Patent Publication Number: US-2019196580-A1

Title: Light emitting apparatus, head-mounted display, and virtual reality system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2017-250579 filed on Dec. 27, 2017. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a light emitting apparatus, a head-mounted display, and a virtual reality system. 
     2. Description of the Related Art 
     In a known virtual reality system, an image in virtual space is displayed on a head-mounted display attached to the head of a user. 
     In a virtual reality system, the position of the head-mounted display attached to the user may be tracked to reflect the position and orientation of the head of the user in real space in an image in the virtual space. In a conventional practice, there is a case in which a base station, for example, is placed in real space in which the user is present and a light receiving unit is attached to the head-mounted display. The base station emits a beam that performs sweeping horizontally and vertically in the real space and also transmits a synchronization signal in the form of a non-directional optical pulse at the start of a sweeping cycle of the beam. The position of the head-mounted display is identified by measuring the length of time from a time at which the synchronization signal had been received to a time at which the beam was received. 
     However, the conventional system has been unable to transmit necessary information from the base station to the head-mounted display, so it has been impossible to flexibly change the operation of the system. For example, the conventional system has been unable to transmit a command to change the sweeping speed of a beam and information about the state of the base station (such as an error) to the head-mounted display at an appropriate time. 
     SUMMARY OF THE INVENTION 
     An exemplary first disclosure in this application is a light emitting apparatus that emits light toward a real space in which a user wearing a head-mounted display is present. The light emitting apparatus includes a first light emitting unit that emits first light, which is used as a reference signal, a second light emitting unit that emits second light, which is used as a scanning signal that scans the real space at a timing based on the reference signal, and a controller that controls the first light emitting unit. The reference signal includes notification information to be submitted to the head-mounted display. 
     An exemplary second disclosure in this application is a head-mounted display that is attached to a user present in a real space and provides a virtual space to the user. The head-mounted display includes a first light sensing unit that senses first light and acquires a reference signal including notification information to be submitted to the head-mounted display, a second light sensing unit that senses second light and acquires a scanning signal that scans the real space at a timing based on the reference signal, and an extracting unit that extracts the notification information from the reference signal. 
     The above and other elements, features, steps, characteristics, and advantages of the present disclosure will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a virtual reality system in a first exemplary embodiment of the present invention. 
         FIG. 2  is a perspective view of a light emitting apparatus in the first exemplary embodiment of the present invention. 
         FIG. 3  is a block diagram illustrating the structure of the light emitting apparatus in the first exemplary embodiment of the present invention. 
         FIG. 4  is a perspective view illustrating a state in which a head-mounted display in the first exemplary embodiment of the present invention is attached to a user. 
         FIG. 5  is a block diagram illustrating the structure of the head-mounted display in the first exemplary embodiment of the present invention. 
         FIG. 6  illustrates an example of the structure of a synchronization signal transmitted from the light emitting apparatus in the first exemplary embodiment of the present invention. 
         FIG. 7  is timing diagrams for various signals in the virtual reality system in the first exemplary embodiment of the present invention. 
         FIG. 8  is a block diagram illustrating the structure of a head-mounted display in a second exemplary embodiment of the present invention. 
         FIG. 9  is a circuit diagram illustrating an example of a band-pass filter included in an extracting unit in the head-mounted display in the second exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A virtual reality system  1  in a first embodiment of the present disclosure will be described below. 
     In the virtual reality system  1  in this embodiment, a head-mounted display  3  is attached to the head of a user and a virtual reality world is provided to the user through the head-mounted display  3 , as an example. That is, the virtual reality system  1  uses a computer to create an artificial digital environment and provides, to the user, a virtual reality world that makes the user feel as if the user were present in the artificial digital environment. In the virtual reality world, control is performed so that a virtual reality image provided to the user through the head-mounted display  3  is changed according to the position of the head of the user. Therefore, it is necessary to accurately track the position of the head-mounted display  3  attached to the user in succession. 
     In this embodiment, the position of the head-mounted display  3  indicates the placement (including the orientation and attitude) of the head-mounted display  3  in three-dimensional real space (space in a room in which the user is present, for example). 
