JPH0720073B2 - Eco-Cancer - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an echo canceller equipped with a two-way call detector.

(Prior Art) Conventionally, this type of device is disclosed in the document “Japanese Patent Laid-Open No. 58-123240”, and a configuration example is shown in FIG.

FIG. 2 is a block diagram showing a configuration of a conventional echo canceller, in which a received signal arriving from the receiving input terminal 1 is spilled from the receiving terminal 2 to the transmitting input terminal 3 and transmitted as an echo transmission signal Sn.
And the pseudo echo signal from this Sn Is subtracted by the subtractor 5, and the erase residual signal En is sent to the transmission output terminal 4 and the correction circuit 9. In the correction circuit 9, the correction amount ▲ Δh ⁿ _j ▼ (the j-th tap coefficient correction amount at time n) for correcting the echo path characteristic by a constant algorithm using the erasure residual signal En and the received signal Rn is calculated. Calculate and update the contents of the second memory 10.

Pseudo echo signal Is calculated by passing the received signal sequence stored in the first memory and the tap coefficient sequence stored in the second memory through the convolution integration circuit 7. Since a true transmission signal and an echo transmission signal are simultaneously applied to the transmission terminal 3 during a two-way call, the echo transmission signal is erased from the output of the subtractor 5, and only the true transmission signal remains. However, if the tap coefficient is corrected using this, the correction operation must be stopped because it is erroneously corrected. As an example of the method of detecting the two-way call, in the example of FIG. 2, the two-way call detection circuit 11 compares the transmission signal Sn with the output of the square sum circuit 8 of the reception signal stored in the first memory 6. Then, when the transmission signal Sn is larger, it is determined to be a two-way call and detected, and thereby the correction operation of the correction circuit 9 is stopped. The same effect can be obtained by using the sum of squares of the transmission signal Sn. In addition, the transmission signal Sn
There is also a method of using the erasure residual signal En instead of the above, and when the erasure residual signal En exceeds a certain threshold value, the bidirectional call detection is performed.

(Problems to be Solved by the Invention) However, the method described above is effective to some extent when the echo path wraparound amount is a relatively small signal level, but has the following drawbacks and problems.

(1) When the true transmission signal is small, it is difficult to detect the bidirectional call state.

(2) If the wraparound amount in the echo path is large, the transmission signal for the echo becomes large, and the echo path is not correctly estimated due to false detection as a two-way communication state detection.

(3) Even when the echo path changes, the erasure residual signal becomes large, and it is erroneously detected as the two-way communication state detection, and the necessary echo path estimation cannot be performed.

The object of the present invention is to solve the above-mentioned drawbacks and has been achieved by focusing on the flat delay of the echo.

(Means for Solving Problems) The present invention relates to an adaptive control echo canceller device, in which an adaptive digital filter (Adaptive Digital Filter) is used.
This is a two-way call detector that focuses on the low-order tap coefficient of ter (hereinafter referred to as ADF) and observes the magnitude of this tap coefficient value so that the two-way call state can be detected without error.

(Function) The low-order stage of the ADF corresponds to the flat tee of the echo, and the tap coefficient of the low-order stage is small when only the echo component is present (when there is no true transmission signal), and the bidirectional communication state (true (When there is a transmission signal of), it changes to a large value.

This characteristic is not significantly affected by the size of the true transmission signal, the size of the echo, or the progress thereof, so the low-order tap coefficient value of the ADF is set to an appropriate threshold value using an evaluation function such as the sum or square sum of them. By comparing with, it is possible to determine the two-way communication state.

(Embodiment) FIG. 1 is a block diagram showing an embodiment of the present invention, in which 1 is a receiving input terminal, 2 is a receiving terminal, 3 is a transmitting input terminal, 5 is a subtractor, 11 is a bidirectional call detecting circuit, and 12 Is a tap coefficient correction amount calculation circuit, 14 to 16 are adders, many 18 are unit delay elements,
Many 19 are multipliers, 20 are adders, 12 to 19 are AD
Make up F.

In FIG. 1, the signal arriving from the reception input terminal 1 wraps around from the reception terminal 2 to the transmission input terminal 3 and becomes the echo transmission signal Sn. Pseudo echo signal that cancels this signal Sn Is the partial pseudo echo signal that is the output of the ADF in the part of the first-stage tap coefficients a _{O to} a _L. And the ADF output of the tap coefficient part of the part of a _{L + 1} ~ a _M And is added. The erasure residual signal En ₁ is sent to the tap coefficient correction amount calculation circuit 12 and the correction amount ▲ Δa ⁿ _j ▼ (ojM) of the ADF tap coefficients a _{O to} a _{L to} a _M is calculated. Next, from the cancellation residual signal En ₁ to the pseudo echo signal The erase residual signal En ₂ obtained by adding is sent to the transmission output terminal 4.

