Abstract:
An encoding device performing an encoding process according to an encoding scheme using a CTS indicating time at which presentation or reproduction is performed, comprising: an encapsulator that generates a media unit in which one or more access units encoded by the encoding scheme are capsulated, and a transmitter that transmits information indicating an absolute time of an access unit included at a top portion of the media unit, in addition to the media unit, wherein the encapsulator adds a CTS relative value to the n th  access unit included in the media unit, the CTS relative value being a difference between the CTS of the n th  access unit and the DTS of the n+1 th  access unit.

Description:
RELATED APPLICATIONS 
     This application is a continuation of PCT/JP2013/058247 filed on Mar. 22, 2013, which claims priority to Japanese Application No. 2012-073062 filed on Mar. 28, 2012. The entire contents of these applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an encoding device, a decoding device and programs for the same which encapsulate one or more access units into a media unit. 
     BACKGROUND ART 
     In a related art encoding device, as a format of an access unit correlated with a time stamp, PES (Packetized Elementary Stream) prescribed by MPEG (Moving Picture Experts Group)-2 System is known (Non Patent Literature 1). 
     As illustrated in  FIG. 19( a ) , this PES  90  includes a field indicating a PES packet length (PES_packet_length), a field indicating a DTS (Decoding Time Stamp) and a field indicating a PTS (Presentation Time Stamp). Further, this PES  90  can encapsulate (i.e., store) one access unit (AU: Access Unit). In  FIGS. 19( a ) and 19( b ) , a part of the fields is not illustrated. 
     Further, in a related art encoding device, a format of RTP (Real-time Transport Protocol) payload prescribed by IETF (Internet Engineering Task Force) is also known (Non Patent Literature 2). 
     As illustrated in  FIG. 19( b ) , this RTP payload  91  includes a field indicating a base time stamp (Base_Timestamp). Further, in this RTP payload  91 , a plurality of NAL (Network Adaptation Layer) units which are smaller units than the access units can be encapsulated. At this time, the RTP payload  91  includes, for each NAL unit, a field indicating the size of this NAL unit (NALu1_length, NALu2_length) and a field indicating an offset value from a time stamp (NALu1_TS_offset, NALu2_TS_offset). The RTP payload  91  does not include a concept of decoding timing and, therefore, the time stamp indicates presentation timing. 
     CITATION LIST NONPATENT LITERATURE 
     
         
         Non Patent Literature 1: ITU-T Rec. H.222.0|ISO/IEC 13818-1 
         Non Patent Literature 2: IETF RFC 3984 “RTP Payload Format for H.264 Video” 
       
