Patent Publication Number: US-2010122310-A1

Title: Digital broadcast receiver

Description:
This application is a U.S. National Phase Application of PCT International Application PCT/JP2008/001042. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a digital broadcast receiver. 
     BACKGROUND ART 
     Digital broadcast deals with the possibility of reception during rainfall time and in a mobile environment. As a result, digital broadcast allows hierarchical transmission simultaneously transmitting two types of services: high hierarchical service with high transmission capacity and low transmission error resistance; and low hierarchical service with low transmission capacity and high transmission error resistance. Such examples include BS digital broadcast and terrestrial digital broadcast in Japan. 
     A digital broadcast receiver capable of such hierarchical transmission allows viewing high hierarchical service in a favorable reception state, and then switches to low hierarchical service when the reception state deteriorates for continuous viewing. 
       FIG. 8  is a block diagram showing the configuration of conventional digital broadcast receiver  800 . As shown in  FIG. 8 , conventional digital broadcast receiver  800  has a hierarchical switching function by using hierarchical switching determining unit  810 . Meanwhile, digital broadcast receiver  800  is equipped with only one set of video decoder  806  and audio decoder  807 . Hereinafter, a detailed description is made of the configuration of conventional digital broadcast receiver  800 . 
     As shown in  FIG. 8 , conventional digital broadcast receiver  800  includes digital broadcast receiving unit  801 , transport decoding unit  802 , video/audio data detecting unit  803 , video buffer  804 , audio buffer  805 , video decoder  806 , audio decoder  807 , video output unit  808 , audio output unit  809 , hierarchical switching determining unit  810 , and system control unit  811 . 
       FIG. 9  is an explanatory drawing of operation timing of hierarchical switching by conventional digital broadcast receiver  800 .  FIG. 10  is a flowchart illustrating operation of video switching when conventional digital broadcast receiver  800  executes hierarchical switching. Next, a description is made of hierarchical switching operation for video by conventional digital broadcast receiver  800  using  FIGS. 8 ,  9 , and  10 . Here, an example is shown where conventional digital broadcast receiver  800  switches video from high hierarchical service to low one. 
     First, a description is made of the mechanism of MPEG-2 Systems method adopted in digital broadcast. In digital broadcast, a program clock reference and a presentation time stamp are transmitted from a broadcast station as information indicating output timing of video and audio. Here, a program clock reference becomes a reference clock for digital broadcast receiver  800  and is abbreviated as PCR hereinafter. A presentation time stamp is embedded in video/audio data for each frame and is abbreviated as PTS hereinafter. Digital broadcast receiver  800  has a system time clock (STC hereinafter) as a reference for controlling decoding video and audio and output timing of video and audio. To generate an STC, digital broadcast receiver  800  copies a PCR value in a PCR packet received to the STC counter inside. After that, the STC counter is incremented by a 27-MHz clock to reproduce the STC. If the STC has exceeded the PTS for each frame, digital broadcast receiver  800  outputs video/audio signals contained in the frames. Consequently, digital broadcast receiver  800  can output video and audio at timing intended by the broadcast station. 
     Thus in digital broadcast, video/audio data is typically transmitted after a certain time interval after video/audio data arrives at digital broadcast receiver  800  before an output time point indicated by the PTS. This is to allow for decoding time by the decoders (i.e. transport decoding unit  802 , audio decoder  807 , and video decoder  806 ), and for delay time in such as video buffer  804  and audio buffer  805 . 
     Here, assumption is made that the video coding method is MPEG-2 video method for high hierarchical service and H.264 MPEG-4 AVC method for low hierarchical service. In MPEG-2 video method, video data video-decodable by itself is called an I frame. Similarly, in H.264 MPEG-4 AVC method, video data video-decodable by itself is called an IDR frame. Digital broadcast receiver  800  can start decoding video only after receiving an I frame or IDR frame. 
     Assuming that the audio coding method is MPEG-2 AAC method, the head of audio data is called an ADTS header in MPEG-2 AAC method. Digital broadcast receiver  800  can start decoding audio only after receiving an ADTS header. 
     Hereinafter, a further concrete description is made of timing where digital broadcast receiver  800  receives digital broadcast, and then decodes and outputs video and audio signals, using  FIG. 9 . Data reception timing diagram  951  of  FIG. 9  shows data reception timing in high hierarchical service with the system time clock (STC) represented with the horizontal axis. The first “I” shown in data reception timing diagram  951  represents an I frame as a reference of the STC (STC=0) for convenience of description. Operation timing diagram  952  shows timing of outputting video and audio signals in high hierarchical service, based on a PTS embedded in video/audio data for each frame. 
     In other words, the first “I” shown in operation timing diagram  952  represents the first I frame for starting to output a video signal in the high hierarchical service. Thus in digital broadcast, transmission is made after a certain time interval between the first “I” shown in data reception timing diagram  951  and the first “I” shown in operation timing diagram  952 , allowing for decoding time by video decoder  806  and for delay time in video buffer  804 . 
     Further, data reception timing diagram  953  shows data reception timing in low hierarchical service for data received when it cannot be received by high hierarchical service. The first “IDR” shown in data reception timing diagram  953  represents the first IDR frame (a frame at the time point indicated by broken line  961  in the diagram) that is video-decodable by itself after starting to receive the low hierarchical service. Operation timing diagram  954  shows timing of outputting a video signal in the low hierarchical service, based on a PTS embedded in video/audio data for each frame. 