     In view of this, the virtual reality system  1  in this embodiment includes a light emitting apparatus  2  that periodically repeats emission of non-directional light emitting diode (LED) light to a real world in which the user is present and emission of laser light (beam) with which the scanning of the real world in which the user is present is started in synchronization with the emission of the LED light. When, for example, the room in which the user is present is scanned, the whole of the room is illuminated by planar laser light at a constant scanning speed. A light sensing unit, which will be described later, is included in the head-mounted display  3 . A plurality of light sensing units are preferably included. 
     In the virtual reality system  1  in this embodiment, an angle between a position from which scanning by laser light starts and the position of the light sensing unit in the head-mounted display  3  is identified from a difference between a time at which the light sensing unit in the head-mounted display  3  received LED light and a time at which the light sensing unit received laser light. 
     Preferably, laser light scans in succession along two directions that are mutually orthogonal. Thus, an angle from the position from which scanning by the light sensing unit in the head-mounted display  3  is started is identified in each of the two mutually orthogonal directions, enabling the position of the head-mounted display  3  to be identified in the three-dimensional real space. 
     The directions of scanning by laser light are not limited to two mutually orthogonal directions. For example, an angle formed by two scanning directions may be smaller than 90 degrees or may be larger than 90 degrees. 
       FIG. 1  illustrates the virtual reality system  1  in the first embodiment. 
     As illustrated in  FIG. 1 , the virtual reality system  1  includes the light emitting apparatus  2  and the head-mounted display  3  attached to a user U 1 . The light emitting apparatus  2  and user U 1  are present in predetermined real space RS. The real space RS is preferably closed space such as in the interior of a room. 
     The light emitting apparatus  2  emits light L toward the real space RS in which the user U 1  to which the head-mounted display  3  is attached is present. The light emitting apparatus  2  is placed at a position at which the emitted light L can cover as wide a range in the real space RS as possible. The user U 1  freely can move in the real space RS and can view a virtual reality image of a predetermined content through the head-mounted display  3 . 
     Although described later in detail, the light L emitted from the light emitting apparatus  2  includes LED light Ls, laser light Lh, and laser light Lv. The LED light Ls is used as a synchronization signal (an example of a reference signal), which is a type of pulse signal, that becomes a starting point of a timing at which to emit laser light Lh and laser light Lv. The laser light Lh and laser light Lv are used as scanning signals that scan the real space RS starting from a time at which the LED light Ls is emitted (that is, at a timing based on the synchronization signal). The laser light Lh horizontally scans the real space RS. The laser light Lv vertically scans the real space RS. 
     Next, the light emitting apparatus  2  in this embodiment will be described with reference to  FIGS. 2 and 3 .  FIG. 2  is a perspective view of the light emitting apparatus  2  in this embodiment.  FIG. 3  is a block diagram illustrating the structure of the light emitting apparatus  2  in this embodiment. 
     As illustrated in  FIG. 2 , the outline shape of the light emitting apparatus  2  is formed by a casing  2 B, which is a substantially rectangular parallelepiped. A light emitting surface  2   p , which is open, is formed in the casing  2 B. For explanation purposes, a coordinate system indicated in  FIG. 2  is defined. 
     As illustrated in  FIG. 2 , the light emitting apparatus  2  has two rotors denoted  243   h  and  243   v  through the light emitting surface  2   p . The rotor  243   h  has a light emitting opening  243   ha  and a rotational mechanism that rotates around the z axis and horizontally moves the light emitting opening  243   ha . The rotor  243   v  has a light emitting opening  243   va  and a rotational mechanism that rotates around the y axis and vertically moves the light emitting opening  243   va.    
     As illustrated in  FIG. 3 , the light emitting apparatus  2  has a controller  21 , a communication interface  22 , and light emitting units  23 ,  24 H and  24 V. The light emitting unit  23  is an example of a first light emitting unit. The light emitting units  24 H and  24 V are each an example of a second light emitting unit. 
     The controller  21 , which has a microcontroller, controls the entire operation of the light emitting apparatus  2 . For example, the controller  21  determines timings at which to emit LED light Ls, laser light Lh and laser light Lv, and controls the rotors  243   h  and  243   v  so that they rotate at rotational speed settings. 
     The communication interface  22  is linked to an external computer apparatus (not illustrated) so that wireless or wired communication with it is possible. The communication interface  22  receives a command from the computer apparatus and transmits the received command to the controller  21 . Commands are used to, for example, change various parameters including the rotational speeds of the rotors  243   h  and  243   v  in the light emitting apparatus  2 . 
