Abstract:
An efficient decoding of reversible variable length codes of the type implemented for H.263++ and MPEG-4 DCT coefficients using lookup tables. Simple-profile MPEG-4 also has the option of using reversible variable length codewords (RVLC) that can be decoded in either the forward or backward direction, for error recovery. Because of the special structure of RVLC codewords, the decoding techniques for regular VLC are less efficient with RVLC. A new method for RVLC decoding is described.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from provisional application Serial No. 60/184,854, filed Feb. 25, 2000. The following copending U.S. patent applications Ser. Nos. 09/631,261, filed Aug. 2, 2000, disclose related subject matter. 
     
    
     
       BACKGROUND  
         [0002]    The present application relates to information encoding for transmission over noisy channels and/or storage, and more particularly to error resilient coding.  
           [0003]    The current rapid expansion of digital communication (speech, video, data) relies on increasingly economical digital signal processing. For example, video communication has general functionality as illustrated in FIG. 8 a , and increasingly includes a link through the air interface (FIG. 8 b ) which can introduce noise and bit errors to the digital signal. Attempts to mitigate bit errors include the use of reversible codes as described in the following.  
           [0004]    Commonly used video compression methods (e.g., MPEG) have block-based motion compensation to remove temporal redundancy (code only (macro)block motion vectors plus the corresponding quantized DCT residuals (texture) as in FIG. 8 c ) and use variable length coding (VLC) to increase coding efficiency. However, variable length coding often is highly susceptible to transmission channel errors, and a decoder easily loses synchronization with the encoder when uncorrectable errors arise. Further, the predictive nature of motion compensation makes matters much worse because the uncorrectable errors in one video frame quickly propagate across the entire video sequence and rapidly degrade the decoded video quality.  
           [0005]    The typical approach to such uncorrectable errors includes the steps of: error detection (e.g., out-of-range motion vectors, invalid VLC table entry, or invalid number of residuals in a block), resynchronization of the decoder with the encoder, and error concealment by repetition of previously transmitted correct data in place of the uncorrectable data. For example, video compressed using MPEG1-2 has a resynchronization marker (start code) at the start of each slice of macroblocks (MBs) of a frame, and an uncorrectable error results in all of the data between correctly decoded resynchronization markers being discarded. This implies degradation in quality of the video stream.  
           [0006]    VLC tables prove to be particularly sensitive to bit errors because bit errors can make one codeword be incorrectly interpreted to be another codeword of a different length, and the error is not detected. This makes the decoder lose synchronization with the encoder. Although the error may finally be detected due to an invalid VLC table entry, usually the location in the bitstream where the error is detected is not the same as the location where the error occurred. Hence, when the decoder detects an error, it has to seek the next resynchronization marker and discard all the data between this and the previous resynchronization marker. Thus, even a single bit error can sometimes result in a loss of a significant amount of data, and this is a problem of the known coding schemes.  
           [0007]    Enhanced error concealment properties for motion compensated compression, such as MPEG, can be achieved by using data partitioning. Consider a “video packet” to consist of the data between two consecutive resynchronization markers. In a data partitioning approach, the motion data and the texture (DCT) data within each of the video packets are separately encoded in the bitstream. Another resynchronization word (Motion Resync. Word) is imbedded between the motion data and the DCT data to signal the end of the motion data and the beginning of the DCT data. This data partitioning allows the decoder to use the motion data even if the DCT data is corrupted by undetectable errors. This provides advantages including partial recovery over uncorrectable error in a packet of compressed video data with little additional overhead. The error concealment that is made possible by the use of motion compensation by applying decoded motion vectors results in a much better decoded video quality. And this extends to video packets for intra-coded frames in that the DCT dc coefficients can be separated from the other, less important texture data (DCT ac coefficients) by a DC resynchronization word.  