     In other words, the first “IDR” shown in operation timing diagram  954  represents the first IDR frame for starting (the time point indicated by broken line  965  in the figure) to output video signals in the low hierarchical service. Thus in digital broadcast, transmission is made at a certain time interval between the first “IDR” shown in data reception timing diagram  953  and the first “IDR” shown in operation timing diagram  954 , allowing for decoding time by video decoder  806  and for delay time in video buffer  804 . As shown in  FIG. 9 , decoding time by video decoder  806  is different between the high hierarchical service and the low one, and thus the above-described two certain time intervals are assumed to be different. 
     As described above, according to conventional digital broadcast receiver  800 , the frame “I” in the high hierarchical service is reproduced in the first place as shown in operation timing diagram  955 . When the reception state deteriorates, it is to be determined (the time point indicated by broken line  960  in the diagram) that switching to low hierarchical service is required. Consequently, a video signal cannot be output after reception in the high hierarchical service ceases to be received and before the IDR (the time point shown by broken line  965  in the diagram) at the first PTS after switching to the low hierarchical service. 
     Next, a detailed description is made of hierarchical switching operation for video by conventional digital broadcast receiver  800 , using the flowchart of  FIG. 10 . Here, an example is shown where conventional digital broadcast receiver  800  switches video from high hierarchical service to low one. 
     Digital broadcast receiver  800 , while decoding the high hierarchical service (step  1001 ), is always monitoring information related to the reception state by digital broadcast receiving unit  801  (step  1002 ). Then, digital broadcast receiving unit  800  is sending information related to the reception state to hierarchical switching determining unit  810 . Information related to the reception state includes reception level, C/N ratio, and bit error rate. Hierarchical switching determining unit  810  determines whether or not the reception state has deteriorated from information related to the reception state, to determine whether or not hierarchical switching is executed (step  1003 ). If the reception state is favorable and hierarchical switching is not needed (No), digital broadcast receiver  800  continues decoding the high hierarchical service without switching the hierarchical service to be received (step  1001 ). Meanwhile, if hierarchical switching determining unit  810  determines that the reception state has deteriorated from information related to the reception state (Yes), hierarchical switching determining unit  810  directs system control unit  811  to switch to the low hierarchical service. 
     In other words, when switching to the low hierarchical service, system control unit  811  directs video decoder  806  to stop video decoding in the high hierarchical service before switching. System control unit  811  directs video decoder  806  to stop video output to video output unit  808  (step  1004 ). Subsequently, system control unit  811  directs transport decoding unit  802  to change setting for outputting video packets from for the high hierarchical service before switching to for the low hierarchical service after switching (step  1005 ). Further, system control unit  811  directs video buffer  804  to change setting for accumulating video data from for the high hierarchical service before switching to for the low hierarchical service after switching (step  1006 ). 
     Next, video/audio data detecting unit  803  of transport decoding unit  802  determines whether or not an IDR frame has been detected in video data in the low hierarchical service after switching (step  1007 ). If an IDR frame has not been detected (No), the process flow returns to step  1007  to repeat the operation of determining whether or not an IDR frame has been detected in video data in the low hierarchical service after switching. 
     Meanwhile, if an IDR frame has been detected (Yes), video/audio data detecting unit  803  informs system control unit  811  of the detection and acquires the PTS of the IDR frame from the video data (step  1008 ). 
     Next, when system control unit  811  receives the notice that an IDR frame has been detected, system control unit  811  directs video decoder  806  to start video decoding in the low hierarchical service after switching (step  1009 ). Then, system control unit  811  determines whether or not the STC has exceeded the PTS of the IDR frame (step  1010 ). If the STC has not exceeded the PTS of the IDR frame (No), the process flow returns to step  1010  to repeat the operation of determining whether or not the STC has exceeded the PTS of the IDR frame. 
     Meanwhile, if the STC has exceeded the PTS of the IDR frame (Yes), system control unit  811  directs video output unit  808  to start outputting video of the IDR frame (step  1011 ). After that, video decoding in the low hierarchical service continues (step  1012 ). 
     The above conventional example describes hierarchical switching operation for video from high hierarchical service to low one. Switching from low hierarchical service to high one follows the completely same procedure if an IDR frame is replaced with an I frame in MPEG-2 Video. Further, hierarchical switching of audio follows the completely same procedure if an IDR frame is replaced with an ADTS header. 
     In this way, in the above-described conventional digital broadcast receiver  800 , having only one decoder, decoding of video and audio needs to be stopped once at hierarchical switching. Consequently, with digital broadcast receiver  800 , video/audio output is to be stopped after decoding before switching is stopped immediately after switching determination and video output are stopped (step  1004 , the time point shown by broken line  960  in  FIG. 9 ) before given data (e.g. I frame, IDR frame, ADTS header) is received and decoded to start outputting (step  1011 , the time point shown by broken line  965  in  FIG. 9 ). 
     Some digital broadcast receivers have two decoders for high hierarchical service and low one to reduce time for hierarchical switching by merely switching the output (refer to patent literature 1 for example). 
     On the other hand, the following method is devised with a digital broadcast receiver having one decoder. That is, another video data retained in memory is used to decode video as an I frame and IDR frame until an I frame and IDR frame are received to reduce time during which video output is interrupted (refer to patent literature 2 for example). 
     However, with the above-described conventional digital broadcast receiver, having only one decoder, decoding of video and audio needs to be stopped once at hierarchy switching. Hence, outputting video and audio is undesirably interrupted over a long time until given data (e.g. I frame, IDR frame, ADTS header) is received and decoded to start outputting according to the PTS, after stopping decoding before switching immediately after switching determination and stopping video output. 
     Also, a digital broadcast receiver described in patent literature 1, having two decoders for high hierarchical service and low one, involves problems of its large scale and expensiveness 
     Further, a digital broadcast receiver described in patent literature 2, having one decoder, uses video data different from an actual I frame or IDR frame, and thus decoding video using the video data as a reference image results in a disturbed image to be decoded. 
