JPS6130134A - Echo erasing system - Google Patents

Abstract

PURPOSE:To apply this system to a device having various echo route characteristics to converge an echo erasing control loop by detecting the case where the gain of an echo route exceeds one and setting the gain of the echo route equivalently to <=1. CONSTITUTION:The reception signal from a reception signal input terminal 1 of the echo erasing system is applied to a speaker 4 through an amplifier 3 for speaker to louden the acoustic signal in a room. This signal is gathered by a microphone 5 and is converted to an electric signal by an amplifier 6 for microphone and is applied to a transmission signal input terminal 7. On the other hand, the reception signal is digitized by an A/D converter 10, and the digital signal is delayed and is applied to a register 11 for reception signal. The convolution between the signal stored in the register 11 and an impulse response of the echo route stored in a false impulse response register 12 is operated by a means 13, and the output is subjected to D/A conversion by a D/A converter 14 and is applied to an adder 15. This adder 15 is provided between a variable gain attenuator 18 connected to the terminal 7 and a variable gain amplifier 19 connected to a transmission signal output terminal 8 to set the gain of the echo route equivalently to <=1.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for canceling echoes in a four-wire communication line. In particular, the present invention relates to an echo cancellation method that operates well when abnormal coupling occurs between a received signal and a transmitted signal on a four-wire line, with an echo gain exceeding rl (OdB) J. The present invention provides, when connecting a four-line line to a subscriber device,
It is suitable for use in subscriber equipment to cancel high-level echoes mainly generated by acoustic coupling.

[Conventional technology]

Conventionally, various methods have been known for canceling echoes occurring in long-distance four-wire lines, but these echoes are mainly caused by impedance mismatching of four-wire to two-wire converters. Therefore, the echo gain is, for example −
30 (dB), and no phenomenon occurs in which this value exceeds 0 (dB).

On the other hand, in recent years, when four-line lines such as dedicated lines have come to be directly connected to subscriber equipment, it has been experienced that echoes that were not predicted due to the line design are generated in the subscriber equipment. For example, the output of the receiving device of a subscriber device connected to a four-wire line is amplified into a room as an acoustic signal from a speaker, and the input of the transmitting device of the subscriber device is connected to a microphone placed in the room. In this case, if the acoustic output of the speaker is directly input to the microphone, an echo with an echo gain exceeding O' (dB) will be generated when viewed from the four-wire line side. The line enters a so-called paralleling state, which has an extremely negative effect on other multiplexed lines.

[Problem that the invention seeks to solve]

However, the conventional echo cancellation method was not designed to perform well against echoes with an echo gain exceeding O (dB).

The present invention solves this problem, and the echo gain is 0 (dB) between the receiving device and the measuring device connected to the four-wire line.
) An object of the present invention is to provide an echo cancellation method that operates satisfactorily even when an echo exceeding .

[Means for solving problems]

In the present invention, when the gain of the echo path exceeds "1",
It is characterized by suppressing it isometrically below rlJ or by multiplying the estimated impulse response of the pseudo echo path by a certain amount of gain.

That is, a transmission signal input terminal connected to the transmission device,
Branches and delays signals arriving at the transmitting signal output terminal connected to the transmitting line, the receiving signal input terminal connected to the receiving line, the receiving signal output terminal connected to the receiving device, and the receiving signal input terminal. a first means for converting the level thereof; a second means for adding the output signal of this means to the signal at the transmitting signal input terminal with its opposite phase; and a second means for branching the signal transmitted to the transmitting signal output terminal. and a third means for controlling the signal of the first means so that an echo caused by coupling between the receiving device and the transmitting device is canceled by the second means. means for detecting that the gain of the echo path to the transmission signal input terminal exceeds "1"; and an output signal of the first means to be input to the second means and transmission according to the detection output of the means for detecting. The present invention is characterized by comprising relative level control means for controlling the relative level of the signal at the signal input terminal.

The relative level control means includes a variable gain attenuation circuit inserted between the transmission signal input terminal and the second means, a variable gain amplification circuit inserted between the second means and the transmission signal output terminal, It is preferable to include a control circuit that controls the variable gain attenuation circuit and the variable gain amplification circuit based on the detection output of the detection means.