     The light emitting unit  23  emits LED light Ls (an example of first light), which is used as a synchronization signal. The light emitting units  24 H and  24 V respectively emit laser light Lh and laser light Lv (which are each an example of second light), which are used as scanning signals that scan the real space RS at a timing based on the synchronization signal. 
     The light emitting unit  23  includes an LED driving unit  231  and an LED unit  232 . The LED unit  232  has one or a plurality of LEDs. The LED driving unit  231  has a driving circuit that receives a control signal from the controller  21  and creates driving signals that drive the LEDs included in the LED unit  232  from the received control signal. When each LED in the LED unit  232  is turned on, non-directional LED light Ls is emitted from the light emitting unit  23 . 
     As described above, the LED light Ls is used as a synchronization signal. In this embodiment, a synchronization signal is a pulse signal including notification information to be submitted to the head-mounted display  3 . To create a pulse signal including notification information, the LED unit  232  is controlled by the controller  21  so as to blink in a predetermined pattern. Notification information will be described later. 
     The light emitting unit  24 H includes a laser diode (LD) driving unit  241   h , a laser diode (LD) unit  242   h , the rotor  243   h , a motor driving unit  244   h , and a motor  245   h.    
     The LD driving unit  241   h  creates a predetermined constant current used to drive the LD unit  242   h . The LD driving unit  241   h  has a switching element connected to the LD unit  242   h . The LD driving unit  241   h  receives a control signal from the controller  21  and turns on and off the switching element according to the received control signal to control current carrying to the LD unit  242   h.    
     The LD unit  242   h  has, for example, one or more laser diodes. When the LD unit  242   h  has a single laser diode, the LD unit  242   h  directly emits laser light Lh. When the LD unit  242   h  has a plurality of laser diodes, the LD unit  242   h  uses a condenser to focus laser light from the plurality of laser diodes and outputs the focused laser light Lh. 
     The motor driving unit  244   h  receives a control signal from the controller  21  and creates a driving current that drives the motor  245   h  from the received control signal. Although the motor  245   h  may be any type of motor, the motor  245   h  is, for example, a direct-current (DC) brushless motor. The motor  245   h  starts to rotate the rotor  243   h  at a predetermined timing by using the driving current created by the motor driving unit  244   h , after which the motor  245   h  rotates the rotor  243   h  at a predetermined rotational speed. 
     An optical system  2430   h  is incorporated into the rotor  243   h . The optical system  2430   h  has one or a plurality of lenses that lead laser light transmitted from the LD unit  242   h  to the light emitting opening  243   ha . The optical system  2430   h  is provided to output planar laser light. 
     The light emitting unit  24 H having the structure described above starts to emit planar laser light Lh at a timing commanded by the controller  21  and also emits laser light Lh used as a scanning signal that horizontally scans while the rotor  243   h  is being rotated horizontally. 
     The light emitting unit  24 V includes an LD driving unit  241   v , an LD unit  242   v , a rotor  243   v , a motor driving u nit  244   v , and a motor  245   v . The light emitting unit  24 V, having a structure similar to the structure of the light emitting unit  24 H, operates similarly to the light emitting unit  24 H, but differs from the light emitting unit  24 H in that the light emitting unit  24 V emits laser light Lv used as a scanning signal that vertically scans while the rotor  243   v  is being rotated vertically. 
     That is, the scanning signals in this embodiment include a first scanning signal, implemented by laser light Lh, and a second scanning signal, implemented by laser light Lv, each of which scans the real space RS along one of two mutually orthogonal directions. 
     Next, the head-mounted display  3  in this embodiment will be described with reference to  FIGS. 4 to 6 .  FIG. 4  is a perspective view illustrating a state in which the head-mounted display  3  in this embodiment is attached to the user U 1 .  FIG. 5  is a block diagram illustrating the structure of the head-mounted display  3  in this embodiment.  FIG. 6  illustrates an example of the structure of a synchronization signal transmitted from the light emitting apparatus  2  in this embodiment. 
