JPS6328390B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a branching and coupling circuit for conference calls that branches and couples a plurality of four-wire telephone lines to enable mutual communication.

[Conventional technology]

General telephone lines consist of two lines for subscribers and terminal exchanges, and four lines for toll exchanges. The loop gain of this 4-wire section is designed so that it does not exceed a predetermined value in the worst case because it takes into consideration the reflected signal from the 2-wire 4-wire conversion point (hybrid coil) and does not generate any noise or quasi-sound. . There is no particular problem when making mutual calls between two parties using a telephone line configured in this way, but in the case of a so-called conference call where three or more parties make mutual calls, there is a problem in the four-wire section. A branching and coupling circuit is required to interconnect multiple lines.

Conventionally known methods for branching and combining 4-wire lines include multi-stage connection of branch amplifier circuits used in configurations such as meeting lines, or methods in which multiple branch amplifiers and coupling amplifiers are coupled using a matrix network. . In this way, when branching and coupling multiple 4-wire lines with each other using a branching/coupling circuit, even if the branching gain and coupling gain are configured to 0 dB, any 4-wire line will have many other 4-wire lines connected to it. By combining the loop signals at the 2-wire 4-wire conversion part (hybrid coil) of the line, this corresponds to a substantial increase in the gain of this 4-wire loop, and no ringing or quasi-ringing noise is generated. If the conditions are no longer met, it becomes difficult to maintain good communication.

As a countermeasure to this problem, a loss of a predetermined amount or more is inserted into at least one of the transmitting side or the receiving side of each 4-wire line, and by comparing the magnitude of the transmitting or receiving signal of each 4-wire line, the loss circuit is A voice switch-type branching and coupling circuit for controlling insertion has been considered (for example, ``Japanese Patent Application No.
Publication No. 157164) “Conference Telephone System”).

[Problem that the invention seeks to solve]

However, although this method can avoid ringing and quasi-ringing, it causes unnaturalness due to the voice switch operation.

The present invention improves this point by connecting echo canceling circuits to the input and output lines of the 4-wire line and canceling the echoes that circulate around the subscriber side. 4 for conference calls that can prevent gain increase and provide natural call quality without ringing or quasi-ringing.
The purpose of this invention is to provide a wire branching and coupling circuit.

[Means for solving problems]

The present invention provides N (N is a positive integer of 3 or more) 4
a mutual branching and coupling circuit for branching and coupling wire lines; an echo cancellation circuit including a pseudo echo generation circuit connected to a 4-wire input and a 4-wire output of the mutual branching and coupling circuit; this echo cancellation circuit and the mutual branching and coupling; a variable loss circuit connected to a four-wire line between the circuit;
and a reference signal generation circuit connected to one of the four-wire lines, and after the conference line is connected, the reference signal generation circuit sends a test signal to each reception line, and the pseudo echo generation circuit of the echo cancellation circuit constant 2
The present invention is characterized by comprising control means for setting a value at which the echo signal of the test signal from the four-wire conversion point is eliminated, and then removing the insertion loss of the variable loss circuit to set a speech state. .

[Effect]

Each line is given additional loss by a variable loss circuit until echo cancellation is completed.

When a plurality of subscribers are connected, a test signal for echo cancellation is sent to each line via a mutual branching and coupling circuit. This test signal appears as the echo signal of each line at the receiving input of the echo cancellation circuit and is canceled by the echo cancellation circuit.

In the echo cancellation circuit, the echo cancellation control circuit changes the constant of the pseudo echo generation circuit so as to increase the level difference between the signal before cancellation and after cancellation.

When the echo signal canceled by the echo cancellation circuit becomes less than a predetermined level compared to the signal before cancellation, the echo cancellation is completed, and the insertion loss caused by the variable loss circuit is removed to form a conference telephone coupling circuit with no additional loss. be done.

〔Example〕

An embodiment of the present invention will be described below based on the drawings.

