Abstract:
A virtual speaker apparatus and a virtual speaker processing method is disclosed. The virtual speaker apparatus uses the closest surround speaker when a virtual speaker to be virtually generated is a back left virtual speaker or a back right virtual speaker.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2009-0098072, filed on Oct. 15, 2009, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    One or more example embodiments relate to a virtual speaker apparatus and a virtual speaker processing method. 
         [0004]    2. Description of the Related Art 
         [0005]    In the past, audio technology tended to be driven by quality-centric development. However, consumer demand for a reality-centric audio technology steadily increases as the capacity of current systems rapidly increases. Recently, a general audio-play environment has at least one of a stereo speaker system and a 5.1 channel speaker system. However, an existing configuration may lack the capacity to play a number of channels that is greater than a number of channels played by the 5.1 channel system. Thus, a virtual speaker technology that enables a user to experience audio beyond the capabilities of the 5.1 channel system through the 5.1 channel system has been developed. 
       SUMMARY 
       [0006]    According to example embodiments of the present disclosure, a virtual speaker apparatus may be provided. The virtual speaker apparatus includes a first adder to add a first virtual channel signal and a second virtual channel signal, a second adder to subtract the second virtual channel signal from the first virtual channel signal, a first filter to perform filtering of a signal outputted from the first adder based on a ratio of a sum of an ipsilateral transfer function and a contralateral transfer function in a virtual location to a sum of an ipsilateral transfer function and a contralateral transfer function in a location of an actual speaker, a second filter to perform filtering of a signal outputted from the second adder based on a ratio of a subtraction between the ipsilateral transfer function and the contralateral transfer function in the virtual location to a subtraction between the ipsilateral transfer function and the contralateral transfer function in the location of the actual speaker, a third adder to add a signal outputted from the first filter and a signal outputted from the second filter, and a fourth adder to subtract the signal outputted from the second filter from the signal outputted from the first filter. In this instance, the first filter performs the filtering of sum of the first virtual channel signal and the second virtual channel signal based on ratio of a sum of the ipsilateral transfer function and the contralateral transfer function in the virtual location to a sum of the ipsilateral transfer function and the contralateral transfer function in the location of the actual speaker. Also, the second filter performs the filtering of subtraction between the first virtual channel signal and the second virtual channel signal based on ratio of a subtraction between the ipsilateral transfer function and the contralateral transfer function in the virtual location to a subtraction between the ipsilateral transfer function and the contralateral transfer function in the location of the actual speaker. 
         [0007]    Also, the speaker apparatus further includes a fast Fourier transform (FFT) unit being installed on a front-end of the first adder and the second adder, and performing a FFT of the first virtual channel signal and the second virtual channel signal, an inverse FFT (IFFT) unit being installed on a back-end of the third adder and the fourth adder, and performing an IFFT of a signal outputted from the third adder and a signal outputted from the fourth adder, a plurality of delay units to delay each of signals outputted from a plurality of actual speakers, a fifth adder to add a signal outputted from one of the plurality of delay units and a first signal outputted from the IFFT unit, and a sixth adder to add a signal outputted from one of the plurality of delay units and a second signal outputted from the IFFT unit. In this instance, the plurality of delay units includes a first delay unit to delay a front first direction channel signal, a second delay unit to delay a front second direction channel signal, a third delay unit to delay a front third direction channel signal, a fourth delay unit to delay a low frequency effect channel signal, a fifth delay unit to delay a surround first direction channel signal, and a sixth delay unit to delay a surround second direction channel signal, and the fifth adder adds a signal outputted from the fifth delay unit and the first signal outputted from the IFFT unit, and outputs a result of the addition via the surround first direction speaker, and the sixth adder adds a signal outputted from the sixth delay unit and the second signal outputted from the IFFT unit, and outputs a result of the addition via the surround second direction speaker. 
         [0008]    Also, the virtual speaker apparatus further includes a fifth adder to add a surround first direction channel signal to a signal outputted from the third adder, a sixth adder to add a surround second channel signal to a signal outputted from the fourth adder, and an Inverse Modified Discrete Cosine Transform (IMDCT) unit to receive a front first direction channel signal, a front second direction channel signal, a front third direction channel signal, a low frequency effect channel signal, a signal outputted from the fifth adder, and a signal outputted from the sixth adder, and to perform an IMDCT of the received signals. 
         [0009]    Also, the virtual speaker apparatus further includes an IMDCT unit to receive a front first direction channel signal, a front second direction channel signal, a front third direction channel signal, a low frequency effect channel signal, a first signal outputted from the third adder, and a second signal outputted from the fourth adder, and to perform an IMDCT of the received signals, a first delay unit to phase-delay the IMDCT-transformed first signal, a second delay unit to phase-delay the IMDCT-transformed second signal, a fifth adder to add an IMDCT-transformed surround first direction channel signal and a signal outputted from the first delay unit, and a sixth adder to add an IMDCT-transformed surround second direction channel signal and a signal outputted from the second delay unit. 
         [0010]    According to other example embodiments of the present disclosure, a method of processing a virtual speaker may be provided. The method includes a first adding operation to add a first virtual channel signal and a second virtual channel signal, a second adding operation to subtract the second virtual channel signal from the first virtual channel signal, a first filtering operation to perform filtering of a result signal of the summation in the first adding operation, based on a ratio of a sum of an ipsilateral transfer function and a contralateral transfer function in a virtual location to a sum of an ipsilateral transfer function and a contralateral transfer function in a location of an actual speaker, a second filtering operation to perform filtering of a result signal of the subtraction in the second adding operation, based on a ratio of a subtraction between the ipsilateral transfer function and the contralateral transfer function in the virtual location to a subtraction between the ipsilateral transfer function and the contralateral transfer function in the location of the actual speaker, a third adding operation to add a first signal filtered in the first filtering operation and a second signal filtered in the second filtering operation, and a fourth adding operation to subtract the second signal filtered in the second filtering operation from the first signal filtered in the first filtering operation. 
         [0011]    In this instance, the first filter performs the filtering of sum of the first virtual channel signal and the second virtual channel signal based on ratio of a sum of the ipsilateral transfer function and the contralateral transfer function in the virtual location to a sum of the ipsilateral transfer function and the contralateral transfer function in the location of the actual speaker. Also, the second filter performs the filtering of subtraction between the first virtual channel signal and the second virtual channel signal based on ratio of a subtraction between the ipsilateral transfer function and the contralateral transfer function in the virtual location to a subtraction between the ipsilateral transfer function and the contralateral transfer function in the location of the actual speaker. 
         [0012]    Also, the virtual speaker processing method further includes an FFT operation being performed in advance of the first adding operation and the second adding operation, and performing an FFT of the first virtual channel signal and the second virtual channel signal, an IFFT operation being performed after the third adding operation and the fourth adding operation, and performing an IFFT of a result signal of the third adding operation and a result signal of the fourth adding operation, a plurality of delaying operations delaying each of signals outputted from a plurality of actual speakers, a fifth adding operation adding a result signal from one of the plurality of delaying operations and a first result signal of the IFFT operation, and a sixth adding operation adding a result signal from one of the plurality of delaying operations and a second result signal of the IFFT operation. In this instance, the plurality of delaying operations respectively delay a front first direction channel signal, a front second direction channel signal, a front third direction channel, a low frequency effect channel signal, a surround first direction channel signal, and a surround second direction channel signal. 
         [0013]    Also, the virtual speaker processing method further includes a fifth adding operation to add a surround first direction channel signal and a result signal of the third adding operation, a sixth adding operation to add a surround second channel signal and a result signal of the fourth adding operation, and an IMDCT performing operation to receive a front first direction channel signal, a front second direction channel signal, a front third direction channel signal, a low frequency effect channel signal, a result signal of the fifth adding operation, and a result signal of the sixth adding operation, and to perform an IMDCT of the received signals. 
         [0014]    Also, the virtual speaker processing method includes an IMDCT performing operation to receive a front first direction channel signal, a front second direction channel signal, a front third direction channel signal, a low frequency effect channel signal, a first signal outputted from the third adder, and a second signal outputted from the fourth adder, and to perform an IMDCT of the received signals, a first delaying operation to phase-delay the IMDCT-transformed first signal, a second delaying operation to phase-delay the IMDCT-transformed second signal, a fifth adding operation to add an IMDCT-transformed surround first direction channel signal to a result signal of the first delaying operation, and a sixth adding operation to add an IMDCT-transformed surround second direction channel signal to a result signal of the second delaying operation. 
         [0015]    Additional aspects and/or advantages will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    These and/or other aspects and advantages will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
           [0017]      FIG. 1  is a diagram illustrating an example of a configuration of a virtual 7.1 channel speaker system using a 5.1 channel speaker system according to an embodiment; 
           [0018]      FIG. 2  is a block diagram illustrating a back-left virtual speaker generating apparatus and a back-right virtual speaker generating apparatus of a 7.1 channel speaker system, when a scheme is asymmetric according to an embodiment; 
           [0019]      FIG. 3  is a block diagram illustrating a back-left virtual speaker generating apparatus and a back-right virtual speaker generating apparatus of a 7.1 channel speaker system, when a scheme is symmetric according to an embodiment; 
           [0020]      FIG. 4  is a diagram illustrating a configuration of a virtual speaker circuit according to an embodiment; 
           [0021]      FIG. 5  is a diagram illustrating an example of a virtual speaker apparatus according to an embodiment; 
           [0022]      FIG. 6  is a diagram illustrating an example of an overlap-adding scheme according to an embodiment; 
           [0023]      FIG. 7  is a diagram illustrating an example of a virtual speaker apparatus that is a combination of a virtual speaker circuit and an audio decoder according to an embodiment; and 
           [0024]      FIG. 8  is a diagram illustrating another example of a virtual speaker apparatus that is a combination of a virtual speaker circuit and an audio decoder according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Reference will now be made in detail to example embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. Example embodiments are described below to explain the present disclosure by referring to the figures. 
         [0026]    Generally, a technology that enables a user to experience a virtual audio channel in addition to a physical sound source is described as a virtual speaker. The virtual speaker technology utilizes a technology that compares sounds that come in both ears and determines a direction from which the sound is transferred. 
         [0027]    Two factors used for evaluating a spatial location of a sound source in a free field are an Inter-aural Time Difference (ITD) and an Inter-aural Intensity Difference (IID). The ITD is defined as a difference between the arrival time of sound waves arriving at left and right ears, and the IID is defined as a difference between sound pressure levels of sound waves arriving at left and right ears. Generally, a recognized lateral drift is proportional to a phase difference between sounds arriving at both ears. However, when a wave length of about 15 kHz that corresponds to a diameter of a human head, the recognized lateral drift may occupy more than the wave length of the diameter of the human head in a frequency of more than 15 kHz, and thus, aliasing may occur. Accordingly, for frequencies greater than 15 kHz, the phase is of no use in determining the spatial location. 
         [0028]    However, for frequencies greater than 15 kHz, the head of the human shields the ear that is most distant from the sound, and thus, the ear receives a weaker sound intensity than the other ear. In this instance, an intensity difference of the sound at both ears is defined as the IID, and an audio signal having a bandwidth greater than 15 kHz is more affected by the IID than by ITD. 
         [0029]    The concepts of IID and ITD may explain much and enhance understanding of the mechanism that humans use to recognize a location based on the sound. However, determining location based on only the IID and ITD may cause a cone of confusion that is not able to provide a unique spatial position. The cone of confusion may occur because diffraction or absorption of the perceived sound caused by the human body, such as hair, an external ear, or a shoulder, is not considered. Although it is not easy to mathematically and accurately understand and predict a sound phenomenon occurring in a real life, a direction-dependent frequency response of an ear may be obtained by a simulation, an empirical measurement, or a physical model based on a large amount of research data, as a first step for understanding a spectral cue. The measured data is referred to as Head-related transfer function (HRTF), and the HRTF synthesizes a direction dependent acoustic filter in a free field. The binaural synthesis through the HRTF is performed by convolving an input signal by using a pair of HRTFs. 
         [0000]    
       
