Abstract:
A method of locating a talker in a reverberant environment comprises receiving multiple audio signals from a microphone array that include direct path audio signal and reverberation signal components. The direct path audio signal components of the multiple audio signals are detected and are used to weight the multiple audio signals. A position estimate based on the weighted audio signals is then calculated. Periods of speech activity are detected and a final position estimate is generated during the periods of speech activity.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to audio systems and in particular to a method and system for improving talker localization in a reverberant environment.  
         BACKGROUND OF THE INVENTION  
         [0002]    Localization of audio sources is required in many applications, such as teleconferencing, where the audio source position is used to steer a high quality microphone towards the talker. In video conferencing systems, the audio source position may additionally be used to steer a video camera towards the talker.  
           [0003]    It is known in the art to use electronically steerable arrays of microphones in combination with location estimator algorithms to pinpoint the location of a talker in a room. In this regard, high quality and complex beamformers have been used to measure the power at different positions. In such systems, location estimator algorithms locate the dominant audio source using power information received from the beamformers. The foregoing prior art methodologies are described in  Speaker localization using a steered Filter and sum Beamformer, N. Strobel, T. Meier, R. Rabenstein,  presented at the Erlangen work shop 99, vision, modeling and visualization, Nov. 17-19th, 1999, Erlangen, Germany.  
           [0004]    U.K. Patent Application No. 0016142 filed on Jun. 30, 2000 for an invention entitled “Method and Apparatus For Locating A Talker” discloses a talker localization system that includes an energy based direction of arrival (DOA) estimator. The DOA estimator estimates the audio source location based on the direction of maximum energy at the output of the beamformer over a specific time window. The estimates are filtered, analyzed and then combined with a voice activity detector to render a final position estimate of the audio source location.  
           [0005]    In highly reverberant environments, reflected acoustic signals can result in miscalculation of the direction of arrival of the audio signals generated by the talker. This is due to the fact that the energy of the audio signals picked up by the beamformer can be stronger in the direction of the reverberation signals than for the direct path audio signals. The effects of reverberation have most impact on audio source localization at the beginning and the end of a speech burst. Miscalculation of the direction of arrival of the audio signals at the beginning of a speech burst can be caused by a strong reverberation signal having a short delay path. As a result, the direct path audio signal may not have dominant energy for a long enough period of time before being masked by the reverberation signal. In this situation, the DOA estimator can miss the beginning of the speech burst and lock on to the reverberation signal. Miscalculation of the direction of arrival of the audio signals at the end of a speech burst can caused by a reverberation signal that masks the decaying tail of the direct path audio signal resulting in beam steering in the wrong direction until the next speech burst occurs.  
           [0006]    In an attempt to deal with the effects of reverberation during talker localization, two approaches have been considered. One approach uses a priori knowledge of the room geometry and the reverberation (interference) and noise sources therein. Different space regions within the room are pre-classified as containing a reverberation or noise source. The response of the beamformer is then minimized at locations corresponding to the locations of the pre-classified reverberation and noise sources.  
           [0007]    The second approach uses a computationally complex Crosspower Spectrum Phase (CPS) analysis to calculate Time Delay Estimates (TDE) between the microphones of the microphone array. Unfortunately, it is known that performance of TDE methods degrade dramatically in the highly reverberant conditions.  
           [0008]    As will be appreciated, the above-described approaches to deal with the effects of reverberation suffer disadvantages. Accordingly, a need exists for an improved method for talker localization in a reverberant environment. It is therefore an object of the present invention to provide a novel method and system for talker localization in a reverberant environment.  
         SUMMARY OF THE INVENTION  
         [0009]    Accordingly, in one aspect of the present invention there is provided a method of locating a talker in a reverberant environment comprising the steps of:  
           [0010]    receiving multiple audio signals from a microphone array, said audio signals including direct path audio signal and reverberation signal components;  
           [0011]    calculating position estimates of a source of said audio signals based on said audio signals;  
           [0012]    rapidly detecting the direction of the direct path audio signal component of said multiple audio signals based on said calculated position estimates;  
           [0013]    using the rapidly detected direction to weight the calculated position estimates;  
           [0014]    detecting periods of speech activity; and  
           [0015]    generating a final position estimate of said source during said periods of speech activity based on the weighted position estimates.  
