Abstract:
The present invention provides a method for improving playback consistency of an encoded video stream. The method of the present invention first determines whether a series of values of the progressive_frame flag of consecutive frames of the encoded video stream are arranged in a predetermined pattern. If the values are arranged in the predetermined pattern, an image of at least one of the frames is displayed progressively regardless of the value of the progressive_frame flag of that frame. As a result, the video stream is played back as if the movie had been consistently encoded in the first place.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to multimedia processors and particularly to MPEG decoders. 
     2. Related Art 
     Encoded digital video streams are used in a variety of applications that allow videos to be distributed on a variety of media. For example, movies can be stored on Digital Video Disc (DVD) for distribution or rental purposes. Similarly, digital video streams can be broadcast (DVB) to multiple subscribers on pay-per-view systems. Alternatively, digital video streams can be transmitted over a computer network, such as the Internet, and viewed on remote computers without the need for a dedicated DVD player or a DVB receiver. 
     Several encoding standards, such as MPEG, MPEG-2, MPEG-4, AVI, QuickTime, etc. have been developed for encoding and distribution of digital video streams. Different standards allow for varying degrees of functionality versus storage requirements. For example, MPEG-2 is primarily designed for encoding movies and other audio-visual works. Similarly, MPEG-4 is designed to handle video streams transmitted over low bandwidth communication channels. 
     Movies, on the other hand, are typically shot on optical film at rate of 24 frames/second by means of a photographic process, while audio is captured on a separate medium, with audio-video synchronization added to either recording medium. To store movies as digital video streams, therefore, each frame must be first captured in electronic format and then encoded according to a pre-selected encoding standard. 
     A system  100  for capturing and displaying an encoded digital video stream is illustrated in FIG.  1 . First, a video camera  110  captures a scene  120  on film  130 . 
     An encoder  140  then transfers the images stored on film  130  onto an encoded video stream  150  stored on an electronic storage medium. A decoder  160 , in turn, decodes video stream  150  that is thus displayed on a screen of a display device  170 . Decoder  160  can be either a dedicated device such as a DVD player or a general purpose device such as a computer. The display device  170  can be either a television set or a computer monitor. 
     Video streams encoded according to the MPEG-2 standard, for example, are often encoded for playback at a rate of 60 fields/second. A field  220  (FIG. 2) consists of only half the pixels of an image  210 . By displaying successive fields  220  in rapid succession, the human eye perceives the entire image  210  as being displayed at once, while in reality only half image  210  is refreshed at any one time. Since only 24 frames where shot in the original movie for all 60 fields to be displayed during playback, each movie frame must be displayed an average of 2.5 times (60/24). This is typically, accomplished by displaying images  210  of successive frames  200  for a duration of 3 and 2 fields  220 ,in an alternating pattern, as shown in FIG.  2 . This technique is typically referred to as “3-2 pulldown.” The MPEG-2 standard provides a repeat_first_field flag  310  on each frame  200  to indicate whether image  210  is to be displayed for  2  or 3 fields  220 ,as shown in FIG.  3 . For example, in FIG. 3, when repeat_first_field flag  310  has a value “0” image  210  is displayed for two fields  220 , while when repeat_first_field flag  310  has a value “1” image  210  is displayed for three fields  220 . 
     In addition, a time stamp (not shown) is included in each frame  200  of the encoded video stream representing a desired playback time for each frame  200 . The decoder, in turn, examines the time stamps of each frame  200  to determine whether the timing relationship among the frames in the encoded video stream is preserved during playback or whether playback timing must be adjusted to compensate for variations in the decoding/display process. 
     The MPEG-2 standard also provides a progressive_frame flag  410  (FIG. 4) that indicates whether each encoded image  210  represents an interlaced or a non-interlaced image. An interlaced image is one in which consecutive fields  220  are captured at successive points in time. Thus, to preserve correct timing, each field  220  in an interlaced image must be displayed at a different point in time. Interlaced images are typically displayed correctly on interlaced display devices (e.g., televisions sets, interlaced computer monitors, etc.). However, displaying interlaced images on non-interlaced (progressively-scanned) monitors is problematic. Progressive displays, in fact, display complete images  210  at each refresh interval. Therefore, each field  220  in an interlaced image must be “filled” by interpolating values for the missing lines in each field  220 . The interpolation process, however, results in a loss of image resolution and the introduction of artifacts which, in turn, degrade the overall image quality. 
     A non-interlaced image, by contrast, is an image  210  in which consecutive fields  220  are captured at the same time. Thus, playback of non-interlaced images on progressive monitors results in higher quality images because there are no missing lines. 
