Patent Publication Number: US-6665411-B2

Title: DVE system with instability detection

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The invention relates to digital voice enhancement, DVE, communication systems, and more particularly to feedback instability detection and corrective action. 
     The invention may be used in duplex systems, for example as shown in U.S. Pat. No. 5,033,082, and U.S. application Ser. No. 08/927,874, filed Sep. 11, 1997, simplex systems, for example as shown in U.S. application Ser. No. 09/050,511, filed Mar. 30, 1998, all incorporated herein by reference, and in other systems. 
     The DVE communication system includes a first acoustic zone, a second acoustic zone, a microphone at the first zone, and a loudspeaker at the second zone and electrically coupled to the microphone such that the speech of a person at the first zone can be heard by a person at the second zone as transmitted by an electrical signal from the microphone to the loudspeaker. 
     Under adverse conditions, instabilities can inadvertently cause feedback in DVE systems. This feedback causes the DVE controller outputs to diverge unbounded at the frequency of instability. The end result is a loud objectionable tonal squeal or screech that grows in magnitude. This is an abnormal operational state of the DVE system which must be detected and suppressed. 
     The present invention uses signal statistics of the electrical signal transmitted to the loudspeaker to detect a condition of instability. An instability detector detects an unstable acoustic feedback condition from the loudspeaker to the microphone by sensing a condition of the electrical signal transmitted from the microphone to the loudspeaker, and a corrective processor responds to the instability detector to modify the noted electrical signal to reduce unstable acoustic feedback. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a DVE system in accordance with the invention. 
     FIG. 2 illustrates a corrective method in accordance with the invention. 
     FIG. 3 illustrates another corrective method in accordance with the invention. 
     FIG. 4 illustrates another embodiment of a DVE system in accordance with the invention. 
     FIG. 5 illustrates a detection method in accordance with the invention. 
     FIG. 6 illustrates another detection method in accordance with the invention. 
     FIG. 7 illustrates another detection method in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a digital voice enhancement, DVE, communication system  10  including a first acoustic zone  12 , a second acoustic zone  14 , one or more microphones  16 ,  18 ,  20 ,  22 , etc. at the first zone, and one or more loudspeakers  24  at the second zone and electrically coupled by channel or line  26  to the microphones such that the speech of a person at a respective microphone at the first zone can be heard by a person at loudspeaker  24  at the second zone. The microphones may be in the same first zone, or each microphone may be in a different first zone, or some combination thereof. Gate array and switch  28  selects which microphone to connect to loudspeaker  24 , and is preferably provided by a short-time average magnitude estimating function to detect if a voice signal is present from the respective microphone, though other estimating functions may be used, for example  Digital Processing of Speech Signals,  Lawrence W. Rabiner, Ronald W. Schafer, 1978, Bell Laboratories, Inc., Prentice-Hall, pages 120-126, and also as noted in U.S. Pat. No. 5,706,344, incorporated herein by reference. Loudspeaker  24  is acoustically coupled to the microphones as shown at feedback path  30  such that the microphones are subject to acoustic feedback from loudspeaker  24 . An instability detector  32  detects an unstable acoustic feedback condition from loudspeaker  24  to microphone  16  by sensing a condition of the electrical signal transmitted from microphone  16  to loudspeaker  24 , and likewise for the remaining microphones. A corrective processor  34  responds to the instability detector to modify the electrical signal transmitted to loudspeaker  24  to reduce unstable acoustic feedback. Instability detector  32  and corrective processor  34  prevent divergence and unbounded growth of the magnitude of the electrical signal at  26  otherwise caused at frequencies of instability in the noted unstable acoustic feedback condition. The noted sensed condition of the electrical signal may be magnitude of the electrical signal greater than a designated threshold, power (magnitude 2 ) of the electrical signal greater than a designated threshold, or, preferably, the sinusoidal characteristic of the electrical signal, i.e. the electrical signal becoming sinusoidal in nature, to be described. 
