Patent Publication Number: US-2021185175-A1

Title: Call terminal, call system, control method of call terminal, and non-transitory recording medium

Description:
TECHNICAL FIELD 
     The present invention relates to a call terminal, a call system, and a control method of a call terminal, in which a call is performed with multiple people. The present invention also relates to a call program for causing a computer to operate as the call terminal, and a recording medium recording such a call program. 
     The present application claims priority to Japanese Patent Application No. 2018-141664 filed in Japan on 27 Jul. 2018, of which contents are incorporated herein by reference. 
     BACKGROUND ART 
     In recent years, as information terminals as typified by smartphones have become widespread, it is commonplace for each individual to acquire various pieces of information and perform communication via the Internet. As such an information terminal, not only the smartphone, but also a smart television capable of receiving information on the Internet via an interface of the television, a smart speaker capable of receiving such information via voices, and the like have been developed, which are appropriately used depending on their purposes and applications. 
     One example of the communication means using the information terminal is a call. A call, which is communication performed in a conversational style mainly using voices, is capable of implementing natural communication, and is frequently used in the modern life as well. In other words, the information terminal described above is used as a call terminal as well. 
     Incidentally, in a call involving multiple people, it is important to identify call partners and distinguish the call partners during the call in order to understand conversation in the call and have an appropriate call continued. PTLS 1 and 2 disclose techniques of localizing voices corresponding to voice signals of call partners calling a user, through the use of sound pressure panning, frequency characteristics, and the like. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP H11-68977 A 
     PTL 2: JP 2004-274147 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the techniques such as PTLS 1 and 2, in a case where the number of call partners is increased during a call, voices corresponding to the voice signals of each of the call partners cannot necessarily be output to the user so that the user can easily hear and distinguish the voices. 
     An object of one aspect of the present invention is to provide a call terminal that outputs voices corresponding to voice signals of each of call partners to a user so that the user can easily hear and distinguish the voices even when the number of call partners is increased, and techniques pertaining to the call terminal. 
     Solution to Problem 
     A call terminal according to one aspect of the present invention includes: a reception unit configured to receive a voice signal of each of one or more call partners; a localization position determination unit configured to, in a case where one of the one or more call partners is added, determine a localization position corresponding to the voice signal of the added one of the one or more call partners not to overlap with a localization position corresponding to the voice signal of another one of the one or more call partners; and a voice output unit configured to output voice corresponding to each voice signal in manner that the voice is localized at the localization position determined by the localization position determination unit. 
     A call terminal according to one aspect of the present invention includes: a reception unit configured to receive a voice signal of each of one or more call partners; a localization position determination unit configured to, in a case where one of the one or more call partners is eliminated, determine a localization position corresponding to the voice signal of the eliminated one of the one or more call partners not to overlap with the localization position corresponding to the voice signal of another one of the one or more call partners; and a voice output unit configured to output voice corresponding to each voice signal in manner that the voice is localized at the localization position determined by the localization position determination unit. 
     A call system according to one aspect of the present invention is a call system including: a call terminal; and a call server, wherein the call terminal receives a voice signal of each of one or more call partners, the call system includes a localization position determination unit configured to, in a case where one of the one or more call partners is added for the call terminal, determine a localization position corresponding to the voice signal of the added one of the one or more call partners received by the call terminal not to overlap with the localization position corresponding to the voice signal of another one of the one or more call partners, and the call terminal outputs voice corresponding to each received voice signal in manner that the voice is localized at the localization position determined by the localization position determination unit. 
     A control method of a call terminal according to one aspect of the present invention includes: a reception step of using the call terminal to receive a voice signal of each of one or more call partners; a localization position determination step of, in a case where one of the one or more call partners is added, determining a localization position corresponding to the voice signal of the added one of the one or more call partners not to overlap with a localization position corresponding to the voice signal of another one of the one or more call partners; and a voice output step of using the call terminal to output voice corresponding to each voice signal in manner that the voice is localized at the localization position determined in the localization position determination step. 
     Advantageous Effects of Invention 
     According to one aspect of the present invention, the call terminal that outputs voices corresponding to voice signals of each of call partners to a user so that the user can easily hear and distinguish the voices even when the number of call partners is increased, and the techniques pertaining to the call terminal can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a principal configuration of a call terminal according to a first embodiment. 
         FIG. 2  is a diagram illustrating examples of a localizable range of voices corresponding to voice signals of call partners according to the first embodiment. 
         FIG. 3  is a diagram illustrating examples of localization positions corresponding to voice signals of call partners according to the first embodiment. 
         FIG. 4  is a diagram illustrating examples of localization positions corresponding to voice signals of call partners according to the first embodiment. 
         FIG. 5  is a diagram illustrating examples of localization positions corresponding to voice signals of call partners according to the first embodiment. 
         FIG. 6  is a diagram illustrating examples of localization positions corresponding to voice signals of call partners according to the first embodiment. 
         FIG. 7  is a flowchart illustrating an example of the procedure of control processing of the call terminal according to the first embodiment. 
         FIG. 8  is a block diagram illustrating a principal configuration of a call terminal according to a second embodiment. 
         FIG. 9  is a diagram illustrating examples of localization positions corresponding to voice signals of call partners according to the second embodiment. 
         FIG. 10  is a flowchart illustrating an example of the procedure of control processing of the call terminal according to the second embodiment. 
         FIG. 11  is a block diagram illustrating a principal configuration of a call system according to a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Each embodiment of the present invention will be described below in detail. Note that, unless otherwise specifically noted, the configuration described in these embodiments is not to limit the scope of the present invention only to the configuration. 
     First Embodiment 
     A call terminal  1  and a control method of the call terminal  1  according to a first embodiment will be described below with reference to  FIGS. 1 to 7 . 
     Call Terminal  1   
       FIG. 1  is a block diagram illustrating a principal configuration of the call terminal  1  according to the first embodiment. As illustrated in  FIG. 1 , the call terminal  1  includes a call participant count acquisition unit  101 , a voice signal acquisition unit (reception unit)  102 , a control unit  103 , a voice signal reproduction unit (voice output unit)  104 , and a storage unit  105 . 
     Note that the call terminal  1  is configured to be capable of implementing a call by a large number of people (at least three people). The call terminal  1  can be appropriately used for a teleconference system, a call system using a VR space, or the like as well. People who participate in a call made by a large number of people are hereinafter referred to as call participants. Among the call participants, a person who operates the call terminal  1  is hereinafter referred to as a user, and people other than the user are hereinafter referred to as call partners. 
     Call Participant Count Acquisition Unit  101   
     The call participant count acquisition unit  101  acquires the number of call partners (call participant count) who are calling the user from the outside of the call terminal  1 . In the present embodiment, the call participant count refers to the number of call participants (call partners) among the people who are performing the call, except for the user himself/herself. For example, in a case where a one-to-one call is performed with a certain user, the call participant count is one, whereas in a case where a call among three people is performed, the call participant count is two. 
     Note that the call participant count acquisition unit  101  may not be included in the call terminals of all of the call participants. For example, the call terminal  1  of the user may serve as a main terminal, and only the call terminal  1  of the user may include the call participant count acquisition unit  101 . In this case, information of the call participant count acquired by the call participant count acquisition unit  101  in the call terminal  1  of the user may be configured to be transmitted to the call terminals of other call participants (call partners). In a case where the call terminal  1  does not include the call participant count acquisition unit  101 , the call terminal  1  may include, instead of the call participant count acquisition unit  101 , a call participant count reception unit  109  (not illustrated) for receiving information of the call participant count from the call terminal  1  that includes the call participant count acquisition unit  101 . With this configuration, the call participant count can be efficiently acquired while reducing the processing amount of the entire system. Instead of the call terminal of each call participant, a server  110  (not illustrated) may include the call participant count acquisition unit. In this case, information of the call participant count acquired by the call participant count acquisition unit of the server  110  may be configured to be transmitted to the call terminal of each call participant. With this configuration as well, the call participant count can be efficiently acquired while reducing the processing amount of the entire system. 
