Patent Publication Number: US-2012045065-A1

Title: Surround signal generating device, surround signal generating method and surround signal generating program

Description:
TECHNICAL FIELD 
     The present invention relates to a technical field for generating surround signals of plural channels from sound signals of two channels. 
     BACKGROUND TECHNIQUE 
     This kind of technique is proposed in Patent Reference 1, for example. In Patent Reference-1, there is proposed a technique (Adaptive Surround Technology) for extracting a correlated component and an uncorrelated component from a momentarily varying stereo input signal by using ADF (Adaptive Digital Filter) in order to generate surround signals. Concretely, there is proposed the technique for generating the surround signals of 5.1 channels from the stereo input signal. In details, the technique assigns a component having a high correlation with a left channel and a right channel to a front-center channel, and assigns a component having a low correlation with the left channel and the right channel to a rear-left channel and a rear-right channel, in order to generate the surround signals for 5 channels. 
     PRIOR ART REFERENCE 
     Patent Reference 
     Patent Reference-1: Japanese Patent No. 3682032 
     DISCLOSURE OF INVENTION 
     Problem to Be Solved by The Invention 
     By the way, as for a real surround sound source such as a DVD, there is a tendency that a relationship of a correlation value between channels differs according to a genre of music, for example. Additionally, there is a tendency that a level on a rear side channel is smaller than a level on a front side channel . However, the technique disclosed in above Patent Reference-1 does not consider the tendencies of the real surround sound source when the surround signals are generated. Therefore, it is not possible to appropriately control a correlation value between a front channel and a rear channel. 
     The present invention has been achieved in order to solve the above problem. It is an object of the present invention to provide a surround signal generating device, a surround signal generating method and a surround signal generating program which can appropriately generate surround signals of plural channels in accordance with a genre of a content. 
     MEANS FOR SOLVING THE PROBLEM 
     In the invention according to claim  1 , a surround signal generating device which generates surround signals of plural channels from input sound signals of two channels, includes: a setting unit which sets a correlation value between channels in the plural channels and/or a level difference of the plural channels, in accordance with a genre of a content corresponding to the sound signals; and a surround signal generating unit which generates the surround signals of the plural channels based on the correlation value and/or the level difference which are set by the setting unit. 
     In the invention according to claim  13 , a surround signal generating method which generates surround signals of plural channels from input sound signals of two channels, includes: a setting process which sets a correlation value between channels in the plural channels and/or a level difference of the plural channels, in accordance with a genre of a content corresponding to the sound signals; and a surround signal generating process which generates the surround signals of the plural channels based on the correlation value and/or the level difference which are set by the setting process. 
     In the invention according to claim  14 , a surround signal generating program executed by a computer, which generates surround signals of plural channels from input sound signals of two channels, makes the computer function as: a setting unit which sets a correlation value between channels in the plural channels and/or a level difference of the plural channels, in accordance with a genre of a content corresponding to the sound signals; and a surround signal generating unit which generates the surround signals of the plural channels based on the correlation value and/or the level difference which are set by the setting unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an example of a 5.1-channel system. 
         FIG. 2  is a diagram for explaining a method for calculating a correlation value and a level difference between channels related to a surround sound source of 5.1 channels. 
         FIGS. 3A and 3B  show examples of an analysis result of a real surround sound source. 
         FIGS. 4A and 4B  are diagrams for explaining a basic concept of a method for generating surround signals. 
         FIG. 5  is a block diagram showing a surround signal generating device in a first embodiment. 
         FIGS. 6A to 6C  are diagrams for explaining an effect of a surround signal generating method in a first embodiment. 
         FIG. 7  is a flow chart showing a process performed by a control unit in a surround signal generating device according to a first embodiment. 
         FIG. 8  is a block diagram showing a surround signal generating device in a second embodiment. 
         FIG. 9  is a schematic diagram showing process blocks of an uncorrelated signal generating unit. 
         FIGS. 10A to 10C  are diagrams for explaining an effect of a surround signal generating method in a second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to one aspect of the present invention, there is provided a surround signal generating device which generates surround signals of plural channels from input sound signals of two channels, including: a setting unit which sets a correlation value between channels in the plural channels and/or a level difference of the plural channels, in accordance with a genre of a content corresponding to the sound signals; and a surround signal generating unit which generates the surround signals of the plural channels based on the correlation value and/or the level difference which are set by the setting unit. 
     The above surround signal generating device generates the surround signals of the plural channels from the input sound signals of the two channels, and is preferably applied to an audio device, for example. The surround signals correspond to sound signals used for more than 2 channels. Concretely, the setting unit sets the correlation value between channels in the plural channels and/or the level difference of the plural channels, in accordance with the genre of the content. The surround signal generating unit generates the surround signals of the plural channels based on the correlation value and/or the level difference which are set by the setting unit. Therefore, it becomes possible to appropriately generate the surround signals having the correlation value and/or the level difference between the channels which are appropriate for the genre of the content, from a stereo sound source. 
