JPH04113794A - Device for compensating transmission loss in telephone line - Google Patents

Abstract

PURPOSE:To attain excellent talking independently of the length of a line by detecting a DC loop current and compensating a transmission loss while revising a gain with respect to a transmission reception signal in response to the detected value so as to keep a talking level to a level. CONSTITUTION:A subscriber line 11 led from a telephone station 10 is connected to a couple of parallel transmission line terminals TA, TB of an office line trunk 21 of a private branch of exchange 20. A current detection circuit 23 forming a DC loop circuit together with a subscriber line 11 and an AC signal blocking coil L and detecting a DC loop current IL is provided between the terminals TA and TB. The DC loop current IL relates to the transmission loss, and a transmission loss compensation circuit 24 decreases a gain when the detected current is large and increases the gain when the detected current is small to compensate the transmission loss with respect to a reception signal. A transmission loss compensation circuit 25 decreases a gain when the detected current is large and increases the gain when the detected current is small to compensate the transmission loss with respect to a transmission signal.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a telephone line transmission loss compensating device provided in a private branch exchange, a key telephone device, etc., which compensates for transmission loss of a telephone line.

[Prior Art] When transmitting and receiving signals between telephone terminals via a switching network, transmission loss due to circuit lines cannot be avoided, but an allowable transmission loss is specified in order to ensure minimum speech quality. There is. For example, as shown in Figure 2, between the telephone terminal 1 and the telephone station 2 of the terminal station, it is about 7 dB, and between the input and output of the telephone station 2 of the terminal station, it is about 0.5 dB, and from the telephone station 2 of the terminal station to the other party. To the telephone station 2, which is a terminal station, an allowable transmission loss of about 10 dB is specified as an allowable transmission loss, regardless of what kind of station (central station 3, central station 4, or central station 5) is interposed therebetween.

Here, as shown in FIG. In this case, the transmission loss on the path from the terminal telephone station 2 to the telephone set 9 is
In addition to the loss 1 in the subscriber line 6, there is also the loss 2 caused by sequentially switching off the office line trunk 7a of the private branch exchange 7, the communication path changeover switch 7b, and the extension subscriber circuit 7c, as well as the loss 2 in the extension line. The loss at the subscriber line 8 and 3 are added. That is, the subscriber line 6 is connected to the terminal telephone office 2.
The transmission loss is greater than when the telephone 9 is directly connected via the terminal, and problems are more likely to occur.

In station equipment, etc., the loss due to equipment processing is kept to a minimum by its design, and in the subscriber line 6, etc., loss is kept down by appropriately selecting the cable core diameter according to the line length. This ensures minimum call quality, so
Conventionally, no configuration has been provided to directly compensate for transmission loss.

[Problems to be Solved by the Invention] However, even if the core diameter of the cable is appropriately selected, it is inevitable that the transmission loss will increase in proportion to the length of the subscriber line, etc. Although call quality is ensured, if the line length is long, it is unavoidable that the call level will drop and the service to subscribers will deteriorate.

As mentioned above, when the telephone terminal is a private branch exchange 7, even if the loss in the subscriber line 6 satisfies the allowable value, the total loss exceeds the allowable value, so the minimum call quality cannot be ensured. There is a risk that this will happen.

The present invention has been made in consideration of the above points, and is capable of directly compensating for transmission loss in telephone lines.
It is an object of the present invention to provide a telephone line transmission loss compensation device that can ensure a sufficient speech level regardless of the magnitude of transmission loss.

[Means for Solving the Problems] In order to solve the problems, the present invention provides a current detection circuit that detects the amount of DC current flowing through a two-wire telephone line in which the DC current is superimposed on the connection line; A telephone line transmission loss compensation device was constructed with a transmission loss compensation circuit that controls the levels of the transmitted signal and the received signal according to the amount of current.

[Operation] The transmission loss in a two-wire telephone line in which a direct current is superimposed on the connection line has a correlation with the amount of direct current flowing through the line.

Therefore, in the present invention, the current detection circuit estimates the transmission loss by detecting the amount of DC current, and the transmission loss compensation circuit compensates for the transmission loss by controlling the levels of the transmitted signal and the received signal according to this amount of DC current. It was decided to.

[Embodiment] Hereinafter, an embodiment in which the telephone line transmission loss compensation device of the present invention is installed in a private branch exchange will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing a first embodiment. In FIG. 1, a subscriber line 11 extending from a telephone office 10 is connected to a pair of balanced transmission line terminals TA and TB of a central branch trunk 21 in a private branch exchange 20. In FIG.