     Since the two fields in each movie frame are always shot at the same time, progressive_frame flag  410  should always be set to  1 ,as shown in FIG.  4 . However, some encoders only set the progressive_frame flag to  1  when repeat_first_field flag  310  is also set to  1 , as shown in FIG.  5 . Movies encoded in such a fashion, therefore, are played back inconsistently on non-interlaced monitors, due to the alternation of display modes. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for improving playback consistency of an encoded video stream. The method of the present invention first determines whether a series of values of the progressive_frame flag of consecutive images of the encoded video stream are arranged in a predetermined pattern. If the values are arranged in the predetermined pattern, the image of at least one of the frames is displayed progressively regardless of the value of the progressive_frame flag for that frame. As a result, the video stream is played back as if the movie had been consistently encoded in the first place. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a system  100  for capturing and displaying an encoded digital video stream. 
     FIG. 2 is a diagram illustrating the playback timing relationship between movie frames and video fields. 
     FIG. 3 is a diagram illustrating the playback timing relationship between images of an encoded video stream and the fields displayed by an interlaced display device. 
     FIG. 4 is a diagram of the playback timing relationship between images of a consistently encoded video stream and the fields displayed by a progressive display device. 
     FIG. 5 is a diagram of the playback timing relationship between images of an inconsistently encoded video stream and the fields displayed by a display device. 
     FIG. 6 is a flow diagram of inconsistent encoding circumvention operation, in accordance to an embodiment of the present invention. 
     FIG. 7 is a flow diagram of inconsistent encoding correction operation, in accordance to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     To optimize playback consistency, the decoder must first be able to detect that the digital video stream has been inconsistently encoded. Once the inconsistent encoding has been detected, the decoder can then adjust the display operation to circumvent the inconsistency of the encoded playback information. FIG. 6 illustrates an inconsistent encoding circumvention operation  600 , according to an embodiment of the invention. 
     Operation  600  examines the current frame  200  being processed by the decoder as well as the last three decoded frames  200  of the video stream. Stage  610  determines whether progressive_frame flag  410  of the third previously decoded frame  200  (i.e., the first of the four consecutive frames examined by operation  600 ) is set, in which case operation  600  proceeds to stage  620 . Otherwise, operation  600  proceeds to stage  650 . Stage  620  determines whether progressive_frame flag  410  of the second previously decoded frame  200  is set, in which case operation  600  proceeds to stage  650 . Otherwise, operation  600  proceeds to stage  630 . Stage  630  determines whether progressive_frame flag  410  of the first previously decoded frame  200  is set, in which case operation  600  proceeds to stage  640 . Otherwise, operation  600  proceeds to stage  650 . In other words, stages  610 - 640  of operation  600  determine whether progressive_frame flags  410  of the last three previously decoded frames  200  follow the pattern “101”. Such a pattern indicates an encoding scheme where interlaced and non-interlaced images alternate in consecutive frames, a very improbable encoding scheme for movies. Those skilled in the art realize that other techniques known in the art could be used to detect an inconsistent encoding of a digital video stream in accordance to the principles of the invention. For example, the improper encoding could be detected by examining the video stream for a pattern other than “101” (e.g., “10101”, “1010101”, etc.). Accordingly, the invention is not limited to any particular technique for detecting the incorrect encoding of the video stream. 
     If the pattern is not detected, stage  650  determines whether progressive_frame flag  410  of the current frame  200  is set, in which case operation  600  proceeds to stage  640 . Stage  640  displays image  210  of the current frame  200  progressively. Otherwise, stage  660  displays image  210  of the current frame in interlaced mode. Finally, the progressive_frame history is updated in stage  670 . Display of image  210  in non-interlaced (progressively-scanned) mode in stage  640  may be effectuated by any suitable technique known in the art. For instance, each image  210  of the encoded video stream may be displayed as a single frame according to the time stamps provided in the encoded video stream. Alternatively, the time stamps can be adjusted so that each image  210  is displayed for approximately 1/24 of a second, rather than according to the 3-2 pulldown scheme embedded in the time stamps of the encoded streams. 
     Display of image  210  in interlaced mode in stage  660 , on the other hand, is effectuated according to the 3-2 pulldown technique described above. 
     Finally, progressive_frame history data is updated in stage  670  by effectively shifting values of progressive_frame flag  410  of each of the frames  200  examined by operation  600  by one frame position and selecting the next frame  200  to be decoded as the current frame  200 . As a result, stages  610 - 670  can be repeated to display each image  210  of the encoded video stream. 
     A possible software implementation of operation  600  is described in Table 1 below. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 if 
                 (prev1_picture_progressive == TRUE) &amp;&amp; 
               
