JPS60235535A - Echo canceller - Google Patents

Abstract

PURPOSE:To obtain an echo canceller showing excellent converging characteristics and cancelling amounts by allowing the 1st adaptive transversal filter to apply adaptive control so as to minimize the residual signal of an input signal at the reception side and allowing the 2nd adaptive transversal filter to apply adaptive control to minimize an output signal at the transmission side. CONSTITUTION:The 1st adaptive transversal filter 11 applies the adaptive control to minimize the residual signal of the input signal at the reception side. Thus, the residual signal of the input signal at the reception side is a signal subtracting the correlation component corresponding to the tap numbers of the 1st adaptive transversal filter 11 from the input signal at the reception side. The 2nd adaptive transversal filter 13 uses the residual signal of the input signal at the reception side as the input signal and brings sequentially the output signal close to the echo signal generated on the echo pass by applying adaptive control to minimize the said output signal at the transmission side. Thus, this system is effective by using a signal itself having high correlativity such as a voice signal to estimate the transmission system.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention eliminates echo signals generated by impedance mismatching of line converters between two-wire and four-wire telephone lines. This invention relates to an echo canceller used for the purpose of improving the call quality of long-distance telephone lines such as submarine cable lines.

1. Configuration of a conventional example and its problems FIG. 1 shows a configuration diagram of a conventional echo canceller. 1st
In the figure, 1 is a receiving side input terminal, 2 is a receiving side output terminal, 3 is a transmitting side input terminal, 4 is a transmitting side output terminal, 5 is an adaptive transversal filter, and 6 is a subtracter.

The operation and problems of the echo canceller configured as above will be explained below.

In the following explanation, the signal inside the echo canceler will be treated as a discrete time signal sampled with respect to time, but the sampler and hold circuit required for this are well known and will be omitted in the following explanation. .

Further, the prohibition of the adaptive control operation of the superimposed speech detector and the adaptive transversal filter based thereon is also well known and is not a feature of the present invention, so the following explanation will be omitted.

Now, when a receiving side input signal is input to the receiving side input terminal 1, it is inputted to the adaptive transversal filter 5 as an input signal, and at the same time, it is outputted as it is to the receiving side output terminal 2 as a receiving side output signal.

The transmission side input signal input to the transmission side input terminal 3 is outputted to the transmission side output terminal 4 as a transmission side output signal after the output signal (pseudo echo signal) of the adaptive transversal filter 5 is subtracted by the subtracter 6. be done. The adaptive transversal filter 5 successively approximates the output signal of the adaptive transversal filter 6 to the echo signal generated by the echo path by performing adaptive control so as to minimize the transmitting side output signal. Therefore, at the stage when the adaptive control operation has converged, a signal in which the echo signal contained in the transmitting side input signal has been eliminated can be output as the transmitting side output signal.

The above is the operation of the conventional echo canceller shown in FIG.

The echo canceller with the configuration shown in Figure 1 can obtain good characteristics when a signal with no correlation, such as a white noise signal, is input as an input signal on the receiving side. There is a problem in that when a signal having the same characteristics is input, the convergence characteristics and the amount of cancellation, which are the basic characteristics of echo canceling, deteriorate.

As an echo canceller that solves these problems,
An echo canceller based on the method described in Japanese Patent Publication No. 58-31129 has been proposed. In this method, by reducing the correlation of receiving side input signals based on an autoregressive model, it is possible to obtain good convergence characteristics and cancellation amount even for receiving side input signals having strong correlation. However, this method has problems in that the hardware configuration is complex and the amount of calculation is large.

Purpose of the Invention In view of the drawbacks of the above-mentioned conventional examples, the present invention d provides an echo canceller that can obtain good convergence characteristics and cancellation amount even for receiving side input signals having strong correlations such as audio signals. The purpose is to provide a simple configuration suitable for

Structure of the Invention The present invention includes a first adaptive transversal filter that inputs a receiving side input signal manually, and a method for adjusting the receiving side input signal by subtracting the output signal of the first adaptive transversal filter from the receiving side input signal. a first subtracter that creates a residual signal; a second adaptive transversal filter that receives the residual signal of the receiving side input signal; and an output of the second adaptive transversal filter from the transmitting side input signal. a second subtracter that creates a transmitter output signal by subtracting the signal; the first adaptive transversal filter performs adaptive control to minimize the residual signal of the receiver input signal; The second adaptive transversal melter performs adaptive control to minimize the transmitting side output signal, so it has a simple configuration and can be used even for receiving side input signals having strong correlations such as audio signals. The present invention provides an echo canceller that exhibits good convergence characteristics and cancellation amount.