         
           
             
               
                 
                   
                     x 
                     = 
                     hx 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       x 
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 x 
                                 L 
                               
                             
                           
                           
                             
                               
                                 x 
                                 R 
                               
                             
                           
                         
                         ] 
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       h 
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 H 
                                 L 
                               
                             
                           
                           
                             
                               
                                 H 
                                 R 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
         [0030]    Here, x is the input signal, x is a column vector of the binaural signal, and h is a column vector including the pair of HRTFs. The synthesized x is more proper to a play system based on a headphone, and thus, is referred to as a binaural signal. The binaural signal may be represented as a sum of sounds that are mapped with various different locations as given in Equation 2. 
         [0000]    
       
         
           
             
               
                 
                   x 
                   = 
                   
                     
                       ∑ 
                       
                         i 
                         = 
                         1 
                       
                       N 
                     
                      
                     
                         
                     
                      
                     
                       
                         h 
                         i 
                       
                        
                       
                         x 
                         i 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
         [0031]    Here, h i  is an HRTF for x i . 
         [0032]    To transfer the binaural signal via a loudspeaker, a process of appropriately filtering based on a transfer function having a 2×2 matrix C is performed. 
         [0000]    
       
         
           
             
               
                 
                   
                     y 
                     = 
                     Cx 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       y 
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 y 
                                 L 
                               
                             
                           
                           
                             
                               
                                 y 
                                 R 
                               
                             
                           
                         
                         ] 
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       C 
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 
                                   C 
                                   11 
                                 
                                  
                                 
                                   C 
                                   12 
                                 
                               
                             
                           
                           
                             
                               
                                 
                                   C 
                                   21 
                                 
                                  
                                 
                                   C 
                                   22 
                                 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
         [0033]    The loudspeaker signal y vector is referred to as a loudspeaker binaural signal, and a filter C is referred to as a crosstalk canceller. In a standard-stereo listening environment, an ear signal is associated with a speaker signal based on Equation 4. 
         [0000]    
       
         
           
             
               
