Abstract:
A voice activity detector is disclosed for use with a radio transmitter to continuously sense the presence of speech in an audio signal. Initially, the audio signal is processed to produce a train of signal samples. Signal peaks are identified therefrom, which are used to compute respective values for a succession of quasi-pitch periods associated with the signal sample train. The quasi-pitch period values are then selectively compared with one another, in order to determine the presence or absence of a speech component.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention disclosed and claimed herein generally pertains to voice activity detection in a communication system. More particularly, the invention pertains to a voice activity detector which is of reduced complexity and does not require a multiplier. Even more particularly, the invention pertains to a voice activity detector for use with a transmitter disposed to transmit audio signals through an air interface, wherein transmissions are to be discontinued whenever speech is absent from the signal.  
           [0002]    There is increasing interest in providing mobile phones and other small electronic devices with the capability to connect and communicate wirelessly, over short ranges, with one another. Such capability would eliminate or substantially reduce the need for cables between devices such as phones, PC cards, headsets and laptop computers. Moreover, a number of such devices could thereby be joined together, very readily, to form small networks. As an example of this interest, the assignee herein, a major supplier of mobile telecommunication equipment and systems, has initiated a program known as the Bluetooth air interface to develop wireless communication capability of the above type.  
           [0003]    In interconnecting small devices over a short range air interface, it is very important to limit costs, and also to reduce overall interference levels on the interface. Accordingly, in an air interface such as the Bluetooth interface, a transmitter disposed to receive speech is provided with a discontinuous transmission (DTX) capability, which causes the transmitter to be switched off during speech pauses. Such capability reduces cost, by minimizing transmitter power requirements, and also reduces signal interference level. However, the discontinuous transmission functionality requires a Voice Activity Detector (VAD) to continually determine whether or not an audio signal, applied or inputted to the transmitter, contains a speech component In the past, VAD dectectors could require voice or speech coders. As disclosed, for example, by U.S. Pat. No. 6,023,674, issued to one of the inventors herein, such coders may be very complex. Prior art VAD detectors could also require multiplier devices, as disclosed by U.S. Pat. No. 5,970,441, likewise issued to one of the inventors herein. It would be very desirable to provide a VAD for an air interface of the above type which is of substantially reduced complexity, and which does not need either a multiplier or a voice coder of the type described in the prior art.  
         SUMMARY OF THE INVENTION  
         [0004]    In order to achieve the above objectives, the present invention discloses a VAD concept which is based on speech signal properties and on parameters extracted from a wave form disposed to carry the speech signal. In one embodiment, the invention is directed to a method for detecting the presence or absence of a speech component in an audio signal. Such method generally comprises the steps of processing the audio signal to produce a train of signal samples, and computing respective values of a succession of quasi-pitch (Q_Pitch) periods associated with the train of samples. The method further comprises the step of comparing the values of selected Q_Pitch periods with one another, to determine whether or not a speech component is present in the audio signal.  
           [0005]    In a preferred embodiment of the invention, the comparing step comprises determining whether or not successive Q Pitch periods have similar values, or lengths, over a number of frames. It has been recognized that if the Q_Pitch periods over a group of adjacent frames indeed do have similar values, this provides an indication of a sustained pitch, as contained in a voice signal. In accordance with the invention, such determination may be made by comparing respective values of a specified number of adjacent Q_Pitch periods, and concluding that a speech component is present if all of the values of the compared Q_Pitch periods are the same, to within a specified limit or narrow range of values.  
           [0006]    Preferably also, as described herein in further detail, respective Q_Pitch period values are computed by identifying signal peaks in respective frames in a succession of frames, and then calculating the spacings between peaks. In order to reduce the amount of samples needed for the method, as well as to simplify the required signal processing, initial processing of the audio signal includes application of an absolute value function or half-wave rectification to the incoming audio signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a block diagram illustrating an embodiment of the invention.  
         [0008]    [0008]FIG. 2 shows a graph depicting a train of signal samples generated by the embodiment of FIG. 1.  
         [0009]    [0009]FIG. 3 is a flow chart illustrating operation of the embodiment shown in FIG. 1.  
         [0010]    [0010]FIG. 4 is a block diagram showing the embodiment of FIG. 1 incorporated into an air interface.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0011]    Referring to FIG. 1, there is shown a voice detector (VAD) 10  constructed in accordance with the invention. Detector  10  includes a preprocessing component  12  which receives an audio frequency signal s(n) having a voice component from a microphone  14  or other source. Preprocessor  12  samples the audio signal to provide a stream or train of signal samples x(n) Preferably, preprocessor  14  includes an absolute value function |X| or a half-way rectifier (HREC) to reduce the number of samples needed to represent the signal, thereby simplifying subsequent signal processing.  
         [0012]    Referring further to FIG. 1, there is shown a component or function block  16  which is disposed to receive the samples x(n) and to compute the average magnitude {overscore (X)} thereof. More particularly, successive samples x(n) are grouped in sets of frames, each having a length M. Function block  16  computes the average magnitude of the respective M samples in each frame, using the following equation:  
               X   _     =       1   M            ∑     n   =   1     M                          x        (   n   )                        Eqn   .                (   1   )                                 
 
         [0013]    For instance, if Bluetooth is the target system, then a frame length M can be taken to be 2×3.75 ms period length (2×HV3 packet length). HV3 pertains to high quality voice packets utilizing the synchronous communication link (SCO) in the specification of the Bluetooth voice interface, referred to above, and has a packet period of 3.75 milliseconds.  
