the block 48, the count of the counter 38 being reset and adaptation of 
the adaptive filter remaining inhibited. If the selected echo attenuation 
parameter is less than the first threshold, then in the block 56 the 
calculated variation of a selected (same or different) one of the two echo 
attenuation parameters is compared with the second threshold value. As 
described above, the variation Ve of the ERLE is calculated in the 
calculator 34 and is compared with the second threshold value Vt in the 
comparator 36. 
If this variation is not less than the second threshold as determined at 
the block 56, there is again a loop back to the block 48, the counter 38 
being reset and adaptation of the adaptive filter remaining inhibited. If 
the variation is less than the second threshold, the counter 38 is 
incremented at the block 58. At the block 60 it is determined whether the 
maximum count (e.g. 10 as described above) has been reached; if not there 
is a return to the block 50 to wait for the next update time, and 
adaptation of the adaptive filter remains inhibited but the counter is not 
reset. If the maximum count has been reached, there is a return to the 
block 42, it being concluded that there has been an echo path change and 
adaptation of the adaptive filter again being enabled to achieve 
reconvergence accordingly. 
In addition to the alternatives already discussed, as already indicated 
above the variation value Ve can be produced in other ways, and it can be 
appreciated that this value, in the presence of the signal Cs, can also be 
used to provide an indication of double-talk situations, thereby 
supplementing or replacing a conventional double-talk detector. In 
addition, different predetermined counts can be used in the counter 38 for 
producing the signal EPC. 
Thus although particular embodiments of the invention have been described 
in detail, it should be appreciated that numerous modifications, 
variations, and adaptations may be made without departing from the scope 
of the invention as defined in the claims.