JPH01180126A - Echo canceller - Google Patents

Abstract

PURPOSE:To decrease the amplitude of an impulse response of a bypassed signal by controlling the impedance of a balance network of a 2/4-wire conversion circuit based on the tap coefficient of an adaptive filter. CONSTITUTION:A 4-wire side input is connected to an output of a pulse transmission circuit 1 receiving a digital signal S, a 2-wire input/output is connected to a 2-wire subscriber line (l) in a 2/4-wire conversion circuit 10 including a balance network ZB which is provided to the echo canceller. The echo canceller is further provided with an A/D converter 3 receiving a 4-wire output of the conversion circuit 10, an adaptive digital filter 4 receiving a digital signal S and outputting a bypassed cancel signal, and a subtraction circuit 5 subtracting the bypassed cancel signal from the bypassed signal outputted from the converter 3. The impedance of the network ZB is controlled based on the tap coefficient of the filter 4. Thus, the amplitude of the impulse response of the bypassed signal is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention is applied to a conversion circuit between a four-wire line and a two-wire line connected to a long-distance communication network.

The present invention is an integrated service digital network (fsDN).
tegrated 5 services Digital
Network) Suitable for use as an echo canceller for two-wire line transmission used in subscriber line digital transmission.

〔overview〕

The present invention is an echo canceller in which the amplitude of the impulse response of the wraparound signal is reduced by controlling the impedance value of the balance network of the two-wire/four-wire conversion circuit based on the tap coefficient value of the adaptive filter. .

[Conventional technology]

Conventionally, echo cancellers have a configuration in which a hybrid circuit attenuates the echo signal by at least 68 dB, an adaptive digital filter using digital signal processing attenuates the echo signal by at least 60 dB, and only the received signal is guided to the receiving circuit. ing.

In recent years, with the development of digital signal processing, a method has been realized in which an analog-to-digital converter is placed after the hybrid circuit and the signal and echo replica are subtracted using the digital signal. The required bit accuracy is strict, and it is preferable that the echo attenuation amount of the hybrid circuit is as large as possible.

[Problem that the invention seeks to solve]

However, with such conventional echo cancellers, the echo attenuation of the hybrid circuit can only be achieved by a few dB when the impedance of the balance network is one type, due to variations in the impedance of the subscriber line. .

In order to improve this, it is possible to adaptively change the impedance of the balance network or make it switchable, but there is a drawback that there is no simple and effective means for adaptation or switching.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned drawbacks, and it is an object of the present invention to provide an echo canceller in which the amplitude of the impulse response of the wrap-around signal is small and the accuracy required for the analog-to-digital converter can be relaxed.

[Means for solving problems]

The present invention provides an echo canceller in which the balance network of the two-wire/four-wire conversion circuit includes means for controlling its impedance value based on the tap coefficient value of the adaptive filter.

[Effect]

The impedance value of the balance network is controlled based on the tap coefficient value of the adaptive filter so as to reduce the amplitude of the impulse response of the wraparound signal.

〔Example〕

Embodiments of the present invention will be described with reference to the drawings. 1st
The figure is a block diagram of an echo canceller according to an embodiment of the present invention. In FIG. 1, the echo canceller has a four-wire side input connected to the output of a pulse transmitting circuit 1 that inputs a digital signal S, and a two-wire side input/output connected to a two-wire subscriber line l, and is connected to a balance network 2. an analog-to-digital converter 3 which inputs the four-wire output of the two-wire and four-wire converter circuit 10 and performs analog-to-digital conversion, and which inputs the digital signal S and outputs a wraparound cancellation signal. and a subtraction circuit 5 that subtracts a wrap-around cancellation signal from a wrap-around signal output from the analog-to-digital converter 3.

The two-wire four-wire conversion circuit 10 has a four-wire side coil inserted between a resistor R1, one terminal of which is connected to the output of the pulse transmitting circuit 1, and a common potential and the other terminal of the resistor R1. A transformer TR whose side coil is connected to the two-wire subscriber line l, a resistor R2 whose one terminal is connected to the output of the pulse transmitter circuit 1, and the other terminal of the resistor R2 and a common potential are inserted between balanced network 2. and a subtracting circuit 2 connected to the other terminal of the resistor R1 and the other terminal of the resistor R2, which attenuates the wraparound signal and provides a four-wire side output to the analog-to-digital converter 3.

The balance network Z includes a variable resistor R3 whose one terminal is connected to the other terminal of the resistor R2, and a variable resistor R3.
includes a parallel-connected resistor R1 and a capacitor C inserted between the other terminal of and a common potential.

Here, the feature of the present invention is that the balance network Z8 is provided with a variable resistor R3 based on the tap coefficient value corresponding to the pulse center of the adaptive digital filter 4.
The invention also includes means for controlling the resistance value of the resistor.