Further, the relative level control means may include a variable gain amplification circuit inserted between the first means and the second means, and a control circuit that controls the variable gain amplification circuit based on the detection output of the detection means. is preferred.

Preferably, the means for detecting that the gain of the echo path exceeds "1" includes means for detecting discontinuity in the signal at the transmission signal output terminal.

Further, the means for detecting that the gain of the echo path exceeds "1" includes means for transmitting a test signal to the reception output terminal;
means for receiving the test signal arriving at the transmission signal input terminal; and means for comparing the level of the test signal transmitted by the transmitting means with the level of the test signal received by the receiving means, and the method is set in advance. It is preferable.

[Effect]

In the present invention, when the gain of the echo path exceeds "1",
By detecting this and isometrically reducing the gain of the echo path to "1" or less, the control loop for echo cancellation is converged.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram illustrating a first embodiment of the present invention. In FIG. 1, a received signal input from a received signal input terminal 1 connected to a receiving line is sent from a received signal output terminal 2 connected to this received signal input terminal 1 to a speaker amplifier 3 of the receiving device. The sound signal is input, amplified, and amplified from the speaker 4 into the room. On the other hand, this indoor sound is collected by the microphone 5 of the transmitting device. The collected sound is converted into an electrical signal, amplified by a microphone amplifier 6, and inputted to a transmission signal input terminal 7, and then sent to a transmission line from a transmission signal output terminal 8 connected to this transmission signal input terminal 7. Output. If there is acoustic coupling 9 from the loudspeaker 4 to the microphone 5 in this room, this will cause echo generation.

In this device, a received signal is branched and inputted to an analog-to-digital conversion circuit 10, where it is converted into a digital signal, and then inputted into a received signal register 11 which delays the signal and converts its level. This received signal register 11
and the echo path (received signal output terminal 2) stored in the pseudo impulse response register 12.
-Speaker site - Refers to the path from speaker 3 -> speaker 4 -> microphone 5 -> microphone amplifier 6 -> transmission signal input terminal 7. ) is input to the convolution calculator 13. This output is input to the digital-to-analog conversion circuit 14 and converted into an analog signal.
The signal is inserted between the transmission signal input terminal 7 and the transmission signal output terminal 8, and is input to an adder 15 which adds the transmission signal sent out from the transmission signal input terminal 7 in an opposite phase. The output from the adder 15 is branched and input to an analog-to-digital conversion circuit 16 where it is converted into a digital signal and input to a coefficient correction circuit I7, the output of which is connected to the pseudo impulse response register 12.

Here, the present invention is characterized by a variable gain attenuator 18 between the transmission signal input terminal 7 and the adder 15, and a variable gain amplifier 1 between the adder 15 and the transmission signal output terminal 8.
9 is connected, and the gains of the variable gain attenuator 18 and variable gain amplifier 19 are controlled by a gain controller 20. The gain controller 20 receives as input the output of an overflow detector 21 which detects an overflow when the pseudo impulse response register 12 overflows and a discontinuous change is observed in the true impulse response of the echo path.

For the received signal stored in the received signal register 11, the estimated impulse response of the echo path stored in the pseudo impulse response register 12, that is, the gain of the pseudo echo path, is calculated by the convolution calculator 13, and the output is added. 15 and its residual output is taken out. The coefficient correction circuit 17 is controlled so that the residual output becomes "0" to cancel the echo on the echo path.

At this time, the pseudo impulse response register 12 is set when the estimated impulse response is "1" or less.
If the true impulse response exceeds "1", the value of the pseudo impulse response register 12 cannot converge. If the gain of the echo path exceeds "1", the coefficient modification signal output from the coefficient modification circuit 17 causes an overflow in the pseudo impulse response register 12, thus shifting the residual output in the direction of "0". Unable to do anything, he emanates.

That is, the received signal stored in the received signal register 11 at time j is an N-dimensional vector X.

When expressed as
・Given in ttl.

Also, the true impulse response of the echo path is expressed as an N-dimensional vector h, as follows: hJ-(h, hJ!,..., h js)...
(2).

Then, the response Hj of the echo path to Xj is: Hj = Xj *hj =Σ
h jN-4"' (3).