           [0016]    According to another aspect of the present invention there is provided a method of locating a talker in a reverberant environment comprising the steps of:  
           [0017]    receiving multiple audio signals from a microphone array, said audio signals including direct path audio signal and reverberation signal components;  
           [0018]    calculating position estimates of a source of audio signals based on the audio signals received from said microphone array;  
           [0019]    detecting periods of speech activity;  
           [0020]    generating a final position estimate of said source during said periods of speech activity based on said position estimates; and  
           [0021]    inhibiting the final position estimate from being changed if no interval of silence separates the calculated position estimates.  
           [0022]    According to yet another aspect of the present invention there is provided a talker localization system comprising:  
           [0023]    a microphone array receiving multiple audio signals, said audio signals including direct path audio signal and reverberation signal components;  
           [0024]    an estimator calculating position estimates of a source of said audio signals based on said audio signals;  
           [0025]    an early detect module rapidly detecting the direction of the direct path audio signal component of said multiple audio signals based on said calculated position estimates;  
           [0026]    a weighting module using the rapidly detected direction to weight the calculated position estimates;  
           [0027]    a voice activity detector detecting periods of speech activity; and  
           [0028]    decision logic generating a final position estimate of said source during said periods of speech activity based on the weighted position estimates.  
           [0029]    According to still yet another aspect of the present invention there is provided a talker localization system comprising:  
           [0030]    a microphone array receiving multiple audio signals, said audio signals including direct path audio signal and reverberation signal components;  
           [0031]    an estimator calculating position estimates of a source of audio signals based on the audio signals received from said microphone array;  
           [0032]    a voice activity detector detecting periods of speech activity; and  
           [0033]    decision logic generating a final position estimate of said source during said periods of speech activity based on said position estimates and inhibiting the final position estimate from being changed if no interval of silence separates the calculated position estimates.  
           [0034]    The present invention provides advantages in that talker localization in reverberant environments is achieved without requiring a priori knowledge of the room geometry including the reverberation and noise sources therein and without requiring complex computations to be carried out. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    Embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which:  
         [0036]    [0036]FIG. 1 a  is a schematic block diagram of a prior art talker localization system including a voice activity detector, an estimator and decision logic;  
         [0037]    [0037]FIG. 1 b  is a state machine of the decision logic of FIG. 1 a;    
         [0038]    [0038]FIG. 2 a  shows an audio signal energy envelope including two speech bursts in a non-reverberant environment;  
         [0039]    [0039]FIG. 2 b  shows the output of the voice activity detector of FIG. 1 a  generated in response to the audio signal energy envelope of FIG. 2 a;    
         [0040]    [0040]FIG. 2 c  shows the output of the estimator of FIG. 1 a  generated in response to the audio signal energy envelope of FIG. 2 a;    
         [0041]    [0041]FIG. 2 d  shows the position estimate output of the decision logic of FIG. 1 a  generated in response to the output of the voice activity detector and estimator;  
         [0042]    [0042]FIG. 3 a  shows an audio signal energy envelope including two speech bursts and accompanying reverberation signals due to a reverberant environment;  
         [0043]    [0043]FIG. 3 b  shows the output of the voice activity detector of FIG. 1 a  generated in response to the audio signal energy envelope of FIG. 3 a;    
         [0044]    [0044]FIG. 3 c  shows the output of the estimator of FIG. 1 a  generated in response to the audio signal energy envelope of FIG. 3 a;    
         [0045]    [0045]FIG. 3 d  shows the position estimate output of the decision logic of FIG. 1 a  generated in response to the output of the voice activity detector and estimator;  
         [0046]    [0046]FIG. 4 a  shows an audio signal energy envelope including two speech bursts and accompanying reverberation signals due to a moderate reverberant environment;  
         [0047]    [0047]FIG. 4 b  shows the output of the voice activity detector of FIG. 1 a  generated in response to the audio signal energy envelope of FIG. 4 a;    
         [0048]    [0048]FIG. 4 c  shows the output of the estimator of FIG. 1 a  generated in response to the audio signal energy envelope of FIG. 4 a;    
         [0049]    [0049]FIG. 4 d  shows the position estimate of the decision logic of FIG. 1 a  generated in response to the output of the voice activity detector and estimator after filtering;  
         [0050]    [0050]FIG. 5 is a schematic block diagram of a talker localization system that is robust in a reverberant environment in accordance with the present invention including an early detect module, an energy history module and a weighting function module;  
         [0051]    [0051]FIG. 6 is a timing diagram for direct path audio signals and reverberation signals and voice activity detection;  
         [0052]    [0052]FIG. 7 is a state machine of the early detect module shown in FIG. 5;  
         [0053]    [0053]FIG. 8 is a schematic block diagram of the weighting function module shown in FIG. 5;  
         [0054]    [0054]FIG. 9 is a state machine of the decision logic forming part of the talker localization system of FIG. 5;  
         [0055]    [0055]FIGS. 10 a  to  10   d  are identical to FIGS. 4 a  to  4   d;    
         [0056]    [0056]FIG. 10 e  shows the timing of a watchdog timer forming part of the decision logic of FIG. 9; and  
         [0057]    [0057]FIG. 10 f  shows the position estimate output of the decision logic of FIG. 9. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0058]    The present invention relates to a talker localization system and method that is robust in reverberant environments without requiring a priori knowledge of the room geometry and the reverberation and noise sources therein and without requiring complex computations to be carried out. The direction of direct path audio is rapidly detected and the direction is used to weight position estimates output to the decision logic. The decision logic is also inhibited from switching position estimate direction if no interval of silence separates a change in position estimates received by the decision logic. For better understanding, a talker localization system that is accurate in low reverberant environments will firstly be described.  
