Abstract:
The present invention provides method and apparatus that improve processing of acoustic signals by reducing acoustic feedback in an acoustic system. An aspect of the invention is a multi-channel digital feedback reducer (DFR) system that comprises a plurality of channel elements. Each channel element comprises a notch filter configuration having an adaptive notch filter and an operative notch filter. The operative notch filter processes a signal received from an acoustic input device and provides the processed signal to an acoustic output device, in which acoustic feedback between the acoustic input device and the acoustic output device is ameliorated. If acoustic feedback is detected by a channel element, the channel element informs other channel elements of the multi-channel DFR system about the detected feedback to ensure that all channel elements may incorporate the same notch filters. During the notification, the other channel elements may continue searching for feedback on the associated channels.

Description:
This application claims priority to provisional U.S. Application. No. 60/453,318, filed Mar. 10, 2003. 

   FIELD OF THE INVENTION 
   The invention relates to techniques for reducing acoustic feedback, and more particularly relates to such techniques in which a digital notch filter algorithm is employed for a plurality of acoustic channels. 
   BACKGROUND OF THE INVENTION 
   Notch filters are often used to reduce acoustic feedback in sound amplification systems, including public address systems and music delivery systems in which microphones and speakers are deployed. For example, U.S. Pat. No. 4,091,236 (Chen, issued May 23, 1978) describes an analog notch filter for an audio signal to suppress acoustic feedback. The apparatus receives an audio signal that is substantially non-periodic in the absence of acoustic feedback and substantially periodic with an instantaneous dominant frequency in the presence of such feedback. 
   The duration of successive periods are monitored and compared by an up/down counter to determine whether the audio input signal is substantially periodic and to determine the instantaneous dominant frequency of the audio signal. Upon detection of an audio signal that is substantially periodic, the notch filter is tuned to the instantaneous dominant frequency so as to suppress the acoustic feedback. 
   U.S. Pat. No. 4,232,192 (Beex, issued Nov. 4, 1980) describes an integrator/detector that determines when an audio signal has exceeded a threshold for a selected number of cycles. If the threshold is exceeded for the selected number of cycles, a sampler circuit samples a voltage corresponding to the frequency that has exceeded the threshold. The sampled voltage is used by a voltage frequency converter in order to adjust the notch of a notch filter implemented in hardware. 
   U.S. Pat. No. 5,245,665 (Lewis et al., issued Sep. 14, 1993) describes a device for suppressing feedback in which a Fast Fourier Transform is conducted on samples of digitized signals to produce corresponding frequency spectrums. The magnitudes of the spectrum at various frequencies are analyzed to determine one or more peak frequencies that are 33 decibels greater than harmonics or sub-harmonics of the frequency in an attempt to detect resonating feedback frequencies. Two processors are required. A primary processor periodically collects a series of the passing digital signals and conducts a Fast Fourier Transform on each collected series of digital signals. The frequency spectrums produced by the Fast Fourier Transform are examined by the primary processor to discover the presence of any resonating feedback frequency. Filter control signals are passed by the primary processor, along with the digital sound signals, to a secondary processor which operates a digital filtering algorithm in accordance with the filter control signals to attenuate resonating feedback frequencies in the stream of digital signals. 
   U.S. Pat. No. 5,999,631 (Porayath, et al., issued Dec. 7, 1999) employs an algorithm defining a digital filter with a notch adjustable to a plurality of notch values for a single acoustic channel. Feedback is located by comparing values resulting from the processing with the notch adjusted to different notch values. Notch filter coefficients are generated directly by the feedback detector, in which feedback is detected if a first harmonic component is sufficiently small relative to a fundamental component. The notch values are adjusted until the signals processed by the notch filter algorithm result in a minimum mean squared value over a time window. After the feedback has been located using the notch filter algorithm, digital output signals are generated by executing the algorithm with the notch adjusted to the notch value at which the minimum mean squared value results. The digital output signals then are converted to corresponding analog signals that are transmitted to a speaker. 
