Patent Publication Number: US-2009241163-A1

Title: Broadcast picture display method and a digital broadcast receiver using the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a digital broadcast receiver. More particularly, the present invention relates to an apparatus and method for displaying broadcast data during a channel switching period. 
     2. Description of the Related Art 
     A digital broadcast receiver is an apparatus that restores digital data that is received from a broadcast signal transmitted by a broadcast station. The digital data is coded and modulated so as to be broadcast in the form of a broadcast signal by the broadcast station. The digital broadcast receiver demodulates and decodes the received broadcast signal in order to restore the original digital data. Accordingly, the digital broadcast receiver is provided with a tuner, a demodulator, and a decoder. Digital broadcast systems are classified into a digital multimedia broadcast (DMB) system and a digital video broadcasting (DVB) system. These digital broadcast systems provide a plurality of service channels in a frequency bandwidth and each service channel is structured with multiplexed sub-channels of video, audio, and program information data. 
     Currently, mobile terminals provide an enhanced multimedia playback function by using a dedicated multimedia processor, particularly for a digital broadcast playback function. As used herein, the term “mobile terminals” includes mobile phones, laptop computers, Personal Digital Assistants (PDAs), Portable Multimedia Players (PMPs) and the like. Such a mobile terminal may also be provided with a radio frequency unit for supporting radio communication. 
     In a conventional digital broadcast receiver, the pictures represented by received broadcast signals are displayed in a playback mode. However, pictures are not displayed during a channel switching period even though the playback mode is selected. That is, the conventional digital broadcast receiver cannot receive the broadcast signals during the channel switching period such that no picture is displayed on the screen of the digital broadcast receiver. This causes an inconvenience for the user. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to address the above-mentioned problems and/or disadvantages and to provide at least the advantages as described below. Accordingly, an aspect of the present invention is to provide a broadcast picture display method for a digital broadcast receiver that is capable of displaying a broadcast picture represented by broadcast signals received first on a switched service channel as a still picture during a channel switching period. 
     Another aspect of the present invention is to provide a broadcast picture display method for a digital broadcast receiver that is capable of displaying, when a preset amount of broadcast signals are buffered, broadcast pictures represented by broadcast signals buffered on a switched service channel before receiving a synchronization signal by preset time interval during a channel switching period. 
     Still another aspect of the present invention is to provide a broadcast picture display method for a digital broadcast receiver that is capable of displaying a broadcast picture received at first on a switched service channel as a still picture and, when a preset amount of broadcast signals are buffered, displaying broadcast pictures represented by the buffered broadcast signals by preset time interval during a channel switching period, and then, when a synchronization signal is received, displaying broadcast pictures in synchronization with audio. 
     In accordance with an aspect of the present invention, a broadcast picture display method for a digital broadcast receiver is provided. The method includes setting a switched-service channel in a channel switching period, buffering broadcast data received on the switched-service channel and displaying specific video data detected in the broadcast data. 
     In accordance with another aspect of the present invention, a broadcast picture display method for a digital broadcast receiver is provided. The method includes setting a switched-service channel in response to a channel switching request in a channel switching period, displaying specific video data detected on the switched-service channel as a still picture, playing video and audio data buffered after the specific video data with reference to a local time of the digital broadcast receiver, and playing, when timestamp information is received, the video and audio data buffered after the timestamp information in synchronization with each other, the synchronization being acquired by compensating a time offset between the video and audio data. 
     In accordance with yet another aspect of the present, a broadcast picture display method for a digital broadcast receiver is provided. The method includes setting a service channel in response to a broadcast playback request, displaying specific video data detected on the service channel as a still picture, playing video and audio data buffered after the specific video data with reference to a local time of the digital broadcast receiver and playing, when timestamp information is received, the video and audio data buffered after the timestamp information in synchronization with each other, the synchronization being acquired by compensating a time offset between the video and audio data. 
     Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram illustrating a configuration of a digital broadcast receiver according to an exemplary embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating an exemplary broadcast reception unit of the digital broadcast receiver (DVB-H receiver) of  FIG. 1 ; 
         FIG. 3  is a block diagram illustrating an exemplary protocol processing unit of the digital broadcast receiver of  FIG. 1 ; 
         FIGS. 4A-C  are diagrams illustrating an exemplary data frame format of a transport stream for a DVB-H system; 
         FIG. 5  is a diagram illustrating an exemplary structure of a physical channel of a DVB-H system; 
         FIG. 6  is a flowchart illustrating a display method for a digital broadcast receiver according to an exemplary embodiment of the present invention; 
         FIG. 7  is a flowchart illustrating an exemplary I-frame display method of  FIG. 6  in more detail; 
         FIG. 8  is a flowchart illustrating an exemplary broadcast data playback step in more detail; and 
         FIG. 9  is a block diagram illustrating a configuration of a mobile terminal equipped with a digital broadcast receiver according to an exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
     In order to enable a clear and consistent understanding of the following description and claims, basic information on the following terms is provided. Unless otherwise noted, terms are to be understood according to conventional usage by those skilled in the relevant art. 
     In the following description, exemplary broadcast picture display methods of the present invention are described with reference to a digital broadcast receiver and a mobile terminal equipped with a digital broadcast receiver. Digital broadcast systems include Digital Multimedia Broadcasting (DMB), Digital Video Broadcasting (DVB), and Media Forward Link Only (Media FLO) systems. Furthermore, DVB is classified into DVB Terrestrial (DVB-T) and DVB Handheld (DVB-H). The DVB-H broadcasts data using Internet Protocol (IP). 
