Multiplex communications – Duplex – Transmit/receive interaction control
Reexamination Certificate
1997-12-19
2001-01-23
Nguyen, Chau (Department: 2739)
Multiplex communications
Duplex
Transmit/receive interaction control
C379S413020
Reexamination Certificate
active
06178162
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and a system for inhibiting echo in a communication line. The system is particularly useful for communication systems where an echo is susceptible to occur as a result of an acoustic feedback taking place at one end of the communication link (acoustical echo) or as a result of hybrid converters in the telecommunication network electrical echo).
BACKGROUND OF THE INVENTION
In a typical telephone network, two types of echoes may be present: acoustical echo and electrical echo. Acoustical echo is susceptible to occur for example in a telecommunication network when a hands-free telephone terminal is used. The speech signal generated from the speaker in the terminal will be fed back toward the microphone of the terminal for example acoustically or mechanically. This signal will be transmitted back to the talker and therefore creates echo.
Electrical echo results from the presence of an impedance mismatch, such as a hybrid converter that is required to connect a full duplex four wire link to a two wire loop. Hybrids may be found, for example in telephones with two-wire analog interfaces or in network (PSTN) line cards. Due to an impedance mismatch in the hybrid converter, part of the received energy is reflected back to the transmitting port. Consequently, a talker hears his own delayed speech that, of course, is undesirable.
The traditional approach for reduction of echo in communication networks is to use echo suppressors. A typical echo suppress or acts like a switch that monitors the voice signals travelling in both directions. It detects which person is talking and blocks the signal ID travelling in the opposite direction. The drawback of such echo suppressors is that they tend to “chop” speech signals when the subscribers talk back and forth quickly due to the response time for monitoring the speech activities. Moreover, during double talk, i.e., when the subscribers talk simultaneously, the suppressor fails to control the echo.
One possibility to avoid the problems of echo suppressors is to provide circuitry or an algorithm that, instead of blocking speech signals in one direction in the communication link, cancels the echo by using an adaptive filter. In essence, an adaptive echo canceller synthesises the echo that is then subtracted from the composite signal (speech signal plus echo signal). A residual echo from this subtraction can still be at a noticeable level ii a practical echo cancelling system because the adaptive filter may not be able to model perfectly the true echo path as a result of time-varying echo path, insufficient filter length, non-linear effects, finite precision computations, etc. Consequently, a non-linear processor, such as a centre clipper (also called “switched loss device”), is often used with an echo canceller for further reduction of the residual echo. However, practical design of such devices may cause speech clipping or introduce clipping sounds that degrade the quality of the communication. Another difficulty arises as a result of the regulatory echo control targets established for existing echo cancellation devices. The targets are typically expressed as long-term averages, and may be overly conservative with quiet signals. A method is required that addresses the time varying speech levels found in practice.
Thus, there exists a need in the industry to provide an improved echo canceller, particularly well suited for use during a communication session involving at least one hands-free telephone terminal or a communication network comprising a hybrid converter.
OBJECTIVES AND SUMMARY OF THE INVENTION
An object of this invention is to provide an improved method and apparatus for performing echo cancellation.
Another object of the invention is to provide a method and apparatus for performing echo cancellation while allowing reducing the amount of switched loss necessary to maintain an acceptable echo control.
As embodied and broadly described herein, the invention provides an echo canceller for reducing the magnitude of an echo occurring in a return channel of a communication device, said echo canceller including:
first processing means for generating an echo estimate, said first processing means including a first input for receiving a signal that is a precursor of the echo in the return channel of the communication device;
second processing means in operative relationship with said first processing means, said second processing means being responsive to said echo estimate for conditioning a signal input in the return channel of the communication device to generate an output signal in which echo is inhibited, the output signal being a residual echo signal;
a signal attenuation device for receiving and attenuating either one of the residual echo signal and the signal that is a precursor of the echo in the return channel of the communication device, said signal attenuation device manifesting a variable signal attenuation rate.
For the purposes of this specification, the expression “switched loss device” designates a system designed to limit or attenuate a signal in a non-linear manner. The switched loss device is not necessarily a component installed at a single location but it can be constituted by a distributed system including a series of components installed at various locations permitting to effect signal attenuation over one or more separate signals, namely the residual echo signal and the signal precursor of the echo signal.
In a most preferred embodiment of this invention, the switched loss device measures the power of the residual echo signal to determine the amount of attenuation to be effected by the signal attenuation device necessary to reduce the echo to acceptable levels. ost preferably, the signal attenuation device is a witched loss device that can be implemented by a multiplier, multiplying the signal by a value less than one to create loss. If the residual echo signal is weak, less switched loss is used. The amount of switched loss can also be made dependent upon the duration of the residual echo signal. This feature is based on the particular way the human ear perceives sounds. More specifically, when two noise-like signals of different duration but of equal energy level are presented to the ear at different times, and one is approximately less than 750 milliseconds in duration, the one shorter in duration will sound quieter. The echo canceller makes use of this characteristic and reduces the amount of switched loss applied to a residual echo signal when the residual echo has a duration less than approximately 750 milliseconds.
In a specific example, the echo canceller computes two distinct switched loss attenuation levels, one level being based on the power contained in the residual echo signal, the other taking into account the duration of the residual echo signal. After the computations are completed, the switched loss device is set at the lowest attenuation level. This feature allows maintaining an overall good echo cancellation when high levels of residual echo are present while avoiding the undesirable impairments arising from an unnecessary high switched loss when the level of residual echo is low.
As embodied and broadly described herein, the invention also provides a method for reducing a magnitude of an echo occurring in a return channel of a communication device, said method including the steps of:
a) generating an echo estimate, comprising the steps of:
providing a first signal that is a precursor of the echo in the return channel of the communication system;
processing said first signal to produce said echo estimate;
b) utilising said echo estimate for conditioning a signal propagating on the return channel of the communication device to generate an output signal in which echo is inhibited, the output signal being a residual echo signal;
c) attenuating either one the first signal and the residual echo signal in a switched loss device that manifests a variable signal attenuation rate.
As embodied and broadly described herein, the invention also provide
Dal Farra David
Ding Heping
Nguyen Chau
Nguyen Phuongchau Ba
Nortel Networks Corporation
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