     Voice Signal Acquisition Unit  102   
     The voice signal acquisition unit  102  acquires a voice signal of each of one or more call partners. Specifically, the voice signal acquisition unit  102  acquires as many voice signals as the number of the call participant count acquired by the call participant count acquisition unit  101  from the outside of the call terminal  1 . 
     In the present embodiment, the voice signal is a voice signal corresponding to the voice of the call partner(s) with which the user performs a call, and preferably is a voice signal in a monaural format. The voice signal acquisition unit  102  may be configured to acquire the voice signal that is compressed with some compression method. In this case, the voice signal acquisition unit  102  decodes the acquired voice signal with an appropriate decoding method. The voice signal acquisition unit  102  may be configured to acquire the voice signal in a format other than the monaural format, that is, the voice signal having two or more channels. In this case, the voice signal acquisition unit  102  may be configured to downmix the acquired multi-channel voice signal into a monaural signal. The decoding of the compressed voice signal and the downmix of the voice signal into a monaural signal described above may be performed in the voice signal processing unit  108  to be described later. 
     Control Unit  103   
     The control unit  103  controls the call participant count acquisition unit  101 , the voice signal acquisition unit  102 , the voice signal reproduction unit  104 , and the storage unit  105 , and inputs and outputs data to and from each of these units. The control unit  103  is, for example, implemented by a central processing unit (CPU) executing a program stored in a prescribed memory. The control unit  103  includes a call participant count increase/decrease detection unit  106 , a localization position determination unit  107 , and a voice signal processing unit  108 . 
     Call Participant Count Increase/Decrease Detection Unit  106   
     The call participant count increase/decrease detection unit  106  detects an increase or a decrease of the call participant count. Specifically, the call participant count increase/decrease detection unit  106  acquires information of the call participant count from the call participant count acquisition unit  101 , and detects whether the call participant count is increased or decreased with respect to the previously acquired call participant count. The call participant count increase/decrease detection unit  106  transmits the increased or decreased call participant count and information of the increased or decreased call partner(s) to the localization position determination unit  107 , as well as the detection results of the increase or the decrease of the call participant count. 
     Localization Position Determination Unit  107   
     In a case where a call partner is added, the localization position determination unit  107  determines the localization position corresponding to the voice signal of the added call partner so as not to overlap with the localization positions corresponding to the voice signals of other call partners. In a case where a call partner is eliminated, the localization position determination unit  107  determines the position corresponding to the voice signal of the eliminated call partner so as not to overlap with the localization positions corresponding to the voice signals of other call partners. In a case where the call participant count is increased or decreased, the localization position determination unit  107  receives a notification indicating that the call participant count is increased or decreased from the call participant count increase/decrease detection unit  106 . In this case, the localization position determination unit  107  changes the localization positions corresponding to the voice signals of respective call partners so that the localization positions do not overlap with each other, based on the increase or the decrease of the call participant count. With this configuration, even in a case where the server  110  side calculates the localization positions, the call terminal  1  side can determine the localization positions in a case where the voice signals acquired by the voice signal acquisition unit  102  are caused to be reflected in localization of the voices to be output. Further, even when the number of call partners is increased or decreased, the voices corresponding to the voice signals of each of the call partners can be output so that the user can easily hear and distinguish the voices. The details of the determination method of the localization position corresponding to the voice signal of the call partner by the localization position determination unit  107  will be described later. 
     Voice Signal Processing Unit  108   
     The voice signal processing unit  108  configures voices to be reproduced from the voice signal reproduction unit  104 , based on the localization positions corresponding to the voice signals of each of the call partners obtained from the localization position determination unit  107  and the voice signals of each of the call partners obtained from the voice signal acquisition unit  102 . 
     Here, the voices configured by the voice signal processing unit  108  are voices able to have the user perceive the localization positions corresponding to the voice signals of each of the call partners determined by the localization position determination unit  107 . The method of implementing the voice is determined depending on the configuration of the voice signal reproduction unit  104 . For example, in a case where the voice signal reproduction unit  104  is a pair of headphones or earphones, the voice signal processing unit  108  configures a binaural voice signal that is implemented by using the head-related transfer function (HRTF). In this manner, the voice signal processing unit  108  has the user perceive the position of the voice corresponding to the voice signal of each of the call partners. In contrast, in a case where the voice signal reproduction unit  104  is a stereo speaker, the voice signal processing unit  108  may have the user perceive the position of the voice by configuring a transaural voice signal using the head-related transfer function described above. The voice signal processing unit  108  may configure the voice signal by using sound pressure panning such as vector base amplitude panning (VBAP). 
     Note that the above example provides description on the assumption that the voice signal of each of the call partners acquired by the voice signal processing unit  108  is a monaural signal. Note that, in a case where the voice signal of a specific call partner obtained from the voice signal acquisition unit  102  is a stereo (2-ch) voice signal or of higher quality, the voice signal processing unit  108  may convert the voice signal into a monaural signal through downmix or the like. 
     Voice Signal Reproduction Unit  104   
     The voice signal reproduction unit  104  outputs the voice corresponding to each voice signal so that the voice is localized at the localization position determined by the localization position determination unit  107 . In this manner, even when the number of call partners is increased or decreased, the voices corresponding to the voice signals of each of the call partners can be output so that the user can easily hear and distinguish the voices. The voice signal reproduction unit  104  reproduces each voice signal that has been subjected to sound effect processing by the control unit  103  through the speaker, the headphones, the earphones, or the like that is connected to the voice signal reproduction unit  104 . In this manner, the voice signal reproduction unit  104  can appropriately output the voices and have the user hear the voices. 
     Storage Unit  105   
     The storage unit  105  includes a secondary storage apparatus for storing prescribed data used by the control unit  103 . The storage unit  105  is, for example, implemented as a magnetic disk, an optical disc, or a flash memory. Specifically, the storage unit  105  is implemented as a hard disk drive (HDD), a solid state drive (SSD), a Blu-Ray (registered trademark) (BD Disc), or the like. The control unit  103  can read data from the storage unit  105  and record data into the storage unit  105  as necessary. 
     Operation of Localization Position Determination Unit  107   
     Next, the operation of the localization position determination unit  107  will be described below in detail with reference to  FIGS. 2 to 6 . 
     Configuration of Localizable Range 
     The localization position determination unit  107  may configure a localizable range, which is a range in which each voice can be localized, before determining the localization position of the voice corresponding to each voice signal. In this manner, the localization position of each voice can be more appropriately determined. Note that the localization position determination unit  107  may determine the localization position of each voice without configuring the localizable range. The configuration method of the localizable range by the localization position determination unit  107  will be described below with reference to  FIG. 2 .  FIG. 2  is a diagram illustrating examples of the localizable range according to the first embodiment. 
     Configuration Example 1 of Localizable Range 
     In one aspect, for example, as illustrated in (A) of  FIG. 2 , the localization position determination unit  107  may configure a localizable range  202   a , which is defined between a localizable range start position  203  and a localizable range end position  204  within a circle about a user  201  (around the user  201 ). In this case, the localization position determination unit  107  determines the localization position (for example, the localization position  205 ) of each voice in the localizable range  202   a.    
