JPH0212063B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an automatic equalizer, and particularly to a digital equalizer used in a transmission line having one or more branches between a terminal equipment and a terminal office, such as a telephone subscriber line. It relates to automatic equalizers used when transmitting signals.

[Background of the invention]

In order to reduce the amount of new line construction required in the future, bridge taps (hereinafter abbreviated as BT) are installed between the subscriber lines that connect each telephone set and the telephone office. An empty circuit (transmission line with no equipment connected to the terminal) is connected. If this BT is left open at the end, it becomes a total internal reflection line, which greatly affects the frequency characteristics of the transmission line.

Transmission lines such as normal pair cables or coaxial lines have attenuation that increases as the frequency increases due to the skin effect of the line, so-called √
have characteristics. Therefore, when transmitting a signal through such a line, in order to compensate for the above-mentioned √ characteristic of the input section of the repeater or receiver and obtain a flat frequency characteristic, it is necessary to use a line that has a characteristic opposite to the above-mentioned √ characteristic. Provide a heat exchanger. Further, when the √ characteristic fluctuates, a variable equalizer is used that detects the fluctuation and controls the equalizer characteristic.

However, in the line to which the above-mentioned BT is added, the characteristics of the transmission line become complicated as described later, so √
In addition to the equalizer for characteristic compensation (hereinafter referred to as √ equalizer), there is an equalizer for transmission lines with BT (hereinafter referred to as √ equalizer).
(abbreviated as BT equalizer) is required. In a conventional equalizer that combines a √ specializer and a BT equalizer, the √ equalizer corresponding to the line length is adjusted and the BT
The BT equalizer was adjusted for the influence of
When the number of BTs with a length of 300 to 400 m or more increases to two or more, the time domain of the impulse response spreads widely, resulting in a problem that the structure of the equalizer becomes complicated and expensive. In other words, in this type of equalizer, a so-called transversal equalizer is used, which has a structure in which multiple delay elements are connected in cascade, the tap output of each delay element is multiplied by a constant, and the results are added. This is because as the area expands, the number of equalizer taps and the number of multipliers that multiply by a constant increase accordingly.

[Purpose of the invention]

Therefore, the object of the present invention is to provide a √ equalizer and a BT
To realize a narrow equalizer in which the spread of the time domain of the impulse response of the equalizer in combination with the equalizer is less than or equal to about 5 times the sampling period, that is, to realize a narrow equalizer in which the spread of the time domain of the impulse response of the equalizer is less than or equal to about 5 times the sampling period. It is achieved by using a vessel.

[Summary of the invention]

In order to achieve the above object, the present invention uses a √ equalizer and
In the configuration of an automatic equalizer for a line with a bridge tap, which includes BT equalizers in tandem, a transversal filter having a finite impulse response in the return path of the BT equalizer, and the output of the filter and the √ equalizer described above. an error detection circuit that adds the outputs of the filter, one part of which is used as the input of the filter, and the other part of which is used as the equalized output, and an error detection circuit that controls the tap coefficient of the filter from the output of the adder. and an equalization information extraction circuit that controls the √ equalizer from the BT equalizer and an error detection circuit so that the convergence value of the tap highest order coefficient of the filter approaches 0.

[Embodiments of the invention]

The present invention will be explained in detail below using the drawings.

FIG. 1 shows a diagram of a BT-equipped telephone circuit having a BT-equipped transmission line and to which the present invention is applied. A telephone 1, which is a terminal device, and a local telephone office 2 are connected by a subscriber 3, and vacant circuits of bridged taps 4-1 to 4-3 are connected to the subscriber line in preparation for adding more telephones. .

In Figure 2, the length of the subscriber line is 2 as shown on the right.
This is a frequency characteristic diagram of the transmission line in the case where BT is not attached, 500 BT are attached at intervals of 400 m from the telephone set, and 1, 2, 3, and 4 BT are attached as Km. has the well-known √ characteristic, but as the number of BTs increases, the frequency characteristics change complexly, especially in the high range.

Figure 3 shows a 5km subscriber line with an interval of 500m every 1km.
This shows the frequency characteristic a and the impulse response waveform b when four BTs are attached. b The horizontal axis in the figure represents the bit length. In the illustrated calculation example, the time length of one bit is 5.68 μsec. In figure a,
is the √ characteristic of a 5Km subscriber line without BT, is
The frequency characteristics of subscriber line to transmission line with BT are shown.
As is clear from diagram b, many subcomponents 6 and 7 are generated in addition to the main component 5 of the impulse response, and the impulse response does not easily converge to zero. Therefore, when an equalizer for such a waveform is constructed using a transversal type filter, the number of taps of the transversal filter and the number of multiplication circuits are extremely large, which is not desirable in terms of manufacturing the device.

FIG. 4 is a block diagram showing the structure of one embodiment of the equalizer according to the present invention. A signal distorted by the frequency characteristics of the transmission line is applied from the input terminal 9 to the √ equalizer 11 . A part of this output is applied to the level detection circuit 12, and a √ equalization control signal corresponding to the line length is applied to the √ equalizer 11 according to the level. The output of the √ equalizer 11 is further applied to the BT equalizer 13 to correct the frequency characteristics due to the addition of BT, and an equalized signal is output from the output terminal 10. To control the BT equalizer 13, the error detection circuit 15 obtains control information from the previously determined training pattern 14 and the output of the BT equalizer 13 to control the BT equalizer 13. The above part is the same as a conventionally known equalizer, but in this embodiment, the control circuit 16 extracts over-equalization information from the BT equalizer 13, and uses this over-equalization information to perform √ equalization. Vessel 11
control. The control of the √ equalizer using the over-equalization information extracted from the BT equalizer 13 will cause the convergence value of the highest order coefficient of the transversal filter used in the BT equalizer to be 0, as will be explained later. controlled to approach. As a result, the impulse response length of the output of the √ equalizer is made shorter than the impulse response length shown in FIG. The structure of the device is simplified.