    
     SUMMARY OF THE INVENTION 
     However, as illustrated in  FIG. 19( a ) , the related art PES  90  is not able to encapsulate a plurality of access units. Further, in the related art PES  90 , in an audio signal of which access unit size is only hundreds of bytes, since the size of the field indicating the PES packet length is a fixed value of 16 bits, this field becomes redundant and causes overheads. On the other hand, in the related art PES, the number of bits of the field indicating this packet length is sometimes insufficient in high resolution image signals. 
     Further, a field indicating decoding timing is not included in the related art RTP payload  91 . Further, in the related art RTP payload  91 , since a field indicating size information of each NAL unit is needed even in a case in which one NAL unit is encapsulated, this field becomes redundant and causes overheads. 
     As described above, in the techniques described in Non Patent Literature 1 and Non Patent Literature 2, the overheads and the shortage of fields caused during encapsulation of an audio signal and an image signal result in problems. Therefore, there is a strong demand for executing encapsulation in an optimum format. 
     Then, an object of the present invention is to solve the problems described above and to provide an encoding device, a decoding device and programs therefor that are capable of encapsulating in an optimum format. 
     In the light of the above problem, an encoding device according to a first invention of the present application summarized as a encoding device which encodes an input signal according to an encoding scheme using a CTS indicating time at which presentation or reproduction is performed and a DTS indicating time at which decoding is performed, comprising an encoder, an a differential value calculation means, an offset value calculation means, an encapsulation determination means and an encapsulation means. 
     According to the configuration described above, the encoding device generates the access unit correlated with the CTS and the DTS by encoding the input signal using the encoder. Further, the encoding device calculates, by the differential value calculation means, a differential value between the DTS of the access unit and the DTS of another access unit encoded immediately before the access unit. Then the encoding device calculates, by the offset value calculation means, an offset value which is a difference between the DTS and the CTS of the access unit. 
     Further, the encoding device determines, by the encapsulation determination means, as an encapsulation unit, one or more access units of which differential values become identical within predetermined determination time, and offset values become identical. Then the encoding device encapsulates, by the encapsulation means, one or more access units included in the encapsulation unit into a media unit and adds the same differential value and the same offset value to the media unit. 
     Here, in a case in which the encapsulation means encapsulates one access unit, the encapsulation means adds, to the media unit, a value indicating that size information for each access unit is not included as encapsulation determination information. On the other hand, in a case in which the encapsulation means encapsulates a plurality of access units, the encapsulation means adds, to the media unit, a value indicating that size information for each access unit is included as encapsulation determination information and adds the size information to the media unit. 
     Even in a case in which a plurality of access units are encapsulated, only one field indicating decoding timing (for the differential value) and only one field indicating decoding timing (for the offset value) are included in the media unit encapsulated by this encapsulation means. Further, in a case in which one access unit is encapsulated, the media unit includes no field indicating size information. In this manner, since no redundant field is included in the media unit, overheads can be reduced. 
     The media unit is a packet form (i.e., format) used in packetization of the access unit and in which useful information at the time of decoding this access unit (for example, the time stamp indicating decoding timing) is stored. The encapsulation determination information is information for determining the number of encapsulated access units. In other words, the encapsulation determination information is information for determining whether a plurality of access units have been encapsulated. The size information is information indicating size (i.e., data length) of each access unit. 
     In the light of the above problem, an encoding device according to a second invention of the present application summarized as a encoding device which encodes an input signal accordion to an encoding scheme using a CTS indicating time at which presentation or reproduction is performed, comprising an encoder, an a differential value calculation means, an offset value calculation means, an encapsulation determination means and an encapsulation means. 
     According to the configuration described above, the encoding device generates the access unit correlated with the CTS by encoding the input signal by the encoder. Further, the encoding device calculates, by the differential value calculation means, a differential value between the CTS of the access unit and the CTS of another access unit encoded immediately before the access unit. 
     Further, the encoding device determines, by the encapsulation determination means, one or more access units of which differential values become identical within predetermined determination time as the encapsulation unit. Then the encoding device encapsulates, by the encapsulation means, one or more access units included in the encapsulation unit into the media unit and adds the same differential value to the media unit. 
     Here, if one access unit is to be encapsulated, the encapsulation means adds, to the media unit, a value indicating that size information for each access unit is not included as encapsulation determination information and, if the plurality of access units are to be encapsulated, the encapsulation means adds, to the media unit, a value indicating that size information for each access unit is included as the encapsulation determination information and adds the size information to the media unit. 
     Even in a case in which a plurality of access units are encapsulated, only one field indicating decoding timing (for the differential value) is included in the media unit encapsulated by this encapsulation means. Further, in a case in which one access unit is encapsulated, the media unit includes no field indicating size information. In this manner, since no redundant field is included in the media unit, overheads can be reduced. 
     In the encoding device according to a third invention of the present application, the encapsulation means adds, to the media unit, different values for each size range of the predetermined access unit as the encapsulation determination information, if the plurality of access units are to be encapsulated, and adds the size information to the media unit with predetermined number of bits depending on the value of the encapsulation determination information. 
     According to the configuration described above, the encoding device makes the length of the size information to be added to the media unit variable depending on the size of each access unit. Therefore, even in a case in which the access units have various sizes (for example, from hundreds of bytes to several gigabytes), the encoding device can avoid a situation in which the field indicating size information becomes redundant and a situation in which the number of bits of the field indicating this size information becomes insufficient. 
     In the light of the above problem, a decoding device according to a fourth invention of the present application is summarized as a decoding device in which a media unit into which one or more access units are encapsulated is input from the encoding device according to the first invention of the present application and which decodes the access unit according to a decoding scheme using a CTS indicating time at which presentation or reproduction is performed and a DTS indicating time at which decoding is performed, comprising a multicapsule determination means, a media unit extraction means, a DTS reverse calculation means, a CTS reverse calculation means and a decoder. 
     According to the configuration described above, the decoding device determines by the multicapsule determination means whether a plurality of access units have been encapsulated into the media unit based on the encapsulation determination information added to the input media unit. Further, when it is determined by the multicapsule determination means that a plurality of access units have been encapsulated, the decoding device extracts, by the media unit extraction means, a plurality of access units from the media unit and, when it is determined that a plurality of access units have not been encapsulated, the decoding device extracts one access unit from the media unit. 
     Here, even in a case in which a plurality of access units are encapsulated, only one time stamp indicating decoding timing (for the differential value) and only one time stamp indicating decoding timing (for the offset value) are included in the media unit input in the decoding device. On the other hand, in order to decode the access unit of an image, the CTS and the DTS are necessary for each access unit. 
     Therefore, the decoding device reversely calculates, by the DTS reverse calculation means, as a DTS of the access unit, a value obtained by adding a differential value added to the media unit and the DTS of another access unit located immediately before the access unit extracted from the media unit. Further, the decoding device reversely calculates, by the CTS reverse calculation means, as a CTS of the access unit extracted from the media unit, a value obtained by adding an offset value added to the media unit and the DTS of the access unit reversely calculated by the DTS reverse calculation means. Then the decoding device decodes, by the decoder, the access unit of which DTS and CTS have been reversely calculated. 
     In the light of the above problem, a decoding device according to a fifth invention of the present application is summarized as a decoding device in which a media unit into which one or more access units are encapsulated is input from the encoding device according to the second invention of the present application and which decodes the access unit according to a decoding scheme using a CTS indicating time at which presentation or reproduction is performed, comprising a multicapsule determination means, a media unit extraction means, a DTS reverse calculation means, a CTS reverse calculation means and a decoder. 
     According to the configuration described above, the decoding device determines, by the multicapsule determination means, whether a plurality of access units have been encapsulated into the media unit based on the encapsulation determination information added to the input media unit. Further, when it is determined by the multicapsule determination means that a plurality of access units have been encapsulated the decoding device extracts, by the media unit extraction means, a plurality of access units from the media unit and, when it is determined that a plurality of access units have not been encapsulated, the decoding device extracts one access unit from the media unit. 
     Here, even in a case in which a plurality of access units are encapsulated, only one time stamp indicating decoding timing (for the differential value) is included in the media unit input in the decoding device. On the other hand, in order to decode the access unit of audio, the CTS is necessary for each access unit. 
     Therefore, the decoding device reversely calculates, by the CTS reverse calculation means, as the CTS of the access unit, a value obtained by adding a differential value added to the media unit and the CTS of another media unit located immediately before the access unit extracted from the media unit. Further, the decoding device decodes the access unit of which CTS has been reversely calculated by the decoder. 
     In the decoding device according to a sixth invention of the present application, the media unit extraction means extracts the plurality of access units from the media unit based on size information of each of the access units added to the media unit when it is determined by the multicapsule determination means that the plurality of access units have been encapsulated. According to the configuration described above, the decoding device can precisely specify a data field of the media unit into which each access unit is encapsulated by referring to the size information. 
     In the light of the above problem, an encoding device according to a seventh invention of the present application summarized as a encoding device which encodes an input signal according to an encoding scheme using a CTS indicating time at which presentation or reproduction is performed and a DTS indicating time at which decoding is performed, comprising an encoder, an encapsulation determination means, a DTS relative value calculation means, a CTS relative value calculation means, an encapsulation means and a transmission means. 
     According to the configuration described above, the encoding device generates the access unit correlated with the CTS and the DTS by encoding the input signal according to an encoding scheme by the encoder. Further, the encoding device determines, by the encapsulation determination means, one or more access units as an encapsulation unit on a predetermined encapsulation condition. 
     This encapsulation condition is a condition which can be set arbitrarily: for example, a predetermined number of access units are defined as an encapsulation unit, or access units encoded within predetermined time are defined as an encapsulation unit. 
     Further, the encoding device calculates, by the DTS relative value calculation means, zero as the DTS relative value of the access unit which is encoded first and calculates, as a DTS relative value of the access unit which is encoded second or thereafter, a difference between the DTS of the access unit and the DTS of another access unit encoded immediately before the access unit. 
     Further, the encoding device calculates, by the CTS relative value calculation means, as the CTS relative value of the access unit, a difference between the CTS of the access unit and the CTS of another access unit encoded immediately after the access unit. 
     Further, the encoding device encapsulates, by the encapsulation means, one or more access units determined as the encapsulation unit into the media unit and adds the DTS relative value and the CTS relative value to each access unit. Then the encoding device transmits, by the transmitting means, the media unit and a control signal which includes absolute time indicating the head of the media unit. 
     In this manner, in the encoding device, a format in which base absolute time of the DTS and the CTS is added to the control signal, and in which the DTS relative value and the CTS relative value are added to the access unit is adopted. 
     In the light of the above problem, an encoding device according to an eighth invention of the present application summarized as a encoding device which encodes an input signal according to an encoding scheme using a CTS indicating time at which presentation or reproduction is performed, comprising an encoder, an encapsulation determination means, a CTS relative value calculation means, an encapsulation means and a transmission means. 
     According to the configuration described above, the encoding device generates the access unit correlated with the CTS by encoding the input signal according to the encoding scheme by the encoder. Further, the encoding device determines, by the encapsulation determination means, one or more access units as encapsulation unit on a predetermined encapsulation condition. 
     Further, the encoding device calculates, by the CTS relative value calculation means, as the CTS relative value of the access unit, a difference between the CTS of the access unit and the CTS of another access unit encoded immediately after the access unit. Then the encoding device encapsulates, by the encapsulation means, one or more access units determined as the encapsulation unit into the media unit and adds the CTS relative value to each access unit. Further, the encoding device transmits, by the transmitting means, the media unit and a control signal which includes absolute time indicating the head of the media unit in an associated manner. 
     In this manner, in the encoding device, a format in which base absolute time of the CTS is added to the control signal, and in which the CTS relative value is added to the access unit is adopted. 
     In the encoding device according to a ninth invention of the present application, the transmitting means adds, to the media unit and a control signal corresponding to the media unit, an identifier which is common in the predetermined media units, and a sequence number specific to the media units. According to the configuration described above, the encoding device can correlate the media unit with the control signal precisely. 
     In the light of the above problem, a decoding device according to a tenth invention of the present application summarized as a decoding device in which a media unit into which one or more access units are encapsulated is input from the encoding device according to the seventh invention of the present application and which decodes the access unit according to a decoding scheme using a CTS indicating time at which presentation or reproduction is performed and a DTS indicating time at which decoding is performed, comprising a reception means, a media unit extraction means, a DTS reverse calculation means, a CTS reverse calculation means and a decoder. 
     According to the configuration described above, the decoding device receives, by the reception means, the media unit and the control signal which includes absolute time indicating the head of the media unit. Further, the decoding device extracts one or more access units from the media unit by the media unit extraction means. 
     Further, the decoding device reversely calculates, by the DTS reverse calculation means, absolute time of the control signal corresponding to the media unit as the DTS of the access unit located at the head and reversely calculates, as the DTS of the access unit located at the second place or thereafter, a value obtained by adding the DTS relative value of the access unit and the DTS of another access unit located immediately before the access unit. 