     The head-mounted display  3  is a goggle-type display attached to the head of the user U 1  as illustrated in  FIG. 4 . The form of the head-mounted display  3  illustrated in  FIG. 4  is just an example. The head-mounted display  3  may be of a helmet type. Although the head-mounted display  3  illustrated in  FIG. 4  is of a non-transparent type (immersive type) that completely covers the eyes, this is not a limitation; the head-mounted display  3  may a transparent-type display through which the circumference is visible. 
     The head-mounted display  3  has a main body  3 B and a head band  3   h  by which the main body  3 B is attached to the head. The main body  3 B has a plurality of light sensing units denoted  32 - 1  to  32 - 4 . Although four light sensing units are provided in the example in  FIG. 4 , there is no limitation on the number of light sensing units. In a common reference to the light sensing units  32 - 1  to  32 - 4  in the description below, they will be collectively referred to as the light sensing unit  32 . 
     The head-mounted display  3  has a controller  31 , the light sensing units  32 , a storage  34 , a display unit  35 , and a voice output unit  36 , as illustrated in  FIG. 5 . 
     The controller  31 , which has a microcontroller, controls the entire operation of the head-mounted display  3 . 
     Each light sensing unit  32  includes a photodiode used as a light receiving element and also has an electronic circuit that amplifies an electric signal as necessary, the electric signal being obtained through opto-electric conversion by the photodiode. The photodiode is just an example of a light receiving element. Any structure can be used as a light receiving element if the structure has a mechanism that can detect light. For example, a photoresistor may be used as a light receiving element. 
     The light sensing unit  32  is an example of a first light sensing unit and a second light sensing unit. Specifically, the light sensing unit  32  used as the first light receiving element detects LED light Ls emitted from the light emitting apparatus  2 , and acquires a synchronization signal including notification information to be submitted to the head-mounted display  3 . The light sensing unit  32  used as the second light sensing unit detects laser light Lh and laser light Lv emitted from the light emitting apparatus  2 , and acquires scanning signals that scan the real space RS at a timing based on the synchronization signal. 
     Notification information included in a synchronization signal includes at least any of first information and second information. The first information identifies the head-mounted display  3 . The second information is at least any of information about the operation state of the light emitting apparatus  2 , information indicating a setting of the light emitting apparatus  2 , and information indicating a command to change a setting of the light emitting apparatus  2 . 
     Information about the operation state of the light emitting apparatus  2  is, for example, information indicating that the light emitting apparatus  2  is normal or abnormal or in another state. Information indicating a setting of the light emitting apparatus  2  is, for example, information about a setting parameter for the light emitting apparatus  2  such as the rotational speed of the rotor  243   h  or  243   v . Information indicating a command to change a setting of the light emitting apparatus  2  is, for example, information about a command to change the rotational speed of the rotor  243   h  or  243   v.    
     The first information and second information are not limited to the above examples. They may include any information useful for the system. 
     In a first example EX 1  illustrated in  FIG. 6 , notification information composed of a bit string including a start bit, an 8-bit data string (data), and a cyclic redundancy check (CRC) code, which is an error detecting code, is superimposed in a synchronization signal Sync, which is a pulse signal. The start bit is a code that causes the receiving side to recognize that the transmission of notification information has been started. At least any of the first information and second information is included in the 8-bit data string. 
     Since the notification information in the first example is composed of a relatively short bit string, the notification information occupies only the small amount of information in the synchronization signal Sync. 
     In a second example EX 2  illustrated in  FIG. 6 , notification information composed of a bit string including a start ID, a system ID, a data string (data), a stop ID, and a cyclic redundancy check (CRC) code, which is an error detecting code, is superimposed in a synchronization signal Sync. The start ID is a code that causes the receiving side to recognize that the transmission of notification information has been started. The system ID is a particular ID assigned to a system (or content) used by the user at the transmission destination so that the synchronization signal is distinguished from a synchronization signal destined for another user. At least any of the first information and second information is included in the data string (data), as in the first example. The stop ID is a code that causes the receiving side to recognize that the transmission of notification information has been terminated. 
     Since the notification information in the second example is composed of a relatively long bit string, it is preferable to set a high transfer rate. 
     To create the pulse signal indicated in the examples in  FIG. 6 , emission of LED light Ls is enabled and disabled by controlling the light emitting unit  23  under the controller  21 . 