     In a manner of the above surround signal generating device, based on the correlation value between the channels in the plural channels and/or the level difference of the plural channels which are obtained from a real surround sound source for each genre, the setting unit sets the correlation value and/or the level difference. Therefore, it becomes possible to appropriately reflect a tendency of the correlation value and the level difference of the real surround sound source for each genre. 
     In another manner of the above surround signal generating device, the surround signal generating unit includes a mixing processing unit which determines an amount for mixing two signals between two channels in the plural channels based on the correlation value between the two channels which is set by the setting unit, and performs a process of mixing the two signals based on the amount. Therefore, it becomes possible to appropriately generate the surround signals in accordance with the correlation value between the two channels in the plural channels. 
     In a preferred example of the above surround signal generating device, the mixing processing unit uses, as the two signals, a sound signal obtained by performing an addition of the input sound signals of the two channels and a sound signal obtained by performing a subtraction of the input sound signals of the two channels, and performs the process of mixing the two signals. Namely, the mixing process can be performed by using the sound signals having a low correlation with each other. 
     As a preferred example, the two channels comprise a left channel and a right channel, and the plural channels are 5 channels comprising a front-left channel, a front-center channel, a front-right channel, a rear-left channel and a rear-right channel, and the mixing processing unit performs the process based on the correlation value of the front-center channel with respect to the front-left channel and the front-right channel, so as to generate a sound signal used for the front-center channel, and the mixing processing unit performs the process based on the correlation value of a rear channel comprising the rear-left channel and the rear-right channel with respect to the front-left channel and the front-right channel included in a front channel comprising the front-left channel, the front-center channel and the front-right channel, so as to generate sound signals used for the rear-left channel and the rear-right channel. 
     In another manner of the above surround signal generating device, the surround signal generating unit further includes an uncorrelated signal generating unit which generates an uncorrelated signal which is uncorrelated with each channel in the plural channels, based on the input sound signals of the two channels, and the mixing processing unit performs the process by using the uncorrelated signal. Therefore, it becomes possible to arbitrarily control the correlation value between the channels so as to generate the surround signals. 
     In a preferred example of the above surround signal generating device, the uncorrelated signal generating unit can generate the uncorrelated signal based on a linear prediction residual. 
     As a preferred example, the two channels comprise a left channel and a right channel, and the plural channels are 5 channels comprising a front-left channel, a front-center channel, a front-right channel, a rear-left channel and a rear-right channel, and the mixing processing unit performs the process by using the uncorrelated signal so as to generate sound signals used for the rear-left channel and the rear-right channel. Therefore, it becomes possible to generate the surround signals for which the correlation value between the rear-left channel and the rear-right channel is appropriately controlled. 
     In another manner of the above surround signal generating device, the surround signal generating unit controls the sound signals after the process by the mixing processing unit, based on the level difference of the plural channels which is set by the setting unit, so as to generate the surround signals of the plural channels. Therefore, it becomes possible to appropriately generate the surround signals having the correlation value and the level difference between the channels which are appropriate for the genre of the content. 
     Preferably, the setting unit can obtain the genre from an external input. 
     Preferably, the setting unit can obtain the genre from tag information attached to the input sound signals of the two channels. 
     Preferably, the above surround signal generating device further includes a unit which identifies the genre by analyzing the input sound signals of the two channels, and the setting unit can use the identified genre. 
     According to another aspect of the present invention, there is provided a surround signal generating method which generates surround signals of plural channels from input sound signals of two channels, including: a setting process which sets a correlation value between channels in the plural channels and/or a level difference of the plural channels, in accordance with a genre of a content corresponding to the sound signals; and a surround signal generating process which generates the surround signals of the plural channels based on the correlation value and/or the level difference which are set by the setting process. 
     According to still another aspect of the present invention, there is provided a surround signal generating program executed by a computer, which generates surround signals of plural channels from input sound signals of two channels, making the computer function as: a setting unit which sets a correlation value between channels in the plural channels and/or a level difference of the plural channels, in accordance with a genre of a content corresponding to the sound signals; and a surround signal generating unit which generates the surround signals of the plural channels based on the correlation value and/or the level difference which are set by the setting unit. 
     By the surround signal generating method and the surround signal generating program, it becomes possible to appropriately generate the surround signals having the correlation value and/or the level difference between the channels which are appropriate for the genre of the content, from a stereo sound source. 
     EMBODIMENT 
     Preferred embodiments of the present invention will be explained hereinafter with reference to the drawings. 
     [Surround Signal Generating Method] 
     First, a description will be given of a basic concept of a surround signal generating method in the embodiment. Hereinafter, a description will be given of an example in such a case that surround signals for a 5.1-channel system are generated from input sound signals of two channels (namely, the surround signals for 5 channels are generated). 
