Abstract:
A system and method for preprocessing a bitstream of compressed video data is presented herein. The bitstream of compressed video data can include, for example, a bitstream in accordance with the MPEG AVC standard. The bitstream is received and modified by a preprocessor to facilitate multi-row decoding. The modifications to the bitstream can include identification of starting points of macroblock rows with row headers. Additionally, multi-row decoding is further facilitated by generation of decode descriptors which indicate the starting row positions in the modified bit stream. Additionally, the modified bit stream can be formatted in accordance with a simpler coding scheme to simplify decompression.

Description:
PRIORITY DATA  
       [0001]    This application claims the priority benefit of Provisional Application Serial No. 60/380,520 filed May 14, 2002.  
       RELATED APPLICATIONS  
       [0002]    This application is related to Utility Application Serial No. ______ Attorney Docket No. 14095US02, filed Oct. 18, 2002, and Provisional Application Serial No. 60/382,267, filed May 20, 2002, each of which are incorporated herein by reference in their entirety. 
     
    
     
       FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0003]    [Not Applicable] 
         MICROFICHE/COPYRIGHT REFERENCE  
         [0004]    [Not Applicable] 
         BACKGROUND OF THE INVENTION  
         [0005]    The present invention is related to digital video, and more particularly to a system and method for entropy code preprocessing.  
           [0006]    A video sequence includes a series of images represented by frames. The frames comprise two-dimensional grids of pixels. An exemplary video sequence, such as a video sequence in accordance with ITU-656, includes 30 720×480 pixel frames per second. The foregoing results in a bit rate of approximately 165 Mbps for one video sequence.  
           [0007]    Multiple video sequences are transmitted together on a communication medium such as a coaxial cable, using a multiple access scheme. The multiple access scheme can include, for example, frequency division multiple access (FDMA), or time division multiple access (TDMA). In a multiple access scheme each video sequence is associated with a particular channel. As the number of video sequences which are transmitted increases, the bandwidth requirements for the communication medium are further increased.  
           [0008]    Accordingly, a number of data compression standards have been promulgated to alleviate bandwidth requirements. One of the most popular standards was developed by the Moving Pictures Experts Group (MPEG), and is known as MPEG. Pursuant to the MPEG standard, each picture is subdivided into regions of 16×16 pixels, each of which are represented by a macroblock. A macroblock stores luminance and chrominance matrices which are mapped to the 16×16 pixels. The macroblocks are grouped into any number of slice groups or slices. The MPEG standard has been subjected to a number of updates and revisions, resulting in numerous versions.  
           [0009]    In a version known as MPEG-2, each of the slices contain macroblocks which are all in the same row and contiguous with respect to one another. The foregoing property permits transmission of a bit stream of the macroblocks in raster scan order by transmitting the slices in raster scan order. MPEG-2 also includes syntax indicating where each row in a picture begins. The indication of where each row in a picture begins permits decoding of multiple rows in parallel. Parallel decoding of multiple rows, known as multi-row decoding is especially useful for achieving a decoding rate sufficient for presentation of the video sequence on a video display.  
           [0010]    More recent standards, such as the Joint Video Team (JVT) project of ISO-MPEG and ITU-VCEG, known as MPEG AVC or MPEG-4 Part 10, provide much better compression than MPEG-2, in terms of compressed bit rate for a given level of quality. This makes AVC attractive for commercial deployment. However, the AVC standard has a number of properties which make decoding substantially more complicated. For example, the macroblocks forming a slice are not necessarily spatially contiguous. Slice groups can include macroblocks that are throughout the entire picture with macroblocks from other slices groups interspersed therebetween. Additionally, new rows do not necessarily begin in a new slice and an AVC bitstream does not necessarily indicate where each row starts. The foregoing make multi-row decoding difficult.  
           [0011]    Additionally, compressed video standards such as MPEG-2 and MPEG AVC include specifications for encoding various syntax elements using either variable length codes or arithmetic coding; these methods are referred to as entropy coding, since they take advantage of the probabilities of the various values that the syntax elements can take on, and they generally produce different numbers of bits to indicate the information that needs to be conveyed. Some of these formats, particularly adaptive arithmetic coding such as CABAC in AVC, adapt the meaning of each transmitted bit according to the sequence of bits previously transmitted in the same slice. That is, the interpretation of each bit is dependent on previous bits. Therefore it is generally not possible to begin decoding from a mid-point of a slice without having already decoded all the previous bits in the same slice. This makes multi-row decoding of streams encoded using CABAC extremely difficult.  
           [0012]    Multi-row decoding is valuable for a number of reasons. One major reason is achieving the decoding performance requirements. The video sequence is transmitted at rates that may range from less than 1 Mbps up to 20 Mbps in most applications. However, due to the wide variations in the number of bits associated with each picture and with each macroblock of each picture, the peak decoding rate required for displaying a video sequence in real time can be as high as 750-1000 Mbps.  
