JPS642252B2 - - 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 noises caused by turning the line on and off. 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 canceller does not have the drawbacks of an echo blocker because it does not turn the line on or 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 cancellation device, when a normal voice signal or white noise 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 as an identification signal for a long time. 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 is a calculation error in the process of identifying a pseudo echo path due to limitations due to its hardware configuration and noise (disturbance) mixed into the echo signal in the echo path. occurs. This calculation error does not cause a major problem as long as the identification signal includes the entire spectrum of the echo path. 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. This will be explained below 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 and 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 ^(j+1) _i = h ^(j) _i +Δh ^(j) _i (2) where i represents the i-th tap, α represents 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, a major feature of the present invention is that the calculation error generated in the repair amount calculation circuit 108 is very small in the initial stage. The reason is that a minute error signal is forcibly added to the residual signal so that a correction loop is formed. The present _invention provides a pseudo _- echo path for creating a pseudo-echo signal from a received signal Based on the residual echo signal and the correction coefficient α, the correction amount Δh ^j _i of the tap coefficient h ^j _i of the transversal filter constituting the pseudo echo path is calculated and output according to the following equation (1). a correction amount calculation circuit; In the transversal filter, an adaptive type having an addition circuit that adds a tap coefficient output stored in a storage circuit for storing tap coefficients and an output of the correction amount calculation circuit to correct the tap coefficient stored in the storage circuit. In the echo cancellation device, the correction coefficient α in the equation (1), the received signal X _ji , and the residual echo signal are determined depending on the tap coefficient stored in the storage circuit, the received signal, and the residual echo signal. The invention is characterized by comprising a circuit for controlling at least one of e and _j . Examples of the present invention will be described below. FIG. 3 shows a first embodiment of the invention. This device includes a residual echo signal control circuit 601. In this control circuit 601, the polarity signal of the storage signal h ^j-1 _i of the tap coefficient storage circuit 107 is
sgnh ^j-1 _i and the output signal of the reception signal storage circuit 105
The polarity signal sgnX _ji of X _ji is input to the first exclusive OR gate 504. In addition, the output signal of the first gate 504 and the polarity signal of the residual echo signal e _ji
sgne _j-1 is input to the second exclusive OR gate 505. Further, a selection circuit 502 selects constant k ₁ or k ₂ according to the output of the second gate 505.
and a multiplier 501 that multiplies the residual echo signal e _j-1 by the constant selected by the selection circuit 502. Here, k ₂ >k ₁ and these selections follow Table 1 below. Of course, the effect is exactly the same as in the previous embodiment.

[Table] With this configuration, for example, when Δh ^j-1 _i in equation (1) above is +, constant k ₁ is selected, and Δh ^j-1 _i
When is -, constant _k2 is selected. This constant is multiplied by the residual echo signal e _j-1 and the correction amount calculation circuit 108
added to. In this way, control is performed to reduce Δh ^j-1 _i . FIG. 4 shows a second embodiment of the invention. This device is designed to vary the correction coefficient α in equation (1). The polarity signal sgnh ^j _i of the storage signal h ^j _i of the tap coefficient storage circuit 107 and the output signal of the receive signal storage circuit 105
The polarity signal sgnX _ji of X _ji is applied to the first exclusive OR gate 506. Further, the output signal of the first gate 506 and the polarity signal sgne _j of the residual echo signal e _j are connected to the second exclusive OR gate 506.
I am trying to input it to 07. Then, according to the output of the second gate 507, the correction coefficient α ₁ of the first correction coefficient setter 802 and the second correction coefficient setter 80 are adjusted.
Selection circuit 5 for selecting one of the correction coefficients α ₂ of 3
It is equipped with 08. Here, α ₁ >α ₂ and these selections are made according to Table 1 above. FIG. 5 shows a third embodiment of the invention. This device includes a reception signal control circuit 901. In this control circuit 901, the polarity signal sgnh ^j _i of the storage signal h ^j _i of the tap coefficient storage circuit 107 is
and the polarity signal sgnX _ji of the output signal X _ji of the reception signal storage circuit 105 are connected to the first exclusive OR gate 50
I am trying to input it to 4. In addition, the output signal of the first gate 504 and the polarity signal of the residual echo signal e _j
sgne _j is input to the second exclusive OR gate 505. and second gate 505
A selection circuit 502 that selects constant k ₁ or k ₂ with the output of
, and a multiplier 501 that multiplies the reception signal applied to the correction amount calculation circuit 108 by a constant selected by the selection circuit 502. Here, k ₂ > k ₁ and these selections are based on the first
According to the table. As described above, the adaptive echo canceling devices shown in the first, second and third embodiments of the present invention all exhibit the same effect. It is clear that the same effect can also be obtained by controlling the output of the accumulation circuit 205. As explained above, the adaptive echo canceling device according to the present invention is equipped with a circuit that controls at least one of the tap coefficients, the received signal, and the residual echo signal depending on the tap coefficient, the received signal, and the residual echo signal. The correction amount Δh ^j _i of the arithmetic circuit is controlled to be always small. Therefore, according to the present invention, it is possible to realize an adaptive echo cancellation device that provides sufficiently stable operation even when a narrowband signal is applied as a reception signal for a long time.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional adaptive echo canceller, FIG. 2 is a block diagram showing details of the correction amount calculation circuit 108 in FIG. 1, and FIG. 3 is an adaptive echo canceler according to the first embodiment of the present invention. FIG. 4 is a block diagram of an adaptive echo canceling device according to a second embodiment of the present invention, and FIG. 5 is a block diagram of an adaptive echo canceling device according to a third embodiment of the present invention. shows. In the figure, 105... receive signal storage circuit, 1
06... Pseudo echo path, 107... Tap coefficient storage circuit, 108... Correction amount calculation circuit, 109... Subtraction circuit, 110... Integral calculation circuit, 201, 20
3... Multiplier, 202... Division circuit, 204...
... Addition correction circuit, 205 ... Square accumulation circuit, 50
1... Multiplier, 502, 508... Selection circuit, 5
04, 505, 506, 507...Exclusive OR gate, 601...Residual echo signal control circuit, 90
1... Receiving signal control circuit.