     [Patent literature 1] Japanese Patent Unexamined Publication No. 2005-223549
 
[Patent literature 2] Japanese Patent Unexamined Publication No. 2006-174209
 
     SUMMARY OF THE INVENTION 
     A digital broadcast receiver according to the present invention includes a digital broadcast receiving unit receiving digital broadcast containing at least two hierarchical services; a transport decoding unit decoding digital broadcast received by the digital broadcast receiving unit and outputting a video packet in a specific hierarchical service; a video buffer accumulating a video packet output from the transport decoding unit; a video decoder decoding a video packet accumulated in the video buffer; a hierarchical switching determining unit determining a hierarchical service to be received from a reception state of the digital broadcast received by the digital broadcast receiving unit; and a system control unit controlling the hierarchical service for the video packet output from the transport decoding unit, based on the hierarchical service determined by the hierarchical switching determining unit, and controlling operation of the video decoder. The system control unit features that the video decoder stops decoding of the video packet when the transport decoding unit detects a given data after the hierarchical switching determining unit determines switching of the hierarchical service. 
     Such configuration provides a digital broadcast receiver capable of reducing time during which outputting video and audio is interrupted even if only one decoder is provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing the configuration of a digital broadcast receiver according to the first and second exemplary embodiments of the present invention. 
         FIG. 2  is a flowchart for illustrating hierarchical switching operation by a digital broadcast receiver according to the first embodiment of the present invention. 
         FIG. 3  is a flowchart for illustrating hierarchical switching operation by a digital broadcast receiver according to the second embodiment of the present invention. 
         FIG. 4  is a block diagram showing the configuration of a digital broadcast receiver according to the third and fourth exemplary embodiments of the present invention. 
         FIG. 5  is a flowchart for illustrating hierarchical switching operation by a digital broadcast receiver according to the third embodiment of the present invention. 
         FIG. 6  is a flowchart for illustrating hierarchical switching operation by a digital broadcast receiver according to the fourth embodiment of the present invention. 
         FIG. 7  illustrates the effect of time reduction in hierarchical switching by a digital broadcast receiver according to the embodiments first through fourth, where the horizontal axis as the time axis is represented with a system clock. 
         FIG. 8  is a block diagram showing the configuration of a conventional digital broadcast receiver. 
         FIG. 9  is an explanatory drawing of operation timing in hierarchical switching by the conventional digital broadcast receiver. 
         FIG. 10  is a flowchart for illustrating hierarchical switching operation by the conventional digital broadcast receiver. 
     
    
    
     REFERENCE MARKS IN THE DRAWINGS 
     
         
         
           
               100  Digital broadcast receiver 
               101  Digital broadcast receiving unit 
               102  Transport decoding unit 
               103  Video/audio data detecting unit 
               104  Video buffer 
               105  Audio buffer 
               106  Video decoder 
               107  Audio decoder 
               108  Video output unit 
               109  Audio output unit 
               110  Hierarchical switching determining unit 
               111  System control unit 
               112  Decoding continuation determining unit 
               113  Buffer accumulation determining unit 
               400  Digital broadcast receiver 
               1041  First video buffer 
               1042  Second video buffer 
               1051  First audio buffer 
               1052  Second audio buffer 
           
         
       
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, a description is made of some exemplary embodiments of the present invention using the related drawings. 
     First Exemplary Embodiment 
       FIG. 1  is a block diagram showing the configuration of digital broadcast receiver  100  according to the first exemplary embodiment of the present invention. As shown in  FIG. 1 , digital broadcast receiver  100  includes digital broadcast receiving unit  101 , transport decoding unit  102 , video/audio data detecting unit  103 , video buffer  104 , audio buffer  105 , video decoder  106 , audio decoder  107 , video output unit  108 , audio output unit  109 , hierarchical switching determining unit  110 , system control unit  111 , and decoding continuation determining unit  112 . Here, video/audio data detecting unit  103  is contained in transport decoding unit  102 . Decoding continuation determining unit  112  is contained in system control unit  111 . 
     Next, a description is made of operation of digital broadcast receiver  100  thus structured. Digital broadcast receiving unit  101  of digital broadcast receiver  100  receives digital broadcast containing at least two hierarchical services. Transport decoding unit  102  decodes digital broadcast received by digital broadcast receiving unit  101  to output video packets in specific hierarchical service. Video buffer  104  accumulates video packets output from transport decoding unit  102 . Then, video decoder  106  decodes video packets accumulated in video buffer  104 . Hierarchical switching determining unit  110  determines hierarchical service to be received from a reception state of digital broadcast received by digital broadcast receiving unit  101 . Then, system control unit  111  controls the hierarchical service for video packets output from transport decoding unit  102 , based on the hierarchical service determined by hierarchical switching determining unit  110 , and controls operation of video decoder  106 . 
       FIG. 2  is a flowchart for illustrating hierarchical switching operation by a digital broadcast receiver according to the first embodiment. Next, a description is made of hierarchical switching operation for video by digital broadcast receiver  100 , using  FIGS. 1 and 2 . In the first embodiment, an example is shown where digital broadcast receiver  100  executes hierarchical switching for video from high hierarchical service to low one. 
     Digital broadcast receiver  100 , while decoding the high hierarchical service (step  201 ), is always monitoring information related to the reception state by digital broadcast receiving unit  101  (step  202 ). Then, digital broadcast receiving unit  101  is sending information related to the reception state to hierarchical switching determining unit  110 . Information related to the reception state includes reception level, C/N ratio, and bit error rate, for example. Hierarchical switching determining unit  110  determines whether the reception state has deteriorated from information related to the reception state to determine whether hierarchical switching is to be executed (step  203 ). If the reception state is favorable and hierarchical switching is not needed (No), digital broadcast receiver  100  does not switch the hierarchical service to be received but continues decoding the high hierarchical service (step  201 ). Meanwhile, if hierarchical switching determining unit  110  determines that the reception state has deteriorated from information related to the reception state (Yes), hierarchical switching determining unit  110  directs system control unit  111  to switch to the low hierarchical service. 
