JPS6343013B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an adaptive echo canceler for eliminating echo signals in a communication line.

2. Description of the Related Art Conventionally, echo prevention devices have been used as means for preventing communication interference caused by echo signals generated in long-distance communication lines. However, this device has drawbacks such as disconnection at the beginning of the line and clicking noise due to the on/off operation of the line.

Echo canceling devices have been put into practical use to solve the drawbacks of such echo blocking devices. This device measures the echo path characteristics, synthesizes a pseudo echo signal that approximates the echo signal based on the measured characteristics, and cancels the actual echo signal using the pseudo echo signal. Such an echo cancellation device does not have the disadvantages of an echo suppression device because it does not turn the line on and off.

As is well known, the adaptive echo cancellation device can be regarded as a type of system identification device. That is, a received signal is used as a signal for system identification, and a pseudo echo path that is almost the same as the echo path of the unknown system is formed (identified). Here, the received signal needs to have a frequency band that is the same as or greater than that of the echo path to be identified. That is, identification cannot be performed for bands other than the band of the received signal used, and the identification becomes so-called undefined.

In an actual echo canceller, when a normal voice signal, white noise, or the like is used as an identification signal, sufficiently stable identification can be performed because it includes a spectrum in the same band as the echo path. However, when applied to a communication line, a signal with a very narrow band compared to the band of the echo path may be applied for a long time as an identification signal. That is, continuous sine waves, modem signals, etc. may be transmitted. When such a narrowband signal is used as a received signal, the band of the narrowband signal is identified, but the other bands are undefined. That is,
A correction loop is formed for the band of the narrowband signal and identification is possible, but a correction loop is not formed for other bands and identification is not possible.

On the other hand, in an actual echo canceller, there are limitations due to its hardware configuration, and noise (disturbance) is mixed into the echo signal in the echo path, so there is a calculation error in the process of identifying the pseudo echo path. occurs. This calculation error does not cause a major problem as long as the identification signal includes the spectrum of the entire echo path band. but,
When the narrowband signal is applied, the following problems occur.

That is, in a frequency band outside the narrow band where there is no signal, a correct transfer function cannot be estimated, and it becomes undefined. Therefore, if estimation is continued for a long time, calculation errors will gradually accumulate and may exceed the dynamic range of the memory circuit that stores the tap coefficients of the transversal filter that constitutes the pseudo echo path. The generated impulse model differs from the actual echo path characteristics even in the narrow band, making it impossible to cancel the echo.

SUMMARY OF THE INVENTION An object of the present invention is to provide an adaptive echo cancellation device that ensures predetermined cancellation characteristics for all received input signals and at the same time provides stable operation.

The present invention provides a pseudo echo path for creating a pseudo echo signal from a received signal, a subtraction circuit for taking a difference between an echo signal and the pseudo echo signal and outputting a residual echo signal, and a subtraction circuit for outputting a residual echo signal between the received signal and the residual echo signal. a correction amount calculating circuit that calculates and outputs a tap coefficient correction amount of a transversal filter constituting the pseudo echo path from the signal; and a tap coefficient stored in a storage circuit for tap coefficients of the transversal filter in the pseudo echo path. an adaptive echo canceller comprising: an addition correction circuit that corrects the tap coefficient stored in the storage circuit by adding the output of the correction amount calculation circuit; The present invention is characterized by comprising a tap coefficient control circuit that controls tap coefficients stored in the storage circuit depending on the output signal of the memory circuit.

Next, a description will be given with reference to the drawings.

FIG. 1 shows a conventional adaptive echo canceler in block diagram form.

The received signal X _j at time j inputted from the received signal input terminal 101 is input to the received signal storage circuit 105 and simultaneously sent to the echo path through the received signal output terminal 102 . The transmission signal _Yj at time j inputted from the echo path to the transmission signal input terminal 103 is sent to a subtraction circuit 109, and the difference from the pseudo echo signal Y^ _j created in the pseudo echo path 106 is taken. The output signal e _j of the subtraction circuit 109 is sent to the transmitting signal output terminal 104
The signal is sent to a transmission path, and simultaneously sent to a correction amount calculation circuit 108, which will be described later.

