Abstract:
An echo canceller circuit for use in an echo canceller system is set forth. The echo canceller circuit comprises a first digital filter having non-adaptive tap coefficients to simulate an echo response occurring during a call. A second digital filter having adaptive tap coefficients to simulate an echo response occurring during the call is also used. The adaptive tap coefficients of the second digital filter are updated over the duration of the call. A coefficient transfer controller is disposed in the echo canceller circuit to transfer the adaptive tap coefficients of the second digital filter to replace the tap coefficients of the first digital filter when a value, {circumflex over (E)}, is greater than a value, {overscore (E)}, and, concurrently, when {circumflex over (E)} is greater than a value, E max . The value of {overscore (E)} corresponds to the ratio between a signal-plus-echo signal and a first echo compensated signal using the first digital filter. The value of {circumflex over (E)} corresponds to the ratio between the signal-plus-echo signal and a second echo compensated signal using the second digital filter. The value of E max  corresponds to the largest {circumflex over (E)} experienced over at least a portion of the duration of the call and at which a transfer occurred.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The following applications, filed on even date, herewith, are incorporated by reference: U.S. Ser. No. 08/971,116, “Echo Canceller Employing Dual-H Architecture Having Improved Double-Talk Detection”; U.S. Ser. No. 08/970,228, “Echo Canceller Employing Dual-H Architecture Having Improved Non-Linear Echo Path Detection”; U.S. Ser. No. 08/970,874, “Echo Canceller Employing Dual-H Architecture Having Variable Adaptive Gain Settings”; U.S. Ser. No. 08/970,639, “Echo Canceller Employing Dual-H Architecture Having Improved Non-Linear Processor”; U.S. Ser. No. 08/970,229, “Echo Canceller Employing Dual-H Architecture Having Split Adaptive Gain Settings.” 
    
    
     BACKGROUND OF THE INVENTION 
     Long distance telephone facilities usually comprise four-wire transmission circuits between switching offices in different local exchange areas, and two-wire circuits within each area connecting individual subscribers with the switching office. A call between subscribers in different exchange areas is carried over a two-wire circuit in each of the areas and a four-wire circuit between the areas, with conversion of speech energy between the two and four-wire circuits being effected by hybrid circuits. Ideally, the hybrid circuit input ports perfectly match the impedances of the two and four-wire circuits, and its balanced network impedance perfectly matches the impedance of the two-wire circuit. In this manner, the signals transmitted from one exchange area to the other will not be reflected or returned to the one area as echo. Unfortunately, due to impedance differences which inherently exist between different two and four-wire circuits, and because impedances must be matched at each frequency in the voice band, it is virtually impossible for a given hybrid circuit to perfectly match the impedances of any particular two and four-wire transmission circuit. Echo is, therefore, characteristically part of a long distance telephone system. 
     Although undesirable, echo is tolerable in a telephone system so long as the time delay in the echo path is relatively short, for example, shorter than about 40 milliseconds. However, longer echo delays can be distracting or utterly confusing to a far end speaker, and to reduce the same to a tolerable level an echo canceller may be used toward each end of the path to cancel echo which otherwise would return to the far end speaker. As is known, echo cancellers monitor the signals on the receive channel of a four-wire circuit and generate estimates of the actual echoes expected to return over the transmit channel. The echo estimates are then applied to a subtractor circuit in the transmit channel to remove or at least reduce the actual echo. 
     In simplest form, generation of an echo estimate comprises obtaining individual samples of the signal on the receive channel, convolving the samples with the impulse response of the system and then subtracting, at the appropriate time, the resulting products or echo estimates from the actual echo on the transmit channel. In actual practice generation of an echo estimate is not nearly so straightforward. 
     Transmission circuits, except those which are purely resistive, exhibit an impulse response that has amplitude and phase dispersive characteristics that are frequency dependent, since phase shift and amplitude attenuation vary with frequency. To this end, a suitable known technique for generating an echo estimate contemplates manipulating representations of a plurality of samples of signals which cause the echo and samples of impulse responses of the system through a convolution process to obtain an echo estimate which reasonably represents the actual echo expected on the echo path. One such system is illustrated in FIG.  1 . 
     In the system illustrated in FIG. 1, a far end signal x from a remote telephone system is received locally at line  10 . As a result of the previously noted imperfections in the local system, a portion of the signal x is echoed back to the remote site at line  15  along with the signal V from the local telephone system. The echo response is illustrated here as a signal s corresponding to the following equation: 
     