     An exemplary broadcast picture display method of the present invention is described in association with a DVB-H system. However, the present invention is not limited to DVB-H systems but can be adapted to mobile terminals supporting other systems such as DVB, DMB, and Media FLO services. 
     The following definitions are provided to enable a clear and consistent understanding of the description and claims below. 
     A “channel” is a frequency channel selected by a tuner, a “service” is a logical channel assigned a program identifier or product identifier (PID) for a broadcast service, and an “event” means a program provided through the service channel. In order to avoid complication, the term “physical channel” is used for representing the “channel”, and the term “service channel” is used for representing the “service.” The service channels are multiplexed in the physical channel. The physical channel can be represented by the frequency band set by the tuner, and the service channels are represented by the broadcast stations or service providers. The service channel can be identified with a PID in the DMB and DVB-T systems. In the case of DVB-H, the service channel is identified with a combination of PID, IP address, and port number. In the following exemplary embodiments, certain frames are composed of frame video data that are coded without reference to other frame video data except themselves or with reference to other frame video data once. Such a frame is called an Intra (I) frame. 
       FIG. 1  is a block diagram illustrating a configuration of a digital broadcast receiver according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , the digital broadcast receiver includes a broadcast reception unit  110 , a protocol processing unit  120 , a decoding unit  130 , a display unit  150 , a speaker unit  155 , a memory unit  160 , a key input unit  170 , and a control unit  100 . 
     The control unit  100  controls general operations of the digital broadcast receiver. The key input unit  170  generates commands in response to key manipulation and transmits the commands to the control unit  100 . Particularly, the key input unit  170  is configured to generate commands such as a channel selection command, a playback command, a recording command, and a channel switching command. In an exemplary embodiment, if a channel switching command is input at the beginning of a playback mode or during the playback mode, the control unit  100  controls to display the broadcast picture of a switched service channel. That is, the control unit  100  controls switching broadcast pictures during the channel switching period. 
     The memory unit  160  includes a program memory for storing an application program for controlling the digital broadcast receiver and a data memory for storing application data produced while the application programs are executed. The program memory also stores an application program for quickly displaying a broadcast picture of a switched service channel in response to the channel switching command or a broadcast playback mode activation command. 
     The broadcast reception unit  110  receives broadcast streams on the service channel selected by the user under the control of the control unit  100 . The protocol processing unit  120  performs protocol processing on the broadcast streams so as to separately extract audio, video, and broadcast information data from the service channel. In the case of DVB-H, the protocol processing unit  120  decapsulates IP datagrams, and extracts the audio, video, and program data from the IP datagrams. In the case of DMB and DVB-T, the protocol processing unit  120  analyzes a transport stream (TS) and demultiplexes the audio and video data from the TS. 
     The decoding unit  130  is provided with an audio decoder, a video decoder, and a data decoder. The audio decoder performs decoding on the audio data and outputs the decoded audio data. The video decoder performs decoding on the video data and outputs the decoded video data. The data decoder performs decoding on the broadcast information data. In an exemplary embodiment, the decoding unit  130  is provided with the audio and video decoders, and the data decoder is implemented in the control unit  100  in the form of software. 
     In an exemplary embodiment, the digital broadcast receiver is configured to display, when channel switching is requested in the broadcast playback mode, the broadcast picture of a switched service channel. 
     In the case of a conventional digital broadcast receiver, when a user switches service channels, it takes a few seconds for the digital broadcast receiver to normally display the broadcast data received on the switched service channel (which is called channel switching delay). The channel switching delay occurs because the digital broadcast receiver starts decoding the broadcast signals by a preset amount. In some devices, in order to moderate the inconvenience caused by the channel switching delay, the digital broadcast receiver is configured to display a screen that indicates channel switching is occurring or display a preset image during the channel switching period. Also, since the digital broadcast data is composed of audio, video, and broadcast information that is synchronized in time, the conventional digital broadcast receiver performs a synchronization process for acquiring synchronization among the received audio, video, and broadcast information data. This synchronization causes an increase in the channel switching delay time. 
     In an exemplary embodiment, if service channel switching is requested, the control unit  100  determines a specific frame of video data received on the switched service channel and displays a picture represented by the specific frame video data on the screen of the display unit  150  while buffering the received broadcast data. Thus, it is possible to quickly display the picture represented by the broadcast data of the switched service channel. The picture represented by the specific frame can be displayed until a preset amount of broadcast data is buffered. If the received broadcast data reaches the preset amount, the control unit  100  controls to start playing video and audio data extracted from the buffered broadcast data. In an exemplary embodiment, the audio and video data may be played before the synchronization control signal (i.e., “timestamp”) is received. Also, the non-synchronized audio and video data playbacks are maintained until the timestamp is received. Upon receiving the timestamp, the control unit  100  starts playing the audio and video data in synchronization with each other. Here, the specific frame is the I-frame which will be discussed in detail below. 
     The buffer size is preferably set in the range of 1 to 3 seconds. The timestamp may include a decoding timestamp (DTS) for synchronizing decoding start times and a presentation timestamp (PTS) for controlling the output times of the decoded audio and video data. The timestamp may include at least one of DTS and PTS such that the synchronization of the audio and video data can be acquired using the audio and video DTSs or audio and video PTSs. 
     When switching between service channels, the control unit  100  inspects the broadcast data of the switched service channel output by the protocol processing unit  120 . At this time, if the received video data represents a specific frame, the control unit  100  controls the decoding unit  130  to decode the video data of the specific frame and controls the display unit  150  to display the video data. Here, the decoded video data frame can be composed of an Infra frame (I-frame), a Previous frame or Predicted frame (P-frame), and a Bidirectional frame (B-frame). 