The operation of the echo canceller having such a configuration will be explained. FIG. 2 is a diagram showing the impedance of the echo canceller of the present invention when viewed from the subscriber line side. FIG. 3 is a diagram showing the impulse response of the wraparound signal of the echo canceller of the present invention.

In FIG. 1, a resistor R2, a balance network Z,
and the subtraction circuit 2 is a hybrid circuit for preventing a part of the transmitted pulse from going around to the receiving side,
Normally, when the impedance seen on the subscriber line side is ZL, a bridge is constructed by setting R+: ZL = R2: Z++, and the wrap-around signal is made zero.

If there is a limit to the accuracy of the analog-to-digital converter 3, it is possible to perform quantization by setting a dynamic range that is more advantageous for the signal-to-noise ratio (S/N) of the received signal as the surrounding signal is smaller. It is possible by design. The adaptive digital filter (ADF) 4 and the subtraction circuit 5 are circuits that almost completely remove the wraparound signal that has been largely eliminated by the hybrid circuit. The adaptive digital filter 4 is a balance network Z.

is configured to control the resistance value of.

The gist of the present invention is that the resistance value of the balance network Z is controlled by the tap coefficient value of the tap memory corresponding to the pulse center of the adaptive digital filter 4.

When looking at the subscriber line side, the impedance ZL is determined by the variable resistor R3, resistor R4, and capacitor C, as shown in Figure 2 (■), if there is no bridged tap near the line circuit.
The balance network Z has frequency characteristics that can be approximated by three-element impedance, and is matched to this.

By using this, the impulse response of the wraparound pulse can be made small as shown in Figure 3 (■).

However, in rare cases where a bridged tap exists in the immediate vicinity of the line circuit, when using the same balance network Z as in the case where there is no bridged tap, the impulse response of the wraparound pulse will be as shown in Figure 3 [F]. growing. In order to prevent this, since the impedance Zt when a bridged tap exists in the immediate vicinity of the circuit has the properties shown in Figure 2, change to a smaller value. For this reason, it is preferable to configure the variable resistor R0 with an MIS semiconductor element and control its equivalent impedance by the voltage applied to the control electrode. However, by reducing the amount of subtraction by the subtraction circuit 2, excessive subtraction at the center of the pulse can be improved and the amplitude of the impulse response can be reduced.

◎ in FIG. 3 shows the impulse response when the variable resistor R3 is improved to a smaller value.

On the other hand, since the impulse response of the wraparound signal is completely removed by the adaptive digital filter 4 and the subtraction circuit 5, the adaptive digital filter 4 stores an impulse response that is equal to the impulse response of the wraparound signal.

Therefore, it is possible to control the variable resistor R3 by determining whether the variable resistor R3 is too small or too large depending on whether the polarity of the impulse response value at the center of the pulse is positive or negative.

As a result, it is possible to reduce the amplitude of the impulse response at least at the center of the pulse, and to effectively utilize the dynamic range of the analog-to-digital converter 3.

The present invention does not limit what is used for the variable resistor R3 and how it is specifically converted, since there are various methods.

Furthermore, the element whose value is changed is not limited to the resistor, and can be configured to change the value of other reactance elements in the balance network.

Although the above example shows an example using a digital adaptive filter, it is also possible to control the impedance value of the balance network according to its tap coefficient or weighting coefficient in a circuit using a transversal filter or other analog adaptive filter. The present invention can be implemented similarly as a configuration in which:

〔Effect of the invention〕

As described above, the present invention can control the amplitude of the impulse response of the wraparound signal to be small. Moreover, this has the excellent effect of easing the precision required for the analog-to-digital converter and making its design easier.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an echo canceller according to an embodiment of the present invention. FIG. 2 is a diagram showing the impedance of the echo canceller of the present invention when viewed from the subscriber line side. FIG. 3 is a diagram showing the impulse response of the wraparound signal of the echo canceller of the present invention. 1... Pulse transmission circuit, 2.5... Subtraction circuit, 3.
...Analog-digital converter, 4...Adaptive digital filter, 10...Two-wire four-wire conversion circuit,
C...Capacitor, R+, Rz, R4...Resistor, R3...Variable resistor, S...Digital signal, TR
...transformer, Z, ...balance network, l
...Two-wire subscriber line. Patent Applicant: NEC Corporation, 4゜Representative Patent Attorney: Nao Ide Takashi 1゛・:2t
・Figure 1

Claims (1)

[Claims] 1. A two-wire/four-wire conversion circuit including a balance network, and an adaptive filter that outputs a wraparound cancellation signal that attenuates the wraparound signal sent from the four-wire output of the two-wire/four-wire conversion circuit. An echo canceller comprising: an echo canceller, wherein the balance network includes means for controlling an impedance value of the adaptive filter based on a tap coefficient value of the adaptive filter.