Abstract:
An automatic speech recognition apparatus and method with a front end feature extractor that improves recognition performance under adverse acoustic conditions are disclosed. The inventive feature extractor is characterized by a critical bandwidth spectral resolution, an emphasis on slow changes in the spectral structure of the speech signal, and adaptive automatic gain control. In one the feature extractor includes a feature generator configured to compute short-term parameters of the speech signal, a filter system configured to filter the time sequences of the short-term parameters, and a normalizer configured to normalize the filtered parameters with respect to one or more previous values of the filtered parameters. Accordingly, the feature extractor is operable to carry out the following steps: computing short-term parameters of the speech signal; filtering time sequences of the short-term parameters; and normalizing the filtered parameters with respect to one or more previous values of the filtered parameters. The steps of filtering and normalizing preferably are performed independently of one another.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of provisional application Ser. No. 60/116,684, filed Jan. 20, 1999, entitled “Perceptually Inspired Signal-Processing Strategies for Robust Speech Recognition in Reverberant Environments,” which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Automatic speech recognition has only recently emerged from the research laboratory as a viable technology. Among the automatic speech recognition systems that have been developed are large-vocabulary document dictation systems and automated telephone directory systems. Large vocabulary dictation systems typically require the use of head-mounted close-talking microphones, a relatively quiet operating environment and a considerable amount of speaker adaptation. Telephone-based systems, on the other hand, are operable over a wide range of telephone channel conditions with relatively little or no speaker adaptation; however, users typically are limited with respect to the kinds of speech input that is recognizable. For example, such systems typically require discontinuous speech input or limit the grammar or vocabulary of the recognizable speech. 
     The performance of all automatic speech recognition systems degrades when acoustic interference is present in the input speech signal. Such interference may include one or more of the following: extraneous sounds (additive noise) received from the speaker&#39;s environment or the communication channel, spectral shaping or nonlinear distortion imposed by the microphone or communication channel, and reverberation from the room in which the speaker is talking. 
     SUMMARY OF THE INVENTION 
     The invention features an automatic speech recognition apparatus and method with a front end feature extractor that improves recognition performance under adverse acoustic conditions. The inventive feature extractor is characterized by a critical bandwidth spectral resolution, an emphasis on slow changes in the spectral structure of the speech signal, and adaptive automatic gain control. The use of critical-band-like frequency resolution reduces the recognizer&#39;s sensitivity to speaker-dependent signal characteristics and enhances the recognizer&#39;s sensitivity to speech-dependent signal characteristics. The emphasis on slow changes in the spectral structure of the speech signal focuses the recognizer on the primary carrier of linguistic information in the speech signal, thereby improving the accuracy of the recognizer. The use of adaptive automatic gain control reduces the recognizer&#39;s sensitivity to unknown spectral shaping imposed on the speech signal. The combination of these features improves the reliability of the recognizer in the presence of acoustic interference (e.g., reverberation, additive noise, and unknown spectral shaping). 
     In one aspect, the invention features an apparatus for generating a parametric representation of a speech signal, comprising: a feature generator configured to compute short-term parameters of the speech signal; a filter system configured to filter the time sequences of the short-term parameters; and a normalizer configured to normalize the filtered parameters with respect to one or more previous values of the filtered parameters. 
     Embodiments may include one or more of the following features. 
     The feature generator preferably is configured to compute short-term spectral parameters of the speech signal. The feature generator preferably is configured to compute parameters of an auditory-like spectrum. The filter system preferably includes one or more linear filters. In one embodiment, the filter system includes a lowpass filter and a bandpass filter configured to operate in parallel. The lowpass filter may be characterized by a cutoff frequency of about 8 Hz and the bandpass filter may be characterized by a passband of about 8-16 Hz. The lowpass filter may be characterized by a moderate degree (e.g., 5 dB) of DC attenuation. 
     The normalizer preferably is configured to normalize the filtered parameters with respect to an average of preceding parameter values. The normalizer may include one or more feedback automatic gain control (AGC) networks. In one embodiment, each feedback network includes a feedback loop with a feedback lowpass filter. The feedback lowpass filter preferably is a single-pole IIR filter. In one embodiment, the normalizer includes two or more series-connected feedback AGC networks each having a single-pole IIR filter, the single-pole IIR filter of any one of the AGC networks being characterized by a cutoff frequency that is less than or equal to the cutoff frequency of the IIR filters in preceding AGC networks and greater than or equal to the cutoff frequency of the IIR filters in succeeding AGC networks. In accordance with this embodiment, the normalizer preferably includes first and second series-connected feedback AGC networks, the first AGC network having a single-pole lowpass IIR filter characterized by a cutoff frequency of about 1 Hz, and the second AGC network having a single-pole lowpass IIR filter characterized by a cutoff frequency of about 0.5 Hz. 