     Because the I-frame can be decoded independently of other frames, it is preferred to use the I-frame as the specific frame which is displayed as a still picture. In contrast, a P-frame is a video frame encoded with dependence to a closest past frame (here, I-frame), which can be an I-frame. And a B-frame is a video frame encoded relative to the past frame, future frame, or both (I- and P-frames). 
     While the I-frame video data is displayed, the control unit  100  controls the decoding unit  130  to buffer the broadcast data which is continued to be received on the switched service channel in its input buffer. If the buffered broadcast data reaches the preset amount, the control unit  100  controls the audio decoder and video decoder to decode the received audio and video data such that they are output through the speaker unit  155  and the display unit  150 , respectively. 
     In an exemplary implementation, the buffer size is set to an amount of data to be played in the range of 1 to 3 seconds. The DVB system using the Phase Alternating Line (PAL) scheme processes 25 frames per second and the DMB system using the National Television System Committee (NTSC) scheme processes about 30 frames per second. Accordingly, in the case of a DVB system, it is preferred to set the buffer size to a data amount corresponding to 25 to 75 frames. The buffered broadcast data can be played, for example, in the form of motion pictures. In this case, the broadcast data can be output through the display unit  150  and the speaker unit  155  without synchronization to each other. 
     If a timestamp is received while buffering the broadcast data, the control unit  100  acquires synchronization between the audio and video data using an offset of the audio and video timestamps. Once the audio and video are synchronized with each other, the audio and video data are played normally. 
     In an exemplary embodiment, the channel switching is performed with a two- or three-step intermission picture display process. In the case of a two-step intermission picture display process, the second step can be skipped. In this case, the control unit  100  controls to capture and display the first I-frame received in the service channel switching period. Next, the control unit  100  synchronizes the audio and video frame using the audio and video timestamp offsets and then plays audio and video data in synchronization with each other. 
       FIG. 2  is a block diagram illustrating an exemplary broadcast reception unit of a digital broadcast receiver, in this example a DVB-H receiver,  FIG. 3  is a block diagram illustrating an exemplary protocol processing unit of a digital broadcast receiver,  FIGS. 4A-F  are diagrams illustrating an exemplary data frame format of a transport stream for a DVB-H system, and  FIG. 5  is a diagram illustrating an exemplary structure of a physical channel of a DVB-H system. 
     Referring to  FIGS. 4A-F , the broadcast data of a DVB-H system is structured in the form of a Motion Picture Experts Group 2 transport stream (MPEG2-TS) The MPEG2-TS format is implemented as a packet stream, wherein each packet contains 4 bytes of packet header and 184 bytes of payload. The packet header contains packet synchronization and packet identification (PID) information. The PID can be used as a service identifier for identifying a service channel and program data contained in the payload. The payload consists of multi protocol encapsulation (MPE) sections. Each MPE section includes a table identifier (table_ID), MPE forward error correction (MPE-FEC) information for correcting errors of the received data, and information for dividing the received data in time. Each MPE section also contains at least one IP datagram. In  FIGS. 4A-F , an IPv6 datagram is depicted as an example. The IP datagram includes an IP version information, as well as a source IP address and a destination IP address. The IP datagram consists of user datagram protocol (UDP) units and each UDP unit includes port addresses of the transmitter and receiver. Using the destination IP address, the broadcast service can be provided to a specific IP user in an IP datacast rather than the broadcast. The UDP unit contains FLUTE/ALC units and a real time transport protocol (RTP) unit. The FLUTE/ALC unit includes the ESG and files, and the RTP unit includes audio and video data. 
     Referring to  FIG. 5 , a frame consists of a plurality of service channels that are multiplexed using a time division scheme. The frame has a duration (Δt) and is sliced into 10 timeslots. Each time slot is assigned for a service channel. In an exemplary embodiment, the term “burst time” is interchangeably used with the frame duration. In this manner, the service channels are transmitted in the frame duration. 
     In  FIG. 5 , 10 service channels, e.g. CH 1  to CH 10 , are multiplexed in a frame and channel  3  (CH 3 ) is selected as a current service channel. In this case, the timeslot represented by the channel  3  is a burst-on time and the timeslots represented by channel  1 , channel  2 , and channel  4  to channel  10  become burst-off time. The broadcast data of the service channels are transmitted at the respective timeslots for corresponding service channels. When using the time division scheme, the broadcast reception unit  110  can predict the burst time of the current service channel and the burst times of other service channels. In an exemplary embodiment, the broadcast reception unit  110  receives the broadcast data through the channel  3  (CH 3 ) and the broadcast data are decoded for the duration At that is substantially identical with the frame length. The decoded data are output through the display unit  150  as a viewable image. Such a timeslot based-power supply and data decoding technique is called “time division technique” and such a timing control method is called “time division control.” 
     Referring to  FIG. 2 , an exemplary broadcast reception unit  110  includes a tuner  200  and a broadcast data demodulator. The broadcast data demodulator includes an analog/digital (A/D) converter  210 , a demodulator  220 , a PID filter  230 , and a demodulation controller  240 . The tuner  200  is tuned to the physical channel of the DVB-H so as to receive the service channels constituting the physical channel. The tuner  200  includes a phase-locked loop (PLL) circuit for generating a frequency for the physical channel, a mixer for mixing the received signal and the signal generated by the PLL, and a band-pass filter for passing the frequency of the physical channel. 