- On the other hand, the estimated impulse response of the echo path stored in the pseudo impulse response register 2 at time j is N
When expressed as a dimensional vector lj, 11J= C1jls
ljz, . . . , N jH) . . . is given by (4).

At that time, the response of the pseudo echo path to Xj is Lj = Xj ''' j = ΣxJL-IljH-5 (5).

Therefore, from equations (3) and (5), the residual output ej of the echo at time j after passing through the adder 15 is ej
=Hj Lj = NoXji ()17+1-4
JjN-i...(6) becomes. The residual output e of this echo is the coefficient correction circuit 1
7 so that it becomes "0", so in a steady state, h,=ij...(7). That is, the true impulse response of the echo path and the estimated impulse response of the pseudo echo path have the same amplitude.

By the way, the pseudo impulse response register 12 contains "1".
Since it has the following value, the gain of the echo path is "1".
”, overflow occurs and convergence does not occur. Therefore, this overflow is detected by the overflow detector 21, and the gain of the variable gain attenuator 18 is controlled by the gain controller 20 to increase the amount of attenuation.
The gain of the echo path is isometrically controlled to be "1" or less, and the value of the pseudo impulse response register 2 is converged.

That is, the gain of the echo path is set to A (>1), and BAA
For the variable gain attenuator 18 using a constant B that satisfies
When attenuation of l/B is given, the output H,l is j = XJ * (-) (8).

On the other hand, since the impulse response Lj of the pseudo echo path to the input Xj is given as shown in equation (5), equation (5)
By comparing and (8), j □−β, ...(9) is obtained. That is, since B>1, it is possible to suppress the estimated impulse response of the echo path to "1" or less, which is smaller than the true impulse response.

Further, the variable gain amplifier 19 compensates for the transmission signal attenuated by the variable gain attenuator 18.

FIG. 2 is a block diagram illustrating a second embodiment of the present invention.

Since the pseudo impulse response register 12 has a value of "1" or less, if the gain of the echo path exceeds "1", the pseudo impulse response register 12 overflows and does not converge.

In the first embodiment, the problem is solved by attenuating the transmission signal sent from the transmission signal input terminal 7, but in this embodiment, the overflow of the pseudo impulse response register 12 is detected by the overflow detector 21, A gain controller 20 which receives this detection output as an input controls the variable gain amplifier 22 inserted into the output of the convolution calculator 13 to isometrically reduce the gain of the echo path to "1" or less, resulting in a pseudo-impulse response. The value of register 12 is converged.

That is, let the gain of the echo path be A (>1), and C>A
When a gain of C is given to the variable gain amplifier 19 using a constant C that satisfies the following, the output LJ' is L,' =CL, =
CXJ*#j =XJ*(CIJ)...The result is.

On the other hand, since the impulse response Hj of the true echo path with respect to input XJ is given as shown in equation (3), comparing equation (3) and aθ), C7! , -h, ...(11)
is obtained. In other words, since C>1, the estimated impulse response of the echo path is made smaller than the true impulse response.
It is possible to suppress it to 1" or less.

FIG. 3 is a diagram illustrating the outline of the structure of the pseudo impulse response register 12 and overflow detector 21 used in the first and second embodiments of the present invention.

When a coefficient correction signal that cancels the gain of the echo path exceeding "1" is input from the coefficient correction circuit 17 to the pseudo impulse response register 12, an overflow [from the first unit 21 using carry bit detection] is detected. Take out the detection output.

FIG. 4 is a block diagram illustrating a third embodiment of the present invention.

This embodiment is characterized in that the gain of the echo path is measured in advance and the results are used to perform the same control as above.

A test signal source 24 is switched and connected by a switching circuit 23 connected between the received signal input terminal 1 and the received signal output terminal 2. A switching circuit 25 connected between the transmission signal input terminal 7 and the transmission signal output terminal 8 allows the signal of the test signal source 24 to be input to the gain measuring device 26 .

In this way, the gain of the echo path is measured, and if the result exceeds "1" (2), a control signal is output from the gain controller 20 to the variable gain attenuator 18 and the variable gain amplifier 19. The following operations are similar to those in the first embodiment.

Note that the method of measuring the gain of the echo path in advance and using the result to cancel the echo of the echo path according to the third embodiment can be similarly implemented in the method of the second embodiment.