           [0008]    When using data partitioning the data within the video packet is organized to look as shown in FIGS. 6 a - c : FIG. 6 a  shows the fields between two resynchronization markers and FIGS. 6 b - c  illustrate the motion data field and the texture data field in more detail by an example. In particular, the first field (“Resynch Marker”) is a resynchronization marker, the second field (“MB No.”) is the the number in the frame of the first macroblock (16×16 block of pixels) in the video packet, the third field (“QP”) is the default quantization parameter used to quantize the texture data (DCT coefficients) in the video packet, the fourth field (“Motion Data”) is the motion data, the fifth field (“Motion Resynch Word”) is the resynchronization marker between the motion data and the texture data, the sixth field (“DCT Data”) is the texture data, and the last field (“Resynch Marker”) is the ending resynchronization marker.  
           [0009]    [0009]FIG. 6 b  shows the motion data field consisting of a COD field, an MCBPC field, and an MV field for each of the macroblocks in the packet. The COD field indicates whether the macroblock is coded or skipped (COD=0 macroblock is coded, COD=1 macroblock is skipped). The MCBPC field indicates (1) the mode of the macroblock and (2) which of the chrominance blocks in the macroblock are coded and which are skipped: the mode indicates whether the current macroblock is coded INTRA (no motion compensation), INTER (motion compensated with one 16×16 motion vector), or INTER4V (motion compensated with four 8×8 motion vectors). Of course, if COD indicates the macroblock is not coded, then the MCBPC field is not present. The MV field is the actual motion vector data; either one vector or four vectors. Again, if COD indicates that the macroblock is not coded, then the MV field is not present. FIG. 6 c  shows the texture (DCT Data) field as consisting of a CBPY field and a DQUANT field for each of the macroblocks followed by the DCT data for each of the macroblocks. The CBPY field indicates which of the luminance blocks of the macroblock are coded and which are skipped. The DQUANT field indicates the differential increment to the default quantizer value (QP) to compute the quantization value for the macroblock. The DCT fields are run-length-encoded quantized DCT coefficient values of the macroblock.  
           [0010]    MPEG-4 has three kinds of VLCs to encode the DCT coefficients: Table B-16 for encoding INTRA macroblocks, Table B-17 for coding INTER macroblocks, and Table B-23 which is used for coding macroblocks if reversible variable length codes (RVLC) are used. In contrast, H.263 uses only one table for encoding both the INTER and INTRA MBs: Table 13/H263. Table 13/H263 is identical to Table B-17 of MPEG-4.  
           [0011]    Decoding normal VLCs (Table B-16/B-17 of MPEG-4 and Table 13/H263 of H.263) is done using identical techniques, thus consider the decoding of VLCs from Table 13/H263 of H.263. The length of VLC codewords in Table 13/H263 varies from 3 to 13 bits. The last bit is always a sign bit and is not used in variable length decoding, so the number of decodable bits varies up to 12. Variable length decoding is typically carried out in most of the standard decoders by using two different tables: DCT3DtabXval which contains the entries of Table 13/H263 and DCT3DtabXlen which contains the length of the corresponding codewords. Since the length of the VLC codeword is not known in advance, the fastest way to decode a VLC would be by using 2 12  (4096) entries in DCT3DtabXval. If 2 12  elements are used in DCT3DtabXval, then 12 bits from the bitstream can be read and be directly used to index into DCT3DtabXval to obtain the decoded values. The same 12 bits are indexed into DCT3DtabXlen to obtain the length of the codeword. The initial bits in the bitstream corresponding to the length of the codeword are then discarded and the process is repeated on the remaining bits plus the next bits in the bitstream up to 12 bits. Note that there are only 102 entries in Table 13/H263. Hence the DCT3DtabXval table and the DCT3DtabXlen table in sequential memory with index addresses would contain many duplicate entries. To conserve on memory, one may split DCT3DtabXval/DCT3DtabXlen based on the number of leading zeros present in the 12 bits, but this increases index complexity.  