FIG. 1 is a block diagram of essential parts of an embodiment of the present invention. N subscriber telephones 1 ₁ to 1 _o are 2 wires 4
It is converted into four lines by line converters 2 ₁ to 2 _o , and connected to the exchange trunk 3 of a predetermined station via the exchange of each station. The 4-wire transmission lines 4 ₁ to 4 _o from this exchange trunk 3 are echo cancellation circuits 5 ₁ to 5 _o, respectively.
connected to the transmitter input. The transmission outputs of the echo canceling circuits ₅₁ to _5o are connected to a variable loss circuit ₆₁ whose loss is controlled on and off by an external control signal.
~ _6o is connected to the four-wire input of the mutual branching and coupling circuit 7.

This mutual branching and coupling circuit 7 is composed of low output impedance amplifier circuits 8 and 9, etc. The four-wire output of this mutual branching and coupling circuit 7 is connected to the receiving inputs of the echo canceling circuits 5 ₁ to 5 _o . The reception outputs of the echo cancellation circuits 5 ₁ to 5 _o are connected to the exchange trunk 3 .

Further, a reference signal generating circuit 11 is inserted in series to one of the four-wire input/output of the mutual branching/coupling circuit 7 for selecting the amount of echo cancellation of the connected subscriber side line. Further, the transmitting outputs of the echo canceling circuits 5 ₁ to 5 _o are guided to the variable loss circuits 6 ₁ to 6 _o and also to the AND circuit 13 of the control circuit 12, respectively. The output of this AND circuit 13 is led to a set terminal of an RS flip-flop 14. The output of this RS flip-flop 14 is led via an AND circuit 15 to the control terminals of the echo canceling circuits 5 ₁ to 5 _o . This RS flip-flop 1
The output of the exchange trunk 3 is led to the reset terminal 4.

FIG. 2 is a detailed diagram of the echo cancellation circuits 5 ₁ to 5 _o . The echo cancellation circuits ₅₁ to _5o are composed of a pseudo echo generation circuit 17 including a transversal filter, an addition circuit 18, an echo cancellation determination circuit 19, and an echo cancellation control circuit 20.

In other words, this echo cancellation circuit 5 ₁ to 5 _o is a 2-wire 4
The echo signal that goes around from the 4-wire receiving side to the 4-wire transmitting side at the line conversion point is canceled by adding a pseudo-echo signal calculated from the transmitted and received signals on the 4-wire side at the 4-wire transmitting side with opposite polarity. This is detailed on pages 58 to 60 of the January 1978 issue of the Journal of the Institute of Electrical Communication Engineers.

In addition, in the variable loss circuits 6 ₁ to 6 _o , the gain of the echo loop of the 4-wire line may exceed the sound limit during the test period until the echo cancellation in the echo cancellation circuit is completed. Since the test signal may cause ringing and quasi-ringing, a loss is inserted to prevent the ringing and quasi-ringing.

Next, the operation of the present invention will be explained.

With the circuit configuration described above, in order to connect a plurality of subscribers and hold a conference using the subscriber telephones ₁₁ to _1o , the subscribers are connected by a predetermined method. In this state, a drive signal for driving the reference signal generation circuit 11 is applied to the control circuit 12 via the exchange trunk 3. As a result, an echo cancellation test signal (for example, white noise) is applied to one predetermined transmission line into which the reference signal generation circuit 11 is inserted. This signal is passed through the mutual branching and coupling circuit 7 and becomes a receiving input signal of the echo canceling circuits 5 ₁ to 5 _o corresponding to each line as a receiving side signal of the other four-line line.

This received input signal reaches each subscriber line,
A part of it becomes an echo at the 2-wire/4-wire conversion point and becomes the transmission input signal (echo input) of the corresponding echo cancellation circuits 5 ₁ to 5 _o . Each echo canceling circuit ₅₁ to _5o adds the pseudo echo signal obtained by passing the received input signal through the pseudo echo generating circuit 17 and the transmitting input signal in an adding circuit 18 to cancel the echo. conduct. At this time, the echo cancellation control circuit 20 changes the constants (delay time and attenuation amount) of the pseudo echo generation circuit 17 to increase the level difference between the echo signal and the output signal (echo cancellation residual signal) of the addition circuit 18. Control as follows.