     In another aspect, the invention features a method for generating a parametric representation of a speech signal, comprising: computing short-term parameters of the speech signal; filtering time sequences of the short-term parameters; and normalizing the filtered parameters with respect to one or more previous values of the filtered parameters. 
     The steps of filtering and normalizing preferably are performed independently of one another. 
     Among the advantages of the invention are the following. The invention improves speech recognition performance by combining modulation filtering (in the amplitude domain) and automatic gain control processing in the front-end feature generator. The separation of these operations enables each step to be independently optimized, leading to better recognition performance. The invention reduces the error rate of automatic speech recognizers under degraded acoustic conditions, including reverberant conditions, additive noise and unknown spectral shaping. The invention may be applied to small-vocabulary and large-vocabulary recognizers to improve performance under degraded acoustic conditions. 
     Other features and advantages will become apparent from the following description, including the drawings and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an automatic speech recognizer. 
     FIG. 2 is a block diagram of the basic features of a feature extractor for the automatic speech recognition system of FIG.  1 . 
     FIG. 3A is a block diagram of an embodiment of the feature extractor of FIG.  2 . 
     FIG. 3B is a block diagram of two series-connected AGC normalization units. 
     FIG. 3C is a block diagram of a cross-coupled AGC normalization unit. 
     FIG. 3D is a block diagram of a feedforward AGC normalization unit. 
     FIG. 3E is a block diagram of a mean normalizer and a variance normalizer for performing on-line normalization of the acoustic features extracted from an input speech signal. 
     FIG. 4 is a block diagram of another embodiment of the feature extractor of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     I. Automatic Speech Recognizer—System Overview 
     Referring to FIG. 1, an automatic speech recognizer  10  includes a feature extractor  12 , an acoustic modeler  14 , and a search engine  16  that together operate on an input speech signal  18  to produce a recognized speech output  20 . Automatic speech recognizer  10  is configured to identify a sequence of words that best corresponds to input speech signal  18  based upon a similarity metric that accounts for acoustic similarity, phonotactics, syntax, and pragmatics. In particular, automatic speech recognizer  10  identifies the most probable sequence of words as determined by statistical pattern recognition techniques. 
     Feature extractor  12  processes input speech signal  18  to produce a set of acoustic features  22  which describe spectral features of the speech signal. As explained in detail below, the acoustic features produced by feature extractor  12  describe input speech signal  18  in a way that captures a significant amount of linguistic content (the phonetic identity of the incoming speech signal) while suppressing a significant amount of non-linguistic content (e.g., information about speaker identity, such as gender, vocal tract length, accent and age, and information about the acoustic environment and the input speech channel, such as background noise, filtering and reverberation). Feature extractor  12  includes separate processing stages for performing modulation filtering and normalizing the acoustic features extracted from input speech signal  18 . In this way, the modulation filtering and normalization stages may be independently optimized to improve the overall performance of recognizer  10 . 
     Acoustic modeler  14  estimates phone probabilities  24 —the probability of each of the acoustic features  22  corresponding to different phonetic classes. In one embodiment, acoustic modeler  14  is a multilayer perceptron (MLP). The MLP is a two-layer, feedforward MLP that includes an input layer, a variable number (e.g., 300-2000) of hidden units, and as many output units as are needed to represent the context-independent phonemes in the recognizer vocabulary (e.g., 32-54). The hidden units are standard sigmoid units which compute the output activation y as          y        (   x   )       =     1     1   +            -     w   T          x                                  
     where w is the unit&#39;s weight vector and the input x is augmented with a constant element having a value of 1 so that w T x is an affine transform of x. The output layer of the MLP is a softmax layer in which the k th  output unit computes its activation g k  as          g   k     =              w   k   T        x           ∑     i   =   1     K                            w   i   T        x                                  
     where w k  is the weight vector for the k-th output unit and the input vector x is augmented with a constant element having a value of 1. The features are normalized to have zero mean and unit variance before they are input to the MLP. The mean and standard deviation for each feature is estimated from the MLP training set, and the features are normalized by subtracting the means and dividing by the standard deviations. The MLP is trained using on-line error back propagation with a cross-entropy error criterion. Additional information about the structure and operation of acoustic modeler  14  may be found in U.S. Provisional Application Ser. No. 60/116,684, which is entitled “Perceptually Inspired Signal-Processing Strategies for Robust Speech Recognition in Reverberant Environments.” 