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 2 shows a configuration diagram of an echo canceller in an embodiment of the present invention. In FIG. 2, 7 is a receiving side input terminal, 8 is a receiving side output terminal, 9 is a transmitting side input terminal,
10 is a transmission side output terminal, 11 is a first adaptive transversal filter, 12 is a first subtracter, 13 is a second adaptive transversal filter, and 14 is a second subtracter.

In this embodiment, in order to reduce the correlation of the human input signal on the receiving side, an adaptive transversal filter with the same configuration as the adaptive transversal filter that creates the pseudo echo signal but with a small Tauf number and a subtracter are newly added. With this simple configuration, it is possible to obtain good convergence characteristics and cancellation amount even for receiving side input signals having strong correlation such as audio signals.

The operation of the embodiment shown in FIG. 2 will be explained below.

Now, when a receiving side human input signal is input to the receiving side input terminal 7, it is input to the first adaptive transversal filter 11 as an input signal, and at the same time, it is directly outputted to the receiving side output terminal 8 as a receiving side output signal. Ru.

The first subtracter 12 creates a residual signal of the receiving side input signal by subtracting the output signal of the first adaptive transversal filter 11 from the receiving side input signal, and inputs the input signal to the second adaptive transversal filter 13. Enter as .

The first adaptive transversal filter 11 performs adaptive control so as to minimize the residual signal of this receiving side input signal. By doing this, the residual signal of the receiving side input signal becomes a signal obtained by subtracting the correlation component corresponding to the number of taps of the first adaptive transversal filter 11 from the receiving side input signal.

Further, by adaptively removing correlation components according to the degree of correlation of the receiving side input signals, the present invention can be applied to receiving side input signals having various correlations.

The transmitting side input signal inputted to the transmitting side input terminal 9 is
After the output signal (pseudo echo signal) of the second adaptive transversal filter 13 is subtracted by the subtracter 14, the subtracter 14 subtracts the output signal (pseudo echo signal) from the second adaptive transversal filter 13, and then outputs the resultant signal to the transmission side output terminal 10 as a transmission side output signal.

The second adaptive transversal filter 13 converts the residual signal of the input signal on the receiving side into a human signal, and performs adaptive control to minimize the output signal on the transmitting side, thereby converting the output signal into an echo path. It successively approximates the echo signal generated by .

The above operation will be explained in principle below. Now, Z transformation iX■) of the input signal on the receiving side, 2 of the residual signal of the input signal on the receiving side
Conversion Y■), 2 conversion e Y (Z) of the input signal on the transmitting side
, the system function of the echo path is H(Z), and the system function of the first adaptive transversal filter 11 'e A
(Z), equations (1) and (2) hold true.

Y CZ,') = H(Z)X(Z) -...-(1
)X(Z)=(1-mu)X(Z) ・・・・・・@)
Equation (3) holds true from equations (1) and (2).

Therefore, in the second adaptive transversal filter H(Z) 13, the impulse response corresponding to the system function 1-A(Z) is sequentially estimated to create a pseudo echo signal which is an estimated signal of the echo signal. Become.

In the echo canceller according to this embodiment, the first adaptive transversal filter 11 and the second adaptive transversal filter 13 can have the same internal configuration but differ only in the number of taps, and can be used for the same calculation. LSI that uses hardware in a time-sharing manner or that can repeatedly use the basic cell per tap
The configuration is suitable for

Now, the sampling frequency inside the echo canceller is assumed to be s KHz, taking into account the frequency band of the telephone line. The number of taps of the first adaptive transversal filter 11 varies depending on the degree of correlation of the input signals on the receiving side, but in the case of audio signals, 10 taps is sufficient to achieve the effect. The number of taps of the second adaptive transversal filter 13 is determined by the length of the impulse response to the system function9, and for example, in the case of a telephone line in Japan, approximately 40 m5eO1, or 320 taps are required. Therefore, the first step necessary to reduce correlation is
The amount of hardware and calculation required to realize the second adaptive transversal filter 11 is sufficiently smaller than that of the second adaptive transversal filter.