       FIG. 1  is a diagram showing an example of the 5.1-channel system.  FIG. 1  shows the diagram when five speakers are arranged concentrically. As shown in  FIG. 1 , speakers  10  include a front-left speaker  10 L, a front-center speaker  10 C, a front-right speaker  10 R, a rear-left speaker  10 SL and a rear-right speaker  10 SR. In this case, a listening point is located at a position shown by a reference numeral  70 , for example. Though the 5.1-channel system actually includes a subwoofer speaker, the speaker is not shown in  FIG. 1 . 
     In the specification, a channel of the front-left speaker  10 L is referred to as “front-left channel”, and a channel of the front-center speaker  10 C is referred to as “front-center channel”, and a channel of the front-right speaker  10 R is referred to as “front-right channel”, and a channel of the rear-left speaker  10 SL is referred to as “rear-left channel”, and a channel of the rear-right speaker  10 SR is referred to as “rear-right channel”. Additionally, a combination of the front-left channel, the front-center channel and the front-right channel is referred to as “front channel”, and a combination of the front-left channel and the front-right channel in the front channel is referred to as “front-left-right channel”, and a combination of the rear-left channel and the rear-right channel is referred to as “rear channel”. Additionally, the front-left channel is suitably represented by “Lch”, and the front-center channel is suitably represented by “Cch”, and the front-right channel is suitably represented by “Rch”, and the rear-left channel is suitably represented by “SLch”, and the rear-right channel is suitably represented by “SRch”. 
     In the embodiment, a correlation value between channels in the plural channels and a level difference of the plural channels are set in accordance with a genre of an input content, and the surround signals of the plural channels are generated from the sound signals of the two channels based on the correlation value and the level difference. Namely, in the embodiment, the surround signals having the correlation value and the level difference between the channels which are appropriate for the genre of the content are generated from a stereo sound source. In this case, the correlation value and the level difference between the channels which are obtained from a real surround sound source for each genre are used. This is because, as for the real surround sound source such as a DVD, there is a tendency that a relationship of the correlation value between the channels differs according to the genre of music, for example, and there is a tendency that a level of the rear channel is smaller than that of the front channel. 
     Here, a description will be given of a method for calculating the correlation value and the level difference between the channels from the real surround sound source, with reference to  FIG. 2 ,  FIGS. 3A and 3B . 
       FIG. 2  is a diagram for explaining the method for calculating the correlation value and the level difference between the channels related to the real surround sound source of the 5.1 channels. In the embodiment, by analyzing the surround sound source of the 5.1 channels such as DVD contents and broadcast contents, the correlation value and the level difference between the channels are calculated for each genre. Concretely, as shown by a broken line  81 , the correlation value and the level difference between the front-left channel and the front-center channel are used as a physical amount, and the correlation value and the level difference between the front-right channel and the front-center channel are used as a physical amount. As shown by a broken line  82   a,  the correlation value and the level difference between the front-left channel and the rear-left channel are used as a physical amount. As shown by a broken line  82   b,  the correlation value and the level difference between the front-right channel and the rear-right channel are used as a physical amount. Additionally, as shown by a broken line  83 , the correlation value and the level difference between the rear-left channel and the rear-right channel are used as a physical amount. 
     Then, by using the above physical amounts as five-dimensional vectors, the correlation value and the level difference between the channels are calculated for each genre, based on group classification methods such as a cluster classification. For example, a popular music (hereinafter suitably referred to as “pop”), a classical music, a jazz and a fusion are used as the genre. In addition to the genre of the music content, a sport content and a movie content are used as the genre, too. Additionally, the above genre is subdivided (hereinafter, the subdivided genre is referred to as “sub-genre”). For example, the pop is subdivided into the sub-genre such as a vocal music, a nationality (Japan, the U.S. and Europe), a male artist, a female artist, a band and a group. In addition, the classical music is subdivided into the sub-genre such as an orchestra and a small-group session. The Japanese pop may be separately used as “J-pop”. 
       FIGS. 3A and 3B  show examples of the analysis result of the real surround sound source.  FIG. 3A  shows an example of the correlation value and the level difference between the channels of a genre A, and  FIG. 3B  shows an example of the correlation value and the level difference between the channels of a genre B. Here, a description will be given of an example of the correlation value and the level difference (which are shown on the upper side) of the front-center channel with respect to the front-left channel and the front-right channel, and an example of the correlation value and the level difference (which are shown on the right side) between the front-left-right channel and the rear channel, and an example of the correlation value and the level difference (which are shown on the downside) between the rear-left channel and the rear-right channel. 