     Then, decoding continuation determining unit  112  determines that video decoding in the high hierarchical service before switching is continued when system control unit  111  is directed to switch from the high hierarchical service to the low one by hierarchical switching determining unit  110 . Hence, system control unit  111  does not direct video decoder  106  to stop decoding and does not direct video output unit  108  to stop video output even if system control unit  111  is directed to switch from the high hierarchical service to the low one. Meanwhile, system control unit  111  directs transport decoding unit  102  to change setting from for outputting video packets in the high hierarchical service before switching to that in the low hierarchical service after switching (step  204 ). Subsequently, system control unit  111  changes setting from for accumulating video data in the high hierarchical service before switching in video buffer  104  to that in the low hierarchical service after switching (step  205 ). 
     Next, video/audio data detecting unit  103  of transport decoding unit  102  determines whether or not an IDR frame by H.264 MPEG-4 AVC method as given data has been detected from video data in the low hierarchical service after switching (step  207 ). If an IDR frame has not been detected from the video data in the low hierarchical service after switching (No), the process flow returns to step  207  to repeat the operation of determining whether or not an IDR frame has been detected from the video data in the low hierarchical service after switching. Meanwhile, if an IDR frame has been detected from the video data in the low hierarchical service after switching (Yes), video/audio data detecting unit  103  informs system control unit  111  of the detection and acquires the PTS of the IDR frame from the video data (step  208 ). 
     Then, decoding continuation determining unit  112  of system control unit  111  that has been informed that video/audio data detecting unit  103  has detected an IDR frame determines to stop video decoding and video output. System control unit  111  directs video decoder  106  to stop video decoding in the high hierarchical service before switching, and also directs video output unit  108  to stop video output (step  209 ). 
     After that, system control unit  111  directs to start video decoding in the low hierarchical service after switching (step  210 ). Then, system control unit  111  determines whether the STC has exceeded the PTS of the IDR frame (step  211 ). If the STC has not exceeded the PTS of the IDR frame (No), the process flow returns to step  211 . Meanwhile, if the STC has exceeded the PTS of the IDR frame (Yes), system control unit  111  directs video output unit  108  to start video output for the IDR frame (step  212 ). After that, system control unit  111  continues video decoding in the low hierarchical service (step  213 ). 
       FIG. 7  is an explanatory drawing of the effect of time reduction in hierarchical switching by digital broadcast receiver  100  according to the embodiment of the present invention, where the horizontal axis as the time axis is represented with a system clock.  FIG. 7  shows operation timing diagram  703  of hierarchical switching by digital broadcast receiver  100  according to the first embodiment.  FIG. 7  additionally shows data reception timing diagrams  951  and  953  and operation timing diagrams  952  and  954  in digital broadcast already described. As described already, data reception timing diagrams  951  and  953  indicate timing required to decode digital signals for high hierarchical service transmitted from a broadcast station and to output them. Operation timing diagrams  952 ,  954  indicate a reference for digital broadcast receiver  100  to control such as decoding and output timing of video and audio. 
     As described above, using the flowchart of  FIG. 2 , digital broadcast receiver  100  according to the first embodiment continues video decoding and video output before switching the hierarchical service even after determining the hierarchical switching (the time point shown by broken line  960 , step  204  in  FIG. 2 ), when switching from the high hierarchical service to the low one. Then, digital broadcast receiver  100  stops video decoding and video output after detecting an IDR frame in the low hierarchical service after switching the hierarchical service (the time point shown by broken line  961 , step  209  in  FIG. 2 ). 
     Then, digital broadcast receiver  100  determines whether the STC has exceeded the PTS of the IDR frame by system control unit  111 . If the STC has exceeded the PTS of the IDR frame, system control unit  111  directs video output unit  108  to start video output for the IDR frame (the time point shown by broken line  965 , step  212  in  FIG. 2 ). Controlling in this way allows digital broadcast receiver  100  according to the first embodiment to reduce time during which output of video and audio is interrupted at hierarchical switching. Specifically, time during which output of video and audio is interrupted at hierarchical switching can be reduced by the period shown by broken lines  960  and  961 . 
     As described above, system control unit  111  of digital broadcast receiver  100  according to the first embodiment stops decoding video packets by video decoder  106  when transport decoding unit  102  detects an IDR frame as given data, after hierarchical switching determining unit  110  has determined switching the hierarchical service. Controlling in this way allows digital broadcast receiver  100  according to the first embodiment to reduce time during which output of video and audio is interrupted at hierarchical switching. 
     As described above, the first embodiment describes hierarchical switching operation for video from high hierarchical service to low one. However, the present invention is applicable to switching from low hierarchical service to high one in the completely same procedure by replacing an IDR frame as given data with an I frame by MPEG-2 video method. By doing in this way, the present invention provides the same effect in switching from low hierarchical service to high one. 
     For hierarchical switching of audio, the present invention is applicable in the completely same procedure if an IDR frame as given data is replaced with an ADTS header by H.264 MPEG-4 AVC method. That is, as described above, digital broadcast receiver  100  is equipped with audio buffer  105  accumulating audio packets; and audio decoder  107  decoding audio packets accumulated in audio buffer  105 . Then, transport decoding unit  102  decodes digital broadcast received by digital broadcast receiving unit  101  to output audio packets in specific hierarchical service to audio buffer  105 . Then, system control unit  111  stops decoding audio packets by audio decoder  107  when transport decoding unit  102  detects an ADTS header by MPEG-2 AAC method as given data, after hierarchical switching determining unit  110  has determined switching the hierarchy. Controlling in this way allows digital broadcast receiver  100  according to the first embodiment to reduce time during which output of video and audio is interrupted at hierarchical switching. 
