Echo canceler

Details Echo canceler Echo canceler

1998-09-01

2003-09-09

Isen, Forester W. (Department: 2644)

Telephonic communications

Echo cancellation or suppression

C379S406060, C379S406070, C379S406010, C379S406160

Reexamination Certificate

active

06618479

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an echo canceler for suppressing an echo of a received signal generated by impedance mismatching in an analog two-wire/four-wire conversion portion of a telephone circuit.
2. Description of the Related Art
By using a conventional echo canceler, it is possible to suppress an echo of a receive signal contained in a transmit signal by estimating a transfer function of an echo path on the basis of the receive signal and the echo of this receive signal, generating a pseudo echo on the basis of the estimated transfer function and the receive signal, and subtracting this estimated echo from the transmit signal.
Furthermore, as for the above described estimation of the transfer function of the echo path, it is possible to estimate the transfer function of the echo path in the case where a signal in the transmit direction is only the echo of the receive signal. In the case where the signal in the transmit direction contains a mixture of the echo of the receive signal and the transmit signal, however, the transfer function of the echo path cannot be estimated.
In the conventional echo canceler, it is determined whether the current state is a double talk state such as a state of mixed presence of the transmit signal and the receive signal, or a state of only the transmit signal. If the current state is the double talk state, the operation for estimating the transfer function is stopped.
FIG. 5
is a block diagram showing the schematic configuration of the inside of an echo canceler system adopting such a conventional echo canceler.
An echo canceler system
100
shown in
FIG. 5
includes an analog two-wire telephone set
101
, an analog four-wire telephone set
102
, a transmission apparatus
104
having an echo canceler
103
, and an exchange
106
for conducting exchange and connection for the analog two-wire telephone set
101
, the analog four-wire telephone set
102
, and telephone circuits
105
via the transmission apparatus
104
.
Within this exchange
106
, there are included a two-wire/four-wire conversion unit
107
connected to the analog two-wire telephone set
101
to conduct mutual two-wire/four-wire conversion for the transmit and receive signals, an exchange switch
108
for conducting switching connection among the two-wire/four-wire conversion unit
107
, the analog four-wire telephone set
102
, and the transmission apparatus
104
, and a PB (push-button dial) signal transmitter
109
for transmitting a PB signal obtained from the analog two-wire telephone set
101
or the analog four-wire telephone set
102
to a circuit side exchange.
The echo canceler
103
includes an echo cancel circuit
110
for suppressing the echo of the receive signal contained in the transmit signal supplied from the analog two-wire telephone set
101
.
In such an echo canceler system
100
, it is possible to suppress an echo of a receive signal in the echo cancel circuit
110
by estimating a transfer function of an echo path on the basis of the receive signal of the analog two-wire telephone set
101
and the echo of the receive signal, generating a pseudo echo on the basis of the estimated transfer function and the receive signal, and subtracting the pseudo echo from a transmit signal.
However, the conventional echo canceler
103
has such a configuration that the operation of estimating the transfer function of the echo path is stopped if the current state is judged to be a double talk state. In the case where the transmit signal and the receive signal have similar frequency components, there is a fear that the current state is falsely judged not to be a double talk state although the current state is a double talk state. If in such a double talk state the transfer function of the echo path is estimated, an estimated value is disturbed and the speech quality of the transmit signal is degraded.
In the case where such an echo canceler is applied to a telephone circuit having an echo path, the influence caused by the degradation of the speech quality of the transmit signal is slight even if the current state is falsely judged not to be a double talk state and the transfer function of the echo path is estimated although the current state is the double talk state.
In the case where the echo canceler
103
is applied to a telephone circuit having no echo path, however, the current state is falsely judged not to be the double talk state although the current state is the double talk state. Therefore, the transfer function of the echo path which does not exist is estimated, a pseudo echo is generated, and the pseudo echo is subtracted from a transmit signal. Thus there is a fear that the degradation of the speech quality of the transmit signal may be greatly influenced.
In addition, among kinds of telephone circuits, there are circuits which do not generate echoes such as analog four-wire circuits and digital telephone circuits. In such telephone circuits which do not generate echoes, the echo canceler is not required.
On the side of the transmission apparatus
104
having the echo canceler
103
, it is typically impossible to determine whether the kind of the telephone circuit
105
connected to the exchange
106
is a telephone circuit which generates an echo.
In other words, if such a conventional echo canceler
103
is applied to a telephone circuit having no echo path and the current state is a double talk state, then an abnormal rasping noise is added to the transmit signal or the transmit signal is made silent, resulting in a degraded speech quality of the transmit signal, by falsely judging the current state not to be the double talk state, estimating the transfer function of the echo path which does not exist, generating a pseudo echo, and subtracting this pseudo echo from the transmit signal. The conventional echo canceler
103
had such a problem.
When originating a call from the analog two-wire telephone set
101
toward a telephone set of circuit side, the caller of the analog two-wire telephone set
101
unhooks the receiver of the telephone set
101
and obtains the dial tone from the exchange of the circuit side. And the caller presses the dial number of the telephone set of circuit side. In the case where this dial signal system is the PB system, the analog two-wire telephone set
101
of the caller side transmits a PB signal representing dial numbers to the exchange
106
. The exchange
106
temporarily stores this PB signal, and transmits the PB signal from the PB signal transmitter
109
included in the exchange
106
to the circuit side exchange via the transmission apparatus
104
.
In the case where a call is originated from the analog two-wire telephone set
101
to a circuit-side telephone set, an echo path does not exist when a PB signal is transmitted from the PB signal transmitter
109
because of the analog two-wire telephone set
101
. When seen from the conventional echo canceler
103
, there occurs a double talk state with a dial tone serving as the receive signal and the PB signal serving as the transmit signal. It is now assumed that the current state is falsely judged in the echo canceler
103
not to be the double talk state in spite of the double talk state. Then the transfer function of the echo path which does not exist is estimated, a pseudo echo is generated, and the pseudo is subtracted from the transmit signal. Therefore, the PB signal for transferring the dial information between exchanges is degraded. Since thus the dial information is not transferred, misconnection or uncompleted connection is caused. The conventional echo canceler had such a problem.
On the other hand, JP-A-5-14242 discloses a method of controlling an echo canceler. This method is aimed to enable comfortable conversation even if the level of a voice signal changes extremely. However, this document does not disclose preventing speech quality degradation due to the false judgment of double talk.
Also, JP-A-5-218904 discloses a method of controlling an echo canceler. This method is aimed to solve the

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