     In one aspect, for example, the call terminal  1  may include a range input unit  111  (not illustrated), such as a keyboard or a touch panel, that receives input of the localizable range from the user  201 , and the localization position determination unit  107  may configure the range input into the range input unit  111  as the localizable range. For example, the range input unit  111  is configured to receive input of the localizable range start position  203  and the localizable range end position  204 , and the localization position determination unit  107  configures the range defined between the localizable range start position  203  and the localizable range end position  204  as the localizable range  202   a.    
     In this manner, in a case where the number of call partners is small, for example, the range to which attention should be paid during a call can be reduced by limiting the localizable range  202   a , whereas in a case where the number of call partners is large, for example, the voices originating from each of the call partners can be made to be more easily heard and distinguished by widening the localizable range  202   a.    
     Note that the radius of the circle about the user  201  being used for defining the localizable range is not specifically limited, and may be set to any distance. For example, the localization position determination unit  107  may determine the radius of the circle by receiving the distance from the user  201  to the localization position of the voice via any command input unit  112  (not illustrated), such as a keyboard or a touch panel, from the user  201 . 
     Configuration Example 2 of Localizable Range 
     The user  201  may input the localizable range start position  203  and the localizable range end position  204  to be the same. The user  201  may omit the input of the localizable range. In such cases, as illustrated in (B) of  FIG. 2 , the localization position determination unit  107  may configure the localizable range to be a localizable range  202   b , which corresponds to the whole circle about the user  201 . In this case, the localization position determination unit  107  determines the localization position (for example, the localization position  206 ) of each voice in the localizable range  202   b.    
     Configuration Example 3 of Localizable Range 
     The above example describes a case in which the localizable range is a continuous range. However, the localizable range need not necessarily be such a continuous range. For example, as illustrated in (C) of  FIG. 2 , the localization position determination unit  107  may configure multiple non-continuous localizable ranges  202   c  and  202   d  as the localizable range. 
     Configuration Example 4 of Localizable Range 
     In one aspect, the call terminal  1  may include a detection unit  113  (not illustrated) that detects ambient sound of the call terminal  1 , and the localization position determination unit  107  may determine the localization position corresponding to each voice signal so as to avoid the generation source of the sound detected by the detection unit  113 . 
     For example, in a case where the detection unit  113  detects sound such as television sound from the front side of the user  201 , as illustrated in (C) of  FIG. 2 , the localization position determination unit  107  configures the non-continuous localizable ranges  202   c  and  202   d  except for the front side of the user  201  as the localizable range, and determines localization positions  207  to  209  of each voice in the localizable ranges  202   c  and  202   d . In this manner, the localization position determination unit  107  can determine the localization position corresponding to each voice signal so as to avoid the generation source of the sound detected by the detection unit  113 . 
     In this manner, for example, even in a case where sound such as television sound is generated from a certain direction, the voices originating from the call partners can be arranged to be heard from a direction different from the generation source of the sound. In this manner, the user  201  can be made to more easily hear and distinguish the voices originating from each of the call partners. Note that, as long as the configuration of determining the localization position of each voice so as to avoid the generation source of sound is adopted, the configuration is not limited to the configuration of configuring the localizable range so as to avoid the generation source of sound. The localization position determination unit  107  may be configured to determine the localization position of each voice so as to avoid the generation source of sound in a freely configured localizable range. 
     Configuration Example 5 of Localizable Range 
     In one aspect, the localization position determination unit  107  may configure the localizable range, based on the range in which the voice signal reproduction unit  104  can actually localize the output voice. Specifically, the localization position determination unit  107  may configure the localizable range, based on the position of the voice signal reproduction unit  104  or the position of the voice signal reproduction unit  104  and a voice signal configuration method of the voice signal processing unit  108 . 
     For example, as illustrated in (D) of  FIG. 2 , it is assumed that the voice signal reproduction unit  104  is stereo speakers  210  and  211 , and the voice signal configuration method of the voice signal processing unit  108  is VBAP. In this case, the range in which the voice signal reproduction unit  104  can localize the output voice is a range between the stereo speaker  210  and the stereo speaker  211 . In this case, the localization position determination unit  107  may determine a line connecting the user  201  and the stereo speaker  210  as the localizable range start position  203 , and configure a line connecting the user  201  and the stereo speaker  211  as the localizable range end position  204 . 
     As illustrated in (E) of  FIG. 2 , in a case where the voice signal reproduction unit  104  is 5.1-ch multi-channel speakers  212  to  214  disposed adjacently to each other on the circle about the user  201 , and the voice signal configuration method of the voice signal processing unit  108  is VBAP, the voice signal reproduction unit  104  can localize the output voice at omnidirectional positions as seen from the user  201 . In this case, the localization position determination unit  107  may configure, for example, the localizable range  202   b  illustrated in (B) of  FIG. 2  as the localizable range. 
     Configuration Example 6 of Localizable Range 
     In the above example, the localization position determination unit  107  configures the localizable range in advance. However, the present embodiment is not limited to this. In the present embodiment, the localization position determination unit  107  may configure or change (reconfigure) the localizable range during a call. 
     For example, during a call, the call terminal  1  may receive a change command of the localization position from the user  201  via the command input unit  112 , and the localization position determination unit  107  may change the localization position corresponding to each voice signal, based on the change command. In this manner, for example, during a call, in a case where it is difficult to hear the voices originating from each of the call partners due to a reason such as that the range of the localization position is excessively wide or is excessively narrow, by changing the configuration of the localizable range, the localization positions of the voices originating from each of the call partners can be changed to the positions that allow the voices to be more easily heard and distinguished. 
     Configuration Example 7 of Localizable Range 
     In the above example, the localization position determination unit  107  configures at least a part of the range in the circle about the user  201  as the localizable range. However, the present embodiment is not limited to this. In the present embodiment, the localization position determination unit  107  can determine any range as the localizable range. In one aspect, the localization position determination unit  107  may configure at least a part of the range on a semicircle about the user  201  as the localizable range. In this case, the localization position determination unit  107  can determine the above of the user  201  as the localization position of the voice. Further, in one aspect, the localization position determination unit  107  may configure a range on the circumference of the circle about the user  201  as the localizable range, and determine the localization position of each voice on the circumference. Further, in one aspect, the localizable range may have a shape other than a circle. 
     Determination of Localization Position 
     Next, the determination method of the localization position corresponding to the voice signal of the call partner in a case where a call has just been started and the call participant count is acquired for the first time will be described with reference to  FIGS. 3 to 6 . 
     Determination Example 1 of Localization Position 
     An example of the determination method of the localization position of the voice (output voice originating from each call partner) corresponding to the voice signal of each call partner by the localization position determination unit  107  will be described with reference to  FIG. 3 . Note that, in the following description, it is assumed that the localization position determination unit  107  configures the localizable range  202   a.    
     In a case where the number of call partners is one, as illustrated in (A) of  FIG. 3 , the localization position determination unit  107  determines a localization position  301  which is determined in advance in the localizable range  202   a  as the localization position of the voice originating from the call partner. Note that, although the localization position  301  is a position in front of the user  201 , this is not restrictive. The localization position determination unit  107  may determine another position, or the localization position may be determined based on a command of the user  201  via the command input unit  112 . 
     In a case where the number of call partners is two or more, the localization position determination unit  107  determines the localization positions of the voices originating from each of the call partners so that the localization positions of the voices do not overlap with each other. Specifically, the localization position determination unit  107  determines the localization positions of the voices originating from each of the call partners at positions different from each other in the localizable range  202   a , and preferably determines the localization positions of the voices originating from each of the call partners so that the directions in which the voices originating from each of the call partners arrive the user  201  do not overlap with each other. For example, as illustrated in (B) of  FIG. 3 , the localization position determination unit  107  may determine localization positions  302  and  303  of the voices originating from each of the call partners at both the ends of the localizable range  202   a.    