     Second Exemplary Embodiment 
     In the second exemplary embodiment, the block diagram showing the configuration is  FIG. 1  in the same way as in the first embodiment.  FIG. 3  is a flowchart for illustrating hierarchical switching operation by digital broadcast receiver  100  according to the second embodiment. The second embodiment is different from the first one in that video decoding and video output are stopped when video decoder  106  has detected that video data before switching accumulated in video buffer  104  has been emptied. Hence, a detailed description is omitted of a component and its operation same as those in the first embodiment. 
     Next, a description is made of hierarchical switching operation for video by digital broadcast receiver  100  using  FIGS. 1 and 3 . In the second embodiment, an example is shown where digital broadcast receiver  100  switches the hierarchy for video from high hierarchical service to low one. The procedure is common with the first embodiment from when digital broadcast receiver  100  is decoding the high hierarchical service (step  301 ), until when system control unit  111  changes setting from for accumulating video data in the high hierarchical service before switching in video buffer  104  to that in the low hierarchical service after switching (step  305 ), and thus a detailed description for the procedure is omitted. 
     After that, video decoder  106  determines whether video data before switching accumulated in video buffer  104  has been emptied (step  306 ). If video data before switching has not been emptied (No), the control process returns to step  306  to repeat the determining operation. Meanwhile, if video data before switching has been emptied (Yes), video decoder  106  informs system control unit  111  that video data before switching has been emptied. Then, decoding continuation determining unit  112  of system control unit  111  that has received the notice determines stopping video decoding and video output. Then, system control unit  111  directs video decoder  106  to stop video decoding in the high hierarchical service before switching, and directs video output unit  108  to stop video output (step  307 ). 
     Next, video/audio data detecting unit  103  of transport decoding unit  102  determines whether an IDR frame has been detected from video data in the low hierarchical service after switching (step  308 ). If an IDR frame has not been detected, the control flow returns to step  308  to repeat the determining operation. Meanwhile, if an IDR frame is (or has been already) detected from the video data in the low hierarchical service after switching (Yes), video/audio data detecting unit  103  informs system control unit  111  of the detection and acquires the PTS of the IDR frame from the video data (step  309 ). 
     Then, system control unit  111  that has received the notice directs to start video decoding in the low hierarchical service after switching (step  310 ). Next, system control unit  111  determines whether the STC has exceeded the PTS of the IDR frame (step  311 ). If the STC has not exceeded the PTS of the IDR frame (No), the control flow returns to step  311  to repeat the determining operation. Meanwhile, if the STC has exceeded the PTS of the IDR frame (Yes), system control unit  111  directs video output unit  108  to start video output of the IDR frame (step  312 ). After that, video decoding in the low hierarchical service is continued (step  313 ). 
       FIG. 7  is an explanatory drawing of the effect of time reduction in hierarchical switching by digital broadcast receiver  100  according to the embodiment of the present invention, where the horizontal axis as the time axis is represented with a system clock.  FIG. 7  additionally shows operation timing diagram  704  in hierarchical switching by digital broadcast receiver  100  according to the second embodiment. 
     As described above using the flowchart of  FIG. 3 , digital broadcast receiver  100  according to the second embodiment continues video decoding and video output before switching the hierarchical service, even after determining (the time point shown by broken line  960 , step  304  in  FIG. 3 ) hierarchical switching, when switching from the high hierarchical service to the low one. Then, digital broadcast receiver  100  stops video decoding and video output after detecting that video data before switching accumulated in video buffer  104  has been emptied (the time point shown by broken line  962 , step  307  in  FIG. 3 ). 
     Then, digital broadcast receiver  100  determines whether the STC has exceeded the PTS of the IDR frame by system control unit  111 . If the STC has exceeded the PTS of the IDR frame, system control unit  111  directs video output unit  108  to start video output of an IDR frame (the time point shown by broken line  965 , step  312  in  FIG. 3 ). Controlling in this way allows digital broadcast receiver  100  according to the second embodiment to reduce time during which output of video and audio is interrupted at hierarchical switching. Specifically, time during which output of video and audio is interrupted at hierarchical switching can be reduced by the period shown by broken lines  960  and  962 . That is, the second embodiment is more advantageous than the first in reducing time during which output of video and audio is interrupted at hierarchical switching. 
     As described above, system control unit  111  of digital broadcast receiver  100  according to the first embodiment includes: digital broadcast receiving unit  101  receiving digital broadcast containing at least two hierarchical services; transport decoding unit  102  decoding digital broadcast received by digital broadcast receiving unit  101  to output video packets in specific hierarchical service; video buffer  104  accumulating video packets output from transport decoding unit  102 ; video decoder  106  decoding video packets accumulated in video buffer  104 ; hierarchical switching determining unit  110  determining hierarchical service to be received from a reception state of the digital broadcast received by digital broadcast receiving unit  101 ; and system control unit  111  controlling hierarchical service for video packets output from transport decoding unit  102 , based on the hierarchical service determined by hierarchical switching determining unit  110 , and controlling operation of video decoder  106 . Then, system control unit  111  stops decoding video packets by video decoder  106  when video packets in the hierarchical service before switching are emptied from video buffer  104 , after hierarchical switching determining unit  110  has determined switching the hierarchical service. Controlling in this way allows digital broadcast receiver  100  according to the first embodiment to reduce time during which output of video and audio is interrupted at hierarchical switching. 
