Echo canceller and method of controlling the same

Details Echo canceller and method of controlling the same Echo canceller and method of controlling the same

1998-02-18

2001-06-05

Chan, Wing F. (Department: 2643)

Telephonic communications

Subscriber line or transmission line interface

Network interface device

C455S570000

Reexamination Certificate

active

06243462

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an echo canceller for improving a speech quality etc. in a telephone network etc.
2. Description of the Related Art
Echo cancellers are installed for such purposes as preventing the deterioration of a speech quality attributed to echoes which occur in, for example, 2-wire/4-wire converting hybrid devices included in various telephone networks etc. in order to mutually switch 2-wire transmission lines and 4-wire transmission lines.
Heretofore, echo cancellers have been often employed for suppressing echoes which occur in networks of long transmission lines and great transmission delays, such as a satellite communication network, etc.
Further, in recent years, techniques in which a transmission band is compressed by a voice CODEC (voice coder/decoder) so as to reduce the transmission rate of voice have extensively come into practical use, and echo cancellers have been extensively applied in order to eliminate echoes ascribable to processing delays which are involved in such CODECS. Concretely, the echo cancellers have come into wide use in radio networks such as the base station system of mobile terminals, etc.
Besides, as a telephone network and an ATM (asynchronous transfer mode) network are integrated more in the future, the necessity of echo cancellers will rise due to the increases of processing delays attendant upon the disassembly/assembly of ATM cells, etc.
FIG. 1
is a diagram for explaining the applied positions of echo cancellers for suppressing echoes which occur in 2-wire/4-wire converting hybrid devices (hereinbelow, simply termed “hybrid devices”).
Referring to
FIG. 1
, the echo of a speaker “A” signifies a signal which is so produced that part of the voice of the speaker “A” leaks into the reception line of the speaker “A” in the hybrid device (Hybrid)
101
(#B) on the side of a speaker “B”. The echo of the speaker “A” is suppressed by the echo canceller (EC)
102
(#A) (hatched part) which is installed on the speaker-B side. A path which extends from the echo canceller
102
(#A) back to the same
102
(#A) via the hybrid device
101
(#B) on the speaker-B side is called the “echo path”
103
of the echo canceller
102
(#A). In addition, with respect to the echo canceller
102
(#A), the speaker “A” is called a “far-end speaker”, and the speaker “B” is a “near-end speaker”.
Conversely, the echo of the speaker “B” signifies a signal which is so produced that part of the voice of the speaker “B” leaks into the reception line of the speaker “B” in the hybrid device
101
(#A) on the speaker-A side. The echo of the speaker “B” is suppressed by the echo canceller (EC)
102
(#B) which is installed on the speaker-A side. Although not especially shown in the figure, the echo path of the echo canceller
102
(#B) extends from the echo canceller
102
(#B) back to the same
102
(#B) via the hybrid device
101
(#A) on the speaker-A side. In addition, a far-end speaker with respect to the echo canceller
102
(#B) is the speaker “B”, while a near-end speaker is the speaker “A”.
As delays involved between the speakers “A” and “B” are longer, a transmission delay to occur therebetween increases more. Accordingly, the speaker “A” comes to hear later the echo arising from his/her own voice and to more conspicuously perceive the echo as being offensive to the ear.
FIG. 2
is a block diagram showing the prior-art construction of the echo canceller
102
depicted in FIG.
1
.
Generally, in the echo canceller
102
, a tap coefficient updating unit
204
successively updates N tap coefficients exhibiting the characteristics of the echo path
103
and successively held in a tap coefficient memory
203
, on the basis of residual echo signals
212
successively outputted from a subtracter
210
and receive-in signals
205
successively held in a tap memory
202
. It holds the updated tap coefficients in the tap coefficient memory
203
anew.
Besides, an adaptive FIR (Finite Impulse Response) filter
201
executes a convolution calculation for the receive-in signals
205
successively held in the tap memory
202
and the N tap coefficients successively updated by the tap coefficient updating unit
204
, thereby to generate a pseudo or artificial echo signal
209
. That is, the adaptive FIR filter
201
is a filter which realizes the characteristics of the echo path
103
. Here, the echo path
103
is assumed to have linear characteristics, which are estimated as an impulse response. Further, the adaptive FIR filter
201
is implemented as a transversal filter which executes the convolution calculation of a finite impulse response (FIR) approximating the impulse response of the linear characteristics.
The tap coefficient updating unit
204
calculates the updated values of the respective values of the N tap coefficients from the residual echo signals
212
outputted from the subtracter
210
and the receive-in signals
205
successively held in the tap memory
202
, every sampling point and on the basis of, for example, an algorithm called “learning identification”. Subsequently, using the N updated values, the tap coefficient updating unit
204
updates the respective values of the N tap coefficients calculated at the last sampling point and held in the tap coefficient memory
203
. Also, the tap coefficient updating unit
204
outputs the resulting N tap coefficients to the coefficient setting portion of the adaptive FIR filter
201
and holds them in the tap coefficient memory
203
anew.
Here, since a conventional echo canceller assumes white noise as the receive-in signal, the echo cannot be completely removed in a situation where background noise enters the actual voice signal or the near-end speaker side. In particular, when the transmission delay of the echo path increases, the influence thereof becomes conspicuous. As shown in
FIG. 2
, therefore, a processor
211
called “NLP (nonlinear processor)” is inserted on the output side of the subtracter
210
included in the echo canceller
102
. The NLP
211
executes such a process that, if the level of the residual echo signal
212
outputted from the subtracter
210
does not exceed a certain level, the signal is forcibly made zero by way of example. Accordingly, the process is a kind of nonlinear process. However, in a case where the speakers “A” and “B” are talking at the same time or where the near-end speaker is talking, the state is detected by a superposed-talk detecting circuit or the like not especially shown, and the operation of the NLP
211
is stopped.
As stated before, it is premised for the echo canceller
102
that the echo path
103
has-the linear characteristics, which can be simulated by the adaptive FIR filter
201
. Herein, the tap length of the adaptive FIR filter
201
is set at a length which can cover the maximum time period supposable as the transmission delay of the echo path
103
, in consideration of a network to which the echo canceller
102
is connected. A time period corresponding to the length is called an “echo control time”. More concretely, the tap length of the adaptive FIR filter
201
is usually set at a time length which is obtained in such a way that an impulse response time arising in the hybrid device
101
is added to the maximum transmission delay time of the echo path
103
.
By way of example, in a telephone network laid within the State of Japan, the maximum delay time of the echo path
103
is said to be on the order of 40-50 [msec] in and around Tokyo. It is known that the echo does not offend the ear within such limits.
Recently, especially in a mobile-type network such as PHS network or portable telephone network, etc., an overall transmission delay involved in the network tends to increase due to the increases of transmission delays developing in a radio line portion, a voice CODEC and a line multiplexer/demultiplexer which are included in the network. In the mobile-type network, therefore, echo cancellers have come to be often inserte

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