     In another embodiment, the acoustic modeler  14  classifies acoustic features  22  into different phonetic classes based upon Gaussian mixture models. 
     HMM (hidden Markov model) word modeler  26  provides a model of the speech signal in terms of a set of states, a set of time-independent transition probabilities between the states, and a probability distribution for each state describing the distribution of acoustic vectors. The HMM lexicon models the pronunciations and temporal characteristics of the words in the vocabulary of recognizer  10 . The lexicon of HMM word models contains multiple pronunciations for each word in the vocabulary of recognizer  10 . Minimum phoneme duration constraints are enforced by repeating states in the HMMs. The HMM transition probabilities in the lexicon are not trained; instead, the transition probabilities are fixed to 1/T, where T is the number of transitions leaving a given state (including the self loop). 
     In one embodiment, the HMM lexicon is generated as follows. A set of pronunciations that covered about 90% of the pronunciations in the training set is derived from phonetic transcriptions of the training utterances. Average context-dependent phoneme durations are calculated from the hand transcriptions of the training data. An initial HMM lexicon that incorporates all of the derived pronunciations and the calculated context-dependent phonemes is generated. The durations are included in the model by a sequence of n states, where n=d/(2s), d is the average duration of the phoneme, and s is that frame step time. Automatic speech recognizer  10  is trained using the hand transcriptions of the training set as targets and the generated lexicon. The resulting recognizer is used to relabel the training data by forced alignment. The final lexicon is generated by eliminating any of the derived pronunciations that were not used in the relabeling of the training set, computing new phonemes durations from the relabeling and compiling a new lexicon using the pruned set of pronunciations and the newly derived durations. This iterative procedure may be repeated multiple times. 
     Language modeler  28  estimates the probability (P(M)) associated with the sequence of words (M=m 1 , m 2 , . . . , m m ) that preceded the current portion of input speech signal  18  being processed. Language modeler  28  uses a bigram grammar which approximates P(M) as          P        (   M   )       ≈       P        (       m   1        s     )            P        (     e        m   n       )              ∏     i   =   2     n                     P        (       m   i          m     i   -   1         )                                  
     where s is the start symbol, P(m 1 |s) is the probability that m 1  is the first word of an utterance, e is the end symbol, and P(e|m n ) is the probability that m n  is the last word of an utterance. The bigram probabilities P(m i |m i-1 ) are estimated by counting word pairs in the training data. The probabilities of word pairs that never occur in the training set or do not occur frequently enough to permit reliable estimation of the conditional probability are approximated by 
     
       
           P ( y|x )≈ b   x   P ( y ) 
       
     
     where P(y) is the prior probability of word y and b x  is the “backoff weight” for word x. The prior probabilities for the individual words are calculated by counting words in the training set. 
     Methods for calculating backoff weights are described in, for example, K. W. Church et al., “A comparison of the enhanced Good-Turing and deleted estimation methods for estimating probabilities of English bigrams,”  Computer Speech and Language , 5(1): 19-54 (January 1991). 
     Search engine  16  combines phoneme probabilities  24  with the information received from HMM word modeler  16  and language modeler  28  to determine the most probable utterance, which corresponds to recognized speech output  20 . Search engine  16  performs a Viterbi search using a dynamic programming search to compute the best state sequence (the Viterbi approximation) given the scaled acoustic likelihoods from the MLP, the lexicon of HMM models and the bigram probabilities from the language model. 
     The resulting recognized speech output  20 , which is derived from acoustic features  22 , is stable and robust under a wide variety of acoustic conditions, including the presence of acoustic interference such as reverberation, additive noise, and unknown spectral shaping. 
