JPH01827A - Echo canceller for two-wire subscriber line bidirectional transmission - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an echo canceller, and more particularly to an echo canceller for two-wire subscriber line bidirectional transmission using a metallic subscriber line in an integrated services digital network (ISDN). related to

[Conventional technology]

In recent years, field trials of l5DN have been conducted in various countries. As a subscriber interface, a bidirectional transmission system using an echo canceler using existing metallic subscriber lines is being tested.

In such an echo Kinsella circuit, it is usually coupled to the subscriber line using a transformer to exchange the transmitted and received signals, and for the so-called echo signal in which the transmitted signal goes around to the receiver on its own side, a simple balancing network is provided. A hybrid circuit achieves slight attenuation, and an echo canceller section that generates and subtracts echo replicas using an adaptive filter that receives the transmitted symbol as input completely eliminates echo signals with a degree of suppression of approximately 60 dB or more. be done. At this time, the required number of taps for the transversal filter is determined from the length of the impulse response of the isolated waveform of the echo input to the echo canceller, so an echo path equalization filter that shortens this impulse response length is used. It is inserted between the transmission driver and the echo canceller section. Conventionally, as this echo path equalization filter, a bypass filter is used to suppress the low frequency components because the long impulse response tail (echo tail) contains many low frequency components. A completely new DC bypass filter with infinite loss is used.

[Problem to be solved by the invention]

However, when a transmission line code such as 2BIQ, which has been proposed in recent years, in which the transmission symbol sequence is a random sequence and has no DC balance, is input to the above-mentioned bypass filter type echo canceller, this echo canceler has a large number of taps. When is N, echo residuals that cannot be completely removed are generated by convolving the components of the impulse response waveform after the time axis N+1 with the transmission symbol sequence. For example, if the number of taps N = 30, the average power of this echo residual is -10 tesibel (
dB), or about -40 dB when using a complete direct current blocking type echo path equalization filter. These values are insufficient compared to the amount of suppression of -60 dB that is normally required. On the other hand, it is possible to obtain a sufficient amount of suppression by increasing the number N of taps, but this is difficult to realize because the number N of taps of the echo bus equalization filter must be 100 or more.

[Means to solve the problem]

The echo canceller for bidirectional transmission of a two-wire joining line according to the present invention is characterized in that a filter having a zero point that cancels a pole of an echo path transfer function caused by an inductance component of a line coupling transformer is disposed in an echo path.

Further, in the echo canceller of the present invention, the transfer function R(s) of the filter is set to a constant K close to 1 and smaller than 1, and T
is the operating period of the digital filter and the rate, R(g)=
It is characterized by being composed of 1-Ke-s'r Narute 4 Silurua 4 /l/ri.

Further, in the echo canceller of the present invention, when RS is the two-wire side sending impedance and L is the inductance of the line coupling transformer, the above constant is satisfied.

Furthermore, in the echo canceller of the present invention, the transfer function R(3) of the filter is R(s,==(s+ω.)/(
It is characterized by being composed of a function S+ωl).

Moreover, the echo canceller of the present invention has the above-mentioned ω.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

Referring to the model configuration of the hybrid circuit shown in FIG. It is the sum of the component H2<s> that passes through the network (ZB) 13 side.Among them, the component H2(S) is the sum of the impulse response when the balancing network 13 is configured with a resistor or a normally considered three-element impedance. Decay is fast;
Above time LOT (T is Baud period), the component Hl (S
) dominantly determines the waveform. The component I(us) can be written as follows.

Here, R5 represents the two-wire side sending impedance, the impedance when looking at the line side from the zo+i circuit, S represents jω, and L represents the inductance of the transformer.

If we evaluate equation (1) above for low frequencies, we can see that during long-distance transmission, where the amplitude of the echo tail cannot be ignored relative to the received signal level, the gauge 2 length of the line added as Zo, the bridged Since 2°>RS regardless of the presence or absence of taps, equation (1) can be approximated as follows.

It can be seen that equation (2) is a first-order no-path characteristic of the time constant τ=L/RS, and the pole of equation (2) determines the echo tail. The echo tail decay time constant is τ=L/R
, for example, L: 50 mH, RS=135
If Ω, τ = approximately 370 μs, T = 12.5 μ
s, it becomes τ#30T. This corresponds to the portion t)30T of the impulse response waveform (A) of the echo isolated waveform in the case of no echo path equivalent filter in FIG.

Next, when this echo tail waveform is passed through a 1-0-5T filter (not shown), the amplitude becomes the difference between the original echo tail waveforms separated by IT, and the amplitude becomes (1- eBottom) Attenuates twice as much.

In the above numerical example, the attenuation is about 1/30. This is the second
This corresponds to the portion of waveform (B) in the figure where t>30T. Waveform(
In B), the amplitude is attenuated compared to (A), but the time constant is (
Same as A).

Referring to FIG. 3, the basic configuration of the present invention is a hybrid circuit 1. The device includes an echo canceller circuit 2 and an echo pass filter 3. The hybrid circuit 1 is connected to a two-wire subscriber line 15 via a line coupling transformer 11 and performs two-to-four wire conversion. Although this hybrid circuit has various types such as an electronic circuit configuration and a transformer configuration, in the present invention, the subscriber line 15 must be connected by the transformer 11.