     Further, for hierarchical switching of audio, the present invention is applicable in the completely same procedure if an IDR frame as given data is replaced with an ADTS header by H.264 MPEG-4 AVC method. That is, as described above, digital broadcast receiver  100  is equipped with audio buffer  105  accumulating audio packets; and audio decoder  107  decoding audio packets accumulated in audio buffer  105 . Then, transport decoding unit  102  decodes digital broadcast received by digital broadcast receiving unit  101  to output audio packets in specific hierarchical service to audio buffer  105 . System control unit  111  stops decoding audio packets by audio decoder  107  when audio packets in the hierarchical service before switching are emptied from audio buffer  105 , after hierarchical switching determining unit  110  has determined switching the hierarchical service. Controlling in this way allows digital broadcast receiver  100  according to the second embodiment to reduce time during which output of video and audio is interrupted at hierarchical switching. 
     Further, as described above, the present invention is applicable to the second embodiment in the completely same procedure for hierarchical switching of video and audio from low hierarchical service to high one. The second embodiment is more advantageous than the first in reducing time during which output of video and audio is interrupted at hierarchical switching. 
     Third Exemplary Embodiment 
       FIG. 4  is a block diagram of digital broadcast receiver  400  according to the third exemplary embodiment of the present invention. As shown in  FIG. 4 , digital broadcast receiver  400  according to the third embodiment is different from digital broadcast receiver  100  according to the first embodiment shown in  FIG. 1  in that digital broadcast receiver  400  includes first video buffer  1041 , second video buffer  1042 , first audio buffer  1051 , and second audio buffer  1052  so as to accumulate video/audio data both before and after hierarchical switching, and that system control unit  111  includes buffer accumulation determining unit  113 . Here, first video buffer  1041  accumulates video data before hierarchical switching; second video buffer  1042 , after. First audio buffer  1051  accumulates audio data before hierarchical switching; second audio buffer  1052 , after. The other components are the same as those in the first embodiment. Hence, a detailed description is omitted for a component and its operation same as those in the first embodiment. 
       FIG. 5  is a flowchart illustrating hierarchical switching operation by digital broadcast receiver  400  according to the third embodiment. Next, a description is made of hierarchical switching operation for video by digital broadcast receiver  400  using  FIGS. 4 and 5 . In the third embodiment, an example is shown where digital broadcast receiver  400  executes hierarchical switching for video from high hierarchical service to low one. 
     The operation is common with the first embodiment from when digital broadcast receiver  400  is decoding the high hierarchical service (step  501 ), until when system control unit  111  does not direct video decoder  106  to stop decoding and does not direct video output unit  108  to stop video output even if system control unit  111  receives a direction for hierarchical switching. 
     After that, buffer accumulation determining unit  113  of system control unit  111  determines that video data both before and after switching needs to be processed. System control unit  111  directs transport decoding unit  102  to add setting for outputting video packets in the high hierarchical service before switching, and setting for outputting video packets in the low hierarchical service after switching (step  504 ). Then, system control unit  111  directs transport decoding unit  102  to add setting for accumulating video data in the high hierarchical service before switching in first video buffer  1041 , and setting for accumulating video data in the low hierarchical service after switching in second video buffer  1042  (step  505 ). 
     Video/audio data detecting unit  103  of transport decoding unit  102  determines whether an IDR frame has been detected from video data in the low hierarchical service after switching (step  506 ). If an IDR frame has not been detected, the control flow returns to step  506  to repeat the determining operation. Meanwhile, if video/audio data detecting unit  103  detects an IDR frame in the low hierarchical service after switching from the video data (Yes), video/audio data detecting unit  103  informs system control unit  111  of the detection of the IDR frame and acquires the PTS of the IDR frame from the video data (step  507 ). 
     Then, buffer accumulation determining unit  113  of system control unit  111  that has received the notice of the detection of an IDR frame determines that video data both before and after switching does not need to be processed any longer. System control unit  111  directs transport decoding unit  102  to change setting from for outputting video packets for both before and after switching to that in the low hierarchical service after switching (step  508 ). Then, system control unit  111  changes setting from for accumulating video data both before and after switching in first video buffer  1041  and second video buffer  1042 , respectively, to that in the low hierarchical service after switching in second video buffer  1042  (step  509 ). 
     Next, video decoder  106  determines whether video data before switching accumulated in video buffer  1041  has been emptied (step  510 ). If video data before switching has not been emptied (No), the control process returns to step  510  to repeat the determining operation. Meanwhile, if video data before switching has been emptied (Yes), video decoder  106  informs system control unit  111  that video data before switching has been emptied. 
     Then, decoding continuation determining unit  112  of system control unit  111  that has received the notice determines to stop video decoding and video output. System control unit  111  directs video decoder  106  to stop video decoding in the high hierarchical service before switching and directs video output unit  108  to stop video output, based on the determination (step  511 ). Subsequently, system control unit  111  directs to start video decoding in the low hierarchical service after switching (step  512 ). 
     Next, system control unit  111  determines whether the STC has exceeded the PTS of the IDR frame (step  513 ). If the STC has not exceeded the PTS of the IDR frame (No), the control flow returns to step  513  to repeat the determining operation. Meanwhile, if the STC has exceeded the PTS of the IDR frame (Yes), system control unit  111  directs video output unit  108  to start video output of an IDR frame (step  514 ). After that, video decoding in the low hierarchical service is continued (step  515 ). 
       FIG. 7  is an explanatory drawing of the effect of time reduction in the hierarchical switching by digital broadcast receivers  100  and  400  according to the embodiment of the present invention, where the horizontal axis as the time axis is represented with a system clock.  FIG. 7  additionally shows operation timing diagram  705  in hierarchical switching by digital broadcast receiver  400  according to the third embodiment. 