     Referring again to  FIG. 5 , the controller  31  in the head-mounted display  3  converts the synchronization signal, which is a signal sensed by the light sensing unit  32 , from analog to digital, performs CRC to detect an error, and acquires the notification information transmitted from the head-mounted display  3 . The controller  31  also records a time at which the synchronization signal was received. 
     In this embodiment, the controller  31  is an example of an extracting unit that extracts notification information to be submitted to the head-mounted display  3  from the synchronization signal. 
     As described above, in the virtual reality system  1  in this embodiment, serial communication is performed between the light emitting apparatus  2  and the head-mounted display  3  through transmission and reception of a synchronization signal. 
     The light sensing unit  32  detects laser light Lh and laser light Lv emitted from the light emitting apparatus  2  and acquires analog values of scanning signals. 
     The controller  31  converts the scanning signal acquired by the light sensing unit  32  from analog to digital, captures the converted signal, and records a time at which the scanning signal was received. The controller  31  then performs processing (i) to (iv) below. 
     (i) Processing to calculate a difference between the time at which the synchronization signal was received and the time at which the scanning signal was received 
     (ii) Processing to calculate an angle of the light sensing unit  32  from the scanning start point in the real space RS in two directions, horizontal and vertical, according to the rotational speeds of the rotors  243   h  and  243   v  and the difference 
     (iii) Processing to identify the position of the light sensing unit  32  (specifically, the positions of the light sensing units  32 - 1  to  32 - 4 ) from the angles calculated in (ii) above 
     (iv) Processing to identify the position of the head-mounted display  3  according to the results in (iii) above 
     Referring again to  FIG. 5 , the storage  34  stores an application that reproduces a virtual reality content (such as a game content) that the user U 1  views and listens to by wearing the head-mounted display  3 . When the head-mounted display  3  is activated, the controller  31  loads the application from the storage and executes the application, providing a virtual reality content to the user U 1 . 
     The display unit  35  includes a display panel attached to the main body  3 B of the head-mounted display  3 . The display panel displays a virtual reality image according to the result of the application execution by the controller  31 . At that time, an image for which the point of view of the virtual space has been adjusted according to the position of the head-mounted display  3 , the position being calculated by the controller  31  in succession, is displayed on the display unit  35 . 
     The voice output unit  36  includes a speaker (not illustrated), from which the voice output unit  36  outputs a voice according to the result of the application execution by the controller  31 . 
     Next, the operation of the virtual reality system  1  in this embodiment will be described with reference to  FIG. 7 .  FIG. 7  is timing diagrams for various signals in the virtual reality system  1  in this embodiment. 
     The timing diagrams in  FIG. 7  are for the synchronization signal Sync, a horizontal scanning signal Scan_H, a vertical scanning signal Scan_V, and a signal Sens sensed by the light sensing unit  32 . 
     Although,  FIG. 7  illustrates, as an example, a case in which the light emitting apparatus  2  first transmits a synchronization signal Sync and then transmits a horizontal scanning signal Scan_H and a vertical scanning signal Scan_V in succession in that order, this is not a limitation. After having transmitted a synchronization signal Sync, the light emitting apparatus  2  may transmit a horizontal scanning signal Scan_H and then may transmit a synchronization signal Sync again, after which the light emitting apparatus  2  may transmit a vertical scanning signal Scan_V. A horizontal scanning signal Scan_H and a vertical scanning signal Scan_V may not be transmitted in that order. A vertical scanning signal Scan_V may be transmitted first. 
     In the example illustrated in  FIG. 7 , the light emitting apparatus  2  emits LED light Ls, which is used as a synchronization signal Sync, during a period from time t 1  to time t 2 . Then, emission of laser light Lh, which is used as a horizontal scanning signal Scan_H, starts in synchronization with a falling edge of the synchronization signal Sync (at time t 2 ). Emission of laser light Lh, which is used as a horizontal scanning signal Scan_H, is performed during a period from time t 2  to time t 4 . 
     The sensed signal Sens is observed by the light sensing units  32 - 1  to  32 - 4  of the head-mounted display  3  during a period from time td 1  to time td 4  in the period from time t 2  to time t 4 . The controller  31  in the head-mounted display  3  calculates a difference between time t 2  and each of times td 1  to td 4  (in  FIG. 7 , differences between time t 2  and times td 1  to td 4  are collectively indicated as ΔT_H), and calculates the angle of each light sensing unit  32  in the horizontal direction from the scanning start point in the real space RS. 