   The approaches of the prior art, as described heretofore, reduce acoustic feedback on an individual channel basis. However, public address systems and music delivery systems may utilize a plurality of channels, such as a stereophonic (2-channel) acoustic system. A listener is typically very sensitive to any differences of operation between channels (often referred as “stereo image”), even though the differences may seem insignificant in an absolute sense. Furthermore, a notch filter may be deployed on one channel and not the other. Such an occurrence would cause a higher degradation of the stereo image. Thus, it would be an advancement of the art to provide apparatuses and methods that assist in balancing a plurality of acoustic channels for an acoustic delivery system. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention improves processing of acoustic signals by reducing acoustic feedback in an acoustic system, including a public address system, hearing aid, teleconferencing system, hands-free communication interface, and music delivery system. An aspect of the invention is a multi-channel digital feedback reducer (DFR) system that comprises a plurality of channel elements. In an embodiment of the invention, each channel element comprises a notch filter configuration having an adaptive notch filter and an operative notch filter. The operative notch filter processes a signal received from an acoustic input device (e.g. a microphone) and provides the processed signal to an acoustic output device (e.g. a speaker), in which acoustic feedback between the acoustic input device and the acoustic output device is ameliorated. If acoustic feedback is detected by a channel element, the channel element informs other channel elements of the multi-channel DFR system about the detected feedback. In such a case, the other channel elements may continue searching for feedback on the associated channels and configure the associated operative notch filters once filter parameters are sent by the channel element to the other channel elements. Experimental data indicates that a multi-channel DFR system having intra-system communication operates faster than a DFR configuration in which each channel element operates in an independent and isolated fashion. 
   An exemplary embodiment of the invention is provided for a stereophonic DFR system. The stereophonic DFR system comprises a right channel and a left channel. Each channel element has an associated operative notch filter that can be configured for a plurality of notches having different notch depths and notch frequencies. In the exemplary embodiment, if a channel element (left channel element or right channel element) detects an acoustic feedback component on the associated channel, the channel element ameliorates the associated feedback component. Additionally, that channel element informs the other channel element through a communications pathway. The other channel element may continue to detect acoustic feedback on its channel while waiting for filter information. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention and the advantages thereof may be acquired by referring to the following description in consideration of the accompanying drawings, in which like reference numbers indicate like features and wherein: 
       FIG. 1  is an architectural diagram of a dual-channel digital feedback reducer system (DFR) in accordance with an embodiment of the invention; 
       FIG. 2  is a flow diagram illustrating an operation of a dual-channel digital feedback reducer in accordance with an embodiment of the invention; 
       FIG. 3  shows an event scenario for a dual-channel digital feedback reducer system in accordance with an embodiment of the invention; 
       FIG. 4  shows a state diagram for a channel element of a multi-channel digital feedback reducer in accordance with an embodiment of the invention; 
       FIG. 5  is a state diagram that is a continuation of the state diagram that is shown in  FIG. 4 ; 
       FIG. 6  is a flow diagram showing an algorithm for an adaptive notch filter that is utilized by an embodiment of the invention; 
       FIG. 7  is a continuation of the flow diagram that is shown in  FIG. 6 ; 
       FIG. 8  shows a topology of an adaptive notch filter that is utilized by an embodiment of the invention; 
       FIG. 9  shows a functional architecture of an operative notch filter that is utilized by an embodiment of the invention; and 
       FIG. 10  shows exemplary performance data of a dual-channel digital feedback reducer. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is an architectural diagram of a dual-channel digital feedback reducer system (DFR) system  100  in accordance with an embodiment of the invention. DFR  100  is a stereophonic system, supporting a left channel and a right channel. The left channel comprises a left channel microphone  101 , and analog to digital (A/D) converter  103 , a digital to analog (D/A) converter  107 , a left amplifier  109 , and a left speaker. The right channel comprises a right channel microphone  113 , and an analog to digital (A/D) converter  115 , a digital to analog converter (D/A)  117 , a right amplifier  119 , and a right speaker. Additionally a processor  105  processes signals from the left channel and the right channel (from A/D converter  103  and A/D converter  115 , respectively) and provides the processed signals to D/A converter  107  and D/A converter  117 , respectively. 