     II. Feature Extractor—Overview 
     Referring to FIG. 2, feature extractor  12  includes a feature generator  30  characterized by a critical bandwidth spectral resolution, a filter system  32  configured to emphasize slow changes in the spectral structure of input speech signal  18 , and a normalizer  34  characterized by adaptive automatic gain control. The critical-band-like frequency resolution of feature generator  30  reduces the sensitivity of recognizer  10  to speaker-dependent signal characteristics and enhances the sensitivity of recognizer  10  to speech-dependent signal characteristics. The emphasis by filter system  32  on slow changes in the spectral structure of the speech signal enables feature extractor  12  to produce a set of acoustic features  22  that captures a significant amount of the linguistic content of input speech signal  18  while suppressing a significant amount of non-linguistic content to improve the performance of recognizer  10 . The use of adaptive automatic gain control by normalizer  34  reduces the sensitivity of recognizer  10  to unknown spectral shaping which may be imposed on input speech signal  18 . 
     The combination of these features improves the reliability of the recognizer, especially in the presence of acoustic interference (e.g., reverberation, additive noise and unknown spectral shaping). The performance of recognizer  10  is further improved by the ability to independently optimize filter system  32  and normalizer  34 . 
     III. Feature Extractor—Detailed Implementation 
     Referring to FIG. 3A, in one embodiment of feature extractor  12 , feature generator  30  includes a Bark-scale power-spectral filterbank  40  and a bank of square root generators  42 , filter system  32  includes parallel lowpass and bandpass filtering systems  44 ,  46 , and normalizer  34  includes AGC networks  48 ,  50 . 
     Initially, feature generator  30  segments input speech signal  18  into 25 ms frames with a 10 ms frame step, each frame is multiplied by a Hamming window, and the power spectrum for each frame is computed with a Fast Fourier Transform (FFT). The power spectrum is accumulated into critical-band-like frequency channels  52  by convolution with filterbank  40 . Filterbank  40  includes thirteen overlapping, triangular filters that have bandwidths and spacings of 0.95 Bark and cover the frequency range of 230-4000 Hz. The critical-band-like frequency channels  52  are converted into an amplitude spectrum  54  by square root generators  42  which are configured to take the square root of the output of filterbank  40 . 
     Other filterbanks may be used. For example, Bark-scale filterbank  40  may be replaced by an FIR (finite impulse response) filterbank with a resolution (for both filter bandwidth and filter spacing) preferably in the range of ⅛ octave to 1 octave, and more preferably with a resolution of a quarter-octave. Alternatively, the triangular filters of filterbank  40  may be replaced by filters characterized by a trapezoidal magnitude response. 
     The resulting critical band amplitude spectrum  54  is filtered in parallel by lowpass filtering system  44  and bandpass filtering system  46  of filter system  32 . Lowpass filter system  44  includes a bank of FIR lowpass filters with a passband of 0-8 Hz and 5 dB of DC suppression. Bandpass filter system  46  includes a bank of FIR bandpass filters with a passband of 8-16 Hz. Lowpass and bandpass filtering systems  44 ,  46  produce lowpass and bandpass filtered streams  56 ,  58 , respectively. 
     Lowpass and bandpass filtered streams  56 ,  58  are processed through AGC networks  48 ,  50 , respectively. Generally, a moderate degree of DC attenuation of the filters improves automatic-speech-recognition (ASR) performance. For example, in systems having a single filter with non-uniform group delay in its passband, 8, 9, or 11 dB of DC attenuation significantly improves ASR performance under reverberant acoustic conditions. In systems having two or more of the non-uniform group delay filters, 8 dB or less of DC attenuation usually provides good ASR performance under clean and reverberant acoustic conditions. In systems having linear-phase filters, 4 or 5 dB of DC attenuation usually provides the best overall performance. Each AGC network  48 ,  50  may include one or more series-connected feedback AGC units. The inclusion of frequency-local AGC in feature extractor  12  improves recognition robustness by reducing the effects of changes caused by unknown spectral shaping of input speech signal  18  and changes in overall signal level on the speech representation. In the modulation-spectral domain, the AGC normalization suppresses slowly-varying components of input speech signal  18  (i.e., components with very low modulation frequencies). 
     AGC networks having multiple feedback AGC units provide better gain normalization and onset enhancement than AGC networks having only a single feedback AGC unit. This improvement is due to the ability to match most auditory systems, which usually have central regions with adaptation time constants that are longer than adaptation time constants of peripheral regions. Specifically, AGC networks having multiple feedback AGC units can accommodate the need for varying time constants by constraining the time constants of feedback AGC units so that units later in the signal flow have time constants greater than time constants of the preceding feedback AGC units. 