The office line trunk 21 includes a transformer 22 that electromagnetically couples the subscriber line 11 and the internal transmission line.

One end of the primary winding 22a of this coupling transformer 22 is connected to a terminal TA via a DC blocking capacitor C1, and the other end of the primary winding 22 is connected to a terminal TB. Further, a current detection circuit 123 is provided between the terminals TA and TB to form a DC loop circuit together with the subscriber line 11 and the AC signal blocking coil L to detect the magnitude of the DC loop current .

Two transmission loss compensation circuits 2 are provided on the secondary side of the transformer 22.
4 and 25 are provided. That is, the secondary winding 22
One end of the secondary winding 22b for the received signal is connected to the communication path changeover switch 26 via the transmission loss compensation circuit 24 for the received signal, and one end of the secondary winding 22b for the transmitted signal is connected to the transmission signal for the transmitted signal. Communication path changeover switch 2 via loss compensation diagram n25
6. Image transmission loss compensation circuits 24 and 25
is constituted by, for example, a variable amplifier circuit, and the detected current value from the current detection circuit 23 is given as gain control information.

The transmission loss compensation circuit 24 compensates for the transmission loss with respect to the received signal by decreasing the gain when the detected current value is large, and increasing the gain when the detected current value is small. The transmission loss compensation circuit 25 also reduces the gain when the detected current value is large, and increases the gain when the detected current value is small, thereby compensating the transmission loss for the transmission signal.

Note that between the office line trunk 21 and the telephone 13, there is a call path changeover switch 26 and an extension subscriber circuit 27, as in the past.
, extension subscriber lines 12 are interposed therebetween, and transmission signals are transmitted and received via these. Further, a hybrid balance circuit 28 used during data transmission is connected between the intermediate tap of the secondary winding 22b of the transformer 22 and the ground.

As described above, in this first embodiment, the DC loop current I
Transmission loss is compensated for by detecting L and controlling the gain of an amplifier circuit inserted in the transmission signal line.

This is based on the fact that the DC loop current {circle around (2)} corresponds to the transmission loss caused by the subscriber line 11.

In the following, it will be made clear that the DC loop current I [corresponds to the transmission loss.

FIG. 4 shows an equivalent circuit of a DC loop circuit.

The DC loop circuit 30 connects the power supply Vbb of the telephone office 10 and a resistor R3 (fourth
(The figure shows each for sending and receiving separately), and
The line resistance RL in the subscriber line 11 (, symbol R [indicates the combined resistance value of transmission and reception) and the central office line trunk 21
It can be equivalently seen as consisting of a resistor RT.

Therefore, the DC loop current flowing in the subscriber line 11 is the following formula (% formula % (1)) Here, the resistance R3 at the telephone office 10 and the central office line trunk 2
The value of the resistance RT of the DC loop No. 1 is a known value (for example, R3 is 220Ω±15%, and Rd is 50Ω to 300Ω). Therefore, the DC loop current l[ is as shown in Fig. 5(A)
As shown in , it is determined by the line resistance R[, and conversely, the value of the line resistance R[ can be estimated from the value of the DC loop current [.

On the other hand, the subscriber line loss between the telephone office 10 and the central office line trunk 21 is 1, and if 1 is the line line loss per 1 km and 2 is the line resistance per 1 km, then it can be simply expressed as the following % formula. It can be expressed as %(2). That is, subscriber line loss L1 can be estimated by line resistance R[. In addition,
Figure 5 (B) shows these relationships ((2)
Although the relationship is linear according to the equation, there are other factors as well, so in reality it is a nonlinear relationship as shown in FIG. 5(B)).

As mentioned above, the DC loop current I[ is determined by the line resistance R[, and the subscriber line loss 1 is also determined by the line resistance R[, so the subscriber line loss 1 is estimated by the DC loop current I[ can do.

Therefore, as described above, the value of the DC loop current I[ is detected in the current product circuit Nj23, and the output thereof is made into a control signal and inputted to the gain control terminal of the transmission loss compensation circuit 24. Transmission loss can be compensated by changing the gain of 24.25.

Note that both transmission loss compensation circuits 1i'J24.25 may vary the gain stepwise according to the value of the DC loop current ■[, or may vary the gain continuously according to the value of the DC loop current I[. The gain may be varied. Figure 6 shows the DC loop current I
Figure 7 shows the equivalent transmission loss (solid line) after compensation processing when the gain is varied stepwise according to the value of L. Equivalent transmission loss after compensation processing when varying the gain (solid line)
It shows. The broken lines in FIGS. 6 and 7 indicate uncompensated transmission losses.