FIG. 3 is an explanatory diagram of a general echo path impulse response. The adaptive control type echo canceller corrects and updates the tap coefficient value of the ADF by following the change of the impulse response sequence of this echo path. This means that the tap coefficient value continuously changes with time t of the impulse response. It is based on the fact that it has a corresponding relationship with the discrete amplitude values of the subsequent damped vibration. The impulse response of the echo path is a flat delay (Flat delay, the time until the echo canceller confirms the echo component transmission signal. Of course, it depends on the application system, but it is usually several msec). Is generally decided by.

In Fig. 1 again, the low-order stage of ADF (tap coefficient a _O ~
a _L ) corresponds to this flat delay, and the bidirectional speech detection circuit 11 has a tap coefficient a _{O to} a _{L of} a low-order stage.
To monitor. In steady state with no transmitted signal, tap coefficient
Although a _{O to} a _L converge to approximately zero, the tap coefficient a _{O to} a _L becomes large in the bidirectional call state, and the bidirectional call detection circuit 11
Determines that the two-way communication state is achieved when the sum of the tap coefficients a _{O to} a _L exceeds a certain threshold. Even if the echo path fluctuates and the transmitted signal for the echo is large,
Since the low-order tap coefficient in the steady state still converges to zero, erroneous detection of the bidirectional communication state can be avoided.

Next, when it is determined that the two-way call state, the information is sent to the tap coefficient correction amount calculation circuit 12, and the tap coefficient of the higher order stage is
Modification update of a _{L + 1 to} a _M is stopped and fixed so that incorrect modification is not performed. On the other hand, the tap coefficients a _{O to} a _L of the low-order stage are constantly revised and updated, and when the state where the tap coefficient becomes smaller than the threshold value is detected, it is recognized that the two-way communication state has ended, and all tap coefficients a _{O to} a Restart the modification update of _M. Adder 15
The difference signal between the ADF output and the transmission signal from the transmission input terminal 3 is created by the subtractor 5 according to the output of
However, since the tap coefficients a _{O to} ae of the low-order stage are constantly updated, the pseudo echo signal Because the cancellation residual signal En ₁ is disturbed by this, in this embodiment, in the adder 20, the difference signal which is the output of the adder 5 and the pseudo echo signal of the low-order stage are added. And the sum is made and this sum signal is sent from the transmission output terminal to improve the call quality.

(Effects of the Invention) As described above, the present invention determines the bidirectional communication state by paying attention to the flat delay of the impulse response of the echo path. Therefore, when the true transmission signal is small or the echo component is small. Even if it is large or the echo path changes, it is possible to more reliably determine the bidirectional communication state.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is an explanatory diagram of a conventional technique, and FIG. 3 is an explanatory diagram of an impulse response of an echo path. 1 is a reception input terminal, 2 is a reception terminal, 3 is a transmission input terminal,
Reference numeral 5 is a subtractor, 11 is a two-way call detection circuit, 12 is a tap coefficient correction amount calculation circuit, 14 to 16 are adders, 18 is a unit delay element, 19 is a multiplier, and 20 is an adder.

Claims (2)

[Claims] 1. An echo canceller comprising an adaptive digital filter which receives a reception signal from a reception input side terminal and outputs a difference signal between the transmission signal and the output of the filter to the transmission output terminal side. The filter is provided with means for constantly monitoring the low-order tap coefficient and determining the bidirectional call state by exceeding a preset threshold value. Based on the result of the bidirectional call state determination by the determining means, the filter is provided. The echo canceller characterized by stopping the update of the tap coefficient of the high order stage in. 2. An echo canceller comprising an adaptive digital filter which receives a reception signal from a reception input side terminal and outputs a difference signal between the transmission signal and the output of the filter to the transmission output terminal side. A means for outputting the sum signal of the difference signal between the signal and the filter output and the output of the low order stage in the filter to the transmission output terminal side (20), and constantly monitoring the tap coefficient of the low order stage in the filter. Providing a means for judging a two-way communication state by exceeding a preset threshold value, and stopping the update of the tap coefficient of a high-order stage in the filter based on the two-way communication state judgment result by the judging means. , An echo canceller featuring.