Telephonic communications – Diagnostic testing – malfunction indication – or electrical... – Of hybrid or echo suppressor or canceller
Reexamination Certificate
2000-10-25
2003-02-04
Barnie, Rexford (Department: 2643)
Telephonic communications
Diagnostic testing, malfunction indication, or electrical...
Of hybrid or echo suppressor or canceller
C379S406010, C379S406080, C370S286000
Reexamination Certificate
active
06516050
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a double-talk detecting apparatus in which a state of a double talk performed by both a near-end caller and a far-end caller through a transmission path in a two-way communication system is detected. Also, the present invention relates to an echo canceller and an echo suppressor in which an echo occurring on a near-end caller's side in the double-talk detecting apparatus is cancelled or suppressed.
2. Description of Related Art
A conventional double-talk detecting apparatus is, for example, disclosed in the Japanese Published Unexamined Patent Application No. H10-242891. In this application, an echo canceller, in which a first conventional double-talk detecting method A and a second conventional double-talk detecting method B are performed for the purpose of correctly performing the renewal of filter coefficients used in an adaptive filter, are described.
In detail, in a two-way communication system, a transmission signal is received from a near-end caller, a reception signal is received from a far-end caller, a convolution calculation using the filter coefficients is performed for the reception signal in the adaptive filter to produce a pseudo-echo signal, the pseudo-echo signal is subtracted from the transmission signal in a subtracting unit to produce a residual signal (or called an error signal), and the filter coefficients are renewed according to the residual signal. In the first conventional double-talk detecting method A, the judgment whether or not a double-talk is performed by both the near-end caller and the far-end caller is performed by using three equations (1) to (3) using a root-mean-square level (or an average amplitude level, and called a level hereinafter) S of the transmission signal, a level X of the reception signal and a level E of the residual signal.
X<p
1
(1)
S>p
2
*X
(here,
p
2
≦0.5) (2)
E>p
3
*S
(3)
Here, each of the coefficients P
1
, p
2
and p
3
is a constant. Values of the constants P
1
, p
2
and p
3
are determined according to environmental conditions of the double-talk detecting apparatus.
The judgment of the double-talk is performed according to three conditions.
First Condition: the equation (1) is satisfied.
Second Condition: the equation (1) is not satisfied, but the equation (2) is satisfied.
Third Condition: neither the equation (1) nor the equation (2) is satisfied, but the equation (3) is satisfied.
In cases where the first condition is satisfied, a voiceless state of the reception signal is detected, so that a single-talk state is judged. In cases where the second or third condition is satisfied, it is judged that the near-end and far-end callers are in a double-talk state.
In the second conventional double-talk detecting method B, the judgment whether or not a double-talk is performed is performed by using two equations (4) to (5).
Sj/Ej<C
2
*
SM/EM
(
C
2
=½) (4)
Sj/Ej<C
1
*
SM/EM
(
C
1
=¼) (5)
Here, the symbol SM denotes a level of the transmission signal obtained when the filter coefficients of the adaptive filter are sufficiently converged in a single-talk state in which only the voice of the far-end caller is present (that is, the near-end caller is not speaking), the symbol EM denotes a level of the residual signal at the determination time of the level SM, the symbol Sj denotes a current level of the transmission signal, and the symbol Ej denotes a current level of the residual signal. In cases where the equation (4) or the equation (5) is satisfied, it is judged that the near-end and far-end callers are in a double-talk state at the current time.
Also, in the Application No. H10-242891, a first conventional echo canceller and a second and conventional echo canceller are disclosed. In the first conventional echo canceller, the renewal operation for the filter coefficients of the adaptive filter is stopped in cases where the first condition, the second condition or the third condition is satisfied according to the first conventional double-talk detecting method A. Also, in the second conventional echo canceller, the renewal operation for the filter coefficients of the adaptive filter is stopped in cases where the equation (4) or the equation (5) is satisfied according to the second conventional double-talk detecting method B.
Also, a conventional echo suppressor is, for example, disclosed in the International Telecommunication Union (ITU)-T Recommendation G.164. This echo suppressor is composed of a transmission path loss unit, a receiving line loss unit and a double-talk detecting unit. A line loss in the transmission path loss unit and a line loss in the receiving line loss unit are determined according to a detecting result obtained in the double-talk detecting unit.
Also, another conventional double-talk detecting method is disclosed in FIG.
6
/G.164 and FIG.
10
/G.164 of the ITU-T Recommendation G.164. In this method, a double-talk is detected by using only two inputs of a reception signal and a transmission signal. Therefore, the accuracy of the detection in this method is inferior to those of the first conventional double-talk detecting method A and the second conventional double-talk detecting method B.
There are two types of state transitions for which the detection of a double-talk state is required.
In a first state transition, a single-talk state, in which only the voice of the far-end caller is present, is changed to a double-talk state in which a voice of the near-end caller is added to the voice of the far-end caller.
In a second state transition, a single-talk state, in which a voice of the near-end caller only exists, is changed to a double-talk state in which a voice of the far-end caller is added to the voice of the near-end caller.
A double-talk state in the case of the second state transition can be detected according to any conventional double-talk detecting method. The reason is that a double-talk state can be detected by judging that a voiceless state of the reception signal is changed to a voiced state when a single-talk state based on the existence of a near-end caller's voice is changed to a double-talk state. For example, the double-talk state is detected in the first conventional double-talk detecting method A when the second condition is satisfied.
However, the detection of the double-talk state in the case of the first state transition is difficult as compared with that in the case of the second state transition. The reason is that it is required to detect the existence of a near-end caller's voice, on which an echo signal derived from a reception signal is superposed in a transmission signal, when a single-talk state based on the existence of a far-end caller's voice is changed to a double-talk state.
Various drawbacks in the detection of the double-talk state in the case of the first state transition are described hereinafter.
Drawbacks in the first conventional double-talk detecting method A are as follows.
(First Drawback) It is required to determine values of the coefficients P
2
and P
3
used in the equations (2) and (3) in advance according to the environmental conditions. Therefore, the use of the first conventional double-talk detecting method A is limited to a two-way communication system matching with the environmental conditions corresponding to the constants P
2
and P
3
.
(Second Drawback) In cases where a double-talk state is detected because the third condition is satisfied, a ratio of a residual signal level E to a transmission signal level S becomes a value higher than the constant value P
3
. In this condition of the echo canceller disclosed in the Application No. H10-242891, the filter coefficients of the adaptive filter considerably deteriorate as compared with ideal filter coefficients used to perform an impulse response which is almost the same as an actual impulse response of an echo path through which an echo signal of the reception signal is superposed on
Kajiyama Ikuo
Matsuoka Bunkei
Takahashi Shinya
Tasaki Hirohisa
Barnie Rexford
Birch & Stewart Kolasch & Birch, LLP
Mitsubishi Denki & Kabushiki Kaisha
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