Echo canceler and echo path estimating method

Details Echo canceler and echo path estimating method Echo canceler and echo path estimating method

1996-01-16

1999-01-12

Matar, Ahmad F.

Telephonic communications

Subscriber line or transmission line interface

Network interface device

379406, 379411, H04M 100

Patent

active

058599070

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to an echo canceler suited to be used for a mobile communications network and a long-distance telephone line network. Also, the invention relates to an echo path estimating method suited to be used for this echo canceler.


BACKGROUND ART

In a long-distance telephone line via a submarine cable or via a communication satellite, the subscriber's line, in general, connected to both ends of the line is of a two-wire circuit and its long-distance transmission portion is of a four-wire circuit for the purposes of amplification of a signal, etc. Similarly, in the mobile communications network using a mobile telephone (or cellular phone), the subscriber's line of a terrestrial analog telephone is of a two-wire circuit and its portion from a terminal of the mobile telephone to a switch, etc. is of a four-wire circuit. In this case, the connection region between the two-wire and the four-wire is provided with a hybrid circuit for performing a four-wire/two-wire conversion.
This hybrid circuit is designed to match with the impedance of the two-wire circuit. However, since it is difficult to obtain always a good matching condition, a received signal reaching an input side of the four-wire of the hybrid circuit tends to leak toward an output side of the four-wire, thereby generating a so-called echo. Since such an echo is lower in level than the talker's voice and reaches the talker after a delay of a predetermined time period, a speech hindrance is created. Such a speech hindrance caused by echo becomes more significant as the signal propagation time becomes longer. Particularly, in the case of a mobile communication using a mobile telephone, since various processing procedures are carried out in the radio communication section leading to the switch, etc., the delay of signal is increased, thus resulting, particularly, in the problem of speech hindrance caused by echo. FIG. 2 shows one example of the waveform of an echo with respect to impulse response.
As an apparatus for preventing a generation of echo, there are an echo suppressor and an echo canceler. FIG. 1 shows a schematic construction of an echo canceler which can be used in a mobile communications network. The echo canceler 1 illustrated here is located on a front stage of a hybrid circuit 2. In this illustration, the subscriber of an ordinary analog telephone is referred to as the "near-end talker" and the subscriber of a mobile telephone as the "far-end talker". A far-end voice signal input into the echo canceler 1 is represented by Rin; a far-end voice signal output from the echo canceler 1, by Rout; a near-end voice signal input into the echo canceler 1, by Sin; and a near-end voice signal output from the echo canceler 1, by Sout; respectively.
The echo canceler 1 shown in FIG. 1 comprises an echo path estimation/echo replica generation circuit 3, a control unit 4, an adder 5, and a non-linear processor 6. Here, the echo path estimation/echo replica generation circuit 3 detects a response characteristic of the hybrid circuit 2 based on both the far-end voice input Rin and near-end voice input Sin and estimates an echo path (namely, echo propagating line). Then, an anticipated echo (namely, echo replica) from the hybrid circuit 2 is generated through a convolutional operation based on the result of estimation and the far-end voice input Rin. This echo replica is generated by an FIR filter which is constituted of so many taps as 512, for example. A convolutional operation in an echo replica refers to this. In the adder 5, this echo replica is subtracted from the near-end voice input Sin, thereby canceling the echo. As the above-mentioned echo path estimation algorithm, a learning identification algorithm is used. Among many adaptive algorithms, this learning identification algorithm is comparatively small in computational complexity and good in convergence characteristic.
Specifically, the echo path estimation/echo replica generation circuit 3 has an FIR filter. An echo replica signal Y(z) output from the FIR filter

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