As described above, according to the first embodiment, since the configuration is provided to compensate for the transmission loss of the subscriber line, the equivalent transmission loss is reduced, and the speech level can be increased compared to the conventional method. Quality can be further improved.

Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 8 shows this second embodiment on a more concrete level than the first embodiment, and shows the central office line trunk portion. Components corresponding to those in FIG. 1 are designated by the same reference numerals, and their explanation will be omitted.

In this second embodiment, transmission loss compensation is performed in stages (see FIG. 6).

In FIG. 8, a pair of balanced transmission line terminals TA and TB
A resistor RO and an AC signal blocking coil L are connected in series between them. This resistor RO is connected to the central office (not shown)
The DC loop current I[ when the office line trunk 21 is in a talking state is detected as a voltage signal. That is, it constitutes a current detection circuit (current/voltage conversion circuit) 23.

The transmission loss compensation circuits Ji24 and Ji25 have symmetrical configurations as shown in FIG. Therefore, the transmission loss compensation circuit 24 for received signals will be explained below.

The transmission loss compensation circuit 24 is configured around an operational amplifier H1, and functions as a non-inverting amplifier circuit in which a received signal is input to a non-inverting input terminal of the amplifier H1.

The gain A of the transmission loss compensation circuit 24 having such a configuration is:
Using the grounding resistance RB of the inverting input terminal and the feedback resistance R^, it can be expressed as the following formula (3), so the gain A can be changed according to the detected current value (voltage signal V). One possible solution is to change the value of the input resistor RB to the inverting input terminal, or to change the value of the feedback resistor RA. In this second embodiment, the value of the grounding resistor RB (R13) is fixed and the value of the feedback resistor RAO is changed.

The feedback resistor section is connected in series to a resistor R12 that is always connected between the output terminal and the inverting input terminal of the operational amplifier H1, and to the light receiving elements Plr to P3r of the three photocouplers P1 to P3. It is composed of three resistors R6 to R8 that can be connected between the output terminal of H1 and the inverting input terminal.

In this way, the light emitting elements P1t to P3t of the photocouplers P1 to P3, which control the connection of the resistors R6 to R8, respectively constitute a series circuit with the Zener diodes D1 to D3 and the resistors R1 to R3.

Light emitting elements P1t to P3t, Zener diodes D1 to D
A voltage V obtained by a resistor RO detecting the DC loop current rt is applied to both ends of the three series circuits made up of the DC loop current rt and the resistors R1 to R3. In addition, the Zener diode D1~
D3 are selected to have different Zener voltages.

In the above configuration, when the DC loop current I [ is the value of the smallest stage among the four stages, the detected voltage
is small, any of the Zener diodes D1 to D
3 do not break down, so the feedback resistor in the connected state straddles the resistor R1.

As a result, the gain A increases.

When the DC loop current I[ is at the second stage value, for example, the Zener diode D1 is connected based on the detected voltage V.
The only feedback resistors that are in the connected state are resistors R12 and R6. As a result, the feedback resistance value becomes the value due to these resistors R12 and R6 connected in parallel, and is smaller than when only resistor R12 functions as a feedback resistor, and the gain A is also smaller than in that case. becomes smaller.

Thereafter, similarly, as the DC loop current {circle around (2)} increases, the number of parallel resistors functioning as feedback resistors increases, and the gain A gradually decreases.

As described above, since this second embodiment also has a structure that compensates for the transmission loss of the subscriber line, the equivalent transmission loss is reduced, and the speech level can be increased compared to the conventional method. Quality can be further improved.

Note that since the photocouplers P1 to P3 are used, the electrical shielding relationship between the primary side and the secondary side of the coupling transformer 22 is maintained.

Next, a third embodiment of the present invention will be described with reference to FIG. FIG. 9 shows the third embodiment on a more concrete level than the first embodiment, and shows the central office line trunk section. Components corresponding to those in FIG. 1 are designated by the same reference numerals, and their explanation will be omitted.

In this third embodiment, transmission loss compensation is continuous (see FIG. 7).

In FIG. 9, a pair of balanced transmission line terminals TA and TB
A resistor RO and an AC signal blocking coil L are connected in series between them. This resistor RO is connected to the central office (not shown)
and the DC loop current {circle around (2)} when the central office line trunk 21 is in a talking state are detected as voltage signals (V).