     In the noted preferred embodiment, instability detector  32  is provided by a model  36  modeling the noted electrical signal from output  38  of the gate array and switch  28  as a filter model with filter coefficients, for example as in U.S. Pat. Nos. 4,677,676, 4,677,677, 4,987,598, 5,033,082, 5,172,416, 5,206,911, 5,386,477, 5,396,561, 5,621,803, 5,680,337, 5,706,344, 5,710,822, 5,715,320, all incorporated herein by reference. An unstable feedback condition in the DVE system is detected by determining that the DVE output at  38  has become sinusoidal, or tonal, in nature. The tonal condition is identified by continually modeling the DVE output at  38  as a second order all pole filter and monitoring one of the filter coefficients. Under normal voice output conditions, the variation of such filter coefficient is large. At the onset of feedback, the DVE output at  38  becomes sinusoidal, and the variation of the filter coefficient becomes very small. Instability detector  32  includes detection logic  40  monitoring the filter coefficient and outputting a feedback indicator signal at  42  to corrective processor  34  in response to a given condition of the filter coefficient. In contrast to the above noted method of outputting feedback indicator signal  42  when the magnitude or power of the electrical signal is greater than a designated threshold as shown at greater-than sign  44 , the tonal sinusoid sensing of the preferred detection method outputs feedback indicator signal  42  when the variation of the noted filter coefficient is below a designated threshold as shown at less-than sign  46 . Model  36  is preferably a second order all pole filter model, as noted above. Detection logic  40  outputs feedback indicator signal  42  to corrective processor  34  when the variation of the filter coefficient is below a designated threshold. Corrective processor  34  includes a variable gain element  48  applying variable gain to the electrical signal after sensing by instability detector  32 . The corrective processor responds to the noted sensed condition of the electrical signal to vary the gain applied at  48 . The electrical signal at  38  is supplied to parallel branches  50  and  52 . Branch  50  is supplied to variable gain element  48  and loudspeaker  24 . Branch  52  is supplied to instability detector  32  and corrective processor  34 . 
     In one embodiment, corrective processor  34  responds to the noted sensed condition from instability detector  32  by reducing gain, FIG. 2, e.g. setting the DVE variable gain at element  48  to zero, then instituting a delay, e.g. wait 1 to 5 seconds, then resetting the gate array and switch  28  to an initialized condition such that the latter may again sense the active microphone, and then increasing the gain, e.g. setting the DVE variable gain to 1 or back to its value prior to the reducing of the gain. In another embodiment, FIG. 3, the gain is reduced, e.g. by half, and then a delay is instituted, e.g. 0.5 seconds, and then the gate array and switch is reset, and then monitoring of the instability detector is resumed. 
     In preferred form, instability detector  36  uses Prony&#39;s method of sinusoidal identification as described in  Handbook For Digital Signal Processing,  Sanjit K. Mitra and James F. Kaiser, 1993, John Wiley &amp; Sons, pages 1193-1195. This method is used to identify the sinusoidal components of an input signal. FIG. 4 shows implementation and uses like reference numerals from above where appropriate to facilitate understanding. Gate array and switch  28  is broken out into its respective gates  54 ,  56 ,  58 ,  60 , etc., one for each microphone, and DVE switch component  62 . The detector uses the Prony method for a number of poles equal to 2 to match the electrical signal to a single sinusoid, which requires a data sample size of only 4, which small size is considered desirable. 
     Prony&#39;s method with p=2, N=4 gives the a coefficients of an all pole model: 
     