Further, the comparison between the output of the received signal register 11 and the output of the pseudo-impulse response register 12 is not limited to the convolution operation, and the present invention can be similarly carried out by using other loop control methods for controlling the delay gain. be able to.

Furthermore, in this embodiment, an acoustic coupling system consisting of a speaker and a microphone was described as an echo path as a cause of echo, but the echo path that causes echo is not limited to acoustic coupling. With a convenient combination, the invention can be implemented in any case where the gain of the echo path exceeds "1".

〔effect〕

As explained above, according to the method of the present invention, even if the gain of the echo path exceeds "1", the gain of the pseudo echo path can be reduced to "1" or less.

That is, it is possible to construct an echo canceller that can be applied to devices with various echo path characteristics.

Therefore, by equipping a subscriber device directly connected to a four-wire line with the apparatus of the present invention, high-level echoes caused by acoustic coupling can be canceled without affecting the communication line.

[Brief explanation of the drawing]

FIG. 1 is a block diagram illustrating a first embodiment of the echo cancellation method of the present invention. FIG. 2 is a block diagram illustrating a second embodiment of the echo cancellation method of the present invention. FIG. 3 is a schematic diagram of a structure explaining an example of an overflow detection circuit. FIG. 4 is a block diagram illustrating a third embodiment of the echo cancellation method of the present invention. DESCRIPTION OF SYMBOLS 1... Received signal input terminal, 2... Received signal output terminal, 3... Speaker amplifier, 4... Speaker, 5...
...Microphone, 6...Microphone amplifier, 7...Transmission signal input terminal, 8...Transmission signal output terminal, 9...Acoustic coupling, 10...Analog-digital conversion circuit, 11... Received signal register, 12...
Pseudo-impulse response register, 13... Convolution operator, 14... Digital-to-analog conversion circuit, 15...
・Adder, 16...Analog-digital conversion circuit, 1
7... Coefficient correction circuit, 18... Variable gain attenuator, 1
9... Variable gain amplifier, 20... Gain controller, 21
... overflow detector, 22 ... variable gain amplifier, 23.25 ... switching circuit, 24 ... test signal source, 26 ... gain measuring device.

Claims (5)

[Claims] (1) A transmission signal input terminal connected to the transmitter, a transmission signal output terminal connected to the transmission line, a reception signal input terminal connected to the reception line, and a reception signal output terminal connected to the reception apparatus. , a first means for branching and delaying the signal arriving at the received signal input terminal and converting its level; and a second means for adding the output signal of this means to the signal at the transmitting signal input terminal at an opposite phase thereof. and the first means branching the signal transmitted to the transmission signal output terminal so that the echo generated by the coupling between the receiving device and the transmitting device is canceled by the second means. a third means for controlling a signal; means for detecting that the gain of the echo path from the received signal output terminal to the transmitted signal input terminal exceeds "1"; and this detecting means. and relative level control means for controlling the relative level of the output signal of the first means to be input to the second means and the signal of the transmission signal input terminal according to the detection output of the echo. Elimination method. (2) The relative level control means includes a variable gain attenuation circuit inserted between the transmission signal input terminal and the second means, and a variable gain attenuation circuit inserted between the second means and the transmission signal output terminal. The echo cancellation method according to claim 1, comprising: a variable gain amplification circuit; and a control circuit that controls the variable gain attenuation circuit and the variable gain amplification circuit based on the detection output of the detection means. (3) The relative level control means includes a variable gain amplification circuit inserted between the first means and the second means, and control for controlling the variable gain amplification circuit based on the detection output of the detection means. An echo cancellation method according to claim 1, comprising a circuit. (4) The echo cancellation according to claim (1), wherein the means for detecting that the gain of the echo path exceeds "1" includes means for detecting discontinuity of the signal at the transmission signal output terminal. method. (5) The means for detecting that the gain of the echo path exceeds "1" includes: means for transmitting a test signal to the reception output terminal; means for receiving this test signal arriving at the transmission signal input terminal; The echo cancellation method according to claim 1, further comprising means for comparing the level of the test signal transmitted by the receiving means with the level of the test signal received by the receiving means.