Abstract:
The invention relates to a method for determining voice activity in a signal section of an audio signal. The result, i.e., whether voice activity is present in the section of the signal thus observed, depends upon spectral and temporal stationarity of the signal section and/or prior signal sections. In a first step, the method determines whether there is spectral stationarity in the observed signal section. In a second step, the method determines whether there is temporal stationarity in the signal section in question. The final decision as to the presence of voice activity in the signal section observed depends upon the initial values of both steps.

Description:
BACKGROUND 
   The present invention relates to a method for determining speech, or voice, activity in a signal segment of an audio signal, the result of whether speech activity is present in the observed signal segment depending both on the spectral and on the temporal stationarity of the signal segment and/or on preceding signal segments. 
   In the domain of speech transmission and in the field of digital signal and speech storage, the use of special digital coding methods for data compression purposes is widespread and mandatory because of the high data volume and the limited transmission capacities. A method which is particularly suitable for the transmission of speech is the Code Excited Linear Prediction (CELP) method which is known from U.S. Pat. No. 4,133,976. In this method, the speech signal is encoded and transmitted in small temporal segments (“speech frames”, “frames”, “temporal section”, “temporal segment”) having a length of about 5 ms to 50 ms each. Each of these temporal segments or frames is not represented exactly but only by an approximation of the actual signal shape. In this context, the approximation describing the signal segment is essentially obtained from three components which are used to reconstruct the signal on the decoder side: Firstly, a filter approximately describing the spectral structure of the respective signal section; secondly, a so-called “excitation signal” which is filtered by this filter; and thirdly, an amplification factor (gain) by which the excitation signal is multiplied prior to filtering. The amplification factor is responsible for the loudness of the respective segment of the reconstructed signal. The result of this filtering then represents the approximation of the signal portion to be transmitted. The information on the filter settings and the information on the excitation signal to be used and on the scaling (gain) thereof which describes the volume must be transmitted for each segment. Generally, these parameters are obtained from different code books which are available to the encoder and to the decoder in identical copies so that only the number of the most suitable code book entries has to be transmitted for reconstruction. Thus, when coding a speech signal, these most suitable code book entries are to be determined for each segment, searching all relevant code book entries in all relevant combinations, and selecting the entries which yield the smallest deviation from the original signal in terms of a useful distance measure. 
   There exist different methods for optimizing the structure of the code books (for example, multiple stages, linear prediction on the basis of the preceding values, specific distance measures, optimized search methods, etc.). Moreover, there are different methods describing the structure and the search method for determining the excitation vectors. 
   Frequently, the task arises to classify the character of the signal located in the present frame to allow determination of the coding details, for example, of the code books to be used, etc. In this context, a so-called “voice activity decision” (voice activity detection, VAD) is frequently made as well, which indicates whether or not the currently present signal section contains a speech segment. A correct decision of this type must also be made when background noises are present, which makes the classification more difficult. 
   SUMMARY OF THE INVENTION 
   In the approach set forth herein, the VAD decision is equated to a decision on the stationarity of the current signal so that the degree of the change in the essential signal properties is thus used as the basis for the determination of the stationarity and the associated speech activity. Along these lines, for instance, a signal region without speech which, for example, only contains a constant-level background noise which does not change or changes only slightly in its spectrum, is then to be considered stationary. Conversely, a signal section including a speech signal (with or without the presence of the background noise) is to be considered not stationary, that is, non-stationary. Along the lines of the VAD, therefore, the result “non-stationary” is equated to speech activity in the method set forth here while “stationary” means that no speech activity is present. 
   Since the stationarity of a signal is not a clearly defined measurable variable, it will be defined more precisely below. 
   In this context, the presented method assumes that a determination of stationarity should ideally be based on the time rate of change of the short-term average value of the signal energy. However, such an estimate is generally not possible directly because it can be influenced by different disturbing boundary conditions. Thus, the energy also depends, for example, on the absolute loudness of the speaker which, however, should have no effect on the decision. Moreover, the energy value is also influenced, for example, by the background noise. Hence, the use of a criterion which is based on energy considerations is only useful if the influence of these possible disturbing effects can be ruled out. For this reason, the method is made up of two stages: In the first stage, a valid decision on stationarity is already made. If in the first stage, the decision is “stationary”, then the filter describing this stationary signal segment is recomputed and thereby adapted in each case to the last stationary signal. In the second stage, however, this decision is made once more on the basis of another criterion, thus being checked and possibly changed using the values provided in the first stage. In this context, this second stage works using an energy measure. Moreover, the second stage produces a result which is taken into account by the first stage in the analysis of the subsequent speech frame. In this manner, there is feedback between these two stages, ensuring that the values produced by the first stage forn an optimal basis for the decision of the second stage. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows a flow chart of a method for determining speech activity in a signal segment of an audio signal. 
   