       
         
           s=x*h 
         
       
     
     where h is the impulse response of the echo characteristics. As such, the signal sent from the near end to the far end, absent echo cancellation, is the signal y, which is the sum of the telephone signal v and the echo signal s. This signal is illustrated as y at line  15  of FIG.  1 . 
     To reduce and/or eliminate the echo signal component s from the signal y, the system of FIG. 1 uses an echo canceller having an impulse response filter {overscore (h)} that is the estimate of the impulse echo response h. As such, a further signal {overscore (s)} representing an estimate of echo signal s is generated by the echo canceller in accordance with the following equation: 
     
       
         
           {overscore (s)}={overscore (h)}*x 
         
       
     
     The echo canceller subtracts the echo estimate signal {overscore (s)} from the signal s to generate a signal e at line  20  that is returned to the far end telephone system. The signal e thus corresponds to the following equation: 
     
       
         
           e=s+v−{overscore (s)}≈v 
         
       
     
     As such, the signal returned to the far end station is dominated by the signal v of the near end telephone system. As the echo impulse response {overscore (h)} more closely correlates to the actual echo response h, then {overscore (s)} more closely approximates s and thus the magnitude of the echo signal component s on the signal e is more substantially reduced. 
     The echo impulse response model {overscore (h)} may be replaced by an adaptive digital filter having an impulse response {circumflex over (h)}. Generally, the tap coefficients for such an adaptive response filter are found using a technique known as Normalized Least Mean Squares adaptation, although other Mean Squares processes may also be used (e.g., RLS, NLMS, etc.). 
     Although such an adaptive echo canceller architecture provides the echo canceller with the ability to readily adapt to changes in the echo path response h, it is highly susceptible to generating sub-optimal echo cancellation responses in the presence of “double talk” (a condition that occurs when both the speaker at the far end and the speaker at the near end are speaking concurrently as determined from the viewpoint of the echo canceller). 
     To reduce this sensitivity to double-talk conditions, it has been suggested to use both a non-adaptive response and an adaptive response filter in a single echo canceller. One such echo canceller is described in U.S. Pat. No. 3,787,645, issued to Ochiai et al on Jan. 22, 1974. Such an echo canceller is now commonly referred to as a dual-H echo canceller. 
     Although the dual-H echo canceller architecture of the &#39;645 patent provides substantial improvements over the use of a single filter response architecture, the &#39;645 patent is deficient in many respects and lacks certain teachings for optimizing the use of such a dual-H architecture in a practical echo canceller system. The present inventors have recognized the problems associated with the foregoing dual-H architecture and have provided solutions to these problems. 
     BRIEF SUMMARY OF THE INVENTION 
     An echo canceller circuit for use in an echo canceller system is set forth. The echo canceller circuit comprises a first digital filter having non-adaptive tap coefficients to simulate an echo response occurring during a call. A second digital filter having adaptive tap coefficients to simulate an echo response occurring during the call is also used. The adaptive tap coefficients of the second digital filter are updated over the duration of the call. A coefficient transfer controller is disposed in the echo canceller circuit to transfer the adaptive tap coefficients of the second digital filter to replace the tap coefficients of the first digital filter when a value, {circumflex over (E)}, is greater than a value, {overscore (E)}, and, concurrently, when {circumflex over (E)} is greater than a value, E max . The value of {overscore (E)} corresponds to the ratio between a signal-plus-echo signal and a first echo compensated signal using the first digital filter. The value of {circumflex over (E)} corresponds to the ratio between the signal-plus-echo signal and a second echo compensated signal using the second digital filter. The value of E max  corresponds to the largest {circumflex over (E)} experienced over at least a portion of the duration of the call at which a transfer occurred. 