Claims (1)

[Scope of Claims] 1. A pseudo echo path that creates a pseudo echo signal from the received signal X _ji ; a subtraction circuit that takes the difference between the echo signal and the output of the pseudo echo path and outputs a residual echo signal e _j ; a correction amount calculation circuit that calculates and outputs a correction amount of a tap coefficient h _{j i} of a transversal filter constituting the pseudo echo path based on the received signal, the residual echo signal e ^j and the correction coefficient α; and the transformer. Adaptive echo cancellation comprising: an addition circuit that corrects the tap coefficient stored in the storage circuit by adding the tap coefficient stored in the storage circuit that stores the tap coefficient in the versatile filter and the output of the correction amount calculation circuit; In the apparatus, one of two constants K ₁ and K ₂ is selected depending on the polarity signal of the tap coefficient, the polarity signal of the received signal, and the polarity signal of the residual echo signal stored in the storage circuit. and a multiplication circuit that multiplies the residual echo signal e _j applied to the modification amount calculation circuit by the constant K selected by the selection circuit, the modification amount calculation circuit comprising: , the formula below
(1) An adaptive echo canceling device characterized in that it outputs a calculation according to the following. 2. A pseudo echo path that creates a pseudo echo signal from the received signal X _ji , a subtraction circuit that takes the difference between the echo signal and the output of the pseudo echo path and outputs a residual echo signal e _j , and a correction amount calculation circuit that calculates and outputs a correction amount Δh j _i of the tap coefficient h ^j _i of the transversal filter constituting the pseudo echo path based on the difference echo signal and the correction coefficient ^α according to the following equation (1); and, An adaptive type comprising: an addition circuit that corrects the tap coefficient stored in the storage circuit by adding the tap coefficient stored in the storage circuit that stores the tap coefficient in the transversal filter and the output of the correction amount calculation circuit. The echo canceling device is provided with two setters for the correction coefficient α shown in equation (1) above, and further includes a polarity signal of the tap coefficient stored in the storage circuit, a polarity signal of the received signal, and a polarity signal of the residual echo signal. 1. An adaptive echo canceling device comprising: a selection circuit that selects one of the two correction coefficients depending on the polarity signal and outputs the selected one to the correction amount calculation circuit. 3. A pseudo _- echo path for creating a pseudo-echo signal _from the received signal a correction amount calculation circuit that calculates and outputs a correction amount of a tap coefficient h _j ⁱ of a transversal filter constituting the pseudo echo path based on a difference echo signal e _j and a correction coefficient α; In the adaptive echo canceling device, the adaptive echo canceling device has an addition circuit that corrects the tap coefficient stored in the storage circuit by adding the tap coefficient stored in the storage circuit and the output of the correction amount calculation circuit. a selection circuit that selects one of two constants K ₁ and K ₂ depending on the polarity signal of the tap coefficient stored in the tap coefficient, the polarity signal of the received signal, and the polarity signal of the residual echo signal; a control circuit having a multiplier for multiplying the reception signal X _ji by the constant K selected by the selection circuit; An adaptive echo canceling device characterized in that it outputs a calculation according to the following.