                 
                   
                     e 
                     = 
                     Ay 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       e 
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 e 
                                 L 
                               
                             
                           
                           
                             
                               
                                 e 
                                 R 
                               
                             
                           
                         
                         ] 
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       A 
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 
                                   A 
                                   LL 
                                 
                                  
                                 
                                   A 
                                   RL 
                                 
                               
                             
                           
                           
                             
                               
                                 
                                   A 
                                   LR 
                                 
                                  
                                 
                                   A 
                                   RR 
                                 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     4 
                   
                   ] 
                 
               
             
           
         
       
     
         [0034]    Here, e is a column vector of the ear signal, A is an acoustical transfer matrix of the ear signal, and y is a column vector of the speaker signal. The ear signal is assumed to be measured by an ideal transducer that is able to catch all directional features of a head response in an ear canal. Also, an A xy  function provides a transfer function of a speaker Xε{L, R}, and includes a frequency response of the speaker, an air propagation, and a head response. A may be factorized as given in Equation 5. 
         [0000]    
       
         
           
             
               
                 
                   
                     A 
                     = 
                     HS 
                   
                    
                   
                     
 
                   
                    
                   
                     
                       H 
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 
                                   H 
                                   LL 
                                 
                                  
                                 
                                   H 
                                   RL 
                                 
                               
                             
                           
                           
                             
                               
                                 
                                   H 
                                   LR 
                                 
                                  
                                 
                                   H 
                                   RR 
                                 
                               
                             
                           
                         
                         ] 
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       S 
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 
                                   S 
                                   L 
                                 
                                  
                                 
                                   A 
                                   L 
                                 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               
                                 
                                   S 
                                   R 
                                 
                                  
                                 
                                   A 
                                   R 
                                 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
         [0035]    Here, H is an HRTF matrix that is normalized as a free field response from a center of the head, and S, which is a transfer matrix of the speaker and air, explains a frequency response of the speaker and air propagation that is transferred to a listener via air. S x  is a frequency response of a speaker X, and A x  represents a transfer function of air propagation from the speaker X to the center of the head. 
         [0036]    To accurately transfer the binaural signal to an ear of the human, a crosstalk canceller C is determined to be inverse of the transfer function as given in Equation 6. 
         [0000]      C=A −1 =S −1 H −1   [Equation 6]
 
         [0037]    Here, H −1  is an inverse of the head transfer matrix, and S −1  is an inverse filter of a response of each speaker and is represented as Equation 7. 
         [0000]    
       
         
           
             
               
                 
                   
                     S 
                     
                       - 
                       1 
                     
                   
                   = 
                   
                     [ 
                     
                       
                         
                           
                             
                               1 
                               / 
                               
                                 S 
                                 L 
                               
                             
                              
                             
                               A 
                               L 
                             
                           
                         
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                         
                           
                             
                               1 
                               / 
                               
                                 S 
                                 R 
                               
                             
                              
                             
                               A 
                               R 
                             
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     7 
                   
                   ] 
                 
               
             
           
         
       
     
         [0038]    Here, 1/S X  item is an inverse of a speaker frequency response and 1/A X  item is an inverse of air propagation. 
         [0039]    When the listener is located at an equal distance from two well adjusted high quality loudspeakers, the Equations may be omitted. However, when the listener moves to a location where distances between the location and the two high quality loudspeakers are different from each other, a volume of a nearby located loudspeaker is decreased, and also a time delay is given to enable signals from the two loudspeaker to simultaneously arrive at the listener and to have an identical sound pressure. The described process may be performed by correcting 1/A X  in Equation 7. 
         [0040]    Connection of a crosstalk canceller having a sufficient modeling delay to be a causal system is needed, and Equation 8 is formulated when a discrete-time modeling delay m is added. 
         [0000]        C ( z )= z   −m   S   −1 ( z ) H   −1 ( z )  [Equation 8]
 
         [0041]    A desired amount of modeling delay is related to a special embodiment. Also, hereinafter, for ease of description, a modeling delay and a speaker function S −1  item will be omitted and only a head transfer matrix will be considered. Accordingly, a general expression of Equation 5 will be simply represented as Equation 9. 
         [0000]      C=H −1   [Equation 9]
 
         [0042]    Here, an inverse head transfer matrix is expressed as given in Equation 10. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       H 
                       
                         - 
                         1 
                       
                     
                     = 
                     
                       
                         [ 
                         
                           
                             
                               
                                 H 
                                 RR 
                               
                             
                             
                               
                                 - 
                                 
                                   H 
                                   RL 
                                 
                               
                             
                           
                           
                             
                               
                                 - 
                                 
                                   H 
                                   LR 
                                 
                               
                             
                             
                               
                                 H 
                                 LL 
                               
                             
                           
                         
                         ] 
                       
                        
                       
                         1 
                         D 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     D 
                     = 
                     
                       
                         
                           H 
                           LL 
                         
                          
                         
                           H 
                           RR 
                         
                       
                       - 
                       
                         
                           H 
                           LR 
                         
                          
                         
                           H 
                           RL 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     10 
                   
                   ] 
                 
               
             
           
         
       
     
         [0043]    Here, D is a determinant of a matrix H, and an inverse determinant 1/D is an important factor to determine a stability of an inverse filter, since the inverse determinant 1/D is commonly applied to all terms. When the determinant is “0” in a predefined frequency, a head transfer matrix is a singular, and an inverse matrix does not exist. 
         [0044]    A method of embodying the crosstalk canceller assumes a symmetric listening condition, and has an advantage of being easily embodied although a symmetric solution is a unique case of a general solution. When the listening condition is assumed to be symmetric, a transfer function may be defined as given in Equation 11. 
         [0000]      H i =H LL =H RR    
         [0000]      H c =H LR =H RL   [Equation 11]
 
         [0045]    Here, H i  is an ipsilateral transfer function, and H c  is a contralateral transfer function. When a symmetric parameter is used in Equation 10, it is represented as Equation 12. 
         [0000]    
       
         
           
             
               
                 
                   
                     H 
                     
                       - 
                       1 
                     
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
                             
                               H 
                               i 
                             
                           
                           
                             
                               - 
                               
                                 H 
                                 c 
                               
                             
                           
                         
                         
                           
                             
                               - 
                               
                                 H 
                                 c 
                               
                             
                           
                           
                             
                               H 
                               i 
                             
                           
                         
                       
                       ] 
                     
                      
                     
                       1 
                       
                         
                           H 
                           i 
                           2 
                         
                         - 
                         
                           H 
                           c 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     12 
                   
                   ] 
                 
               
             
           
         
       
     
         [0046]    A symmetric formula indicates a crosstalk canceller using a shuffler. The shuffler calculates a sum of a binaural input signal and a subtraction of the binaural input signal, and appropriately performs filtering each signal. The filtered signal is returned again to an original state based on the summation and the subtraction. In the shuffler, the summation and the subtraction is represented as a unitary matrix U, and the unitary matrix U is referred to shuffler matrix. 
         [0000]    
       
         
           
             
               
                 
                   U 
                   = 
                   
                     
                       [ 
                       
                         
                           