     A method for transferring tap coefficients between an adaptive digital filter and a non-adaptive digital filter of a dual-H echo canceller during a call is also set forth. According to the method, a comparison is made between the value of {circumflex over (E)} and the value of {overscore (E)}. Further, a comparison is also made between the value of {circumflex over (E)} the value of E max . Transfer of the adaptive tap coefficients of the adaptive digital filter to replace the tap coefficients of the non-adaptive digital filter occurs when {circumflex over (E)} is greater than {overscore (E)} and, concurrently, {circumflex over (E)} is greater than E max . 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a block diagram of a conventional canceller. 
     FIG. 2 is a schematic block diagram of an echo canceller that operates in accordance with one embodiment of the present invention. 
     FIG. 3 is a flow chart illustrating one manner of carrying out coefficient transfers in accordance with one embodiment of the present invention. 
     FIG. 4 is a flow chart illustrating a further manner of carrying out coefficient transfers in accordance with a further embodiment of the present invention. 
     FIG. 5 illustrates one manner of implementing an echo canceller system employing the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 illustrates one embodiment of a dual-h echo canceller suitable for use in implementing the present invention. As illustrated, the echo canceller, shown generally at  25 , includes both a non-adaptive filter {overscore (h)} and an adaptive filter {circumflex over (h)} to model the echo response h. Each of the filters {overscore (h)} and {circumflex over (h)} are preferably implemented as digital filters, such as finite impulse response (FIR) filters comprising a plurality of taps each having a corresponding tap coefficient. This concept may be extended to IIR filters as well. If FIR filters are used, the duration of each of the FIR filters should be sufficient to cover the duration of the echo response of the channel in which the echo canceller  25  is disposed. 
     The output of the non-adaptive filter {overscore (h)} is available at the line  30  while the output of the adaptive filter {circumflex over (h)} is available at line  35 . Each of the signals at lines  30  and  35  are subtracted from the signal-plus-echo signal of line  40  to generate echo compensated signals at lines  50  and  55 , respectively. A switch  45 , preferably a software switch, may be used to selectively provide either the output signal at the line  50  or the output signal at line  55  to the echo canceller output at line  60 . The switch  45  may be used to provide the echo compensation based on the {circumflex over (h)} filter during convergence and then be switched to provide the echo compensation based on the {overscore (h)} filter after convergence. Further, the switch  45  is directed to provide the echo compensation based on the {overscore (h)} filter in response to the detection of a double-talk condition. 
     A transfer controller  65  is used to transfer the tap coefficients of filter {circumflex over (h)} to replace the tap coefficients of filter {overscore (h)}. As illustrated, the transfer controller  65  is connected to receive a number of system input signals. Of particular import with respect to the present invention, the transfer controller  65  receives the signal-plus-echo response y and each of the echo canceller signals {overscore (e)} and {circumflex over (e)} at lines  50  and  55 , respectively. The transfer controller  65  is preferably implemented in the software of one or more digital signal processors used to implement the echo canceller  25 . 
     As noted above, the art is substantially deficient of teachings with respect to the manner in which and conditions under which a transfer of tap coefficients from {circumflex over (h)} to {overscore (h)} is to occur. The present inventors have implemented a new process and, as such, a new echo canceller in which tap coefficient transfers are only made by the transfer controller  65  when selected criterion are met. The resulting echo canceller  25  has substantial advantages with respect to reduced double-talk sensitivity and increased double-talk detection capability. Further, it ensures a monotonic improvement in the estimates {overscore (h)}. 
     The foregoing system uses a parameter known as echo-return-loss-enhancement (ERLE) to measure and keep track of system performance. Two ERLE parameter values are used in the determination as to whether the transfer controller  65  transfers the tap coefficients from {circumflex over (h)} to {overscore (h)}. The first parameter, {overscore (E)}, is defined in the following manner: 
       {overscore (E)}=y/{overscore (e)}   
     Similarly, the parameter {circumflex over (E)} is defined as follows: 
     