     As described above using the flowchart of  FIG. 5 , digital broadcast receiver  400  according to the third embodiment continues accumulating video data, video decoding, and video output before switching, even after determining (the time point shown by broken line  960 , step  504  in  FIG. 5 ) hierarchical switching. Then, digital broadcast receiver  400  stops accumulating video data after detecting an IDR frame after switching the hierarchical service, and stops video decoding and video output after detecting that video data before switching accumulated in the video buffer has been emptied (the time point shown by broken line  963 , step  511  in  FIG. 5 ). 
     Then, digital broadcast receiver  400  determines whether the STC has exceeded the PTS of the IDR frame by system control unit  111 . If the STC has exceeded the PTS of the IDR frame, system control unit  111  directs video output unit  108  to start video output of an IDR frame (the time point shown by broken line  965 , step  514  in  FIG. 2 ). Controlling in this way allows digital broadcast receiver  400  according to the third embodiment to reduce time during which output of video and audio is interrupted at hierarchical switching. Specifically, time during which output of video and audio is interrupted at hierarchical switching can be reduced by the period shown by broken lines  960  and  963 . That is, the third embodiment is more advantageous than the second in reducing time during which output of video and audio is interrupted at hierarchical switching. 
     As described above, system control unit  111  of digital broadcast receiver  400  according to the third embodiment includes: digital broadcast receiving unit  101  receiving digital broadcast containing at least two hierarchical services; transport decoding unit  102  decoding digital broadcast received by digital broadcast receiving unit  101  to output video packets in high and low hierarchical services; first video buffer  1041  accumulating video packets in the high hierarchical service output from transport decoding unit  102 ; second video buffer  1042  accumulating video packets in the low hierarchical service output from transport decoding unit  102 ; video decoder  106  decoding video packets accumulated in first video buffer  1041  or second video buffer  1042 ; hierarchical switching determining unit  110  determining hierarchical service to be received from a reception state of the digital broadcast received by digital broadcast receiving unit  101 ; and system control unit  111  controlling hierarchical service for video packets output from transport decoding unit  102 , based on the hierarchical service determined by hierarchical switching determining unit  110 , and controlling operation of video decoder  106 . Then, system control unit  111  stops decoding video packets before switching by video decoder  106  when a video buffer before switching out of first video buffer  1041  and second video buffer  1042  is emptied, when transport decoding unit  102  detects an IDR frame as given data, after hierarchical switching determining unit  110  has determined switching the hierarchical service. Controlling in this way allows digital broadcast receiver  400  according to the third embodiment to reduce time during which output of video and audio is interrupted at hierarchical switching. 
     Further, for hierarchical switching of audio, the present invention is applicable in the completely same procedure if an IDR frame as given data is replaced with an ADTS header by H.264 MPEG-4 AVC method. That is, as described above, digital broadcast receiver  400  is equipped with first audio buffer  1051  accumulating audio packets in high hierarchical service; second audio buffer  1052  accumulating audio packets in low hierarchical service; and audio decoder  107  decoding audio packets accumulated in first audio buffer  1051  or second audio buffer  1052 . Then, transport decoding unit  102  decodes digital broadcast received by digital broadcast receiving unit  101  to output audio packets in the high hierarchical service and the low one to first audio buffer  1051  and second audio buffer  1052 , respectively. Then, system control unit  111  stops decoding audio packets before switching by audio decoder  107  when a video buffer before switching out of first video buffer  1051  and second video buffer  1052  is emptied, when transport decoding unit  102  detects an ADTS header by MPEG-2 AAC method as given data, after hierarchical switching determining unit  110  has determined switching the hierarchy. Controlling in this way allows digital broadcast receiver  400  according to the first embodiment to reduce time during which output of video and audio is interrupted at hierarchical switching. 
     As described above, the present invention is applicable to the third embodiment in the completely same procedure for hierarchical switching of video and audio from low hierarchical service to high one. That is, the present invention is applicable to switching from low hierarchical service to high one in the completely same procedure by replacing an IDR frame as given data with an I frame by MPEG-2 video method. The third embodiment is more advantageous than the second in reducing time during which output of video and audio is interrupted at hierarchical switching. 
     Fourth Exemplary Embodiment 
     In the fourth exemplary embodiment, the block diagram showing the configuration is  FIG. 4  similarly to the third embodiment.  FIG. 6  is a flowchart for illustrating hierarchical switching operation by digital broadcast receiver  400  according to the fourth embodiment. The fourth embodiment is different from the third in that system control unit  111  directs video decoder  106  to decode video in the high hierarchical service before switching and directs video output unit  108  to output video until the time point that is the PTS minus first given time a (a time period required for video decoder  106  to decode video of an IDR frame after switching). Hence, a detailed description is omitted for a component and its operation same as those in the third embodiment. 
     First, a description is made of hierarchical switching operation for video from high hierarchical service to low one using  FIGS. 4 and 6 . The operation is common with the third embodiment from when digital broadcast receiver  400  is decoding high hierarchical service (step  601 ), until when video/audio data detecting unit  103  detects an IDR frame after switching and acquires the PTS (step  607 ). 
     After that, determination is made whether the STC has exceeded the time point that is the PTS of an IDR frame minus first given time α (a time period required for the video decoder to decode video of an IDR frame) (step  608 ). If the STC has not exceeded the time point that is the PTS of an IDR frame minus first given time α (No), the control flow returns to step  608  to repeat the determining operation. Meanwhile, if detected that the STC has exceeded the time point that is the PTS of an IDR frame minus first given time α (Yes), buffer accumulation determining unit  113  of system control unit  111  determines that video data both before and after switching does not need to be processed any longer. At this moment, system control unit  111  directs transport decoding unit  102  to change setting from for outputting video packets for both before and after switching to that in the low hierarchical service after switching (step  609 ). Consequently, system control unit  111  changes setting from for accumulating video data both before and after switching in first video buffer  1041  and second video buffer  1042 , respectively, to that in the low hierarchical service after switching in second video buffer  1042  (step  610 ). 