   Processor  105  processes the left channel (corresponding to an input  151  and an output  153 ) with an operative notch filter  123  and the right channel (corresponding to an input  155  and an output  157 ) with an operative notch filter  127 . (A functional architecture for operative notch filters  123  and  127  is discussed in the context of  FIG. 9 , although other filter topologies may be utilized in other embodiments of the invention.) Operative notch filter  123  is configured by an adaptive notch filter  125  through a control path  165  and operative notch filter  129  is configured by an adaptive filter  127  through a control path  167 . (A topology for adaptive notch filters  125  and  127  is discussed in the context of  FIG. 8 , although other topologies may be used in other embodiments of the invention.) Operative notch filter  123  or operative notch filter  129  notifies the other operative filter through communications path  159  whenever the associated adaptive filter determines the occurrence of acoustic feedback between an acoustic output device (e.g. a speaker) and an acoustic input device (e.g. a microphone) of DFR system  100  as is discussed with  FIG. 2 . (Acoustic feedback is typically characterized by a signal having a fundamental component with small harmonic components.) Operative notch filter  123  and adaptive notch filter  125  is collectively referred as a left DFR  161 , and operative notch filter  129  and adaptive notch filter  127  is collectively referred as a right DFR  163 . 
     FIG. 2  is a flow diagram illustrating an operation of dual-channel digital feedback reducer system  100  as shown in  FIG. 1  in accordance with an embodiment of the invention. In order to clarify operation of DFR system  100 , the discussion presents a perspective of the left channel, corresponding to adaptive notch filter  125  and operative notch filter  123 . The “partner” corresponds to adaptive notch filter  127  and operative notch filter  129  (that is associated with the right channel). However, the discussion is similar from the perspective of the right channel. 
   Whenever adaptive notch filter  125  detects acoustic feedback in step  201  and if the other adaptive notch filter  127  is not deploying (i.e. configuring) its operative notch filter  129  (as determined in step  203 ), adaptive notch filter  125  will configure its operative notch filter  123  by ramping an existing or a new notch for operative notch filter  123  in step  207 . Additionally, operative notch filter  123  notifies operative notch filter  129  (its “partner”) that acoustic feedback has been detected as shown in step  201 . In such a case, adaptive notch filter  127  may continue search for acoustic feedback on the right channel as shown in step  209 . However, in such a case, adaptive notch filter  127  does not configure operative notch filter  129  until adaptive notch filter  125  has completed deploying operative notch  123  and operative notch filter  123  has sent the corresponding filter parameters to operative notch filter  129 . In the embodiment, operative notch filters  123  and  129  can operate with 16 notch filters, where the number of notch filters is configurable and where each notch filter corresponds to a notch frequency and a notch depth varying from 0 dB and 18 dB. Notch depths for different notch frequencies may differ in magnitude. However, variations of the embodiment may utilize a different number of notch filters. 
   However, in step  203 , if adaptive notch filter  127  is configuring (deploying) operative notch filter  129  and if adaptive notch filter  125  detects acoustic feedback on the left channel, adaptive notch filter  125  will defer configuring operative notch filter  123  in accordance with detecting acoustic feedback on the left channel. Rather, in step  205 , operative notch filter  123  will wait for receiving filter parameters from operative notch filter  129  (its “partner”) and adaptive notch filter  125  will continue searching for feedback detection on the left channel. When operative notch filter  123  receives the filter parameters, operative notch filter  123  is configured in accordance with the received filter parameters. 