       
         
           {circumflex over (E)}=y/{circumflex over (e)} 
         
       
     
     Each of the values {circumflex over (E)} and {overscore (E)} may also be averaged over a predetermined number of samples to arrive at averaged {circumflex over (E)} and {overscore (E)} values used in the system for the transfer determinations. 
     FIG. 3 illustrates one manner of implementing the echo canceller  25  using the parameters {circumflex over (E)} and {overscore (E)} to control tap coefficients transfers between filter {circumflex over (h)} to {overscore (h)}. As illustrated, the echo canceller  25  provides a default {overscore (h)} set of coefficients at step  80  during the initial portions of the call. After the tap coefficients values for {overscore (h)} have been set, a measure of {overscore (E)} is made at step  85  to measure the performance of the tap coefficient values of filter {overscore (h)}. 
     After the initialization sequence of steps  80  and  85 , or concurrent therewith, the echo canceller  25  begins and continues to adapt the coefficients of {circumflex over (h)} to more adequately match the echo response h of the overall system. As noted in FIG. 3, this operation occurs at step  90 . Preferably, the adaptation is made using a Normalized Least Mean Squares method, although other adaptive methods may also be used (e.g., LMS and RLS). 
     After a period of time has elapsed, preferably, a predetermined minimum period of time, the echo canceller  25  makes a measure of {circumflex over (E)} at step  95 . Preferably, this measurement is an averaged measurement. At step  100 , the echo canceller  25  compares the value of {circumflex over (E)} with the value of {overscore (E)}. If the value of {circumflex over (E)} is greater than the value of {overscore (E)}, the tap coefficients of filter {circumflex over (h)} are transferred to replace the tap coefficients of filter {overscore (h)} at step  105 . If this criterion is not met, however, the echo canceller  25  will continue to adapt the coefficients of the adaptive filter {circumflex over (h)} at step  90 , periodically measure the value of {circumflex over (E)} at step  95 , and make the comparison of step  100  until the condition is met. 
     Although not illustrated, other transfer conditions may be imposed in addition to the foregoing. For example, the echo canceller may impose a requirement that a far end signal exist before a transfer may occur. Additionally, transfers may be inhibited during a double-talk condition. Further conditions may also be imposed based on system requirements. 
     If the echo canceller  25  finds that {circumflex over (E)} is greater than {overscore (E)}, the above-noted transfer takes place. Additionally, the echo canceller  25  stores the value of {circumflex over (E)} as a value E max . This operation is depicted at step  110  of the FIG.  3 . The value of E max  is thus the maximum value of ERLE that occurs over the duration of the call and at which a transfer has taken place. This further value is used thereafter, in addition to the {circumflex over (E)} and {overscore (E)} comparison, to control whether the tap coefficients of {circumflex over (h)} are transferred by the transfer controller  65  to replace the tap coefficients of {overscore (h)}. This further process is illustrated that steps  115 ,  120 , and  125  of FIG.  3 . In each instance, the tap coefficient transfer only occurs when both of the following two conditions are met: 1) {circumflex over (E)} is greater than the current {overscore (E)}, and 2) {circumflex over (E)} is greater than any previous value of {overscore (E)} used during the course of the call. ({circumflex over (E)} is greater than E max ). Each time that both criteria are met, the transfer controller  65  of echo canceller  25  executes the tap coefficient transfer and replaces the previous E max  value with the current {circumflex over (E)} value for future comparison. 
     Requiring that {circumflex over (E)} be greater than any {overscore (E)} value used over the course of the call before the coefficient transfer takes place has two beneficial and desirable effects. First, each transfer is likely to replace the prior tap coefficients of filter {overscore (h)} with a better estimate of the echo path response. Second, this transfer requirement increases the double-talk protection of the echo canceller system. Although it is possible to have positive ERLE {circumflex over (E)} during double-talk, the probability that {circumflex over (E)} is greater than E max  during double-talk decreases as the value of E max  increases. Thus an undesirable coefficient transfer during double-talk becomes increasingly unlikely as the value of E max  increases throughout the duration of the call. 
     The echo canceller  25  may impose both an upper boundary and a lower boundary on the value of E max . For example, E max  may have a lower bounded value of 6 dB and an upper bounded value of 24 dB. The purpose of the lower bound is to prevent normal transfers during double-talk conditions. It has been shown in simulations using speech inputs that during double-talk, a value of greater than 6 dB ERLE was a very low probability event, thus making it an appropriate value for the initial value of E max . The upper bound on E max  is used to prevent a spuriously high measurement from setting E max  to a value at which further transfers become impossible. 
     The value of E max  should be set to, for example, the lower bound value at the beginning of each call. Failure to do so will prevent tap coefficient transfers on a new call until the echo cancellation response of the echo canceller  25  on the new call surpasses the quality of the response existing at the end of the prior call. However, this criterion may never be met during the subsequent call Thereby causing the echo canceller  25  to operate using sub-optimal tap coefficients values. Resetting the E max  value to a lower value increases the likelihood that a tap coefficient transfer will take place and, thereby, assists in ensuring that the {overscore (h)} filter uses tap coefficients for echo cancellation that more closely correspond to the echo path response of the new call. 
