Telephonic communications – Echo cancellation or suppression
Reexamination Certificate
1999-10-04
2003-10-21
Isen, Forester W. (Department: 2644)
Telephonic communications
Echo cancellation or suppression
C379S406020, C379S406050, C379S406060, C379S406070, C379S406080, C379S406100
Reexamination Certificate
active
06636604
ABSTRACT:
The present invention relates to a device for echo suppression in a handsfree apparatus, particularly for a telephone.
BACKGROUND INFORMATION
Echo cancellers having adaptive filters for handsfree apparatuses are known in general, the cancellers altering the amplification of controllable amplifiers disposed in the transmitting and receiving paths of the handsfree apparatus in accordance with the speech activity of the near-end and far-end subscribers. In practice, the familiar room-echo cancellers achieve echo attenuations of between 20 dB and 30 dB. Consequently, owing to for example, an adaptive filter length which is too short, imprecise adjustment because of existing noise, or nonlinearities, there remains a residual echo.
To reduce the residual echo, German Patent No. 41 19 936 proposes a nonlinear filter which can also be considered as a further development of the center clipper (see “Der Fernmeldeingenieur”, 1/95, p. 32). The disadvantages of this arrangement, however, lie in the nonlinearity of the filter, which results in signal distortion at low levels.
A further possibility for reducing the residual echo is the use of the level balance proposed, for example, in German Patent No. 43 05 256, German Patent No. 39 28 805, European Patent No. 0 415 031 and also described in “Der Fernmeldeingenieur”, 10/94, p. 11. In this case, controllable amplifiers are located in the transmitting and receiving paths, respectively. Starting from a quiescent value, the amplifications of the amplifiers are changed in opposite directions, such that the product of the amplifications remains constant. Although the level balance prevents nonlinearities, a disadvantage lies in the fact that, in the quiescent state or during two-way speech, an attenuation is effective in both directions equivalent to half the deviation of the level balance.
European Patent No. 0 471 083, describes a handsfree apparatus having echo cancellers and additional adjustable amplifiers. To control the amplifiers, the received signal and the transmitted signal are evaluated in a control circuit and the amplifiers are switched over between two amplifications.
This arrangement has the disadvantage that the transmitted signal rises in response to parameter changes, and simulates an activity of the near-end subscriber. Although the control circuit was not explained in detail in the patent specification, it can be assumed that the product of both amplifications is kept constant. Consequently, this is a form of level balance in which a high attenuation is set in the receiving path and a low attenuation is set in the transmitting path when the near-end subscriber is speaking. If the far-end subscriber is speaking, the relationships are reversed.
Some disadvantages of this arrangement lie in the fact that, when the near-end subscriber is speaking, he can easily gain the impression that he has a bad connection and, in the case of two-way speech, disturbances arise due to the changeover of the amplification.
SUMMARY OF THE INVENTION
On the object of the present invention is to provide an improvement in the room-echo cancellation of handsfree apparatuses, particularly during two-way speech. An algorithm is made available for adjusting the amplification of the two, amplifiers in the transmitting and receiving paths, such that a large additional attenuation is attained for the residual echo without disturbing the impression of natural speech. At the same time, an intention is to prevent erroneous decisions by the control logic due to parameter changes.
An objective is achieved according to the present invention by a device for echo suppression in a handsfree apparatus, particularly for a telephone, the device having an adaptive filter, a control circuit, as well as adjustable amplifiers disposed in the transmitting and receiving paths, the amplification of the amplifiers being controlled by the control circuit with control signals V
in
, V
out
, the control circuit including a first short-term power estimator which emits a signal P
f
at its output as a measure for the power of the received signal U
f
from the far-end subscriber. Also provided in the control circuit are one or more further power estimators whose output signals, either individually or in combination, form a measure P
n
for the power of the near-end subscriber. The variables P
n
and P
f
are used to form a first control variable U
1
=P
n
+P
f
, as well as a second control variable U
2
=P
n
/P
f
. A control logic processes control variables U
1
, U
2
according to the following rules to form output signals V&tgr;, V
1
and V
2
, wherein S
0
and S
1
are predetermined threshold values, S
1
<1 and &agr;, &bgr;<1:
if U
1
<S
0
, then V&tgr;=0, V
1
=1 and V
2
=&bgr;;
if U
1
>S
0
and U
2
<S
1
, then V&tgr;=0, V
1
=&agr; and V
2
=1;
if U
1
>S
0
and U
2
>1/S
1
, then V&tgr;=1, V
1
=1 and V
2
=&bgr;;
if U
1
>S
0
and 1/S
1
>U
2
>S
1
, then V&tgr;=1, V
1
=1 and V
2
=1.
Signals V
1
and V
2
are supplied in each case to a nonlinear shaping filter allocated to the transmitting and receiving paths of the handsfree apparatus, the output signals V
out
and V
in
of the shaping filters controlling the amplification of the respective amplifier in proportion to their magnitude, and the control signal V&tgr;, depending on its level, setting two different time constants &tgr;
0
, &tgr;
1
of the shaping filter in the transmitting path.
With regard to the method, an objective of the present invention is achieved by a method for echo suppression in a handsfree apparatus with a receiving path, including at least of one controllable amplifier and a loudspeaker, and with a transmitting path composed at least of one controllable amplifier and a microphone, the amplifications V
S
and V
E
respectively, of the controllable amplifiers in the transmitting and receiving paths being adapted to the speech activity of the far-end or near-end subscriber and, in order to determine the speech activity of the far-end subscriber, the power P
f
of the received signal U
f
being determined by a power estimator, the method being characterized by the following features:
a) the speech activity of the near-end subscriber is determined by one or more further power estimators whose output signals, either individually or in combination, represent a measure P
n
for the power of the near-end subscriber;
b) starting from the quiescent values V
S
=1 and V
E
=&bgr;, the amplification of the controllable amplifiers is adjusted when the summation signal U
1
=P
n
+P
f
exceeds a predetermined noise threshold S
0
;
c) if the far-end subscriber is speaking, amplification V
S
in the transmitting path is set instantaneously to the value &agr;<1 and amplification V
E
in the receiving path is set instantaneously to the value 1;
d) if the near-end subscriber is speaking, amplification V
S
in the transmitting path is set to the value 1 and amplification V
E
in the receiving path is set to the value &bgr;<1, the transitions proceeding exponentially with the time constant &tgr;
1
in the transmitting path and with a time constant independent thereof in the receiving path;
e) if both subscribers are speaking, amplifications V
S
and V
E
are equal to 1, the transition in the receiving path being made instantaneously and the transition in the transmitting path being made exponentially with the time constant &tgr;
1
;
f) if neither subscriber is speaking, amplification V
S
is set to the value 1 and amplification V
E
is set to the value &bgr;, the transitions proceeding exponentially with the time constant &tgr;
0
>&tgr;
1
in the transmitting path and with a time constant independent thereof in the receiving path.
The amplifications of the controllable amplifiers are adjusted for echo cancellation according to the different speech modes when the summation signal U
1
=P
n
+P
f
exceeds a predetermined noise threshold S
0
, the speech modes “one-w
Deutsche Telekom AG
Isen Forester W.
Kenyon & Kenyon
Singh Ramnandan
LandOfFree
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