Abstract:
Voiced speech preprocessing employs waveform interpolation or a harmonic model circuit to smooth a transition region and simplify speech coding. At low bit rates, the speech is coded by a system that maintains a high perceptual quality in the transition region from a voiced (quasi-periodic) portion of the speech signal to an unvoiced (non-periodic) portion of the speech signal. Similarly, the transition region from an unvoiced portion to a voiced portion is conditioned to maintain a high perceptual quality at a low bandwidth. The transition region from one type of voiced region to another type of voiced region is also smoothed. The transition region is smoothed to create a quasi-periodic speech signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to speech coding, and more particularly, to a system that performs speech pre-processing.  
           [0003]    2. Related Art  
           [0004]    Speech coding systems often do not operate at low bandwidths. When the bandwidth of a speech coding system is reduced, the perceptual quality of its output, a synthesized speech, is often reduced. In spite of this loss, there is an effort to reduce speech coding bandwidths.  
           [0005]    Some speech coding systems perform strict waveform matching using code excited linear prediction (CELP) at low bandwidths such as 4 kbit/s. The waveform matching used by these systems do not always accurately encode and decode speech signals due to the system&#39;s limited capacity. This invention provides an efficient speech coding system and a method that modifies an original speech signal in transition areas, and accurately encodes and decodes the modified speech signal to keep the perceptually important features of a speech signal.  
         SUMMARY  
         [0006]    A speech codec includes a classifier and a periodic smoothing circuit. The classifier processes a transition region that separates portions of a speech signal. The periodic smoothing circuit uses at least an interpolated pitch lag and/or a constant pitch lag to smooth the transition region that is represented by a residual signal, a weighted signal, or a portion of an unconditioned speech signal. The pitch track corresponds to the voiced portion of the speech signal.  
           [0007]    In one aspect, the periodic smoothing circuit selects either a forward pitch extension or a backward pitch extension to smooth the transition region between two periodic signals. The transition region can extend through multiple frames and may include an unvoiced portion. The periodic smoothing circuit smoothes the transition region between these signals in the time domain using a waveform interpolation circuit, or in the frequency domain using a harmonic circuit. The smoothing may occur when a long term pre-processing circuit or a long term processing circuit fails or when an irregular voiced speech portion is detected.  
           [0008]    In another aspect, the periodic smoothing circuit smoothes the transition region between a periodic portion of a speech signal and other portions of that signal. In this aspect, smoothing occurs in the time domain using the waveform interpolation circuit or in the frequency domain using the harmonic circuit. The classifier uses a pitch lag, a linear prediction coefficient, an energy level, a normalized pitch correlation, and/or other parameters to classify the speech signal.  
           [0009]    Other systems, methods, features and advantages of the invention will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0010]    The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.  
         [0011]    [0011]FIG. 1 illustrates a speech coding system.  
         [0012]    [0012]FIG. 2 illustrates a second speech coding system.  
         [0013]    [0013]FIG. 3 illustrates a speech codec.  
         [0014]    [0014]FIG. 4 illustrates an unvoiced to voiced speech signal onset transition region.  
         [0015]    [0015]FIG. 5 illustrates a voiced to unvoiced speech signal offset transition region.  
         [0016]    [0016]FIG. 6 illustrates a first voice to a second voice speech signal transition region.  
         [0017]    [0017]FIG. 7 illustrates a first voice to a second voice speech signal transition region.  
         [0018]    [0018]FIG. 8 illustrates a periodic/smoothing method.  
         [0019]    [0019]FIG. 9 illustrates a second periodic/smoothing method. 
     
    
       [0020]    The dashed connections shown in FIGS.  1 - 3 ,  8 , and  9 , represent direct and indirect connections. As shown, other circuits, functions, devices, etc. can be coupled between the illustrated blocks. Similarly, the dashed boxes illustrate optional circuits or functionality.  
       DETAILED DESCRIPTION  
       [0021]    A preferred system maintains a smooth transition between portions of a speech signal. During an onset or an offset transition from a voiced speech signal to an unvoiced speech signal, the system performs a periodic smoothing. The system initiates the periodic smoothing when a long term processing (LTP) failure, a pre-processing (PP) failure, and/or an irregular voiced speech portion is detected. A classifier detects the transition region and a smoothing circuit transforms that region into a more periodic signal in the time or the frequency domain.  
         [0022]    [0022]FIG. 1 is a diagram of an embodiment of a speech coding system  100 . The speech coding system  100  includes a speech codec  102  that conditions an input speech signal  104  into an output speech signal  106 . The speech codec  102  includes a classifier  108 , a periodic/smoothing circuit  110 , a time domain circuit  112 , a waveform interpolation circuit  114 , and a transition detection circuit  116 .  
         [0023]    The speech coding system  100  operates in the time and the frequency domains. When operating in the frequency domain, the periodic/smoothing circuit  110  uses a frequency domain circuit  118  and a harmonic model circuit  120 . In the frequency domain, the transition detection circuit  116  initiates a transformation of the input speech signal  104  to a more periodic output speech signal  106  through the harmonic model circuit  120 . In the time domain, the transition detection circuit  116  initiates a transformation of the input speech signal  104  to a more periodic speech signal  106  through the waveform interpolation circuit  114 .  
