Abstract:
A system and method for reducing interlace artifacts during trick mode playback of pre-encoded content. The system and method operate a decoder to receive a video signal, configured as frames, with each frame having a plurality of interlaced fields, and to provide a video output in interlaced format or trick mode format. When operating in a normal mode, the decoder provides the video output in the interlaced format. When operating in trick mode, the decoder is instructed to use one of the fields of a frame as a reference field, and outputs a video output frame in the trick mode format where both fields of the video output frame are the same as the reference field. The video signal can be a streaming video or read from a medium.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a system and method for improving trick mode playback of a video. More particularly, the present invention relates to a system and method for reducing interlace artifacts during trick mode playback of pre-encoded content. 
     BACKGROUND 
     Television systems typically use a pair of interlaced fields to compose an entire frame or picture on the display. According to the National Television Systems Committee (NTSC) standards, in television system in the United States, 29.97 frames are transmitted per second as 59.94 fields, so that the display is updated 59.94 times per second, which is fast enough to avoid the appearance of the display flickering. In such a system, one field will contain the odd numbered scan lines of the image, while the other field contains the even numbered scan lines. 
     When the television program is generated from an interlaced source, such as a video camera, these fields represent different moments in time. For example, an actor walking across the set may be in a different location from one field to the next. Also, camera motion may cause the entire image to shift between fields. On the other hand, if the program is generated from a progressive (non-interlaced) source, such as film, the fields represent the same moment in time. 
     Video on Demand (VOD) servers that generate dynamic trick mode sequences typically use the intra-coded pictures present in the stream to generate these sequences. As understood in the art, a “trick mode” is a mode other than normal playback, such as fast forward, rewind and so on. A trick mode sequence typically consists of the intra-coded picture followed a series of inter-coded pictures (P-repeats) that force the decoder to redisplay the current image. Traditional server architectures that rely on trick mode files will typically re-encode the intra-coded pictures and are thus able to groom them to their own requirements. 
     Content that is encoded for VOD use is typically encoded as a stream of frame pictures, where each frame contains the contents of two fields. However, it is generally not possible to separate the coded frame into its fields without decoding the frame. In a trick mode sequence, both fields are displayed for the original intra-coded frame, and the subsequent repeat frames cause both fields to be redisplayed. 
     Hence, if the original content originated from a progressive source, such as movie film, the two fields represent the same instant in time and the effect of redisplaying the two fields is a stable image. However, if the content originated from an interlaced source, the images in the two fields may be different and the visual effect will be that of redisplaying two alternate, superimposed images, each at 29.27 or 25 times per second, which results in flicker in the picture that can be very distracting. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention. 
         FIG. 1  is a block diagram illustrating an example of video on demand server (“server”), decoder and video display employing an embodiment of the present invention; 
         FIG. 2  is a conceptual block diagram illustrating an example of a normal frame based trick sequence; 
         FIG. 3  is a conceptual block diagram illustrating an example of a field deinterlace trick mode sequence according to an embodiment of the present invention; and 
         FIG. 4  is a flowchart illustrating an example of operations performed in accordance with an embodiment of the present invention as the result of receiving the field deinterlace trick mode sequence as shown in  FIG. 3 . 
     
    
    
     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
     DETAILED DESCRIPTION 
     Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to a system and method for reducing interlace artifacts during trick mode playback of pre-encoded content. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element preceded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. 
     It will be appreciated that embodiments of the invention described herein may be comprised of one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of a system for reducing interlace artifacts during trick mode playback of pre-encoded content described herein. The non-processor circuits may include, but are not limited to, a radio receiver, a radio transmitter, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as steps of a method for reducing interlace artifacts during trick mode playback of pre-encoded content. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Thus, methods and means for these functions have been described herein. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation. 
       FIG. 1  illustrates an example of a server  100  that is operable to reduce interlace artifacts during trick mode playback of pre-encoded content. As can be appreciated by one skilled in the art, the server  100  can include, for example, a processor and associated hardware and software for performing the operations described herein. 
     When video content, such as pre-encoded video content, is to be displayed on a display  102  such as a television, video screen and so on, the server  100  transmits each frame to the decoder  101  as two interleaved fields as discussed in the Background section above. That is, as shown in  FIG. 2 , each frame  200  includes a first field  202  (identified as “1”) and a second field  204  (identified as “2”). It is noted that for illustrative purposes, the first field  202  and second field  204  are shown as being in the top and bottom halves, respectively, of the frame  200 . However, in actuality, the fields  202  and  204  are interlaced as discussed in the Background section above. The video content can be received by the decoder  101  from a storage device, such as a DVD, memory stick, and so on, or could be streaming video data from the server  100 . Each field and frame can be identified in any suitable manner, for example, a pointer, tag or metadata, as described in U.S. patent application Ser. No. 10/730,365, filed on Dec. 8, 2003, the entire disclosure of which is incorporated herein by reference. 
