JP2807054B2 - Gain control method for bidirectional amplifier in telephone circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling a gain of a bidirectional amplifier (a receiving signal amplifier and a transmitting signal amplifier) in a telephone circuit.

[Conventional technology]

As an example of the prior art, FIG. 3 shows an amplifier gain control circuit widely used in telephone circuits.

In FIG. 3, 1 is a line DC current detecting unit, 2 is a 2-wire to 4-wire conversion circuit, 3 is a balanced network, 10 is a subscriber cable,
12 is an exchange, 13 is a transmitting circuit, 14 is a receiving circuit, 15 is a transmitting amplifier, 16 is a receiving amplifier, 17 is a transmitter, and 18 is a receiver.

The transmission signal from the transmitter 17 is transmitted to the transmission circuit 13 and the transmission amplifier.
15, a two-wire / four-wire conversion circuit 2, a line DC current detector 1, and a subscriber cable 10 to an exchange 12, and a received signal from the exchange 12 is conversely transmitted to the subscriber cable 10, line DC current detectors 1, 2, The signal reaches the receiver 18 via the line-to-wire conversion circuit 2, the reception amplifier 16, and the reception circuit 14. At this time, the transmission signal leaks to the reception side in the 2-line to 4-wire conversion circuit 2, Alternatively, the balanced network 3 is selected so that the reception signal does not leak to the transmission side.

Such a conventional gain control circuit has a configuration in which the gains of the transmission amplifier 15 and the reception amplifier 16 are controlled in accordance with the magnitude of the circuit DC current value detected by the circuit DC current detection unit 1. However, the control is performed by utilizing the fact that it is inversely proportional to the line loop resistance of the subscriber cable 10 to be connected.

FIG. 4 is a block diagram showing another conventional example of an amplifier gain control circuit in a telephone circuit.

3, the same components as those in FIG. 3 are denoted by the same reference numerals. In addition, 5 is a transmission signal amplification unit, 8 is a reception signal amplification unit, 2A is a 2-wire 4-wire conversion circuit, 3A is a balanced network, and 11 is a telephone. Although the circuit configuration is slightly different from that of FIG. 3, the operation principle is almost the same, and the gain control of the transmission signal amplification unit 5 and the reception signal amplification unit 8 also depends on the magnitude of the line DC current value. Is being done.

Here, various types of cables having various core diameters are used for the subscriber cable 10 of the telephone line, and the relationship between the line loop resistance and the cable loss of the subscriber cable is as shown in FIG.
Depending on the core diameter, the subscriber cable loss increases as the core diameter increases, even if the line lube resistance is constant. For example, if the line loop resistance is 1000Ω, the core diameter is 0.9mmφ
Has a loss of about 9 dB greater than a 0.4 mmφ subscriber cable.

In the prior art, a line loop resistance value is estimated from a line DC current value, a cable loss is estimated using only the line loop resistance value, and the gain of the amplifier is controlled so as to compensate for the loss estimated value. That is, if the line loop resistance values are the same, the gain of the amplifier is controlled to the same value even when subscriber cables having different core diameters are connected.

That is, assuming the characteristics of a virtual subscriber cable, which indicates the average characteristics of cables of various core diameters, as shown by broken lines in FIG. 2, based on this, the relationship between the line loop resistance and the cable loss is assumed. This is used to control the gain of the amplifier in the past.

Therefore, in the prior art in which only the magnitude of the line DC current value is detected and the gain of the amplifier is controlled, when a subscriber cable having a core diameter larger than the design target subscriber cable is connected, the loss ratio to the line loop resistance is connected. However, it is only possible to obtain a gain smaller than the required gain for accurately compensating for cable loss, the volume is insufficient, good speech quality cannot be ensured, and the level of audible signals such as PB signals However, there is a drawback that the device does not reach a predetermined level and does not operate normally.

For example, if the line loop resistance is 1000Ω,
Since the 0.9 mmφ subscriber cable has a cable loss of about 4.5 dB greater than the average subscriber cable, the core diameter of the prior art circuit designed for the average subscriber cable is 0.9 mm.
When the φ subscriber cable is connected, the gain is controlled to be about 4.5 dB smaller than the gain required to compensate for the cable loss.