           [0013]    Accordingly, it would be beneficial if multi-row decoding of compressed video data could be facilitated.  
           [0014]    Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with embodiments of the present invention as set forth in the remainder of the present application.  
         BRIEF SUMMARY OF THE INVENTION  
         [0015]    A system and method for facilitating multi-row decoding of compressed video data and for facilitating decoding of entropy coded data is presented herein. The bit stream of compressed video data is preprocessed by a preprocessor prior to storage in a compressed data buffer. The preprocessor parses and modifies the bit stream of compressed video data and places the modified bit stream of compressed video data in a compressed data buffer. The modifications facilitate multi-row decoding by a decompression engine.  
           [0016]    In one embodiment, the preprocessor modifies the bit stream of compressed video data by inserting byte aligned slice headers which indicate the start of a macroblock row. The preprocessor can also provide ancillary information which indicate the memory address in the compressed data buffer of the starting points of the macroblock rows.  
           [0017]    In another embodiment, the preprocessor replaces the bit stream of compressed video data with another bit stream. The other bit stream represents the compressed video data in a format that eliminates the dependencies between bits that would otherwise have made multi-row decoding difficult or impossible. The simpler format for decoding allows the decompression engine to decode the bit stream using multi-row decoding.  
           [0018]    These and other advantages and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings. 
       
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a block diagram of an exemplary system for transmitting video data to a display unit;  
         [0020]    [0020]FIG. 2 is a block diagram of an exemplary compression hierarchy;  
         [0021]    [0021]FIG. 3 is a block diagram of an exemplary slice group scheme;  
         [0022]    [0022]FIG. 4A is a block diagram of an exemplary bitstream;  
         [0023]    [0023]FIG. 4B is a block diagram of an exemplary bitstream modified in accordance with an embodiment of the present invention; and  
         [0024]    [0024]FIG. 5 is a block diagram of an exemplary decoder in accordance with an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    Although the illustrated embodiments are described with emphasis on the AVC standard, it should be noted that the present invention is not limited to the AVC standard and is applicable in other contexts. Referring now to FIG. 1, there is illustrated a block diagram of an exemplary transmission system for providing a video sequence  105  to a display unit  110  over a communication medium  125 . A video sequence  105  includes a series of images represented by frames. The frames comprise two-dimensional grids of pixels. An exemplary video sequence  105 , such as a video sequence in accordance with ITU-656, includes 30 720×480 pixel frames per second.  
         [0026]    The communication medium  125  may comprise either a point-to-point link or a network of links, or any combination thereof. The network of links may include either a broadcast network, a switched network, or a packet switched network, such as the internet, or any combination thereof. The links may include, for example, a coaxial cable, an Ethernet connection, a Digital Subscriber Loop (DSL), an optical fiber, a satellite/radio link, or a phone line.  
         [0027]    The video sequence  105  is received by encoder  140 . The encoder  140  encodes the video sequence  105  pursuant to the AVC standard. The AVC standard is described in the MPEG AVC Final Committee Draft (FCD), which is incorporated by reference herein, in its entirety. Pursuant to the AVC standard, the video sequence  105  is represented by a bitstream including a series of data packets, known as AVC packets  142 . The bitstream of AVC packets  142  are transmitted over the communication channel  125  and received by a decoder  145 . The decoder  145  decodes the AVC packets, providing video sequence  105 ′ which is typically imperceptibly different from video sequence  105  to the human eye. The video sequence  105 ′ is provided for display to the display unit  110 .  
         [0028]    Referring now to FIG. 2, there is illustrated a block diagram of the AVC stream hierarchy. A video sequence  105  includes a series of pictures  305 . Each picture  305  comprises encoded data representing a two-dimensional array of pixels. Each picture  305  is further divided into macroblocks  312  wherein each macroblock  312  comprises encoded data representing 16×16 segments of the two dimensional array of pixels.  
         [0029]    A picture  305  may be divided into slices  315 , wherein each slice  315  includes any number of encoded macroblocks  312 . In certain versions of MPEG, such as MPEG-2, slices  315  include contiguous macroblocks  312  from left to right, and consecutive slices are in order from left to right and top to bottom. However, in AVC, the macroblocks  312  forming a slices  315  are not necessarily contiguous. Slices  315  can include macroblocks  312  that are throughout the entire picture  305  with macroblocks  312  from other slices  315  interspersed therebetween. The foregoing is known as Flexible Macroblock Organization (FMO). When FMO is utilized in AVC, slices are generally organized into slice groups.  
