Telephonic communications – Echo cancellation or suppression
Reexamination Certificate
2001-10-31
2004-06-15
Tieu, Binh (Department: 2643)
Telephonic communications
Echo cancellation or suppression
C379S406050, C379S406080, C379S406120, C379S390020, C379S390020, C370S289000, C370S290000, C370S295000
Reexamination Certificate
active
06751313
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to the cancellation of an echo signal in a voice communication system.
BACKGROUND OF THE INVENTION
In worldwide telecommunications systems, echoes arise in various situations and impair communication quality. Echoes occur when a delayed or distorted version of an original audio signal is reflected back to the source. In telecommunications networks, an impedance mismatch is one factor that contributes to the refection of an audio signal back to its source. The reflected audio signal is a delayed or distorted version of the original signal, which causes echoes in speech communication systems.
A hybrid transformer is typically used to connect a two-wire local telephone exchange to a four-wire long distance or mobile telephone network. The imperfect impedance match exhibited by the hybrid transformer generates the echoed signal. In the past two decades several methods have been used to alleviate the echo problem and improve communication quality. These prior art methods are collectively referred to as echo cancellation.
The long distance or mobile telephone from which a voice signal originates is commonly referred to as the “far-end”. The voice signal from the far-end is called the inbound signal and travels through a path called the receive-path. The inbound signal passes through a hybrid transformer located at a local telephone exchange. The hybrid transformer is typically made integral to a device called a Central Office Line Interface Unit. Most of the inbound signal is transferred through the hybrid transformer to the party subscribing to the local telephone exchange that is receiving the phone call. The subscriber using the local telephone exchange is referred to as the “near-end”. The hybrid transformer propagates a signal originating at the near-end, commonly called the “near-end signal”, to the far-end using a second signal path called the “send-path”. An unwanted version of the inbound signal is also coupled into the send path resulting in an echo. This unwanted version of the inbound signal is the echo that needs to be eliminated. The composite of the near-end signal and the reflected inbound signal is referred to as the “outbound signal”.
The echo-path-model is a transfer function that describes the amount of the inbound signal that is reflected back into the outbound signal. In order to determine the echo-path-model, echo cancellation systems monitor the inbound signal and compare that inbound signal to the amount of echo signal observed in the send-path. This process can only be accomplished when the send-path is devoid of any other signals. When the near-end is generating a signal, the presence of that near-end signal in the send-path will preclude any estimation of the echo-path-model.
Once the echo-path-model has been derived, an estimate of the echo signal can be calculated. The estimated echo is subtracted from the send-path leaving only the desired near-end signal. Because the resulting transfer function for the echo-path-model is only an estimate of the actual echo transfer function, some of the echo signal will remain in the send-path. This component is called the residual echo.
Echo cancellers usually use some form of filter to implement the echo-path-model. By subjecting the inbound signal to the filter, an estimate of the echo can be derived. The filter itself is normally an adaptive filter that can be based on one of many different adaptation algorithms. One such algorithm is the Least Mean Squares (LMS) algorithm. To support an LMS based implementation of an echo canceller, a coefficient generator is used to sample both the inbound signal and the outbound signal. From these two signals, a set of filter coefficients are determined and fed to the LMS filter. Again, it is important to note that the coefficient generator cannot perform its function if there is a near-end signal present in the send-path.
As the echo canceller continues to operate, the residual echo is used to adjust the coefficients of the LMS filter that models the echo-path. This process is called adaptation. As the adaptation process continues, the coefficients of the filter assume values that more accurately represent the actual echo-path-model. When the coefficients of the filter no longer change, the echo canceller is said to have converged and a near-perfect echo estimate can be derived.
Because the outbound signal is a composite of the reflected component of the inbound signal and the near-end signal, it is impossible to measure the magnitude of the reflected echo signal in the presence of the near-end signal. To overcome this, echo cancellation systems normally comprise a double-talk detector that senses when the near-end signal is active. The double-talk detector sends a signal to the coefficient generator that causes the coefficient generator to suspend the adaptation process.
The actual echo-path in any given system constantly changes as a result of varying physical phenomenon experienced by the system components themselves. Because of these variations, the adaptation process will seldom converge in a perfect echo-path-model.
One way to improve the accuracy of the echo-path-model is to ensure that the adaptation process is performed as quickly so that any temporal variations in the signal line can be reflected in the resulting filter coefficients. By achieving faster convergence, echo-cancellation systems could reduce the amount of residual echo remaining in the send-path. This would contribute to better voice quality in the communications system.
SUMMARY OF THE INVENTION
In one illustrative embodiment of the present invention, an estimate of the spectral distribution of an inbound signal is used as a basis for filter coefficients for a filter disposed prior to a coefficient generator and an echo-estimation filter. This first filter flattens, or whitens the spectrum of the inbound signal used to generate coefficients and is likewise subjected to the echo-estimation filter. The echo-estimation filter actually implements the transfer function for an echo-path-model that describes the system.
In this same illustrative embodiment, a second filter is placed in send-path prior to a subtractor that is used to subtract an estimated echo from the send-path. This second filter uses the same coefficients used by the first filter. The second filter flattens the spectrum of the outbound signal. Hence, the adaptation filter operates on spectrally equalized versions of the inbound and outbound signals. Once the estimated echo is subtracted from the send-path, the outbound signal is fed through a reconstruction filter in order to introduce the original spectral components of the inbound signal into the equalized outbound signal. By flattening the inbound and the outbound signal, the adaptation filter will converge to a solution of an echo-path-model in less time compared to conventional echo cancellation systems. This contributes to better overall echo cancellation quality.
There are, of course, several brute force mechanisms for achieving faster convergence in an echo cancellation system. These brute force methods rely principally on the use of faster processors in the implementation of the adaptive filters. The present invention exploits the fact that certain adaptive filters converge more rapidly when the input signal presented to the filter has been equalized.
The present invention comprises in the first instance a method for canceling echoes in communications systems. When an inbound signal is received, the method provides that the frequency spectrum of the inbound signal should be determined. Determining the spectrum of the inbound signal can be accomplished several ways. In one example embodiment, the inbound signal is actually measured and the spectrum is determined from the measurement. In an alternative embodiment, the general characteristics of the communications system are monitored over some period of time. Based on the historical observations of the communications systems channel, an exemplary spectrum is determined and subseq
Conexant Systems Inc.
Jmaev Jack
Pham Tuan
Tieu Binh
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