         [0059]    Turning now to FIG. 1 a,  a talker localization system that is accurate in low reverberant environments such as that described in U.K. Patent Application No. 0016142 filed on Jun. 30, 2000 is shown and is generally identified by reference numeral  90 . As can be seen, talker localization system  90  includes an array  100  of omni-directional microphones, a spectral conditioner  110 , a voice activity detector  120 , an estimator  130 , decision logic  140  and a steered device  150  such as for example a beamformer, an image tracking algorithm, or other system.  
         [0060]    The omni-directional microphones in the array  100  are arranged in circular microphone sub-arrays, with the microphones of each sub-array covering segments of a 360° array. The audio signals output by the circular microphone sub-arrays of array  100  are fed to the spectral conditioner  110 , the voice activity detector  120  and the steered device  150 .  
         [0061]    Spectral conditioner  110  filters the output of each circular microphone sub-array separately before the output of the circular microphone sub-arrays are input to the estimator  130 . The purpose of the filtering is to restrict the estimation procedure performed by the estimator  130  to a narrow frequency band, chosen for best performance of the estimator  130  as well as to suppress noise sources.  
         [0062]    Estimator  130  generates first order position or location estimates, by segment number, and outputs the position estimates to the decision logic  140 . During operation of the estimator  130 , a beamformer instance is “pointed” at each of the positions (i.e. different attenuation weightings are applied to the various microphone output audio signals). The position having the highest beamformer output is declared to be the audio signal source. Since the beamformer instances are used only for energy calculations, the quality of the beamformer output signal is not particularly important. Therefore, a simple beamforming algorithm such as for example, a delay and sum beamformer algorithm, can be used, in contrast to most teleconferencing implementations, where high quality beamformers executing filter and sum beamformer algorithms are used for measuring the power at each position.  
         [0063]    Voice activity detector  120  determines voiced time segments in order to freeze talker localization during speech pauses. The voice activity detector  120  executes a voice activity detection (VAD) algorithm. The VAD algorithm processes the audio signals received from the circular microphone sub-arrays and generates output signifying the presence or absence of voice in the audio signals received from the circular microphone sub-arrays. The output of the VAD algorithm is then used to render a voice or silence decision.  
         [0064]    Decision logic  140  is better illustrated in FIG. 1 b  and as can be seen, decision logic  140  is a state machine that uses the output of the voice activity detector  120  to filter the position estimates received from estimator  130 . The position estimates received by the decision logic  140  when the voice activity detector  120  generates silence decision logic output (i.e. during pauses in speech), are disregarded (steps  300  and  320 ). Position estimates received by the decision logic  140  when the voice activity detector  120  generates voice decision logic output are stored (step  310 ) and are then subjected to a verification process. During the verification process, the decision logic  140  waits for the estimator  130  to complete a frame and repeat its position estimate a threshold number of times, n, including up to m&lt;n mistakes.  
         [0065]    A FIFO stack memory  330  stores the position estimates. The size of the FIFO stack memory  330  and the minimum number n of correct position estimates needed for verification are chosen based on the voice performance of the voice activity detector  120  and estimator  130 . Every new position estimate which has been declared as voiced by activity detector  120  is pushed into the top of FIFO stack memory  330 . A counter  340  counts how many times the latest position estimate has occurred in the past, within the size restriction M of the FIFO stack memory  330 . If the current position estimate has occurred more than a threshold number of times, the current position estimate is verified (step  350 ) and the estimation output is updated (step  360 ) and stored in a buffer (step  380 ). If the counter  340  does not reach the threshold n, the counter output remains as it was before (step  370 ). During speech pauses no verification is performed (step  300 ), and a value of 0xFFFF(xx) is pushed into the FIFO stack primary  330  instead of the position estimate. The counter output is not changed.  