   Although the embodiment shown in  FIG. 2  utilizes a digital signal processor (i.e. DSP  105 ), other embodiments of the invention may utilize other types of components such as discrete components (e.g. operational amplifiers) and customized integrated circuits. 
     FIG. 3  shows an event scenario  300  for dual-channel digital feedback reducer system  100  in accordance with an embodiment of the invention. Event scenario  300  is one possible scenario that is supported by the embodiment and includes entries  301 - 321 , where each entry comprises a time duration  323 , an event  325 , a corresponding action  327  that is executed by left DFR  161 , and a corresponding action  329  that is executed by right DFR  163 . During time duration of entry  301 , both left and right channel DFRs (corresponding to adaptive notch filters  125  and  127 , respectively) are searching for acoustic feedback. 
   With entry  301 , both left DFR  161  and right DFR  163  are searching for acoustic feedback (as designated by “detector running”). With entry  303 , right DFR  163  detects acoustic feedback on the right channel (corresponding to an event “feedback detected on right channel”) and consequently sends a semaphore (flag) over communications path  159  to notify left DFR  161  that acoustic feedback has been detected by DFR  163 . Upon receiving the semaphore as shown in entry  305 , left DFR  161  continues searching for acoustic feedback on the left channel. If during the time that DFR  161  is waiting for filter parameters (data) from DFR  163 , DFR  161  will not deploy (configure) operative notch filter  123  in accordance with parameters determined by adaptive notch filter  125  if DFR  161  detects acoustic feedback on the left channel. Rather, adaptive notch filter  125  will continue searching for acoustic feedback on the left channel and will defer configuring operative notch filter  123  until operative notch filter  129  sends filter data (parameters) to operative notch filter  123  over communications path  159  and operative notch filter  123  is configured according to the sent filter data. With entry  305 , operative notch filter  129  either deepens an existing notch or creates a notch at a new frequency. (The operation of operative notch filters  123  and  129  are discussed in the context of  FIG. 9 .) 
   With entry  307 , operative notch filter  129  ramps the notch. (In the embodiment of the invention, the notch is increased 0.5 dB per 10 msec until the notch is deepened 3 dB, which takes approximately 60 msec. Ramping the notch ameliorates a perceptible degradation by a user of DFR system  100  by gradually inducing the notch. (With a plurality of ramping operations corresponding to detecting acoustic feedback at different times, the notch may be deepened to 18 dB in the embodiment.) In entry  309 , left DFR  161  detects acoustic feedback on the left channel and sends a semaphore to right DFR  161  over communications path  159 . However, as previously mentioned, adaptive notch filter  125  defers configuring operative notch filter  123  in accordance with the detected acoustic feedback on the left channel. 
   With entry  311  left DFR  161  waits for filter parameter information from right DFR  163 . Right DFR  163  completes ramping the associated notch and sends filter parameters to DFR  161  over communications path  159 . In the embodiment, filter parameters comprise a frequency of the notch (F i ), a bandwidth of the notch (BW i ), and a gain of the notch (G i ). The filter parameters are discussed in the context of  FIG. 9 . With entry  313 , operative notch filter  123  is configured according to filter parameters that it receives from operative notch filter  129 . With entry  313 , right DFR  163  searches for acoustic feedback on the right channel, in which a notch may be subsequently deepened or a notch at another frequency may be created. 
   With entry  315 , left DFR  161  configures operative notch filter  123  in accordance with acoustic feedback that is detected by adaptive filter  125  on the left channel. Right DFR  163  continues searching for acoustic feedback on the right channel. With entry  317 , operative notch filter  123  completes its configuration and sends filter parameters to operative notch filter  129 . Consequently, with entry  319 , operative notch filter  129  is configured in accordance with the received filter parameters. With entry  321 , both DFR  161  and DFR  163  continue search for acoustic feedback on respective channels. 