   DETAILED DESCRIPTION 
   CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a U.S. National Stage Application under 35 U.S.C. §371 of PCT International Application No. PCT/EP01/03056, filed Mar. 13, 2001, which claims priority to German Patent Application No. 100 20 863.0, filed Apr. 28, 2000, and German Patent Application No. 100 26 872.2, filed May 31, 2000. Each of these applications is hereby incorporated by reference as if set forth in its entirety. 
   The principle of operation of the two stages will be presented separately below. 
   Referring to  FIG. 1 , in a method for determining speech activity in a signal segment of an audio signal, in a first stage it is assessed whether spectral stationarity is present in the signal segment (block  102 ). In a second stage it is assessed whether temporal stationarity is present in the signal segment (block  104 ). A decision on the presence of speech activity in the signal segment is made based on outputs of the first and second stages (block  106 ). 
   Initially, the first stage is presented which produces a first decision based on the analysis of the spectral stationarity. If the frequency spectrum of a signal segment is looked at, it has a characteristic shape for the observed period of time. If the change in the frequency spectra of temporally successive signal segments is sufficiently low, i.e., the characteristic shapes of the respective spectra are more or less maintained, then one can speak of spectral stationarity. 
   The result of the first stage is denoted by STAT 1  and the result of the second stage is referred to as STAT 2 . STAT 2  also corresponds to the final decision of the here presented VAD method. In the following, lists including a plurality of values in the form “list name [ 0  . . . N−1]” will be described; a single value being denoted via list name [k], k=0 . . . N−1, namely the value indexed by k of the list of values “list name”. 
   Spectral Stationarity (Stage 1) 
   This first stage of the stationarity method obtains the following quantities as input values:
         linear prediction coefficients of the current frame
           a) (LPC_NOW[ 0  . . . ORDER−1]; ORDER=14)   
           a measure for the voicedness of the current frame (STIMM[ 0  . . .  1 ])   the number of frames (N_INSTAT 2 , values =0, 1, 2, etc.) which have been classified as “non-stationary” by the second stage of the algorithm in the analysis of the preceding frames   different values (STIMM_MEM[ 0  . . .  1  ], LPC_STAT 1 [ 0  . . . ORDER−1]) computed for the preceding frame       

   The first stage produces, as output, the values
         first decision on stationarity: STAT 1  (possible values: “stationary”, “non-stationary”   linear prediction coefficients of the last frame classified as “stationary” (LPC_STAT 1 )       

   The decision of the first stage is primarily based on the consideration of the so-called “spectral distance” (“spectral difference”, “spectral distortion”) between the current and the preceding frames. The values of a voicedness measure which has been computed for the last frames are also considered in the decision. Moreover, the threshold values used for the decision are influenced by the number of immediately preceding frames classified as “stationary” in the second stage (i.e., STAT 2 =“stationary”). The individual calculations are explained below: 
   a) Calculation of the Spectral Distance: 
   The calculation is given by: 
           SD   =           1     2   ⁢   π       ⁢       ∫     -   π     π     ⁢         (       10   ⁢     log   ⁡     [     1            A   ⁡     (     ⅇ   jω     )            2       ]         -     10   ⁢     log   ⁡     [     1              A   ′     ⁡     (     ⅇ   jω     )            2       ]           )     2     ⁢           ⁢     ⅆ   ω             .           
In this context,
 
           10   ⁢     log   ⁡     [     1            A   ⁡     (     ⅇ   jω     )            2       ]             
denotes the logarithmized frequency response envelope of the current signal segment which is calculated from LPC_NOW.
 
           10   ⁢     log   ⁡     [     1              A   ′     ⁡     (     ⅇ   jω     )            2       ]             
denotes the logarithmized frequency response envelope of the preceding signal segment which is calculated from LPC_STAT 1 .
 