     Additionally, the size of the circles in  FIGS. 3A and 3B  represents the magnitude of level of each channel. For example, since the size of the circles of the Lch and the Rch is larger than that of the circle of the Cch, it indicates that the level of the Lch and the Rch is larger than that of the Cch. In  FIG. 3A , such a situation that the level difference is a positive value indicates that the level of the Lch and the Rch is “8 (dB)” larger than that of the Cch, and the level of the front-left-right channel is “5 (dB)” larger than that of the rear channel. Therefore, it can be understood that the correlation value and the level difference between the channels differ according to the genre. 
     Next,  FIGS. 4A and 4B  are diagrams for explaining a basic concept of a method for generating the surround signals in the embodiment.  FIG. 4A  shows an input of the stereo sound source including the left channel and the right channel.  FIG. 4B  shows the 5.1 channels for which the surround signals should be generated from the stereo sound source. In  FIG. 4B , the size of the circles represents the magnitude of level of each channel, and the width of the arrows represents the magnitude of the correlation value. The correlation value and the level difference are examples. 
     In the embodiment, based on the correlation value and the level difference between the channels which are obtained from the real surround sound source for each genre, the surround signals having the correlation value and the level difference between the channels which are appropriate for the genre are generated from the stereo sound source. Namely, the surround signals for the 5 channels are generated so that the tendency of the correlation value and the level difference of the real surround sound source for each genre is reflected. 
     [First Embodiment] 
     Next, a description will be given of a surround signal generating method in a first embodiment. In the first embodiment, based on the correlation value between the two channels in the plural channels which is preliminarily obtained by the above analysis, the amount (hereinafter referred to as “mixing amount”) for mixing two signals between the two channels is determined, and a process (hereinafter referred to as “mixing process”) of mixing the two signals is performed based on the mixing amount. Concretely, a sound signal obtained by performing an addition of the input sound signals of the two channels and a sound signal obtained by performing a subtraction of the input sound signals of the two channels are used as the above two signals. Namely, the mixing process is performed by using the sound signals having a low correlation with each other. 
     Specifically, in the first embodiment, a sound signal used for the front-center channel is generated by performing the mixing process based on the correlation value of the front-center channel with respect to the front-left channel and the front-right channel, and sound signals used for the rear-left channel and the rear-right channel are generated by performing the mixing process based on the correlation value between the front-left-right channel and the rear channel. 
     Then, in the first embodiment, the sound signals after the mixing process are controlled based on the level difference of the channels which is preliminarily obtained by the above analysis, so as to generate the surround signals for the 5 channels. In details, the surround signals for the front-left channel and the front-right channel are generated by controlling not the sound signals after the mixing process but a sound signal of the front-left channel and a sound signal of the front-right channel which are included in the input sound signals of the two channels, based on the level difference. 
     Next, a description will be given of a concrete configuration of a surround signal generating device which can realize the above surround signal generating method, with reference to  FIG. 5 . 
       FIG. 5  is a block diagram showing a surround signal generating device  50  in the first embodiment. The surround signal generating device  50  mainly includes adders  11  and  14   a  to  14   c,  a subtractor  12 , correlation value control units  13   a  to  13   c,  a level difference control unit  15 , a control unit  20  and a coefficient table  30 . The surround signal generating device  50  generates the surround signals used for the front-left channel, the front-center channel, the front-right channel, the rear-left channel and the rear-right channel, from the input sound signals of the two channels. 
     The surround signal generating device  50  is provided with the sound signals for the two channels which are obtained from the stereo sound sources such as the CD (Compact Disc) and the MP3 (MPEG Audio Layer-3), as a sound signal S 1 L of the left channel and a sound signal S 1 R of the right channel. The sound signals S 1 L and S 1 R are provided to the adder  11 , the subtractor  12  and the level difference control unit  15 . The adder  11  provides the correlation value control units  13   a  to  13   c  with a sound signal S 2  obtained by adding the sound signal S 1 L of the left channel to the sound signal S 1 R of the right channel. The subtractor  12  provides the correlation value control units  13   a  to  13   c  with a sound signal S 3  obtained by subtracting the sound signal S 1 R of the right channel from the sound signal S 1 L of the left channel. Namely, the adder  11  and subtractor  12  generate the sound signals S 2  and S 3  having a low correlation with each other. 
     The control unit  20  obtains a coefficient in accordance with the genre of the content from the coefficient table  30 , and controls the correlation value control units  13   a  to  13   c  and the level difference control unit  15  based on the obtained coefficients. The coefficient in accordance with the correlation value and the level difference between the channels which are preliminarily obtained by the above analysis is stored in the coefficient table  30 , for each genre. Concretely, mixing coefficients al, a 2  and a 3  for controlling the correlation value and gain amounts ATT_L, ATT 13  R, ATT_C, ATT_SL and ATT_SR for controlling the level difference are stored in the coefficient table  30 , for each genre. 