         [0024]    [0024]FIG. 2 illustrates a second embodiment of a speech coding system  200 . The speech coding system  200  includes a speech codec  202  that conditions an input speech signal  204  into the output speech signal  206 . The speech codec  202  includes a classifier  210 , a periodic/smoothing circuit  212 , and a failure detection circuit  214 . The failure detection circuit  214  detects the failure of a long term pre-processing (PP) circuit  216  and a long term processing (LTP) circuit  218 . The classifier  210  includes a transition detection circuit  220  that processes transition parameters. The transition parameters preferably include a pitch lag stability  222 , a linear prediction coefficient (LPC)  224 , an energy level indicator  226 , and a normalized pitch correlation  228 .  
         [0025]    As shown in FIG. 2, the periodic/smoothing circuit  212  includes a waveform interpolation circuit  232  that is a unitary part of or is integrated within a time domain circuit  230 . The transition detection circuit  220  initiates a temporal transformation of the input speech signal  204  to a more periodic output speech signal  206 . When the failure detection circuit  214  detects a long term pre-processing (PP) circuit  216  failure, a long term processing (LTP) circuit  218  failure, and/or an irregular voiced speech portion, the failure detection circuit  214  initiates a waveform interpolation in the time domain. Once initiated, the waveform interpolation circuit  232  performs a transformation of the input speech  204  to a more periodic output speech signal  206 . The periodic smoothing circuit  212  can employ an interpolated pitch lag and/or a constant pitch lag.  
         [0026]    When the speech coding system  200  operates in the frequency domain, the periodic/smoothing circuit  212  performs a frequency transformation. In the frequency domain, the transition detection circuit  220  initiates the transformation of the input speech  204  to a more periodic speech signal using a harmonic model circuit  234 . When desired, the failure detection circuit  214  initiates the harmonic model circuit  234  to transform the input speech  204  to a more periodic speech signal  206  in the frequency domain.  
         [0027]    [0027]FIG. 3 is a diagram illustrating an embodiment of a speech codec  300 . A speech signal  302 , such as an unconditioned speech signal, is transformed into a weighted speech signal  304  at block  306 . The weighted speech signal  304  is conditioned by a periodic/smoothing circuit at block  308 . The periodic/smoothing circuit, block  308 , includes a pitch-preprocessing block  310 , a waveform interpolation block  312 , and an optional harmonic interpolation block  314 . The operation of the waveform interpolation block  312  or the harmonic interpolation block  314  can be performed before or after the pitch preprocessing block  310 . The weighted speech signal  304  is transformed into a speech signal  316  at block  318  which is fed to a subtracting circuit  320 .  
         [0028]    As shown in FIG. 3, a pitch lag of one  324  is received by an adaptive codebook  326 . A code-vector  328 , shown as v a , is selected from the adaptive codebook  326 . After passing through a gain stage  330 , shown as g p , the amplified vector  332  is fed to a summing circuit  334 . Preferably, a pitch lag, such as a pitch lag of two  336 , is provided to a fixed codebook  338 . In alternative embodiments, the pitch lag received by the fixed and the adaptive codebooks  326  and  338  may be equal or have a range of other values. A code-vector  340 , shown as v c , is generated by the fixed codebook  338 . After being amplified by a gain stage  342 , shown as g c , the amplified vector  344  is received by the summing circuit  334 .  
         [0029]    When the two input signals V a g p    332  and V c g c    344  are added by the summing circuit  334 , the combined signal  346  is filtered by a synthesis filter  348  that preferably has a transfer function of (1/A(z)). The output of the synthesis filter  348  is received by the subtracting circuit  320  and subtracted from the transformed speech signal  316 . An error signal  350  is generated by this subtraction. The error signal  350  is received by a perceptual weighting filter W(z)  352  and minimized at block  354 . Minimization block  354  can also provide optional control signals to the fixed codebook  338 , the gain stage g c    342 , the adaptive codebook  326 , and the gain stage g p    330 . The minimization block  354  can also receive optional control information.  
         [0030]    [0030]FIG. 4 illustrates an embodiment of an unvoiced to voiced speech signal onset transition  400 . As shown, certain portions of a speech signal are separated into two classified regions  402  and  404  that extend through multiple frames. The speech signal comprises an unvoiced (non-periodic) portion  408  and a voiced (quasi-periodic) portion  406  that are linked through a transition region  412 . A coded pitch track  410  that corresponds to the voiced  406  portion is used to perform backward pitch extension. The backward pitch extension is attenuated through time into the unvoiced portion  408  of the speech signal to ensure a smooth transition between the unvoiced portion  408  and the voiced portion  406 . The classifier  210  detects the classified regions  402  and  404 . The slope of the backward pitch extension is adaptable to many parameters that define the speech signal such as the difference in amplitude between the classified regions  402  and  404 .  