     In one aspect, the localization positions corresponding to each voice signal may be determined so that the intervals between the adjacent localization positions are uniform in the localizable range  202   a . For example, as illustrated in (C) and (D) of  FIG. 3 , the localization position determination unit  107  determines the localization positions so that the intervals between two adjacent localization positions are uniform in the localizable range  202   a  of the voices. For example, in a case where the number of call partners is four, as illustrated in (C) of  FIG. 3 , the localization position determination unit  107  determines each of the localization positions of the voices originating from the four call partners at the positions of both the ends of the localizable range  202   a  of the voices and the positions to equally divide the localizable range  202   a  into three parts. For example, in a case where the number of call partners is five, as illustrated in (D) of  FIG. 3 , the localization position determination unit  107  determines each of the localization positions of the voices originating from the five call partners at the positions of both the ends of the localizable range  202   a  of the voices and the positions to equally divide the localizable range  202   a  into four parts. In this manner, the voices originating from each of the call partners can be more easily heard and distinguished. 
     As described above, when the localization position determination unit  107  determines the localization positions of the voices originating from each of the call partners based on the number of call partners, the voices of each of the call partners can be output so that the user  201  can more easily hear and distinguish the voices, depending on the number of call partners. 
     Determination Example 2 of Localization Position 
     Another example of the determination method of the localization position of the voice (output voice originating from each call partner) corresponding to the voice signal of the call partner by the localization position determination unit  107  will be described with reference to  FIG. 4 . Note that, in the following description, it is assumed that the localization position determination unit  107  configures the localizable range  202   b.    
     In a case where the number of call partners is one, as illustrated in (A) of  FIG. 4 , the localization position determination unit  107  determines a localization position  401  which is determined in advance and is a position in front of the user  201  in the localizable range  202   b , for example, as the localization position. Note that the position of the localization position  401  is not limited to this. 
     In a case where the number of call partners is two or more, the localization position determination unit  107  determines the localization positions of the voices originating from each of the call partners so that the localization positions of the voices do not overlap with each other. Specifically, the localization position determination unit  107  determines the localization positions of the voices originating from each of the call partners at positions different from each other in the localizable range  202   b , and preferably determines the localization positions of the voices originating from each of the call partners so that the directions in which the voices originating from each of the call partners arrive the user  201  do not overlap with each other. 
     In one aspect, the localization positions corresponding to each voice signal may be determined so that the intervals between the adjacent localization positions are uniform in the localizable range  202   b.    
     For example, as illustrated in (B) and (C) of  FIG. 4 , the localization position determination unit  107  determines the localization positions so that the intervals between two adjacent localization positions are uniform in the localizable range  202   b  of the voices. For example, in a case where the number of call partners is two, as illustrated in (B) of  FIG. 4 , the localization position determination unit  107  determines the localization positions of the voices originating from the two call partners at the positions to equally divide the localizable range  202   b  of the voices into two parts. For example, in a case where the number of call partners is five, as illustrated in (C) of  FIG. 4 , the localization position determination unit  107  determines the localization positions of the voices originating from the five call partners at the positions to equally divide the localizable range  202   b  of the voices into five parts. In this manner, the voices originating from each of the call partners can be more easily heard and distinguished. 
     When equally dividing the localizable range  202   b  into parts, the localization position determination unit  107  can equally divide the localizable range  202   b  in any manner of equal division. For example, when the call participant count is two and the localization position determination unit  107  determines the positions to equally divide the localizable range  202   b  into two parts as the localization positions of the voices originating from each of the call partners, the localization position determination unit  107  may determine localization positions  409  and  410  illustrated in (D) of  FIG. 4  instead of localization positions  402  and  403  illustrated in (B) of  FIG. 4 . When the call participant count is five and the localization position determination unit  107  determines the positions to equally divide the localizable range  202   b  of the voices into five parts as the localization positions of the voices originating from each of the call partners, the localization position determination unit  107  may determine localization positions  411  to  415  illustrated in (E) of  FIG. 4  instead of localization positions  404  to  408  illustrated in (C) of  FIG. 4 . 
     Determination Example 3 of Localization Position 
     Another example of the determination method of the localization position of the voice (output voice originating from each call partner) corresponding to the voice signal of the call partner by the localization position determination unit  107  will be described with reference to  FIG. 5 . Note that, in the following description, it is assumed that the localization position determination unit  107  configures the localizable ranges  202   c  and  202   d.    
     In a case where the number of call partners is one, the localization position determination unit  107  determines the localization position which is determined in advance in any of the localizable ranges  202   c  and  202   d  as the localization position of the voice originating from the call partner. For example, as illustrated in (A) of  FIG. 5 , the localization position determination unit  107  may determine a localization position  501  which is determined in advance in the localizable range  202   c  as the localization position of the voice originating from the call partner. 
     In a case where the number of call partners is two or more, the localization position determination unit  107  determines the localization positions of the voices originating from each of the call partners so that the localization positions of the voices do not overlap with each other. In this case, the localization position determination unit  107  determines the localization positions so that the localization positions are distributed in both of the localizable range  202   c  and the localizable range  202   d.    
     For example, in a case where the number of call partners is two, as illustrated in (B) of  FIG. 5 , the localization position determination unit  107  determines one localization position in each of the localizable ranges  202   c  and  202   d  (the localization position  502  in the localizable range  202   c  and the localization position  503  in the localizable range  202   d ). 
     Further, for example, in a case where the number of call partners is three, as illustrated in (C) of  FIG. 5 , the localization position determination unit  107  may determine two localization positions ( 504  and  505 ) in the localizable range  202   c  and determine one localization position ( 506 ) in the localizable range  202   d . As illustrated in (D) of  FIG. 5 , the localization position determination unit  107  may determine one localization position ( 507 ) in the localizable range  202   c  and determine two localization positions ( 508  and  509 ) in the localizable range  202   d.    
     Further, for example, in a case where the number of call partners is five, the localization position determination unit  107  determines the localization positions so that the intervals between the adjacent localization positions of the voices of the call participants are uniform in each of the localizable ranges  202   c  and  202   d . In this case, for example, as illustrated in (E) of  FIG. 5 , the localization position determination unit  107  may determine four localization positions  510  to  513  in the localizable range  202   c  and determine one localization position  514  in the localizable range  202   d . As illustrated in (F) of  FIG. 5 , the localization position determination unit  107  may determine three localization positions  515  to  517  in the localizable range  202   c  and determine two localization positions  518  and  519  in the localizable range  202   d.    
     In this case, as illustrated in (E) and (F) of  FIG. 5 , it is preferable that the localization position determination unit  107  determine each of the localization positions so that the intervals between the adjacent localization positions of the voices of the call participants are uniform in at least one of the localizable ranges  202   c  and  202   d  of the voices. 
     In this manner, by determining the localization positions so that the intervals between the adjacent localization positions of the voices of the call participants are uniform in at least one of the localizable ranges  202   d  and  202   d  in a case where the call participant count is three or more, the voices of each of the call participants can be easily localized at the positions where the user  201  can easily hear and distinguish the voices. 
     Determination Example 4 of Localization Position 
     In the above example, in a case where the number of call partners is a prescribed number or more, the localization position determination unit  107  determines each of the localization positions in the localizable range so that the intervals between the adjacent localization positions are uniform. Note that the localization position determination unit  107  need not determine each of the localization positions so that the intervals between the adjacent localization positions are uniform. 
