JPH0527317B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for automatically terminating line circuits in a four-wire telephone exchange such as a time division subscriber exchange.

In a time-division subscriber exchange, unlike a space-division type exchange such as a crossbar exchange, in principle, it is not possible to pass voice current (voltage) through the call. Therefore, in a time division switch, the input/output circuit of the switch itself has a specific impedance,
This will terminate the line. However, in the past, 600Ω (or 900Ω) was customary.
Ω) Depending on the termination, even if some consideration was given, only a certain amount of pseudo-line (BON) was inserted, which resulted in insufficient impedance matching with the line, resulting in deterioration of speech quality. This point will be explained below using figures.

Figures 1 to 4 are explanatory diagrams of the conventional method.
The figure is a schematic diagram of line switching using a space division type switch.

In FIG. 1, reference numeral 1 denotes a space division type exchange, 2 a voice input line group, 3 a voice output line group, and 4 and 5 impedance conversion transformers. The space-division type exchange 1 is simply a switch that connects lines to each other, and its configuration is based on the basic principle of straight-through operation to prevent quality deterioration caused by the exchange. Therefore, if the exchange is ideally constructed, the deterioration in speech quality caused by impedance mismatch during space-division line switching would exist on the line side.

Currently, the types of metallic lines in Japan have been integrated and have already become several types. That is, it is sufficient to target typical B-type loaded cable systems and unloaded cable systems, as well as 600Ω lines of transmission equipment (including PCM systems).

FIG. 2 is an example of a frequency locus of each line impedance. In the figure, the characteristic impedance of the unloaded cable is indicated by Z ₁ , and the characteristic impedance of the loaded cable, Z ₂ , is converted to around 600Ω by the impedance conversion transformer shown in 4 or 5 in Fig. 1, and Z ₂ ' It is indicated by.

Also, the mismatch attenuation RL at the connection point of the line interconnection whose line impedances are Zm and Zn, respectively
is generally expressed by the following equation (1).

RL = 20log | Zm + Zn / Zm - Zn | (dB) ...(1) Therefore, the mismatch attenuation RL ₁ when connecting the 600Ω line and the loaded cable line is: RL ₁ = 20log | 600 + Z ₂ '/600- Z ₂ ′ | (dB) ...(2), and the mismatch attenuation when connecting a 600Ω line and an unloaded cable line is RL ₂ , and the mismatch when connecting a loaded cable line and an unloaded cable line is RL 2 . If the attenuation is RL ₃ , then RL ₂ = 20log | 600 + Z ₁ / 600 − Z ₁ | (dB) …(3) RL ₃ = 20log | Z ₁ + Z ₂ ′ / Z ₁ − Z ₂ ′ | (dB) ...(4) is given.

The minimum value within the audio frequency band of the amount of mismatch attenuation during line connection shown by equations (3) and (4) is approximately 6 to 12 dB. Note that since the mismatch attenuation is expressed as a logarithm of the reflection coefficient, 1/2 to 1/4 of the voltage or current is actually reflected at the connection point.

Next, Fig. 3 shows an example of a conventional time division switch with an input/output impedance of 600Ω. In FIG. 3, 10 and 11 are two-wire voice lines, 12 is a time-division local exchange, 13 is a time-division switch of exchange 12, 14 and 15 are 2-wire to 4-wire conversion circuits, 16,
17 is a balanced connection network for balancing the two-wire lines 10 and 11, and 18 is a telephone set. Also A-
A' is a time-sharing local exchange 12 and a two-wire voice line 1.
Similarly, B-B' is the demarcation point with the two-wire audio line 11. The two-wire audio line 10 is assumed to be an unloaded cable line, and its characteristic impedance is Z ₁ , and the two-wire audio line 11 is assumed to be a loaded cable line, and its characteristic impedance is Z 1 .
Z ₂ ', and if this is connected via the time-division local exchange 12, then the direct reflection at the demarcation point A-A' is given by equation (3) above, RL ₂ = 20log | 600 + Z ₁ / 600−Z ₁ |=XdB…(3)′. In addition, it is assumed that there is no exchange transmission loss, and the hybrid balance return loss shown by the dashed line 19 in the figure, which is caused by the imbalance between the two-wire audio line 11, which is the loaded cable line, and the balanced connection network 17, is defined as YdB. For example, two-wire audio line 10
The total amount of mismatch attenuation toward the side ΣRL _IN is expressed as ΣRL _IN =X _~ 〓 ⁱ Y (5). Note _that 〓 ⁱ is an operational symbol indicating the sum of currents taking into account interactions. Naturally, the total mismatch attenuation ΣRL _IN shown by equation (5) is generally smaller than that shown by equation (3), and therefore the reflection is large. Further, the reflection at the demarcation point BB' is exactly the same as above.