The echo canceller circuit 2 is connected to the 4-wire side of the hybrid circuit. This echo canceller circuit 2 is
For bidirectional digital transmission of two-wire subscriber lines, there are usually two types: transversal filter type and memory type. Figure 3 shows an outline of the transversal filter type, and its details can be found in, for example, USP 4,088.
7,654 can be applied. The echo pass filter 3 is connected between the hybrid circuit 1 and the echo canceller circuit 2, and has a transfer function R(3, as described above).
・An operation is performed to accelerate the attenuation of the echo tail generated by the DC cutoff characteristic of the transformer 11 of the Evelint circuit 1.

FIG. 4 shows a first embodiment of the present invention, in which the echo pass filter 3 having the basic configuration shown in FIG. 3 is described in more detail. The hybrid circuit 1 and the F-canceller circuit 2 are as explained in FIG. 3 above, and their explanation will be omitted. In this embodiment, the echo pass filter 3 includes delay elements 31. It is composed of a digital filter having an adder 32 and a multiplier 33, and has a transfer function R(8)=1−(1
--T) e-'' characteristic. This echo pass filter 3 and the hybrid circuit 1 are connected via an A/D converter 4.

The echo pass filter 3 receives the output of the A/D converter 4 and inputs it to a delay element 31 and an adder 32. The multiplier 33 receives an input separately from the output e-9T from the delay element 31. The adder 32 adds the output of the multiplier 33 and the output of the A/D converter 4, and transfer function R(S)=1 (1-T
) e-” is output to the echo canceller circuit 2.

Note that the echo pass filter 3 includes delay elements 31 . The adder 32 and the multiplier 33 may be replaced by a memory and a microprocessor, and may be controlled by a microprogram. At this time, the microprocessor can also perform calculations for the echo canceller circuit 2.

The transmission function R(3) of the echo pass filter 3 has the following form, but if e-9T is expanded in the low frequency range, it can be approximated as follows.

However, L>RST From equation (4), the transfer function RS3 has a zero point at 5=--c, and its frequency is the same as the pole in equation (2) above. Second
The waveform in (C) in the figure is the waveform when the waveform in (A) is passed through this echo pass filter 3. Transfer function RC
s cancels the pole in equation (2) above at the zero point, so it has the effect of eliminating a waveform with a long time constant τ. Therefore, the attenuation of waveform (C) is rapid, and the amplitude is smaller than that of waveform (B) at time 20T and above.

For waveform (C), assuming the number of taps in the echo canceller circuit is N, and calculating the echo residual generated by convolving the impulse response component after (N+1)T with the transmission symbol sequence, even when N = 30, the degree of suppression is more than 60 dB. The waveform (A) can be realized.

An echo canceller with sufficient suppression can be realized with a smaller number of taps than in case (B).

Note that since the inductance of the transformer is usually selected so that −>T, 1−−T is a constant close to l but smaller than 1. As shown in equation (2) above, the number of holes caused by the transinductance is 5=-- under long-distance conditions, and the transmission distance is sometimes optimal.

FIG. 5 shows a second embodiment of the invention. In FIG. 5, the echo pass filter 3 is realized by an analog filter made up of resistors R1, R2 and a capacitor C, followed by an A/F filter.
A D converter 4 is arranged.

This hog transfer function R(s) is expressed by equation (5) using resistors R1, R2, and capacitor C.

CR1R2 When the pole of equation (2) and the zero point of equation (5) match, that is, RS 1 1 □ = ω. (6) LCR
1, there is no long time constant term in the characteristics of the entire echo path, and the echo impulse response exhibits a fast decay characteristic almost similar to that shown in FIG. 2(C). Similarly to the example in FIG. 4, under the long-distance condition, ω. =-C becomes optimal.

〔Effect of the invention〕

As explained above, the echo canceller of the present invention cancels out the poles of the echo path transfer function caused by the inductance component of the line coupling transformer, thereby eliminating poles with long time constants and speeding up the attenuation of the echo tail.

Furthermore, the echo canceller of the present invention can reduce the number of taps of the filter that cancels the echo path transfer function.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an example of a general hybrid circuit, Fig. 2 is a diagram showing an echo tail waveform, Fig. 3 is a block diagram showing the basic configuration of the present invention, and Fig. 4 is a diagram showing the basic configuration of the present invention. 1
FIG. 5 is a block diagram showing a second embodiment of the present invention. l...Hybrid circuit, 2...Echo can along Figure 1 Figure 4/

Claims (5)

[Claims] (1) An echo canceler for bidirectional transmission on a two-wire subscriber line, characterized in that a filter having a zero point that cancels a pole of an echo path transfer function caused by an inductance component of a line coupling transformer is arranged in the echo path. (2) In the echo canceller according to claim 1, the transfer function R_(_S_) of the filter is such that K is close to 1 and 1
An echo canceller for bidirectional transmission on a two-wire subscriber line, characterized in that it is configured with a digital filter where R_(_S_)=1-Ke^-^S^T, where T is a smaller constant and T is the operating period of the digital filter. . (3) In the echo canceller according to claim 2, the constant K is defined as K=1−, where R_S is the two-wire side sending impedance and L is the inductance of the line coupling transformer.
An echo canceller for bidirectional transmission on a two-wire subscriber line, characterized in that it is configured to provide (R_S/L)T. (4) In the echo canceller according to claim 1, the transmission function R_(_S_) of the filter is R_(_S_
)=(s+ω_0)/(s+ω_1). (5) In the echo canceller according to claim 4, the ω_0 is ω_0 when R_S is the two-wire side sending impedance and L is the inductance of the line coupling transformer.
1. An echo canceller for two-way subscriber line bidirectional transmission, characterized in that it is configured so that =R_S/L.