     The demodulation controller  240  controls the tuner  200  to be tuned to the physical channel and controls the PID filter  230  to be set with PIDs of the primary and secondary service channels, under the control of the control unit  100 . At this time, the control unit  100  analyzes a Program Specific Information/Service Information (PSI/SI) output from the broadcast reception unit  110  and Service Description Table (SDT) information included in an Electronic Service Guide (ESG) so as to extract the PIDs, IP addresses, and port numbers associated with the selected service channels. If the PID filtered by the PID filter  230  is a Network Information Table (NIT) PID, Service Description Table (SDT) PID, or Event Information Table (EIT) PID, the control unit  100  can determine the physical channel and PIDs of the service channels from the PSI/SI carried in the tables. The protocol processing unit  120  extracts SDT from the ESG (see  FIG. 4E ) and the control unit  100  can determine the PIDs, IP addresses, and port numbers for indentifying the service channels and video and audio data. The control unit  100  determines the PIDs, IP addresses, and port numbers of the service channels and audio and video data of each service channel on the basis of the information of the SDT. After the service channels are determined, the control unit  100  outputs control signals to the demodulation controller  240  for filtering the physical channel and PIDs of the service channels. 
     The demodulation controller  240  sets the tuner  200  with the frequency of the DVB-H and sets the PID filter  230  with the PIDs of the primary and secondary service channels. The tuner  200  is tuned to the DVB-H frequency to receive the broadcast signals, the A/D converter  210  converts the broadcast signals into digital broadcast streams, and the demodulator  220  demodulates the broadcast streams to recover the original broadcast data. The demodulator  220  can be implemented with an Orthogonal Frequency Division Multiplexing (OFDM) demodulator or a Coded OFDM (COFDM) demodulator  220 . The broadcast streams output by the demodulator  220  can be the MPEG2-TS (see  FIG. 4A ) and each TS packet, in this case, includes a PID for identifying the service channel carried by the TS packet. The PID filter  230  filters the broadcast data having the PIDs of the primary and secondary service channels from the IP datagram (see  FIG. 4C ) and transmits the PSI/SI information to the control unit  100 . The broadcast data passed through the PID filter  230  can include the MultiProtocol Encapsulation-Forward Error Correction (MPE-FEC) sections and time division information. In this case, the demodulation controller  240  performs time division control on the burst data. That is, the demodulation controller  240  controls the tuner  110  and demodulator  220  to be switched on using the time division information. The time division information includes the burst-on time information of the primary and secondary service channels such that the demodulation controller  240  controls the tuner  200  and the demodulator  220  to be switched on at the burst-on times and switched off at the burst-off times. Also, the demodulation controller  240  performs the MPE-FEC functions on the broadcast data of the primary and secondary service channels output by the PID filter  230  using the MPE section information (see  FIG. 4B ). 
     As described above, the demodulation controller  240  controls the tuner  200  to be tuned to the DVB-H frequency and the PID filter  230  to be set with the PIDs of the primary and secondary service channels. Also, the demodulation controller  240  performs the time division to reduce power consumption of the digital broadcast receiver on the basis of the information of the MPE section and performs the MPE-FEC function for correcting reception errors. The data output by the demodulation controller  240  is the IP datagram ( FIG. 4C ). 
     In the above-structured digital broadcast receiver, the tuner  200  is tuned to the DVB-H frequency, the A/D converter  210  converts the broadcast signal received through DVB-H frequency into digital broadcast streams, and the demodulator  220  demodulates the digital broadcast streams in the OFDM or COFDM scheme. The demodulated broadcast stream has the MPEG2-TS packet stream format (see  FIG. 4A ) of which each TS packet includes a PID for identifying the service channel. The PID filter  230  determines the PIDs of TS packets and passes only the packet having the PIDs of PSI/SI and the primary and secondary service channels. The data carried by the packet having the PIDs associated with the PSI/SI are forwarded to the control unit  100 , and the broadcast data of the primary and secondary service channels and the broadcast information data including ESG are output to the demodulation controller  240 . The demodulation controller  240  analyzes the MPE sections (see  FIG. 4B ) and performs time division control and error corrections on the data of the PID filtered service channels. 
       FIG. 3  is a block diagram illustrating a configuration of a protocol processing unit of a DVB-H receiver according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 3 , the protocol processing unit  120  determines the IP and other protocol information of the selected service channel data and extracts the video and audio data from the service channel data. The decoding unit  130  includes a video decoder  500  and an audio decoder  590  such that the video decoder  500  decodes the video data output from the protocol processing unit  120  and outputs the video data through the display unit  150  as a viewable image. Similarly, the audio decoder  590  decodes the audio data output from the protocol processing unit  120  and outputs the audio data through the speaker unit  155  in the form of audible sound wave. 
     The protocol processing unit  120  includes an IP decapsulator  310 , a UDP decapsulator  320 , a FLUTE deliverer  330 , and an RTP deliverer  340 . The IP decapsulator  310  extracts an IP datagram of  FIG. 4C  from the MPE section of  FIG. 4B  and decapsulates the IP datagram to determine the source and destination IP addresses. The UDP decapsulator  320  extracts a UDP unit of  FIG. 4E  from the IP datagram and decapsulates the UDP unit to determine the source and destination port numbers. If the data carried by the UDP unit is FLUTE/ALC protocol data, the UDP decapsulator  320  transports the FLUTE/ALC protocol data to the FLUTE deliverer  330 . If the data carried by the UDP unit is RTP data, the UDP decapsulator  320  transports the RTP data to the RTP deliverer  340 . The FLUTE/ALC protocol data may include ESG or other type of data such as XML, SDT, HTML, JPG, and POL. The RTP data may include the audio and video data. The RTP deliverer  340  performs demultiplexing of the RTP data so as to output the video data to the video decoder  500  and the audio data to the audio decoder  590 . 
