Telephonic communications – Subscriber line or transmission line interface – Network interface device
Reexamination Certificate
1997-05-07
2001-02-06
Chan, Wing F. (Department: 2743)
Telephonic communications
Subscriber line or transmission line interface
Network interface device
C379S406010, C370S290000
Reexamination Certificate
active
06185300
ABSTRACT:
BACKGROUND
The present invention relates to communications systems, and more particularly, to echo cancelation in a bi-directional communications link.
In many communications systems, for example landline and wireless telephone systems, voice signals are often transmitted between two system users via a bi-directional communications link. In such systems, speech of a near-end user is typically detected by a near-end microphone at one end of the communications link and then transmitted over the link to a far-end loudspeaker for reproduction and presentation to a far-end user. Conversely, speech of the far-end user is detected by a far-end microphone and then transmitted via the communications link to a near-end loudspeaker for reproduction and presentation to the near-end user. At either end of the communications link, loudspeaker output detected by a proximate microphone may be inadvertently transmitted back over the communications link, resulting in what may be unacceptably disruptive feedback, or echo, from a user perspective. Furthermore, if the round-trip loop gain is greater than unity at any audible frequency, then the system will tend to “howl” as is well known in the art.
Therefore, in order to avoid transmission of such undesirable echo signals, the microphone acoustic input should be isolated from loudspeaker output as much as possible. With a conventional telephone handset, in which the handset microphone is situated close to the user's mouth while the handset speaker essentially covers the user's ear, the requisite isolation is easily achieved. However, as the physical size of portable telephones has decreased, and as hands-free speaker-phones have become more popular, manufacturers have moved toward designs in which the acoustic path from the loudspeaker to the microphone is not blocked by the user's head or body. As a result, the need for more sophisticated echo suppression techniques has become paramount in modern systems.
The need is particularly pronounced in the case of hands-free automobile telephones, where the closed vehicular environment can cause multiple reflections of a loudspeaker signal to be coupled back to a high-gain hands-free microphone. Movement of the user in the vehicle and changes in the relative directions and strengths of the echo signals, for example as windows are opened and closed or as the user moves his head while driving, further complicate the task of echo suppression in the automobile environment. Additionally, more recently developed digital telephones process speech signals through vocoders which introduce significant signal delays and create non-linear signal distortions. As is well known, these prolonged delays tend to magnify the problem of signal echo from a user perspective, and the additional non-linear distortions can make echo suppression difficult once a speech signal has passed through a vocoder.
Traditionally, echo suppression has been accomplished using echo canceling circuits which employ adaptive filters to estimate and remove echo signals from a microphone output so that only near-end speech and noise are transmitted over the communications link. Such systems are described, for example, in U.S. Pat. No. 5,475,731, entitled “Echo-Canceling System and Method Using Echo Estimate to Modify Error Signal” and issued Dec. 12, 1995, and U.S. patent application Ser. No. 08/578,944, entitled “Gauging Convergence of Adaptive Filters” and filed Dec. 27, 1995, each of which is incorporated herein by reference. While the systems described in the cited documents are generally effective in canceling or suppressing echo signals, certain characteristics of those systems make them impractical in some contexts.
For example, as is described in detail below, such systems are not well suited for double-talk situations in which a near-end user and a far-end user are speaking simultaneously. Additionally, the above-mentioned systems are not well suited for situations in which ambient noise, for example road and traffic noise, is prevalent at a microphone input. Thus, there is a need for an improved echo suppression system.
SUMMARY OF THE INVENTION
The present invention fulfills the above-described and other needs by providing an echo canceler in which effective transfer function coefficients of an adaptive echo-canceling filter are adjusted periodically to track changing system conditions. Periodic adjustment of the effective coefficients is accomplished using a set of more frequently adjusted update coefficients and an update gain which is varied based on a strategic and novel combination of system parameter measurements. A key system parameter measurement, referred to herein as a system status gauge, comprises a ratio of a peak update coefficient value and a baseline update coefficient value. The system status gauge indicates, among other things, a level of system convergence and exhibits a number of beneficial characteristics which make it extremely useful in dynamically adjusting a coefficient update gain.
Whereas conventional echo canceling devices provide adaptive filters in which the effective filtering coefficients are updated continuously or on a sample-to-sample basis, an echo canceling device constructed in accordance with the teachings of the present invention provides an adaptive filter in which the effective filtering coefficients are updated less frequently, for example on a sample-block-to-sample-block basis. By updating the effective filtering coefficients periodically rather than continuously, and by using the system status gauge in making soft, fuzzy-logic-type decisions with respect to the magnitude of the update gain, embodiments of the present invention optimize system speed and adaptability without sacrificing stability.
Generally, embodiments of the present invention apply relatively large update gains in situations where only a far-end user is speaking and the near-end signal is relatively noise-free. However, in near-end single talk, double-talk, and high near-end noise situations, a more conservative approach is used so that the adaptive filter does not become unstable or cause distortion in the near-end speech and noise signals. In either event, the update gain is reduced as the adaptive filter converges to match a prevailing steady-state echo environment so that erroneous perturbations of an already properly-adapted filter are minimized. The decision as to whether to apply high or low update gains during periods of nonconvergence is based in part on the system status gauge and in part on measurements of energy existing in various system signals.
In exemplary embodiments, relatively high variable update gain is applied during unconverged, far-end single-talk situations by employing a modified form of the well known normalized least-mean-squares (NLMS) approach. In other unconverged situations, the update gain is adjusted more conservatively based on a normalized version of the system status gauge. By dynamically providing relatively large update gains while being careful not to create system instabilities, the exemplary embodiments quickly and robustly adapt to successfully cancel echoes in a wide variety of system conditions and environments.
In a first embodiment, an echo canceling device for estimating an echo component of an input signal and for subtracting a resulting echo component estimate from the input signal to provide an echo-canceled output signal, wherein the echo component of the input signal results from an echo-causing signal, the echo canceling device comprises an adaptive filter for filtering the echo-causing signal to provide the echo component estimate. An adaptive filter transfer function of the adaptive filter is adapted in dependence upon the echo-causing signal and the echo-canceled output signal. The echo canceling device also includes a storage device coupled to the adaptive filter for storing an update corresponding to changes in the adaptive filter transfer function and a processor coupled to the storage device for computing a status indicator, wherein the status indica
Burns Doane Swecker & Mathis L.L.P.
Chan Wing F.
Ericsson Inc.
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