           [0012]    Reversible variable length codes (RVLC) are designed such that they can be decoded in either the forward or backward direction; see FIGS.  1 - 4  illustrating error detection possibilities. These codes are useful when the data may be corrupted by errors. MPEG-4 video includes the option to use RVLC for the DCT data. The RVLC for MPEG-4 is formed by concatenating a variable-length code (VLC) with a fixed-length code (FLC) where the FLC part has 2 bits, including the sign bit. The VLC part either starts and ends with a 1 with all, if any, zeroes in between; or starts and ends with a 0 with exactly one 0 in between (the rest of the bits, if any, equal 1). That is, the VLC part is either 100 . . . 001 (with possibly no 0s) or 01 . . . 101 . . . 10 with position of the interior 0 anywhere among the 1s. The longest valid RVLC codeword is 15 bits plus a sign bit, so the VLC part can be as long as 14 bits. Contrarily, the VLC part can be as short as 2 bits (11 if starting with a 1) or 3 bits (000 if starting with a 0). Because of the way the RVLC is designed, the codebook is very sparse. Also, not all RVLC codewords with this structure are used; see the following table which shows codewords 01 1111 1101 1110 through 01 1111 1111 1100 are not used among the 14-bit VLC part. Codewords are defined for the 169 most commonly occurring events (combinations of last, level, and run), and an escape codeword for all other cases. The RVLC structure does not lend itself to the type of table lookup strategy that is used for regular VLCs. With the typical VLC lookup strategy, 15 bits (without the sign bit) would be read from the bitstream. A single table would require 2 15 =32K (0x0000 to 0x7FFF in hexadecimal) entries, or the table could be partitioned according to the number of leading zeroes. However, for the MPEG-4 RVLC, this doesn&#39;t help much: there are 24 codewords beginning with a 1, 22 codewords with two leading 0&#39;s, only two codewords with three or more leading 0&#39;s, and all other 122 codewords have exactly one leading 0. The lookup table for codewords with exactly one leading 0 would have to cover indices 010 00xx xxxx xxxx through 011 1111 0111 1101, requiring 0x3F7D−0x2000=0x1F7D (=8061 decimal) entries for the 122 codewords that begin with a single leading 0. Thus, the normal VLC decoding approach is very inefficient for RVLC decoding. Because RVLC codewords require so much memory to use the typical VLC decoding approach, the known MoMuSys software uses a brute force approach for decoding RVLC of MPEG-4 Table B-23 as follows. A test is made, comparing with each possible codeword (not counting the sign bit). When the match is found, the index into a densely-packed lookup table is hard-coded. The MoMuSys RVLC decode is implemented using one gigantic case statement. For efficiency, it is best to test for the shorter, more common codewords first. However, the worst-case cycles can be quite high, if all 169 cases are tested before finding a match. And this is not sufficiently efficient for effective use of RVLC decoding with MPEG-4.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention uses a codeword hashing index to access reversible VLC (RVLC) tables such as in MPEG and H.263..  
           [0014]    This has the advantage of better performance and smaller memory requirements for RVLC decoding. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The drawings are heuristic for clarity.  
         [0016]    FIGS.  1 - 4  shows error detection with reversible coding.  
         [0017]    [0017]FIG. 5 illustrates the fixed length code for events outside of the preferred embodiment code tables.  
         [0018]    [0018]FIGS. 6 a - 6   c  indicate video bitstream structure.  
         [0019]    [0019]FIG. 7 is a flow diagram.  
         [0020]    [0020]FIGS. 8 a - 8   c  illustrate systems.  
         [0021]    [0021]FIG. 9 shows a header structure.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Overview  
         [0023]    The preferred embodiments store/decode the reversible variable length codes of MPEG-4 Table B-23 (used for DCT coefficients) with a hash-type function of the codeword providing an index (address) for compact memory storage and efficient decoding. Thus preferred embodiment systems such as teleconferencing or Internet video reception would have a receiver with a memory storing the Table B-23 in a preferred embodiment indexed manner and available for use for decoding received video.  
         [0024]    Note that these video systems and decoding methods may be implemented on special integrated circuits or on programmable microprocessors or digital signal processors such as the TMS320C5000 or C6000 families from Texas Instruments.  