     Further, the decoding device reversely calculates, by the CTS reverse calculation means, as the CTS of another access unit located immediately after the access unit, a value obtained by adding absolute time of the control signal corresponding to the media unit and a sum of the CTS relative values from the access unit located at the head to the access unit. Then the decoding device decodes the access unit of which DTS and CTS have been reversely calculated by the decoder according to the decoding scheme. 
     In this manner, in the decoding device, a format in which base absolute time of the DTS and the CTS is added to the control signal, and in which the DTS relative value and the CTS relative value are added to the access unit is adopted. 
     In the light of the above problem, a decoding device according to an eleventh invention of the present application summarized as a decoding device in which a media unit into which one or more access units are encapsulated is input from the encoding device according to the seventh invention of the present application and which decodes the access unit according to a decoding scheme using a CTS indicating time at which presentation or reproduction is performed, comprising a reception means, a media unit extraction means, a CTS reverse calculation means and a decoder. 
     According to the configuration described above, the decoding device receives, by the reception means, the media unit and the control signal which includes absolute time indicating the head of the media unit. Further, the decoding device extracts one or more access units from the media unit by the media unit extraction means. 
     Further, the decoding device reversely calculates, by the CTS reverse calculation means, as the CTS of another access unit located immediately after the access unit, a value obtained by adding absolute time of the control signal corresponding to the media unit and a sum of the CTS relative values from the access unit located at the head to the access unit. Then the decoding device decodes the access unit of which CTS has been reversely calculated by the decoder according to the decoding scheme. 
     In this manner, in the decoding device, a format in which base absolute time of the CTS is added to the control signal, and in which the CTS relative value is added to the access unit is adopted. 
     In the decoding device according to the twelfth invention of the present application, in the reception means, an identifier which is common in the predetermined media units and a sequence number specific to the media unit have been added to the received media unit and to a control signal corresponding to the media unit. According to the configuration described above, the decoding device can correlate the media unit with the control signal precisely. 
     The encoding device according to the first, second, seventh and eighth inventions of the present application may also be implemented as an encoding program which causes a hardware resource provided in a computer, such as a central processing unit (CPU), a memory and a HDD (Hard Disk Drive) to cooperatively operate as each means described above. This encoding program may be distributed via a communication line or may be distributed by being written in a recording medium, such as a CD-ROM and a flash memory. 
     The decoding device according to the fourth, fifth, tenth and eleventh inventions of the present application may also be implemented as a decoding program which causes a hardware resource provided in a computer, such as a CPU, a memory and a HDD, to cooperatively operate as each means described above. This decoding program may be distributed via a communication line or may be distributed by being written in a recording medium, such as a CD-ROM and a flash memory. 
     According to the present invention, the following excellent effects are exhibited. According to the first and fourth inventions of the present application, even in a case in which a plurality of access units are encapsulated, only one field indicating decoding timing (for the differential value) and only one field indicating decoding timing (for the offset value) are included in the media unit. Further, according to the first and fourth inventions of the present application, in a case in which one access unit is encapsulated, a field indicating size information of the access unit is not included in the media unit. Therefore, according to the first and fourth inventions of the present application, since no redundant field is included in the media unit, overheads in this media unit can be reduced and encapsulation in an optimum format becomes possible. 
     According to the second and fifth invention of the present application, even in a case in which a plurality of access units are encapsulated, only one field indicating decoding timing (for a differential value) is included in a media unit. According to the second and fifth invention of the present application, in a case in which one access unit is encapsulated, a field indicating size information of the access unit is not included in the media unit. Therefore, according to the second and fifth invention of the present application, since no redundant field is included in the media unit, overheads in this media unit can be reduced and encapsulation in an optimum format becomes possible. 
     According to the third invention of the present application, a situation in which the field indicating size information becomes redundant and a situation in which the number of bits of the field indicating this size information becomes insufficient can be avoided. According to the sixth invention of the present application, since the data field of the media unit into which a plurality of access units are encapsulated can be specified precisely, data missing during extraction of the access unit can be prevented. 
     According to the seventh and tenth inventions of the present application, a format in which base absolute time of the DTS and the CTS is added to the control signal, and in which the DTS relative value and the CTS relative value are added to the access unit is adopted. Therefore, according to the seventh and tenth inventions of the present application, encapsulation in an optimum format becomes possible with which a change in time at which the access unit is decoded, a change in time at which the access unit is presented or reproduced, commonization of the media unit, and switching of the transmission paths are implemented easily. 
     According to the eighth and eleventh inventions of the present application, a format in which base absolute time of the CTS is added to the control signal, and in which the CTS relative value is added to the access unit is adopted. Therefore, to the eighth and eleventh inventions of the present application, access unit enables encapsulation in an optimum format becomes possible with which a change in time at which the access unit is presented or reproduced, commonization of the media unit, and switching of the transmission paths are implemented easily. 
     According to the ninth and twelfth inventions of the present application, since the media unit and the control signal can be correlated precisely, accurate DTS and CTS can be reversely calculated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating configurations of an encoding device and a decoding device according to a first embodiment of the present invention. 
         FIG. 2( a )  is a diagram illustrating a format of a media unit in which one access unit is encapsulated in the first embodiment of the present invention and FIG.  2 ( b ) is a diagram illustrating a format of a media unit in which a plurality of access units are encapsulated in the first embodiment of the present invention. 
         FIG. 3  is a diagram illustrating differential values and offset values in the first embodiment of the present invention. 
         FIG. 4  is a flowchart illustrating an operation of the encoding device of  FIG. 1 . 
         FIG. 5  is a flowchart illustrating an operation of the decoding device of  FIG. 1 . 
         FIG. 6  is a block diagram illustrating configurations of an encoding device and a decoding device according to a second embodiment of the present invention. 
         FIG. 7( a )  is a diagram illustrating a format of a media unit in which one access unit is encapsulated in the second embodiment of the present invention and  FIG. 7( b )  is a diagram illustrating a format of a media unit in which a plurality of access units are encapsulated in the second embodiment of the present invention. 
         FIG. 8  is a diagram illustrating differential values in the second embodiment of the present invention. 
         FIG. 9  is an explanatory view illustrating commonization of media units and switching of transmission paths in a third embodiment of the present invention. 
         FIG. 10  is a block diagram illustrating configurations of an encoding device and a decoding device according to the third embodiment of the present invention. 
         FIG. 11  is a diagram illustrating a format of a media unit in which a plurality of access units are encapsulated and a control signal in the third embodiment of the present invention. 
         FIG. 12  is an explanatory view illustrating a relationship between absolute time and a CTS relative value in the third embodiment of the present invention. 
         FIG. 13  is a flowchart illustrating an operation of the encoding device of  FIG. 10 . 
         FIG. 14  is a flowchart illustrating an operation of the decoding device of  FIG. 10 . 
         FIG. 15  is a block diagram illustrating configurations of an encoding device and a decoding device according to a fourth embodiment of the present invention. 
         FIG. 16  is a flowchart illustrating an operation of the encoding device of  FIG. 15 . 
         FIG. 17  is a flowchart illustrating an operation of the decoding device of  FIG. 15 . 
         FIG. 18  is a diagram illustrating a format of a media unit in which a plurality of access units are encapsulated and a control signal in the fourth embodiment of the present invention. 
         FIG. 19( a )  is a diagram illustrating a related art PES and  FIG. 19( b )  is a diagram illustrating a related art RTP payload. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     [Outline of Content Providing System] 
     Hereinafter, each embodiment of the present invention will be described in detail with reference to the drawings as necessary. In each embodiment, means having the same functions are denoted by the same reference numeral and description thereof is omitted. 
     As illustrated in  FIG. 1 , a content providing system  1000  according to an embodiment of the present invention provides content (i.e., an image) and includes an encoding device  1  and a decoding device  2 . In this content providing system  1000 , the encoding device  1  and the decoding device  2  are connected to each other via a network N. This network N is an IP packet transmission path, such as the Internet and an intranet. In the present embodiment, it is supposed that the content providing system  1000  includes one encoding device  1  and one decoding device  2 . 
     [Configuration of Encoding Device] 
     Hereinafter, a configuration of the encoding device  1  will be described. The encoding device  1  encodes an image signal as an input signal and provides the encoded image signal to the decoding device  2 . Therefore, the encoding device  1  includes an encoder  11 , a time stamp calculation means  12 , an encapsulation determination means  15  and an encapsulation means  16 . 
     The encoder  11  generates an access unit with (to) which a CTS and a DTS are correlated (added) by encoding an image signal input from outside according to an encoding scheme in which the CTS and the DTS are used. 
     Here, the encoder  11  encodes the input image signal using an image encoding scheme, such as MPEG (Moving Picture Experts Group)-2 Video or MPEG-4 AVC (Advanced Video Coding), and generates an access unit of an image. Here, in a case in which a decoding order and a presentation order of the access unit differ as in the case of the image signal, the CTS and the DTS are correlated with each access unit. The encoder  11  sequentially outputs, to the time stamp calculation means  12 , the access unit correlated with the CTS and the DTS. 
     The CTS (Composition Time Stamp) is information indicating time at which presentation or reproduction is performed, and the CTS and the PTS are sometimes used with the same meaning. The DTS is information indicating time at which decoding is performed. The access unit is a collection of codes (i.e., input signals) having identical DTS. The input signals are signals to be encoded and decoded, such as image signals and audio signals. 
     The time stamp calculation means  12  calculates a time stamp to be added to a media unit (i.e., a differential value of the DTS and an offset value) and includes a differential value calculation means  13  and an offset value calculation means  14 . 
     The differential value calculation means  13  calculates a differential value between the DTS of the access unit input from the encoder  11  and a DTS of another access unit encoded immediately before this access unit. 
     Here, in a case in which the access unit is the head access unit, since no preceding access unit exists immediately before the head access unit, the differential value calculation means  13  calculates the differential value as ‘0.’ At this time, since the differential value calculation means  13  sequentially performs processing to the access units which are continuous in the time direction, the differential value calculation means  13  temporarily stores in an unillustrated memory, as a DTS of another access unit, the DTS of the access unit for which differential value has been calculated. Further, in a case in which the access unit is the second access unit or thereafter, the differential value calculation means  13  calculates a differential value between the DTS of the input access unit and the DTS of another access unit which is stored temporarily. 
     The offset value calculation means  14  calculates an offset value which is a difference between the DTS and the CTS of the access unit input from the encoder  11 . That is, the offset value calculation means  14  calculates, as an offset value, a difference between the DTS and the CTS which are correlated with an identical access unit. 
     Then, the time stamp calculation means  12  sequentially outputs, to the encapsulation determination means  15 , the access unit input from the encoder  11 , the differential value calculated by the differential value calculation means  13  and the offset value calculated by the offset value calculation means  14 . 
     The encapsulation determination means  15  determines, as an encapsulation unit, one or more access units which satisfy a later-described determination condition within predetermined determination time among the access units input from the time stamp calculation means  12 . This determination condition is a condition in which the differential values become identical and, at the same time, the offset values become identical. Further, the encapsulation unit indicates an access unit that can be encapsulated in the same media unit. 
     That is, the encapsulation determination means  15  determines whether sequentially input access units satisfy a determination condition while counting a timer. The encapsulation determination means  15  then determines, as an encapsulation unit, one or more access units which satisfy the determination condition when determination time elapsed on the timer. 
     For example, in a case in which two access units input within the determination time satisfy the determination condition, the encapsulation determination means  15  determines the two access units as the identical encapsulation unit. On the other hand, in a case in which two access units input within the determination time do not satisfy the determination condition, the encapsulation determination means  15  determines the two access units as separate encapsulation units. Then, the encapsulation determination means  15  outputs, for each encapsulation unit, a differential value, an offset value and an access unit included in the encapsulation unit. 
     The encapsulation means  16  encapsulates (i.e., stores), in a media unit, one or more access units determined as an encapsulation unit by the encapsulation determination means  15  and describes (i.e., adds) the identical differential value and the identical offset value in (to) the media unit. 
     &lt;Format of Medium Unit&gt; 
     With reference to  FIG. 2( a ) , a media unit  100  in which one access unit  200  is encapsulated will be described. The encapsulation means  16  describes an input differential value in a DTS field  101 . Further, the encapsulation means  16  describes an input offset value in a CTS field  102 . Then the encapsulation means  16  describes in an encapsulation determination information field (Length_flag)  103 , as encapsulation determination information, a value indicating that size information is not included (for example, ‘0’). This encapsulation determination information field  103  is secured, for example, as 2 bit width. The encapsulation means  16  encapsulates one access unit  200  into the media unit  100  (AU of  FIG. 2( a ) ). 
     Although the media unit  100  includes fields for a time stamp flag, an extension header flag, a random access point flag and the like, these fields are not directly related to the present invention and, therefore, description and illustration thereof will be omitted. Further, details of the media unit  100  are described, for example, in a reference “Media Transport System in Hybrid Broadcasting” Information Processing Society of Japan Research Report, Vol. 2011-AVN-72 No. 1 2011/3/11. 
     Next, as illustrated in  FIG. 2( b ) , the media unit  100  in which two access units  200  are encapsulated will be described. In this case, the encapsulation means  16  describes an input differential value in the DTS field  101 . Further, the encapsulation means  16  describes an input offset value in the CTS field  102 . 
     Further, the encapsulation means  16  describes in the encapsulation determination information field  103 , as encapsulation determination information, a value indicating that size information is included (for example, other than ‘0’). At this time, the encapsulation means  16  may describe, in the encapsulation determination information field  103 , different values for each media unit  100  so that the maximum size can be expressed among one or more access units  200  included in the same media unit  100 . 