     The mixing coefficient al is set based on the correlation value of the front-center channel with respect to the front-left channel and the front-right channel. The mixing coefficients a 2  and a 3  are set based on the correlation value between the front-left-right channel and the rear channel. Based on a relationship (map) between the mixing coefficient and the correlation value which are obtained by preliminarily performing an experiment, for example, the mixing coefficients a 1  to a 3  are determined in accordance with the correlation value between the channels. Additionally, the mixing coefficients a 1  to a 3  correspond to the above mixing amount, and the mixing coefficients a 1  to a 3  are equal to or smaller than “1”. 
     The gain amounts ATT_L, ATT_C, ATT_R, ATT_SL and ATT_SR are set in accordance with the level difference of the front-left channel, the front-center channel, the front-right channel, the rear-left channel and the rear-right channel, respectively. 
     In the coefficient table  30 , the mixing coefficients and the gain amounts are stored in association with the genre such as the pop, the classical music, the jazz, the fusion, the sport content and the movie content. Additionally, in the coefficient table  30 , the mixing coefficients and the gain amounts are stored in association with the sub-genre for which the above genre is subdivided. For example, as for the pop, the mixing coefficients and the gain amounts are stored in association with the sub-genre such as the vocal music, the nationality, the male artist, the female artist, the band and the group. 
     For example, the control unit  20  obtains the mixing coefficients and the gain amounts in accordance with the genre, from the coefficient table  30 , by obtaining the genre input by a user, or by obtaining the genre from tag information of the MP3. In this case, the control unit  20  can obtain the mixing coefficients and the gain amounts in accordance with the sub-genre, by obtaining information of the sub-genre for which the above genre is subdivided, from the user. 
     The correlation value control unit  13   a  performs a control in accordance with the mixing coefficient al from the control unit  20 . Concretely, the correlation value control unit  13   a  provides the adder  14   a  with a sound signal obtained by multiplying the sound signal S 2  by “a 1 ” and a sound signal obtained by multiplying the sound signal S 3  by “ 1 -a 1 ”. The adder  14   a  generates a sound signal S 4 C by adding the two signals provided by the correlation value control unit  13   a,  and provides the level difference control unit  15  with the sound signal S 4 C. The correlation value control unit  13   b  performs a control in accordance with the mixing coefficient a 2  from the control unit  20 . Concretely, the correlation value control unit  13   b  provides the adder  14   b  with a sound signal obtained by multiplying the sound signal S 2  by “a 2 ” and a sound signal obtained by multiplying the sound signal S 3  by “ 1 -a 2 ”. The adder  14   b  generates a sound signal S 4 SL by adding the two signals provided by the correlation value control unit  13   b,  and provides the level difference control unit  15  with the sound signal S 4 SL. The correlation value control unit  13   c  performs a control in accordance with the mixing coefficient a 3  from the control unit  20 . Concretely, the correlation value control unit  13   c  provides the adder  14   c  with a sound signal obtained by multiplying the sound signal S 2  by “a 3 ” and a sound signal obtained by multiplying the sound signal S 3  by “ 1 -a 3 ”. The adder  14   c  generates a sound signal S 4 SR by adding the two signals provided by the correlation value control unit  13   c,  and provides the level difference control unit  15  with the sound signal S 4 SR. Thus, the correlation value control units  13   a  to  13   c  and the adders  14   a  to  14   c  function as the mixing processing unit. 
     The level difference control unit  15  is provided with the above sound signals S 1 L, S 4 C, S 1 R, S 4 SL and S 4 SR. The level difference control unit  15  multiplies the sound signals S 1 L, S 4 C, S 1 R, S 4 SL and S 4 SR by the gain amounts ATT_L, ATT_C, ATT_R, ATT_SL and ATT_SR, respectively, so as to generate sound signals S 5 L, S 5 C, S 5 R, S 5 SL and S 5 SR. Then, the level difference control unit  15  provides the front-left speaker  10 L, the front-center speaker  10 C, the front-right speaker  10 R, the rear-left speaker  10 SR and the rear-right speaker  10 SR with the sound signals S 5 L, S 5 C, S 5 R, S 5 SL and S 5 SR, respectively. 
     Next, a description will be given of an effect of the surround signal generating method in the first embodiment, with reference to  FIGS. 6A to 6C .  FIG. 6A  shows data of the surround signals which is used as a target. Namely, the data (target data) of the surround signals which should be realized is shown. As shown in  FIG. 6A , the correlation value of the front-center channel with respect to the front-left channel and the front-right channel is “0.1”, and the level difference is “8.0 (dB)”. Additionally, the correlation value between the front-left-right channel and the rear channel is “0.5”, and the level difference is “5.0 (dB)”. 