Various configurations have been proposed for the configuration of the adaptive transversal filter, but these are not a feature of the present invention.
Examples of the present invention will be briefly described. For details on the operation, see the following documents (D, D.

Duttweiler: ” A Twelve-Ch
annel DigitalEcho Cancele
r”, IEEE Trans, on Common
,.

Vol, C0M-26, NO, 5, PP, 647-65
3. M2L7 1978) etc.

FIG. 3 shows the internal configurations of the first adaptive transversal filter 11 and the second adaptive transversal filter 13 according to an embodiment of the present invention.

In Figure 3, 1oO is an adaptive transversal filter, 1
01 is an input signal terminal, 102 is an output signal terminal, 103 is an adaptive control signal input terminal, 104-1 to 104-Nld adaptive French universal filter main component, 106-
1 to 105-N are delay elements, 108-1 to 106-N are multipliers, 107-1 to 107-N are constant multipliers, and 108
-1 to 108-N are adders, 109-1 to 109-N are delay elements, 110-1 to 110-NFi multipliers, 111
is an adder, and N represents the number of taps.

Multipliers 110-1 to 110-N and adder 111 for multiplying delay elements 105-1 to 105-N having a delay time equal to the sampling period and tap coefficients are 10
A transversal filter with N taps is constructed, with 1 as an input signal terminal and 102 as an output signal terminal. Third
The other components in the figure are for updating tap coefficients based on adaptive control.

Since the operation of updating the tap coefficient is common to each tap, the operation of the first tap 104-1 will be described here.

The adaptive control signal input from the adaptive control signal input terminal 103 is multiplied by the output of the delay element 105-1 in the multiplier 106-1, and then multiplied by the constant multiplier 107-1 (K is a constant).
This becomes the correction amount when updating the tap coefficient. Adder 108
-1, the amount of correction at the time of updating the tap coefficient determined as above and the current tap coefficient are added, and the result is outputted via the delay element 109-1i having a delay time equal to the sampling period. be the new tap coefficient at the sampling point in time.

The configuration described above is an example of the configuration of an adaptive transversal filter, and for example, the following document (J, Nagu
mo and A, Noda: ” A Learni
ng Method for System Ident
ification”, IHEE Trans.

on Automatic ControlJol,A
C-12,) Jo, 3°PP, 282-287. June
It is also possible to adopt a configuration based on the method described in J. E. 1967).

Effects of the Invention As described above, according to the present invention, by newly providing an adaptive transversal filter and a subtracter for the purpose of reducing the correlation of input signals on the receiving side, reception with strong correlation such as audio signals can be performed. It is possible to provide an echo canceller that exhibits good convergence characteristics and cancellation amount even for side input signals.

Furthermore, the newly provided adaptive transversal filter can have the same configuration as the conventional adaptive transversal filter, and has a simple configuration suitable for LSI implementation, with a small amount of calculation and hardware.

The echo canceller according to the present invention is effective when estimating a transmission system using highly correlated signals such as voice signals. is also effective.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a conventional echo canceller, FIG. 2 is a configuration diagram of an echo canceller according to an embodiment of the present invention, and FIG. 3 is an internal configuration diagram of the adaptive transversal filter. 11...first adaptive transversal filter,
12...First subtractor, 13...Second adaptive transversal filter, 14...Second
subtractor, 100...adaptive transversal filter, 1o1...human input signal terminal, 102...
...Output signal terminal, 103...Adaptive control signal input terminal, 104-1 to 104-N...Adaptation 1
- Basic components of Lanceversal filter, 105-1
~105-N...delay element, 106-1~106
-N... Multiplier, 107-1 to 107-N...
...Constant multiplier, 108-1 to 108-N...
Adder, 109-1 to 109-N...delay element,
110-1 to 110-N... Multiplier, 111.
...Adder. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure 2

Claims (1)

[Claims] a first adaptive transversal filter that receives the receiving side input signal; and a first adaptive transversal filter that creates a residual signal of the receiving side input signal by subtracting the output signal of the first adaptive transversal filter from the receiving side input signal. 1, a second adaptive transversal filter which inputs the residual signal of the input signal on the receiving side, and subtracting the output signal of the second adaptive transversal filter from the input signal on the transmitting side. a second subtracter for creating an output signal, the first adaptive transversal filter performs adaptive control to minimize the residual signal of the receiving side input signal, and the second adaptive transversal filter An echo canceller characterized in that the filter performs adaptive control so as to minimize the transmitting side output signal.