     Referring to FIG. 3B, in one AGC embodiment, each channel  56 ,  58  of AGC networks  48 ,  50  includes upstream and downstream series-connected feedback AGC units  60 ,  62 . Each AGC unit  60 ,  62  includes a divider unit  64 ,  66  and a single-pole UR (infinite impulse response) lowpass filter  68 ,  70  in the feedback loop. Filter  68  of upstream AGC unit  60  is characterized by a time constant of 160 ms, and filter  70  of downstream AGC  62  unit is characterized by a time constant of 320 ms. In terms of cut-off frequency (which is equal to the inverse of the quantity of the product of time constant and 2 π), filter  68  is characterized by a cutoff frequency of 1 Hz, and filter  70  is characterized by a cutoff frequency of 0.5 Hz. In operation, the absolute values  72 ,  74  of the outputs of divider units  64 ,  66  are fed back to the divider units  64 ,  66  through filters  68 ,  70  to permit the processing of both positive and negative input signals. If the input to one of the AGC units is x(t), the output of the AGC unit is y(t), the output of the corresponding lowpass filter is g(t), and the transfer function of the lowpass filter is            H        (   z   )       =       1   -   a       1   -     az     -   1             ,                          
     then the AGC obeys the following relationships 
     
       
           x ( t )= y ( t ) g ( t ) 
       
     
     
       
           g ( t )=(1− a )| y ( t )|+ ag ( t −1). 
       
     
     From these equations, the output y(t) of the AGC unit may be given as follows:                y        (   t   )       =         -     ag        (     t   -   1     )         +           a   2            g   2          (     t   -   1     )         +     4        (     1   -   a     )          x        (   t   )                 2        (     1   -   a     )                     if                   x        (   t   )         ≥   0     ,                 y        (   t   )       =         ag        (     t   -   1     )       -           a   2            g   2          (     t   -   1     )         -     4        (     1   -   a     )          x        (   t   )                 2        (     1   -   a     )                 otherwise   .                                
     For a steady-state input, the magnitude of the AGC output is the square root of the magnitude of the input, and the sign of the AGC output is the sign of the input. When the input varies, the AGC output is nearly proportional to the square root of the magnitude of the input, but the constant of proportionality is greater than one. Thus, the AGC is essentially a square-root compressor with a variable gain that depends upon the dynamics of the input. To prevent large transients on start-up, the AGC gain g(t) is initialized so that g( 0 )=(x( 0 )) ½ . 
     In another AGC embodiment, AGC network  48  may include two series-connected AGC units characterized by time constants of 160 and 320 ms, respectively. Additionally, AGC network  50  may include two-series connected AGC units characterized by time constants of 160 and 640 ms, respectively. This configuration provides the best general performance of automatic speech recognition under clean and reverberant acoustic conditions. 
     In another AGC embodiment, normalizer  34  may include three series-connected AGC units in the lowpass and bandpass channels. In this embodiment, the time constants of the AGC units increase from the upstream AGC to the downstream AGC unit. For example, the upstream AGC unit may have a time constant of 80 ms, the intermediate AGC unit may have a time constant of 320 ms, and the downstream AGC unit may have a time constant of 640 ms. 
     In an alternative AGC embodiment, the lowpass and bandpass channels of normalizer  34  may include respective cross-coupled AGC systems in which the signal in a given channel is normalized by the gain that is estimated from the signal itself and from signals in neighboring channels. By coupling neighboring channels, the AGC systems emphasize spectral peaks and preserve spectral shape information that might otherwise be eliminated by fast-acting, per-channel AGC processing. One implementation of this embodiment is illustrated in FIG.  3 C. In each channel the signal x i (t) is normalized by a factor g i (t) which is a temporally smoothed, weighted average of the signal level in the channel itself (y i (t)) and in the other channels (y j (t), wherein j≠i). This embodiment enhances energy peaks in time and frequency by performing a form of lateral inhibition in addition to automatic gain control. The unit delays (z −1 ) are included to simplify the coupled AGC computation. The w i,j  factors are the coupling weights between channels. In FIG. 3C, only coupling between neighboring channels is portrayed; additional channel coupling may be used. 
     Referring to FIG. 3D, in another alternative AGC embodiment, the feedback AGC units may be replaced by a feedforward AGC unit  76 . As with the above-described feedback AGC units, this AGC implementation enhances signal onsets and the output converges to 1 for a constant input. In this embodiment, the time constant of IIR low pass filter  77  preferably is set to 320 ms or 640 ms for both the lowpass and bandpass channels, and E preferably is set to a value of 1. One or more series-connected feedforward units  70  may be used in the lowpass and bandpass channels of normalizer  34 . 