     FIG. 4  shows a state diagram  400  for a channel element (e.g. left DFR  161  and right DFR  163  as shown in  FIG. 1 ) of a multi-channel digital feedback reducer in accordance with an embodiment of the invention. As with DFR  161  and DFR  163 , the channel element may comprise an operative notch filter and an adaptive notch filter. In the embodiment, each constituent channel element conforms to state diagram  400  and a state diagram  500  (that is shown in  FIG. 5 ). In state  401 , the channel element is initiated. For example, the constituent notch filters of the associated operative notch filters are reset so that any existing notches are removed. Consequently, the channel element transitions to state  403 , in which the channel element searches for acoustic feedback on its associated channel. If the channel element detects acoustic feedback on its associated channel before another channel element detects acoustic feedback on its associated channel, in state  407  the channel element configures the associated operative notch filter in accordance with filter parameters that are determined from acoustic feedback detected on its channel. Once the filter configuration is completed, the channel element returns to state  403 . While in state  403 , if the channel element receives filter parameters from another channel element, the channel element configures its operative notch filter in accordance with the received filter parameters. The channel element returns to state  403  when the configuration of the operative notch filter is completed. (In  FIGS. 4 and 5 , the notation “NOT(A)” represents a negation of the statement “A”.) 
     FIG. 5  is a state diagram that is a continuation of state diagram  400  that is shown in  FIG. 4 . If the channel element receives a semaphore (that indicates a detection of feedback on another channel) from another channel element while the channel element is in state  403 , the channel element transitions to state  501 , in which the channel element continues searching for feedback on its channel. However, in channel  501 , if the channel element detects acoustic feedback, the channel element will not configure its associated operative notch filter until the channel element receives filter data from the other channel element that sent the semaphore. Consequently, as shown in state  503 , the channel element waits for filter parameter information from the other channel element (that sent the previous semaphore) and sends a semaphore to the other channels elements. In state  507 , the channel element receives filter parameters from the other channel element that previously sent a semaphore (as shown in transition  409  in  FIG. 4 ), and configures the associated operative notch filter in state  507 . When the configuration is completed, the channel element enters state  509 , in which the associated operative notch filter is configured in accordance with filter parameters that are derived form the detected acoustic feedback on its channel. The channel element then returns to state  401  through transition  411 . 
   If the channel element does detect acoustic feedback on its channel subsequent to receiving a semaphore from another channel element, the channel element enters state  505 , in which the channel element configures the associated operative filter in accordance with filter parameters that are received from the other channel element and returns to state  403  through transition  411 . 
   With the embodiment of the invention, interaction among the channel elements ensures that each channel element incorporates the same notch filters as the other channel elements. However, with other embodiments of the invention, varying spatial separation between channel elements of an N-channel DFR system may cause a varying acoustic correlation between different channel elements. In such a case, the channel elements may be grouped so that a channel element does not interact with all other channel elements but only with a subset of channel elements having a sufficiently high acoustic correlation. For example, channel elements that are the most spatially separated may not interact, while adjacent channel elements may interact in order to share filter information. 
     FIG. 6  is a flow diagram  600  showing an algorithm for an adaptive notch filter that is utilized by an embodiment of the invention. The algorithm is disclosed in U.S. Pat. No. 5,999,631 (“Acoustic Feedback Elimination Using Adaptive Notch Filter Algorithm”), which is incorporated by reference. Processor  105  (that executes a program corresponding to adaptive notch filters  125  and  127  as shown in  FIG. 1 ) receives a new digital input sample from A/D converters  103  and  115  every 21 microseconds as shown in step  610 . In step  612 , processor  105  performs an automatic gain control function that includes a digital peak detector with a rapid attack and slow decay. The peak detector creates a control signal that keeps the value of the signals from A/D converters  103  and  115  normalized to the digital clipping level. This feature maintains a maximum undistorted signal for processing by an adaptive filter algorithm even in the presence of weak feedback signals. 