     One manner of implementing the E max  value change is illustrated in the echo canceller operations flow-chart of FIG.  4 . When all transfer conditions are met except {circumflex over (E)} greater than E max , and this condition persists for a predetermined duration of time, the echo canceller  25  will reset the E max  value to, for example, the lower bound value. In the exemplary operations shown in FIG. 4, the echo canceller  25  determines whether {circumflex over (E)} is greater than the lower bound of E max  at step  140  and less than the current value of E max  at step  145 . If both of these condition remain true for a predetermined period of time as determined at step  150 , and all other transfer criterion have been met, the echo canceller  25  resets the E max  value to a lower value, for example, the lower bound of the E max  value, at step  155 . This lowering of the E max  value increases the likelihood of a subsequent tap coefficient transfer. 
     Choosing values for the lower and upper bound of E max  other than 6 dB and 24 dB, respectively, is also possible in the present system. Choosing a lower bound of E max  smaller than 6 dB provides for a relatively prompt tap coefficient transfer after a reset operation or a new call, but sacrifices some double-talk protection. A value greater than 6 dB, however, inhibits tap coefficient transfer for a longer period of time, but increases the double-talk immunity of the echo canceller. Similarly, varying the value of the predetermined wait time T before which E max  is reset may also be used to adjust echo canceller performance. A shorter predetermined wait time T produces faster reconvergence transfers, but may sacrifice some double-talk immunity. The opposite is true for larger predetermined wait time values. 
     A further modification of the foregoing echo canceller system relates to the value stored as E max  at the instant of tap coefficient transfer. Instead of setting E max  equal to the {circumflex over (E)} value at the transfer instant, E max  may be set to a value equal to the value of {circumflex over (E)} minus a constant value (e.g., one, three, or 6 dB). At no time, however, should the E max  value be set to a value that is below the lower bound value for E max . Additionally, a further condition may be imposed in that a new softened E max  is not less than the prior value of E max . The foregoing “softening” of the E max  value increases the number of transfers that occur and, further, provides more decision-making weight to the condition of {circumflex over (E)} being larger than {overscore (E)}. 
     As will be readily recognized, the echo canceller of the present invention may be implemented in a wide range of manners. Preferably, the echo canceller system is implemented using one or more digital signal processors to carry out the filter and transfer operations. Digital-to-analog conversions of various signals are carried out in accordance with known techniques for use by the digital signal processors. 
     There are some circumstances when the foregoing transfer criterion should be defeated. For example, the transfer criterion is preferably defeated when 1) the long-term ERLE remains low, and 2) a small but measurable performance advantage of {circumflex over (h)} over {overscore (h)} is sustained over a long period of time. 
     One case in which it should be defeated is when the steady-state ERLE remains below the lower value of E max . Such a case may occur when there is a high-level, constant background noise entering from the near-end. Since the foregoing process prevents transfers from occurring unless the ERLE is greater than the lower bound of E max , no transfers are possible in low ERLE situations. Since the {overscore (h)} may contain the solution to a previous call at the start of a new, low ERLE call, defeating the foregoing transfer criterion is preferable in some cases. 
     The first condition for defeating the foregoing transfer criterion is a sustained low ERLE measurement over a relatively long period of time (e.g. 150 to 500 msec) of adaptation. Since a low ERLE call will tend to have a smaller difference between the ERLEs of {overscore (h)} and {circumflex over (h)} (a 1 dB difference may be the largest difference observed), the required ERLE difference between {circumflex over (h)} and {overscore (h)} for a transfer to occur should be reduced (e.g. to 0 or 1 dB) once the long-term ERLE is confirmed to be low. To compensate, a requirement may be imposed whereby the small ERLE difference between {overscore (h)} and {circumflex over (h)} is maintained for a long period of time (e.g. 75 to 200 msec) before the transfer is allowed. 
     FIG. 5 illustrates one embodiment of an echo canceller system, shown generally at  700 , that maybe used to cancel echoes in multi-channel communication transmissions. As illustrated, the system  700  includes an input  705  that is connected to receive a multi-channel communications data, such as a T 1  transmission. A central controller  710  deinterleaves the various channels of the transmission and provides them to respective convolution processors  715  over a data bus  720 . It is within the convolution processors  715  that a majority of the foregoing operations take place. Each convolution processor  715  is designed to process at least one channel of the transmission at line  730 . After each convolution processor  715  has processed its respective channel(s), the resulting data is placed on the data bus  720 . The central controller  710  multiplexes the data into the proper multichannel format (e.g., T 1 ) for retransmission at line  735 . User interface  740  is provided to set various user programmable parameters of the system. 
     Numerous modifications may be made to the foregoing system without departing from the basic teachings thereof. Although the present invention has been described in substantial detail with reference to one or more specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the scope and spirit of the invention as set forth in the appended claims.