     Referring back to FIG. 3A, the spectral resolution of bandpass stream  58  is halved by a spectral smoother  78  which is configured to sum the features from adjacent channels. By reducing the spectral resolution of feature extractor  12 , spectral smoother  78  reduces the complexity of the acoustic model and reduces the sensitivity of recognizer  10  to speaker-dependent signal characteristics without detrimentally affecting the description of input speech signal  18 . 
     Referring to FIGS. 3A and 3E, all features are normalized by an on-line normalizer  80  to have means of zero and variances of one. On-line normalizer  80  reduces the effects of slowly varying additive offsets and multiplicative gain terms caused, for example, by additive noise and spectral shaping. On-line normalizer  80  includes a mean normalizer  82  with a lowpass IIR filter  84  that computes an estimate of the signal mean (with an exponentially decaying window into the past) which is subtracted from the signal (x). On-line normalizer  80  also includes a variance normalizer  86  with a lowpass IIR filter  88  that computes an estimate of the standard deviation of the signal. The signal is normalized with respect to the estimated standard deviation by a divider unit  90 . A small offset e is added to the estimated standard deviation to avoid division by zero. For the first utterance in a speech signal, the estimates of the mean and standard deviation are initialized from estimates previously computed over training data. For each following utterance, the final estimates from the previous utterance are used. 
     The time constants of the lowpass filters in normalizer  80  control the duration of the processing memory. The longer the time constant, the more reliable the estimates of the mean and variance; however, longer time constants also entail slower adaptation to changes in the acoustic environment. In one embodiment, the means and variances are estimated using single-pole lowpass filters with a time constant preferably in the range of 250 ms to 4 seconds, and more preferably a time constant of 2 seconds, and the offset ε to the standard deviation estimate is set to 1. 
     Other embodiments are within the scope of the claims. For example, referring to FIG. 4, another embodiment of feature extractor  12  includes a Bark-scale power-spectral filterbank  100  and a bank of square root generators  102 , filter system  32  includes parallel lowpass and bandpass filtering systems  104 ,  106 , and normalizer  34  includes AGC networks  108 ,  110 . 
     Initially, feature generator  30  segments input speech signal  18  into 32 ms frames with a 16 ms frame step, each frame is multiplied by a Hamming window, and the power spectrum for each frame is computed with a Fast Fourier Transform (FFT). The power spectrum is accumulated into critical-band-like frequency channels  112  by convolution with filterbank  100 . Filterbank  100  includes fourteen overlapping, triangular filters that have bandwidths and spacings of 1.0 Bark and cover the frequency range of 160-4000 Hz. The critical-band-like power spectrum is converted into an amplitude spectrum  114  by square root generators  102  which are configured to take the square root of the output of filterbank  100 . 
     Other filterbanks may be used, as explained above in connection with the embodiment of FIG.  3 A. 
     The resulting critical band amplitude spectrum  114  is filtered in parallel by lowpass filtering system  104  and bandpass filtering system  106  of filter system  32 . Lowpass filter system  104  includes a bank of FIR lowpass filters with a passband of 0-16 Hz. Bandpass filter system  106  includes a bank of FIR bandpass filters with a passband of 2-16 Hz and 5 dB of DC suppression. Lowpass and bandpass filtering systems  104 ,  106  produce lowpass and bandpass filtered streams  116 ,  118 , respectively. 
     Lowpass and bandpass filtered streams  116 ,  118  are processed through AGC networks  108 ,  110 , respectively. Each AGC network  108 ,  110  may include one or more series-connected feedback AGC units. In one AGC embodiment, each channel of AGC networks  108 ,  110  includes two upstream and downstream series-connected feedback AGC units, as described above in connection with the embodiment of FIG.  3 B. In one implementation, in the lowpass stream the upstream AGC unit has a time constant of 320 ms and the downstream AGC unit has a time constant of 320 ms, and in the bandpass stream the upstream AGC unit has a time constant of 160 ms and the downstream AGC unit has a time constant of 640 ms. Other AGC implementations may be used, as explained above in connection with the embodiment of FIG.  3 A. 
     On-line normalizer  80  (FIG.  3 A), which normalized the feature means and variances in the embodiment of FIG. 3A, is replaced by an off-line per-utterance normalization step in the embodiment of FIG.  4 . 
     Still other embodiments are within the scope of the claims.