       FIG. 6B  shows an example of an analysis result of the surround signals generated by a method of a comparative example. As for the method of the comparative example, without consideration of the correlation value and the level difference between the channels, a component having a high correlation with the left channel and the right channel is assigned to the front-center channel and a component having a low correlation with the left channel and the right channel is assigned to the rear-left channel and the rear-right channel, in order to generate the surround signals (The same will apply hereinafter). As shown in  FIG. 6B , by the method of the comparative example, “0.88” is obtained as the correlation value of the front-center channel with respect to the front-left channel and the front-right channel, and “0 (dB)” is obtained as the level difference. Additionally, “0.24” is obtained as the correlation value between the front-left-right channel and the rear channel, and “0 (dB)” is obtained as the level difference. Therefore, by the method of the comparative example, it can be understood that the surround signals as shown in  FIG. 6A  are not appropriately realized. 
       FIG. 6C  shows an example of an analysis result of the surround signals generated by the method of the first embodiment. As shown in  FIG. 6   c,  by the method of the first embodiment, “0.05” is obtained as the correlation value of the front-center channel with respect to the front-left channel and the front-right channel, and “8.0 (dB)” is obtained as the level difference. Additionally, “0.46” is obtained as the correlation value between the front-left-right channel and the rear channel, and “5.0 (dB)” is obtained as the level difference . Therefore, by the method of the first embodiment, it can be understood that the surround signals as shown in  FIG. 6A  are appropriately realized. 
     Next, a description will be given of a concrete process performed by the surround signal generating device  50  in the first embodiment, with reference to  FIG. 7 .  FIG. 7  is a flow chart showing the process performed by the control unit  20  in the surround signal generating device  50 . For example, the process is performed when the genre of the reproduced content changes and/or the reproduced sound source changes. 
     First, in step S 101 , the control unit  20  determines whether the reproduced sound source is the CD or the MP3. When the reproduced sound source is the CD, the process goes to step S 102 . In step S 102 , the control unit  20  requires the user to input the genre, and obtains the genre input by the user. For example, the user inputs the genre of the content stored in the reproduced CD, by a key input. Then, the process goes to step S 104 . 
     In contrast, when the reproduced sound source is the MP3, the process goes to step S 103 . In step S 103 , the control unit  20  refers to the tag information of the MP3, and obtains the genre of the reproduced content. Then, the process goes to step S 104 . 
     In step S 104 , the control unit  20  requires the user to input the sub-genre, and obtains the sub-genre input by the user. For example, the control unit  20  displays the sub-genre related to the genre obtained in step S 102  or S 103 , so as to make the user select the sub-genre. As an example, when the genre is “J-pop”, the user selects one of the male artist, the female artist, the band and the group, as the sub-genre. Then, the process goes to step S 105 . 
     In step S 105 , by referring to the coefficient table  30 , the control unit  20  obtains the mixing coefficients and the gain amounts in accordance with the sub-genre obtained in step S 104 , and sets the obtained mixing coefficients and the gain amounts. Concretely, the control unit  20  sets the obtained mixing coefficients and the obtained gain amounts, to the correlation value control units  13   a  to  13   c  and the level difference control unit  15 . Then, the process goes to step S 106 , and the content is reproduced. After that, the process ends. 
     By the above first embodiment, it becomes possible to appropriately generate the surround signals having the correlation value and the level difference between the channels which are appropriate for the genre of the content. 
     While the above embodiment shows such an example that the control unit  20  obtains the genre input by the user and such an example that the control unit  20  obtains the genre from the tag information of the MP3, it is not limited to these examples. As another example, the control unit  20  can obtain the genre from a server. As still another example, instead of obtaining the genre as described above, the genre can be identified by analyzing the input sound signals. 
     [Second Embodiment] 
     Next, a description will be given of a surround signal generating method in a second embodiment. The second embodiment is different from the first embodiment in that an uncorrelated signal which is uncorrelated with each channel is generated from the input sound signals of the two channels so as to perform the mixing process by using the uncorrelated signal. Concretely, in the second embodiment, the mixing process is performed by using the uncorrelated signal so as to generate sound signals used for the rear-left channel and the rear-right channel. Therefore, it becomes possible to appropriately control the correlation value between the rear-left channel and the rear-right channel. 
       FIG. 8  is a block diagram showing a surround signal generating device  51  in the second embodiment. The surround signal generating device  51  mainly includes adders  11 ,  14   a  to  14   c  and  18   a,  subtractors  12  and  18   b,  correlation value control units  13   a  to  13   c,  a level difference control unit  15 , flame dividing units  16 L and  16 R, an uncorrelated signal generating unit  17 , a control unit  20  and a coefficient table  30 . The components to which the same reference numerals as those of the surround signal generating device  50  (see  FIG. 5 ) are given basically have same functions as those of the surround signal generating device  50 , and explanations thereof are omitted. 
     As for the surround signal generating device  51 , the sound signal S 1 L of the left channel and the sound signal S 1 R of the right channel are provided to the flame dividing units  16 L and  16 R, respectively. The flame dividing units  16 L and  16 R divide the sound signals S 1 L and S 1 R by a predetermined time unit. Then, the flame dividing units  16 L and  16 R provide the adder  11 , the subtractor  12 , the level difference control unit  15  and the uncorrelated signal generating unit  17  with the divided sound signals S 1 La and S 1 Ra. The flame dividing units  16 L provides the correlation value control unit  13   b  with the sound signal S 1 La, too. 