     The protocol processing unit  120  operates under the control of the control unit  100 . The control unit  100  can integrate an ESG engine (XML engine and ESG decoder), an SDT parser, and a PSI/SI decoder. The control unit  100  also can integrate a protocol information controller and manager for controlling the protocol process and management. The control unit  100  processes the protocol information and data received from the protocol processing unit  120 . That is, the control unit  100  analyzes the PSI/SI table (NIT, SDT, and EIT) extracted by the broadcast reception unit  110  so as to determine the PSI/SI according to the MPEG-2 and DVB-SI standards, parses the ESG data received from the protocol processing unit  120 , and then controls the overall operation of the broadcast reception unit  110  on the basis of this information. The service channels, ESG per service channel, and audio and video data are identified using the PID, IP, port information. That is, the PSI/SI and SDT is provided with tables defining information on the service channel identifiers, audio and video identifiers, and ESG identifiers. The control unit  100  can identify the service channels, audio data, video data, and ESG data with reference to the decoding result of the PSI/SI and the SDT. The control unit  100  may integrate the protocol processing unit  120 . 
     The control unit  100  also controls the operation of the protocol processing unit  120 . Typically, the MPEG-TS stream carries audio and video data. That is, the burst data is composed of the video and audio data. Accordingly, the control unit  100  analyzes the types of data received through the service channel and controls the internal elements to cooperate with each other according to the analysis result. That is, if an MPE section is received, the control unit  100  controls the demodulation controller  240  to receive the burst information such that the demodulation controller  240  analyzes the MPE section data and performs time division and MPE-FEC functions according to the analysis result. If an IPv6 datagram is contained in the MPE section, the control unit  100  controls the IP decapsulator  310  to extract the IP information. If a UDP unit is detected from the IPv6 datagram, the control unit  100  controls the UDP decapsulator  320  to extract the port numbers. If a FLUTE/ALC unit is contained in the UDP unit, the control unit  100  controls the FLUTE deliverer  330  for processing the ESG and equivalent file formats. If the RTP unit is contained in the UDP unit, the control unit  100  controls the RTP deliverer  340  for processing the RTP unit. The video and audio data output from the RTP deliverer  340  are delivered to the video decoder  500  and the audio decoder  590 , respectively. The control unit  100  analyzes the data and activates internal processing blocks required for processing the data. Other internal elements that are not involved in the data processing bypass the data, resulting in improvement of data processing speed. 
     The video and audio data are output from the protocol processing unit  120  after being demultiplexed such that the video decoder  500  can decode the video data and output the decoded video data through the display unit  150  as a visual image, and the audio decoder  590  can decode the audio data and output the decoded audio data through the speaker unit  155  in the form of audible sound. The video decoder can be an H.264 standard decoder or an MPEG series decoder, and the audio decoder can be an advanced audio coding (AAC) decoder. 
     In the case of DMB, the broadcast reception unit can be implemented with only the tuner  200  and demodulator  220 . 
     In this case, the broadcast reception unit outputs the packet streams in the structure of MPEG-TS. Also, the protocol processing unit  120  is composed of a demultiplexer. The demultiplexer analyzes the PID contained in the packet header of the received packet stream, selects the packets on the selected service channel on the basis of the PID, and determines the selected packets have the PES header. The PES header includes information indicating audio and video data, whereby the demultiplexer can distinguish the audio and video data packets from each other. Also, the PES header includes timestamp information so as to adjust the time offsets of the audio and video data. In the case of video packet, it is possible to determine whether the video data is I-frame or P-frame with reference to the PES header. 
     The operation of a DVB-H receiver for displaying the broadcast pictures during the channel switching period is described hereinafter. 
     The broadcast data output from the RTP deliverer  340  may include audio and video data, or timestamp and variable length of data. The variable length of data may be the audio and video data. The video data can be classified into I- and P-frame video data. In the case that the video data is coded using an H.264 coding scheme, the packet is composed of an 8-bit network abstraction layer (NAL) header and variable length of video data as shown in table 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 NAL header 
                 Data 
               
               
                   
                   
               
            
           
         
       
     
     The NAL header consists of 3-bit priority and 5-bit NAL type. By analyzing the NAL type information, it is possible to predict whether the next frame is an I-frame or a P-frame. If a channel switching command is input through the key input unit  170 , the control unit  100  changes the channel control information (in DVB-H, PID, IP address, and port number) for setting the switched service channel and controls such that the broadcast reception unit  110  is set with the PID of the switched service channel. Next, the broadcast reception unit  110  decodes the broadcast data on the switched service channel and outputs the decoded broadcast data to the protocol processing unit  120  such that the RTP deliverer  340  of the protocol processing unit  120  outputs the video data structured as shown in table 1. At this time, the control unit  100  captures the first I-frame received on the switched service channel. That is, the control unit  100  inspects the NAL type in the header of the video data structured as shown in table 1. If it is determined that the following data is I-frame data, the control unit  100  controls the video decoder  590  of the decoding unit  130  to decode the I-frame video data. The decoded I-frame video data is output to the display unit  105  so as to be displayed as a still picture. 
