Telephonic communications – Subscriber line or transmission line interface – Network interface device
Reexamination Certificate
1997-01-06
2002-09-03
Isen, Forester W. (Department: 2747)
Telephonic communications
Subscriber line or transmission line interface
Network interface device
C379S406010, C379S413020, C370S286000
Reexamination Certificate
active
06445792
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an echo canceller for canceling an echo signal which becomes a deterioration factor of a speech quality in satellite communications and telephones with loudspeakers.
2. Description of the Prior Art
An example of conventional echo cancellers is disclosed in (1) “Echo canceller technique”, Japan Industrial Technical Center, Dec. 20, 1986, pp. 129-130. An example of the filter coefficient substitution method of conventional echo cancellers is disclosed in (2) the laid-open patent publication No. 62-24726. The construction of the conventional echo canceller is explained below, along with
FIGS. 8-14
.
FIG. 8
is a block diagram of the conventional echo canceller
30
explained in the above literature (1). In
FIG. 8
, Rin (k) represents a reception signal at k (where, k represents a certain time of digital signal), in other words it represents the voice of the caller at the far end. In case of an echo canceller used for a telephone with a speaker, an echo path
1
corresponds to a path in which the voice of the far-end caller, reproduced from the speaker, goes into a microphone via sound space. And in case of the echo canceller used for satellite communication channel, the echo path
1
represents the path in which the voice of the far-end caller leaks out to that of the near end talker by impedance mismatching of a hybrid transformer used for two-line/four line conversion.
Since the echo signal passes through the echo path
1
, the reception signal Rin (k) is superposed on the near-end voice Nin (k). Therefore, a transmission signal Tin (k) contains not only the near-end voice but also the superposed far-end voice echo. If Tin (k) is transmitted to the far-end as it is, the communication quality deteriorates. In the echo canceller
30
, an adaptive filter
3
generates a pseudo echo signal Tin′ (k). An echo subtracter
2
subtracts the pseudo echo signal Tin′ (k) from the transmission signal Tin (k) to generate a residual signal Res (k) in which the echo component is canceled. The residual signal Res (k) is transmitted to the far-end.
An adaptive controller
4
controls the adaptive filter
3
. When the double-talk is detected, in other words when the far-end caller and the near-end caller are both speaking simultaneously, the adaptive controller
4
stops the adaptation. When only the far-end caller is speaking, the adaptive controller
4
outputs an adaptive control flag FLG to the adaptive filter
3
in order to activate the adaptation.
The adaptive controller
4
will hereinafter be described further in detail, using FIG.
9
.
FIG. 9
shows the adaptive controller
4
in the conventional echo canceller which is disclosed in the above literature (1). A level calculator
5
calculates logarithmically converted power of the reception signal Rin (k), in other words calculates a reception signal level Lrin (k) from a time k to a time prior to the L-th sampling according to formula (1), and outputs its result. A level calculator
6
calculates logarithmically converted power of the residual signal Res (k), in other words calculates the residual signal level Lres (k) from a time k to a time prior to the L-th sampling according to formula (2), and outputs its result. Then a level difference calculator
7
calculates DL (k), a level difference between the reception signal level Lrin (k) and the residual signal level Lres (k), according to formula (3). In the following formulas, L denotes a duration from a time k to a time prior to the L-th sampling, N denotes an average number to ensure the level difference DL (k).
Lrin
(
k
)
=
log
10
{
∑
i
=
k
-
L
+
1
k
Rin
(
i
)
2
}
(
1
)
Lres
(
k
)
=
log
10
{
∑
i
=
k
-
L
+
1
k
Res
(
i
)
2
}
(
2
)
DL
(
k
)
=
∑
i
=
k
-
N
+
1
k
{
Lrin
(
i
)
-
Lres
(
i
)
}
/
N
(
3
)
The level difference DL (k) is equal to the sum of the amount of echo loss via the echo path and the amount of echo canceling by the echo canceller. A level compensator
8
calculates an estimated residual signal level Lres′ (k) according to formula (4).
Lres′(
k
)=Lrin (
k
)−DL(
k
)
Where, Lres′ (k) represents an estimated value of the residual signal level Lres (k) at the time when only the echo signal of the reception signal exists in the transmission signal Tin (k), in other words when only the far-end caller is speaking.
The comparator
9
compares the residual signal level Lres (k) and the estimated residual signal Lres′ (k). In case that the result meets the conditional formula (5) below, the comparator
9
judges that both the far-end caller and the near-end caller are speaking, that is to say, that double-talk condition is occurred. Then the comparator
9
sets the adaptive control flag FLG to “0” in order to stop the adaptation of the adaptive filter.
Lres
k
)>Lres′(
k
)
Where, FLG=“1” represents the activation order of the adaptation, and FLG=“0” represents the deactivation order of the adaptation.
Adaptation control is a process in which whether the near-end voice level reaches a level for stopping the adaptation processing is judged, then the activation or the deactivation of the adaptation is controlled. The activation and the deactivation of the adaptation should be controlled according to a relationship between the echo signal level in the transmission signal Tin (k) and the near-end voice level. When the level of near-end voice exceeds the level of the echo signal by the far-end signal, the adaptation processing is deactivated. On the other hand, as shown in FIGS.
10
A~
10
D, in case that the echo signal level exceeds the near-end voice level, or in case that the echo signal level exceeds the value added to the near-end voice level by a certain margin, the adaptation should be activated.
FIG. 10A
shows a relationship between the near end voice level Nin (k) and the echo signal level Rin′ (k).
FIG. 10B
shows a near-end voice sections in the reception signals Tin (k).
FIG. 10C
shows an echo signal section in the reception signals Tin (k).
FIG. 10D
shows sections where the adaptation of the conventional adaptive filter should be executed.
FIG. 11A
shows a relationship between the near end voice level Nin (k) and the echo signal level Rin′ (k).
FIG. 11B
shows near-end voice sections in the reception signals Tin (k).
FIG. 11C
shows an echo signal section in the reception signals Tin (k).
FIG. 11D
shows a relationship between residual signal level Lres (k) and the estimated residual signal level Lres′ (k).
FIG. 11D
shows sections where the adaptation of the conventional adaptive filter is executed.
In the conventional art, the double talk is detected from the residual signal level Lres (k) and the estimated residual signal level Ires′ (k) in the comparator
9
. Therefore, if the residual signal level Lres (k) and the estimated residual signal level Lres′ (k) are nearly equal, as shown in
FIG. 11D
, the adaptation is deactivated by partly mis-detecting the condition as double talk condition even in a section (a) where only the echo signal is contained in the transmission signal Tin (k). In addition, if the residual signal level Lres (k) exceeds the estimated residual signal level Lres′ (k) adaptation is deactivated, even in a section (b) shown in
FIG. 11
where the adaptation should be carried out because echo signal level exceeds the near-end voice level.
FIGS.
12
A~
12
E shows another example of relationship between the near end voice level and the echo signal level and sections where the adaptation of the conventional adaptive filter is executed. In FIGS.
12
A~
FIG. 12E
, if the residual signal level Lres (k) exceeds the estimated residual signal level Lres′ (k), adaptation is deactivated, even in a section (b) shown in
FIG. 12C
where the adaptation should be carried out because echo signal level exceeds the near-end vo
Kajiyama Ikuo
Shiraki Koichi
Harold Jefferey
Isen Forester W.
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