CROSS REFERENCE TO RELATED APPLICATION(S) 
     The present disclosure relates to the subject matters contained in Japanese Patent Application No. 2009-139934 filed on Jun. 11, 2009, which are incorporated herein by reference in its entirety. 
     FIELD 
     The present invention relates to a video stream decoding control technique that is suitably applied for example, a personal computer having a TV function for recording and reproducing digital broadcast program data. 
     BACKGROUND 
     Various electronic apparatus capable of handling a video image in the form of digital data are widely used nowadays. For example, if carrying a mobile electronic apparatus having a wireless network function, the user can view latest news images etc. even when the user is out or is moving. Furthermore, recently, personal computers having a TV function for viewing, recording, and reproducing digital TV broadcast data have became widely used. 
     Digital processing on a video image, that is, coding, transmission/reception, and decoding of a video image, imposes a heavy load on a processor and a network. In view of this, various schemes for reducing the load of digital processing on a video image have been proposed so far (refer to JP-A-2004-357205, for example). 
     In the publication, JP-A-2004-357205, when the transmission bandwidth is limited in stream delivery, image quality deterioration that is caused by decoding on the reception side is suppressed by giving higher priority to packets (corresponding to slices) having large coding information amounts per macroblock irrespective of the coding prediction method, selecting transmittable video packets from the video packets in each frame according to priority ranks, and sending them. In performing N-fold fast reproduction, fast reproduction frames are constructed and reproduced by selecting video packets that can be reproduced fast are selected according to packet priority ranks in both of P frames and B frames. Therefore, serious image quality deterioration occurs if coding information of an I frame (intrapicture predictive coding data) or a P frame as a reference frame is lost according to priority ranks. Furthermore, this conventional technique is associated with a problem that it cannot accommodate a case that necessary processing cannot be performed on the transmission side (coding side) as in the case of handling broadcast data because it is necessary to determine priority ranks on the decoding side and reconstruct data on the reception side. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A general configuration that implements the various feature of the invention will be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not to limit the scope of the invention. 
         FIG. 1  is a perspective view showing an appearance of a video decoder according to an embodiment of the present invention. 
         FIG. 2  shows an example system configuration of the is video decoder according to the embodiment. 
         FIG. 3  is a block diagram showing a functional configuration of the video decoder according to the embodiment. 
         FIG. 4  is a conceptual diagram showing a relationship between a frame, slices, and macroblocks in a compression-coded video stream in the embodiment. 
         FIG. 5  shows header information of a NAL unit in H.264 which is used in the embodiment. 
         FIG. 6  is a flowchart of an ordinary slice-by-slice decoding process used in the embodiment. 
         FIG. 7  is a flowchart of a slice-by-slice decoding process for a non-reference frame according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     An embodiment of the present invention will be hereinafter described with reference to  FIGS. 1-7 . 
     A configuration of a video decoder according to the embodiment of the invention will be described with reference to  FIGS. 1 and 2 . The video decoder according to the embodiment is configured to be a notebook personal computer  10 . 
     The computer  10  has a function of reproducing a video stream that was compression-coded by a method that complies with the H.264 standard and, for example, is received over a network or read from a recording medium. This function is realized by, for example, a video decoder application program that is preinstalled in the computer  10 . 
       FIG. 1  is a perspective view of the computer  10  in a state that a display unit  12  is opened. The computer  10  is composed of a main unit  11  and the display unit  12 . An LCD (liquid crystal display)  13  is incorporated as a display device in the display unit  12 . 
     The display unit  12  is attached to the main unit  11  so as to be rotatable between an open position where it exposes the top surface of the main unit  11  and a closed position where it covers the top surface of the main unit  11 . The main unit  11  has a thin, box-shaped body, and a keyboard  14 , a power button  15  for powering on/off the computer  10 , a touch pad  16 , speakers  17 A and  17 B, etc. are provided on the top surface of the main unit  11 . 
     Next, a system configuration of the computer  10  will be described with reference to  FIG. 2 . As shown in  FIG. 2 , the computer  10  is equipped with a CPU  101 , a northbridge  102 , a main memory  103 , a southbridge  104 , a display controller  105 , a video memory (VRAM)  105 A, a sound controller  106 , a BIOS-ROM  107 , a LAN controller  108 , an HDD (hard disk drive)  109 , an ODD (optical disc drive)  110 , a wireless LAN controller  111 , an IEEE 1394 controller  112 , an embedded controller/keyboard controller (EC/KBC)  113 , an EEPROM  114 , etc. 