Figure 4 shows a pseudo line (BON) for the input and output of the exchange.
This is an example in which a 2-wire line is inserted to match the 2-wire line. In FIG. 4, 20 and 21 are pseudo lines (BON). In FIG. 4, the same parts as in FIG. 3 are given the same reference numerals. In this BON insertion method, the BON insertion amount (attenuation amount) lm, ln
The larger the value, the better the matching, and therefore it is possible to provide a CdB gain to the switch. However, although this amount of gain is commensurate with the degree of improvement in mismatched attenuation, it generally has the drawback that it cannot provide enough gain to cancel out the amount of BON insertion.

Note that Figure 2 shows an ideal case in which the line impedances Z ₁ and N ₂ match the terminal impedance.In the current subscriber line, an unloaded cable system is generally used, and the line impedance Z ₁ is It is terminated with a telephone impedance Z _T that does not necessarily match. Therefore, the line impedance at the switching point will be different from _Z1 , and therefore cannot be simply calculated using equations (2) to (5) above.

The purpose of the present invention is to prevent the occurrence of backflow at the input/output points of a 4-wire telephone exchange having a 2-wire to 4-wire conversion function, and in particular to improve the sidetone characteristics of a time-division local exchange. The object of the present invention is to provide an effective automatic line circuit termination method.

The present invention estimates the line length from the line type and the DC current value applied to the line, automatically sets the input/output impedance of the exchange to the optimum value, and at the same time, the 2-wire to 4-wire conversion circuit. This system improves hybrid balance return loss to prevent backflow and improve sidetone characteristics. In addition, by creating a feedback loop on the 4-wire side of the 2-wire to 4-wire conversion circuit and changing the transfer function of the feedback loop to arbitrarily synthesize the termination impedance, the system can be configured economically. It is something to do.

Hereinafter, the present invention will be explained in detail with reference to the drawings. FIG. 5 shows an embodiment in which the automatic line circuit termination method in an exchange according to the present invention is applied to accommodate subscriber lines, and the same parts as in FIGS. 3 and 4 are given the same reference numerals.

In FIG. 5, 27 is an electronic 2-wire to 4-wire conversion circuit, 22 is a current supply circuit for the subscriber telephone 18, and 23 is an isolation transformer that separates the subscriber line side from the office side. - Also serves as an unbalanced conversion circuit. 25 is a coder, 26 is a decoder, and 24 is a transfer function control unit, which receives information on the sending current value from a communication current monitoring unit (not shown) provided in the current supply circuit 22.
The transfer function H of the feedback loop configuration filter (H) 28 for setting the exchange input/output impedance in the line-to-four line conversion circuit 27 is varied. Also, the transfer function control section 24
, the conditions necessary for setting the optimum input/output impedance of the exchange, such as line type and telephone type, are separately given as station information as necessary, and this is combined with the information on the sending current value to determine the transfer function H. A function is set to be variable.

Regarding the electronic 2-wire to 4-wire conversion circuit 27,
Let me briefly explain its operation.

The voice signal of the other subscriber passes through the time division switch 13, is decoded into a voice communication current by the decoder 26, and reaches point a within the decoder 26. This communication current passes through point b, point c, operational amplifier OP, and resistor R, then d
The signal reaches the point and is sent to the subscriber telephone 18 via the isolation transformer 23.

In addition, the voice communication current from the subscriber telephone 18 is
After passing through the isolation transformer 23, it reaches point d in the electronic 2-wire to 4-wire conversion circuit 27, and then reaches point e,
The signal passes through point f and reaches point g, is encoded by the coder 25, and is sent to the other subscriber via the time division switch 13.

Also, an adder is inserted at point c and a subtracter is inserted at point f, points e and c are filtered by a filter (H) 28 having a transfer function H, and points b and f are filtered by a filter (G) having a transfer function G. It is tied at 29.

As a result, the impedance Z _IN seen into the station from the demarcation point A-A' can be arbitrarily set using the transfer function H of the filter 28. In addition, the loop current at the 2-wire to 4-wire conversion point of the voice communication current from the other subscriber, indicated by the dashed line 30 in the figure, can be determined by arbitrarily setting the transfer function G of the filter 29.
In the figure, point b indicated by the two-dot chain line 100 → filter (G) 2
9 is canceled by the subtracter at point f, and the hybrid balance return loss is improved.

Next, a method for automatically setting the input/output impedance Z _IN of the exchange to a required impedance, which is the main part of the present invention, will be explained.

The subscriber is now using telephone 1 to make or answer a call.
8 is turned off, the power supply circuit 22
Electric current is supplied to the telephone set 18 via the subscriber line (two-wire voice line) 10 . At this time, the output current value is read out from the power supply circuit 22, and is combined with the line type and telephone type information separately given by the transfer function control unit 24, and the transfer function H of the filter 28 is varied via the information line 31. . This sets the switch input/output impedance Z _IN .