A pseudo echo path 106 consisting of a transversal filter includes a received signal storage circuit 105 that stores a received signal, a storage circuit 107 that stores tap coefficients of the transversal filter, and a convolution operation of the contents of these storage circuits 105 and 107. and an integral calculation circuit 110. The output signal of the integral calculation circuit 110 becomes the pseudo echo signal Y^ _j .

Output signal e _j of subtraction circuit 109 and memory circuit 105
The receiving signal from the transversal filter is applied to a correction amount calculation circuit 108, and the correction amount of the tap coefficient of the transversal filter is calculated. Furthermore, the correction amount calculation circuit 108
The output of is applied to an addition correction circuit 204 for modifying the tap coefficients.

FIG. 2 is a block diagram of the adaptive echo canceler shown in FIG. 1, in which the learning identification method is used as a modification algorithm.

In FIG. 2, the correction amount calculation circuit 108 is composed of multipliers 201, 203, a divider 202, and a square accumulation circuit 205, and calculates the following equation (1). Further, the tap coefficient is modified by the addition modification circuit 204 based on equation (2).

h _i ^(j+1) = h _i (j) + Δh _i (j) (2) where i represents the i-th tap, α is the correction coefficient, and N represents the number of taps of the transversal filter.

Following the above learning identification correction algorithm,
When a normal voice signal, white noise, or the like is used as the identification signal, sufficiently stable identification can be performed because it includes a spectrum in the same band as the echo path. However, when a continuous sine wave or modem signal, which has a very narrow band compared to the echo path band, is transmitted as an identification signal, calculation errors may cause unstable operation.

In contrast, the major feature of the present invention is that the calculation error generated in the correction amount calculation circuit 108 is very small in the initial stage, and the calculation error is prevented from accumulating in the same direction. The purpose is to provide stable operation.

FIG. 3 shows a basic embodiment of the invention.

This device differs from the configuration shown in FIG. 2 in that the contents of the tap coefficients of the transversal filter depend on both the storage signal of the storage circuit 107 that newly stores the tap coefficients and the output signal of the correction amount calculation circuit 108. A tap coefficient control circuit 401 is provided at the output of the memory circuit 107 to control the output from running out of control in one direction.

FIG. 4 shows a first embodiment of the tap coefficient control circuit 401 in detail.

The polarity signal sgn h _i ^(j-1) of the tap coefficient h i ^(j-1 ) stored in the storage circuit 107 and the polarity signal sgn _{Δh i (} j-1) of the output signal Δh _i (j ^-1) of the correction amount calculation circuit ^{108 -1)} is applied to the exclusive OR gate 503.
If the polarities of these signals are the same, then K ₁ ,
A selection circuit 50 that selects K ₂ if the polarity is different.
2. Also, the output of this selection circuit 502 is stored in the storage circuit 107 as a tap coefficient h _i ^(j-1).
It is provided with a multiplier 501 that multiplies by. here K ₁
and K ₂ is a constant and K ₂ >K ₁ .

With this configuration, if the signal polarity of the storage circuit 107 and the signal polarity of the correction amount calculation circuit 108 are the same, the signal h obtained by multiplying the tap coefficient h _i ^(j-1) of the storage circuit 107 by the constant K _{1 i} ^(j-1) ′ is added to the addition circuit 204
On the other hand, if the polarity is different, the adder circuit 204
A signal obtained by multiplying the output of the memory circuit 107 by a constant K ₂ is applied to. That is, the tap coefficients are controlled to reduce calculation errors.

This makes it possible to prevent the tap coefficient of the transversal filter from going out of control in one direction due to the accumulation of calculation errors.

FIG. 5 shows a tap coefficient control circuit 401 of the present invention.
A second example is shown below.

