Telephonic communications – Echo cancellation or suppression
Reexamination Certificate
2000-08-18
2004-01-13
Barnie, Rexford (Department: 2643)
Telephonic communications
Echo cancellation or suppression
C379S406080, C379S406020
Reexamination Certificate
active
06678376
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an echo canceling apparatus in which an echo occurring on a near-end caller's side is canceled in a system in which a speech communication is performed between the near-end caller and a far-end caller through a transmission path.
2. Description of Related Art
FIG. 10
is a block diagram showing the configuration of a conventional echo canceling apparatus disclosed in the Published Unexamined Japanese Patent Application No. H10-242891 (1998) as an example. In
FIG. 10
,
1
indicates a reception signal received from a far-end caller,
2
indicates a transmission signal transmitted from a near-end caller to the far-end caller,
3
indicates a first adaptive filter unit,
4
indicates a first subtracting unit,
5
indicates a second adaptive filter unit,
6
indicates a second subtracting unit,
7
indicates an adaptive filter control unit,
8
indicates a filter selecting unit,
9
indicates a, tap coefficient storing unit,
10
indicates a pseudo-echo producing unit,
11
indicates a third subtracting unit,
12
indicates a delaying unit,
13
indicates a delaying unit, and
14
indicates an echo canceled output signal.
Next, an operation of a conventional echo canceling apparatus is described.
In this apparatus, a group of first tap coefficients, a group of second tap coefficients and a group of third tap coefficients are used. The group of first tap coefficients is renewed by using a first convergence factor in the first adaptive filter unit
3
for each sample cycle. Also, a first pseudo-echo signal is produced by using the group of first tap coefficients and the reception signal
1
in the first adaptive filter unit
3
and is output to the first subtracting unit
4
. In the first subtracting unit
4
, the first pseudo-echo signal is subtracted from the transmission signal
2
to produce a first error signal, and the first error signal is output to the adaptive filter control unit
7
. Also, the first error signal is returned to the first adaptive filter unit
3
to renew the group of first tap coefficients. The group of first tap coefficients of the first adaptive filter unit
3
is renewed according to an equation (1).
(
Hk,j+
1)=(
Hk,j
)+&mgr;×
E
1
j×{Xj−k}/&agr;
(1)
Here the symbol k denotes a tap number (k=0,1, - - - ,N), the symbol j denotes a j-th time point, the symbol Hk,j denotes a tap coefficient of a k-th tap at the j-th time point, and the symbol Hk,j+1 denotes a tap coefficient of a k-th tap at the (j+1)-th time point. In other words, the symbol Hk,j+1 denotes a tap coefficient at a sample time point next to that of the tap coefficient Hk,j. Also, the symbol E
1
j denotes a value of the first error signal at the j-th time point, and the symbol Xj−k denotes the reception signal
1
at the (j−k)-th time point. The symbol &mgr; is called a convergence factor and denotes a parameter for determining a convergence speed of each tap coefficient. In cases where the convergence factor &mgr; is heightened, the convergence speed of the group of first tap coefficients is heightened. In contrast, in cases where the convergence factor &mgr; is lowered, the convergence speed of the group of first tap coefficients is lowered. A factor &agr; is indicated according to an equation (2).
α
=
∑
i
=
0
N
&LeftBracketingBar;
Xi
&RightBracketingBar;
/
N
(
2
)
The symbol N denotes the number of tap coefficients.
The second group of tap coefficients is renewed by using a second convergence factor lower than the first convergence factor in the second adaptive filter unit
5
for each sample cycle according to the equation (1) in the same manner as the renewal of the group of first tap coefficients. Also, a second pseudo-echo signal is produced by using the group of second tap coefficients and the reception signal
1
in the second adaptive filter unit
5
and is output to the second subtracting unit
6
. In the second subtracting unit
6
, the second pseudo-echo signal is subtracted from the transmission signal
2
to produce a second error signal, and the second error signal is output to the adaptive filter control unit
7
. Also, the second error signal is returned to the second adaptive filter unit
5
to renew the group of second tap coefficients.
Also, one group of tap coefficients is selected in the filter selecting unit
8
from the group of first tap coefficients of the first adaptive filter unit
3
, the group of second tap coefficients of the second adaptive filter unit
5
and a group of tap coefficients stored in the tap coefficient storing unit
9
, and the selected group of tap coefficients is stored in the tap coefficient storing unit
9
as a group of third tap coefficients.
In the adaptive filter control unit
7
, the first error signal, the second error signal, the reception signal
1
and the transmission signal
2
are input for each sample cycle (or each sample period), an average power level of M samples of each signal is calculated for each tap coefficient selection cycle corresponding to M sample cycles, selection information is determined according to equations (3) to (6) and four selection conditions, and the selection information is output to the filter selecting unit
8
to select one group of tap coefficients from the group of first tap coefficients, the group of second tap coefficients and the group of third tap coefficients of the tap coefficient storing unit
9
.
X<p
1
(3)
S>p
2
×
X
(here,
p
2
≦0.5) (4)
P
3
×|
E
1
|>|
E
2
| (5)
P
4
×|
E
2
|>|
S|
(6)
Here the symbol S denotes an average power level (which indicates a root-mean-square level or an average amplitude level) of the transmission signal
2
, the symbol X denotes an average power level of the reception signal
1
, the symbol E
1
denotes an average power level of the first error signal, and the symbol E
2
denotes an average power level of the second error signal. Also, the symbols p
1
, p
2
, p
3
and p
4
are respectively a constant and are determined according to environmental conditions of the conventional echo canceling apparatus.
The selection conditions of one group of tap coefficients using the equations (3) to (6) are as follows.
First Selection Condition: the equation (3) is satisfied, or the equation (4) is satisfied on condition that the equation (3) is not satisfied. In this case, the group of third tap coefficients of the tap coefficient storing unit
9
is selected.
Second Selection Condition: the equation (5) is satisfied on condition that either the equation (3) or the equation (4) is not satisfied. In this case, the group of first tap coefficients is selected.
Third Selection Condition: the equation (6) is satisfied on condition that each of the equations (3), (4) and (5) is not satisfied. In this case, the group of second tap. coefficients is selected.
Fourth Selection Condition: each of the equations (3), (4), (5) and (6) is not satisfied. In this case, the group of third tap coefficients of the tap coefficient storing unit
9
is selected.
In the pseudo-echo producing unit
10
, a third pseudo-echo signal is produced by using the group of third tap coefficients stored in the tap coefficient storing unit
9
and the reception signal
1
delayed in the delaying unit
13
, and the third pseudo-echo, signal is output to the third subtracting unit
11
. In the third subtracting unit
11
, the third pseudo-echo signal is subtracted from the transmission signal
2
delayed in the delaying unit
12
to produce an echo canceled output signal
14
, and the echo canceled output signal
14
is output to the far-end caller.
Echo canceling apparatuses similar to the above-described conventional echo canceling apparatus are disclosed in the Published Unexamined Japanese Patent Application No. H9-148965 (1997) and the Published Unexamined Japanese Patent Applicati
Kajiyama Ikuo
Takahashi Shinya
Barnie Rexford
Mitsubishi Denki & Kabushiki Kaisha
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