     With reference to  FIG. 6 , an example of the determination method of the localization position of the voice of the call participant by the localization position determination unit  107  will be described below.  FIG. 6  is a diagram illustrating an example of the localization position of the voice according to the first embodiment. 
     For example, in a case where the number of call partners is five, the localization position determination unit  107  may equally divide the localizable range  202   b  into five parts as illustrated in (A) of  FIG. 6 , or may not equally divide the localizable range  202   b  into five parts as illustrated in (B) of  FIG. 6 . In particular, in a case where the localizable range  202   b  is divided into a front region  601  and a rear region  602  by a boundary line  603 , the user  201  may perceive the voices from the rear less clearly than the voices from the front. In this case, as illustrated in (B) of  FIG. 6 , the localization position determination unit  107  can output each voice to the user more appropriately by determining the interval between localization positions  607  and  608  in the rear region  602  to be wider than each of the intervals between adjacent ones of localization positions  604  to  606  in the front region  601 . 
     The localization position determination unit  107  may determine each of the localization positions so that the localization positions of the voices originating from each of the call partners are separated away from each other at least by a prescribed angle or more as seen from the user  201 . Although the prescribed angle is not particularly limited, the prescribed angle may be set to 1 degree, 5 degrees, 10 degrees, 15 degrees, 20 degrees, 25 degrees, 30 degrees, or the like, as appropriate. In this manner as well, the localization positions can be determined in a range where the user  201  can easily hear the voices of the call partners. 
     Change of Localization Position 
     Next, the change method of the localization positions of the voices originating from the call partners by the localization position determination unit  107  in a case where the call participant count is increased or decreased will be described. As described above, the localization position determination unit  107  changes the localization positions so that the localization positions of the voices originating from each of the call partners do not overlap with each other, based on the increase or the decrease of the call participant count. 
     In a conventional call that assumes a one-to-one call, as a matter of course, when one of the call partners ends the call, the call itself ends. In contrast, in a call involving a large number of people including the user (multi-person call), even when any one of the call participants (call partners) ends the call, the call itself is maintained by the remaining call participants. It is conceivable that a new call partner is added to an ongoing multi-person call. In this manner, in the multi-person call, a call partner may be increased or decreased in the middle of the call. In this case, in a voice-only call without an image, as compared to a call with an image, it is more difficult to hear and distinguish the voices corresponding to the voice signals of the call partners. In view of this, as described above, by changing the localization positions corresponding to the voice signals of each of the call partners so that the localization positions do not overlap with each other based on the increase or the decrease of the call participant count, the voices corresponding to the voice signals of each of the call partners can be output so that the user can easily hear and distinguish the voices even when the call participant count is increased or decreased. 
     Case where Call Partner is Increased (Added) 
     In the present embodiment, in a case where the call participant count increase/decrease detection unit  106  detects that the call partner has been increased (added), the localization position determination unit  107  ( i ) determines the localization position corresponding to the voice signal of the added call partner so as not to overlap with the localization positions corresponding to the voice signals of other call partners (call partners before the addition), and (ii) changes the localization positions corresponding to the voice signals of such other call partners according to the number of call partners after the addition while maintaining the relative positional relationship (order) thereof. 
     For example, in one aspect, the localization position determination unit  107  determines the localization positions corresponding to the voice signals of each of the call partners after addition in a manner similar to determination examples 1 to 4 of the localization position described above. In this case, it is arranged that the relative positional relationship (order) be maintained between the localization positions corresponding to the voice signals of the call partners existing before addition. The following is a description provided by taking specific examples. 
     For example, as illustrated in (C) of  FIG. 3 , it is assumed that the number of call partners has increased to five from a call state in which the number of call partners is four. It is assumed that the call partners before the increase are call partners A to D, and a call partner E is added. Further, localization positions  304  to  307  illustrated in (C) of  FIG. 3  respectively correspond to the voice signals of the call partners A to D. 
     In this case, as illustrated in (D) of  FIG. 3 , the localization position determination unit  107  changes the localization positions corresponding to the voice signals of the call partners A to D respectively to localization positions  308  to  311 , and determines the localization position corresponding to the voice signal of the call partner E at the localization position  312 . 
     Here, the localization positions  308  to  312  are determined in the localizable range  202   a  so that the intervals between the adjacent localization positions are uniform, in a manner similar to determination example 1 of the localization position. Note that the determination method of the localization positions corresponding to the voice signals of the call partners A to E is not limited to this, and the intervals between the adjacent localization positions may not be uniform as in determination example 4 of the localization position. In any case, the localization positions  308  to  312  are determined based on the number of call partners after addition. 
     The localization position determination unit  107  changes the localization positions so as not to change the relative positional relationship of the localization positions corresponding to the voice signals of each of the call partners illustrated in (C) of  FIG. 3 . Specifically, the localization position determination unit  107  changes the localization positions so that the voice localized at the localization position  304  is localized at the localization position  308 , the voice localized at the localization position  305  is localized at the localization position  309 , the voice localized at the localization position  306  is localized at the localization position  310 , and the voice localized at the localization position  307  is localized at the localization position  311 . Then, the localization position determination unit  107  determines the localization position corresponding to the voice signal of the newly added call partner at the localization position  312  being a right end position for the user  201 . 
     Note that the present embodiment is not limited to this, and the localization position determination unit  107  may determine the localization position corresponding to the voice signal of the newly added call partner at another position for the user  201 . For example, the localization position determination unit  107  may determine the localization position corresponding to the voice signal of the call partner newly added to the localization position  308  illustrated in (D) of  FIG. 3 . In this case, the localization position determination unit  107  changes the localization positions so that the voice localized at the localization position  304  is localized at the localization position  309 , the voice localized at the localization position  305  is localized at the localization position  310 , the voice localized at the localization position  306  is localized at the localization position  311 , and the voice localized at the localization position  307  is localized at the localization position  312 . In this manner, the relative positional relationship of the localization positions corresponding to the voice signals of each of the call partners before the call partner is added can be maintained in a manner similar to the above example. 
     Note that the above example describes a case in which the number of call partners is increased by one between four and five. However, the present embodiment is not limited to this. In the present embodiment, the number of call partners may be increased in any number. In such a case as well, in a manner similar to the above example, it is only necessary that the localization position determination unit  107  determine the localization positions corresponding to the voice signals of each of the call partners after addition in a manner similar to determination examples 1 to 4 of the localization position described above, and maintain the relative positional relationship (order) between the localization positions corresponding to the voice signals of the call partners existing before addition. 
     As described above, in a case where the call partner is added, by (i) determining the localization position corresponding to the voice signal of the added call partner so as not to overlap with the localization positions corresponding to the voice signals of other call partners (call partners before the addition) and (ii) changing the localization positions corresponding to the voice signals of such other call partners according to the number of call partners after the addition while maintaining the relative positional relationship (order) thereof, (i) the user can appropriately hear and distinguish the voice originating from the newly added call partner, and by (ii) maintaining the relative positional relationship (order) of the localization positions of the voices originating from the previously existing call partners, the user can be prevented from incorrectly recognizing the call partners. 
     Case where Call Partner is Reduced (Eliminated) 
     In the present embodiment, in a case where the call participant count increase/decrease detection unit  106  detects that the call partner has been reduced (eliminated), the localization position determination unit  107  changes the localization positions corresponding to the voice signals of the remaining call partners according to the number of call partners after the elimination while maintaining the relative positional relationship (order) thereof. 