     For example, in a case in which the size of the access unit  200  exceeds 0 byte and is equal to or smaller than 64 KB (i.e., an expression range of 16 bits), the encapsulation means  16  describes ‘1’ in the encapsulation determination information field  103 . At this time, the encapsulation means  16  secures a 16 bit width for a size information field (AU1_length, AU2_length)  104 . Further, for example, the size of the access unit  200  exceeds 64 KB and is equal to or smaller than 16 MB (i.e., an expression range of 24 bits), the encapsulation means  16  describes ‘2’ in the encapsulation determination information field  103 . Further, the encapsulation means  16  secures a 24 bit width for the size information field  104 . Further, for example, the size of the access unit  200  exceeds 16 MB and is equal to or smaller than 4 GB (i.e., an expression range of 32 bits), the encapsulation means  16  describes ‘3’ in the encapsulation determination information field  103 . Further, the encapsulation means  16  secures a 32 bit width for the size information field  104 . 
     Then the encapsulation means  16  describes size information of each access unit  200  in the size information field  104  secured by the predetermined bit width. Further, the encapsulation means  16  encapsulates a plurality of access units  200  into the media unit  100  (AU1 and AU2 of  FIG. 2( b ) ). 
     &lt;Difference Values and Offset Values in Media Unit&gt; 
     Next, with reference to  FIG. 3 , the differential values and the offset values described in the media unit  100  will be described (see  FIG. 1  as necessary). 
     In this  FIG. 3 , it is supposed that six access units  200   1  to  200   6  have been generated continuously (AU1 to AU6 of  FIG. 3 ). Further, it is supposed that the access unit  200   1  is encapsulated in a first media unit  100   1  and the access units  200   2  and  200   3  are encapsulated in a second media unit  100   2 . Further, it is supposed that the access units  200   4  and  200   5  are encapsulated in a third media unit  100   3  and the access unit  200   6  is encapsulated in a fourth media unit  100   4 . 
     In the media unit  100   1 , since no other preceding access unit exists immediately before the access unit  200   1 , a differential value ‘0’ is described in a DTS field  101   1 . Further, in the media unit  100   1 , an offset value indicating a difference between a DTS and a CTS of the access unit  200   1  is described in a CTS field  102   1 . 
     In the media unit  100   2 , a differential value of a DTS of the access unit  200   2  and the DTS of the access unit  200   1  is described in a DTS field  101   2 . Further, in the media unit  100   2 , an offset value between the DTS and a CTS of the access unit  200   2  is described in a CTS field  102   2 . 
     Here, the differential value between a DTS of the access unit  200   3  and the DTS of the access unit  200   2  is identical to the differential value between the DTS of the access unit  200   2  and the DTS of the access unit  200   1 . Further, the offset value between the DTS and a CTS of the access unit  200   3  is identical to the offset value of the access unit  200   2 . Therefore, in the media unit  100   2 , the DTS field  101   2  and the CTS field  102   2  can be shared by the access units  200   2  and  200   3 . In other words, since the DTS field  101  and the CTS field  102  are shared in the media unit  100 , the media unit  100  is not able to encapsulate access units  200  which have different differential values and offset values. 
     In the media unit  100   3 , a differential value between a DTS of the access unit  200   4  and the DTS of the access unit  200   3  is described in a DTS field  101   3  (α of  FIG. 3 ). In the media unit  100   4 , a differential value between a DTS of the access unit  200   6  and a DTS of the access unit  200   5  is described in a DTS field  101   4  (β of  FIG. 3 ). In addition, since the media units  100   3  and  100   4  are similar to the media units  100   1  and  100   2 , description thereof will be omitted. Further, in  FIG. 3 , since a part of the fields of the media unit  100  is not directly related to the present invention, illustration thereof is omitted. 
     Then, the encoding device  1  transmits, by a transmitting means (not illustrated), the media unit  100  generated by the encapsulation means  16  to the decoding device  2  via the network N. For example, the transmitting means converts the media unit  100  into IP packets, performs transmission path encoding processing and modulation processing in accordance with the network N, and transmits the generated IP packets. 
     [Configuration of Decoding Device] 
     Returning to  FIG. 1 , a configuration of the decoding device  2  will be described (see  FIG. 2  and  FIG. 3  as necessary). The decoding device  2  extracts one or a plurality of access units  200  from the media unit  100  transmitted by the encoding device  1  and decodes the extracted access units  200 . Therefore, the decoding device  2  includes a multicapsule determination means  21 , a reverse encapsulation means (i.e., a media unit extraction means)  22 , a time stamp reverse calculation means  23  and a decoder  26 . 
     Here, the decoding device  2  receives, by a reception means (not illustrated), a media unit  100  from the encoding device  1  via the network N. For example, the reception means performs demodulation processing and transmission path decoding processing in accordance with the network N and receives the IP packets. Then the reception means extracts a media unit  100  from the received IP packets and sequentially outputs the extracted media unit  100  to the multicapsule determination means  21 . 
     The multicapsule determination means  21  determines whether a plurality of access units  200  have been encapsulated into the media unit  100  based on the input encapsulation determination information field  103  of the media unit  100 . 
     Here, in a case in which the value of the encapsulation determination information field  103  is other than ‘0’ (for example, ‘1’ to ‘3’), the multicapsule determination means  21  determines that a plurality of access units  200  have been encapsulated into the media unit  100 . On the other hand, in a case in which the value of the encapsulation determination information field  103  is ‘0,’ the multicapsule determination means  21  determines that a plurality of access units  200  have not been encapsulated into the media unit  100 . Then the multicapsule determination means  21  sequentially outputs, to a reverse encapsulation means  22 , the determination result indicating whether a plurality of access units  200  have been encapsulated and the media unit  100 . 
     When the determination result of the multicapsule determination means  21  shows that a plurality of access units  200  have not been encapsulated, the reverse encapsulation means  22  extracts (i.e., reverse encapsulates) one access unit  200  from the input media unit  100 . In this case, since one access unit  200  can be extracted when the entire media unit  100  is read, it is not necessary for the reverse encapsulation means  22  to refer to the size information field  104 . The entire size of the media unit  100  can be specified by the reception means described above from a length information field included in a UDP header of the IP packet, for example. 
     Further, when the determination result of the multicapsule determination means  21  shows that a plurality of access units  200  have been encapsulated, the reverse encapsulation means  22  extracts a plurality of access units  200  from the input media unit  100 . In this case, it is necessary for the reverse encapsulation means  22  to specify, with reference to the size information field  104 , a data field of the media unit  100  into which each access unit  200  is encapsulated. 
     Then the reverse encapsulation means  22  extracts, from the media unit  100 , the differential value described in the DTS field  101  and the offset value described in the CTS field  102 . Then the reverse encapsulation means  22  sequentially outputs, to the time stamp reverse calculation means  23 , the access unit  200 , the differential value and the offset value extracted from the media unit  100 . 
     The time stamp reverse calculation means  23  reversely calculates the DTS and the CTS of the access unit  200  from a time stamp (i.e., a differential value and an offset value) and includes a DTS reverse calculation means  24  and a CTS reverse calculation means  25 . The DTS reverse calculation means  24  performs reverse operation of the differential value calculation means  13 . The CTS reverse calculation means  25  performs reverse operation of the offset value calculation means  14 . 
     &lt;Reverse Calculation of DTS and CTS&gt; 
     Returning to  FIG. 3 , the reverse calculation of the DTS by the DTS reverse calculation means  24  and the reverse calculation of the CTS by the CTS reverse calculation means  25  will be described (see  FIG. 1  as necessary). 
     In the first media unit  100   1 , since no other preceding access unit exists immediately before the access unit  200   1  and, therefore, ‘0’ is described in the DTS field  101   1  as the differential value. Therefore, the DTS reverse calculation means  24  performs the reverse calculation with an absolute value of the DTS of the access unit  200   1  being ‘0’. At this time, since the DTS reverse calculation means  24  performs processing to the access units  200  which are continuous in the time direction, the DTS reverse calculation means  24  temporarily stores the value of reversely calculated DTS in an unillustrated memory as a DTS of the latest access unit (i.e., another access unit)  200   1 . The CTS reverse calculation means  25  adds the offset value described in the CTS field  102   1  as it is to the value of the reversely calculated DTS in the access unit  200   1  and then reversely calculates as a CTS of the access unit  200   1 . 
     In the second media unit  100   2 , the differential value between the DTS of the access unit  200   2  and the DTS of the access unit  200   1  is described in the DTS field  101   2 . Therefore, the DTS reverse calculation means  24  reversely calculates, as the DTS of the access unit  200   2 , a value obtained by adding the DTS of the access unit  200   1  temporarily stored in the memory and the differential value described in the DTS field  101   2 . Then the DTS reverse calculation means  24  temporarily stores, in the memory described above, the value of the reversely calculated DTS of the access unit  200   2  and updates memory content. That is, in this memory, each time the DTS of the latest access unit  200  included in the media unit  100  is reversely calculated, the temporarily stored DTS is updated. Further, the CTS reverse calculation means  25  reversely calculates, as the CTS of the access unit  200   2 , a value obtained by adding the offset value described in the CTS field  102   2  and the DTS of the access unit  200   2  reversely calculated by the DTS reverse calculation means  24 . 
     Here, the second media unit  100   2  further includes the access unit  200   3 . Therefore, the DTS reverse calculation means  24  and the CTS reverse calculation means  25  reversely calculate the DTS and the CTS of the access unit  200   3 , respectively. That is, the DTS reverse calculation means  24  adds the differential value described in the DTS field  101   2  to the value of the DTS temporarily stored in the memory (i.e., the value of the DTS of the access unit  200   2 ) and reversely calculates the DTS of the access unit  200   3 . Further, the CTS reverse calculation means  25  adds the value of the DTS reversely calculated by the DTS reverse calculation means  24  to the offset value described in the CTS field  102   2  and reversely calculates the CTS of the access unit  200   3 . 
     Since processing in the media units  100   3  and  100   4  is similar to that of the media units  100   1  and  100   2 , description thereof will be omitted. Then the time stamp reverse calculation means  23  correlates the DTS reversely calculated by the DTS reverse calculation means  24  and the CTS reversely calculated by the CTS reverse calculation means  25  with the access unit  200  and outputs the correlated result to the decoder  26 . 
     Returning to  FIG. 1 , description about the configuration of the decoding device  2  will be continued. The decoder  26  decodes the access unit  200  input from the time stamp reverse calculation means  23  according to an image decoding scheme corresponds to the encoder  11  (for example, MPEG-2 Video or MPEG-4 AVC). Since both the CTS and the DTS are correlated with the access unit  200 , this access unit  200  can be decoded according to the image decoding scheme described above. 
     [Operation of Encoding Device] 
     An operation of the encoding device  1  will be described with reference to  FIG. 4  (see  FIG. 1  to  FIG. 3  as necessary). The encoding device  1  encodes, by the encoder  11 , an image signal input from outside to generate an access unit  200  correlated with a CTS and a DTS (step S 11 ). 
     The encoding device  1  calculates, by the differential value calculation means  13 , a differential value of the DTS for each access unit  200 . Further, the encoding device  1  calculates, by the offset value calculation means  14 , an offset value between the DTS and the CTS for each access unit  200  (step S 12 ). 
     The encoding device  1  determines, by the encapsulation determination means  15 , whether a plurality of access units  200  satisfy a determination condition within determination time (step S 13 ). Here, in a case in which a plurality of access units  200  satisfy a determination condition (step S 13 : Yes), the encoding device  1  determines these plurality of access units  200  as an encapsulation unit and proceeds to a process of step S 14 . 
     The encoding device  1  describes, by the encapsulation means  16 , a differential value and an offset value in the media unit  100 . Further, the encoding device  1  encapsulates, by the encapsulation means  16 , a plurality of access units  200  included in the encapsulation unit into the media unit  100 . Further, the encoding device  1  describes, by the encapsulation means  16 , encapsulation determination information (for example, any of ‘1’ to ‘3’) and size information in the media unit  100  (step S 14 ). 
     On the other hand, in a case in which one access unit  200  satisfies the determination condition (step S 13 : No), the encoding device  1  determines this one access unit  200  as an encapsulation unit and proceeds to a process of step S 15 . 
     The encoding device  1  describes, by the encapsulation means  16 , a differential value and an offset value in the media unit  100 . Further, the encoding device  1  encapsulates, by the encapsulation means  16 , one access unit  200  included in the encapsulation unit into the media unit  100 . The encoding device  1  describes, by the encapsulation means  16 , encapsulation determination information (for example, ‘0’) in the media unit  100  (step S 15 ). 
     [Operation of Decoding Device] 
     An operation of the decoding device  2  will be described with reference to  FIG. 5  (see  FIG. 1  to  FIG. 3  as necessary). The decoding device  2  determines, by the multicapsule determination means  21 , whether a plurality of access units  200  are encapsulated into the media unit  100  based on encapsulation determination information described in the media unit  100  input from the encoding device  1  (step S 21 ). 
     Here, in a case in which a plurality of access units  200  are encapsulated (step S 21 : Yes), the decoding device  2  proceeds to a process of step S 22 . The decoding device  2  extracts, by the reverse encapsulation means  22 , a plurality of access units  200  from the input media unit  100  based on size information (step S 22 ). 
     On the other hand, in a case in which a plurality of access units  200  are not encapsulated (step S 21 : No), the decoding device  2  proceeds to a process of step S 23 . The decoding device  2  extracts, by the reverse encapsulation means  22 , one access unit  200  from the input media unit  100  (step S 23 ). 
     The decoding device  2  reversely calculates, by the DTS reverse calculation means  24 , a DTS of the access unit  200 . Further, the decoding device  2  reversely calculates, by the CTS reverse calculation means  25 , a CTS of the access unit  200  (step S 24 ). The decoding device  2  decodes, by the decoder  26 , the access unit  200  of which DTS and CTS have been reversely calculated (step S 25 ). 
     As described above, in the encoding device  1  and the decoding device  2  according to the first embodiment of the present invention, even in a case in which a plurality of access units  200  are encapsulated, only one field indicating decoding timing (for the differential value of the DTS) and only one field indicating decoding timing (for the offset value) are included in the media unit  100  ( FIG. 2( b ) ). Further, in a case in which one access unit  200  is encapsulated, the encoding device  1  and the decoding device  2  include no size information field in the media unit ( FIG. 2( a ) ). In this manner, since no redundant field is included in the media unit  100 , the encoding device  1  and the decoding device  2  can reduce overheads in this media unit  100  and encapsulation can be performed in an optimum format. Especially the encoding device  1  and the decoding device  2  can substantially reduce overheads in the media unit  100  compared with a case in which the fields of the DTS and the CTS are simply added to each access unit  200 . 
     Further, the encoding device  1  secures the size information field  104  with the number of bits in accordance with the size of the access unit  200 . Therefore, the encoding device  1  can avoid a situation in which this size information field  104  becomes redundant and a situation in which the number of bits of the size information field  104  becomes insufficient. 
     Further, even in a case in which a plurality of access units  200  are encapsulated, the decoding device  2  can correctly specify a data field of each access unit  200  encapsulated into the media unit  100  by referring to the size information field  104 . Therefore, the decoding device  2  can prevent data missing during extraction of the access unit  200 . 