Note that the value of Z _IN is not necessarily equal to the matching impedance with the line, and in some cases, it is set to an impedance that matches the design conditions of the side sound protection circuit of the telephone. In fact, when considering actual system conditions, it is desirable to set the impedance to match the design conditions.

In addition, in the present invention, the loop resistance of the line is determined from the sending current value, the line impedance is estimated from this loop resistance, and the line is terminated at a predetermined impedance, but the correlation between the loop resistance and line impedance is generally It is known to be strong (for example, 1981, National Conference of the Institute of Electronics and Communication Engineers, paper number 1758, a study on constant current feeding system of BORSHT circuit), and is the easiest method for estimating line impedance. Therefore, if the two-wire voice line is limited to a subscriber line and the telephones are of the same type, it becomes necessary to provide station information to the transfer function control section 24. Furthermore, since the line impedance changes as the line length changes, unless the transfer function G is also set variable at the same time as the terminal impedance is set, the subtracter at point f will perform 2-wire to 4-wire conversion. This causes a breakdown in cancellation of the roundabout current at the point, a so-called hybrid balance breakdown. If this continues, it will be necessary to increase the transmission loss of the exchanger and satisfy the matching condition in order to prevent the generation of whistling noise. Similarly, for this,
The hybrid return loss is improved by variably setting the transfer function G of the filter 29 according to instructions from the transfer function control unit 24 via the information line 32. That is, as a result of the reduction in the amount of leakage from 4-wire input to 4-wire output that occurs during 2-wire to 4-wire conversion, generation of whistling noise can be prevented without increasing transmission loss in the exchange.

FIG. 6 shows another embodiment, in which the same parts as in FIG. 5 are given the same reference numerals. In FIG. 6, 34 is an impedance switching unit in the electronic 2-wire to 4-wire conversion circuit 33, which prepares several types of assumed exchange input/output impedances (Z ₁ to Zn), and controls the transfer function control unit in FIG. 5. When a switching instruction is given from the impedance control section 35 instead of 24 via the information line 36, the impedance switching section 35 selectively inserts and connects a predetermined impedance according to the instruction.

In addition, in the case of a hybrid imbalance caused by a change in the line length, correction instructions are sent from the impedance control section 35 to the information line 37, as in FIG.
The signal may be applied to the filter 29 via the filter 29.

As described above, the present invention is a method for estimating line impedance from line loop resistance and automatically terminating exchange input/output. Figures 5 and 6 show examples of application to subscriber lines. . Similarly, it goes without saying that the present invention can be applied as is to relay lines in which a DC signal system is used, as long as the loop resistance can be detected from the DC current value.

As described above, the present invention uses the loop resistance of the line to automatically set the switch input/output impedance to the optimal termination impedance, and at the same time corrects the disruption of hybrid balance due to line length differences, and is superior to the conventional BON insertion method. , which has the advantage of satisfying matching conditions without increasing exchange transmission loss. Furthermore, since the optimum termination is automatically selected at all times when a line is connected, it has various effects such as great convenience in daily maintenance and operation.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of line switching using a conventional space-division type switch, Fig. 2 is a graph showing the frequency locus of line impedance, and Fig. 3 is a diagram for explaining the backflow phenomenon of a conventional time-division type switch. Fig. 4 shows an example of the configuration of a conventional time-sharing switch in which impedance matching is achieved by inserting a BON into the input/output section, and Fig. 5 shows a configuration example using the automatic line circuit termination method according to the present invention. A diagram showing an example of a split switch,
FIG. 6 is a diagram showing another embodiment of the automatic termination system according to the present invention. 10...2-wire voice line, 12...Time division local exchange, 13...Time division switch, 27, 33...2 wire-
4-wire conversion circuit, 18... Telephone, 22... Current supply circuit, 23... Transformer, 24... Transfer function control unit, 2
5... Coder, 26... Decoder, 28, 29... Filter, 34... Impedance switching section, 35... Impedance control section.

Claims (1)

[Claims] 1. An automatic termination method for a line circuit for a two-wire line housed in a four-wire telephone exchange, with regard to the loop resistance on the two-wire line, the type of the circuit, etc. The line impedance for the two-wire line is estimated based on the station information of
In order to terminate the wire line, the variable impedance means for setting the exchange input/output impedance provided in the 2-wire to 4-wire conversion circuit is automatically set and controlled to be in the predetermined impedance state, and at the same time, Based on the above estimation results, the transfer function of the filter provided on the 4-wire side of the 2-wire to 4-wire conversion circuit is variable in order to correct the loss of hybrid balance in the 2-wire to 4-wire conversion circuit due to line length differences. An automatic termination method for line circuits that is automatically set and controlled. 2. One of the multiple types of fixed impedance means provided in the line circuit as a variable impedance means for setting exchanger input/output impedance is automatically selected based on the line impedance estimation result for the two-wire line. The automatic termination system for a line circuit according to claim 1, wherein the line circuit is connected in a switched manner.