     For example, in one aspect, the localization position determination unit  107  determines the localization positions corresponding to the voice signals of each of the call partners after elimination in a manner similar to determination examples 1 to 4 of the localization position described above. In this case, it is arranged that the relative positional relationship (order) be maintained between the localization positions corresponding to the voice signals of the remaining call partners. The following is a description provided by taking specific examples. 
     For example, as illustrated in (D) of  FIG. 3 , it is assumed that the call partner that emits the voice localized at the localization position  311  has ended the call and the call participant count is decreased to 4 from a call state in which the call participant count is five. In this case, the localization position determination unit  107  changes the localization positions corresponding to the voice signals of the call partners to the localization positions  304  to  307  illustrated in (C) of  FIG. 3 . Here, the localization position determination unit  107  changes the localization positions so as not to change the relative positional relationship (order) of the localization positions corresponding to the voice signals of each of the call partners illustrated in (D) of  FIG. 3 . Specifically, the localization position determination unit  107  changes the localization positions so that the voice localized at the localization position  308  is localized at the localization position  304 , the voice localized at the localization position  309  is localized at the localization position  305 , the voice localized at the localization position  310  is localized at the localization position  306 , and the voice localized at the localization position  312  is localized at the localization position  307 . 
     Note that the above example describes a case in which the number of call partners is decreased by one between four and five. However, the present embodiment is not limited to this. In the present embodiment, the number of call partners may be decreased in any number. In such a case as well, in a manner similar to the above example, it is only necessary that the localization position determination unit  107  determine the localization positions corresponding to the voice signals of each of the call partners after elimination in a manner similar to determination examples 1 to 4 of the localization position described above, and maintain the relative positional relationship (order) between the localization positions corresponding to the voice signals of the remaining call partners. 
     As described above, in a case where the call partner is eliminated, by changing the localization positions corresponding to the voice signals of the remaining call partners according to the number of call partners after the elimination while maintaining the relative positional relationship (order) thereof, (i) the user can appropriately hear and distinguish the voices originating from the remaining call partners, and (ii) the user can be prevented from incorrectly recognizing the call partners. 
     Change of Localization Position During Call 
     The localization position determination unit  107  may change the localization position of the voice originating from the call partner during a call. In this manner, even in a case where it is difficult for the user  201  to hear and distinguish the voices localized at the localization positions of the voices determined in advance, the user  201  can cause the localization position determination unit  107  to change the localization positions of the voices originating from each of the call partners afterwards by inputting a change command via the command input unit  112 , for example. As a result, the localization positions of the voices originating from each of the call partners can be determined at appropriate positions where the user  201  can more easily hear and distinguish the voices. 
     In a case where the change command via the command input unit  112  is a rotation command of the localization positions, the localization position determination unit  107  may rotate the localization positions of the voices originating from each of the call partners about the user  201  (listener of each voice), based on the rotation command. 
     For example, it is assumed that the number of call partners is two, and the localization position determination unit  107  determines the localizable range to be the localizable range  202   b . In this case, based on the command of the user  201 , the localization position determination unit  107  may perform rotation from the localization positions  402  and  403  illustrated in (B) of  FIG. 4  to the localization positions  409  and  410  illustrated in (D) of  FIG. 4  about the user  201  (listener of each voice). Then, the localization position determination unit  107  may determine the localization positions of the voices originating from each of the call partners at the localization positions  409  and  410  after the rotation. 
     Further, for example, it is assumed that the number of call partners is five, and the localization position determination unit  107  determines the localizable range to be the localizable range  202   b . In this case, based on the command of the user  201 , the localization position determination unit  107  may rotate the localization positions  404  to  408  of the voices originating from each of the call partners illustrated in (C) of  FIG. 5  to the localization positions  411  to  415  illustrated in (E) of  FIG. 5  about the user  201  (listener of each voice). Then, the localization position determination unit  107  may determine the localization positions of the voices originating from each of the call partners at the localization positions  411  to  415  after the rotation. 
     In this manner, for example, even in a case where it is difficult to hear and distinguish the voices originating from each of the call partners heard from the localization positions of the voices determined in advance during a call with the call partners, the localization positions of the voices originating from the call partners can be changed to the positions where the user  201  can more easily hear and distinguish the voices originating from each of the call participants. 
     Control Processing of Call Terminal  1   
     Next, with reference to  FIG. 7 , the procedure of the control processing of the call terminal  1  (control method of the call terminal) according to the present embodiment will be described.  FIG. 7  is a flowchart illustrating an example of the procedure of the control processing of the call terminal  1  according to the first embodiment. 
     In Step S 101 , the call participant count acquisition unit  101  acquires the number of call partners from the outside of the call terminal  1 . The voice signal acquisition unit  102  acquires the voice signals of each of one or more call partners (reception step). 
     In Step S 102 , the call participant count increase/decrease detection unit  106  acquires the number of call partners from the call participant count acquisition unit  101 , and determines whether or not the number of call partners has increased or decreased. In a case where the call participant count increase/decrease detection unit  106  determines that the number of call partners has increased or decreased (YES in Step S 102 ), the processing proceeds to Step S 106 . In a case where the call participant count increase/decrease detection unit  106  determines that the number of call partners has not increased or decreased (NO in Step S 102 ), the processing proceeds to Step S 103 . 
     In Step S 103 , in a case where the localization position determination unit  107  has not yet determined the localization positions corresponding to the voice signals of each of the call partners (NO in Step S 103 ), the processing proceeds to Step S 104 . In Step S 103 , in a case where the localization position determination unit  107  has already determined the localization positions corresponding to the voice signals of each of the call partners (YES in Step S 103 ), the processing proceeds to Step S 105  without changing the current localization positions corresponding to the voice signals of the call partners. 
     In Step S 104 , the localization position determination unit  107  determines the localization positions corresponding to the voice signals of each of the call partners, based on the number of call partners. For example, in a case where the number of call partners is one, the localization position determination unit  107  determines the localization positions corresponding to the voice signals of the call partners at the localization positions determined in advance. In a case where the number of call partners is two or more, the localization position determination unit  107  determines the localization positions corresponding to the voice signals of each of the call partners so as not to overlap with the localization positions corresponding to the voice signals of other call partners (localization position determination step). In this case, the localization position determination unit  107  may determine the localization positions so that the intervals between the adjacent localization positions corresponding to the voice signals of the call partners are uniform. Subsequently, the processing proceeds to Step S 105 . 
     In Step S 105 , the voice signal reproduction unit  104  outputs the voices corresponding to each of the voice signals so that the voices are localized at the localization positions determined in the localization position determination step, and ends the processing (voice output step). 
     In Step S 106 , the localization position determination unit  107  changes the localization positions corresponding to the voice signals of each of the call partners, based on the increase or the decrease of the number of call partners (localization position change step). In this case, the localization position determination unit  107  changes the localization positions of the voice signals of each of the call partners so that the relative positional relationship (order) of the localization positions of the voice signals is not changed between the previously existing call partners. 
     For example, in one aspect, in a case where the call partner is added, the localization position determination unit  107  may change the localization positions corresponding to each of the voice signals according to the number of call partners after the addition while maintaining the order of localization positions corresponding to the voice signals of the call partners before the addition. In one aspect, in a case where the call partner is eliminated, the localization position determination unit  107  may change the localization positions corresponding to each of the voice signals according to the number of call partners after the elimination while maintaining the order of localization positions corresponding to the voice signals of the remaining call partners. 
     Note that one example of an aspect in which the localization position determination unit  107  changes the localization positions corresponding to each of the voice signals according to the number of call partners after the increase or the decrease (after the addition or after the elimination) is an example in which the localization position determination unit  107  changes each of the localization positions so that the intervals between the localization positions corresponding to the voice signals of each of the call partners are uniform. 
