Telephonic communications – Echo cancellation or suppression – Using digital signal processing
Reexamination Certificate
1998-04-14
2004-03-02
Isen, Forester W. (Department: 2644)
Telephonic communications
Echo cancellation or suppression
Using digital signal processing
C379S406010, C379S406050, C379S406060, C379S406090, C379S406120, C379S406020
Reexamination Certificate
active
06700977
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to method and apparatus for cancelling an echo in a system having a plurality of received signals and a single or a plurality of transmission signal or signals.
Regarding multi-channel echo cancelling method and apparatus for cancelling an echo occurring by transmission of the received signal through a spatial acoustic path in the system having a plurality of received signals and a single or a plurality of transmission signal or signals, there have been proposed two types of systems, such as a cascade connection type and a linear combination type, in the technical Report of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan Vol. 84, No. 330, pp. 714, CS-84-714 (hereafter, referred as Reference 1). According to the Reference 1, since the cascade connection type has a restriction of a constitution, an echo suppression performance thereof is inferior to that of the linear combination type. Accordingly, there will be described a case where a linear combination type multi-channel echo cancelling apparatus (an echo canceller) is applied to a two channel system having a pair of both reception and transmission signals.
FIG. 24
shows a linear combination type multi-channel echo canceller. A first received signal
1
is reproduced by a first speaker
3
and is going through a spatial acoustic path to a first microphone
9
so as to generate a first echo
5
. A second received signal
2
is reproduced by a second speaker
4
and is going through a spatial acoustic path to the first microphone
9
so as to generate a second echo
6
. A first mixed signal
14
is generated by adding the first and second echoes
5
and
6
and a first transmission signal
12
which is inputted to the first microphone
9
corresponding to a voice of a talker
11
. As the same manner, the first received signal
1
is reproduced by the first speaker
3
and is going through the spatial acoustic path to a second microphone
10
so as to generate a third echo
7
. The second received signal
2
is reproduced by the second speaker
4
and is going through a spatial acoustic path to the second microphone
10
so as to generate a fourth echo
8
. A second mixed signal
15
is generated by adding the third and fourth echoes
7
and
8
and a second transmission signal
13
which is inputted to the second microphone
10
corresponding to a voice of the talker
11
.
In order to cancel an echo which is mixed in the first mixed signal
14
, an echo replica
125
corresponding to the first echo
5
is generated by inputting the first received signal
1
in a first adaptive filter
121
, and an echo replica
126
corresponding to the second echo
6
is generated by inputting the second received signal
2
in a second adaptive filter
122
. A first subtracter
129
subtracts the echo replicas
125
and
126
respectively corresponding to the first and second echoes
5
and
6
from the first mixed signal
14
. The first and second adaptive filters
121
and
122
are controlled such that the first subtracter
129
has the minimum output. An output of the first subtracter
129
is a first output signal
16
of an echo canceller
120
.
In order to cancel an echo which is mixed in the second mixed signal
15
, an echo replica
127
corresponding to the third echo
7
is generated by inputting the first received signal
1
in a third adaptive filter
123
, and an echo replica
128
corresponding to the fourth echo
8
is generated by inputting the second received signal
2
in a fourth adaptive filter
124
. A second subtracter
130
subtracts the echo replicas
127
and
128
respectively corresponding to the third and fourth echoes
7
and
8
from the second mixed signal
15
. The third and fourth adaptive filters
123
and
124
are controlled such that the second subtracter
130
to has the minimum output. An output of the second subtracter
130
is a second output signal
17
of the echo canceller
120
.
In a multi-channel television conference system as one of the main applications of multi-channel echo cancellers, since the voice of a talker is recorded by a plurality of microphones, the received signal recorded by each microphone may be approximated to have an attenuation and a delay corresponding to a distance between the talker and the microphone compared to the other received signal. Accordingly, inter-channel correlation of the received signals becomes high.
It this application, a second received signal
2
which is a delayed version of the first received signal
1
, an echo path which can be modeled as an FIR (Filter impulse response) filter and an echo canceller based on linear combination are assumed.
