JPH0541681A - Echo canceller - Google Patents

Abstract

(57) [Abstract] [Purpose] An object of the present invention is to provide an echo canceller capable of reliably canceling an echo signal even if a non-linear characteristic is present in the echo path. A pseudo-echo signal is provided with a non-linear characteristic table 111 having substantially the same non-linear characteristic existing in an echo path composed of a D / A converter 103, an amplifier 104, a speaker 105, an amplifier 106 and an A / D converter 108. An echo canceller in cascade connection with the generating adaptive digital filter 112.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to satellite communication, public telephone,
The present invention relates to an echo canceller for canceling an echo signal which causes quality deterioration in a video conference, a hands-free call in a car phone, and the like.

[0002]

2. Description of the Related Art Generally, in a hands-free call in a car telephone or the like, a voice from a far-end speaker is received through a speaker and a voice from a near-end speaker is transmitted through a microphone. At this time, the received voice is transmitted as an echo signal to the far-end speaker through the microphone after being affected by the transfer characteristics such as reflection in the vehicle. The echo canceller is for canceling such an echo signal, and the one for voice is particularly called an acoustic echo canceller.

FIG. 4 is a block diagram showing a typical structure of a conventional acoustic echo canceller. For example, "Application of Digital Signal Processing", edited by The Institute of Electronics, Information and Communication Engineers (Sho 58).
It is described in. In FIG. 4, the received signal from the far-end speaker is input from the terminal 201, and the A / D converter 2
It is converted into a digital signal sequence by 02. The received signal converted into the digital signal sequence is the D / A converter 20.
After being converted to an analog signal by 3, the signal is amplified by the amplifier 204 and supplied to the speaker 205. On the other hand, the voice of the near-end speaker is input through the microphone 206 and the amplifier 2
After being amplified by 07, it is converted into a digital signal sequence by the A / D converter 208, and further, it is passed through the subtracter 212 and D
The analog signal is converted back into an analog signal by the A / A converter 209, then output as a transmission signal from the terminal 210, and transmitted to the far-end talker side.

The speaker 205 reproduces the received signal from the far-end speaker as voice and provides the received voice to the near-end speaker.
At that time, a part of the received voice is influenced by the transfer characteristic in the vehicle, then wraps around to the transmission side through the microphone 206, and is superimposed as an echo signal on the transmission signal from the near-end speaker. After this echo signal is amplified by the amplifier 207,
The digital signal sequence is converted by the A / D converter 208 and input to the subtractor 212.

The adaptive digital filter 210 includes an A / D
The received signal of the digital signal series from the converter 202 is input, and the received signal reproduced from the D / A converter 203 via the amplifier 204 and the speaker 205 is an in-vehicle acoustic path, a microphone 206, an amplifier 207 and A / D conversion. The transfer characteristic (echo transfer characteristic) of the echo path up to the transmission side is estimated by the device 208, and a pseudo echo signal is generated.

The subtractor 212 cancels the echo signal contained in the transmission signal by subtracting the pseudo echo signal generated by the adaptive digital filter 211 from the output signal of the A / D converter 208. At this time, the residual echo signal appearing at the output of the subtracter 212 is fed back to the adaptive digital filter 211 for estimating the echo transfer characteristic.

A linear digital filter is used as the adaptive digital filter 211. That is, if the transfer function is H (z), the transversal form (FIR
[Formula 1] in the case of the shape) and [Formula 2] in the case of the IIR form
Therefore, in any case, it is a linear filter.

[0008]

[Equation 1]

[0009]

[Equation 2]

When the echo transfer characteristic is a linear characteristic described by the above transfer function, the echo transfer characteristic can be simulated by the adaptive digital filter 211 to sufficiently attenuate the echo signal.

However, the D / A is in the echo transmission path.
Converter 203, amplifier 204, speaker 205, microphone 206, amplifier 207 and A / D converter 208
There are elements with non-linear characteristics such as, and the echo transfer characteristics are non-linear. Adaptive digital filter 211
Since such a non-linear characteristic cannot be simulated, it is impossible for the conventional echo canceller to cancel the residual echo signal due to the non-linear characteristic inherent in the echo transmission path. In particular, the amplifier 204 and the speaker 20
5. It is difficult for the microphone 206 and the amplifier 207 to achieve linearity with high precision due to cost reduction and miniaturization requirements, and the echo resulting from these non-linear characteristics is a factor causing deterioration of speech quality.

[0012]

As described above, in the conventional echo canceller, since the adaptive digital filter for generating the pseudo echo signal is the linear filter, the residual echo signal caused by the non-linear characteristic inherent in the echo path is generated. However, there is a problem that the call quality is deteriorated because the call cannot be deleted.

It is an object of the present invention to provide an echo canceller capable of surely canceling an echo signal even if a nonlinear characteristic exists in the echo path.

[0014]

In order to solve the above-mentioned problems, the present invention is characterized in that a non-linear element having a non-linear characteristic which is substantially the same as the non-linear characteristic existing in the echo path is cascade-connected to the pseudo echo generating means. To do.

[0015]

The pseudo echo generating means is composed of, for example, an adaptive digital filter, estimates the linear transfer characteristic of the echo path, and generates a pseudo echo signal based on the transfer characteristic. On the other hand, the non-linear element reproduces the non-linear characteristic existing in the echo path and generates a pseudo echo signal based on the non-linear characteristic. By using the pseudo echo signal based on the linear and non-linear transfer characteristics of the echo path thus generated, the echo signal is reliably removed even if the non-linear transfer characteristic of the echo path exists.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an acoustic echo canceller for a mobile phone according to an embodiment of the present invention.