                             1 
                           
                           
                             1 
                           
                         
                         
                           
                             1 
                           
                           
                             
                               - 
                               1 
                             
                           
                         
                       
                       ] 
                     
                      
                     
                       1 
                       
                         2 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     13 
                   
                   ] 
                 
               
             
           
         
       
     
         [0047]    A column of the matrix U is an eigenvector of a symmetric 2×2 matrix, and thus, the shuffler matrix U is digitalized into a symmetric matrix H −1  through a similarity transformation. 
         [0000]    
       
         
           
             
               
                 
                   
                     H 
                     
                       - 
                       1 
                     
                   
                   = 
                   
                     
                       
                         U 
                         
                           - 
                           1 
                         
                       
                        
                       
                         [ 
                         
                           
                             
                               
                                 1 
                                 
                                   
                                     H 
                                     i 
                                   
                                   + 
                                   
                                     H 
                                     c 
                                   
                                 
                               
                             
                             
                               0 
                             
                           
                           
                             
                               0 
                             
                             
                               
                                 1 
                                 
                                   
                                     H 
                                     i 
                                   
                                   - 
                                   
                                     H 
                                     c 
                                   
                                 
                               
                             
                           
                         
                         ] 
                       
                     
                      
                     U 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     14 
                   
                   ] 
                 
               
             
           
         
       
     
         [0048]    Accordingly, the crosstalk canceller calculates a shuffler filter Σ that is an inverse of a sum of an ipsilateral response and a contralateral response, and a shuffler filter Δ that is an inverse of a subtraction between the ipsilateral response and the contralateral response, the shuffler filters Σ and Δ being embodied as given in Equation 15. 
         [0000]      Σ=1/( H   i   +H   c )
 
         [0000]      Δ=1/( H   i   −H   c )  [Equation 15]
 
         [0049]      FIG. 1  is a diagram illustrating an example of a configuration of a virtual 7.1 channel speaker system using a 5.1 channel speaker system according to an embodiment. 
         [0050]    Referring to  FIG. 1 , the virtual stereo system needs to consider a speaker which is capable of providing a highest quality, since the virtual stereo system uses a 5.1 channel loudspeakers  101  through  105 , and generates two virtual speakers  106  and  107  to embody a 7.1 channel. A first speaker  101  is installed on a location at a zero degrees, namely, a center of a front of a listener  100 , and a second speaker  102  is installed on a location at −30 degrees, namely, a front-left of the listener  100 , and a third speaker  103  is installed on a location at 30 degrees, namely, a front-right of the listener  100 . A fourth speaker  104  is a surround left speaker installed in a location at −110 degrees based on the front of the listener  100 , and a fifth speaker  105  is a surround right speaker installed in a location at 110 degrees based on the front of the listener  100 . A first virtual speaker  106  is a virtual speaker that enables the listener  100  to experience as though a speaker is installed in a location at −140 degrees, namely a back-left based on the front of the listener  100 , and the first virtual speaker  106  enables a first virtual channel signal to be played via the fifth speaker  105 . The second virtual speaker  107  is a virtual speaker that is recognized as though being installed in a location at 140 degrees, namely a back-right of the listener  100 , and enables a second virtual channel signal to be played via the fourth speaker  104  and the fifth speaker  105 . 
         [0051]      FIG. 2  is a block diagram illustrating a back-left virtual speaker generating apparatus and a back-right virtual speaker generating apparatus of a 7.1 channel speaker system, when a scheme is asymmetric according to an embodiment. 
         [0052]    Referring to  FIG. 2 , an HRTF localization unit  210  outputs signals x L  and x R  that have a back-directional feature by using an HRTF filtering that matches sound with a back-left direction (B L ) and with a back-right direction (B R ). 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       x 
                       B 
                     
                     = 
                     
                       
                         h 
                         B 
                       
                        
                       B 
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       
                         x 
                         B 
                       
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 x 
                                 L 
                               
                             
                           
                           
                             
                               
                                 x 
                                 R 
                               
                             
                           
                         
                         ] 
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       h 
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 H 
                                  
                                 
                                     
                                 
                                  
                                 
                                   140 
                                   near 
                                 
                               
                             
                             
                               
                                 H 
                                  
                                 
                                     
                                 
                                  
                                 
                                   140 
                                   far 
                                 
                               
                             
                           
                           
                             
                               
                                 H 
                                  
                                 
                                     
                                 
                                  
                                 
                                   140 
                                   far 
                                 
                               
                             
                             
                               
                                 H 
                                  
                                 
                                     
                                 
                                  
                                 
                                   140 
                                   near 
                                 
                               
                             
                           
                         
                         ] 
                       
                     
                     , 
                     
                       
 
                     
                      
                     
                       B 
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 B 
                                 L 
                               
                             
                           
                           
                             
                               
                                 B 
                                 R 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     16 
                   
                   ] 
                 
               
             
           
         
       
     
         [0053]    Here, x L  and x R  are inputted into a crosstalk canceller  220 , and are outputted as speaker outputs y L  and y R  which are constituted as signals from which crosstalk is eliminated by the crosstalk canceller. When the virtual back listening environment is in a symmetric condition based on two surround speakers, a shuffler crosstalk circuit may be used. An HRTF location measurement and crosstalk canceller circuit constructed by using the shuffler may be represented as illustrated in  FIG. 3 . 
         [0054]      FIG. 3  is a block diagram illustrating a back-left virtual speaker and a back-right virtual speaker of a 7.1 channel speaker system, when a scheme is symmetric according to an embodiment. 
         [0055]    Referring to  FIG. 3 , an HRTF localization unit  310  outputs signals x L  and x R  having a back-directional feature by using HRTF filtering that matches sound with a back-left direction (B L ) and with a back-right direction (B R ). 
         [0056]    A Sigma  321  and a Delta  322  of a crosstalk canceller  320  are defined as Equation 17. 
         [0000]    
       
         
           
             
               
                 
                   
                     Sigma 
                     = 
                     
                       1 
                       
                         2 
                          
                         
                           ( 
                           
                             
                               H 
                               i 
                             
                             + 
                             
                               H 
                               c 
                             
                           
                           ) 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     Delta 
                     = 
                     
                       1 
                       
                         2 
                          
                         
                           ( 
                           
                             
                               H 
                               i 
                             
                             - 
                             
                               H 
                               c 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     17 
                   
                   ] 
                 
               
             
           
         
       
     
         [0057]    The crosstalk canceller  320 , in a case of symmetric state, may reduce, by two, a number of filterings performed compared with a number of filterings performed in the crosstalk canceller  220  in a case of an asymmetric state. Here, to further simplify the operation of eliminating crosstalk of a virtual back channel, an expression may be formulated to have a minimal filtering by combining an HRTF localization process and a crosstalk cancellation process, and a final output y may be expressed as given in Equation 18. 
         [0000]    
       
         
           
             
               
                 
                   y 
                   = 
                   
                     
                       
                         
                           [ 
                           
                             
                               
                                 1 
                               
                               
                                 1 
                               
                             
                             
                               
                                 1 
                               
                               
                                 