         [0025]    First Preferred Embodiments  
         [0026]    Preliminarily consider modification of the MoMuSys code for RVLC decoding to reduce the worst-case computation. Because of the RVLC structure, it is possible to find the end of the codeword during parsing. The codeword can be extracted without any trailing “don&#39;t care” bits before it is decoded. The worst-case computation can be reduced by testing for ranges of code values before testing for matches. For instance, the first preferred embodiments modify the MoMuSys code to test according to the position of the MSB (after finding the end of the codeword and storing it in a separate memory location), i.e.,  
                                                                       if (code &lt; 0 × 8) { }           else if (code &lt; 0 × 10) { }           else if (code &lt; 0 × 20) { }           else if (code &lt; 0 × 40) { }                . . .                else if (code &lt; 0 × 2000) { }           else { }                      
 
         [0027]    This approach breaks the codewords into 12 classes of similar size. But even after the codewords are broken into these 12 classes, a table lookup is not memory-efficient. For example, the 24 codewords between 0x1000 and 0×2000 range from 0x1002 to 0x1FF9, which would require a table of size 0x1FF9−0x1002=4087 (decimal) entries. Thus, the code is written to test for matches against every possible codeword, but some matches are skipped by dividing the codewords into classes. The worst case requires 35 “if” tests (compared to 169), or one more, for an invalid codeword. Note, similar techniques could be applied for backward decoding of RVLC data.  
         [0028]    Note that the events coded for an 8×8 block of DCT coefficients are a combination of three fields: a 1-bit LAST field which indicates whether the coefficient is the last nonzero coefficient in the block, a 6-bit RUN which indicates the number of successive zeros preceding the coded coefficient, and an 11-bit LEVEL which is the nonzero value of the coded coefficient. As illustrated in FIG. 5 a 5-bit ESCAPE code (0000x) begins and ends the event code with the last bit of the end ESCAPE code reflecting the sign of the nonzero coefficient. Table B-23 provides RVLC for the 169 most common events plus the ESCAPE for less common events.  
         [0029]    The sparse nature of RVLC codewords makes it inefficient to use table lookups indexed by the codeword itself, but the structure of the codewords is used by the preferred embodiments to define a sort of hashing function to access a table that can be more densely packed. In particular, the preferred embodiments define an index (for a codeword table) based on the codeword itself as follows.  
         [0030]    (1) For codewords starting with 1 (VLC part of form 100 . . . 001), form an index into a separate table of such codewords:  
         [0031]    Index=2*(number of 0&#39;s in VLC part)+first bit (non-sign bit) of FLC part. Thus this index runs from 0 to 23. The index can also be expressed as a 5-bit concatenation of 4 bits to indicate the up-to-eleven 0&#39;s in the VLC part followed by the 1 bit equal to the non-sign bit of the FLC part. And the indexing running from 0 to 23 implies a 24-word block of memory can contain each of the inter and intra tables.  
         [0032]    (2) For codewords starting with 0 (VLC part of form 01 . . . 101 . . . 10), similarly form an index into a table for such codewords:  
         [0033]    Index=32*(number of 1&#39;s in VLC part)+2*(number of 1&#39;s preceding the interior 0 in VLC part)+first bit (non-sign bit) of FLC.  
         [0034]    Thus this index runs from 0 to 365 and can be expressed as a 9-bit concatenation of 4 bits indicating the up to eleven interior 1&#39;s of the VLC part followed by 4 bits indicating the up to ten 1&#39;s preceding the interior 0 in the VLC part and completed by the 1 bit equal to the non-sign bit of the FLC part. The index running from 0 to 365 implies a 366-word block of memory can contain each of the inter and intra tables.  
         [0035]    The computation of the appropriate index can be done fairly efficiently as a byproduct of the parsing, especially on devices (such as DSPS) which offer a single-cycle EXP-like instruction to count the number of leading 0&#39;s or 1&#39;s (after stripping off the starting bit). The following table shows the MPEG-4 RVLC table B-23, with the corresponding preferred embodiment “hash” indices shown beside the codeword for the 0-starting and 1-starting codewords.  