The voltage at one end of the resistor RO is input to the inverting input terminal of the operational amplifier H3 via the resistor R21, and the voltage at the other end of the resistor RO is input to the non-inverting input terminal of the operational amplifier H3 via the resistor R22. . This operational amplifier H3 has a feedback resistor R23 to the inverting input terminal and a grounding resistor R23 to the non-inverting input terminal.
24, forming a differential amplifier circuit. That is, the DC loop current obtained by the resistor RO [
It amplifies the voltage according to the voltage. This amplified voltage is
Noise components (such as AC signals that could not be blocked by the AC signal blocking coil) are removed and applied to transmission loss compensation circuits 824 and 25 via a low-pass filter circuit consisting of a resistor R25 and a capacitor C2.

As described above, in this third embodiment, the current detection circuit 23 is constituted by the processing system from the current/voltage conversion resistor RO to the low-pass filter circuit.

The transmission loss compensation circuits 24 and 25 have symmetrical configurations as shown in FIG. Therefore, the transmission loss compensation circuit f¥424 for the received signal will be explained below.

The transmission loss compensation circuit 24 receives an input signal (received signal) consisting of a resistor R26 and a field effect transistor (FET) Ql.
Operational amplifier H4 having a level divider, a feedback resistor R27 to the inverting input terminal, and a grounding resistor R28 to the inverting input terminal.
It consists of a non-inverting amplifier circuit section centered on.

Transistor Q1 functions as a variable resistor, and is connected in series with resistor R26 between one end of secondary winding 22b of coupling transformer 22 and ground. That is, the resistor R26 and the transistor Q1 constitute a ladder resistor, and the division level of the input signal (received signal) level is obtained from the connection point thereof as described above.

This divided level is applied to the non-inverting input terminal of operational amplifier H4 and amplified with a fixed gain. The non-inverting amplifier circuit section centered on the operational amplifier H4 secures a basic gain that compensates for transmission loss, and the difference between this basic gain and the required gain is determined by the above-mentioned input signal (received signal).
I try to get it in the level division part.

The output voltage of the current detection circuit 23 is inputted to the gate of the transistor Q1 which functions as a variable resistor, thereby varying the resistance value. Here, the bias point of the transistor Q1 is selected so that the detection voltage of the current detection circuit 23 and the resistance value of the transistor Q1 correspond linearly.

In addition to the above configuration, surge lightning protection diodes D11 to D14 are provided.

In the above configuration, the larger the DC loop current ■ becomes, the higher the control voltage output from the current detection circuit 23 becomes. becomes smaller. Therefore, the gain of the entire transmission loss compensation circuit 24 becomes smaller as the DC loop current I[ becomes larger, and the transmission loss is continuously compensated for as shown in FIG.

As described above, since this third embodiment also has a structure that compensates for the transmission loss of the subscriber line, the equivalent transmission loss is reduced, and the speech level can be increased compared to the conventional method. Quality can be further improved.

In the above-described embodiment, a case was explained in which transmission loss between a telephone office and a central office line trunk is compensated for, but the present invention is not limited to this, and the present invention is not limited to this. It can be widely applied to compensate for transmission loss in telephone lines.

[Effects of the Invention] As described above, according to the telephone line transmission loss compensation circuit of the present invention, the transmission loss is compensated for while detecting the DC loop current and changing the gain for the transmitted and received signals according to the detected value. As a result, the call level can be maintained at a certain value, making it possible to make good calls regardless of line length.

[Brief explanation of drawings]

FIG. 1 shows a first diagram of a telephone line transmission loss compensation device according to the present invention.
A block diagram showing the embodiment, Figures 2 and 3 are explanatory diagrams of transmission loss, Figure 4 is an equivalent diagram of the DC loop circuit in the first embodiment, and Figure 5 is the relationship between transmission loss and DC loop current. FIG. 6 and FIG. 7 are explanatory diagrams showing equivalent transmission loss after transmission loss compensation, FIG. 8 is a block diagram showing the second embodiment of the present invention, and FIG. FIG. 3 is a block diagram showing a third embodiment. 11... Subscriber line, 21... Office line trunk, 23
...Current detection circuit, 24.25...Transmission loss compensation circuit. Loss compensation characteristic diagram (1) Figure 6 Loss compensation characteristic diagram (2) Figure 7

Claims (1)

[Claims] A current detection circuit that detects the amount of direct current flowing through a two-wire telephone line in which the direct current is superimposed on the connection line, and a level of a transmitted signal and a received signal according to the detected amount of current. A telephone line transmission loss compensation device comprising a transmission loss compensation circuit that controls the transmission loss compensation circuit.