This invention relates generally to the cancellation of an echo signal in a voice communication system.
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 xe2x80x9cfar-endxe2x80x9d. 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 xe2x80x9cnear-endxe2x80x9d. The hybrid transformer propagates a signal originating at the near-end, commonly called the xe2x80x9cnear-end signalxe2x80x9d, to the far-end using a second signal path called the xe2x80x9csend-pathxe2x80x9d. 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 xe2x80x9coutbound signalxe2x80x9d.
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.
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 subsequently used in the process. Once the frequency spectrum of the inbound signal is determined, the inbound signal is itself equalized.
In this example embodiment of the method for canceling echoes, an outbound communications signal is also equalized based on the frequency spectrum of the inbound signal. The outbound communications signal typically comprises at least two components. These are the actual near-and signal that must be propagated to a far-end and some derivative of the inbound communications signal; i.e. the echo component. Using the flattened inbound communications signal and the flattened outbound communications signal, filter coefficients are generated for an adaptive filter. The adaptive filter is a convenient means for implementing the echo-path-model that the communications system exhibits. As such, subjecting the inbound communications signal to the adaptive filter results in an estimate of the echo component found in the outbound communications signal.
Once an estimate of the echo component is generated by the adaptive filter, the method provides for subtracting the estimated echo component from the flattened outbound communications signal. In theory, the outbound communications signal should be devoid of any echo component at this stage. Prior to directing the echo-canceled outbound signal to the far-end, the signal must be reconstructed so as to include the original spectral envelope representative of the original inbound signal. This reconstruction is accomplished based on the spectral distribution for the original inbound signal.
In one example embodiment, the method of the present invention provides for monitoring the frequency spectrum of the inbound signal by buffering the inbound communications signal and then calculating correlation coefficients for the buffered signal. The correlation coefficients are used to create a set of normal equations that can then in turn be solved to determine the frequency spectrum of the inbound signal.
The invention further comprises a system for canceling echoes. In one example embodiment, an echo cancellation system comprises a receiver capable of excepting external signal and then propagating that signal to other components in the system. The echo cancellation system further comprises a spectrum estimator that is able to create filter coefficients. These filter coefficients are used to configure a first whitening filter that accepts the inbound signal from the receiver and creates an equalized rendition of the inbound signal.
The receiver also propagates the inbound signal to a hybrid transformer. The hybrid transformer, having received the non-equalized inbound signal, directs the non-equalized inbound signal to a near-end subscriber. Unfortunately, not all of the inbound signal is propagated to the subscriber. Some of the inbound signal is coupled together with a near-end signal that originates with the subscriber. The inbound signal that is coupled together with the near-end signal is the echo signal that needs to be cancelled. The combination of the near-end signal and the echo signal is called the outbound signal. The invention further comprises a second whitening filter that receives the outbound signal and equalizes the outbound signal based on the filter coefficients created by the spectrum estimator.
The flattened inbound signal and the flattened outbound signal are initially used to create filter coefficients for an adaptive filter that further comprises the invention. The adaptive filter, as configured by these filter coefficients, implements the echo-path-model transfer function that can be used to estimate the amount of echo that should be found in the outbound signal. The invention further comprises a subcontractor that subtracts the predicted echo from the outbound signal.
Because the outbound signal has been equalized, it must be reconstructed in order to reflect the spectral envelope of the original inbound signal. This is accomplished by a reconstruction filter that further comprises the invention and whose filter coefficients may be based on the inverse of the inbound signal""s spectral density.
In one example variant of the present invention, the spectrum estimator comprises a spectrum analyzer that actually measures the inbound signal and determines the level of energy at various frequencies. Such a spectrum analyzer may comprise a buffer that captures the inbound communications signal and a correlation calculator that creates correlation coefficients based on the inbound signal stored in the buffer. In this example embodiment, the equation generator creates a set of normal equations based on the coefficients created by the correlation calculator. A matrix analyzer solves this set of normal equations; the resulting matrix defines the spectral distribution of the inbound signal.
In yet a second illustrative variation of the present invention, the spectrum estimator comprises a memory for storing a set of anticipated spectral distributions. In this example embodiment, the invention further comprises a selection unit that selects the estimated spectrum from the memory. The anticipated spectral distributions may be created off-line and can be based on a priori knowledge of the condition of the inbound communications line or can be based on an extrapolation of historical observations of line condition.
In all of these example embodiments, any of the whitening filters can be implemented as filter processors that accepts coefficients from the spectrum estimator. The reconstruction filter may also comprise a filter processor that accepts filter coefficients from the spectrum estimator.
The present invention further comprises a central office line interface that can be used in a telephone-switching center or like application. In one illustrative embodiment, the central office line interface unit according to the present invention comprises a hybrid transformer, the first whitening filter, a second whitening filter, a coefficient generator, an adaptive filter, a subtractor, and a reconstruction filter. The central office line interface unit may further comprise a double-talk detector.
In this example embodiment, the central office line interface unit receives the inbound signal from a remote exchange using a four-wire interface provided by the hybrid transformer. The hybrid transformer directs the inbound signal to a two-wire interface that is used to service a local subscriber. The four-wire interface provided by the hybrid transformer itself comprises a two-wire send-path and a two-wire receive-path. In operation, a near-end signal is received from the local subscriber and is directed into the two-wire send-path by the hybrid transformer.
The inbound signal arrives at the central office line interface unit by way of the two-wire receive-path provided by the hybrid transformer. The first whitening filter equalizes the inbound signal and creates a flattened inbound signal. The second whitening filter concurrently flattens the outbound signal emanating from the hybrid transformer on the two-wire send-path. It should be noted that this outbound signal comprises a near-end signal originating with the local subscriber and an echo signal that is coupled into the send-path by the hybrid transformer.
The coefficient generator receives the flattened inbound signal and the flattened outbound signal and creates filter coefficients for the adaptive filter. The adaptive filter, as configured by these filter coefficients, implements the echo-path-model transfer function that is used to predict the nature and quality of the echo signal coupled into the send-path by the hybrid transformer. In operation, the adaptive filter will create the estimated echo signal. The subcontractor receives the flattened outbound signal from the second whitening filter and subtracts the estimated echo signal therefrom.
In another example embodiment of the present invention, the coefficient generator may continually refine the coefficients that define the echo-path-model in a process called adaptation. This is done by receiving a residual echo signal from the subcontractor. Such adaptation, and for that matter the creation of the original echo-path-model coefficients can only be accomplished in the absence of any near-end signal. This is due to the fact that the near-end signal obscures the echo signal necessary to determine the echo-path-model. The central office line interface unit of the present invention may in this instance further comprise a double-talk detector. The double-talk detector monitors the state of the near-end signal source. When the near-end signal is active, the double-talk detector issues a signal that prevents the coefficient generator from updating its coefficients.
In yet another example embodiment of the present invention, the first and second whitening filters and the reconstruction filter are configured based on the estimate of the spectral distribution of the inbound signal. The estimate of the spectral distribution of the inbound signal may be obtained either through measurement of the inbound signal and a determination of the spectral distribution thereof or by simply anticipating what the spectral distribution of the inbound signal would be under some given circumstance.