     At time t 4  at which the emission of laser light Lh, which is used as a horizontal scanning signal Scan_H, is terminated, emission of laser light Lv, which is used as a vertical scanning signal Scan_V, is started. Laser light Lv, which is used as vertical scanning signal Scan_V, is emitted during a period from time t 4  to time t 6 . 
     The sensed signal Sens is observed by the light sensing units  32 - 1  to  32 - 4  of the head-mounted display  3  during a period from time td 5  to time td 8  in the period from time t 4  to time t 6 . The controller  31  in the head-mounted display  3  calculates a difference between time t 4  and each of times td 5  to td 8  (in  FIG. 7 , differences between time t 4  and times td 5  to td 8  are collectively indicated as ΔT_V), and calculates the angle of each light sensing unit  32  in the vertical direction from the scanning start point in the real space RS. 
     Processing in one cycle is completed in a period from time t 1  to time t 6 . At time t 6 , the controller  31  in the head-mounted display  3  identifies the horizontal and vertical positions, in the real space RS, of each light sensing unit  32  attached to the head-mounted display  3  with respect to the scanning start position. Therefore, the position of the head-mounted display  3  can be identified. 
     At time t 6 , emission of LED light Ls, which is used as a synchronization signal Sync, is started again. After that, processing to identify the position of the head-mounted display  3  is repeatedly performed. The light emitting apparatus  2  operates at, for example 60 Hz (the length of one cycle is about 16.7 ms). 
     As described above, in the virtual reality system  1  in this embodiment, a cycle is repeatedly performed in which the light emitting apparatus  2  emits LED light, which is used as a synchronization signal, and then emits laser light, which is used as a scanning signal, in synchronization with the synchronization signal to track the position of the head-mounted display  3  in succession. The display of a virtual reality image reproduced by the head-mounted display  3  is controlled according to the position of the head-mounted display  3 , reproducing a virtual reality world matching the orientation of the head of the user U 1 . 
     In this embodiment, notification information to be submitted to the head-mounted display  3  is included in a synchronization signal that is periodically transmitted from the light emitting apparatus  2  to the head-mounted display  3 . Therefore, useful information can be transmitted to the head-mounted display  3  at an appropriate time. Examples of this type of useful information include information indicating whether there is an error in the light emitting apparatus  2  and information as to, for example, a setting change in the light emitting apparatus  2 . Since the head-mounted display  3  can acquire this type of information at an appropriate time, the head-mounted display  3  can execute error handling at the time of an emergency, processing to respond to a change in the rotational speed of a rotor in the light emitting apparatus  2 , and other processing that is needed immediately. 
     Next, a second embodiment of the virtual reality system  1  in the present disclosure will be described. The description below will mainly focus on differences from the first embodiment. 
     This embodiment is intended to, even if there is optical noise (such as light from a fluorescent lamp or infrared light from a remote control) in real space in which a user is present, prevent a problem from occurring in communication between the light emitting apparatus  2  and a head-mounted display  3 A in this embodiment. 
     Conventionally, there has been the following problem caused by optical noise in real space in which a user is present. For example, there has been the possibility that when light due to optical noise interferes with LED light emitted from a light emitting apparatus, a light sensing unit in a head-mounted display cannot correctly sense light from the light emitting apparatus or incorrectly senses light. 
     Another problem has been that, in a situation in which two or more light emitting apparatuses are used in a single room and two or more users view and listen to different virtual reality contents, if a light sensing unit in a head-mounted display attached to one user receives LED light directed to another user (or intended for to another content), a timing to receive a synchronization signal may be delayed or advanced. In this case, each user failed to correctly view and listen to a content. 
     In view of this, in this embodiment, notification information to be submitted to the head-mounted display  3 A is superimposed in a synchronization signal, which is a pulse signal, transmitted from the light emitting apparatus  2  as a high-frequency signal having a higher frequency than the pulse signal. The high-frequency signal to be superimposed in the synchronization signal has a frequency assigned to the head-mounted display  3 A at the transmission destination in advance. When a filter is used to extract the high-frequency signal from the synchronization signal sensed by the light sensing unit  32 , the head-mounted display  3 A can reliably obtain intended notification information (that is, notification information destined for the head-mounted display  3 A). That is, since optical noise other than from the light emitting apparatus  2  and a signal due to LED light directed to another user (or intended for another content) can be removed by a filter, it is possible to more reliably transmit notification information to the head-mounted display  3 A through a synchronization signal. 