         [0066]    The output of the decision logic  140  is a verified final position estimate, which is then used by the steered device  150 . If desired, the decision logic  140  need not wait for the estimator  130  to complete frames. The decision logic  140  can of course process the outputs of the voice activity detector  120  and estimator  130  generated for each sample.  
         [0067]    Turning now to FIGS. 2 a  to  2   d , an example of how the talker localization system  90  determines the audio source location of a single talker that is located in the Z direction assuming no noise or reverberation sources are present is shown. As can be seen, FIG. 2 a  illustrates an audio signal energy envelope including two speech bursts SB 1  and SB 2  picked up by the array  100  and fed to the voice activity detector  120  and estimator  130 . When the voice activity detector  120  receives the speech bursts, the speech bursts are processed by the VAD algorithm. FIG. 2 b  illustrates the output of the voice activity detector  120  indicating detected voice and silence segments of the audio signal energy envelope. FIG. 2 c  illustrates the output of the estimator  130 , where N is the number of equally spaced segments, each having a size equal to 2π/N. The position estimates generated by the estimator  130  during the silence periods are derived from background noise and therefore may vary from one time point to another. FIG. 2 d  illustrates the audio source location result (final position estimate) generated by the decision logic  140  in response to the output of the voice activity detector  120  and estimator  130 .  
         [0068]    Turning now to FIGS. 3 a  to  3   d , an example of how the talker localization system  90  attempts to determine the audio source location of a single talker in a reverberant environment is shown. As can be seen, FIG. 3 a  illustrates an audio signal energy envelope including two speech bursts SB 3  and SB 4  accompanied by two reverberation signals RS 1  and RS 2 . The two speech bursts SB 3  and SB 4  are assumed to arrive at the array  100  from the Z direction while the reverberation signals are assumed to arrive at the array  100  from the Y direction. FIG. 3 b  illustrates the output of the voice activity detector  120  indicating detected voice and silence segments of the audio signal energy envelope. FIG. 3 c  illustrates the output of the estimator  130 . As can seen, the estimator  130  classifies the speech bursts SB 3  and SB 4  as an audio source location for the interval Td. FIG. 3 d  illustrates the audio source location result generated by the decision logic  140  in response to the output of the voice activity detector  120  and estimator  130 . Although the estimator  130  classifies the speech bursts SB 3  and SB 4  as the audio source location for the interval Td, the interval Td is not sufficient for the decision logic  140  to select the Z direction as the valid audio source location. Since the reverberation signals RS 1  and RS 2  have dominant energy most of the time, the decision logic  140  incorrectly selects the Y direction as the valid audio source location.  
         [0069]    [0069]FIG. 4 a  illustrates an audio signal energy envelope in a moderate reverberant environment that may result in incorrect position estimates being generated by the talker localization system  90 . As can be seen, the audio signal energy envelope includes two speech bursts SB 5  and SB 6  accompanied by two reverberation signals RB 3  and RB 4 . The two speech bursts SB 5  and SB 6  are assumed to arrive at array  100  from the Z direction while the reverberation signals RB 3  and RB 4  are assumed to arrive at the array  100  from the Y direction. FIG. 4 b  illustrates the output of the voice activity detector  120  indicating detected voice and silent segments of the audio signal energy envelope. FIG. 4 c  illustrates the output of estimator  130 . FIG. 4 d  illustrates the position estimate generated by the decision logic  140  after filtering.  
         [0070]    In this situation, although the reverberation signals may have low energy, the long delay of the reverberation signals RS 3  and RS 4  may result in the decision logic  140  selecting the direction of the reverberation signals as the valid audio source location at the end of the speech bursts. This is due to the fact that even though the direct path audio signals having a higher energy for almost the entire duration of the speech bursts, the decaying tails of the speech bursts SB 5  and SB 6  fall below the energy level of the reverberation signals RS 3  and RS 4  resulting in the estimator  130  locking onto the Y direction if the delay path of the reverberation signals exceeds the decision logic threshold.  