Conversely, when a subscriber cable having a core diameter smaller than the designed subscriber cable is connected to the line loop resistance, the gain is set to be larger than the required gain for accurately compensating for the cable loss, The signal is amplified more than necessary, and there is a drawback that excessive volume, crosstalk, overload, and the like occur. For example, if the line loop resistance value is 1000Ω, the conventional cable designed for the average subscriber cable has a core diameter of 0.4 mmφ because the cable loss is about 4.5 dB smaller than the average subscriber cable. 0.4 m core diameter in circuit
When a subscriber cable of mφ is connected, the gain is controlled to about 4.5 dB larger than the gain required to compensate for the cable loss.

Further, in the related art, an AGC circuit that controls the gain so that the output signal level of the amplifier is always constant may be used. With this AGC circuit, the loss to the input terminal of the AGC circuit is accurately compensated regardless of the core diameter of the subscriber cable in order to always control the output signal level to a constant value regardless of the magnitude of the input signal level. However, there is a disadvantage that the loss of the cable connected first from the output terminal of the AGC circuit cannot be compensated. That is, the AC signal arriving from the subscriber cable can be compensated, but the AC signal arriving from the telephone cannot be compensated.

[Problems to be solved by the invention]

In a conventional amplifier gain control method in a telephone circuit,
If the core diameter of the connected subscriber cable is different from the core diameter to be designed, there is a problem that the gain of the amplifier cannot be controlled so as to always accurately compensate for the subscriber cable loss.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a gain control method for a bidirectional amplifier in a telephone circuit capable of accurately compensating for a subscriber cable loss irrespective of a core diameter of a subscriber cable to be connected. .

[Means for solving the problem]

In order to achieve the above object, according to the present invention, one circuit from a switch to a two-wire / four-wire conversion circuit via a subscriber cable and branching therefrom to a reception signal amplifier, and the other from a transmission signal amplifier to a transmission signal amplifier. A telephone circuit comprising:
A detecting unit for detecting a line direct current in a subscriber cable, a selecting unit for selecting a balanced network for minimizing signal leakage between a receiving side and a transmitting side in a two-wire / four-wire conversion circuit, and a receiving signal amplifier And a control unit for controlling the amplification gain of the transmission signal amplifier.

[Operation] The detection unit detects a line direct current in a subscriber cable. The selector selects a balanced network that minimizes signal leakage between the receiving side and the transmitting side in the two-wire / four-wire conversion circuit. The control unit is configured to control the reception based on the line loop resistance estimated from the detected line DC current value and the subscriber cable core diameter estimated from the selected balanced network. The amplification gain of the signal amplifier and the transmission signal amplifier is controlled.

As a result, gain control of a bidirectional amplifier in a telephone circuit capable of accurately compensating for a subscriber cable loss is realized irrespective of the core diameter of the connected subscriber cable.

〔Example〕

FIG. 1 is a block diagram showing one embodiment of the present invention. In the figure, 1 is a line DC current detecting unit for detecting a DC current value of a telephone line, 2, 2A is a 2-wire 4-wire conversion circuit, 4 is a plurality of balanced networks, 3A is a balanced network for telephones, 5 Is a signal generation circuit, 6 is an optimum balanced network selection section, 7 is a transmission signal amplification section, 8 is a reception signal amplification section, 9 is an amplifier gain control section,
10 is a subscriber cable, 11 is a telephone, and 12 is an exchange.

The circuit operation will be described with reference to FIG. First, when the telephone circuit is connected to the subscriber cable 10, a DC loop is formed, and a DC current flows through the subscriber cable 10. This DC current value is detected by the line DC current detector 1.

Next, the signal generation circuit 5 is operated to send an AC signal to the subscriber cable side via the two-wire / four-wire conversion circuit 2.
A part of this signal leaks as a side tone to the receiving signal amplifying unit 8 of the two-wire / four-wire conversion circuit 2, and the amount of the leak is determined by the impedance of the balanced network 4 connected to the two-wire / four-wire conversion circuit 2. It depends on the degree of impedance matching of the subscriber cable 10.

That is, the impedance of the balanced network and the subscriber cable
As the difference between the 10 impedances decreases, the amount of leakage decreases. The optimum balanced network selector 6 switches the plurality of balanced networks 4 to measure the leakage signal level to the receiving side, and the optimum balanced network in which the leakage signal level is minimized, that is, of the connected subscriber cable. The balanced network having the impedance closest to the impedance is selected from among four.