       
         a=[1 a1 a2] 
       
     
     where                x        (   n   )       ≡                [       x        (     k   -   3     )            x        (     k   -   2     )            x        (     k   -   1     )            x        (   k   )         ]     ≡     [       x        (   0   )            x        (   1   )            x        (   2   )            x        (   3   )         ]                   a                 2     =                      -     x        (   3   )         ·     x        (   1   )         +       x        (   2   )       2             x        (   1   )       2     -       x        (   0   )       ·     x        (   2   )                           a                 1     =                    -     x        (   2   )         -     a                   2   ·     x        (   0   )               x        (   1   )                               
     The roots of a tell the pole locations, and the angle of the pole is the frequency of the sinusoid. 
     The DVE output is continually modeled using Prony&#39;s method, looking for a trend in the results that indicate a tone is present. The “results” to be monitored can be the a1 &amp; a2 coefficients, the location of the poles, the amplitude of the poles, etc., all of which will stabilize when the signal is sinusoidal. In the preferred embodiment, only the a2 coefficient need be calculated. The present detection method is based on the fact that under feedback conditions when the DVE output  38  is sinusoidal, the a2 coefficient becomes very stable compared to all other normal operating conditions, i.e. under normal operating conditions the a2 coefficient is random. This method of feedback detection offers the following advantages over other detection methods: a) such method creates a single parameter whose value answers the question as to whether the output is sinusoidal; b) such method differentiates between abnormal sinusoidal signals and normal voice signals; c) such method is not prone to false detections that occur in output power monitoring methods under conditions of wind noise, door slams and microphone thumps; and d) such method requires a buffer size of only four data samples, as compared to buffer sizes of 512 or more data samples required by fast Fourier transform techniques or correlation based statistical methods. 
     In one form, the detection method compares the a2 coefficient to 1.0, FIG.  5 . In a pure tone, the second order all pole model is of the form 
     
       
           a ( z )=1 +2 cosθ· z   −1   +z   −2  or [ a 0  a 1  a 2 ]=[1 2 cosθ1] 
       
     
     Therefore, when the signal is tonal in nature, a2 will equal 1. The detection method observes the average magnitude of the difference of a2 and 1.0. The average magnitude is obtained using a typical averaging equation: 
     
       
           avg   —   mag ( k+ 1)= avg   —   mag ( k )+1/( tau*fs )* ( abs (input( k ))− avg   —   mag ( k ))  
       
     
     wherein input(k)=a2(k)−1.0 and a2(k) is calculated from Prony&#39;s equation shown above. 
     In another form, FIG. 6, the method uses the fact that under sinusoidal conditions the a2 coefficient is very stable, i.e. its difference about its mean value is small. This characteristic is used to detect tonal or periodic signals by measuring the average magnitude of a2 (k)−a2(k−1). The gate truth and gate energy signals indicate whether there is voice activity and the amount of power on the respective microphone, respectively, and the active mic gate truth and active mic gate energy signals provide the noted signals for comparison for the active microphone. The gate information could be used to only enable the detection logic when there is signal or voice activity from the microphone and/or when signal power or energy from the microphone is above a given level, i.e. the detection logic is enabled to output the feedback indicator signal to the corrective processor only by an activity signal from the microphone, i.e. active mic or gate truth signal, and/or signal energy or power from the microphone above a given level, i.e. active mic gate energy. This will avoid detection “falses” when the input signal is zero or near zero. 
     FIG. 7 shows a modification of the above method of FIG.  6  and is more robust. FIG. 7 measures the variance of the a2 coefficient. The variance of a signal is defined as the E{X 2 }−(E{X}) 2 . For zero mean signals, (E{X}=0), the variance is simply E{X 2 }, which is the average power. Since X=a2(k)−a2(k−1) is a simple high pass filter, mean(X)=0, and its variance can be monitored by monitoring its average power E{X 2 }. The average power of the difference is monitored using a typical averaging scheme: 
     
       
           avg   —   pwr ( k+ 1)= avg   —   pwr ( k )+1/( tau*fs )*(input( k )  2   −avg   —   pwr ( k ))  
       
     
     wherein input(k)=a2(k)−a2(k−1) and a2(k) is calculated from Prony&#39;s equation shown above. 
     It is recognized that various equivalents, alternatives and modifications are possible within the scope of the appended claims.