     The timestamp is not contained in all the video and audio data. That is, as shown in line (a) of  FIG. 5 , the timestamp may be included in a single burst or no timestamp may be included in multiple bursts. In order to synchronize and normally play the audio and video data together, the audio and video timestamps are required. However, only one of the audio and video data timestamps may be received. Furthermore, it may occur that, even when the preset amount of the broadcast data is received (here, the size of the broadcast data received at the burst time), neither the audio timestamp nor the video timestamp is received. In this case, the synchronization between the audio and video data fails. In an exemplary embodiment, the buffered video and audio data are played at a preset time interval before acquiring the synchronization between the audio and video data. That is, the buffered audio and video data are played at a preset frame rate in accordance with a local clock of the digital broadcast receiver before the synchronization between the audio and video data is acquired. If the timestamp is received, the control unit  100  adjusts the time offsets of the buffered audio and video data and plays the audio and video data normally in synchronization with each other. 
       FIG. 6  is a flowchart illustrating a display method for a digital broadcast receiver according to an exemplary embodiment of the present invention. In this embodiment, it is assumed that the display method is implemented with a DVB-H receiver. 
     Referring to  FIG. 6 , if a channel switching command is input through the key input unit  170  while playing a current service channel, the control unit  100  detects the channel switching command in step S 411  and sets control information for selecting a switched service channel to be switched thereto in step S 413 . The control information includes the PID, IP, and port number of the switched service channel. The PID is transferred to the broadcast reception unit  110  such that the PID filter  230  is set with the PID of the switched service channel. The control unit  100  also informs the broadcast reception unit  110  of the time division information for receiving burst data of the switched service channel. The broadcast reception unit  110  controls such that the power is supplied at burst-on time of the switched service channel with reference to the time division information. Thus, the broadcast reception unit  110  selectively decodes the broadcast data on the switched service channel and outputs the decoded broadcast data to the protocol processing unit  120 . The protocol processing unit  120  performs the protocol processing on the broadcast data received from the broadcast reception unit  110  and outputs audio, video, and broadcast data extracted from the broadcast data to their respective decoders of the decoding unit  130 . 
     In step  415 , the control unit  100  analyzes the video data of the switched service channel and captures the first I-frame video data such that the I-frame video data is displayed on the screen of the display unit  150 . 
       FIG. 7  is a flowchart illustrating an exemplary I-frame display step in more detail. 
     Referring to  FIG. 7 , if the video data separated from the broadcast data is output from the protocol processing unit  110 , the control unit detects the video data in step S 511  and analyzes the header of the video data. In an exemplary embodiment, it is assumed that the video data is coded by an H.264 coding scheme and thus the video decoder  500  is an H.264 decoder. In this case, the video data output by the RTP deliverer  340  of the protocol processing unit  120  has a structure of table 1 composed of an NAL tag and data. The NAL tag includes NAL type information of 5 bits. The NAL type information also indicates the frame type of video data, i.e., I-frame or P-frame. Accordingly, the control unit  100  inspects the video data output by the protocol processing unit  120  in step S 513  and skips displaying the video data that is not I-frame video data in step S 515  and continues video data inspection. When an I-frame is detected during the video data inspection in step S 513 , the control unit  100  controls the video decoder  500  of the decoding unit  130  to decode the I-frame video data in step S 517 . That is, the video decoder  500  decodes the I-frame video data received on the switched service channel and outputs the decoded video data to the display unit  150  under the control of the control unit  100 . 
     As described above, the control unit  100  controls such that the first I-frame data received on the switched service channel is captured and displayed, thereby quickly displaying the broadcast picture of the switched service channel during the channel switching period. Since the I-frame video data can be decoded without reference to other video data, unlike the P- and B-frame data, it can be used for fast broadcast picture display, resulting in moderation of an inconvenience caused by channel switching delay. Although it was described that the first I-frame is captured and displayed in this embodiment, the present invention includes embodiments in which any I-frame data following the first one is processed and displayed. 
     Once the first I-frame video data is decoded, other frame data (e.g. P-frame video data and B-frame video data) following the first I-frame video can also be decoded. Accordingly, the control unit  100  controls such that the broadcast data following the first I-frame video data is buffered at step S 417  in  FIG. 6 . The broadcast data following the first I-frame video data includes the audio and video data. If the buffered broadcast data reach a preset amount, the control unit  100  controls the decoding unit to decode the buffered audio and video data at a preset decoding rate. 
     Once decoded, the buffered audio and video data may be played in step S 419 . The buffered broadcast data can be played through in a two-step procedure. The audio and video data of the broadcast data should be synchronized with each other in order to be played normally. For this reason, the video and audio data contain playback time control information such as DTS and PTS. However, the DTS and PTS timestamps are received irregularly. Accordingly, the control unit  100  controls the audio and video data to be decoded at the preset decoding rate on the basis of the amount of the buffered broadcast data and once a timestamp is received, the control unit  100  adjusts the time offsets of the audio and video data on the basis of the timestamp such that the audio and video data are normally played in synchronization with each other. 
       FIG. 8  is a flowchart illustrating an exemplary broadcast data playback step. 
     Referring to  FIG. 8 , after receiving the first I-frame of the switched service channel, the control unit  100  initializes a video input buffer (Vbcnt=0) in step S 551  and starts buffering the audio and video data in the audio and video buffers (not shown). The input video and audio buffers are preferably arranged at the front ends of the respective video and audio decoders  500  and  590 . While buffering the video data, the control unit  100  determines whether the amount of the video data buffered in the input video buffer is equal to or greater than a preset amount (in this exemplary embodiment, a buffer size of the input video buffer) in step S 553 . The buffer size can be set to a size of burst of the service channel, or 1 to 3 seconds in time. In the case of DVB-H system according to this exemplary embodiment, the DVB-H receiver processes 25 frames per second. Accordingly, the buffer size is preferably set for 25 to 75 frames. In this exemplary embodiment, it is assumed that the buffer size is set to 2 seconds for 50 frames. Before the input buffer is full, the control unit  100  detects that the input video buffer is not full in step S 553 . Accordingly, the control unit  100  continues buffering the video and audio data in the respective input buffers in step S 555  and increases the count value (Vbcnt) in step S 557 . Furthermore, the control unit  100  controls such that the I-frame video data is continued to be displayed on the screen of the display unit  150  in step S 559 . 