         [0031]    [0031]FIG. 5 illustrates an embodiment of a voiced  406  to unvoiced  408  speech signal offset transition  500 . As shown, portions of the speech signal are separated into classified regions  506  and  508  that extend through multiple frames. The speech signal comprises a voiced portion  406  and an unvoiced portion  408  that are linked through a transition region  510 . A pitch track  512  corresponding to the voiced portion  406  is used to perform a forward pitch extension. The forward pitch extension  512  is attenuated through time between the voiced portion  406  and the unvoiced portion  408 . The classifier  210  detects the classified regions  506  and  508 . The slope of the forward pitch extension  512  is adaptable to many parameters that define the speech signal such as the difference in amplitude between the classified regions  506  and  508 .  
         [0032]    [0032]FIG. 6 illustrates a transition  600  between a first voice (voice 1)  602  and a second voice (voice 2)  604  speech signal. As shown, certain portions of the speech signal are separated into classified regions  606  and  608  that extend through multiple frames. The speech signal comprises voice 1 speech  602  and voice 2 speech  604  linked through a transition region  610 . A pitch track  614  corresponding to the voice 1 speech portion  602  and the voice 2 speech portion  604  is used to perform waveform interpolation or harmonic interpolation, which combines both forward and backward pitch extensions. The interpolation smoothes the harmonic structure, the energy level, and/or the spectrum in the transition region  610  between the two voiced speech portions  602  and  604  in time. In other words, the extensions and interpolation from both directions from one of the voiced speech portions to the other speech portion ensures a smooth transition between the voice 1 speech  602  and the voice 2 speech  604 .  
         [0033]    Two examples of a pitch track  614  are shown in FIG. 6. One pitch track  618  smoothly transitions from a lower pitch track level to a higher pitch track level through the transition region  610  between the voice 1 speech  602  and the voice 2 speech  604 . This transition occurs when a voice 1 lag is less than a voice 2 lag. Another pitch track  616  smoothly transitions from a higher pitch track level to a lower pitch track level through the transition region  610  between voice 1 speech  602  and voice 2 speech  604 . This transition occurs when the voice 1 lag is greater than the voice 2 lag. The classifier  210  is used to detect the classified regions  606  and  608 . The smoothing and interpolation are adaptable to many parameters including the relative magnitude and frequency differences between the classified regions  606  and  608 .  
         [0034]    [0034]FIG. 7 illustrates another embodiment of a voice 1 to a voice 2 speech signal transition  610 . As shown, certain portions of a speech signal are classified into classified regions  606  and  608  that extend through multiple frames. A pitch track  702  corresponding to the voice 1 speech portion  602  and the voice 2 speech portion  604  is used to perform the interpolation, smoothing, or forward and backward pitch extension that ensure a smooth transition between the voice 1 speech portion  602  and the voice 2 speech portion  604 .  
         [0035]    Two examples of the pitch track  702  are shown in FIG. 7. One pitch track  704  smoothly transitions from a lower pitch track level to a higher pitch track level through the transition region  610  separating voice 1 speech  602  from voice 2 speech  604 . This transition occurs when the voice 1 lag is less than the voice 2 lag. Another pitch track  706  smoothly transitions from a higher pitch track level to a lower pitch track level through the transition region  610 . This transition occurs when the voice 1 lag is greater than the voice 2 lag. The classifier  210  is used to detect the classified regions  606  and  608 . The smoothing and interpolation are adaptable to many parameters including the relative magnitude and frequency differences between the classified regions  606  and  608 .  
         [0036]    [0036]FIG. 8 illustrates a periodic/smoothing method  800 . At block  802 , a transition region is detected. At block  804 , the transition type is derived and either a frequency or time domain smoothing is selected. At block  806 , waveform interpolation is performed on the transition region in the time domain. If desired, at optional block  808 , a harmonic model interpolation is performed on the transition region in the frequency domain.  
         [0037]    [0037]FIG. 9 is a block diagram illustrating an embodiment of a sequential periodic/smoothing method  900 . At block  902 , a transition region is detected. At block  904 , the transition type is determined. Once the transition type is known, the transition region is smoothed by decision criteria. For example, if the detected transition type is of a voice 1 speech  602  to a voice 2 speech  604  type signal, then block  908  performs a forward and backward pitch extension using the pitch interpolation between two pitch lags. The two pitch lags are defined by the current and the previous speech frames of the signal. If it is determined that the transition type is from an unvoiced speech signal  408  to a voiced speech signal  406  at block  910 , then at block  912  a backward pitch extension using a single pitch lag is performed using the current frame of the speech signal. If it is determined that the detected transition type is from a voiced speech signal  406  to an unvoiced speech signal  408  at block  914 , then at block  916  a forward pitch extension using a single pitch lag is performed using the previous frame of the speech signal. If none of the decision blocks  906 ,  910 , or  914  detect the speech segment type, then the periodic/smoothing method  900  is re-initiated at block  918 .  
         [0038]    While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.