     Next, the head-mounted display  3 A in this embodiment will be described with reference to  FIGS. 8 and 9 .  FIG. 8  is a block diagram illustrating the structure of the head-mounted display  3 A in this embodiment.  FIG. 9  is a circuit diagram illustrating an example of a band-pass filter included in an extracting unit  33  in the head-mounted display  3 A in this embodiment. 
     As illustrated in  FIG. 8 , the head-mounted display  3 A in this embodiment differs from the head-mounted display  3  (see  FIG. 5 ) in the first embodiment in that the head-mounted display  3 A has the extracting unit  33 . 
     In the example in  FIG. 8 , the extracting unit  33  has band-pass filters  331  and  332  and a switch  333 . The band-pass filters  331  and  332  each extract a high-frequency signal (that is, notification information) from a synchronization signal, which is a pulse signal. The switch  333  is an analog switch that can be made switchable by the controller  31 . 
     An example of the circuit structure of the band-pass filters  331  and  332 , which are each an analog band-pass filter, is illustrated in  FIG. 9 . 
       FIG. 9  illustrates just an example of an analog band-pass filter. Various circuit structures of analog band-filters are known. The band-pass filters used in this embodiment may have any circuit structure if a desired pass center frequency f c  is obtained. 
     The transfer function G(s) of the circuit illustrated in  FIG. 9  is represented as in expression (1). The pass center frequency f c  is represented as in expression (2). 
     
       
         
           
             
               
                 
                   
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     In the extracting unit  33  in  FIG. 8 , the band-pass filters  331  and band-pass filter  332  are set so that they have different pass center frequencies f c . Specifically, the pass center frequency f c  of the band-pass filter  331  is F 1  and the pass center frequency f c  of the band-pass filter  332  is F 2  (F 2  is not equal to F 1 ). 
     In this embodiment, the output terminal of the band-pass filter  331  is connected to one terminal of the switch  333  and the output terminal of the band-pass filter  332  is connected to another terminal of the switch  333 . The connection state of the switch  333  is controlled by the controller  31 . 
     In the structure illustrated in  FIG. 8 , the reason why band-pass filters having different pass center frequencies f c  are provided so as to be selectively used is to enable notification information included in a synchronization signal to be selectively transmitted in two different contents or systems. 
     In a case in which two users view and listen to different virtual reality contents CT 1  and CT 2  in a single room, for example, two light emitting apparatuses are provided in the room. Then, a light emitting apparatus corresponding to the virtual reality content CT 1  transmits a synchronization signal in which a notification signal at the frequency F 1  is superimposed and a light emitting apparatus corresponding to the virtual reality content CT 2  transmits a synchronization signal in which a notification signal at the frequency F 2  is superimposed. 
     The controller  31  in the head-mounted display  3 A corresponding to the virtual reality content CT 1  controls the switch  333  so that the band-pass filter  331 , the pass center frequency of which is F 1 , is selected. The controller  31  in the head-mounted display  3 A corresponding to the virtual reality content CT 2  controls the switch  333  so that the band-pass filter  332 , the pass center frequency of which is F 2 , is selected. When band-pass filters having different pass center frequencies f c  are provided in the head-mounted display  3 A so as to be selectively used as described above, it is possible to prevent incorrect sensing between head-mounted displays  3 A that reproduce different contents in a single real space RS. 
     As described above, in the virtual reality system  1  in this embodiment, notification information is superimposed in a synchronization signal, which is a pulse signal, transmitted by the light emitting apparatus  2  as a high-frequency signal having a higher frequency than the pulse signal. The extracting unit  33  in the head-mounted display  3 A uses filters to remove signals due to light from another light source other than the light emitting apparatus  2  and LED light directed to another user (or intended for to another content). Therefore, it is possible to more reliably transmit notification information to the head-mounted display  3 A through a synchronization signal. 
     Another advantage is that since notification information is superimposed in a synchronization signal as a high-frequency signal, the notification information is transmitted highly efficiently. 