                                   - 
                                   1 
                                 
                               
                             
                           
                           ] 
                         
                          
                         
                           [ 
                           
                             
                               
                                 Sigma 
                               
                               
                                 0 
                               
                             
                             
                               
                                 0 
                               
                               
                                 Delta 
                               
                             
                           
                           ] 
                         
                       
                        
                       
                         [ 
                         
                           
                             
                               1 
                             
                             
                               1 
                             
                           
                           
                             
                               1 
                             
                             
                               
                                 - 
                                 1 
                               
                             
                           
                         
                         ] 
                       
                     
                      
                     
                       x 
                       B 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     18 
                   
                   ] 
                 
               
             
           
         
       
     
         [0058]    Here, when Equation 16 is substituted in Equation 18, it is represented as given in Equation 19. 
         [0000]    
       
         
           
             
               
                 
                   y 
                   = 
                   
                     
                       
                         
                           
                             [ 
                             
                               
                                 
                                   1 
                                 
                                 
                                   1 
                                 
                               
                               
                                 
                                   1 
                                 
                                 
                                   
                                     - 
                                     1 
                                   
                                 
                               
                             
                             ] 
                           
                           [ 
                           
                               
                           
                            
                           
                             
                               
                                 Sigma 
                               
                               
                                 0 
                               
                             
                             
                               
                                 0 
                               
                               
                                 Delta 
                               
                             
                           
                           ] 
                         
                       
                     
                     
                       
                         
                           
                             
                               [ 
                               
                                   
                               
                                
                               
                                 
                                   
                                     1 
                                   
                                   
                                     1 
                                   
                                 
                                 
                                   
                                     1 
                                   
                                   
                                     
                                       - 
                                       1 
                                     
                                   
                                 
                               
                               ] 
                             
                             [ 
                             
                                 
                             
                              
                             
                               
                                 
                                   
                                     H 
                                      
                                     
                                         
                                     
                                      
                                     
                                       140 
                                       near 
                                     
                                   
                                 
                                 
                                   
                                     H 
                                      
                                     
                                         
                                     
                                      
                                     
                                       140 
                                       far 
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     H 
                                      
                                     
                                         
                                     
                                      
                                     
                                       140 
                                       far 
                                     
                                   
                                 
                                 
                                   
                                     H 
                                      
                                     
                                         
                                     
                                      
                                     
                                       140 
                                       near 
                                     
                                   
                                 
                               
                             
                             ] 
                           
                            
                           B 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     19 
                   
                   ] 
                 
               
             
           
         
       
     
         [0059]    When a third item and a fourth item of a right term in Equation 19 are combined and expanded, it is represented as given in Equation 20. 
         [0000]    
       
         
           
             
               
                 
                   y 
                   = 
                   
                     
                       
                         
                           
                             [ 
                             
                               
                                 
                                   1 
                                 
                                 
                                   1 
                                 
                               
                               
                                 
                                   1 
                                 
                                 
                                   
                                     - 
                                     1 
                                   
                                 
                               
                             
                             ] 
                           
                            
                           
                             [ 
                             
                               
                                 
                                   Sigma 
                                 
                                 
                                   0 
                                 
                               
                               
                                 
                                   0 
                                 
                                 
                                   Delta 
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                     
                       
                         
                           
                             [ 
                             
                               
                                 
                                   
                                     
                                       H 
                                        
                                       
                                           
                                       
                                        
                                       
                                         140 
                                         near 
                                       
                                     
                                     + 
                                     
                                       H 
                                        
                                       
                                           
                                       
                                        
                                       
                                         140 
                                         far 
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       H 
                                        
                                       
                                           
                                       
                                        
                                       
                                         140 
                                         far 
                                       
                                     
                                     + 
                                     
                                       H 
                                        
                                       
                                           
                                       
                                        
                                       
                                         140 
                                         near 
                                       
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     
                                       H 
                                        
                                       
                                           
                                       
                                        
                                       
                                         140 
                                         near 
                                       
                                     
                                     - 
                                     
                                       H 
                                        
                                       
                                           
                                       
                                        
                                       
                                         140 
                                         far 
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       H 
                                        
                                       
                                           
                                       
                                        
                                       
                                         140 
                                         far 
                                       
                                     
                                     - 
                                     
                                       H 
                                        
                                       
                                           
                                       
                                        
                                       
                                         140 
                                         near 
                                       
                                     
                                   
                                 
                               
                             
                             ] 
                           
                            
                           B 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     20 
                   
                   ] 
                 
               
             
           
         
       
     
         [0060]    When a second item and a third item of a right term in Equation 20 are combined and expanded, it is represented as given in Equation 21. 
         [0000]    
       
         
           
             
               
                 
                   y 
                   = 
                   
                     
                       
                         
                           [ 
                           
                             
                               
                                 1 
                               
                               
                                 1 
                               
                             
                             
                               
                                 1 
                               
                               
                                 
                                   - 
                                   1 
                                 
                               
                             
                           
                           ] 
                         
                       
                     
                     
                       
                         
                           
                             [ 
                             
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           H 
                                            
                                           
                                               
                                           
                                            
                                           
                                             140 
                                             near 
                                           
                                         
                                         + 
                                         
                                           H 
                                            
                                           
                                               
                                           
                                            
                                           
                                             140 
                                             far 
                                           
                                         
                                       
                                       ) 
                                     
                                      
                                     Sigma 
                                   
                                 
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           H 
                                            
                                           
                                               
                                           
                                            
                                           
                                             140 
                                             far 
                                           
                                         
                                         + 
                                         
                                           H 
                                            
                                           
                                               
                                           
                                            
                                           
                                             140 
                                             near 
                                           
                                         
                                       
                                       ) 
                                     
                                      
                                     Sigma 
                                   
                                 
                               
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           H 
                                            
                                           
                                               
                                           
                                            
                                           
                                             140 
                                             near 
                                           
                                         
                                         - 
                                         
                                           H 
                                            
                                           
                                               
                                           
                                            
                                           
                                             140 
                                             far 
                                           
                                         
                                       
                                       ) 
                                     
                                      
                                     Delta 
                                   
                                 
                                 
                                   
                                     
                                       ( 
                                       
                                         
                                           H 
                                            
                                           
                                               
                                           
                                            
                                           
                                             140 
                                             far 
                                           
                                         
                                         - 
                                         
                                           H 
                                            
                                           
                                               
                                           
                                            
                                           
                                             140 
                                             near 
                                           
                                         
                                       
                                       ) 
                                     
                                      
                                     Delta 
                                   
                                 
                               
                             
                             ] 
                           
                            
                           B 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     21 
                   
                   ] 
                 
               
             
           
         
       
     
         [0061]    When Equation 21 is decomposed, it is represented as given in Equation 22. 
         [0000]    
       
         
           
             
               
                 
                   
                     y 
                     = 
                     
                       
                         
                           
                             [ 
                             
                               
                                 
                                   1 
                                 
                                 
                                   1 
                                 
                               
                               
                                 
                                   1 
                                 
                                 
                                   
                                     - 
                                     1 
                                   
                                 
                               