   Upon calculation, the value of SD is downward limited to a minimum value of 1.6. The value limited in this manner is then stored as the current value in a list of previous values SD_MEM[ 0  . . .  9 ], the oldest value being previously removed from the list. 
   Besides the current value for SD, an average value of the previous 10 values of SD is calculated as well, which is stored in SD_MEAN, the values from SD_MEM being used for the calculation. 
   b) Calculation of the Mean Voicedness: 
   The results of a voicedness measure (STIMM[ 0  . . .  1 ]) were also provided as an input value to the first stage. (These values are between 0 and 1 and were previously calculated as follows: 
   
     
       
         
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   The generation of the short-term average value of χ over the last 10 signal segments (m cur : index of the momentary signal segment) produces the values: 
               STIMM   ⁡     [   k   ]       =       1   10     ⁢       ∑     i   =       m   cur     -   10         m   cur       ⁢           ⁢     χ   i           ,     k   =   0     ,   1         
two values being calculated for each frame; STIMM[ 0 ] for the first half frame and STIMM[ 1 ] for the second half frame. If STIMM[k] has a value near 0, then the signal is clearly unvoiced whereas a value near 1 characterizes a clearly voiced speech region.)
 
   To first exclude disturbances in the special case of signals of very low volume (for example, prior to the signal start), the very small values of STIMM[k] resulting therefrom are set to 0.5, namely when their value was below 0.05 (for k=0, 1) up to that point. 
   The values limited in this manner are then stored as the most current values at point  19  in a list of the previous values STIMM_MEM[ 0  . . .  19 ], the most previous values being previously removed from the list. 
   Now, the mean is taken over the preceding 10 values of STIMM_MEM, and the result is stored in STIMM_MEAN. 
   The last four values of STIMM_MEM, namely values STIMM_MEM[ 16 ] through STIMM_MEM[ 19 ], are averaged once more and stored in STIMM  4 . 
   c) Consideration of the Number of Possibly Existing Isolated “Voiced” Frames: 
   If non-stationary frames should occasionally have occurred in the analysis or the preceding frames, then this is recognized from the value of N_INSTAT 2 . In this case, a transition into the “stationary” state has occurred only a few frames before. The LPC_STAT 1  values required for the second stage which are provided in the first stage, however, should not immediately be forced to a new value in this transition zone but only after several “safety frames” to be waited for. For the case that N_INSTAT 2 &gt;0, therefore, internal threshold value TRES_SD_MEAN which is used for the subsequent decision is set to a different value than otherwise.
         TRES_SD_MEAN=4.0 (if N_INSTAT 2 &gt;0)   TRES_SD_MEAN=2.6 (otherwise)
 
d) Decision
       

   To make the decision, initially, both SD itself and its short-term average value over the last 10 signal segments SD_MEAN are looked at. If both measures SD and SD_MEAN are below a threshold value TRES_SD and TRES_SD_MEAN, respectively, which are specific for them, then spectral stationarity is assumed. 
   Specifically, it applies for the threshold values that:
         TRES_SD=2.6 dB   TRES_SD_MEAN=2.6 or 4.0 dB (compare c)
 
and it is decided that
   STAT 1 =“stationary” if
           (SD&lt;TRES_SD) AND (SD_MEAN&lt;TRES_SD_MEAN),   
           STAT 1 =“non-stationary” (otherwise).       

   However, within a speech signal which should be classified as “non-stationary” according to the objective of VAD, segments can also occur for a short time which are considered to be “stationary” according to the above criterion. However, such segments can then be recognized and excluded via voicedness measure STIMM_MEAN. If the current frame was classified as “stationary” according to the above rule, then a correction can be carried out according to the following rule:
         STAT 1 =“non-stationary” if
           (STIMM_MEAN≧0.7) AND (STIMM 4 &lt;=0.56)   or (STIMM_MEAN&lt;0.3) AND (STIMM 4 &lt;=0.56)   or STIMM_MEM[ 19 ]&gt;1.5.
 