     The adder  11  provides the correlation value control unit  13   a  with a sound signal S 2   a  obtained by adding the sound signal S 1 La to the sound signal S 1 Ra, and the subtractor  12  provides the correlation value control unit  13   a  with a sound signal S 3   a  by subtracting the sound signal S 1 Ra from the sound signal S 1 La. The correlation value control unit  13   a  provides the adder  14   a  with a sound signal obtained by multiplying the sound signal Sts by “a 1 ” and a sound signal obtained by multiplying the sound signal S 3   a  by “ 1 -al”. The adder  14   a  generates a sound signal S 4 Ca by adding the two signals provided by the correlation value control unit  13   a,  and provides the level difference control unit  15  with the sound signal S 4 Ca. 
     The control unit  20  obtains the mixing coefficients a 1 , a 2  and a 3  and the gain amounts ATT_L, ATT_R, ATT_C, ATT_SL and ATT_SR in accordance with the genre of the content, from the coefficient table  30 , and controls the correlation value control units  13   a  to  13   c  and the level difference control unit  15  based on the mixing coefficients and the gain amounts. The mixing coefficients a 1 , a 2  and a 3  and the gain amounts ATT_L, ATT_R, ATT_C, ATT_SL and ATT_SR are preliminarily stored in the coefficient table  30 , for each genre (including the sub-genre). 
     The uncorrelated signal generating unit  17  generates uncorrelated signals S 17 L and S 17 R which are uncorrelated with each channel, based on the sound signals S 1 La and S 1 Ra, and provides the adder  18   a  and the subtractor  18   b  with the uncorrelated signals S 17 L and S 17 R. The adder  18   a  provides the correlation value control unit  13   c  with a signal S 18   a  obtained by adding the uncorrelated signal S 17 L to the uncorrelated signal S 17 R, and the subtractor  18   b  provides the correlation value control unit  13   b  with a signal S 18   b  obtained by subtracting the uncorrelated signal S 17 R from the uncorrelated signal S 17 L. 
     The correlation value control unit  13   b  performs a control in accordance with the mixing coefficient a 2  from the control unit  20 . Concretely, the correlation value control unit  13   b  provides the adder  14   b  with a sound signal obtained by multiplying the sound signal S 1 La by “a 2 ” and a signal obtained by multiplying the signal S 18   b  by “ 1 -a 2 ”. The adder  14   b  generates a sound signal S 4 SLa by adding the two signals provided by the correlation value control unit  13   b,  and provides the correlation value control unit  13   c  and the level difference control unit  15  with the sound signal S 4 SLa. 
     The correlation value control unit  13   c  performs a control in accordance with the mixing coefficient a 3  from the control unit  20 . Concretely, the correlation value control unit  13   c  provides the adder  14   c  with a sound signal obtained by multiplying the sound signal S 4 SLa by “a 3 ” and a signal obtained by multiplying the signal S 18   a  by “ 1 -a 3 ”. The adder  14   c  generates a sound signal S 4 SRa by adding the two signals provided by the correlation value control unit  13   c,  and provides the level difference control unit  15  with the sound signal S 4 SRa. 
     The level difference control unit  15  is provided with the above sound signals S 1 La, S 4 Ca, S 1 Ra, S 4 SLa and S 4 SRa. The level difference control unit  15  multiplies the sound signals S 1 La, S 4 Ca, S 1 Ra, S 4 SLa and S 4 SRa by the gain amounts ATT_L, ATT_C, ATT_R, ATT_SL and ATT_SR, respectively, so as to generate sound signals S 5 La, S 5 Ca, S 5 Ra, S 5 SLa and S 5 SRa. Then, the level difference control unit  15  provides the front-left speaker  10 L, the front-center speaker  10 C, the front-right speaker  10 R, the rear-left speaker  10 SR and the rear-right speaker  10 SR with the sound signals S 5 La, S 5 Ca, S 5 Ra, S 5 SLa and S 5 SRa, respectively. 
     Next, a description will be given of a concrete example of the uncorrelated signal generating unit  17 , with reference to  FIG. 9 .  FIG. 9  is a schematic diagram showing process blocks of the uncorrelated signal generating unit  17 . The uncorrelated signal generating unit  17  mainly includes a prediction residual calculating unit  17   a  and a linear prediction analyzing unit  17   b.  Basically, the uncorrelated signal generating unit  17  generates the uncorrelated signals based on a linear prediction residual. 