     Repeating the above steps, if the count value becomes equal to or greater than the buffer size, the control unit  100  control the audio decoder  590  to decode and play the audio data buffered in the input audio buffer in step S 561 . Next, the control unit  100  checks the local clock of the digital broadcast receiver in step S 571  and sets a previous playback time to the current local time in step S 573 . In this state, the timestamp may not yet have been received. In this case, the control unit  100  monitors to detect the timestamp in step S 575 . If no time stamp is received, the control unit  100  determines the current local time in step S 577  and determines whether the time obtained by summing the previous playback time and the playback interval is less than or equal to the current local time in step S 579 . 
     If the time obtained by summing the previous playback time and the playback interval is less than or equal to the current local time, the control unit  100  controls the video decoder  500  to decode the video data buffered in the input video buffer in step  58   1 )and, otherwise, repeats step S 577 . 
     If the buffered video data reaches the preset size while displaying the I-frame video data, the control unit  100  determines the local time of the digital broadcast receiver and controls to play the buffered video and audio data by a preset time interval. For example, if the digital broadcast receiver is a DVB-H receiver processing 25 frames per second and the play interval is one second, the control unit  100  determines the local time of the digital broadcast receiver and controls the video decoder  500  to decode the video data of 25 frames every one second at steps S 577  and S 579 . Next, the control unit  100  changes the current local time to the previous playback time by repeating steps S 571  and S 573 . Repeating the above steps, the control unit  100  controls to play the buffered broadcast data with reference to the local time of the digital broadcast receiver until the timestamp is received. In this case, a time offset can occur between the audio and video data playback times. 
     At this time, the video data decoded and output by the video decoder  500  is displayed on the screen of the display unit  150 , while the audio data decoded by the audio decoder  590  is output through the speaker unit  155 . In this manner, once the video and audio data are buffered as much as the preset amount, the digital broadcast receiver plays the buffered video and audio data at a preset play rate with reference to the local time of the digital broadcast receiver. 
     The timestamp information can be obtained from the RTCP output by the RTP deliverer  340 . That is, the control unit  100  can determine the timestamp of the video data with reference to the RTCP. In the case that the RTCP is not received, the control unit  100  plays the audio and video data with reference to the local time of the digital broadcast receiver while performing steps S 571  to S 581 . The RTCP is not carried regularly. That is, more than two RTCP can be contained in a single burst, or no RTCP may be contained in several continuous bursts. The control unit  100  can adjust the time offset between audio and video data when both the audio and video RTCPs are received. The audio RTCP includes the time information (audio DTS or PTS) for controlling the playback of the audio data, and the video RTCP includes the time information (video DTS or PTS) for controlling the playback of the video data. Accordingly, the control unit  100  calculates the time offset between the audio and video timestamps and acquires synchronization between the audio and video data by adjusting the time offset. 
     If the timestamp information is received at step S 575 , the control unit  100  determines the offset between the audio and video timestamps and compensates the offset to acquire the synchronization between the audio and video data in step S 591 . After acquiring the synchronization between the audio and video data, the control unit  100  controls the decoding timing of the video and audio decoders  500  and  590  (or the audio and video output buffer that are not shown in the drawings) such that the audio and video data are normally played in step S 593 . 
     As described above, an exemplary digital broadcast receiver of the present invention captures the first I-frame video data received during the channel switching period and displays the I-frame video data in the form of a still picture while buffering the broadcast data. If the buffered broadcast data reaches a preset amount, the digital broadcast receiver plays the buffered broadcast data with reference to device local time until the timestamp information of the broadcast data is received. If the timestamp information is received, the digital broadcast receiver compensates for the offset between the audio and video play times so as to play the audio and video data in synchronization with each other. 
     In an alternative embodiment, the local time-based playback process can be omitted. In this case, the digital broadcast receiver waits until the timestamp is received while playing the audio signal and displaying the first I-frame received on the switched service channel as a still picture. If the timestamp information is received such that the synchronization of the audio and video data is acquired, the video data is normally displayed in synchronization with the audio data. 
     In another alternative embodiment, the audio playback can be omitted during the display of the first I-frame video data. In this case, the digital broadcast receiver performs only the displaying of the first I-frame video data as a still picture before acquiring the synchronization of the video and audio data. 
     Although the exemplary broadcast picture display method of  FIGS. 6 to 8  is described with reference to a DVB-H receiver, the present invention can be adopted to other receivers such as a DMB receiver. In a case in which the broadcast picture display method is implemented with a DMB receiver, the demodulation part of the broadcast reception unit  110  is implemented without a PID filter  230  and the demodulation controller  240 , and the protocol processing unit  120  is implemented with a demultiplexer. Here, the information output by the broadcast reception unit  110  can be the TS packet stream such that the demultiplexer analyzes the video PES information contained in the TS packet stream for detecting the I-frame video data. Other structures and operations of the DMB receiver are similar to those of the DVB-H receiver. 
     Although the exemplary broadcast picture display method is described in association with a channel switching operation, the present invention is not limited thereto. For example, the broadcast picture display method can be implemented in association with the channel setting procedure in response to a broadcast playback mode activation request. That is, the broadcast picture display method of the present invention can be applied in the broadcast channel-setting step. 