The configuration operation will be explained in more detail below.

FIG. 5 shows an example of the operating waveform of a conventional BT equalizer, that is, a case where the characteristics of the √ equalizer are not controlled by information from the BT equalizer. In the figure, the vertical axis represents amplitude, the horizontal axis represents time, and T represents the bit period. In the figure, A is the input waveform of the BT equalizer, and B is the desired waveform as the BT equalizer output. However, in order to simplify the device, the number of taps in the BT equalizer is simply reduced, which means that even though the impulse response of the input signal is long,
When the number of taps in the BT equalizer is reduced, the cancellation waveform generated by the BT equalizer becomes as shown in C, and in the figure, it is impossible to equalize intersymbol interference of 5T or more.

Figure 6 shows the route conditions similar to those shown in Figure 3.
This shows the characteristics when using a √ equalizer for 6.3 km, where a is the frequency characteristic of the line with BT, and 6.3 is the frequency characteristic of the line with BT.
Shows the √characteristics of a Km subscriber line. Figure b shows the impulse response waveform in that case.Similar to Figure 3b, secondary peaks 6 and 7 occur, but the impulse response after peak 7 rapidly converges to 0. I understand that. That is, 5
If equalization with the √ characteristic of 6.3 km is applied to a line of Km, even an equalizer with a short impulse response length (equalizes only up to 4T) as shown in Figure 5C will have a symbol interval of 5T or more. It can be seen that it is possible to realize BT equalization without interference.

The extent to which this √ characteristic should be over-equalized is determined by √ equalization so that the response value approaches 0 after the time domain where the impulse response of the line with BT becomes a monotonically decreasing characteristic (more than 4T in Figure 5A). All you have to do is control the device.

Figure 7 shows the outline of the BT equalizer.19
is the terminal output on the transmitting side, 20 is the output on the receiving side, 21 is the line represented by the transfer function F(Z), 13 is the BT equalizer, and 22 is the transfer relation G(Z) inserted in the return path.
We show a finite impulse length transversal filter with Now the voltage at terminal 19 is V ₁ and the voltage at terminal 20 is
When the voltage of V 2 is V ₂ , the overall transfer function V ₂ /V ₁ is expressed as V ₂ /V ₁ =F(Z)/1+G(Z). When complete equalization is performed, V ₂ /V ₁ is 1, so G(Z) = F(Z) - 1, and G(Z) is F(Z) except for the term Z ⁰ . ) matches. Therefore, if the transversal filter that constitutes G (Z) in Fig. 7 is longer than the time range in which the impulse response of the BT line decreases monotonically, then
Intersymbol interference can be minimized by controlling the √ equalizer so that the coefficient of the highest degree of G(Z) (ie, the coefficient at t=4T in FIG. 5C) becomes 0.

FIG. 8 is a diagram showing the configuration of a line equalizer with BT used to implement the present invention. 23 in the figure is input 2
25 is a digital delay circuit, 26 is a variable gain buffer circuit, 27 is a discriminator, and 15 is an error rate detection circuit constituted by the subtracter. other 9,1
0, 11, 16, and 13 indicate the same parts as those with the same numbers in FIG. G shown at 22 in Figure 7
The transversal filter that creates (Z) includes a delay circuit 25, a variable gain buffer circuit 26, and a subtracter 2.
Consisting of 3. This highest order tap coefficient of G(Z) is output to line 28. This 28
The control circuit 16 keeps the signal output to 0 at all times.
By controlling the √ equalizer 11 via
As mentioned above, it is possible to make the impulse response length of the BT line shorter than the operating time length of the transversal filter (4T in the above embodiment).

As described above, by using the present invention, it is possible to easily equalize a line with BT without increasing the transversal filter impulse response length for BT equalization. Lower prices can be achieved.

[Brief explanation of drawings]

Figure 1 is a diagram showing the configuration of a subscriber line with a bridged tap, Figure 2 is a frequency characteristic diagram of a line with a bridged tap, and Figures 3a and b are frequency characteristics of a subscriber line using a conventional equalizer. FIG. 4 is a block diagram showing the configuration of an embodiment of the automatic equalizer according to the present invention, FIG. 5 is an impulse response waveform diagram for explaining the operation of the present invention, and FIG. Figures 6a and b are a frequency characteristic diagram and an impulse response waveform diagram when using the equalizer according to the present invention, respectively, and Figure 7 is a diagram of the impulse response waveform.
Block diagram showing the general configuration of the BT equalizer, No. 8
The figure shows the configuration of an embodiment of an automatic equalizer according to the present invention. 1...Telephone, 2...Local telephone office, 3...Subscriber line, 4...Bridged tap, 9, 19...Input terminal,
10, 20...output terminal, 11...√equalizer, 12
...Level detector, 13...BT equalizer, 14...Training pattern generation circuit, 15...Error rate detection circuit, 16...Control circuit.

Claims (1)

[Claims] 1. In an automatic equalizer formed by cascading a √ equalizer and a BT equalizer, the BT equalizer is a transversal filter having a finite length impulse response provided in a return path. , an adder circuit that adds the output of the transversal filter and the output of the √ equalizer;
an identification circuit that identifies the output of the addition circuit; an error detection circuit that detects an error from the input/output of the identification circuit; a circuit that uses the output of the identification circuit as an input of the transversal filter; It consists of a circuit that calculates the coefficients of the transversal filter from the output of the discrimination circuit, and the √ equalizer controls the √ equalizer so that the convergence value of the highest order coefficient of the tap of the transversal filter approaches 0. An automatic equalizer comprising a control circuit.