The first and second received signals
1
and
2
at the time n denoted as x
1
(n) and x
2
(n), and an echo which is mixed in the first mixed signal
14
as d(n). When a time difference between the first and the second received signals is n
d
(a natural number) samples, equation (1) can be obtained:
X
2
(
n
)=
X
1
(
n−n
d
) (1)
For simplicity, it is assumed that the entire spatial acoustic paths from the first and the second speakers
3
and
4
to the first and second microphones
9
and
10
have the same length N for its impulse response. Further, a symbol h
1
,
i
denotes an impulse response sample of the acoustic path from the speaker
3
to the microphone
9
, and a symbol h
2
,
i
denotes an impulse response sample of the acoustic path from the speaker
4
to the microphone
9
. Here, i is an integer between 0 and N−1. The echo d(n) which is mixed in the mixed signal
14
can be obtained as a sum of the echoes
5
and
6
according to a equation (2) as follows:
d
(
n
)
=
∑
i
=
0
N
-
1
h
1
,
i
x
1
(
n
-
i
)
+
∑
i
=
0
N
-
1
h
2
,
i
x
2
(
n
-
i
)
(
2
)
When equation (1) is combined with equation (2) to eliminate x
2
(n), equation (3) can be obtained as follows:
d
(
n
)
=
∑
i
=
0
nd
-
1
h
1
,
i
x
1
(
n
-
i
)
+
∑
i
=
nd
N
-
1
(
h
1
,
i
+
h
2
,
i
-
nd
)
x
1
(
n
-
i
)
+
∑
i
=
N
-
nd
N
-
1
h
2
,
i
x
1
(
n
-
n
d
-
i
)
(
3
)
If the i-th filter coefficient of the adaptive filters
121
and
122
are respectively denoted as w
1
,
i
(n) and w
2
,
i
(n), the echoe replica d {circumflex over ( )}(n) (d(n) hat({circumflex over ( )}) ) which is generated by the adaptive filters
121
and
122
, can be obtained by a equation (4) as follows:
d
^
(
n
)
=
∑
i
=
0
N
-
1
w
1
,
i
(
n
)
x
1
(
n
-
i
)
+
∑
i
=
0
N
-
1
w
2
,
i
(
n
)
x
2
(
n
-
i
)
(
4
)
When the equation (1) combined with equation (4) to eliminate x
2
(n), it is possible to obtain equation (5) as follows:
d
^
(
n
)
=
∑
i
=
0
nd
-
1
w
1
,
i
(
n
)
x
1
(
n
-
i
)
+
∑
i
=
nd
N
-
1
{
w
1
,
i
(
n
)
+
w
2
,
i
-
nd
(
n
)
}
x
1
(
n
-
i
)
+
∑
i
=
N
-
nd
N
-
1
w
2
,
i
(
n
)
x
1
(
n
-
n
d
-
i
)
(
5
)
A redidual echo e(n) can be obtained by equation (6) as follows:
e
(
n
)
=
∑
i
=
0
nd
-
1
{
h
1
,
i
-
w
1
,
i
(
n
)
}
x
1
(
n
-
i
)
+
∑
i
=
nd
N
-
1
{
h
1
,
i
+
h
2
,
i
-
nd
-
w
1
,
i
(
n
)
-
w
2
,
i
-
nd
(
n
)
}
x
1
(
n
-
i
)
+
∑
i
=
N
-
nd
N
-
1
{
h
2
,
i
-
w
2
,
i
(
n
)
}
x
1
(
n
-
n
d
-
i
)
(
6
)
To completely cancel the echo, the following conditions must be satisfied:
h
1
,
i
=
w
1
,
i
(
n
)
i
=
0
,
⋯
,
n
d
-
1
h
1
,
i
+
h
2
,
i
-
nd
=
w
1
,
i
(
n
)
+
w
2
,
i
-
nd
(
n
)
i
=
n
d
,
⋯
,
N
-
1
h
2
,
i
=
w
2
,
i
(
n
)
i
=
N
-
n
d
,
⋯
,
N
-
1
(
7
)
According to a equation (7),
w
1,0
(
n
), . . . ,
w
1,nd−1
(
n
) and
w
2,N−nd
(
n
), . . . ,
w
2,N−1
(
n
)
is uniquely determined, however solutions to
w
1,nd
(
n
), . . . ,
w
1,N−1
(
n
) and
w
2,0
, . . . , w
2,N−nd−1
(
n
)
include an infinite number of combinations. Spec
Dickstein Shapiro Morin & Oshinsky LLP.
Isen Forester W.
NEC Corporation
Singh Ramnandan
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