                                                                                                                                                                             B-23   intra   inter       1-start   0-start                index   LAST   RUN   LEVEL   LAST   RUN   LEVEL   BITS   hash Index   hash Index   RVLC_CODE                    0   0   0   1   0   0   1   4    0       110s       1   0   0   2   0   1   1   4    1       111s       2   0   1   1   0   0   2   5        1   0001s       3   0   0   3   0   2   1   5    2       1010s       4   1   0   1   1   0   1   5    3       1011s       5   0   2   1   0   0   3   6        32   00100s       6   0   3   1   0   3   1   6        33   00101s       7   0   1   2   0   4   1   6        34   01000s       8   0   0   4   0   5   1   6        35   01001s       9   1   1   1   1   1   1   6    4       10010s       10   1   2   1   1   2   1   6    5       10011s       11   0   4   1   0   1   2   7        64   001100s       12   0   5   1   0   6   1   7        65   001101s       13   0   0   5   0   7   1   7        66   010100s       14   0   0   6   0   8   1   7        67   010101s       15   1   3   1   1   3   1   7        68   011000s       16   1   4   1   1   4   1   7        69   011001s       17   1   5   1   1   5   1   7    6       100010s       18   1   6   1   1   6   1   7    7       100011s       19   0   6   1   0   0   4   8        96   0011100s       20   0   7   1   0   2   2   8        97   0011101s       21   0   2   2   0   9   1   8        98   0101100s       22   0   1   3   0   10   1   8        99   0101101s       23   0   0   7   0   11   1   8       100   0110100s       24   1   7   1   1   7   1   8       101   0110101s       25   1   8   1   1   8   1   8       102   0111000s       26   1   9   1   1   9   1   8       103   0111001s       27   1   10   1   1   10   1   8   8       1000010s       28   1   11   1   1   11   1   8   9       1000011s       29   0   8   1   0   0   5   9       128   00111100s       30   0   9   1   0   0   6   9       129   00111101s       31   0   3   2   0   1   3   9       130   01011100s       32   0   4   2   0   3   2   9       131   01011101s       33   0   1   4   0   4   2   9       132   01101100s       34   0   1   5   0   12   1   9       133   01101101s       35   0   0   8   0   13   1   9       134   01110100s       36   0   0   9   0   14   1   9       135   01110101s       37   1   0   2   1   0   2   9       136   01111000s       38   1   12   1   1   12   1   9       137   01111001s       39   1   13   1   1   13   1   9   10       10000010s       40   1   14   1   1   14   1   9   11       10000011s       41   0   10   1   0   0   7   10       160   001111100s       42   0   5   2   0   1   4   10       161   001111101s       43   0   2   3   0   2   3   10       162   010111100s       44   0   3   3   0   5   2   10       163   010111101s       45   0   1   6   0   15   1   10       164   011011100s       46   0   0   10   0   16   1   10       165   011011101s       47   0   0   11   0   17   1   10       166   011101100s       48   1   1   2   1   1   2   10       167   011101101s       49   1   15   1   1   15   1   10       168   011110100s       50   1   16   1   1   16   1   10       169   011110101s       51   1   17   1   1   17   1   10       170   011111000s       52   1   18   1   1   18   1   10       171   011111001s       53   1   19   1   1   19   1   10   12       100000010s       54   1   20   1   1   20   1   10   13       100000011s       55   0   11   1   0   0   8   11       192   0011111100s       56   0   12   1   0   0   9   11       193   0011111101s       57   0   6   2   0   1   5   11       194   0101111100s       58   0   7   2   0   3   3   11       195   0101111101s       59   0   8   2   0   6   2   11       196   0110111100s       60   0   4   3   0   7   2   11       197   0110111101s       61   0   2   4   0   8   2   11       198   0111011100s       62   0   1   7   0   9   2   11       199   0111011101s       63   0   0   12   0   18   1   11       200   0111101100s       64   0   0   13   0   19   1   11       