     That is, as understood in the art, an MPEG video stream encoded for VOD includes predictive frame pictures (P-frames), and intra-coded frame pictures or intraframes (I-frames). Each of the P-frames includes two fields (P-fields) and each of the I-frames includes two fields (I-fields). 
       FIG. 2  further illustrates an example of the manner in which a normal frame based trick mode sequence operates, such as for a rewind, fast forward or pause operation. For example, as can be appreciated by one skilled in the art, the intra-coded frame picture (I-frame)  200  is transmitted followed by a series of “P-repeat” frames  200 - 1 ,  200 - 2 ,  200 - 3 ,  200 -n. A P-repeat frame (e.g.,  200 - 1 ) is an inter-coded frame picture that indicates to the decoder in the display  102  that nothing has changed since the previous picture, forcing the display  102  to display the same image again. Each field (e.g., fields  202  and  204 ) will contain the same image as the corresponding field in the previous picture. Hence, each of the P-repeat frames  200 - 1 ,  200 - 2 , and  200 - 3  . . . includes P-repeat fields  202 - 1 / 204 - 1 ,  202 - 2 / 204 - 2 ,  202 - 3 / 204 - 3  . . .  202 -n/ 204 -n, respectively. 
     However, as discussed in the Background section above, this type of trick mode operation can result in undesirable flicker. 
     Hence, a field deinterlace trick mode sequence according to an embodiment of the present invention changes the sequence to use pairs of P-repeat field pictures instead of the P-repeat frame pictures as discussed above. In this type of field P-repeat technique, one of the previously displayed fields (e.g., field  204 ) of a P frame picture is used as the reference field for both P-repeat field pictures in the pair as shown in  FIG. 3 . The first field (e.g.,  204 - 1 ) of the pair, is coded to use the second field (e.g.,  204 ) of the previous I-frame or P-frame as its reference, thus making it appear identical to the second field  204 . The second P-repeat field  204 - 11  of the pair also references the second field  204  of the previous I-frame or P-frame, so now both displayed fields contain the same image (e.g., the image in field  204 ). That is, the image displayed by the second P-repeat field  204 - 11  is identical to the image displayed by first P-repeat field  204 - 1 . Naturally, in a further example, the other field  202  can instead be used as the reference field, with the first and second P-repeat fields displaying an image identical to the image displayed by first field  202 . Hence, as illustrated, the P-repeat frames  210 - 1 ,  210 - 2 ,  210 - 3  . . .  210 -n include identical P-repeat field pairs  204 - 1 / 204 - 11 ,  204 - 2 / 204 - 12 ,  204 - 3 / 204 - 13  . . .  204 -n/ 204 - 1 n. 
       FIG. 4  is a flowchart illustrating examples of operations performed by the decoder  101  when it is receiving a field deinterlace trick mode sequence according to an embodiment of the present invention. For example, in step  400 , the decoder  101  receives the video signal. In step  402 , the decoder  101  receives a P-repeat picture if a trick mode operation (e.g., reverse, fast forward, pause) is being performed. If not, in step  404 , the decoder  101  passes the interlaced fields to the display  102  for display and stores the fields for future use as references. However, if a trick mode operation is being performed, the decoder  101  in step  406  is instructed to use one of the fields (e.g., field  204 ) of a frame  200  as a reference field, which is used in step  408  to create the first field of the frame to be displayed in the trick mode. In step  410 , the decoder  101  is instructed to use the same field (field  204 ) as the reference for the second field of the frame to be displayed in the trick mode. The process then repeats for each next frame in the trick mode, until the trick mode operation no longer being performed. 
     As can be understood by one skilled in the art, the above technique can use standard MPEG syntax elements to force the client (e.g., decoder  101 ) to deinterlace the content as the decoder  101  is decoding the content. The technique is applicable to, for example, content encoded in MPEG-2 or H.264/AVC and, since it uses only standard syntax elements defined by the relevant standards, it can be applied to any decoder that complies with those standards The MPEG-2 related technique can use macroblocks coded to reference the opposite field in the cross field P-repeat. An implementation for H.264/AVC can use Memory Management Control Operations (MMCOs) in the P-repeat pictures to manipulate the reference picture list to achieve the cross field reference. As can be appreciated by one skilled in the art, this technique can be more flexible and requires less memory than the coded macroblock approach used for MPEG-2. 
     In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.