         [0014]    Referring to FIG. 2, there are shown signal samples x(n) respectively included in frames F 1 , F 2  and F i  . . . F i+4 , where each frame is of length M as stated above. Moreover, it is to be emphasized that FIG. 2 shows only the samples x(n) of a frame which are equal to or greater than the average magnitude thereof, computed in accordance with Equation (1). Samples x(n) which are less than the average magnitude for a frame are disregarded, for reasons set forth hereafter. FIG. 2 further shows the maximum value of x(n) for frame F i  occurring at n=m i , that is, m i  is the maximum value index of frame F i . Thus, x(m i ) is a signal peak for frame F i . Similarly, frames F i+1  . . . F i+3  have signal peaks at n=m i+1  . . . m i+3 , respectively.  
         [0015]    The term quasi-pitch (Q_pitch), as used herein, is the period between adjacent signal peaks. Thus, in FIG. 2 m i+1 -m i  is the Q_pitch period L i  between signal peaks x(m i ) and x(m i+1 ). In the invention, it has been recognized that if successive Q_pitch periods have similar lengths or values over a number of frames, this is an indication of a sustained pitch contained in a voice signal. It has been recognized further that by detecting the signal peaks of successive frames, it becomes possible to provide a comparatively simple yet effective, technique to continually monitor the audio signal for the presence of speech. Accordingly, VAD  10  of FIG. 1 is further shown provided with a peak detection and Q_pitch period computation component  18 . Component  18  is coupled to receive the signal samples x(n) and also to receive the average magnitude {overscore (X )}for successive signal frames.  
         [0016]    Referring to FIG. 3, there are shown several process blocks depicting operation of computation component  18  in detecting the value of a Q_pitch period associated with the signal samples x(n) of the frame F i . Process block  20  receives respective samples x(n) from component  12 , which preprocesses audio signal s(n) as described above to provide x(n). For the i th  frame, block  20  provides the vector X i  which comprises respective samples x k  of frame F i , k=1 . . . M. The respective samples are coupled to process block  22 , which also receives the average magnitude (Avg Magn.) computed for frame F i  by component  16 .  
         [0017]    Process block  22  compares each received x(n) sample with the average magnitude computed for the i th  frame, and selects only the samples which are equal to or greater than the average magnitude. The remaining samples, those which are less than the average magnitude, are disregarded Moreover, process block  22  functions to compare the values of respective samples x(n) received thereby with one another. The results of such comparison provide max x(n), that is, the signal sample of maximum value or magnitude for the i th  frame, which is the signal peak for the frame as stated above. It will be seen that by disregarding all signal samples which are less than average magnitude {overscore (X)}, the processing task which must be carried out by block  22  is significantly simplified. Clearly, all such signals of lesser value are irrelevant to a process which seeks to identify the sample of maximum value. Identifying the sample x(n) of maximum value also identifies the max value index mi thereof. Thus, max x(n)=x(mi) for the i th  frame.  
         [0018]    Referring further to FIG. 3, there is shown max value index mi furnished to process block  24 , which computes Q-pitch period Li from the quantity (m i -m i−1 ), wherein L i−1 =(m i+1           m i ). m i+1  is the max index value for the signal peak of the frame F i+1 , which immediately follows frame F i . After L i  for frame F i  has been determined, function block  25  of computation component  18  updates component  20  to receive respective samples for frame F i+1  to proceed with computation of L i+1 , the value of the next—following Quasi_pitch period.  
         [0019]    Referring again to FIG. 1, there is shown VAD  10  provided with a decision block  26  coupled to receive respective Q_pitch periods from computation component  18 , as well as average magnitudes respectively determined by block  16 . Decision block  26  operates in accordance with the principles set forth above, to detect successive Q_pitch periods having similar values over a number of frames. More specifically, decision block  26  is provided with specific criteria. For example, block  26  may be set up to conclude that a succession of Q_pitch periods indicate the presence of a speech component whenever the calculated Q_pitch periods for 5 successive frames remain close in value to each other within a tolerance limit of +/− 8 samples. Thus, when decision block  26  detects this condition from input Q_pitch period values, it generates a logic 1 flag to denote the presence of a voice or speech component in the audio signal. Otherwise, block  26  generates a logic 0 to indicate a silent condition.  
         [0020]    Usefully, block  26  is further constructed to determine whether successively received average magnitude values are above or below a specified threshold. In such embodiment, if a succession of average magnitude values are below the threshold, block  20  will conclude that speech is not present and generate a logic 0 flag notwithstanding successive Q-pitch periods which meet the above criteria.  
         [0021]    Referring to FIG. 4, there is shown an air interface for signal transmissions, which comprises a receiver  28  and a transmitter  30  incorporating VAD  10 . Transmitter  30  is provided with a Continuously Variable Slope Deltamodulation encoder (CVSD-enc.), usefully of 64 kb/s, which is used to implement the voice encoder algorithm. Receiver  28  is provided with a corresponding CVSD decoder  34 .  
         [0022]    Transmitter  30  is further provided with a DTX mechanism  36 , comprising VAD  10  and a DTX component  38  responsive to the flag generated by decision block  26  of VAD  10 . Thus, DTX enables transmission to occur when a flag 1 is produced, indicating a speech present or voice condition. The DTX discontinues transmission when a flag 0 is produced, indicating a speech absent or silent condition. Moreover, the DTX uses the VAD flag to extract silent or background information for use at the receiver  28 . At the receiver, a comfort noise generator (BLT-CN)  40  uses this information to generate a noise signal similar to that which occurs during periods of silence. The comfort noise replaces the voice decoder output during periods of silence.  
         [0023]    Obviously, other modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the disclosed concept, the invention may be practiced otherwise than as specifically described.