Thus, the result of the first stage is known.
 
e) Preparation of the Values for the Second Stage
   
               

   The second stage works using a list of linear prediction coefficients which is prepared in this stage, the linear prediction coefficients describing the signal portion that has last been classified as “stationary” by this stage. In this case, LPC_STAT 1  is overwritten by the current LPC_NOW (update):
         LPC_STAT 1 [k]=LPC_NOW[k], k=0 . . .  0 RDER−1 if   STAT 1 =“stationary”       

   Otherwise, the values in LPC_STAT 1  are not changed and thus still describe the last signal section that has been classified as “stationary” by the first stage. 
   Temporal Stationarity (Stage 2): 
   If a signal segment is observed in the time domain, then it has an amplitude or energy profile which is characteristic of the observed period of time. If the energy of temporally successive signal segments remains constant or if the deviation of the energy is limited to a sufficiently small tolerance interval, then one can speak of temporal stationarity. The presence of a temporal stationarity is analyzed in the second stage. 
   The second stage uses as input the following values
         the current speech signal in sampled form
           (SIGNAL [ 0  . . . FRAME_LEN−1], FRAME_LEN=240)   
           VAD decision of the first stage: STAT 1  (possible values: “stationary”, “non-stationary”)   the linear prediction coefficients describing the last “stationary” frame (LPC_STAT 1 [ 0  . . .  13 ])   the energy of the residual signal of the previous stationary frame (E_RES_REF)   a variable START which controls a restart of the value adaptation (START, values=“true”, “false”)       

   The second stage produces, as output, the values
         final decision on stationarity: STAT 2  (possible values: “stationary”, “non-stationary”)   the number of frames (N_INSTAT 2 , values=0, 1, 2, etc.) which have been classified as “non-stationary” by the second stage of the algorithm in the analysis of the preceding frames and the number of immediately preceding stationary frames N_STAT 2  (values=0, 1, 2, etc.).   variable START which was possibly set to a new value.       

   For the VAD decision of the second stage, the time rate of change of the energy of the residual signal is used which was calculated with LPC filter LPC_STAT 1  adapted to the last stationary signal segment and with current input signal SIGNAL. In this context, both an estimate of the most recent energy of the residual signal E_RES_REF as well as a lower reference value and a previously selected tolerance value E_TOL are considered in the decision. Then, the current energy value of the residual signal must not exceed reference value E_RES_REF by more than E_TOL if the signal is to be considered “stationary”. 
   The determination of the relevant quantities is described below. 
   a) Calculation of the Energy of the Residual Signal 
   Input signal SIGNAL[ 0  . . . FRAME_LEN−1] of the current frame is inversely filtered using the linear prediction coefficients stored in LPC_STAT 1  [ 0  . . . ORDER−1]. The result of this filtering is denoted as; “residual signal” and stored in SPEECH_RES[ 0  . . . FRAME_LEN−1]. 
   Thereupon, the energy E_RES of this residual signal SIGNAL_RES is calculated:
 
E_RES=Sum{SIGNAL_RES[k]*SIGNAL_RES[k]/FRAME_LEN},
         k=0 . . . FRAME_LEN−1
 
and then expressed logarithmically:
 
E_RES=10*log(E_RES/E_MAX),
 
Where
 
E_MAX=SIGNAL MAX*SIGNAL_MAX
       

   SIGNAL_MAX describes the maximum possible amplitude value of a single sample value. This value is dependent on the implementation environment; in a prototype based on an embodiment of the present invention, for example, it amounted to
         SIGNAL_MAX=32767; in other application cases, one would possibly have to put, for example:   SIGNAL_MAX =1.0       

   Value E_RES calculated in this manner is expressed in dB relative to the maximum value. Consequently, it is always below 0, typical values being about −100 dB for signals of very low energy and about −30 dB for signals with comparatively high energy. 
   If calculated value E_RES is very small, then an initial state exists, and the value of E_RES is downward limited:
         if (E_RES&lt;−200):   E_RES=−200   START=true       

   Actually, this condition can be fulfilled only at the beginning of the algorithm or in the case of very long very quiet pauses, so that it is possible to set value START=true only at the beginning. 
   Under this condition, the value of START is set to false:
         if (N_INSTAT 2 &gt;4):   START=false       

   To ensure the calculation of the reference energy of the residual signal also for the case of low signal energy, the following condition is introduced:
         if (START=false) AND (E_RES&lt;−65.0):   STAT 1 =“stationary”       