   The input sample values resulting from automatic gain control in step  612  (i.e., values x(n)) are operated on by an adaptive notch filter algorithm in step  614 .  FIG. 8  shows a topology of adaptive notch filters  125  and  127  that are utilized by an embodiment of the invention.  FIG. 8  illustrates the adaptive notch filter algorithm in conventional filter notation. The algorithm includes addition terms A 10 -A 17 , multiplication terms M 10 -M 17 , and one clock cycle delays represented by D 10 -D 13 . During each clock cycle, a new value of k 0  is calculated and substituted in multiplication terms M 14 -M 15 . The value of k 1  is fixed at 1. In  FIG. 8 , a 0 =k 0 , a 1 =α*k 1 , therefore a 1 =α. 
   The notch filter algorithm adapts parameter k 0  until the presence of acoustic feedback, if any, is detected. A value of k is calculated according to the following equation: 
                 k   =       -     C   ⁡     (     n   +   1     )           D   ⁡     (     n   +   1     )                 (     EQ   .           ⁢   1     )               
from which k 0 (n) is calculated, where
   k′   0 ( n )=(1−γ) k′   0 ( n− 1)+γ k   0 ( n ),   C ( n+ 1)=λ C ( n )+ A ( n+ 1) B ( n+ 1),   D ( n+ 1)=λ D ( n )+ A ( n+ 1) A ( n+ 1),   A ( n+ 1)=2* s   0 ( n ),   B ( n+ 1)= s   0 ( n+ 1)+ s   0 ( n− 1), 
and
   s   0 ( n+ 1)= x ( n+ 1)− k   0 ( n )(1+α) s   0 ( n )−α* s   0 ( n− 1), 
where α is a parameter that may range in value from 0.99 to 0.999 and corresponds to the phase angle bandwidth of the notch filter that may vary from 0.0375 to 0.075 degrees.
 
   In step  614 , the value of k 0  converges on a first value at which the values resulting from the notch filter algorithm described in  FIG. 8  represent a minimum mean squared value over a time window. The time window is determined by the value of λ which is set to a value less than one, such as 0.9. Stated differently, the value of parameter k 0  converges on a first notch value at which the value of s 2   2  is minimized over a time period determined by the value of λ which preferably lies within the range 0.9 to 0.05. The algorithm illustrated in  FIG. 8  results in a value s 2  at the end of step  614 . 
   In step  616 , value s 2  is used to generate first remainder values by subtracting the values of s 2  from the input values x(n). In step  618 , a first resultant value is calculated by taking the absolute value of the first remainder values and averaging them over time. Averaging is achieved by calculating the average of the absolute value signals using the following equation:
 
 z ( n )=β* y ( n )+(1−β)* y ( n− 1)+(1−β) 2   *y ( n− 2)+ . . . +(1−β) 10   *y ( n− 10)+  (EQ. 2)
 
   The term β determines the averaging ratio, viz. the most recent sample is multiplied by the value of β and the previous value of the average output is multiplied by a term (1−β). This is the same concept as multiplying older values of y by a smaller term. Values of β are chosen for optimum performance and determine the value to which z would average to for a given signal input. 
   In step  620 , the value of k 0  for the algorithm illustrated in  FIG. 8  is set to the relationship −2k 0   2 +1, where the value of k 0  is the value obtained in step  614 . If k 0  is represented by the −cos x, then the new second value of k 0  is set equal to cos 2x. With the new second value of k 0 , the algorithm illustrated in  FIG. 8  is again executed and the resulting output value s 2  is subtracted from the input x(n) in step  622  to create second remainder values. In step  624 , a second resultant value is calculated by taking the absolute value of the second remainder values and averaging them over time as in step  618 . 