               
                   
                 (prev2_picture_progressive == FALSE) &amp;&amp; 
               
               
                   
                 (prev3_picture_progressive == TRUE) ) 
               
               
                   
                 display_picture_progressive = TRUE; 
               
               
                 else 
               
               
                   
                 display_picture_progressive = current_picture_progressive; 
               
               
                   
                 prev3_picture_progressive == prev2_picture_progressive; 
               
               
                   
                 prev2_picture_progressive == prev1_picture_progressive; 
               
               
                   
                 prev1_picture_progressive == current_picture_progressive; 
               
               
                   
               
             
          
         
       
     
     Table 1 contains a listing of computer source code written in the C programming language. An alternative implementation of operation  600  is illustrated in Table 2 below. 
     
       
         
               
             
               
               
             
               
             
               
               
             
               
             
               
               
             
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                 progressive_history = (progressive_history &lt;&lt; 1) + 
               
             
          
           
               
                   
                 current_picture_progressive; 
               
             
          
           
               
                 if(progressive_history == 10) //this corresponds to TFTF 
               
             
          
           
               
                   
                 display_picture_progressive = TRUE; 
               
             
          
           
               
                 else 
               
             
          
           
               
                   
                 display_picture_progressive = current_picture_progressive; 
               
             
          
           
               
                 progressive_history &amp;=7; //truncates history to last 3 pics 
               
               
                   
               
             
          
         
       
     
     Table 2 also contains a listing of a computer source code written in the C programming language. The implementation of Table 2 takes advantage of the bit manipulation feature of the C programming language to reduce the number of variables required to perform operation  600  with respect to the implementation of Table 1. 
     Those skilled in the art realize that the method of the present invention can be easily modified to correct the values of progressive_frame flag  410 , as shown in FIG. 5, rather than overriding the values of progressive_frame flag  410 . Table 3 contains a computer source code listing of a possible implementation of such a method. 
     
       
         
               
               
             
               
               
               
             
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
             
               
                   
                 last_picture_progressive == current_picture_progressive; 
               
             
          
           
               
                   
                 if 
                 (prev1_picture_progress:.ve == TRUE) &amp;&amp; 
               
               
                   
                   
                 (prev2_picture_progressive == FALSE) &amp;&amp; 
               
               
                   
                   
                 (prev3_picture_progressive == TRUE) ) 
               
               
                   
                   
                 current_picture_progressive = TRUE; 
               
             
          
           
               
                   
                 prev3_picture_progressive = prev2_picture_progressive; 
               
               
                   
                 prev2_picture_progressive = prev1_picture_progressive; 
               
               
                   
                 prev1_picture_progressive = last_picture_progressive; 
               
               
                   
                   
               
             
          
         
       
     
     The computer source code segments of Tables 1, 2 and 3 can be implemented, for example, as digital circuitry embedded in a processor, as microcode executed by a graphics accelerator operatively coupled to a CPU, or as an application program executed by a general purpose computer. 
     FIG. 7 illustrates an inconsistent encoding correction operation  700 , according to an embodiment of the invention. 
     Operation  700  examines the current frame  200  being processed by the decoder as well as the last three decoded frames  200  of the video stream. First, the value of progressive_frame flag  410  for the current frame  200  is saved in last_picture. Stage  720  then determines whether progressive_frame flag  410  of the third previously decoded frame  200  (i.e., the first of the four consecutive frames  200  examined by operation  700 ) is set, in which case operation  700  proceeds to stage  730 . Otherwise, operation  700  proceeds to stage  760 . Stage  730  determines whether progressive_frame flag  410  of the second previously decoded frame  200  is set, in which case operation  700  proceeds to stage  760 . Otherwise, operation  700  proceeds to stage  740 . Stage  740  determines whether progressive_frame flag  410  of the first previously decoded frame  200  is set, in which case operation  700  proceeds to stage  750 . Otherwise, operation  700  proceeds to stage  760 . Progressive_frame flag  410  for the current frame  200  is set in stage  750 . Finally, the progressive history is updated in stage  760  using the value stored in last_picture, rather than the value stored in current_picture. 
     Embodiments described above illustrate but do not limit the invention. In particular, the invention is not limited by any particular test for detecting an inconsistent encoding of the video stream. In fact, detection schemes other than the ones described herein can be used in accordance to the principles of the invention. In addition, the invention is not limited to any hardware or software implementation. Those skilled in the art realize that the present invention can be implemented entirely in hardware, entirely in software or in any combination of hardware and software. Other embodiments and variations are within the scope of the invention, as defined by the following claims.