201   0111101101s       65   0   0   14   0   20   1   11       202   0111110100s       66   1   21   1   1   21   1   11       203   0111110101s       67   1   22   1   1   22   1   11       204   0111111000s       68   1   23   1   1   23   1   11       205   0111111001s       69   1   24   1   1   24   1   11   14       1000000010s       70   1   25   1   1   25   1   11   15       1000000011s       71   0   13   1   0   0   10   12       224   00111111100s       72   0   9   2   0   0   11   12       225   00111111101s       73   0   5   3   0   1   6   12       226   01011111100s       74   0   6   3   0   2   4   12       227   01011111101s       75   0   7   3   0   4   3   12       228   01101111100s       76   0   3   4   0   5   3   12       229   01101111101s       77   0   2   5   0   to   2   12       230   01110111100s       78   0   2   6   0   21   1   12       231   01110111101s       79   0   1   8   0   22   1   12       232   01111011100s       80   0   1   9   0   23   1   12       233   01111011101s       81   0   0   15   0   24   1   12       234   01111101100s       82   0   0   16   0   25   1   12       235   01111101101s       83   0   0   17   0   26   1   12       236   01111110100s       84   1   0   3   1   0   3   12       237   01111110101s       85   1   2   2   1   2   2   12       238   01111111000s       86   1   26   1   1   26   1   12       239   01111111001s       87   1   27   1   1   27   1   12   16       10000000010s       88   1   28   1   1   28   1   12   17       10000000011s       89   0   10   2   0   0   12   13       256   001111111100s       90   0   4   4   0   1   7   13       257   0011111111010       91   0   5   4   0   2   5   13       258   010111111100s       92   0   6   4   0   3   4   13       259   010111111101s       93   0   3   5   0   6   3   13       260   011011111100s       94   0   4   5   0   7   3   13       261   011011111101s       95   0   1   10   0   11   2   13       262   011101111100s       96   0   0   18   0   27   1   13       263   011101111101s       97   0   0   19   0   28   1   13       264   011110111100s       98   0   0   22   0   29   1   13       265   011110111101s       99   1   1   3   1   1   3   13       266   011111011100s       100   1   3   2   1   3   2   13       267   0111110111010       101   1   4   2   1   4   2   13       268   011111101100s       102   1   29   1   1   29   1   13       269   011111101101s       103   1   30   1   1   30   1   13       270   011111110100s       104   1   31   1       31   1   13       271   011111110101s       105   1               32   1           272   011111111000s       106   1   33   1   1   33   1   13       273   011111111001s       107   1   34   1   1   34   1   13   18       100000000010s       108   1   35   1   1   77   1   13   19       100000000011s       109   0   14   1   0   0   13   14       288   0011111111100s       110   0   15   1   0   0   14   14       289   0011111111101s       111   0   11   2   0   0   15   14       290   0101111111100s       112   0   8   3   0   0   16   14       291   0101111111101s       113   0   9   3   0   1   8   14       292   0110111111100s       114   0   7   4   0   3   5   14       293   0110111111101s       115   0   3   6   0   4   4   14       294   0111011111100s       116   0   2   7   0   5   4   14       295   0111011111101s       117   0   2   8   0   8   3   14       296   0111101111100s       118   0   2   9   0   12   2   14       297   0111101111101s       119   0   1   11   0   30   1   14       298   0111110111100s       120   0   0   20   0   31   1   14       299   0111110111101s       121   0   0   21   0   32   1   14,       300   0111111011100s       122   0   0   23   0   33   1   14       301   0111111011101s       123   1   0   4   1   0   4   14       302   0111111101100s       124   1   5   2   1   5   2   14       303   0111111101101s       125   1   6   2   1   6   2   14       304   0111111110100s       126   1   7   2   1   7   2   14       305   0111111110101s       127   1   8   2   1   8   2   14       306   0111111111000s       128   1   9   2   1   9   2   