     Although, in the structure illustrated in  FIG. 8 , two band-pass filters that can be selectively used are provided, there is no limit on the number of selectively used band-pass filters. As would be apparent to one skilled in the relevant art, three or more band-pass filters each of which has a different pass center frequencies f c  can be provided so that they are selectively used. 
     From the viewpoint of removing light coming from another light source, the extracting unit  33  may be a single filter. When light from a fluorescent lamp is to be removed as noise, since domestic fluorescent lamps generally blink at 100 or 120 Hz, it suffices to provide a single filter that adequately attenuates a light pulse signal at 100 or 120 Hz (the filter is not limited to a band-pass filter, but may be a low-pass filter or a high-pass filter may). 
     Although, in the examples in  FIGS. 8 and 9 , a case in which analog band-pass filters are provided has been described, digital filters may be used. In this case, two digital filters (each of which is an example of the extracting unit) equivalent to the band-pass filters  331  and  332  are provided in the controller  31 . The controller  31  converts a signal sensed by the light sensing unit  32  from analog to digital and obtains a digital value, after which the controller  31  performs filtering on the digital value by using the above two digital filters. The controller  31  then selects one of outputs from the two digital filters and obtains target notification information (that is, notification information destined to the controller  31 ). 
     The digital filter may be of a finite impulse response (FIR) type or an infinite impulse response (IIR) type. 
     An advantage of using digital filters is that the setting of the pass center frequency f c  can be easily changed. 
     Instead of the examples illustrated in  FIGS. 8 and 9 , a method may be used by which a timer included in a microcomputer in the controller  31  is used to acquire target notification information. In this method, the head-mounted display  3 A needs to be assigned the frequency of a signal including the target notification information in advance. 
     Specifically, the controller  31  converts a signal sensed by the light sensing unit  32  from analog to digital and obtains a digital value, after which the controller  31  measures a time Ts between two adjacent rising edges (or two adjacent falling edges) of the sensed signal with the timer. If the time Ts is a value within a predetermined range stipulated by the reciprocal of the assigned frequency, the controller  31  decides that the received sensed signal is the target signal. 
     Since the sensed signal may include a noise component or an unintended signal, a structure is preferably used together with the analog filters or digital filters to remove the noise component or unintended signal. 
     So far, a plurality of embodiments of the present disclosure have been described. However, the present disclosure is not limited to the above embodiments. The above embodiments can be improved or modified in various forms without departing from the intended scope of the present invention. 
     For example, in the second embodiment, a case has been described in which notification information to be submitted to the head-mounted display  3 A is superimposed in a synchronization signal, which is a pulse signal, transmitted from the light emitting apparatus  2  as a high-frequency signal having a higher frequency than the pulse signal. An aspect to include notification information in a synchronization signal is not limited to the superimposition of a high-frequency signal; another method may be used. For example, the amplitude of a synchronization signal may be modulated to include notification information in the synchronization signal. 
     In the embodiments described above, a case has been described in which notification information is included in a synchronization signal transmitted from the light emitting apparatus  2 . However, the notification information may also be included in a scanning signal transmitted from the light emitting apparatus  2 . Specifically, in  FIG. 3 , the controller  21  in the light emitting apparatus  2  may control the light emitting units  24 H and  24 V so that notification information is included in a scanning signal through laser light Lh and laser light Lv. Thus, in a case in which a plurality of light emitting apparatuses  2  are placed in a single room, when an ID that identifies the head-mounted display  3  is included in a scanning signal as notification information, the head-mounted display  3  can reliably receive the target scanning signal (that is, the scanning signal destined for the head-mounted display  3 ). 
     In the embodiments described above, a case has been described in which the light emitting units  24 H and  24 V, each of which is an example of the second light emitting apparatus, use a laser diode (semiconductor laser). However, this is not a limitation. Another type of laser (such as, for example, a gas laser or a liquid laser) may be used. 
     In the embodiments described above, a case has been described in which four light sensing units  32  are provided in the head-mounted display  3  or  3 A. However, there is no limitation on the number of light sensing units  32 . The number of light sensing units  32  can be appropriately determined according to the capacity of calculation and/or a system request. 
     Features of the above-described preferred embodiments and the modifications thereof may be combined appropriately as long as no conflict arises. 
     While preferred embodiments of the present disclosure have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present disclosure. The scope of the present disclosure, therefore, is to be determined solely by the following claims.