                             
                             ] 
                           
                            
                           
                             [ 
                             
                               
                                 
                                   Σ 
                                 
                                 
                                   0 
                                 
                               
                               
                                 
                                   0 
                                 
                                 
                                   Δ 
                                 
                               
                             
                             ] 
                           
                         
                          
                         
                           [ 
                           
                             
                               
                                 1 
                               
                               
                                 1 
                               
                             
                             
                               
                                 1 
                               
                               
                                 
                                   - 
                                   1 
                                 
                               
                             
                           
                           ] 
                         
                       
                        
                       
                         x 
                         B 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     Σ 
                     = 
                     
                       
                         ( 
                         
                           
                             H 
                              
                             
                                 
                             
                              
                             
                               140 
                               near 
                             
                           
                           + 
                           
                             H 
                              
                             
                                 
                             
                              
                             
                               140 
                               far 
                             
                           
                         
                         ) 
                       
                        
                       Sigma 
                     
                   
                    
                   
                     
 
                   
                    
                   
                     Δ 
                     = 
                     
                       
                         ( 
                         
                           
                             H 
                              
                             
                                 
                             
                              
                             
                               140 
                               near 
                             
                           
                           - 
                           
                             H 
                              
                             
                                 
                             
                              
                             
                               140 
                               far 
                             
                           
                         
                         ) 
                       
                        
                       Delta 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     22 
                   
                   ] 
                 
               
             
           
         
       
     