     The CPU  101 , which is a processor for controlling the operations of the computer  10 , runs an operating system (OS)  200  and various application programs such as a video decoder application program  300  which operate under the OS  200 . The OS  200  and the various application programs are loaded into the main memory  103  from the HDD  109 . The video decoder application program  300  performs processing of reproducing a video stream that was compression-coded by a method that complies with the MPEG-2 standard and, for example, is received by the LAN controller  108  or the wireless LAN controller  111  over a network or read from a DVD (digital versatile disk) or the like by the ODD  110 . The CPU  101  also runs a BIOS (basic input/output system) that is stored in the BIOS-ROM  107 . The BIOS is a program for hardware control. 
     The northbridge  102  is a bridge device which connects a local bus of the CPU  101  to the southbridge  104 . The northbridge  102  incorporates a memory controller for access-controlling the main memory  103 . The northbridge  102  also has a function of communicating with the display controller  105  via, for example, a serial bus that complies with the PCI Express standard. 
     The display controller  105  is a device for controlling the LCD  13  which is used as a display monitor of the computer  10 . A display signal is generated by the display controller  105  and supplied to the LCD  13 . 
     The southbridge  104  controls individual devices on a PCI (peripheral component interconnect) bus and an LPC (low pin count) bus. The southbridge  104  incorporates an IDE (integrated drive electronics) controller for controlling the HOD  109  and the ODD  110 . The southbridge  104  also has a function of communicating with the sound controller  106 . 
     The sound controller  109 , which is a sound source device, outputs reproduction subject audio data to the speakers  17 A and  17 B. 
     The wireless LAN controller  111  is a wireless communication device which performs a wireless communication according to the IEEE 802.11 standard, for example. The IEEE 1394 controller  112  communicates with an external apparatus via a serial bus that complies with the IEEE 1394 standard. 
     The EC/KBC  113  is a one-chip microcomputer in which an embedded controller for power management and a keyboard controller for controlling the keyboard (KB)  14  and the touch pad  16  are integrated together. The EC/KBC  113  has a function of powering on/off the computer  10  in response to a operation of the power button  15  by the user. 
     The video decoder application  300 , which operates as one piece of software on the above-configured computer  10  on the same level as other application programs and serves to reproduce a video stream, has a mechanism for reducing the amount of processing that relates to reproduction of a video stream while suppressing image quality deterioration, irrespective of the load state of the CPU  101 . This feature will be described below in detail. 
     First, a functional configuration of a software decoder which is realized by the video decoder application  300  will be described with reference to a block diagram of  FIG. 3 . 
     Header information, slice information, macroblock (MB) information of an input video stream are decoded in a decoding data processor  301  (controller). The decoding data processor  301  is equipped with a memory  311  for decoding information including these three kinds of information, a decoding section  321  for performing a syntax analysis etc. using this information, a counter  331  for counting skip MBs, and a decoding control unit  341  for controlling the above components. 
     On the other hand, in a signal processor  310 , inverse quantization/inverse DCT (discrete cosine transform) section  302  inverse-quantizes quantized coefficient information on the basis of quantization characteristic information and performs inverse quadrature transform on an inverse quantization result. A result is added to a past reproduction image by an adder  304  on the basis of motion vector information, and an addition result is stored in a reference image memory  307 . At the same time, video is output using the image stored in the reference image memory  307 . In this manner, video that is intended by the coding side can be output. For bidirectional prediction, the reference image memory  307  holds reference images of plural I and P frames. The signal processor  310  is also equipped with an interframe predicting section  303 , an intraframe predicting section  305 , a filter  306 , and a skip processing section  308 . 
     A description will be made of the I, P, and B frames. The I frame is an intraframe coding image and is a frame that can be decoded using its own information. The P frame is a predictive coding image (interframe forward predictive coding image) and is a frame that can be decoded by referring to information of an immediately preceding I frame. The B frame is a bidirectionally predictive coding image and is a frame that can be decoded by referring to information of preceding and following I and P frames. I, P, and B frames are arranged as follows: I, B, B, P, B, B, P, B, B, . . . I, B, B, P, B, B, . . . . Input/output is controlled while I and P frame before and after each set of two B frames (in the case of the above arrangement) are stored in the reference image memory  307 . 
     That is, whereas information of an I and/or P frame is referred to in decoding another frame, information of a B frame is not referred to in decoding another frame. Therefore, first, the video decoder application program  300  employs B frames as candidates on which load reduction processing is to be performed. More specifically, the load reduction processing is prevented from affecting decoding of other frames. 
     Next, the background of load reduction processing that the video decoder application program  300  performs on P frames will be described with reference to  FIGS. 4 and 5 . 
     In a standard relating to H.264, each image is divided into macroblocks of 16×16 pixels and coding is performed on a macroblock-by-macroblock basis. One or more macroblocks (in the maximum case, all macroblocks of a frame) are combined into a slice.  FIG. 4  is a conceptual diagram showing a relationship between an image (frame), slices, and macroblocks in a case that all macroblocks arranged in each row are combined into a slice. 
     As shown in  FIG. 4 , each frame a is divided into plural slices b and each slice b consists of plural macroblocks c. 