     Further, decoding continuation determining unit  112  of system control unit  111  determines to stop video decoding and video output at this time. System control unit  111  directs video decoder  106  to stop video decoding in the high hierarchical service before switching and directs video output unit  108  to stop video output based on the determination (step  611 ). Subsequently, system control unit  111  directs to start the low hierarchical service after switching (step  612 ). Determination is made whether the STC has exceeded the PTS of the IDR frame (step  613 ). 
     If the STC has not exceeded the PTS of the IDR frame, (No), the control flow returns to step  613  to repeat the determining operation. Meanwhile, if detected that the STC has exceeded the PTS of the IDR frame (Yes), system control unit  111  directs video output unit  108  to start video output of an IDR frame (step  614 ). After that, video decoding in the low hierarchical service is continued (step  615 ). 
       FIG. 7  is an explanatory drawing of the effect of time reduction in hierarchical switching by digital broadcast receiver  400  according to the fourth embodiment of the present invention, where the horizontal axis as the time axis is represented with a system clock.  FIG. 7  additionally shows operation timing diagram  706  in hierarchical switching by digital broadcast receiver  400  according to the fourth embodiment. 
     As described above using the flowchart of  FIG. 6 , digital broadcast receiver  400  according to the fourth embodiment continues video data accumulation, video decoding, and video output before switching, even after determining hierarchical switching when switching from the high hierarchical service to the low one. Then, digital broadcast receiver  400  stops data accumulation, video decoding, and video output before switching at a given time point that is the PTS minus a time period required to decode video of an IDR frame after switching (the time point shown by broken line  964 , step  611  in  FIG. 6 ). This reduces time during which output of video and audio is interrupted at hierarchical switching. Specifically, time during which output of video and audio is interrupted at hierarchical switching can be reduced by the period shown by broken lines  960  and  964 . That is, the fourth embodiment is more advantageous than the third in reducing time during which output of video and audio is interrupted at hierarchical switching. 
     As described above, digital broadcast receiver  400  according to the fourth embodiment includes: digital broadcast receiving unit  101  receiving digital broadcast containing at least two hierarchical services; transport decoding unit  102  decoding digital broadcast received by digital broadcast receiving unit  101  to output video packets for high and low hierarchical services; first video buffer  1041  accumulating video packets in the high hierarchical service output from transport decoding unit  102 ; second video buffer  1042  accumulating video packets in the low hierarchical service output from transport decoding unit  102 ; video decoder  106  decoding video packets accumulated in first video buffer  1041  or second video buffer  1042 ; hierarchical switching determining unit  110  determining hierarchical service to be received from a reception state of the digital broadcast received by digital broadcast receiving unit  101 ; and system control unit  111  controlling hierarchical service for video packets output from transport decoding unit  102 , based on the hierarchical service determined by hierarchical switching determining unit  110 , and controlling operation of video decoder  106 . Then, system control unit  111  stops decoding video packets before switching by video decoder  106  at the given time point, after hierarchical switching determining unit  110  has determined to switch the hierarchical service. Then, transport decoding unit  102  has detected an IDR frame as given data. Here, the given time is the time point that is the PTS of the IDR frame minus first given time α. First given time α is a time period required for video decoder  106  to decode video of an IDR frame by H.264 MPEG-4 AVC method. Controlling in this way allows digital broadcast receiver  400  according to the fourth embodiment to reduce time during which output of video and audio is interrupted at hierarchical switching. 
     Further, for hierarchical switching of audio, the present invention is applicable in the completely same procedure if an IDR frame as given data showed in  FIG. 7 , is replaced with an ADTS header by H.264 MPEG-4 AVC method. That is, as described above, digital broadcast receiver  400  is equipped with first audio buffer  1051  accumulating audio packets in high hierarchical service; second audio buffer  1052  accumulating audio packets in low hierarchical service; and audio decoder  107  decoding audio packets accumulated in first audio buffer  1051  or second audio buffer  1052 . Then, transport decoding unit  102  decodes digital broadcast received by digital broadcast receiving unit  101  to output audio packets in the high hierarchical service and the low one to first audio buffer  1051  and second audio buffer  1052 , respectively. System control unit  111  stops decoding audio packets by the audio decoder  107  at the given time point, after hierarchical switching determining unit  110  has determined to switch the hierarchical service. Then, transport decoding unit  102  has detected an IDR frame as given data. Here, the given time is the time point that is the PTS of the IDR frame minus second given time β. Second given time  3  is a time period required for audio decoder  107  to decode audio of an ADTS header by MPEG-2 AAC method. Second given time  13  for audio decoder  107  corresponds to first given time α for video decoder  106 . Controlling in this way allows digital broadcast receiver  400  according to the fourth embodiment to reduce time during which output of video and audio is interrupted at hierarchical switching. 
     As described above, the present invention is applicable to the fourth embodiment in the completely same procedure for hierarchical switching of video and audio from low hierarchical service to high one. The fourth embodiment is more advantageous than the third in reducing time during which output of video and audio is interrupted at hierarchical switching. 
     INDUSTRIAL APPLICABILITY 
     A digital broadcast receiver of the present invention offers an advantage in that time during which output of video and audio is interrupted at hierarchical switching can be reduced, and is industrially applicable to a digital broadcast receiver allowing stress-free, continuous viewing at hierarchical switching.