     Owing to the configuration described above, the relative positional relationship (order) of the localization positions corresponding to the voice signals of each of the call partners is not changed even when the number of call partners is changed, and therefore the voices originating from each of the call partners can be appropriately distinguished, and by changing the localization positions corresponding to each of the voice signals according to the number of call partners after the increase or the decrease, the voices originating from each of the call partners can be made to be easily heard. 
     Modifications 
     In one aspect, the localization position determination unit  107  may perform calculation for determining how to change the localization positions corresponding to the voice signals of each of the call partners itself. Alternatively, the calculation itself may be performed in the server  110  connected to the call terminal  1  via a network, the call terminal  1  may receive the calculation results by the server  110 , and the localization position determination unit  107  may change the localization positions corresponding to the voice signals of each of the call partners, based on the received results. 
     In the above example, as illustrated in Step S 101  and Step S 102  of  FIG. 7 , the call participant count increase/decrease detection unit  106  determines whether or not the number of call partners is increased or decreased by acquiring the number of call partners from the call participant count acquisition unit  101 . However, the present embodiment is not limited to this. In the present embodiment, the determination of the increase or the decrease of the number of call partners may be performed with another method. For example, only when the user  201  participates in a call, the call participant count increase/decrease detection unit  106  may acquire the number of call partners from the call participant count acquisition unit  101  as illustrated in Step S 101  of  FIG. 7 , and may determine the increase or the decrease of the number of call partners, based on a log-in or log-off event of the call partner regarding the call. 
     Second Embodiment 
     In the call terminal  1  according to the first embodiment described above, the localization position determination unit  107  changes the localization positions so as not to change the relative positional relationship of the localization positions corresponding to the voice signal of each of the call partners before the call participant count is increased or decreased. Note that, as does a localization position determination unit  1070  in a control unit  1030  of a call terminal  10  according to a second embodiment, the localization position determination unit may change the localization positions corresponding to the voice signals of the call partners so as not to change the absolute positions of each of the call partners before the call participant count is increased or decreased. 
     The call terminal  10  according to the second embodiment will be described below with reference to  FIGS. 8 to 10 . Note that, for the sake of convenience of description, members having the same functions as the members described in the first embodiment are denoted by the same reference signs, and the description thereof will be omitted. 
     Call Terminal  10   
       FIG. 8  is a block diagram illustrating a principal configuration of the call terminal  10  according to the second embodiment. 
     As illustrated in  FIG. 8 , the call terminal  10  includes a control unit  1030  instead of the control unit  103  of the call terminal  1  according to the first embodiment. Except for this configuration, the call terminal  10  has the same configuration as the call terminal  1  according to the first embodiment. 
     Control Unit  1030   
     As illustrated in  FIG. 8 , the control unit  1030  includes a localization position determination unit  1070  instead of the localization position determination unit  107  according to the first embodiment. Except for this configuration, the control unit  1030  has the same configuration as the control unit  103  according to the first embodiment. 
     Localization Position Determination Unit  1070   
     Based on the increase or the decrease of the call participant count, the localization position determination unit  1070  changes the localization positions corresponding to the voice signals of the call partners so as not to change the absolute positions of each of the call partners before the call participant count is increased or decreased. 
     In a case where the call partner is added, the localization position determination unit  1070  determines the localization position corresponding to the voice signal of the added call partner according to the localization positions corresponding to the voice signals of the call partners before the addition while maintaining the localization positions corresponding to the voice signals of the call partners before the addition. In one aspect, the localization position determination unit  1070  identifies an available position not filled by the localization positions corresponding to the voice signals of the call partners before the addition, and determines the identified available position as the localization position corresponding to the voice signal of the added call partner. 
     In a case where the call partner is eliminated, the localization position determination unit  1070  maintains the localization positions corresponding to the voice signals of the remaining call partners. For example, in a case where the number of call partners is reduced such as in a case where there is a call partner who ends the call, the localization position determination unit  1070  changes the state of the localization position corresponding to the voice signal of the call partner to be eliminated to an available state. Here, the available state refers to a state in which the voice corresponding to the voice signal of the call partner is not assigned. 
     Note that the localization position determination unit  1070  may determine the upper limit for the number of call partners and candidate positions for the localization position in advance. In this case, in a case where the number of call partners exceeds the upper limit, even in a case where the call partner is increased, the localization position determination unit  1070  does not determine the localization position corresponding to the voice signal of the increased call partner and does not let the call partner participate in the call. The localization position determination unit  1070  selects the localization positions corresponding to the voice signals of each of the call partners only from the candidate positions for the localization position determined in advance. Note that the present invention is not limited to this. The localization position determination unit  1070  need not set the upper limit for the number of call partners and may determine the localization position at any positions. 
     Change of Localization Position 
     The change method of the localization positions corresponding to the voice signals of the call partners by the localization position determination unit  1070  in a case where the number of call partners is increased or decreased will be described below with reference to  FIG. 9 .  FIG. 9  is a diagram illustrating an example of the localization positions corresponding to the voice signals of the call partners according to the second embodiment. The following will describe a case in which the localization position determination unit  1070  sets the upper limit for the call participant count to be five, and determines localization positions  901  to  905  illustrated in  FIG. 9  as the candidate positions for the localization position determined in advance. 
     Here, the localization positions  901  to  905  are assigned identifiers 1 to 5, respectively. In one example, call partner information related to each call partner has a structure illustrated in the following table 1. Specifically, the call partner information includes an identifier of the call partner for uniquely identifying each call partner, and an identifier of the localization position indicating the localization position corresponding to the voice signal of the call partner. Note that, in a case where the localization position is not determined yet, the identifier of the localization position may be an identifier indicating that the localization position has not been determined yet. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 IDENTIFIER OF CALL PARTNER 
               
               
                   
                 IDENTIFIER OF LOCALIZATION POSITION 
               
               
                   
                   
               
            
           
         
       
     
     In a case where the number of call partners is increased or decreased, the localization position determination unit  1070  receives, from the call participant count increase/decrease detection unit  106 , the call partner information of each call partner as well as a notification indicating that the number of call partners has increased or decreased. The localization position determination unit  1070  can operate the localization positions corresponding to the voice signals of each of the call partners by processing the call partner information. 
     In  FIG. 9 , for example, it is assumed that the call partner assigned identifier 1 corresponding to the localization position  901  ends the call, and the number of call partners is decreased by one. In this case, the localization position determination unit  1070  changes the state of the localization position  901  to the available state, and maintains the states of the localization positions corresponding to the voice signals of the rest of the call partners. It is assumed that, in a state in which the call partners corresponding to the localization positions  901  and  903  participate in the call, one call partner newly participates in the call and the number of call partners is increased by one. In this case, the localization position determination unit  1070  determines the localization position corresponding to the identifier having the smallest value out of the numbers of the identifiers corresponding to the localization positions currently in the available state as the localization position corresponding to the voice signal of the call partner who has newly participated. Specifically, the localization position determination unit  1070  determines the localization position  902  corresponding to identifier 2 having the smallest value out of identifiers 2, 4, and 5 corresponding to the localization positions  902 ,  904 , and  905  currently in the available state as the localization position corresponding to the voice signal of the call partner who has newly participated. 