         [0071]    Turning now to FIG. 5, a talker localization system that is robust in reverberant environments in accordance with the present invention is shown and is generally identified by reference numeral  390 . As can be seen, talker localization system  390 , similar to that of the previous embodiment, includes an array  400  of omni-directional microphones, a spectral conditioner  410 , a voice activity detector  420 , an estimator  430 , decision logic  440  and a steered device  450 .  
         [0072]    However, unlike the talker localization system  90 , talker localization system  390  further includes a mechanism to detect rapidly the direction of direct path audio and to weight position estimates output by the estimator. As can be seen, the mechanism includes an early detect module  500 , an energy history module  510  and a weighting function module  520 . Early detect module  500  receives the position estimates output by estimator  430  and the voice/silence decision logic output of the voice activity detector  420 . Energy history module  510  communicates with the estimator  430 . Weighting function module  520  receives the position estimates output by estimator  430  and the output of the early detect module  500 . The output of the weighting function module  520  is fed to the decision logic  440  together with the output of the voice activity detector  420  to enable the decision logic  440  to generate audio source location position estimates.  
         [0073]    The energy history module  510  accumulates output energy values for all beamformer instances of the estimator  130  in a circular buffer and thus, provides a history of the energy for a time interval T. Time interval T is sufficient so that energy values are kept for a period of time that is expected to be longer than the reverberation path in the room. The early detect module  500  calculates a position estimate for the direct path audio signal based on the rapid detection of a new speech burst presence. The weighting function module  520  performs weighting of the position estimates received from the estimator  130  and from the early detect module  500 . The weighting is based on the energies of the relevant position estimates provided by the energy history module  510 .  
         [0074]    The early detect module  500 , energy history module  510  and weighting function module  520  allow the talker localization system  390  to determine reliably audio source location in reverberant environments. Specifically, the early detect module  500 , energy history module  510  and weighting function module  520  exploit the fact that when a silence period is interrupted by a speech burst, the direct path audio signal arrives at the array  100  before the reverberation signals. If the direction of the direct path audio signals is determined on a short time interval relative to the delay of the reverberation signals, then the correct audio source location can be identified at the beginning of the speech burst. Once the early detection of the direct path audio signal direction is complete, the position estimates output by estimator  130  are weighted through the weighting function module  520  based on the output energy of the corresponding beamformer. Thus, the location corresponding to the early detect position estimate generated by the early detect module  500  is assigned a higher weight than all others. The energy of the reverberation signals even in the highly reverberant rooms rarely exceeds the energy of the direct path audio signal. As a result, the reverberation signals are filtered out by the weighting function module  520 .  
         [0075]    [0075]FIG. 6 is a timing diagram for a direct path audio signal and a reverberation signal together with voice activity detection, where:  
         [0076]    T d  is the time interval when the direct path audio signal has dominant energy;  
         [0077]    T r  is the time interval when the reverberation signal has dominant energy;  
         [0078]    T loc  is the minimum time interval required for an audio source to have dominant energy in order for the decision logic  440  to yield a position estimate; and  
         [0079]    T ed  is the minimum time interval required for an audio signal to have dominant energy in order for the early detect module  500  to yield a position estimate.  
         [0080]    The early detect module  500  operates on principles similar to those of the decision logic  440 . Specifically, the early detect module  500  is a state machine that combines the output of the voice activity detector  420  and the estimator  430  as shown in FIG. 7. The early detect module  500  accumulates a number of position estimates provided by the estimator  430  (step  610 ) and stores the position estimates in a FIFO stack memory (step  630 ). A check is then made to determine if the early detect module  500  is in a hunt state (step  700 ). If so, the early detect module  500  waits for the localization algorithm of the estimator  430  to repeat its estimation a predetermined number of times (M) out of a total accumulated estimates (N) (step  640 ). The early detect module  500  disregards the position estimates during speech pauses (steps  600  and  620 ). The numbers N and M are significantly smaller than the corresponding numbers in the decision logic  440 . Typically the decision logic  440  yields a final position estimate after a duration Tl oc =30-40 ms. The early detect module  500  provides its position estimate after a duration T ed =10-15 ms.  
         [0081]    A counter  650  counts how many times the latest position estimate has occurred in the past within the size restriction M. When the current position estimate has occurred more than a first threshold number of times, the state of the early detect module  500  is set to a confirm state (step  710 ) and the early detect position estimate is output (step  670 ).  