       FIG. 5  shows the structure and header information of a NAL unit in H.264. As shown in  FIG. 5 , a coding image signal of H.264 is configured in such a manner that start codes a and packets called NAL units b are arranged alternately and continuously. 
     The start code a is a common code in this coding image signal and is fixed to “0x000001” (hexadecimal). 
     Each NAL unit b consists of a NAL header c and NAL data d which is a data body. The NAL header c contains parameters “forbidden_zero_bit,” “nal_ref_idc,” and “nal_unit_type,” and the contents of the NAL data d is determined from the parameter “nal_unit_type” e (hereinafter referred to as a NAL type) which indicates a type of the NAL data d. 
       FIG. 6  is a flowchart of an ordinary slice-by-slice decoding process which is executed by the decoding data processing section  301  (see  FIG. 3 ) of the H.264 video decoder. 
     In a reference frame, slice information is decoded at step S 601 , MB skip information is decoded at step S 602 , and whether the decoding of all pieces of MB information of the slice has been completed is determined at step S 603 . If the decoding of all the pieces of MB information has not been completed yet, at step S 604  another piece of MB information (MB type information, prediction information, quantization characteristic information, and quantized coefficient information) is decoded. It is again determined at step S 605  whether the decoding of all the pieces of MB information has been completed. If the decoding of all the pieces of MB information has not been completed yet, at step S 602  MB skip information is decoded again. After the decoding of all the pieces of MB information of the slice has been completed (S 603 : yes or S 605 : yes), the slice is subjected to signal processing. 
       FIG. 7  is a flowchart of a slice-by-slice decoding process for a non-reference frame according to the embodiment of the invention which is executed by the decoding data processing section  301  (see  FIG. 3 ) of the H.264 video decoder. Part of this process is executed by the skip processing section  308 . 
     A NAL header c is decoded by the decoding data processing section  301  of the H.264 video decoder, whereby a NAL type and whether the subject data is data of a reference frame becomes known. If “nal_ref_idc” of the NAL header c is “0,” it becomes known that the subject frame is a frame that is not referred to (i.e., non-reference frame). 
     The slice-by-slice decoding process for a non-reference image according to the embodiment of the invention will be described below with reference to the flowchart of  FIG. 7 . 
     In a non-reference frame, slice information is decoded at step S 701 , MB skip information is decoded at step S 702 , and the number of skip MBs in the slice is counted at step S 703 . If the number of skip MBs in the slice is larger than or equal to a threshold value (e.g., 80% of the number of skip MBs in one slice) (S 704 : yes), all the MBs of the slice are set as skip MBs at step S 705  and the slice is subjected to signal processing. If the number of skip MBs in the slice is smaller than the threshold value (S 704 : no), it is determined at step S 706  whether the decoding of all pieces of MB information of the slice has been completed. If the decoding of all the pieces of MB information has not been completed yet, at step S 707  another piece of MB information (MB type information, prediction information, quantization characteristic information, and quantized coefficient information) is decoded. It is again determined at step S 708  whether the decoding of all the pieces of MB information has been completed. If the decoding of all the pieces of MB information has not been completed yet, at step S 702  MB skip information is decoded again. After the decoding of all the pieces of MB information of the slice has been completed (S 706 : yes or S 708 : yes), the slice is subjected to signal processing. 
     When the number of skip MBs in a slice is larger than or equal to the threshold value, following MB information decoding data processing is not performed. The processing amount is thus reduced. Since all the MBs in the slice are processed as skip MBs, the amount of such signal processing as inverse quantization and DCT process can be reduced. Since this type of processing is limited to non-reference frames, the image quality deterioration can be made lower than in a case that ordinary decoding data processing and signal processing are performed. 
     As described above, the use of information of NAL headers c of an H.264 video stream makes it possible to apply the processing specific to the embodiment to even interframe-prediction-coded P frames if they are not referred to. Applying the processing specific to the embodiment to only non-reference frames and setting all MBs in a slice as skip MBs make it possible to reduce the amount of decoding processing and to suppress image quality deterioration. 
     Although the embodiment according to the present invention has been described above, the present invention is not limited to the above-mentioned embodiment but can be variously modified. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.
A video decoder including: an input module configured to receive a video stream that is coded based on macroblocks; a frame determination module configured to determine whether or not a decoding subject image in the video stream that is input to the input module is a non-reference frame image that is not referred to when decoding another image; a slice analyzing module configured to determine, for each slice being configured by arranging the macroblocks, whether or not skip macroblocks each of which has no coding information of its own exist in a predetermined number or more when the frame determination module determines that the decoding subject image is a non-reference frame image; and a slice editing module configured to set, as skip macroblocks, all macroblocks in a slice for which the slice analyzing module determines that skip macroblocks exist in the predetermined number or more.