     In another exemplary implementation, the above-structured digital broadcast receiver can be integrated into a mobile terminal.  FIG. 9  is a block diagram illustrating a configuration of a mobile terminal equipped with a digital broadcast receiver according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 9 , a mobile terminal equipped with a digital broadcast receiver includes a control unit  100 , a broadcast reception unit  110 , a protocol processing unit  120 , a decoding unit  130 , a display unit  150 , a speaker unit  155 , a memory unit  160 , a key input unit  170 , and a radio frequency (RF) unit  190 . In such configuration, the control unit  100  controls both the radio communication and broadcast playback functions of the mobile terminal. The RF unit  190  includes a frequency converter for performing frequency up-conversion on transmitted signals and frequency down-conversion on received signals. The RF unit  190  also includes a modem for modulating transmitted signals and demodulating received signals and a codec for coding transmitted signals and decoding received signals. The modem and codec can be integrated into the control unit  100 . 
     In an exemplary implementation, when the mobile terminal operates in a communication mode, the digital broadcast receiver may be disabled and the RF communication function is enabled with the operation of the RF unit  190 . 
     In another exemplary implementation, the digital broadcast receiver embedded in the mobile terminal plays the broadcast data of a service channel selected by the user and enables the RF unit  190 , when it is required to communicate with a service provider or a broadcast station, to establish a return channel. 
     As described above, the digital broadcast receiver may include the broadcast reception unit  110 , protocol processing unit  120 , and decoding unit  130 . If a channel establishment or channel switching request is detected in the above-structured digital broadcast receiver, the control unit  100  changes the service channel information and resets the broadcast reception unit  110  and the protocol processing unit  120  with new service channel information. If a service channel is selected, the broadcast reception unit  110  performs the time division control for receiving the burst data on the selected service channel and receives the burst data on the service channel set with the PID of the received burst data. The protocol processing unit  120  performs the protocol processing on the received burst data. At this time, the control unit  100  captures the first I-frame video data received on the selected service channel and displays the I-frame video data on the screen of the display unit  150  while buffering the video and audio data constituting the broadcast data following the first I-frame video data in the respective video and audio input buffers. 
     Next, the control unit  100  starts playing the buffered video data, when the buffered video data reaches a preset amount, with reference to the local time of the mobile terminal until the audio and video timestamps are received. If the audio and video timestamps are received, the control unit  100  compensates the time offset between the audio and video data such that the video and audio data are normally played in synchronization with each other. 
     The mobile terminal can be set with an incoming call alert mode. Accordingly, if an incoming call is detected while the mobile terminal performs a broadcast picture display, the control unit  100  generates an incoming call alert. The incoming call alert mode includes a normal ringer mode and a mute mode. When the normal ringer mode is set, the control unit  100  indicates the incoming call using a ring tone such as a melody, a bell, music and the like together with display of caller information. The mute mode includes a vibrating mode and an announcement mode. In the case of the vibrating mode, the control unit  100  indicates the incoming call by vibrating the mobile terminal using a motor (not shown) together with a display of the caller information. When the announcement mode is set, the control unit  100  displays an announcement message indicating an incoming call together with the caller information. The announcement message can be cycled on and off for attracting the user&#39;s attention. The announcement message and caller information can be displayed on the multiplexed broadcast screen. The incoming call alert mode can be automatically set as the announcement message mode in the broadcast playback mode, since the user is likely to watch the display screen. 
     The mobile terminal can be configured such that an outgoing call request can be input by the user during the broadcast playback mode. This is because the digital broadcast can support an interactive program requiring user feedback. For example, the user may need to communicate with another party to order goods or services while watching a shopping program of a specific service channel. Also, the user can participate in an entertainment or gaming program. Since the digital broadcast receiver supports unidirectional communication, the program-related communication channel is established using the RF unit  190 . This is because the mobile terminal supports the radio communication using the RF unit  190 . The control unit  100  can check the information on the program provider such as phone number and IP address of a department managing the program on the service channel. That is, in the case of DVB-H enabled mobile terminal, the control unit  100  can obtain the phone number and IP address associated with the program of the service channel from the ESG. In the case of DMB-enabled mobile terminal, such information can be obtained from the EPG. Accordingly, if an outgoing call request is input by the user while the mobile terminal operates in the broadcast playback mode, the mobile terminal establishes a communication channel to the phone number by means of RF unit  190  such that the user can transmit a message (for example, product order or vote message) through a communication channel. Also, the mobile terminal can receive a response to the message through the communication channel. 
     In an exemplary implementation, the user can make a call by positioning a cursor on a service channel item displayed on the broadcast picture screen by manipulating the key input unit  170 . For example, the service channel item may include an icon displayed on the display unit  150 . The control unit  100  can recognize the position of the cursor on the broadcast picture screen of the display unit  150 . Accordingly, if the user places the cursor at a specific position on the screen and requests an outgoing call, the control unit  100  determines the current service channel and program information on the basis of the position of the cursor on the screen and makes a call on the basis of the program information (for example, phone number and IP address) of the current service channel. 
     If an outgoing call is made in the broadcast playback mode, the user can communicate with a person associated with the current service channel. 
     Although exemplary embodiments of the present invention are described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims. 
     As described above, the broadcast picture display method of the present invention allows for quickly capturing broadcast data received on a service channel switched or reset in response to a channel switching and channel reset request and displaying the captured broadcast data during the channel switching or channel reset period, thereby moderating an inconvenience felt during conventional channel switching and channel reset delay. Also, the broadcast picture display method of the present invention enables broadcast data buffered during a channel switching or channel reset period to be played before acquiring synchronization between the video and audio data, resulting in improvement of display functionality of the digital broadcast receiver.