     The prediction residual calculating unit  17   a  and the linear prediction analyzing unit  17   b  are provided with the sound signals S 1 La and S 1 Ra for one frame, which are processed by the flame dividing units  16 L and  16 R. The linear prediction analyzing unit  17   b  performs a framing of the sound signals S 1 La and S 1 Ra so as to generate a variance-covariance matrix, and performs a linear prediction based on a Levinson-Durbin method. In this case, the linear prediction analyzing unit  17   b  calculates a linear prediction filter coefficient. The prediction residual calculating unit  17   a  performs a filter process of the sound signals S 1 La and S 1 Ra by using the linear prediction filter coefficient. Then, the prediction residual calculating unit  17   a  outputs a value (linear prediction residual) obtained by subtracting the signal after the filter process from the sound signals S 1 La and S 1 Ra, as the uncorrelated signals S 17 L and S 17 R. 
     Next, a description will be given of an effect of the surround signal generating method in the second embodiment, with reference to  FIGS. 10A to 10C .  FIG. 10A  shows data of the surround signals which is used as a target. Namely, the data (target data) of the surround signals which should be realized is shown. As shown in  FIG. 10A , the correlation value of the front-center channel with respect to the front-left channel and the front-right channel is “0.1”, and the level difference is “8.0 (dB)”. The correlation value between the front-left-right channel and the rear channel is “0.5”, and the level difference is “5.0 (dB)”. Additionally, the correlation value between the rear-left channel and the rear-right channel is “0.7”. 
       FIG. 10B  shows an example of an analysis result of the surround signals generated by the method of the comparative example. As shown in  FIG. 10B , by the method of the comparative example, “0.88” is obtained as the correlation value of the front-center channel with respect to the front-left channel and the front-right channel, and “0 (dB)” is obtained as the level difference. Additionally, “0.24” is obtained as the correlation value between the front-left-right channel and the rear channel, and “0 (dB)” is obtained as the level difference. Additionally, “−0.20” is obtained as the correlation value between the rear-left channel and the rear-right channel. Therefore, by the method of the comparative example, it can be understood that the surround signals as shown in  FIG. 10A  are not appropriately realized. 
       FIG. 10C  shows an example of an analysis result of the surround signals generated by the method of the second embodiment. As shown in  FIG. 10   c,  by the method of the second embodiment, “0.05” is obtained as the correlation value of the front-center channel with respect to the front-left channel and the front-right channel, and “8.0 (dB)” is obtained as the level difference. Additionally, “0.49” is obtained as the correlation value between the front-left-right channel and the rear channel, and “5.0 (dB)” is obtained as the level difference. Additionally, “0.67” is obtained as the correlation value between the rear-left channel and the rear-right channel. Therefore, by the method of the second embodiment, it can be understood that the surround signals as shown in  FIG. 10A  are appropriately realized. Specifically, it can be understood that the correlation value between the rear-left channel and the rear-right channel is appropriately realized. 
     According to the above second embodiment, since the mixing process is performed based on the uncorrelated signals, it becomes possible to generate the surround signals for which the correlation value between the rear-left channel and the rear-right channel is appropriately controlled. 
     [Modification] 
     The above embodiments show such an example that the surround signals are generated based on the correlation value of the front-center channel with respect to the front-left channel and the front-right channel, the correlation value between the front-left-right channel and the rear channel, and the correlation value between the rear-left channel and the rear-right channel. However, the surround signals may be generated based on the correlation value between channels other than the above-mentioned channels. 
     While the above embodiments show such an example that the present invention is applied to the 5.1-channel system (namely, such an example that the surround signals for 5 channels are generated from the sound signals of the two channels), it is not limited to this. Other than the 5.1-channel system, the present invention can be applied to a configuration which generates surround signals for more than 2 channels from the sound signals of the two channels. 
     While the above embodiments show such an example that the surround signals are generated based on both the correlation value and the level difference, the surround signals may be generated based on either the correlation value or the level difference. Namely, the surround signals may be generated by only controlling the correlation value without controlling the level difference, or the surround signals may be generated by only controlling the level difference without controlling the correlation value. 
     In the above embodiments, it is assumed that the calculation for generating the surround signals is basically performed by a circuit. However, the calculation may be performed by a software process. For example, the functions of the surround signal generating devices  50  and  51  are realized by a program executed by a CPU (computer). As an example, the program may be preliminarily stored in a hard disk or a ROM. As another example, the program may be provided via a computer-readable recording medium such as a CD-ROM, and the program for which a CD-ROM drive reads the recording medium may be stored in the hard disc. 
     INDUSTRIAL APPLICABILITY 
     This invention can be used for various kinds of audio devices. 
     DESCRIPTION OF REFERENCE NUMBERS 
       10  Speaker 
       11 ,  14   a  to  14   c  Adder 
       12  Subtractor 
       13   a  to  13   c  Correlation Value Control Unit 
       15  Level Difference Control Unit 
       17  Uncorrelated Signal Generating Unit 
       20  Control Unit 
       30  Coefficient Table 
       50 ,  51  Surround Signal Generating Device