     Second Embodiment 
     With reference to  FIG. 6 , a content providing system  1000 A according to a second embodiment of the present invention will be described with respect to a difference from the first embodiment. The content providing system  1000 A provides content (i.e., audio) and includes an encoding device  1 A and a decoding device  2 A. 
     [Configuration of Encoding Device] 
     The encoding device  1 A encodes an audio signal as an input signal and provides the encoded audio signal to the decoding device  2 A. 
     Therefore, the encoding device  1 A includes an encoder  11 A, a time stamp calculation means  12 A, an encapsulation determination means  15 A and an encapsulation means  16 A. 
     The encoder  11 A generates an access unit correlated with a CTS by encoding an audio signal input from outside according to an encoding scheme in which the CTS is used. 
     Here, the encoder  11 A encodes the input audio signal using an audio encoding scheme, such as MPEG-2 AAC (Advanced Audio Coding), and generates an audio access unit. Here, in a case in which a decoding order and a presentation order of the access unit are identical to each other as in the case of the audio signal, only the CTS is correlated with each access unit. Then the encoder  11 A outputs, to the time stamp calculation means  12 A, the access unit correlated with the CTS. 
     The time stamp calculation means  12 A calculates a time stamp to be added to a media unit (i.e., a differential value of the CTS) and includes a differential value calculation means  13 A. 
     The differential value calculation means  13 A calculates, for each access unit, a differential value between the CTS of the access unit and the CTS of another access unit encapsulated in the immediately preceding media unit. Since the differential value calculation means  13 A is similar to the differential value calculation means  13  of  FIG. 1  except that the CTS is used instead of the DTS, detailed description of the differential value calculation means  13 A will be omitted. 
     The encapsulation determination means  15 A determines, as an encapsulation unit, one or more access units which satisfy a later-described determination condition within determination time among the access units input from the time stamp calculation means  12 A. Since the encapsulation determination means  15 A is similar to the encapsulation determination means  15  of  FIG. 1  except that a determination condition in which differential values are the same is used, detailed description of the encapsulation determination means  15 A will be omitted. 
     The encapsulation means  16 A encapsulates (i.e., stores), in a media unit, one or more access units included in the encapsulation unit determined by the encapsulation determination means  15 A and adds the same differential values to the media unit. 
     In the present embodiment, since the audio signal is encoded, the DTS is not correlated with the access unit  200  but only the CTS is correlated with the access unit  200 . Therefore, as illustrated in  FIGS. 7( a ) and 7( b ) , no DTS field exists in the media unit  100 A output by the encapsulation means  16 A but only a CTS field  102  exists in that media unit  100 A. Then, as illustrated in  FIG. 8 , a differential value of the CTS calculated by the differential value calculation means  13 A is described in the CTS field  102  of this media unit  100 A. In addition, since the encapsulation means  16 A is similar to the encapsulation means  16  of  FIG. 1 , detailed description of the encapsulation means  16 A will be omitted. 
     [Configuration of Decoding Device] 
     Hereinafter, a configuration of the decoding device  2 A will be described. The decoding device  2 A extracts one or a plurality of access units  200  from the media unit  100 A transmitted by the encoding device  1 A and decodes the extracted access units  200 . Therefore, the decoding device  2 A includes a multicapsule determination means  21 , a reverse encapsulation means (i.e., a media unit extraction means)  22 , a time stamp reverse calculation means  23 A and a decoder  26 A. 
     The time stamp reverse calculation means  23 A reversely calculates a CTS corresponding to the access unit  200  from a time stamp (i.e., a differential value of the CTS) and includes a CTS reverse calculation means  25 A. The CTS reverse calculation means  25 A reversely calculates, as a CTS of this access unit  200 , a value obtained by adding a differential value added to the media unit  100 A and a CTS of another access unit located immediately before the access unit  200 . Since the CTS reverse calculation means  25 A is similar to the DTS reverse calculation means  24  of  FIG. 1  except that the CTS is used instead of the DTS, detailed description of the CTS reverse calculation means  25 A will be omitted. 
     The decoder  26 A decodes the access unit  200  input from the time stamp reverse calculation means  23 A according to an audio decoding scheme corresponding to the encoder  11 A (for example, MPEG-2 AAC). Since the CTS is correlated with the access unit  200 , this access unit  200  can be decoded according to the audio decoding scheme described above. Since the DTS is not correlated with the decoder  26 A, the decoder  26 A performs decoding considering that the DTS and the CTS are identical to each other. 
     As described above, in the encoding device  1 A and the decoding device  2 A according to the second embodiment of the present invention, the same effect as that of the first embodiment can be obtained for the audio content. 
     Third Embodiment 
     Commonization of Media Unit, Switching of Transmission Paths 
     With reference to  FIG. 9 , commonization of a media unit  100 B and switching of transmission paths will be described. As illustrated in  FIG. 9 , a content providing system  1000 B provides content (i.e., images) and includes a ground distribution station (i.e., an encoding device)  1 B, two receivers (i.e., decoding devices)  2 B 1  and  2 B 2 , and a broadcasting satellite  92 . 
     The ground distribution station  1 B transmits a media unit  100 B in which the content is stored to the receivers  2 B 1  and  2 B 2  via a network N. The receivers  2 B 1  and  2 B 2  receive the media unit  100 B from the ground distribution station  1 B via the network N and reproduce the content. Further, the receivers  2 B 1  and  2 B 2  receive the media unit  100 B from the broadcasting satellite  92  via a broadcast wave W and reproduce the content. 
     The broadcasting satellite  92  receives (i.e., uplinks) the media unit  100 B from an unillustrated uplink station. Then the broadcasting satellite  92  transmits (i.e., downlinks) the received media unit  100  to the receivers  2 B 1  and  2 B 2  via the broadcast wave W. 
     In this manner, a transmission path by the network N is established between the ground distribution station  1 B and the receiver  2 B 1  and between the ground distribution station  1 B and the receiver  2 B 2 . Further, a transmission path by the broadcast wave W is established between the broadcasting satellite  92  and the receiver  2 B 1  and between the broadcasting satellite  92  and the receiver  2 B 2 . 
     First, commonization of the media unit  100 B will be described. A case in which the same content is provided from the ground distribution station  1 B to both the receivers  2 B 1  and  2 B 2  in a conventional system will be considered. In the conventional system, a DTS and a CTS are included in each media unit in the form of absolute time. Therefore, in the conventional system, there has been a problem that it is necessary to prepare media units corresponding to each of the receivers  2 B 1  and  2 B 2  and thus processing load becomes high. 
     Next, switching of the transmission paths will be described. For example, a case will be considered in which, when the broadcasting satellite  92  is transmitting the media unit to the receiver  2 B 1 , the transmission path is switched from the broadcast wave W to the network N due to rainfall attenuation and the media unit  100 B is transmitted from the ground distribution station  1 B. In the conventional system, the DTS and the CTS are included in the media unit in the form of the absolute time. Therefore, in the conventional system, there has been a problem that it is necessary to prepare media units for each transmission path, such as the broadcast wave W and the network N, and thus the processing load becomes high. 
     Then, an object of the invention of this application is to solve the problems described above and to provide a ground distribution station (i.e., an encoding device), a receiver (i.e., a decoding device) and programs therefor that are capable of encapsulating in an optimum format. 
     In view of the problem described above, the content providing system  1000 B adopts a format to add the base absolute time of the DTS and the CTS to a control signal (not illustrated) and to add a DTS relative value and a CTS relative value to an access unit (not illustrated). 
     Thus, in the content providing system  1000 B, if control signals corresponding to each of the receivers  2 B 1  and  2 B 2  are prepared, the same media unit  100 B can be used in common by the receivers  2 B 1  and  2 B 2 . That is, the media unit  100 B can be used as the minimum usage unit of the image medium and the audio medium. 
     Further, in the content providing system  1000 B, since the format described above is adopted, if the control signals are prepared for each transmission path, the same media unit  100 B can be transmitted to the receiver  2 B 1  while switching the transmission paths. 
     [Configuration of Encoding Device] 
     A configuration of an encoding device  1 B will be described with reference to  FIG. 10 . The encoding device  1 B encodes an image signal as an input signal and provides the encoded image signal to a decoding device  2 B. Therefore, the encoding device  1 B includes an encoder  11 , a time stamp calculation means  12 B, an encapsulation determination means  15 B, and an encapsulation means  16 B and a transmitting means  19 . Although a network N is illustrated as the transmission path in this  FIG. 10 , the transmission path may instead be a broadcast wave W. 
     The encoder  11  generates an access unit with (to) which a CTS and a DTS are correlated (added) by encoding an image signal input from outside according to an encoding scheme in which the CTS and the DTS are used. Then the encoder  11  sequentially outputs, to the encapsulation determination means  15 B, the access unit correlated with the CTS and the DTS. 
     The encapsulation determination means  15 B determines, as encapsulation unit, one or more access units input from the encoder  11  on a predetermined encapsulation condition. Then the encapsulation determination means  15 B outputs the access units to the time stamp calculation means  12 B in an order in which the access units have been encoded for each encapsulation unit. 
     This encapsulation condition is a condition which can be set arbitrarily: for example, a predetermined number of access units are defined as an encapsulation unit, or access units encoded within predetermined time are defined as an encapsulation unit. For example, if a delay is to be reduced, the encapsulation condition is set in advance with one access unit being defined as an encapsulation unit. Further, the encapsulation condition may be set in advance with, in the case of image signals, the number of access units corresponding to GOP (Group of Picture) (for example, 15) being defined as an encapsulation unit. 
     The time stamp calculation means  12 B calculates time stamps (i.e., the DTS relative value and the CTS relative value) for each access unit and includes a DTS relative value calculation means  17  and a CTS relative value calculation means  18 . 
     The DTS relative value calculation means  17  calculates ‘0’ as a DTS relative value of the access unit which is encoded first. Further, the DTS relative value calculation means  17  calculates, as a DTS relative value of the access unit which is encoded second or thereafter, a difference between the DTS of this access unit and the DTS of another access unit encoded immediately before this access unit. 
     The CTS relative value calculation means  18  calculates, as the CTS relative value of the access unit, a difference between the CTS of this access unit and the CTS of another access unit encoded immediately after this access unit. Details of the DTS relative value calculation means  17  and the CTS relative value calculation means  18  will be described later. 
     Then, the time stamp calculation means  12 B outputs, to the encapsulation means  16 B, the access units input from the encapsulation determination means  15 B, the DTS relative value calculated by the DTS relative value calculation means  17 , and the CTS relative value calculated by the CTS relative value calculation means  18  in an order of encoding in each encapsulation unit. 
     The encapsulation means  16 B encapsulates the access units input from the time stamp calculation means  12 B into the media unit  100 B in an order of encoding in each encapsulation unit. Further, the encapsulation means  16 B adds the DTS relative value and the CTS relative value to each access unit and outputs the encapsulated media unit  100 B to the transmitting means  19 . 
     The transmitting means  19  transmits the media unit  100 B input from the encapsulation means  16 B and a control signal  300  to the decoding device  2 B via the network N. 
     The control signal  300  includes absolute time which indicates the head of the media unit  100 B. For example, the control signals  300  is information indicating a configuration of content and an acquisition source of a necessary component, and start-up control meta data in which absolute time is described (see the reference). As the absolute time, for example, the time of UTC (Coordinated Universal Time) or elapsed time based on the head of the content is set in advance. Reference: Aoki et al., “Media Transport System in Hybrid Broadcasting” Information Processing Society of Japan Research Report, 2011. 
     Here, it is desirable that the transmitting means  19  adds an ID (identifier) and a sequence number to the media unit  100 B and the control signal  300  corresponding to the media unit  100 B. That is, an ID is set in advance in each transmission path and the transmitting means  19  adds, to the media unit  100 B and to the control signal  300 , an ID in accordance with the transmission path along which the media unit  100 B is transmitted. Further, transmitting means  19  includes a management table (not illustrated) with which a sequence number already added to the media unit  100 B is managed. Then the transmitting means  19  increments the sequence number of this management table and adds the incremented sequence number as a sequence number of a new media unit  100 B. 
     The ID is identification information which is common in predetermined media units  100 B. For example, since the same value is given to the media unit  100 B of the same transmission path, the ID is a unique value which neither overlaps other distribution environments nor depends on the distribution environment. The sequence number is identification information specific to each media unit  100 B. That is, each media unit  100 B can be uniquely identified by a set of the ID and the sequence number. 
     &lt;Concrete Example of Encapsulation&gt; 
     With reference to  FIG. 11 , a concrete example of encapsulation by the encoding device  1 B will be described. In  FIG. 11 , the CTS field  102  is illustrated as an “AU display period” and a sequence number field  106  is illustrated as “Seq_No.” 
     In this  FIG. 11 , it is supposed that six access units  200   1  to  200   6  have been generated continuously (AU1 to AU6 of  FIG. 11 ). Further, in  FIG. 11 , it is supposed that an encapsulation condition has been set with three access units  200  being defined as an encapsulation unit. Therefore, the encapsulation determination means  15 B determines the encapsulation unit of the first media unit  100 B 1  as the access units  200   1  to  200   3  and determines the encapsulation unit of the second media unit  100 B 2  as the access units  200   4  to  200   6 . Further, in  FIG. 11 , it is supposed that the media units  100 B 1  and  100 B 2  and control signals  300   1  and  300   2  corresponding to these media units  100 B 1  and  100 B 2  are transmitted along the same transmission path. 
     The DTS relative value calculation means  17  calculates ‘0’ as the DTS relative value of the head access unit  200   1  and describes ‘0’ in the DTS field  101   1 . Further, the DTS relative value calculation means  17  calculates, as the DTS relative value of the second access unit  200   2 , a difference between the DTS of this access unit  200   2  and the DTS of the head access unit  200   1 , and describes the calculated value in the DTS field  101   2 . Further, the DTS relative value calculation means  17  calculates, as the DTS relative value of the third access unit  200   3 , a difference between the DTS of this access unit  200   3  and the DTS of the second access unit  200   2 , and describes the calculated value in the DTS field  101   3 . 
     Here, the DTS relative value calculation means  17  calculates a DTS relative value between the access units  200   3  and  200   4  encapsulated in the different media units  100 B 1  and  100 B 2  if these media units  100 B 1  and  100 B 2  have the same ID. That is, the DTS relative value calculation means  17  calculates, as a DTS relative value of the fourth access unit  200   4 , a difference between the DTS of this access unit  200   4  and the DTS of the third access unit  200   3  and describes the calculated difference in the DTS field  101   4 . 
     Further, the DTS relative value calculation means  17  calculates, as a DTS relative value of fifth access unit  200   5 , a difference between the DTS of this access unit  200   5  and the DTS of the fourth access unit  200   4  and describes the calculated value in a DTS field  101   5 . Further, the DTS relative value calculation means  17  calculates, as a DTS relative value of sixth access unit  200   6 , a difference between the DTS of this access unit  200   6  and the DTS of the fifth access unit  200   5  and describes the calculated value in a DTS field  101   6 . 
     That is, the DTS relative value calculation means  17  calculates the DTS relative values as illustrated in following Expression (1) to (6) and describes the calculated DTS relative values in the DTS field  101 .
 