         [0030]    Referring now to FIG. 3, there is illustrated an exemplary macroblock map. The macroblocks  312  are mapped to a picture  305  forming any number of sequential rows  360  of sequential macroblocks  312 . The macroblocks  312  in the present example are grouped into two slice groups  315 , slice group 0, and slice group 1. In the illustration, the macroblocks  312  which are grouped into slice group 0 are labeled with the reference numeral 0, while the macroblocks  312  which are grouped into slice group 1 are labeled with the reference numeral 1.  
         [0031]    As can be seen in the present illustrated example, both slice group 0, and slice group 1 are non-contiguous and cover the entire picture  305 . Additionally, macroblocks  312  of slice group 0 are interspersed between the macroblocks  312  of slice group 1 and vice versa. Additionally, new rows  360  do not necessarily coincide with changes in slice groups. For example, macroblock  312  (2,0) is the first macroblock of row  360  (2). However the sequentially preceding and succeeding macroblocks  312  (1,m−1) and  312  (2,1) are also in the same slice group  315 , e.g., slice group 1. The macroblocks  312  are transmitted in a bitstream one slice group at a time, e.g., slice group 0, and then slice group 1, over the communication medium  125  to the decoder  145 .  
         [0032]    Referring now to FIG. 4, there is illustrated an exemplary bitstream  142  transmitting the macroblocks  312  shown in FIG. 3. The macroblocks  312  are indicated by vertical dashed lines, however, the macroblocks  312  do not necessarily begin or end on identifiable bit or byte boundaries. As noted above, the macroblocks  312  of slice group 0 are followed by the macroblocks  312  of slice group 1.  
         [0033]    The macroblocks  312  are represented by a set of variable length codes. There is no indication in the bitstream  142  indicating which macroblock  312  starts a new row. For example, macroblock  312  (0,m−1) and macroblock  312  (1,0) are sequentially encoded with no indicator indicating that row 1 starts at the location of macroblock  312  (1,0) in the bitstream  142 .  
         [0034]    Discontinuities in the sequence of macroblocks  312  within the slice group  315  are indicated by an entropy coded syntax element  415 , mb_skip_run (skipped macroblocks), which indicates the number of skipped macroblocks until the next macroblock in the bitstream  142 . For example, macroblock  312  (1,0) is followed by variable syntax element  415  indicating m+1 skipped blocks.  
         [0035]    Referring now to FIG. 5, there is illustrated a block diagram describing an exemplary decoder  505  in accordance with an embodiment of the present invention. The decoder  505  receives and decompresses the bitstream  142 . Decompression of the bitstream is achieved by a video decompression engine  535 . The video decompression engine  535  decompresses a bitstream of compressed video data, and thereby produces a video sequence  105 ′. The video sequence  105 ′ is displayable by a display unit, such as display unit  110 . The bitstream  142  is received at a rate that may vary from less than 1 Mbps to 20 Mbps. However, due to the wide variation in the number of bits associated with each macroblock and with each picture, the peak decoding rate for the video decompression engine  535  for providing a video sequence  105 ′ for display in real time can be as high as 750-1000 Mbps. Due to the high peak performance requirements for the video decompression engine  535 , it is preferable for the decompression engine to multi-row decode the macroblocks  312  in parallel. Accordingly, the bitstream  142  is preprocessed prior to decompression to facilitate multi-row decoding and to facilitate simpler processing of the data stream.  
         [0036]    The bitstream  142  is received by the system layer processor  510 . The system layer processor  510  parses the system layer, extracting elementary video stream data and any system layer information that may be necessary for decoding and display, such as time stamps. The output of the system layer processor is a bitstream  142  comprising video elementary stream data  142 .  
         [0037]    The bitstream  142  can be written to a smoothing buffer  520 . The smoothing buffer  520  may be implemented as an Static Random Access Memory (SRAM) on-chip or as a region of Dynamic RAM (DRAM) off-chip. The smoothing buffer  520  stores the data temporarily and smoothes the data rate.  
         [0038]    The bitstream  515  is read by a preprocessor  525 . The preprocessor  525  parses and modifies the bitstream  142  and places a modified bitstream  142 ′ in a compressed data buffer  530 . The modifications to the bitstream  515  facilitate multi-row decoding by a decompression engine  535 .  
         [0039]    The preprocessor  525  segments the bitstream  142 ′ into data groupings of interest which may be selected to facilitate parallel decoding operations, such as macroblock rows. The remainder of this description will be illustrated with an emphasis on data groupings that include macroblock rows, with the understanding that other data groupings are also possible.  