In FIG. 1, the A / D converter 10 is provided in the received signal path from the far-end speaker input from the terminal 101.
2, a D / A converter 103, an amplifier 104 and a speaker 105 are provided. A microphone 106, an amplifier 107, an A / D converter 108, a subtractor 113, a D / A converter 109, and a terminal 110 are provided in the transmission signal path from the near-end speaker.

On the other hand, a non-linear characteristic table 111 and an adaptive digital filter 112 are connected in series between the output terminal of the A / D converter 102 and the subtractor 113. The nonlinear characteristic table 111 is used for the D / A converter 10
3, the received signal reproduced through the amplifier 104 and the speaker 105 is almost the same as the non-linear characteristics inherent in the acoustic path in the vehicle, and the path (echo path) around the microphone 106, the amplifier 107 and the A / D converter 108 to the transmission side. It is a non-linear element having the same non-linear characteristic (input / output characteristic). Specifically, the transfer characteristic (non-linear characteristic) of the echo path is measured in advance and stored in ROM (read-only memory) or the like in the form of a table, which is the non-linear characteristic table 111, which is an A / D converter. When the received signal from 102 is input, this nonlinear characteristic is reproduced and an output signal is output.

FIG. 2 shows an example of the non-linear characteristic of the echo path, which is reproduced by the non-linear characteristic table 111.

The output signal of the nonlinear characteristic table 111 is
It is input to the adaptive digital filter 112. The adaptive digital filter 112 estimates the transfer characteristic (echo transfer characteristic) of the echo path and generates a pseudo echo signal. The pseudo echo signal output from the adaptive digital filter 112 is a pseudo echo signal based on the nonlinear transfer characteristic reproduced by the nonlinear characteristic table 111,
This is a combination of a pseudo echo signal based on the linear transfer characteristic of the echo path estimated by the adaptive digital filter 112 itself.

The output signal of the adaptive digital filter 112 is input to the subtractor 113, where the A / D converter 10
The echo signal contained in this transmission signal is canceled by subtracting it from the transmission signal from 8. This subtracter 1
The residual echo signal, which is the output signal of 13, is fed back to the adaptive digital filter 112 and used for estimating the echo characteristic, and after being converted into an analog signal by the D / A converter 109, it is passed through the terminal 110 to the far end. Sent to the speaker side.

The adaptive digital filter 112 is, for example, a transversal filter of the 240th to 320th order, and a learning identification method is used as a learning algorithm of its filter coefficient. Adaptive digital filter 1
Assuming that the 12 input signals are x (n) and the residual echo signal is e (n), the i-th order filter coefficient h _i (n) is corrected as in [Equation 3].

[0023]

[Equation 3]

However, P is the order of the filter, and P = 240 to 320 as described above. Further, μ is a parameter that determines the update step size, and μ = 1.
In the case of FIG. 1, the filter coefficient h _i (n) is modified according to the residual echo signal output from the subtractor 113.

In FIG. 1, the A / D converter 102 and the D / A converter 109 are used for analog communication and are unnecessary for digital communication. Further, in FIG. 1, the non-linear characteristic table 111 is arranged in the preceding stage of the adaptive digital filter 112, but the same effect can be obtained by arranging it in the latter stage as shown in FIG.
In addition, the nonlinear characteristic table is set to the adaptive digital filter 1
The 12 input and output sides may be distributed and arranged. In addition, the present invention can be variously modified and implemented without departing from the spirit of the invention.

[0026]

According to the present invention, a non-linear element having a non-linear characteristic substantially the same as the non-linear characteristic existing in the echo path is cascade-connected to the pseudo echo generating means constituted by the adaptive digital filter which is a linear filter. , It is possible to cancel not only the normal linear echo signal but also the echo signal caused by the non-linear characteristic existing in the echo path. Especially, when an inexpensive speaker or microphone with poor linearity is used, A great effect can be obtained in preventing deterioration.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an echo canceller according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a nonlinear transfer characteristic existing in the echo path of FIG.

FIG. 3 is a block diagram showing a configuration of an echo canceller according to another embodiment of the present invention.

FIG. 4 is a block diagram showing a configuration of a conventional acoustic echo canceller.

[Explanation of symbols]

101 ... Received signal input terminal 102 ... A /
D converter 103 ... D / A converter 104 ... Amplifier 105 ... Speaker 106 ... Microphone 107 ... Amplifier 108 ... A /
D converter 109 ... D / A converter 110 ... Transmission signal output terminal 111 ... Non-linear characteristic table 112 ... Adaptive digital filter 113 ... Subtractor

Claims (2)

[Claims] 1. A received signal from a far-end talker is input, and the received signal and a transmitted signal from a near-end talker on which an echo signal generated by the received signal passing through an echo path is superimposed. Pseudo-echo signal generating means for estimating a transfer characteristic of the echo path based on the pseudo-echo signal, and pseudo-echo signal generating means arranged on the input side of the received signal of the pseudo-echo generating means, and having substantially the same non-linear characteristic inherent in the echo path. An echo canceller, comprising: a non-linear element having the non-linear characteristic of 1), and a subtracting means for eliminating the echo signal by subtracting the pseudo echo signal generated by the pseudo echo generating means from the transmission signal. 2. A received signal from a far-end speaker is input, and the received signal and a transmitted signal from a near-end speaker on which an echo signal generated by the received signal passing through an echo path is superimposed. Pseudo-echo signal generation means for estimating the transfer characteristic of the echo path based on the pseudo-echo signal generation means, and a pseudo-echo signal generation means arranged on the output side of the pseudo-echo signal of the pseudo-echo generation means and having a substantially non-linear characteristic inherent in the echo path. An echo canceller comprising: a non-linear element having the same non-linear characteristic; and subtraction means for eliminating the echo signal by subtracting the pseudo echo signal that has passed through the non-linear element from the transmission signal.