AU1_DTS relative value=0  Expression (1)
 
AU2_DTS relative value=AU2_DTS-AU1_DTS  Expression (2)
 
AU3_DTS relative value=AU3_DTS-AU2_DTS  Expression (3)
 
AU4_DTS relative value=AU4_DTS-AU3_DTS  Expression (4)
 
AU5_DTS relative value=AU5_DTS-AU4_DTS  Expression (5)
 
AU6_DTS relative value=AU6_DTS-AU5_DTS  Expression (6)
 
     The CTS relative value calculation means  18  calculates, as a CTS relative value of the head access unit  200   1 , a difference between the CTS of this access unit  200   1  and the CTS of the second access unit  200   2  and describes the calculated value in the CTS field  102   1 . Further, the CTS relative value calculation means  18  calculates, as a CTS relative value of the second access unit  200   2 , a difference between the CTS of this access unit  200   2  and the CTS of the third access unit  200   3  and describes the calculated value in the CTS field  102   2 . 
     Here, the CTS relative value calculation means  18  calculates a CTS relative value between the access units  200   3  and  200   4  encapsulated in the different media units  100 B 1  and  100 B 2  if these media units  100 B 1  and  100 B 2  have the same ID. That is, the CTS relative value calculation means  18  calculates, as a CTS relative value of the third access unit  200   3 , a difference between the CTS of this access unit  200   3  and the CTS of the fourth access unit  200   4  and describes the calculated value in a CTS field  102   3 . 
     Further, the CTS relative value calculation means  18  calculates, as a CTS relative value of the fourth access unit  200   4 , a difference between the CTS of this access unit  200   4  and the CTS of the fifth access unit  200   5  and describes the calculated value in a CTS field  102   4 . Further, the CTS relative value calculation means  18  calculates, as a CTS relative value of the fifth access unit  200   5 , a difference between the CTS of this access unit  200   5  and the CTS of the sixth access unit  200   6  and describes the calculated value in a CTS field  102   5 . 
     Further, the CTS relative value calculation means  18  calculates, as a CTS relative value of the sixth access unit  200   6 , a difference between the CTS of this access unit  200   6  and the CTS of a seventh access unit  200  (not illustrated) and describes the calculated value in a CTS field  102   6 . In a case in which the sixth access unit  200   6  is the last access unit, it is only necessary for the CTS relative value calculation means  18  to consider any of the time when the input signal is interrupted, when the encoder  11  stops its operation and when time is out as the CTS of the seventh access unit  200  and to calculate a CTS relative value. 
     That is, the CTS relative value calculation means  18  calculates the CTS relative values as expressed in the following expressions (7) to (12) and describes the calculated CTS relative values in the CTS field  102 .
 