         [0062]    Here, H140 near  is a head transfer function transferred to an ear relatively close to a sound source in a direction of 140 degrees, and H140 far  is a head transfer function transferred to an ear relatively far from the sound source in a direction of 140 degrees. Sigma and Delta are values illustrated in Equation 17. 
         [0063]    Equation 22 may be represented as a virtual speaker circuit  400  as illustrated in  FIG. 4 . 
         [0064]      FIG. 4  is a diagram illustrating a configuration of a virtual speaker circuit according to an embodiment. 
         [0065]    Referring to  FIG. 4 , the virtual speaker circuit  400  includes four adders  410 ,  420 ,  450 , and  460 , and two filters  430  and  440 . 
         [0066]    The first adder  410  adds a first virtual channel signal and a back second virtual channel signal. As an example, the first adder  410  may calculate a sum (B L +B R ) of a signal (B L ) that is intended to be experienced at a location of −140 degrees, namely a location of the first speaker  106 , and a signal (B R ) that is intended to be experienced at a location of 140 degrees, namely, a location of the second virtual speaker  107 . The first virtual channel signal is a signal to be outputted from the first virtual speaker and the second virtual channel signal is a signal to be outputted from the second virtual speaker. 
         [0067]    The second adder  420  subtracts the second virtual channel signal from the first virtual channel signal. As an example, the second adder  420  may calculate subtraction (B L −B R ) between the signal (B L ) that is intended to be experienced at a location of −140 degrees, namely a location of the first speaker  106 , and the signal (B R ) that is intended to be experienced at a location of 140 degrees, namely, a location of the second virtual speaker  107 . 
         [0068]    The first filter  430  performs filtering of a signal outputted from the first adder  410  based on a ratio of a sum of an ipsilateral transfer function and a contralateral transfer function in a virtual location to a sum of an ipsilateral transfer function and a contralateral transfer function in a location of an actual speaker. As an example, the first filter  430  may perform the filtering of a sum of the first virtual channel signal and the second virtual channel signal based on ratio of a sum of the ipsilateral transfer function and the contralateral transfer function in the virtual location to a sum of the ipsilateral transfer function and the contralateral transfer function in the location of the actual speaker. 
         [0069]    The second filter  440  performs filtering of a signal outputted from the second adder based on a ratio of a subtraction between the ipsilateral transfer function and the contralateral transfer function in the virtual location to a subtraction between the ipsilateral transfer function and the contralateral transfer function in the location of the actual speaker. As an example, the second filter  440  may perform the filtering of subtraction between the first virtual channel signal and the second virtual channel signal based on a ratio of a subtraction between the ipsilateral transfer function and the contralateral transfer function in the virtual location to a subtraction between the ipsilateral transfer function and the contralateral transfer function in the location of the actual speaker. 
         [0070]    The third adder  450  adds a signal outputted from the first filter  430  and a signal outputted from the second filter  440 . As an example, the third adder  450  may add a signal outputted from the first filter  430  and a signal outputted from the second filter  440 , and outputs a summation result signal yL via the fifth speaker  105  that is a surround left speaker. 
         [0071]    The fourth adder  460  subtracts the signal outputted from the second filter  440  from the signal outputted from the first filter  430 . 
         [0072]    As described above, the virtual speaker circuit  400  according to example embodiments may decrease a complexity and may greatly decrease a coefficient value of a filter to be stored, since a total amount of filtering to be performed is relatively small, when compared, for example, with a configuration where the asymmetric or symmetric HRTF localization unit  210  or  310  and the crosstalk canceller  220  or  320  are connected based on a cascade. 
         [0073]      FIG. 5  is a diagram illustrating an example of a virtual speaker apparatus according to an embodiment. 
         [0074]    Referring to  FIG. 5 , in the virtual speaker apparatus  500 , channels played through an actual speaker include a front left (FL) channel, a front right (FR) channel, a front center (FC) channel, a low frequency effect (LFE) channel, a surround left (SL) channel, and a surround right (SR) channel. Channels which use the surround left speaker (S SL ) and the surround right speaker (S SR ) and enable a user to experience a virtual speaker through a virtual speaker process, are a back left channel (BL) and a back right (BR) channel. 
         [0075]    The virtual speaker  500  may include, for example, a first delay unit  501 , a second delay unit  502 , a third delay unit  503 , a fourth delay unit  504 , a fifth delay unit  505 , a sixth delay unit  506 , an FFT unit  510 , a virtual speaker circuit  520 , an IFFT unit  530 , a fifth adder  531 , and a sixth adder  532 . 
         [0076]    The first delay unit  501  delays a front right channel signal (F R ). That is, the first delay unit  501  delays the front right channel signal (F R ) as long as signals are under virtual speaker process, so that a time when the virtual speaker processed signals are outputted via the surround left speaker (S SL ) and the surround right speaker (S SR ) is identical to a time when the front right channel signal (F R ) is outputted via the front right speaker (S FR ). 
         [0077]    The second delay unit  502  delays a front left channel signal (F L ). That is, the second delay unit  502  delays the front left channel signal (F L ) as long as signals are under virtual speaker process, so that a time when the virtual speaker processed signals are outputted via the surround left speaker (S SL ) and the surround right speaker (S SR ) is identical to a time when the front left channel signal (F L ) is outputted via the front left speaker (S FL ). 
         [0078]    The third delay unit  503  delays a front center channel signal (F C ). That is, the third delay unit  503  delays the front center channel signal (F C ) while signals are under a virtual speaker process, so that a time when the virtual speaker processed signals are outputted via the surround left speaker (S SL ) and the surround right speaker (S SR ) is identical to a time when the front center channel signal (F C ) is outputted via the front center speaker (S FC ). 
         [0079]    The fourth delay unit  504  delays a low frequency effect channel signal (LFE). That is, the fourth delay unit  504  delays the low frequency effect channel signal (LFE) as long as signals are under virtual speaker process, so that a time when the virtual speaker processed signals are outputted via the surround left speaker (S SL ) and the surround right speaker (S SR ) is identical to a time when the low frequency effect channel signal (LFE) is outputted via a low frequency speaker (S LEF ). 
         [0080]    The fifth delay unit  505  delays a surround left channel signal (S L ). That is, the fifth delay unit  505  delays the surround left channel signal (S L ) as long as signals are under a virtual speaker process, so that a time when the virtual speaker processed signals are outputted via the surround left speaker (S SL ) and the surround right speaker (S SR ) is identical to a time when the surround left channel signal (S L ) is outputted via the surround left speaker (S SL ). 
         [0081]    The sixth delay unit  506  delays a surround right channel signal (S R ). That is, the sixth delay unit  506  delays the surround right channel signal (S R ) as long as signals are under a virtual speaker process, so that a time when the virtual speaker processed signals are outputted via the surround left speaker (S SL ) and the surround right speaker (S SR ) is identical to a time when the surround right channel signal (S R ) is outputted via the surround right speaker (S SR ). 
         [0082]    A temporal domain is transformed into a frequency domain to be used for filtering in the virtual speaker process. Generally, a filtering performed in the frequency domain is performed quicker than a convolution performed in the temporal domain, and thus, a fast Fourier transform (FFT) unit  510  and an inverse fast Fourier transform (IFFT) unit  530  are used at a front end and a back end of the virtual speaker circuit  520  to transform from the temporal domain to the frequency domain and vice-versa. 
         [0083]    The FFT unit  510  performs a FFT of a back left channel signal (B L ) and a back right channel signal (B R ). 
         [0084]    The virtual speaker circuit  520  may include, for example, a first adder  521 , a second adder  522 , a first filter  523 , a second filter  524 , a third adder  525 , and a fourth adder  526 , and may process signals that are felt as though being outputted via a virtual speaker, by using the FFT-transformed back left channel signal (B L ) and FFT-transformed back right channel signal (B R ). 
         [0085]    The first adder  521  adds the FFT-transformed back right channel signal and the FFT-transformed back left channel signal outputted from the FFT unit  510 . 
         [0086]    The second adder  522  subtracts the FFT-transformed back right channel signal from the FFT-transformed back left channel signal outputted from the FFT unit  510 . 
         [0087]    The first filter  523  perform filtering of a signal outputted from the first adder  521  based on a ratio of a sum of an ipsilateral transfer function and a contralateral transfer function in a virtual location to a sum of an ipsilateral transfer function and a contralateral transfer function in a location of an actual speaker. 
         [0088]    The second filter  524  perform filtering of a signal outputted from the second adder  522  based on a ratio of a subtraction between the ipsilateral transfer function and the contralateral transfer function in the virtual location to a subtraction between the ipsilateral transfer function and the contralateral transfer function in the location of the actual speaker. 
         [0089]    The third adder  525  adds a signal outputted from the first filter  523  and a signal outputted from the second filter  524 . 
         [0090]    The fourth adder  526  subtracts the signal outputted from the second filter  524  from the signal outputted from the first filter  523 . 
         [0091]    The IFFT unit  530  performs an IFFT of a signal outputted from the third adder  525  and a signal outputted from the fourth adder  526 . 
         [0092]    The fifth adder  531  adds a signal outputted from the fifth delay unit  505  and a first signal (y L ) outputted from the IFFT unit  530 . 
         [0093]    The sixth adder  532  adds a signal outputted from the sixth delay unit  506  and a second signal (y R ) outputted from the IFFT unit  530 . 
         [0094]    The virtual speaker circuit  520  is performed in a frame unit, and an overlap-add method as illustrated in  FIG. 6  is widely used to reflect a response of a previous frame to a current frame. 
         [0095]      FIG. 6  is a diagram illustrating an example of an overlap-adding scheme according to an embodiment. 
         [0096]    Referring to  FIG. 6 , to maximally reflect the response, overlapping is performed, and a frame boundary is smoothed by windowing on an input and an output by using a sine window. 
         [0097]    As an example, in a configuration using the overlap-adding method, when 50% data is overlapped, an actual data may have a delay as long as one frame. Accordingly, the first through sixth delay unit  501  through  506  may delay as long as a number of samples in one frame of the signals (B L  and B R ) that are transformed into the frequency domain, compared with signals (F L , F R , F C , LFE, S L , S R ) that are not transformed into a frequency domain. 