Here, if a plurality of balanced networks, each simulating the subscriber cable impedance for each core diameter, are prepared as 4, if any of the balanced networks is connected, the leakage signal level to the receiving side is reduced. By examining the minimum value, it is possible to estimate the core diameter of the connected subscriber cable.

The amplifier gain control unit 9 estimates the line loop resistance of the subscriber cable from the line DC current value by the line DC current detection unit 1 and determines that the leakage signal level to the receiving side by the optimal balanced network selection unit 6 is minimum. The core diameter of the connected subscriber cable is estimated from the selection result of the balanced network.

Next, a second value is obtained from the line loop resistance estimated value and the core diameter estimated value.
Using the relationship between the line loop resistance and the subscriber cable loss for subscriber cables of various core diameters shown in the figure, it is necessary to accurately estimate the loss of the connected subscriber cable and compensate for the cable loss. The gains of the transmission signal amplification unit 7 and the reception signal amplification unit 8 are controlled so that an optimal gain can be obtained.

By performing such gain control, the gain of the amplifier can be controlled so as to always accurately compensate for the cable loss regardless of the core diameter and loss of the connected subscriber cable.

In the present embodiment, the signal generating circuit 5 is used to determine the level of the leakage signal from the transmitting side to the receiving side. However, the same function is used even if a telephone signal or the like input to the transmitting signal amplifier is used. Can be realized. Further, the present invention can be realized by either automatic selection or manual selection of the optimum balanced network that minimizes the leakage signal level to the receiving side from the plurality of balanced networks.

In the present embodiment, an example of installation on the inside of the house has been described.

〔The invention's effect〕

As described above, according to the present invention, it is possible to select an optimum balanced network that minimizes a line loop resistance value estimated from a line DC current value and a leakage signal level from a transmission side to a reception side of a 2-wire 4-wire conversion circuit. The amplifier gain control unit controls the gains of the transmission amplifier and the reception amplifier using both the cable diameters of the subscriber cables estimated from the results, so that the cable gain is always constant regardless of the loss of the connected subscriber cable. The gain required to compensate for the loss can be set accurately.

For this reason, when a bidirectional amplifier using the gain control method according to the present invention is used, an excessive volume or an excessive volume does not occur, and an optimal volume is always obtained regardless of the core diameter and loss of the connected subscriber cable. It is possible to provide call quality. Also, since an excessive level signal is not transmitted, no overload of the transmission device or crosstalk in the transmission line occurs, and audible signals such as PB signals are always transmitted at an appropriate level. Various receivers and the like do not malfunction or operate.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIG. 2 is a characteristic diagram showing a relationship between line loss and loop resistance, FIG.
FIG. 4 is a block diagram showing a gain control method of a bidirectional amplifier in a conventional telephone circuit. DESCRIPTION OF SYMBOLS 1 ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………. 6 ... Optimal balanced network selection unit 7 ... Transmission signal amplification unit 8 ... Reception signal amplification unit 9
…… Amplifier gain control unit, 10 …… Subscriber cable, 11 ……
Telephone, 12 ... Exchange

Continuation of the front page (56) References JP-A-63-139493 (JP, A) JP-A-63-153993 (JP, A) JP-A-2-256357 (JP, A) JP-A-53-56903 (JP) (A) JP-A-53-91509 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H04M 1/00 H04M 1/58 H04M 1/76 H04Q 1/54 H04Q 1/56 H04Q 3/42

Claims (1)

(57) [Claims] 1. A two-wire system from a switch via a subscriber cable.
Detecting a line DC current in the subscriber cable in a telephone circuit including one circuit leading to a line conversion circuit, branching therefrom and leading to a reception signal amplifier, and another circuit leading to a transmission signal amplifier. Selecting a balanced network for minimizing signal leakage between the receiving side and the transmitting side in the two-wire / four-wire conversion circuit; and a line loop estimated from the detected line DC current value. Controlling the amplification gains of the receive signal amplifier and the transmit signal amplifier depending on a resistance and a subscriber cable core diameter estimated from the selected balanced network. A gain control method for a bidirectional amplifier in a telephone circuit.