     As described above, also by determining the localization positions corresponding to the voice signals of the call partners so as not to change the absolute positions of each of the call partners before the call participant count is increased or decreased, the change of the positional relationship of the localization positions corresponding to the voice signals of each of the call partners can be reduced to the minimum necessary. This configuration can prevent a significant alteration in the positional relationship of the localization positions corresponding to the voice signals of each of the call partners that the user has recognized before the call participant count is increased or decreased. As a result, the user can be prevented from incorrectly recognizing the call partners. Therefore, even when the number of call partners is increased or decreased, the voices corresponding to the voice signals of each of the call partners can be output so that the user can more easily hear and distinguish the voices. By determining the upper limit for the call participant count and the localization positions in advance by using the identifiers of the call partners or the like, the localization positions corresponding to the voice signals of the call partners can be more appropriately changed so as not to change the absolute positions of each of the call partners before the call participant count is increased or decreased. 
     Note that, in the above example, the localization position determination unit  1070  determines the localization position corresponding to the identifier having the smallest value out of the numbers of the identifiers corresponding to the localization positions currently in the available state as the localization position corresponding to the voice signal of the call partner who has newly participated. Note that, in the present embodiment, the localization position determination unit  1070  may determine the localization position corresponding to any identifier as the localization position corresponding to the voice signal of the call partner who has newly participated in so far as the localization positions corresponding to the voice signals of the call partners can be changed so as not to change the absolute positions of each of the call partners before the call participant count is increased or decreased. 
     Control Processing of Call Terminal  10   
     Next, with reference to  FIG. 10 , the procedure of the control processing of the call terminal  10  (control method of the call terminal) according to the present embodiment will be described.  FIG. 10  is a flowchart illustrating an example of the procedure of the control processing of the call terminal  10  according to the second embodiment. 
     Here, Step S 201  to Step S 205  are the same as Step S 101  to Step S 105  of the control processing of the call terminal  1  according to the first embodiment, and thus description thereof will be omitted. 
     In Step S 206 , the localization position determination unit  1070  acquires the call partner information from the call participant count increase/decrease detection unit  106 . 
     In Step S 207 , in a case where the call participant count is increased (YES in Step S 207 ), that is, in a case where there is a call partner who newly participates in the call, the processing proceeds to Step S 208 . In a case where the call participant count is changed without being increased (NO in Step S 207 ), that is, in a case where there is a call partner who ends the call and the call participant count is reduced, the processing proceeds to Step S 209 . 
     In Step S 208 , the localization position determination unit  1070  determines the localization position corresponding to the identifier corresponding to the call partner who has newly participated out of the localization positions currently in the available state as the localization position corresponding to the voice signal of the call partner who has newly participated (localization position change step). 
     In Step S 209 , the localization position determination unit  1070  changes the state of the localization position corresponding to the voice signal of the call partner who has ended the call to the available state (localization position change step). 
     Third Embodiment 
     The function of the call terminal  1  according to the first embodiment may be implemented by a call system  100  according to a third embodiment. 
     The call system  100  according to the third embodiment will be described below with reference to  FIG. 11 . Note that, for the sake of convenience of description, members having the same functions as the members described in the embodiments described above are denoted by the same reference signs, and the description thereof will be omitted. 
     Call System  100   
       FIG. 11  is a block diagram illustrating a principal configuration of the call system  100  according to the third embodiment. The call system  100  includes a call terminal  200  and a call server  300 . The call server  300  includes the localization position determination unit  107 . 
     In this manner, in the call system  100 , the call terminal  200  includes a control unit  10300  not including the localization position determination unit  107  instead of the control unit  103  including the localization position determination unit  107  in the call terminal  1  according to the first embodiment, and the call system  100  further includes the call server  300  including the localization position determination unit  107 . 
     In the call system  100 , the call terminal  200  receives the voice signals of each of one or more call partners, and the call system  100  includes the localization position determination unit  107  that, in a case where the call partner is added for the call terminal  200 , determines the localization position corresponding to the voice signal of the added call partner received by the call terminal  200  so as not to overlap with the localization positions corresponding to the voice signals of other call partners, and the call terminal  200  outputs the voices corresponding to each of the received voice signals so that the voices are localized at the localization positions determined by the localization position determination unit  107 . 
     More specifically, as illustrated in  FIG. 11 , in the call system  100 , the call participant count acquisition unit  101  of the call terminal  200  acquires the number of call partners, and the voice signal acquisition unit  102  acquires the voice signals of each of the call partners. The call participant count increase/decrease detection unit  106  of the call terminal  200  acquires the information of the call participant count from the call participant count acquisition unit  101 , and detects whether the call participant count has increased or decreased from the previously acquired call participant count. In a case where the call partner is added or eliminated, the localization position determination unit  107  of the call server  300  determines the localization position corresponding to the voice signal of the call partner who has been added or eliminated so as not to overlap with the localization positions corresponding to the voice signals of other call partners, based on detection results of the call participant count increase/decrease detection unit  106  of the call terminal  200 . The voice signal processing unit  108  of the call terminal  200  configures the voices to be reproduced from the voice signal reproduction unit  104 , based on the voice signals of each of the call partners obtained from the voice signal acquisition unit  102  of the call terminal  200  and the localization positions corresponding to each of the voice signals obtained from the localization position determination unit  107  of the call server  300 . The voice signal reproduction unit  104  of the call terminal  200  outputs each of the voices corresponding to each of the voice signals so that the voices are localized at the localization positions determined by the localization position determination unit  107  of the call server  300 . 
     In this manner, the call system  100  functions in a manner similar to the call terminal  1  according to the first embodiment as a whole. According to the call system  100 , with the call server  300  performing the processing of the localization position determination unit  107 , the processing amount of the call terminal  200  can be reduced. 
     Note that the above example describes a case in which, in the call system  100 , the call server  300  includes the localization position determination unit  107  instead of the call terminal  200 . However, the present embodiment is not limited to this. In the present embodiment, it is only necessary that the call terminal  200  include at least the voice signal reproduction unit  104 , and the call server  300  may include other members instead of the call terminal  200 . For example, the call server  300  may include the storage unit  105 , the localization position determination unit  107 , and the control unit  10300 , that is, the storage unit  105  and the control unit  103  of  FIG. 1 , instead of the call terminal  200 , and the call server  300  may further include the call participant count acquisition unit  101  and the voice signal acquisition unit  102  in addition to the control unit  103  and the storage unit  105  instead of the call terminal  200 . In this case as well, the call system  100  can function in a manner similar to the call terminal  1  according to the first embodiment as a whole while reducing the processing amount of the call terminal  200 . 
     Implementation Example by Software 
     Control blocks of the call terminal  1  or  10  (in particular, the call participant count increase/decrease detection unit  106 , the localization position determination unit  107  or  1070 , and the voice signal reproduction unit  104 ) may be implemented by logic circuits (hardware) formed in integrated circuits (IC chips) and the like, or may be implemented by software. 
     In the latter case, the call terminal  1  or  10  is provided with a computer that executes commands of a call program, which is software for implementing each function. The stated computer includes at least one processor (control apparatus), for example, and includes at least one computer-readable recording medium having stored the call program therein. In the computer, the processor reads out the call program from the recording medium and executes the call program, thereby accomplishing the object of the present embodiment. For example, a Central Processing Unit (CPU) may be used as the processor. As the recording medium, a “non-transitory tangible medium” such as a tape, a disk, a card, a semiconductor memory, and a programmable logic circuit may be used in addition to a Read Only Memory (ROM). A Random Access Memory (RAM) on which the call program is loaded, or the like may be further provided. The program may be supplied to the computer via any transmission medium (communication network, broadcast wave, or the like) capable of transmitting the call program. Note that an aspect of the present invention may be implemented in a form of data signal embedded in a carrier wave, which is embodied by electronic transmission of the call program.