         [0098]    Also, when an audio decoder that is an audio source is of a type performing a decoding in a transform domain, there is no need for performing a transformation of a virtual speaker process. Generally, an audio codec performs an encoding process and a decoding process in the frequency domain. Particularly, a modified discrete cosine transform (MDCT) is excellent in coding efficiency, compared with an FFT or a DCT, and for this reason, various audio codecs, such as AAC, MP3, Dolby Digital, Dolby Digital Plus, AAC+, and the like, perform an encoding process and a decoding process in the MDCT domain. 
         [0099]      FIG. 7  is a diagram illustrating an example of a virtual speaker apparatus that is a combination of a virtual speaker circuit and an audio decoder according to an embodiment. 
         [0100]    Referring to  FIG. 7 , an audio codec (not illustrated) includes a virtual speaker circuit  710  to perform a virtual speaker process just before an operation of transforming a frequency domain into a temporal domain, and thus may omit a frequency domain transforming operation that is additionally performed at a front end of the virtual speaker process circuit and an a temporal domain transforming operation that is additionally performed at a back end of the virtual speaker process, unlike the 7.1 channel speaker system  500  of  FIG. 5   
         [0101]    The virtual speaker apparatus  700  includes, for example, a virtual speaker circuit  710 , a fifth adder  721 , a sixth adder  722 , and an IMDCT unit  730 . 
         [0102]    The virtual speaker circuit  710  includes, for example, a first adder  711 , a second adder  712 , a first filter  713 , a second filter  714 , a third adder  715 , and a fourth adder  716 . 
         [0103]    The first adder  711  adds a first virtual channel signal and a second virtual channel signal. As an example, the first adder  711  adds a back left channel signal and a back right channel signal. The back left channel signal may be a signal that is perceived as being outputted from a virtual speaker located at a back left of a listener, and the back right channel signal may be a signal that is perceived as being outputted from a virtual speaker located at a back right of the listener. 
         [0104]    The second adder  712  subtracts the second virtual channel signal from the first virtual channel signal. As an example, the second adder  712  may subtract the back right channel signal from the back left channel signal. 
         [0105]    The first filter  713  performs filtering of a signal outputted from the first adder  711  based on a ratio of a sum of an ipsilateral transfer function and a contralateral transfer function in a virtual location to a sum of an ipsilateral transfer function and a contralateral transfer function in an actual speaker location. 
         [0106]    The second filter  714  performs filtering of a signal outputted from the second adder  712  based on a ratio of a subtraction between the ipsilateral transfer function and the contralateral transfer function in the virtual location to a subtraction of the ipsilateral transfer function and the contralateral transfer function in the actual speaker location. 
         [0107]    The third adder  715  adds a signal outputted from the first filter  713  and a signal outputted from the second filter  714 . 
         [0108]    The fourth adder  716  subtracts the signal outputted from the second filter  714  from the signal outputted from the first filter  713 . 
         [0109]    The fifth adder  721  adds a signal outputted from a surround first direction channel signal and a signal outputted from the third adder  715 . 
         [0110]    The sixth adder  722  adds a signal outputted from a surround second direction channel signal and a signal outputted from the fourth adder  716 . 
         [0111]    The IMDCT unit  730  receives a front right channel signal (F R ), a front left channel signal (F L ), a front center channel signal (F C ), a low effect channel signal (LFE), a signal outputted from a fifth adder  721 , and a signal outputted from a sixth adder  722 , and performs an inverse modified discrete cosine transform (IMDCT) of the received signals. 
         [0112]    The front left channel signal (F R ), a left right channel signal (F L ), a front center channel signal (F C ), and a low frequency effect channel signal (LFE) are respectively decoded by an audio decoder (not illustrated) are transformed into a temporal domain by the IMDCT unit  730 . 
         [0113]    The back left channel signal (B L ) and the back right channel signal (B R ) are virtual-speakers processed by the virtual speaker circuit  710 . The virtual speaker processed left signal (y L ) is added to a surround left channel signal (S L ) by the fifth adder  721 , is transformed into the temporal domain by the IMDCT unit  730 , and is outputted via a surround left speaker (S SL ). The virtual speaker processed right signal (y R ) is added to a surround right channel signal (S R ) by the sixth adder  722 , is transformed into the temporal domain by the IMDCT unit  730 , and is outputted via a surround right speaker (S SR ). The virtual speaker circuit  710  has an advantage of being applied to a configuration where a phase is difficult to be applied, such as an MDCT domain, since a filtering is simplified compared with configurations in  FIG. 2  and  FIG. 3 . 
         [0114]    When the first filter  713  and the second filter  714  are designed based on a minimum phase, a phase component is reflected after the IMDCT is performed. When the first filter  713  and the second filter  714  have linear phases and are designed based on the minimum phase, the first filter  713  and the second filter  714  may be embodied as illustrated in  FIG. 8 . The linear phase may be simply embodied as a sample delay in a temporal domain. 
         [0115]      FIG. 8  is a diagram illustrating another example of a virtual speaker apparatus that is a combination of a virtual speaker circuit and an audio decoder according to an embodiment. 
         [0116]    Referring to  FIG. 8 , the virtual speaker circuit  810  includes a first adder  811 , a second adder  812 , a first filter  813 , a second filter  814 , a third adder  815 , and a fourth adder  816 . 
         [0117]    The first adder  811  adds a first virtual channel signal and a second virtual channel signal. As an example, the first adder  811  adds a back left channel signal (B L ) and a back right channel signal (B R ). The back left channel signal (B L ) may be a signal that is intended to be felt as though being outputted from a virtual speaker located in a back left, and the back right channel signal (B R ) may be a signal that is intended to be felt as though being outputted from a virtual speaker located in a back right. 
         [0118]    The second adder  812  subtracts the second virtual channel signal from the first virtual channel signal. As an example, the second adder  712  may subtract the back right channel signal (B R ) from the back left channel signal (B L ). 
         [0119]    The first filter  813  performs filtering of a signal outputted from the first adder  811  based on a ratio of a sum of an ipsilateral transfer function and a contralateral transfer function in a virtual location to a sum of an ipsilateral transfer function and a contralateral transfer function in an actual speaker location. 
         [0120]    The second filter  814  performs filtering of a signal outputted from the second adder  812  based on a ratio of a subtraction between the ipsilateral transfer function and the contralateral transfer function in the virtual location to a subtraction of the ipsilateral transfer function and the contralateral transfer function in the actual speaker location. 
         [0121]    The third adder  815  adds a signal outputted from the first filter  813  and a signal outputted from the second filter  814 . 
         [0122]    The fourth adder  816  subtracts the signal outputted from the second filter  814  from the signal outputted from the first filter  813 . 
         [0123]    The IMDCT unit  820  receives a front right channel signal (F R ), a front left channel signal (F L ), a front center channel signal (F C ), a low effect channel signal (LFE), a surround left channel signal (S L ), a surround right channel signal (S R ), a first signal outputted from a third adder  815 , and a second signal outputted from a fourth adder  816 , and performs IMDCT of the received signals. 
         [0124]    The first delay unit  831  delays the first signal that is IMDCT-transformed by the IMDCT unit  820 . That is, the first delay unit  831  delays the first signal IMDCT-transformed by the IMDCT unit  820  by as much as δ, and thereby reflects a phase factor. 
         [0125]    The second delay unit  832  delays the second signal that is IMDCT-transformed by the IMDCT unit  820 . That is, the second delay unit  832  delays the second signal IMDCT-transformed by the IMDCT unit  820  by as much as δ, and thereby reflects a phase factor. 
         [0126]    The fifth adder  841  adds a signal outputted from the first delay unit  831  and a surround left channel signal that is IMDCT-transformed by the IMDCT unit  820 , and outputs the added signal via a surround left speaker. 
         [0127]    The sixth adder  842  adds a signal outputted from the second delay unit  832  and a surround right channel signal that is IMDCT-transformed by the IMDCT unit  820 , and outputs the added signal via a surround right speaker. 
         [0128]    Also, the virtual speaker processing method according to an embodiment may be understood based on the description related to operations of the virtual speaker circuit and the virtual speaker apparatus illustrated in  FIGS. 4 through 6 . 
         [0129]    As described above, example embodiments of the present disclosure may decrease a complexity of a virtual speaker circuit based on a relatively small amount of filtering performed in total, and may provide a virtual speaker apparatus and a virtual speaker processing method that may greatly decrease a coefficient value of a filter to be stored. 
         [0130]    The virtual speaker processing method according to exemplary embodiments of the present invention include computer-readable media including program instructions to implement various operations embodied by a computer. The media may also include, alone or in combination with the program instructions, data files, data structures, tables, and the like. The media and program instructions may be those specially designed and constructed for the purposes of the present invention, or they may be of the kind well known and available to those having skill in the computer software arts. Examples of computer-readable media include magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD ROM disks; magneto-optical media such as optical disks; and hardware devices that are specially configured to store and perform program instructions, such as read-only memory devices (ROM) and random access memory (RAM). Examples of program instructions include both machine code, such as produced by a compiler, and files containing higher level code that may be executed by the computer using an interpreter. The described hardware devices may be configured to act as one or more software modules in order to perform the operations of the above-described embodiments of the present invention, or vice versa. The instructions may be executed on any processor, general purpose computer, or special purpose computer such as a virtual speaker apparatus. Further, the software modules may be controlled by any processor. 
         [0131]    Although a few example embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these example embodiments without departing from the principles and spirit of the disclosure, the scope of which is defined in the claims and their equivalents.