Telephonic communications – Echo cancellation or suppression
Reexamination Certificate
1999-03-10
2002-09-10
Isen, Forester W. (Department: 2644)
Telephonic communications
Echo cancellation or suppression
C379S406030, C379S406040, C379S406050, C379S406080, C379S406060, C379S406020
Reexamination Certificate
active
06449361
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a control method and device for echo canceller and more particularly, to an echo canceller control method and an echo canceller control device for use in hands-free telephones, live videoconference systems and the like.
2. Prior Art
In echo cancellers applied to hands-free telephones and employing adaptive filters, an adaptation operation of the adaptive filter is normally performed under a single talk condition. However, when a double talk condition (two-way simultaneous conversations) occurs, the adaptation operation is performed in a manner such as to remove not only an echo signal but also a talker's speech, resulting in confused estimate of an echo path impulse response. Hence, a learning operation must be suspended in the presence of the double talk condition.
In past, a variety of techniques have been proposed for detecting the double talk condition by using the input/output signal level, correlation function and the like.
FIG. 13
shows a prior-art echo canceller
2
including a double talk detecting circuit
1
(see FIG. 2 of Japanese Unexamined Patent Publication No. 5(1993)-102887).
In the double talk detecting circuit
1
, a level of a digital receive path signal RS is detected by an absolute value detecting circuit
3
and a differential circuit
5
while a level of a digital transmit path signal TS is detected by an absolute value detecting circuit
4
and a differential circuit
6
.
Next, a comparator
7
performs an arithmetic operation of R=log{(a level of receive path signal RS)/(a level of transmit path signal TS)} for determination of a ratio R of the detected signal levels, which ratio is compared with a predetermined threshold value.
When the level ratio R becomes smaller than the threshold value, the double talk condition is determined to be present. In a period during which the double talk condition is determined to be present, the adaptation operation of an adaptive filter (ADF)
8
is suspended. This prevents the echo canceller
2
from making a false estimate of an impulse response of an echo path EP under the double talk condition, thus ensuring reliable echo cancellation operations.
In such an echo canceller
2
, however, when the transmit path signal level becomes lower in conjunction with the movement of a near end talker, the level ratio R of the transmit path signal versus the receive path signal exceeds the threshold value. This may lead to a false determination of the single talk condition despite the actual presence of the double talk condition.
Thus, the prior-art double talk detecting circuit
1
is adversely affected by variations of the signal level due to the movement of the near end talker, a line failure and the like, suffering degraded accuracies of the double talk detection. Hence, unreliable echo cancellation operations result.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the invention to provide a control method and device for controlling the echo canceller which ensure the reliable echo cancellation operations.
In accordance with the invention, a first method of controlling the echo canceller for providing an echo cancellation signal by subtracting an echo replica signal either from a transmit signal or a receive signal, which method comprises the steps of: a first step of performing a speech detection based on the echo cancellation signal provided by the echo canceller; a second step of detecting a double talk condition based on a result of the speech detection performed by the first step; and a third step of controlling a learning operation of the echo canceller based on a result of the double talk detection performed by the second step.
In the first step, the speech detection is achieved by, for example, comparing a level of the echo cancellation signal provided by the echo canceller with a threshold value adaptively determined according to a level of background noise.
In the second step, the double talk condition is determined to be present when, for example, the echo cancellation signal provided by the echo canceller is determined to include speech and continues to indicate the presence of the speech for more than a predetermined period of time.
In the third step, the learning operation of the echo canceller is suspended while the double talk condition is detected by the second step.
In accordance with the invention, a second method of controlling an echo canceller for providing an echo cancellation signal by subtracting an echo replica signal either from a transmit signal or a receive signal, which method comprises the steps of: a first step of determining whether a learning operation of the echo canceller is permitted or not based on either of the transmit signal and the receive signal that is not removed of echo by the echo canceller; a second step of performing a speech detection based on the echo cancellation signal provided by the echo canceller; a third step of detecting a double talk condition based on a result of the speech detection performed by the second step; and a fourth step of controlling the learning operation of the echo canceller based on a determination made by the first step and a result of the double talk detection performed by the third step.
In the second step, the speech detection is achieved by, for example, comparing a level of the echo cancellation signal provided by the echo canceller with a threshold value adaptively determined according to a level of background noise.
In the third step, the double talk condition is determined to be present when, for example, the echo cancellation signal provided by the echo canceller is determined to include speech and continues to indicate the presence of the speech for more than a predetermined period of time.
In the fourth step, the learning operation of the echo canceller is suspended while the first step determines the learning operation of the echo canceller to be inhibited or while the double talk condition is detected by the third step.
In accordance with the invention, a third method of controlling an echo canceller for providing an echo cancellation signal by subtracting an echo replica signal either from a transmit signal or a receive signal, which method comprises the steps of: a first step of performing a speech detection based on the echo cancellation signal provided by the echo canceller; a second step of performing a speech detection based on either of the transmit signal and the receive signal that is not removed of echo by the echo canceller; a third step of performing a double talk detection based on a result of the speech detection performed by the first step and a result of the speech detection performed by the second step; and a fourth step of controlling a learning operation of the echo canceller based on a result of the double talk detection performed by the third step.
In the first step, the speech detection is achieved by, for example, comparing a level of the echo cancellation signal provided by the echo canceller with a first threshold value adaptively determined according to a level of background noise.
In the second step, the speech detection is achieved by, for example, comparing a level of either of the transmit signal and the receive signal that is not removed of the echo by the echo canceller with a second threshold value adaptively determined according to a level of background noise.
In the third step, the double talk condition is determined to be present when, for example, both the echo cancellation signal provided by the echo canceller and either of the transmit signal and the receive signal that is not removed of the echo by the echo canceller are determined to include speeches and continue to indicate the presence of the speeches for more than a predetermined period of time.
In the fourth step, the learning operation of the echo canceller is suspended while the double talk condition is detected by the third step.
In accordance with the invention, a fourth method of controlling an
Isen Forester W.
Sanyo Electric Co,. Ltd.
Singh Ramnandan
Smith , Gambrell & Russell, LLP
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