Abstract:
An extended impulse response characteristic is obtained by switchably connecting (via 13, 14, 15) at least first (10) and second (12) adaptive echo cancelers in a tandem connection (FIG. 1). A delay unit (11) is associated with a receive input (X) of the second adaptive echo canceler (12) so that each filter models a different portion of the extended impulse response characteristic. The relative positions of the adaptive echo cancelers (10, 12) in the tandem connection are controllably switched (via 13, 14, 15 and 16) to insure so-called &#34;good&#34; adaptation of the individual adaptive echo cancelers (10, 12) to minimize misalignment noise. Both adaptive echo cancelers (10, 12) are allowed to adapt initially but upon switching only the adaptive echo canceler (12) in a so-called preferred position in the tandem connection is allowed to adapt while the other adaptive echo canceler (10) is inhibited from further adapting.

Description:
TECHNICAL FIELD 
     This invention relates to adaptive echo cancelers and, more particularly, to a tandem adaptive echo conceler arrangement for obtaining an extended impulse response characteristic. 
     BACKGROUND OF THE INVENTION 
     Adaptive echo cancelers operate on an incoming signal to generate an impulse response characteristic in accordance with a prescribed algorithm. Existing adaptive echo cancelers are able to model an impulse response of a limited interval, for example, 16 milliseconds. In some applications it is desirable to model an impulse response having an interval greater than the available interval of an individual adaptive echo canceler. One example is in applications where the round trip electrical delay encountered in a telephone transmission channel is greater than the impulse response interval of an individual echo canceler. 
     One solution to this problem is to use a &#34;truly&#34; cascadable adaptive echo canceler. In such an arrangement each canceler uses a common error signal to update the impulse response estimate. In order to do this the individual adaptive echo cancelers must be arranged circuit wise to facilitate inputting the common error signal to the impulse response updating circuitry. Such a truly cascadable echo canceler including an adaptive filter is manufactured by Western Electric Company. However, most presently available adaptive echo cancelers do not have this capability. 
     Consequently, in order to address the need for a longer impulse response interval with most existing adaptive echo cancelers a tandem arrangement is required. Attempts at tandeming echo cancelers have heretofore yielded less than desirable results. One arrangement includes having two cancelers connected in tandem with a fixed delay connected in the receive side of one of them. It was hoped that this arrangement would extend the impulse response to twice the interval of an individual echo canceler. A serious problem with this arrangement is that the echo canceler which is the first one to subtract its estimate of a reference signal from an actual reference signal has poor adaptation. This poor adaptation results because the error signal used for updating the impulse response includes the portion of the reference signal which the other echo canceler should estimate. As is known in the art, this results in unwanted misalignment noise. 
     SUMMARY OF THE INVENTION 
     An extended impulse response characteristic is realized, in accordance with an aspect of the invention, by switchably connecting a plurality of adaptive echo cancelers in a tandem connection. More specifically, a first adaptive echo canceler arranged to generate a first portion of the extended impulse response interval is switchably connected in tandem with at least a second adaptive echo canceler arranged to generate a second portion of the extended impulse response interval. A switching control circuit is employed to control switching of the relative positions of the adaptive echo cancelers in the tandem connection in accordance with prescribed criteria. 