     FIG. 7  is a continuation of flow diagram  600  that is shown in  FIG. 6 . In step  726 , the ratio of the first and second resultant values obtained in steps  618  and  624  are calculated (referring to  FIG. 6 ). In step  728 , if the ratio exceeds 30 decibels, a software counter is incremented in step  732 . If the ratio does not exceed 30 decibels, then the software counter is reset in step  730 . In steps  734  and  736 , the algorithm determines whether the software counter exceeds a predetermined threshold count (F_COUNT as shown in  FIG. 7 ). The count corresponds to a time period preferably lying in the range of 50 to 100 milliseconds. If the count is exceeded, then the notch value k 0  of the filter algorithm shown in  FIG. 8  is set to the same value obtained in step  614 . In step  738 , the filter algorithm shown in  FIG. 8  is executed with the value of k 0  obtained from step  614 . Step  738  results in a substantial decrease in the magnitude of the feedback signal detected in steps  610 - 734 . Step  738  is executed as many times as necessary with k 0  set to different values corresponding to feedback detected in steps  610 - 734  at different values of k 0 . 
   The output digital signals resulting from step  738  are sent to a digital to analog converter ( 107  or  117  as shown in  FIG. 1 ). In step  740 , the algorithm waits for the next sample and returns via path  741  to step  610  (as shown in  FIG. 6 ) in order to execute another cycle of the algorithm. 
   The flow diagrams shown in  FIGS. 6 and 7  should be construed in the context of a parallel topology that is shown in  FIG. 1 . As shown in  FIG. 1 , an adaptive notch filter (e.g. adaptive filter  125 ) and an operative notch filter (e.g. operative notch filter  123 ) of a channel element (e.g. DFR element  161 ) process an acoustic signal in a parallel fashion. However, other embodiments of the invention may support other channel element topologies. 
     FIG. 9  shows a functional architecture of an operative notch filter  900  (which correspond to operative filters  123  and  129  as shown in  FIG. 1 ) that is utilized by an embodiment of the invention. In the embodiment, operative notch filter  900  comprises sixteen constituent notch filters NF 1   901 , NF 2   903 , NF 3 -NF 15  (not shown), and NF 16   905 . (The exemplary embodiment supports 16 notch filters; however, variations of the embodiment may support a different number of notch filters.) Operative notch filter  900  has a serial topology, although other embodiments may utilize other topologies such as a parallel topology. Other embodiments may utilize a different number of constituent notch filters that may be greater than or less than ten. Each consistent notch filter (e.g. NF 1   901 ) receives filter parameters (e.g. F 1 , BW 1 , and G 1  corresponding to NF 1   901 ) from a control module  907 . Control module  907  receives filter parameters F i , BW i , and G i  through communications path  159 , control path  165 , or control path  167  (as shown in  FIG. 1 ). Control module  907  stores the filter parameters for the active constituent notch filters. (If a constituent notch filter is not active, the corresponding notch filter is not configured for a notch.) Operative notch filter  900  inputs an input signal at input  151  or  155  and outputs a processed signal at output  151  or  157 . 
     FIG. 10  shows exemplary performance data  1000  of a dual-channel digital feedback reducer. The exemplary performance data corresponds to three trials  1003 ,  1005 , and  1007 , in which acoustic feedback (corresponding to frequency  1001 ) is induced. With exemplary performance data  1000 , response times are measured in which a notch of 18 dB is configured. In trials  1003 ,  1005 , and  1007 , the associated response times for a stereophonic DFR configuration is faster than that for a mono DFR configuration. In fact, with performance data  1000 , the average response time for a stereophonic DFR is approximately 0.5 seconds faster than the average response time for a mono DFR. 
   The embodiment of the invention, as shown in state diagrams  400  and  500 , may utilize different notch filter configurations. The notch filters shown in  FIGS. 1 ,  8  and  9  illustrate exemplary embodiments. 
   As can be appreciated by one skilled in the art, a computer system with an associated computer-readable medium containing instructions for controlling the computer system can be utilized to implement the exemplary embodiments that are disclosed herein. The computer system may include at least one computer such as a microprocessor, digital signal processor, and associated peripheral electronic circuitry.