14       307   011111111,100ts       129   1   36   1   1   36   1   14   20       1000000000010s       130   1   37   1   1   37   1   14   21       1000000000011s       131   0   16   1   0   0   17   15       320   00111111111100s       132   0   17   1   0   0   18   15       321   00111111111101s       133   0   18   1   0   1   9   15       322   01011111111100s       134   0   8   4   0   1   10   15       323   01011111111101s       135   0   5   5   0   2   6   15       324   01101111111100s       136   0   4   6   0   2   7   15       325   01101111111101s       137   0   5   6   0   3   6   15       326   01110111111100s       138   0   3   7   0   6   4   15       327   01110111111101s       139   0   3   8   0   9   3   15       328   01111011111100s       140   0   2   10   0   13   2   15       329   01111011111101s       141   0   2   11   0   14   2   15       330   01111101111100s       142   0   1   12   0   15   2   15       331   01111101111101s       143   0   1   13   0   16   2   15       332   01111110111100s       144   0   0   24   0   34   1   15       333   01111110111101s       145   0   0   25   0   35   1   15       334   01111111011100s       146   0   0   26   0   36   1   15       335   01111111011101s       147   1   0   5   1   0   5   15       336   01111111101100s       148   1   1   4   1   1   4   15       337   0111111110110ts       149   1   10   2   1   10   2   15       338   01111111110100s       150   1   11   2   1   11   2   15       339   01111111110101s       151   1   12   2   1   12   2   15       340   01111111111000s       152   1   38   1   1   38   1   15       341   01111111111001s       153   1   39   1   1   39   1   15   22       10000000000010s       154   1   40   1   1   40   1   15   23       10000000000011s       155   0   0   27   0   0   19   16       352   00111111111110       156   0   3   9   0   3   7   16       353   001111111111101s,       157   0   6   5   0   4   5   16       354   010111111111100s       158   0   7   5   0   7   4   16       355   010111111111101s       159   0   9   4   0   17   2   16       356   011011111111100s       160   0   12   2   0   37   1   16       357   011011111111101s       161   0   19   1   0   38   1   16       358   011101111111100s       162   1   1   5   1   1   5   16       359   011101111111101s       163   1   2   3   1   2   3   16       360   011110111111100s       164   1   13   2   1   13   2   16       361   011110111111101s       165   1   41   1   1   41   1   16       362   011111011111100s       166   1   42   1   1   42   1   16       363   011111011111101s       167   1   43   1   1   43   1   16       364   011111101111100s       168   1   44   1   1   44   1   16       365   011111101111101s            169   ESCAPE   5        0   0000s                  
 
         [0036]    The foregoing table would be stored as two separate tables: one for the 1-starting codewords and having a 5-bit index running from 0 to 23 and one for the 0-starting codewords with a 9-bit index running from 0 to 366.  
         [0037]    [0037]FIG. 7 illustrates the decoding flow.  
         [0038]    Other Preferred Embodiments  
         [0039]    Second preferred embodiments modify the foregoing hash indices in the MPEG-4 B-23 table by combining the 0-start and 1-start indices to a single index:  
         [0040]    Index=32*(number of bits in VLC part)+2*(the position number of the first repeat of the first bit in the VLC part)+non-sign bit of FLC−68.  
         [0041]    This index runs from 0 to 409: the number of bits in the VLC part ranges from 2 to 14 (so the first number can be expressed with four bits), the position of the first repeat is also in the range from 2 to 14 (another four bits), and the last number is one bit. Thus a concatenated 9-bit index suffices. Note that the number of bits in the VLC part equals the position of the first repeat for the codewords starting with a 1; but this is not possible for the codewords starting with a 0. Also, the number of bits and the position number both have a minimum of 2, so the −68 normalizes the index to have a minimum of 0.  
