JPS6129178B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an echo canceling device among echo control devices intended to prevent communication disturbances due to echoes occurring in long-distance telephone lines.

Echo control devices, such as echo cancellers or echo suppressors, are allowed to have a very small noise distribution compared to devices serving other parts of the system. In order to solve the problems of currently used echo suppression devices (cutting off the beginning of speech, clicking noise, etc.), conventional echo cancellation devices have an analog-to-digital converter (A/D) and a digital converter between the input and output on the transmitting side. - Many devices include an analog converter (D/A), and a problem has arisen in that the quantization noise generated during each conversion exceeds the above-mentioned allowable noise. The reason why A/D and D/A are used is to implement all internal calculations digitally and increase calculation accuracy in order to increase the erasure ability, but also to suppress quantization noise to below the above-mentioned allowable noise. A/D, D/A with fairly high accuracy, in other words, A/D with accuracy exceeding the accuracy required for echo cancellation,
D/A will be required.

Due to this problem, internal calculations have recently been changed to analog
Achieved by a hybrid system that includes digital signals, and does not use A/D or D/A between the input and output on the transmitting side, converting the digitally synthesized pseudo-reverberations to D/A and subtracting them from the reverberations in an analog manner. There is also an example of a method in which the echo cancellation is performed by using an additional attenuator, such as a center clipper, to remove the residual echo caused by the low precision of the analog subtracter, thereby maintaining the overall echo cancellation ability. However, the center clipper described above is configured to be bypassed when there is no input signal on the receiving side or during two-way communication in order to pass the voice of the near-end speaker, that is, the signal on the transmitting side without distortion. The quantization noise contained in the signal will be sent as is to the transmitting side output. As a result, when there is no signal on the receiving side and during two-way communication, noise, which is not much different from the all-digital system, will be superimposed on the transmitting side signal. During a two-way call, both parties are talking together, so the noise is thought to be masked, so even if there is no problem, the noise superimposed when there is no signal on the receiving side degrades the quality of the call for the listener at the far end. I will let you do it.

An object of the present invention is to eliminate unnecessary noise by placing an attenuator, such as a center clipper, at the output of the pseudo echo path and reducing the output of the pseudo echo path to zero when there is no input signal on the receiving side. The purpose of this invention is to provide an echo cancellation device that solves the problem.

Inserting an attenuator with variable attenuation into the pseudo echo path output creates nonlinearity in the correction loop for convergence of the pseudo echo path, that is, the loop consisting of the pseudo echo path, echo cancellation subtractor, and pseudo echo path correction circuit. Therefore, it is necessary to stop the function of the attenuator at the time of modification. In principle, pseudo-echo path modification is effective only when the signal on the receiving side is large, so the objective can be achieved by monitoring the signal level on the receiving side, stopping the correction when the level is small, and controlling the attenuator to increase the amount of attenuation. reach

Next, a more detailed explanation will be given using the drawings. FIG. 1 shows a conventional basic echo canceler with a modification loop. The signal from the far end speaker is input to the receiver side 100
from the receiving side output 200 to the near-end speaker. Signals from the near-end talker and echoes from the far-end talker enter through the transmitter input 300 and pass through the transmitter output 400 to the far-end talker. Echo cancellation is performed by subtracting the pseudo echo output from the pseudo echo path 20 from the transmitter input signal in the subtracter 30 . The pseudo echo path 20 is corrected by the correction circuit 40, and the operation of the correction circuit 40 is controlled by the output of the two-way communication detector 10 to stop during two-way communication and one-way communication on the transmitting side. In FIG. 1, the loop constituted by the pseudo echo path 20, the subtracter 30, and the modification circuit 40 is called a modification loop or an adaptive loop. By increasing the precision of this correction loop, the echo cancellation ability is strengthened, but
To do this, it is necessary to place an A/D converter at the transmitting side input 300 and a D/A converter at the transmitting side output 400, and to perform the correction loop calculation digitally. A/D,
D/A is included and quantization noise becomes a problem.

Figure 2 shows a center filter after the echo cancellation subtractor to increase the echo cancellation ability without increasing the accuracy of the correction loop.
This is an example with a clipper added. The difference from FIG. 1 is that the center clipper 50 connected to the bidirectional communication detector 10 is connected to the subtracter 30 and the transmitter output 400.
The rest is the same as in Figure 1 except that it is installed between the The center clipper 50 is controlled by the two-way call detector so as to operate only during one-way communication on the receiving side, and outputs the input signal as it is without any modification during one-way communication and two-way communication on the sending side. ing. Therefore, when the center clipper 50 is not operating, that is, during one-way communication on the transmitting side and during two-way communication, the pseudo echo path 20
The noise generated in the transmitter side output 400
will be sent to. In the pseudo-echo path 20, an A/D converter and a D/A converter are often used for input and output, respectively, to perform digital calculations to stabilize internal calculations, so the quantization noise is This occurs even if the receiving signal is completely zero. Further, even if no noise is generated in the pseudo echo path 20, the noise existing on the receiving side is sent to the transmitting side, and the noise at the transmitting side output increases.

FIG. 3 shows an embodiment of the invention. The difference from FIG. 2 is that the output of the pseudo echo path 20 and the subtractor 3
The variable attenuator 60 provided between the receiver and the receiver is controlled by the two-way conversation detector 10 to operate only when the receiving side signal is below a predetermined level and there is no signal. At the same time, the correction circuit 40 is configured to stop operating. In FIG. 3, the problem of increased noise during one-way transmission and two-way communication, which was seen in the example of FIG. 2, is solved, and it is possible to maintain good call quality. If the pseudo echo path 20 is realized digitally,
The variable attenuator 60 can be incorporated as a gate of a digital circuit inside the pseudo-reverberation path 20, and the effect is almost the same except for D/A converter drift and the like. If the noise generated in the pseudo echo path 20 is very small, it may be possible to insert a variable attenuator 60 into the receiving side input of the pseudo echo path 20.
In that case, it is clear that it is possible to prevent noise components present in the receiving side signal from being superimposed on the transmitting side.

There is no problem in using the controllable attenuator provided in the present invention together with the above-mentioned attenuator for enhancing echo canceling ability, and as a result, an echo canceling device with high echo canceling ability that generates less noise can be obtained. It goes without saying that this can be achieved.

[Brief explanation of the drawing]

FIG. 1 shows an example of a basic conventional echo canceling device, FIG. 2 shows an example of an improved version of the conventional echo canceling device, and FIG. 3 shows an embodiment of the present invention. 100...Receiving side input terminal, 200...Receiving side output terminal, 300...Sending side input terminal, 400...Sending side output terminal, 10...Two-way conversation detector, 20...Pseudo echo path, 30...Subtractor, 40...Modification Circuit, 50...
Center clipper, 60...variable attenuator.

Claims (1)

[Claims] 1. A receiving side transmission path, a sending side transmission path, a pseudo echo path which receives the receiving side transmission path signal as input and outputs the pseudo echo signal, and a pseudo echo signal from the pseudo echo path and the sending side transmission path input signal. In an echo canceling device having a subtracter that takes two inputs as input, performs subtraction between the inputs, and outputs a transmitting side transmission line output signal, the echo canceler is inserted in series into the pseudo echo path and its output is connected to the subtracter. a variable attenuator, and a level detector for detecting a reception side transmission line signal level, and when the level detector detects an input signal below a predetermined threshold, the variable attenuator attenuates the signal into the pseudo echo signal. An echo canceling device characterized in that it is controlled so as to give a certain amount of echo.