     In one example, the first adaptive echo canceler is initially connected in a first so-called preferred position in the tandem connection and the at least second adaptive echo canceler including a predetermined fixed delay in its receive side is connected in a second position in the tandem connection. A received signal is supplied to the receive side of the first adaptive echo canceler and via the delay to the receive side of the at least second adaptive echo canceler, while a transmit signal is first supplied to a transmit input of the at least second adaptive echo canceler and a transmit output signal from the at least second adaptive echo canceler is supplied to a transmit input of the first adaptive echo canceler. A transmit output signal from the first adaptive echo canceler is the initial echo canceler or echo canceler output. Initially, both adaptive echo cancelers are adapting. Upon detection of prescribed canceler output signal conditions the relative positions of the adaptive echo cancelers are controllably reversed, i.e., the at least second adaptive echo canceler including the delay is switched to the preferred position, and the first adaptive echo canceler is inhibited from further adapting and is switched to the second position in the tandem connection. The at least second adaptive echo canceler is allowed to continue adapting. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be more fully understood from the following detailed description of an embodiment thereof taken in conjunction with the appended figures in which: 
     FIG. 1 shows in simplified block diagram form a tandem connection of adaptive echo cancelers including an embodiment of the invention, and 
     FIG. 2 depicts in simplified block diagram form details of the control unit used in the embodiment of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates in simplified block diagram form one embodiment of the invention. Accordingly, a received signal X(k) is supplied to receive input X of adaptive echo canceler 10 and via delay unit 11 to receive input X of adaptive echo canceler 12. Adaptive echo cancelers 10 and 12 may be any one of a number known in the art. In this example, adaptive echo cancelers 10 and 12 are echo cancelers each includes an adaptive transversal filter and are of a type broadly disclosed in U.S. Pat. No. 3,500,000 and described in an article by D. L. Duttweiler and Y. S. Chen entitled, &#34;A Single-Chip VLSI Echo Canceler&#34;, The Bell System Technical Journal, Vol. 59, No. 2, February 1980, pages 149-160. Briefly, each of adaptive echo cancelers 10 and 12 has a receive input (X), transmit input (Y), transmit output (E), and update inhibit input (INH). The adaptive echo cancelers (echo cancelers) operate in known fashion to generate a signal estimate Y which is algebraically combined with a signal supplied to transmit input Y to generate an error signal at the transmit output E. The error signal is used internally in the adaptive echo canceler to update filter tap coefficient values in order to better model the impulse response characteristic being generated and drive the error signal toward a zero value. 
     Delay unit 11 is employed to fix the portion of the desired extended impulse response characteristic generated by each of adaptive echo cancelers 10 and 12. The delay interval of delay unit 11 is dependent on the number of coefficient taps (Z) in adaptive echo canceler 10 and the sampling interval k being employed. Typically, delay unit 11 includes a shift register having Z stages which is clocked at the sampling rate k -1 . In one example, the number of taps (Z) is 128 and the sample rate (k -1 ) is 8 kHz. Thus, adaptive echo canceler 10 typically has a zero delay and models a first fixed portion of the extended impulse response, for example, a 16 millisecond interval and adaptive echo canceler 12 has delay unit 11 associated with its receive input (X) and, consequently, models a second fixed portion of the extended impulse response characteristic, for example, an interval from 16 to 32 milliseconds. 
     Adaptive echo cancelers 10 and 12 are switchably connected in a tandem connection via controllable selector units 13, 14 and 15. As shown, the transmit output (E) of adaptive echo canceler 10 is connected to the A IN  input of selector unit 13 and to the B IN  input of selector unit 15. Similarly, the transmit output (E) of adaptive echo canceler 12 is connected to the A IN  input of selector unit 14 and to the B IN  input of selector 13. A signal to be transmitted, for example, Y(k), is supplied to the A IN  input of selector unit 15, to the B IN  input of selector unit 14 and to control unit 16. Signal Y(k) is, for example, an outgoing signal to be transmitted in an echo canceler application including near-end speech and the echo signal to be cancelled or an arbitrary system signal in an adaptive filter application. An output from selector unit 15 is supplied to transmit input Y of adaptive echo canceler 12. Similarly, an output from selector unit 14 is connected to transmit input Y of adaptive echo canceler 10. An output from selector unit 13 is connected to control unit 16 and is the desired transmit output or error signal E(k). The output from each of selector units 13, 14 and 15 is normally the A IN  input and is the B IN  input when enabled by a true, i.e., logical 1, switch control signal SW being supplied to the select B input from control unit 16. 