   In this manner, the condition for the adaptation of E_RES_REF is enforced also for very quiet signal pauses. 
   By using the energy of the residual signal, an adaptation to the spectral shape which has last been classified as stationary is carried out implicitly. If the current signal should have changed with respect to this spectral shape, then the residual signal will have a measurably higher energy than in the case of an unchanged, uniformly continued signal. 
   b) Calculation of the Reference Energy of the Residual Signal E_RES_REF 
   Besides the frequency response envelope described by LPC_STAT 1  of the frame that has last been classified as “stationary” by the first stage, in the second stage, the residual energy of this frame is stored as well and used as a reference value. This value is denoted by E_RES_REF. The residual energy is always redetermined exactly when the first stage has classified the current frame as “stationary”. In this case, previously calculated value E_RES is used as a new value for this reference energy E_RES_REF:
         If STAT 1 =“stationary” then set   E_RES_REF=E_RES if
           (E_RES&lt;E_RES_REF+12 dB) OR   (E_RES_REF&lt;−200 dB) OR   (E_RES&lt;−65 dB)   
               

   The first condition describes the normal case: Consequently, an adaptation of E_RES_REF almost always takes place when STAT 1 =“stationary”, because the tolerance value of 12 dB is intentionally selected to be large. The other conditions are special cases; they cause an adaptation at the beginning of the algorithm as well as a new estimate in the case of very low input values which are in any case intended to be taken as a new reference value. 
   c) Determination of Tolerance Value E_TOL 
   Tolerance value E_TOL specifies for the decision criterion a maximum permitted change of the energy of the residual signal with respect to that of the previous frame in order that the current frame can be considered “stationary”. Initially, one sets
         E_TOL=12 dB
 
Subsequently, however, this preliminary value is corrected under certain conditions:
   if N_STAT 2 &lt;=10:   E_TOL=3.0
 
otherwise
   if E_RES&lt;−60:   E_TOL=13.0
 
otherwise
   if E_RES&gt;−40:   E_TOL=1.5
 
otherwise
   E_TOL=6.5       

   The first condition ensures that a stationarity which, until now, has only been present for a short period of time, can be exited very easily in that the decision of “non-stationary” is made more easily due to low tolerance E_TOL. The other cases include adaptations which provide most suitable values for different special cases, respectively (it should be more difficult for segments of very low energy to be classified as “non-stationary”; segments with comparatively high energy should be classified as “non-stationary” more easily). 
   d) Decision 
   The actual decision now takes place using the previously calculated and adapted values E_RES, E_RES_REF and E_TOL. Moreover, both the number of consecutive “stationary” frames N_STAT 2  and the number of preceding non-stationary frames N_INSTAT 2  are set to current values. 
   The decision is made as follows:
         if (E_RES&gt;E_RES_REF+E_TOL):
           STAT 2 =“non-stationary”   N_STAT 2 =0   N_INSTAT 2 =N_INSTAT 2 +1
 
otherwise
   
           STAT 2 =“stationary”   N_STAT 2 =N_STAT 2 +1   If N_STAT 2 &gt;16:
           N_INSTAT=0   
               

   Thus, the counter of the preceding stationary frames N_STAT 2  is set to 0 immediately when a non-stationary frame occurs whereas the counter for the preceding non-stationary frames N_INSTAT 2  is set to 0 only after a certain number of consecutive stationary frames are present (in the implemented prototype: 16). N_INSTAT 2  is used as an input value of the first stage where it influences the decision of the first stage. Specifically, the first stage is prevented via N_INSTAT 2  from redetermining coefficient set LPC_STAT 1  describing the envelope spectrum before it is guaranteed that a new stationary signal segment is actually present. Thus, short-term or isolated STAT 2 =“stationary” decisions can occur but it is only after a certain number of consecutive frames classified as “stationary” that coefficient set LPC_STAT 1  describing the envelope spectrum is also redetermined in the first stage for the then present stationary signal segment. 
   According to the principle of operation described for the second stage and the introduced parameters, the second stage will never change a STAT 1 =“stationary” decision of the first stage to “non-stationary” but will always make the decision STAT 2 =“stationary” in this case as well. 
   A “STAT 1 =“non-stationary” decision of the first stage, however, can be corrected by the second stage to a STAT 2 =“stationary” decision or also be confirmed as STAT 2 =“non-stationary”. This is the case, in particular, when the spectral non-stationarity which has resulted in STAT 1 =“non-stationary” in the first stage was caused only by isolated spectral fluctuations of the background signal. However, this case is decided anew in the second stage, taking account of the energy. 
   It goes without saying that the algorithms for determining the speech activity, the stationarity and the periodicity must or can be adapted to the specific given circumstances accordingly. The individual threshold values and functions mentioned above are only exemplary and generally have to be found by separate trials.