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 in a telecommunication network when a hands-free telephone terminal is used. The speech signal generated from the speaker in the terminal propagates in the form of an acoustic wave through an acoustic environment (air) and part of it will be fed back toward the microphone of the terminal. This signal will be transmitted back to the talker and therefore creates echo.
Electrical echo results from the presence of a hybrid converter that is required to connect the unidirectional four wire link from the public switched telephone network (PSTN) to the local two wire loop. The basic function of the hybrid converter is to separate the transmitted signal originating in the local loop from the received signal in the PSTN section, and vice versa. This process requires the energy of the received signal to pass fully in the local loop. However, due to an impedance mismatch in the hybrid converter, part of the received energy is reflected back to the transmitting port. As a result, a talker hears his own delayed speech which, of course, is undesirable.
The traditional approach for reduction of echo in communication networks is to use echo suppressors. A typical echo suppressor acts like a switch that monitors the voice signals travelling in both directions. It detects which person is talking and blocks the signal 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 levels in 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 room impulse response, insufficient filter length, non-linear effects, finite precision computations, etc. Consequently, a switched loss device is often used in an echo canceller for further reduction of the residual echo.
Another difficulty arises as a result of the overly conservative target established for existing echo cancellation devices. The problem arises primary in situations where the local loop on an analog CPE (customer premises equipment) manifests a significant signal attenuation characteristics and inherently attenuates the signals propagating in the loop, including echo signals. Thus, in a loop with a higher intrinsic signal attenuation characteristics the existing strategy of echo control results in excessive echo attenuation, that reduces the audio quality of the link. Since the signal attenuation characteristics are different from one CPE to another CPE the current approach is to set the echo canceller target conservative enough so the loops with low signal attenuation characteristics (low loss) will still inhibit echo to a desired degree. This, however, results into a overly conservative echo suppression in cases where the loops manifest a high signal attenuation characteristics.
Thus, there exists a need in the industry to provide an improved echo canceller that can avoid excessive signal attenuation, particularly by operation of a switched loss device, when the communication channel through which the echo propagates manifests a significant level of inherent signal attenuation.
OBJECTIVES AND SUMMARY OF THE INVENTION
An object of this invention is to provide an improved method and apparatus for performing echo inhibition.
Another object of the invention is to provide a method and apparatus for performing echo inhibition that can tailor the level of signal attenuation for inhibiting echo in dependence upon the degree of inherent signal loss manifested by the communication channel through with the echo signal is susceptible to propagate.
As embodied and broadly described herein, the invention provides an echo canceller for managing echo signals occurring in a return channel of a communication device, said communication device being coupled to a switch mechanism through a local loop, said signal processing device including:
a sensor for generating an output signal indicative of a degree of inherent signal attenuation manifested by the local loop;
an echo attenuation device for reducing a magnitude of an echo occurring in the return channel of the communication device, said echo attenuation device being responsive to said output signal to vary a level of echo attenuation in dependence upon the degree of inherent signal attenuation of the local loop observed by said sensor.
In a most preferred embodiment the communication device is a CPE (Customer Premises Equipment) provided with an echo attenuation device that includes two main components, namely an adaptive filter to model the echo path and generate an echo estimate signal that is subtracted from the echo corrupted signal propagating in the return path of the CPE, and a switched loss device that complements the echo attenuation process implemented by the adaptive filter. In a specific embodiment, a switched loss device can be implemented by a multiplier, multiplying the signal by a value less than one to create loss. The switched loss device reduces acoustic levels by selectively inserting variable losses on the received and/or transmitted audio signals. More specifically, the switched loss device includes a first switched loss module on the forward channel of the CPE, the one that carries the signal that is a precursor of the echo, and a switched loss module on the return channel of the CPE. A switched loss controller controls the degree of signal attenuation implemented by the individual switched loss modules. A loop loss sensor is provided for measuring or estimating the inherent signal attenuation in the local loop. The loop loss sensor issues an output signal that is directed to the switched loss controller. In response to that output signal, the switched loss controller adjusts the level of signal attenuation implemented by the switched loss modules to take into account the loop loss. As a result, a more precise echo attenuation mechanism is provided that reduces the likelihood of excessive echo control.
The loop loss sensor may take various forms. For instance the loop loss sensor may be a current measuring device that estimates the resistance of the local loop conductors, hence loop loss, by sensing the loop current. Since the voltage impressed across the loop conductors at the far end of the loop (the end that connects to the switch) is well known and generally constant and the impedance of the CPE also known, one can establish a nominal loop current value. Any deviation from this value (disregarding the components tolerance factor) is primarily due to a varying loop length. If the current observed by the loop loss sensor is significantly below the nominal value, one may conclude that the loop is long, thus it manifests a high degree of signal attenuation. In contrast, a higher current is indicative of a shorter loop, hence the loop manifests a lesser degree of signal attenuation. The switched loss controller receives the output signal of the loop loss sensor that is indicative of the degree of loop loss manifested by the loop. In response to that output signal, the switched loss controller adjusts the set-points of the individual switched loss modules so as to vary the degree of echo attenuation.
Practically, the loop loss sensor may be placed in the CPE. At that location, the loop loss sensor is capable of determining the degree of loss manifested by the local loop. In this example, the local loop is the conductor connecting the CPE to the switch. Thus the CPE, when connected to a certain local loop will adjust the level of echo control in dependence of the loop length in that particular installation. In one possible embodiment the switch may be in the central office of the PSTN network, or be part of a PBX installation.
In a possible variant that is particularly useful in situations where the local loop terminates on a line card in the switch device that injects a constant current (the line card acts as a current source) in the local loop, a voltage sensor can be used to measure the voltage at an appropriate location and determine the level of inherent signal attenuation in the loop. One possibility is to measure the voltage across the conductors of the local loop where they connect with the line card. The voltage variation with respect to a nominal value is indicative of the inherent local loop loss characteristics. Under this embodiment, the sensor is remote from the CPE. As a practical matter, the entire echo canceller can be separated from the CPE.
The CPE terminal can be a telephone, a teleconference device or any other communication system that is intended to be connected to the local loop for voice communications purposes.
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, the communication device being coupled to a switch mechanism through a local loop, said method including the steps of:
a) assessing a level of inherent signal attenuation manifested by the local loop;
b) utilizing the level of inherent signal attenuation manifested by the local loop assessed at step a to reduce a magnitude of the echo occurring in the return channel of the communication device.
As embodied and broadly described herein, the invention also provides a CPE for coupling to a switch mechanism through a local analog loop, said CPE comprising:
an echo canceller for reducing the magnitude of an echo occurring in a return channel of said CPE, said echo canceller including:
a sensor for generating an output signal indicative of a degree of inherent signal attenuation manifested by the local analog loop;
an echo attenuation device for reducing a magnitude of an echo occurring in the return channel of said CPE, said echo attenuation device being responsive to said output signal to vary a level of echo attenuation in dependence upon the degree of inherent signal attenuation of the local loop observed by said sensor.
As embodied and broadly described herein, the invention also provides an echo attenuation device for reducing a magnitude of an echo occurring in a communication device, said echo attenuation device being responsive to a signal indicative of an inherent loss characteristics of a pathway transporting a signal precursor of the echo signal and the echo signal to vary a level of echo attenuation in dependence of the signal.