         [0082]    At step  700 , if the early detect module  500  is in the confirm state (i.e. the early detect module  500  has previously determined an early detect position estimate), a counter  680  counts additional occurrences of the early detect position estimate (step  675 ). In this state, when the early detect position estimate occurs less than a second threshold number of times within a predetermined window, the state of the early detect module  500  is changed back to the hunt state (step  720 ) and the output of the early detect module  500  to the weighting function module  52  is turned off (step  730 ).  
         [0083]    The weighting function module  520  is responsive to the early detect module output state. When the early detect module  500  is not in the confirm state (i.e. it does not have a valid position estimate at its output), the weighting function module  520  is transparent meaning that the output of the estimator  430  is passed directly to the decision logic  440 . When the early detect module  500  is in the confirm state and has a valid position estimate at its output, the weighting function module  520  generates position estimates (PE) as following:  
         P                 E     =     {                          E                 S                 T     ,       if                   Energy        [   EST   ]         &gt;     k   *   max        {     Energy        [     ED                 _                 EST     ]       }         ,                                ED                 _                 E                 ST     ,   otherwise                                     
 
         [0084]    where:  
         [0085]    Energy[EST] is the energy of the beamformer instance positioned in the direction of the position estimate at the output of the estimator  430 ;  
         [0086]    max{Energy[ED EST]} is the maximum energy of the beamformer instance positioned in the direction of the position estimate generated by the early detect module  500  over a time interval T (Interval T is significant to accommodate for the longest expected delay due to reverberations signals); and  
         [0087]    k is the weighting coefficient (value less than 1, depends on the reverberant conditions).  
         [0088]    [0088]FIG. 8 is a state machine of the wieghting function module  520 .  
         [0089]    [0089]FIG. 9 better illustrates the decision logic  440  and as can be seen, decision logic  440  is a state machine that uses the output of the voice activity detector  420  to filter the position estimates received from the weighting function  520 . Decision logic  440  is similar to decision logic  140  but further includes a mechanism to inhibit its final position estimate output from switching direction if no interval of silence separates a change in position estimates received from the weighting function  520 . The position estimates received by the decision logic  440  when the voice activity detector  420  generates silence decision logic output are disregarded (steps  800  and  820 ). Position estimates received by the decision logic  440  when the voice activity detector  420  generates voice decision logic output are stored (step  810 ) and are then subjected to a verification process. During the verification process, the decision logic  440  waits for the estimator  430  to complete a frame and repeat its position estimate a predetermined number of threshold times.  
         [0090]    A FIFO stack memory  830  stores the position estimates. A counter  840  counts how many times the latest position estimate has occurred in the past within the size restriction N of the FIFO stack memory  830 . At each count, a watchdog timer is incremented (step  900 ). The period of the watchdog timer is set to value that is expected to be longer than the delay of the reverberation signal path. If the current position estimate has occurred more than M times, the current position estimate is verified provided the current position estimate repeats for a time interval that is longer than the delay of the reverberation path (step  910 ). If the time interval of the current position estimate is longer than that delay of the reverberation path, the watchdog timer is reset (step  920 ), the final position estimate is updated ( 860 ) and is stored in a buffer (step  880 ).  
         [0091]    If the time interval of the current position estimate is less than the period of the watchdog timer, which is expected to be more than delay of the reverberation path, the watchdog timer is examined (step  930 ) to determine if it has expired. If so, the watchdog timer is reset (step  920 ) and the decision logic state machine proceeds to step  860 . If the watchdog timer has not expired, the watch dog timer is incremented (step  900 ).  
         [0092]    As will be appreciated, the watchdog timer is only activated if new position estimates follow a previous position estimate without any interval of silence therebetween. This inhibits an extra delay in localization during the new speech burst and thus, preserves fast reaction on new speech bursts while avoiding any extraneous switching due to long delay reverberation signals. FIG. 10 e  illustrates the timing of the watchdog timer and FIG. 10 f  illustrates the decision logic output in response to the watchdog timer and to the signals of FIGS. 10 a  to  10   d.    
         [0093]    Although the talker localization system is described as including both the mechanism to detect rapidly the direction a speech burst and the mechanism to inhibit position estimate switching in the event of reverberation signals with long delay paths, those of skill in the art will appreciate that either mechanism can be used in a talker localization system to improve talker localization in reverberant environments.  
         [0094]    Although a preferred embodiment of the present invention has been described, those of skill in the art will appreciate that variations and modifications may be made without departing from the spirit and scope thereof as defined by the appended claims.