AU1_CTS relative value=AU2_CTS-AU1_CTS  Expression (7)
 
AU2_CTS relative value=AU3_CTS-AU2_CTS  Expression (8)
 
AU3_CTS relative value=AU4_CTS-AU3_CTS  Expression (9)
 
AU4_CTS relative value=AU5_CTS-AU4_CTS  Expression (10)
 
AU5_CTS relative value=AU6_CTS-AU5_CTS  Expression (11)
 
AU6_CTS relative value=AU7_CTS-AU6_CTS  Expression (12)
 
     The transmitting means  19  describes an ID common in the media units  100 B 1  and  100 B 2  and the control signals  300   1  and  300   2  (for example, ‘1’) in ID fields  105   1  and  105   2  of the media units  100 B 1  and  100 B 2  and in ID fields  301   1  and  301   2  of the control signals  300   1  and  300   2 . 
     Further, the transmitting means  19  describes a sequence number of the media unit  100 B 1  (for example, ‘1’) in a sequence number field  106   1  of the media unit  100 B 1  and in a sequence number field  302   1  of the control signal  300   1  corresponding to the media unit  100 B 1 . 
     Further, the transmitting means  19  describes absolute time indicating the head of the media unit  100 B 1  (i.e., absolute time indicating the head of MU1 (T_mu1)) in an absolute time field  303   1  of the control signal  300   1 . 
     Further, the transmitting means  19  describes a sequence number of the media unit  100 B 2  (for example, ‘2’) in a sequence number field  106   2  of the media unit  100 B 2  and in a sequence number field  302   2  of a control signal  300   2  corresponding to the media unit  100 B 2 . 
     Further, the transmitting means  19  describes absolute time indicating the head of the media unit  100 B 2  (i.e., absolute time indicating the head of MU2 (T_mu2)) in an absolute time field  303   2  of the control signal  300   2 . 
     Although not illustrated in  FIG. 11 , the media units  100 B 1  and  100 B 2  may include an encapsulation determination information field  103  and a size information field  104  ( FIG. 2 ) as in the case of the first embodiment. In this case, the encapsulation means  16 B describes in the encapsulation determination information field  103 , as encapsulation determination information, a value indicating that size information is included (for example, other than ‘0’). Further, the encapsulation means  16 B describes size information of each access unit  200  in the size information field  104 . 
     &lt;Relationship Between Absolute Time and CTS Relative Value&gt; 
     With reference to  FIG. 12 , additional explanation on a relationship between the absolute time “T_mu” and the CTS relative value will be given (see  FIG. 10  and  FIG. 11  as necessary). 
     The CTS of the head access unit  200   1  and the absolute time are the same. In other words, the CTS and the DTS of the head access unit  200   1  become identical. 
     The CTS of the second access unit  200   2  is a value obtained by adding ΔT_au1 to the CTS of the head access unit  200   1 . In other words, ΔT_au1 represents a difference between the CTS of the second access unit  200   2  and the CTS of the head access unit  200   1 . Therefore, this ΔT_au1 is described in the CTS field  102   1  as the CTS relative value of the head access unit  200   1 . 
     The CTS of the third access unit  200   3  is a value obtained by adding ΔT_au2 to the CTS of the second access unit  200   2 . In other words, ΔT_au2 represents a difference between the CTS of the third access unit  200   3  and the CTS of the second access unit  200   2 . Therefore, this ΔT_au2 is described in the CTS field  102   2  as the CTS relative value of the second access unit  200   2 . Since the fourth access unit  200   4  or thereafter are similar to those described above, description will be omitted. 
     [Configuration of Decoding Device] 
     Returning to  FIG. 10 , a configuration of the decoding device  2 B will be described. The decoding device  2 B extracts one or more access units  200  from the media unit  100 B transmitted by the encoding device  1 B and decodes the extracted access units  200 . Therefore, the decoding device  2 B includes a reverse encapsulation means (i.e., a media unit extraction means)  22 B, a time stamp reverse calculation means  23 B, a decoder  26  and a receiving means  27 . 
     The reception means  27  receives a media unit  100 B and a control signal  300  from the encoding device  1  via the network N or the broadcast wave W. Then the reception means  27  sequentially outputs the received media unit  100 B and the control signal  300  to the reverse encapsulation means  22 B. 
     The reverse encapsulation means  22 B extracts (i.e., reverse encapsulates) one or more access units  200  from the media unit  100 B input from the receiving means  27 . Here, the reverse encapsulation means  22 B specifies, with reference to the size information field  104 , a data field of the media unit  100 B into which each access unit  200  is encapsulated. Then the reverse encapsulation means  22 B outputs the access units  200  to the time stamp reverse calculation means  23 B in an order in which the access units  200  have been extracted from the media unit  100 B. Further, the reverse encapsulation means  22 B outputs the control signal  300  input from the receiving means  27  to the time stamp reverse calculation means  23 B. 
     The time stamp reverse calculation means  23 B reversely calculates the DTS and the CTS of the access unit  200  from a time stamp (i.e., the DTS relative value and the CTS relative value) and includes a DTS reverse calculation means  24 B and a CTS reverse calculation means  25 B. 
     The DTS reverse calculation means  24 B reversely calculates absolute time of the control signal  300  corresponding to the media unit  100 B as the DTS of the access unit  200  located at the head. Further, the DTS reverse calculation means  24 B reversely calculates, as a DTS of the access unit  200  located at the second place or thereafter, a value obtained by adding the DTS relative value of the access unit  200  and the DTS of another access unit located immediately before the access unit  200 . 
     The CTS reverse calculation means  25 B reversely calculates, as a CTS of another access unit located immediately after the access unit  200 , a value obtained by adding absolute time of the control signal  300  corresponding to the media unit  100 B and a sum of the CTS relative values of from the access unit  200  located at the head to this access unit  200 . 
     &lt;Concrete Example of Reverse Encapsulation&gt; 
     With reference to  FIG. 11 , a concrete example of reverse encapsulation by the decoding device  2 B will be described (see  FIG. 10  as necessary). 
     The DTS reverse calculation means  24 B obtains a correlation between the media unit  100 B and the control signal  300  with reference to the ID and the sequence number. In the example of  FIG. 11 , the DTS reverse calculation means  24 B correlates a media unit  100 B 1  of which ID=‘1’ and sequence number=‘1’ with the control signal  300   1 . Further, the DTS reverse calculation means  24 B correlates a media unit  100 B 2  of which ID=‘1’ and sequence number=‘2’ with the control signal  300   2 . 
     Further, the DTS reverse calculation means  24 B reversely calculates, as the DTS of the head access unit  200   1 , absolute time described in the absolute time field  303   1  of the control signal  300   1 . Further, the DTS reverse calculation means  24 B reversely calculates, as a DTS of the second access unit  200   2 , a value obtained by adding the DTS relative value described in the DTS field  101   2  of this access unit  200   2  and the already obtained DTS of the head access unit  200   1 . Further, the DTS reverse calculation means  24 B reversely calculates, as a DTS of the third access unit  200   3 , a value obtained by adding the DTS relative value described in the DTS field  101   3  of this access unit  200   3  and the already obtained DTS of the second access unit  200   2 . 
     Here, the DTS reverse calculation means  24 B reversely calculates a DTS between the access units  200   3  and  200   4  encapsulated in the different media units  100 B 1  and  100 B 2  if these media units  100 B 1  and  100 B 2  have the same ID. That is, the DTS reverse calculation means  24 B reversely calculates, as a DTS of the fourth access unit  200   4 , a value obtained by adding the DTS relative value described in the DTS field  101   4  of this access unit  200   4  and the already obtained DTS of the third access unit  200   3 . 
     Further, the DTS reverse calculation means  24 B reversely calculates, as a DTS of the fifth access unit  200   5 , a value obtained by adding the DTS relative value described in the DTS field  101   5  of this access unit  200   5  and the already obtained DTS of the fourth access unit  200   4 . Further, the DTS reverse calculation means  24 B reversely calculates, as a DTS of the sixth access unit  200   6 , a value obtained by adding the DTS relative value described in the DTS field  101   6  of this access unit  200   6  and the already obtained DTS of the fifth access unit  200   5 . 
     That is, the DTS reverse calculation means  24 B reversely calculates the DTS as expressed in the following Expressions (13) to (17).
 