     Control unit 16 operates to generate switch control signal SW for controllably switching, in accordance with an aspect of the invention, the relative positions of adaptive echo cancelers 10 and 12 in the tandem connection. Control unit 16 in response to an ON-HOOK to OFF-HOOK transition indicating initiation of a call generates a false, i.e., logical 0, switch control signal SW. Upon a prescribed relationship between error signal E(k) and transmit input signal Y(k) occurring control unit 16 generates a true, i.e., logical 1, switch control signal SW. In one example, control unit 16 compares the error signal E(k) power to the transmit input signal Y(k) power to obtain a measure of the tandem adaptive echo canceler &#34;advantage&#34;. Specifically, a scaled version of the transmit input signal power is compared to the error signal power and when the error signal power becomes less than the scaled transmit input signal power the adaptive echo canceler in a preferred position in the tandem connection, initially adaptive echo canceler 10, is assumed to be converged. At this instant, control unit 16 generates a true, i.e., logical 1, SW signal. Consequently, the relative positions of adaptive echo cancelers 10 and 12 are controllably switched, in accordance with an aspect of the invention, in the tandem connection and adaptive echo canceler 10 is inhibited from further adapting. This is realized by selector units 13, 14 and 15 being controlled via signal SW to select input B IN  as their outputs and by adaptive echo canceler 10 being inhibited by signal SW. Thus, transmit input signal Y(k) is supplied to transmit input Y of adaptive echo canceler 10, transmit output E of adaptive echo canceler 10 is supplied to transmit input Y of adaptive echo canceler 12 and transmit output E of adaptive echo canceler 12 is error signal E(k). 
     The reason for switching of the positions of adaptive echo cancelers 10 and 12 is to minimize the effect of noise generated by the adaptive echo cancelers on their convergence. It has been determined that the adaptive in a so-called preferred position in the tandem connection will converge relatively well and cause relatively small amounts of unwanted noise. In this example, adaptive echo canceler 10 is initially in the preferred tandem connection position. Since adaptive echo canceler 10 converges relatively well, it then can be frozen, i.e., inhibited from further adapting, and switched to the other position in the connection while adaptive echo canceler 12 is switched to the preferred position and is allowed to adapt. Since adaptive echo cancelers 12 is now in the preferred position, it will converge relatively well and the misalignment noise introduced by adaptive filters 10 and 12 is minimized. 
     FIG. 2 shows in simplified block diagram form details of control unit 16. Accordingly, shown are power value measurement units 20 and 21, amplifier 22, comparator 23 and set-reset flip-flop 24. Transmit input signal Y(k) is supplied to power value measurement unit 20 which generates an output Y p  (k) representative of the power of Y(k). Similarly, error signal E(k) is supplied to power value measurement unit 21 which generates an output E p  (k) representative of the power of E(k). Power value measurement units 20 and 21 are reset in response to an ON-HOOK-OFF-HOOK transition. Signal Y p  (k) is supplied via scaling amplifier 22 to one input of comparator 23 while signal E p  (k) is supplied to another input of comparator 23. The output from comparator 23 is negative, i.e., representative of a logical 0, until E p  (k)&lt;αY p  (k). When E p  (k) becomes less than αY p  (k) initial convergence of adaptive filter 10 (FIG. 1) is assumed to be complete and comparator 23 yields a positive output, i.e., representative of a logical 1. To this end, scaling factor α is the expected echo return loss enhancement factor, i.e., the adaptive filter transmit input power less the adaptive filter output power after, for example, echo cancellation in an echo canceler application. In one example of an echo canceler application, α is 20 dB power. An output from comparator 23 is supplied to a set (S) input of flip-flop 24 when the ON-HOOK-OFF-HOOK signal is supplied to a reset (R) input of flip-flop 24. In operation, flip-flop 24 is reset by an ON-HOOK-OFF-HOOK transition to generate a false, i.e., logical 0, SW signal at output Q. Then, when E p  (k) becomes less than αY p  (k) flip-flop 24 is set by a logical 1 output from scaling amplifier 23 to generate a true, i.e., logical 1 SW signal at output Q. 
     Each of power measurement units 20 and 21 includes apparatus (not shown) for squaring the supplied input sample values and a low pass filter for obtaining smooth versions of the desired power measurement values in well known fashion.