         [0040]    Where the bitstream  142  comprises AVC encoded data, the preprocessor  525  preferably modifies the bitstream  142  to remove dependencies between bits within a slice, as necessary to facilitate multi-row decoding. For example, where the bitstream  142  comprises data encoded with the AVC CABAC format, the bitstream may be transcoded to a modified bitstream  142 ′ conveying the same information in a more simplified variable length coding format, or a fixed length format that can be decoded starting from a point other than the beginning of a slice. In one embodiment, the bitstream  142  can be transcoded to modified bitstream  142 ′ as is described in further detail in “System and Method for Transcoding Entropy-Coded Bitstreams”, by MacInnis, et. al., U.S. application for patent Ser. No. ______, Attorney Docket No. 14095US02, filed Oct. 18, 2002, which is hereby incorporated by reference in its entirety.  
         [0041]    Referring now to FIG. 4B, there is illustrated an exemplary modified bitstream  142 ′ representing modifications to bitstream  142  of FIG. 4A. The syntactical portions of the bitstream are parsed to locate the points therein where macroblock rows start. The syntax is modified to include row headers  430  where the macroblock rows  360  start and to byte align each row. The byte alignment and headers  430  indicating the start of a row preferably follow the same syntax followed by the slice header as specified in the AVC standard, although a wide variety of different syntaxes can be followed.  
         [0042]    The beginning of each row is found by parsing the bitstream  142  from the beginning of each slice until at least the beginning of each row that is to be identified. For example, the choice of which variable length code table is used to decode a particular element may depend on the value of one or more previously decoded elements. In some cases the values of the elements may be discarded once each syntax element has been parsed.  
         [0043]    Additionally, the preprocessor  525  also performs the inverse of the anti-emulation process specified in AVC. The forward anti-emulation process inserts data according to a specified algorithm as a means to prevent video elementary stream data from accidentally having strings of bits that match the start code prefix. The inverse of the anti-emulation process is advantageously performed by the preprocessor  525  because of the high peak performance rates required of the decompression engine  535  and because performing this function in the pre-processor helps to facilitate multi-row decoding.  
         [0044]    As noted above, the macroblocks  312  of the slice groups  315  encoded together are not continuous with respect to the raster scan order. The foregoing discontinuities are indicated by the skipped macroblocks parameter  415 . In some cases, the value of the skipped macroblock parameter  415  and its location in the bitstream  142  may be such that the run of skipped macroblocks falls on two or more macroblock rows  360 . In order to mark the beginning of the second and succeeding rows, the preprocessor  525  changes the skipped macroblocks parameter  415 . The skipped macroblocks parameter  415  is changed to include a first skipped macroblocks parameter  415  a indicating a number of skipped macroblocks which extends to the end of the first row. The first skipped macroblocks parameter  415  is followed by an inserted a row header  430 . Following the row header  430 , the preprocessor  525  inserts another skipped macroblocks parameter  415   b  indicating the remaining number of macroblocks that are skipped.  
         [0045]    If the remaining number of macroblocks that are skipped from the row header  430  extend into another row, macroblock parameter  415   b  indicates the number of macroblocks extending to the end of the row, another row header  430  is inserted, and another macroblock parameter  415   b  indicates the number of skipped macroblocks from the next row header  430 .  
         [0046]    For the exemplary macroblock of FIG. 3, a row header  430  is inserted immediately after the portion of the bitstream  142  encoding macroblock  312  (0,m−1), indicating the start of row 1. In the bitstream  142 , macroblock  312  (1,0) is followed by the skipped macroblock parameter  415  indicating that m+1 macroblocks are to be skipped, i.e., macroblock  312  (2,2) is the next macroblock in the bitstream  142 .  
         [0047]    The preprocessor  525  replaces the skipped macroblock parameter  415  with a first skipped macroblock parameter  415  a indicating that m−1 macroblocks are skipped. The first skipped macroblock parameter  415   a  is followed by a row header  430 , indicating the start of row 2. The row header  430  indicating the start of row 2 is byte aligned and followed by a second skipped macroblock parameter  415   b  indicating that 2 macroblocks are skipped. After the second macroblock parameter  415   b , the macroblock  312  (2,2) follows.  
         [0048]    Referring again to FIG. 5, the resulting modified bitstream  142 ′ is written to the compressed data buffer  530 . The compressed data buffer  530  is typically a large buffer implemented in DRAM, with a size of typically several hundred kilobytes to one or more megabytes. Additionally, in an exemplary embodiment the preprocessor  525  generates decode descriptors indicating the addresses in the compressed data buffer  530  where each of the macroblock rows  360  begin. The decode descriptors are stored in a decode descriptor buffer  540 .  
         [0049]    The video decompression engine  535  can then decompress bitstream  142 ′ in a parallel, multi-row fashion. Multi-row decompression is described in greater detail in “System for and Method of Decoding of Interleaved Macroblocks of Video”, Provisional Application for Patent Serial No. 60/382,267, filed May 20, 2002, by MacInnis, et. al, which is incorporated by reference herein, in its entirety.  
         [0050]    While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.