That is, the polarity signal sgn h _i ^(j-1) of the tap coefficient h _i ^(j-1 ) stored in the storage circuit 107 and the polarity signal sgn Δh of the output signal Δh _i ^(j-1) of the correction amount calculation circuit 108 _i ^(j-1
) are led to the exclusive OR gate 506.
Further, the output of the memory circuit 107 is branched into two branches, one of which is connected to a first multiplier 504 for multiplying by a constant _K1 , and the other to a second multiplier ₅₀₅ for multiplying by a constant K2. Furthermore, a selection circuit 507 is provided which selects one of the outputs of the multipliers 504 and 505 according to the output of the gate 506.

As in the previous embodiment, the selection circuit 507 selects, for example, the signal polarity of the storage circuit 107 and the correction amount calculation circuit 108.
If the signal polarities are the same, the output from the multiplier 504 obtained by multiplying the output signal of the memory circuit 107 by a constant K ₁ is applied to the adder circuit 204 . Of course,
K ₁ <K ₂ , and the effect is exactly the same as in the above embodiment.

As can be seen from the above description, even if a narrow band signal is applied as a reception signal for a long time, calculation errors are not accumulated, and an adaptive echo canceling device that provides sufficiently stable operation can be realized.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional adaptive echo canceler, FIG. 2 is a block diagram of a correction amount calculation circuit shown in FIG. 1, FIG. 3 is a block diagram of an adaptive echo canceler according to the present invention, and FIG. This figure shows a first embodiment of the present invention, showing the configuration of the main part of FIG. 3, and FIG. 5 shows a second embodiment of the present invention. In the figure, 105...reception signal storage circuit, 10
7...Tap coefficient storage circuit, 109...Subtraction circuit, 1
10... Integral calculation circuit, 201, 203... Multiplier,
202... Divider, 204... Adder, 205... Square accumulation circuit, 106... Pseudo echo path, 401... Tap coefficient control circuit, 501, 504, 505... Multiplier, 502, 507... Selection circuit.

Claims (1)

[Scope of Claims] 1. A pseudo echo path that creates a pseudo echo signal from a received signal, a subtraction circuit that takes the difference between the echo signal and the pseudo echo signal and outputs a residual echo signal, and a subtraction circuit that generates a residual echo signal from the received signal. a correction amount calculating circuit that calculates and outputs a tap coefficient correction amount of a transversal filter constituting the pseudo echo path from the difference echo signal; and a tap stored in a storage circuit for tap coefficients of the transversal filter in the pseudo echo path. In the adaptive echo canceling device, the adaptive echo canceling device includes an addition correction circuit that adds the coefficient and the output of the correction amount calculation circuit to correct the tap coefficient stored in the storage circuit. a tap coefficient control circuit that outputs either a value obtained by multiplying the tap coefficient stored in the storage circuit by a first constant or a value obtained by multiplying the tap coefficient by a second constant depending on the polarity of the output signal of the correction amount calculation circuit; An adaptive echo canceling device characterized by comprising: 2. In the adaptive echo canceling device according to claim 1, the tap coefficient control circuit is controlled depending on the polarity of the tap coefficient stored in the storage circuit and the polarity of the output signal of the correction amount calculation circuit. An adaptive echo cancellation device comprising: a circuit for selecting one of two constants; and a multiplier for multiplying the constant selected by the selection circuit by a tap coefficient stored in the storage circuit. 3. In the adaptive echo canceling device according to claim 1, the tap coefficient control circuit is configured to have a tap coefficient control circuit configured to operate a tap coefficient control circuit using a first constant that multiplies one of two signals obtained by branching the tap coefficient output stored in the storage circuit by a first constant. a second multiplier that multiplies the other by a second constant;
Depending on the polarity of the tap coefficient stored in the storage circuit and the polarity of the output signal of the correction amount calculation circuit,
An adaptive echo cancellation device comprising: a circuit for selecting the output of either the first or second multiplier.