The echo cancellation determination circuit 19 determines the echo cancellation state by comparing their signal levels, and when the echo cancellation residual signal exceeds a predetermined level difference (for example, 20 dB) compared to the echo signal, The echo cancellation completion determination output n is given to each of the correspondingly connected variable loss circuits 6 ₁ to 6 _o to automatically eliminate the loss. At the same time, it is applied to the echo cancellation completion determination output circuit 12. Control circuit 12
When receiving all the erasure completion determination signals, it gives a signal to the reference signal generation circuit 11 to stop the echo cancellation test signal. After the echoes generated at the subscriber side 2-4 wire conversion point of the connected 4-wire line are eliminated in this way, each echo canceling circuit 5 ₁
~ _5o is stopped and held, and at the same time, the variable loss circuit ₆₁ ~ _6o , which had been inserted in the transmission line of each 4-wire circuit to prevent ringing, is removed, creating a conference phone system without additional loss. A four-wire mutual branching and coupling circuit is constructed.

In addition, since the echo canceling circuits ₅₁ to _5o can provide an echo return loss enhancement of about 20 to 30 dB, it is almost impossible to eliminate the echo canceling circuit for the line to which the test signal is applied, as shown by the solid line in Figure 1. Not a problem. Furthermore, as shown by the dotted line in FIG. 1, if the line to which the test signal is applied also has an echo canceling circuit _1o , the echo signal may be configured to be applied to the receiving line of this line as well.

As connection information from the exchange trunk 3, for example, an external control signal is instantaneously applied at the same time as the telephone hooks of all subscribers to be conferenced are raised (for example, a signal of "1" when the line is connected, and "0" otherwise). ), and at the same time a test signal is applied to cancel the echo. The time it takes to complete erasing is about 1 second at most.

〔Effect of the invention〕

As explained above, according to the present invention, the echo canceling circuit is connected to the input and output lines of the 4-line line to cancel echoes circulating on the subscriber side. Therefore, it is possible to prevent the gain of each 4-wire line loop from increasing, so ringing and quasi-ringing noises do not occur, and moreover, natural call quality can be obtained. A branch-coupling circuit can be realized. Furthermore, since the switch is not changed every time a call is made, there is an effect that the unnaturalness of the call caused by the switch change is eliminated.

[Brief explanation of drawings]

FIG. 1 is a block diagram of essential parts of an embodiment of the present invention. Figure 2 is a block diagram of the echo cancellation circuit. 1 ₁ - 1 _o ... subscriber telephone, 2 ₁ - 2 _o ... 2-wire 4-wire converter, 3 ... switching trunk, 4 _{1 -} 4 _o ...
4-wire transmission line, 5 ₁ ~ 5 _o ... Echo cancellation circuit, 6 ₁
~6 _o ... Variable loss circuit, 7... Mutual branching and coupling circuit, 8, 9... Amplifying circuit, 11... Reference signal generation circuit, 12... Control circuit, 13, 15... AND circuit, 14... RS flip-flop, 17...
Pseudo-echo generation circuit, 18... Addition circuit, 19...
Echo cancellation determination circuit, 20...Echo cancellation control circuit.

Claims (1)

[Claims] 1. A mutual branching and coupling circuit for branching and coupling N (N is a positive integer of 3 or more) four-wire lines, and a four-wire input and a four-wire output connected to the mutual branching and coupling circuit. an echo cancellation circuit including a simulated echo generation circuit; a variable loss circuit connected to a four-wire line between the echo cancellation circuit and the mutual branching and coupling circuit; and a variable loss circuit connected to one of the four-wire lines. a reference signal generation circuit; after the conference line is connected, the reference signal generation circuit sends a test signal to each receiving line, and the constant of the pseudo echo generation circuit of the echo cancellation circuit is calculated from the 2-wire 4-wire conversion point. A four-wire branching and coupling circuit, characterized in that it is provided with a control means that sets a value at which the echo signal of the test signal is eliminated, and then removes the insertion loss of the variable loss circuit to set a communication state.