AU1_DTS=absolute time of the control signal  300   1  
 
AU2_DTS=AU2_DTS relative value+AU1_DTS  Expression (13)
 
AU3_DTS=AU3_DTS relative value+AU2_DTS  Expression (14)
 
AU4_DTS=AU4_DTS relative value+AU3_DTS  Expression (15)
 
AU5_DTS=AU5_DTS relative value+AU4_DTS  Expression (16)
 
AU6_DTS=AU6_DTS relative value+AU5_DTS  Expression (17)
 
     The CTS reverse calculation means  25 B obtains a correlation between the media unit  100 B and the control signal  300  with reference to the ID and the sequence number as in the case of the DTS reverse calculation means  24 B. Further, the CTS reverse calculation means  25 B reversely calculates, as a CTS of the head access unit  200   1 , absolute time described in the absolute time field  303   1  of the control signal  300   1 . 
     Further, the CTS reverse calculation means  25 B reversely calculates, as a CTS of the second access unit  200   2 , a value obtained by adding absolute time of the control signal  300   1  and the CTS relative value of the head access unit  200   1 . Further, the CTS reverse calculation means  25 B calculates a sum of the CTS relative values of the access units  200   1  and  200   2 . Then the CTS reverse calculation means  25 B reversely calculates a value obtained by adding absolute time to this sum as a CTS of the third access unit  200   3 . 
     Here, the CTS reverse calculation means  25 B reversely calculates a CTS between the access units  200   3  and  200   4  encapsulated in the different media units  100 B 1  and  100 B 2  if these media units  100 B 1  and  100 B 2  have the same ID. That is, the CTS reverse calculation means  25 B calculates a sum of the CTS relative values of the access units  200   1  to  200   3 . Then the CTS reverse calculation means  25 B reversely calculates a value obtained by adding absolute time to this sum as a CTS of the fourth access unit  200   4 . 
     Further, the CTS reverse calculation means  25 B calculates a sum of the CTS relative values of the access units  200   1  to  200   4 . Then the CTS reverse calculation means  25 B reversely calculates a value obtained by adding absolute time to this sum as a CTS of the fifth access unit  200   5 . Further, the CTS reverse calculation means  25 B calculates a sum of the CTS relative values of the access units  200   1  to  200   5 . Then the CTS reverse calculation means  25 B reversely calculates a value obtained by adding absolute time to this sum as a CTS of the sixth access unit  200   6 . 
     Here, additional explanation on the absolute time “T_mu2” of the control signal  300   2  will be given. In a case in which a broadcast wave W is used, decoding devices  2 B which begin receiving at various timings exist in the content providing system  1000 B. Here, a case in which a certain decoding device  2 B has not been able to receive a control signal  300   1  will be considered. In this case, the decoding device  2 B receives a subsequent control signal  300   2  and reversely calculates, using absolute time “T_mu2” of this control signal  300   2 , the DTS and the CTS of the access units  200   4  to  200   6  encapsulated into the media unit  100 B 2 . In this manner, the decoding device  2 B begins reproduction of content starting at the media unit  100 B 2  corresponding to the control signal  300   2  which the decoding device  2 B has been able to receive. That is, in the content providing system  1000 B, in order that the decoding device  2 B may begin receiving at arbitrary timing, it is desirable to periodically transmit, to the decoding device  2 B, the control signal  300  to which absolute time is added. 
     Further, in a case in which the decoding device  2 B receives the media units  100 B continuously, absolute time of the control signal  300   2  is not necessary if the fourth access unit  200   4  is promptly displayed after the third access unit  200   3 . However, in the content providing system  1000 B, a clock frequency of the encoding device  1 B and a clock frequency of the decoding device  2 B do not necessarily coincide precisely. Therefore, in the content providing system  1000 B, there is a possibility that an error (i.e., a clock drift) occurs in absolute time if the CTS relative value of each access unit  200  is accumulated. Therefore, in the content providing system  1000 B, mapping to absolute time is performed at relatively short intervals to prevent serious errors from occurring. That is, in the content providing system  1000 B, in order to reduce errors in the CTS in the access unit  200 , it is desirable to periodically transmit, to the decoding device  2 B, the control signal  300  to which absolute time is added. 
     Then the time stamp reverse calculation means  23 B sequentially outputs, to the decoder  26 , the access units  200  for which the DTS and the CTS have been reversely calculated. 
     Returning to  FIG. 10 , description about the configuration of the decoding device  2 B will be continued. The decoder  26  decodes the access unit  200  input from the time stamp reverse calculation means  23 B according to an image decoding scheme corresponding to the encoder  11 . Since both the CTS and the DTS are correlated with the access unit  200 , this access unit  200  can be decoded according to the image decoding scheme described above. 
     [Operation of Encoding Device] 
     An operation of the encoding device  1 B will be described with reference to  FIG. 13  (see  FIG. 10  to  FIG. 12  as necessary). 
     The encoding device  1 B encodes, by the encoder  11 , an image signal input from outside to generate an access unit  200  correlated with a CTS and a DTS (step S 31 ). The encoding device  1 B determines, by the encapsulation determination means  15 B, one or more access units as an encapsulation unit on a predetermined encapsulation condition (step S 32 ). 
     The encoding device  1 B calculates the DTS relative value by the DTS relative value calculation means  17  (step S 33 ). The encoding device  1 B calculates the CTS relative value by the CTS relative value calculation means  18  (step S 34 ). 
     The encoding device  1 B encapsulates, by the encapsulation means  16 B, the access units  200  into the media unit  100 B and adds the DTS relative value and the CTS relative value to each access unit  200  (step S 35 ). The encoding device  1 B transmits, by the transmitting means  19 , the encapsulated media unit  100 B and the control signal  300  corresponding to this media unit  100 B to the decoding device  2 B via the network N (step S 36 ). 
     [Operation of Decoding Device] 
     An operation of the decoding device  2 B will be described with reference to  FIG. 14  (see  FIG. 10  to  FIG. 12  as necessary). 
     The decoding device  2 B receives, by the receiving means  27 , the media unit  100 B and the control signal  300  from the encoding device  1 B via the network N or the broadcast wave W (step S 41 ). The decoding device  2 B extracts, by the reverse encapsulation means  22 B, one or more access units  200  from the media unit  100 B (step S 42 ). 
     The decoding device  2 B reversely calculates the DTS by the DTS reverse calculation means  24 B (step S 43 ). The decoding device  2 B reversely calculates the CTS by the CTS reverse calculation means  25 B (step S 44 ). The decoding device  2 B decodes, by the decoder  26 , the access units  200  of which the DTS and the CTS are reversely calculated (step S 45 ). 
     As described above, in the encoding device  1 B and the decoding device  2 B according to the third embodiment of the present invention, a format in which the base absolute time of the DTS and the CTS are added to a control signal  300  and in which the DTS relative value and the CTS relative value are added to the access unit  200  is adopted. Therefore, the encoding device  1 B and the decoding device  2 B can easily change the time when the access unit  200  is decoded and the time when the access unit  200  is presented or reproduced only by rewriting the absolute time of the control signal  300 . Further, the encoding device  1 B and the decoding device  2 B can easily implement commonization of the media unit  100 B and switching of the transmission paths. 
     Fourth Embodiment 
     With reference to  FIG. 15  to  FIG. 18 , a content providing system  1000 C according to a fourth embodiment of the present invention will be described with respect to a difference from the third embodiment. The content providing system  1000 C provides content (i.e., audio) and includes an encoding device  1 C and a decoding device  2 C. 
     [Configuration and Operation of Encoding Device] 
     The encoding device  1 C encodes an audio signal as an input signal and provides the encoded audio signal to the decoding device  2 C. As illustrated in  FIG. 15 , since a configuration of the encoding device  1 C is similar to that of the encoding device  1 B of  FIG. 10  except that an encoder  11 A is provided instead of the encoder  11  and that the DTS relative value calculation means  17  is excluded, detailed description of the configuration of the encoding device  1 C will be omitted. Further, as illustrated in  FIG. 16 , since an operation of the encoding device  1 C is similar to that illustrated in  FIG. 13  except that step S 33  is not performed, detailed description of the operation of the encoding device  1 C will be omitted. 
     [Configuration and Operation of Decoding Device] 
     The decoding device  2 C extracts an access unit  200  from a media unit  100 C transmitted by the encoding device  1 C and decodes the extracted access unit  200 . As illustrated in  FIG. 15 , since a configuration of the decoding device  2 C is similar to that of the decoding device  2 B of  FIG. 10  except that a decoder  26 A is provided instead of the decoder  26  and that the DTS reverse calculation means  24 B is excluded, detailed description of the configuration of the decoding device  2 C will be omitted. Further, as illustrated in  FIG. 17 , since an operation of the decoding device  2 C is similar to that illustrated in  FIG. 14  except that step S 43  is not performed, detailed description of the operation of the decoding device  2 C will be omitted. 
     [Format of Media Unit and Control Signal] 
     As illustrated in  FIG. 18 , since the media unit  100 C and the control signal  300  are similar to those illustrated in  FIG. 11  except that the DTS field  101  is not included in the media unit  100 C, detailed description of the media unit  100 C and the control signal  300  will be omitted. 
     As described above, in the encoding device  1 C and the decoding device  2 C according to the fourth embodiment of the present invention, the same effect as that of the third embodiment can be obtained for the audio content. 
     Functions of the encoding device and the decoding device according to each embodiment may be implemented by a computer. In that case, the present invention may implement the functions by recording a program for implementing these functions on a computer-readable recording medium, and causing a computer system to read and execute the program recorded on the recording medium. 
     The “computer system” here should include an OS and hardware, such as peripheral equipment. Further, the “computer-readable recording medium” should include a portable medium, such as a flexible disk, a magnetic-optical disk, a ROM and a CD-ROM, and a storage device, such as a hard disk incorporated in a computer system. Further, the “computer-readable recording medium” may include a medium which dynamically retains a program for a short time, such as a communication line on which the program is transmitted like a network, such as the Internet, and a communication line, such as a telephone line, and a medium which retains a program for a certain period of time, like a volatile memory incorporated in a computer system used as a server or a client in the case described above. 
     Further, the program described above may be for implementing a part of the function described above and, moreover, may implement the function described above in combination with a program already recorded on a computer system. 
     (Modification) 
     Although each embodiment of the present invention has been described, the present invention is not limited to the same and can be implemented in a range without changing the purport thereof. Modification of the embodiments will be described below. 
     Although the image is handled by the content providing system  1000  and the audio is handled by the content providing system  1000 A in the foregoing description, the present invention may also handle both the image and the audio. In this case, in the content providing side, the encoding device  1  of  FIG. 1  generates a media unit of the image and the encoding device  1 A of  FIG. 6  generates a media unit of the audio. Then, in the content providing side, the media unit of the image and the media unit of the audio are multiplexed and transmitted to the content receiving side. Further, in the content receiving side, the multiplexed media unit is divided into the media unit of the image and the media unit of the audio. Then, in the content receiving side, the media unit of the image is decoded by the decoding device  2  of  FIG. 1  and the media unit of the audio is decoded by the decoding device  2 A of  FIG. 6 . 
     Further, the content providing systems  1000  and  100 A may include two or more encoding devices  1  and  1 A and two or more decoding devices  2  and  2 A. Further, in the content providing systems  1000  and  100 A, the media unit generated by the encoding devices  1  and  1 A may be recorded on a magneto-optical recording medium and may be provided to the decoding devices  2  and  2 A in an offline manner, such as mailing. Further, in the content providing systems  1000  and  100 A, the encoding devices  1  and  1 A and the decoding devices  2  and  2 A may be provided in the same device, the media units generated by the encoding devices  1  and  1 A may be accumulated, and the accumulated media units may be provided to the decoding devices  2  and  2 A.