         [0042]    Third preferred embodiments fold the 0-starting codeword table (subtable of foregoing MPEG-4 table) over onto itself to fill in the unused spaces. That is, the Index has an unused gap between 1 and 32, so the 14 Index values 352-365 can be mapped into this gap; similarly the unused gap between 35 and 64 can be filled with the 22 Index values 320-341, the unused gap between 69 and 96 can be filled with the 20 Index values 288-307, and so forth up to the gap between 171 and 192 filled with the 14 Index values 192-205. This requires a little more computation:  
         [0043]    Index=32*(number of 1&#39;s in VLC part)+2*(number of 1&#39;s before interior 0 of VLC part)+non-sign bit of FLC.  
         [0044]    if (Index&gt;=192) Index=383−Index.  
         [0045]    Thus Index values 352-365 map to 18-31, 320-341 map to 42-63, 288-307 map to 76-95, and so forth up to 192-205 map to 178-191. That is, the folded 0-starting codeword table has 146 entries, but the index for the folded table runs from 0 to 191 and thus requires a 192-word memory block for each of the intra and inter tables.  
         [0046]    Fourth Fourth preferred embodiments further reduce the gaps in the folded 0-starting codeword table, using the index:  
               Index     =                    26   *          (number of 1&#39;s in    VLC    part)       +                                  2   *          (number of 1&#39;s before interior 0 of    VLC    part)       +                                non-sign bit of           FLC   .                                   
 
         [0047]    If (Index&gt;=156) Index=311−Index.  
         [0048]    The intermediate Index (before folding) can no longer be formed as the concatenation of three fields, because 26 is not a power of 2, so a little more computation is required. The Index values 286-299 map to 12-25, 260-281 map to 30-51, 234-253 map to 58-77, and so forth up to 156-160 map to 142-155. That is, the compact folded 0-starting codeword table has 146 entries, but the index for the compact folded table runs from 0 to 155 and thus requires a 156-word memory block for each of the intra and inter tables.  
         [0049]    Fifth preferred embodiments further reduce the gaps in the nonfolded 0-starting codeword table, using the index:  
               Index     =                    22   *          (number of 1&#39;s in         VLC                     part   )         +                                  2   *          (number of 1&#39;s before interior 0 of         VLC                   part)       +                                non-sign bit of           FLC   .                                   
 
         [0050]    This Index can not be formed as the concatenation of 3 fields, because 22 is not a power of 2, so a little more computation is required, (but not as much as the compact folded approach). The index running from 0 to 255 implies a 256-word block of memory can contain each of the inter and intra tables.  
         [0051]    In summary, the preferred embodiment indexings and table(s) for MPEG4 B-23 (RVLC for DCT coefficients) reduce the code size significantly, as well as the number of cycles required for execution compared to using a gigantic case statement or nested if-statements, particularly in the worst case. The C code reduces from roughly 900 lines to about 150 lines, at the expense of increasing table storage from 2*170=340 words (inter and intra tables) to 2*24+2*366=780 words. If the 0-start table is folded over, only 2*24+2*192=432 words are required. If the 0-start table is compacted (without folding), only 2*24+2*256=560 words are required. If the 0-start table is folded over and compacted, only 2*24+2*156=360 words are required.  
         [0052]    H.263 preferred embodiments  
         [0053]    The H.263++ standard has an optional data partitioning mode in which the MBs in a video picture segment (packet) are rearranged analogously to MPEG4 so that header information for all MBs in the segment is followed by MVs for all MBs in the segment and then followed by DCT coefficients for all MBs. The header, MV, and DCT partitions are separated by markers, allowing for resynchronization when the end of the partition in which an error occurred is reached; see FIG. 9 illustrating the HD (header data), HM (header marker), MVD (motion vector data), LMW (last MV value), MM (MV marker), and DCT (DCT data) fields.  
         [0054]    The HD field includes reversible variable length code (RVLC) coded data for combined COD and MCBPC for all MBs in a packet. The standard has five tables which apply to the cases of INTRA MBs, INTER MBs, B MBs, EP MBs, and EI MBs, respectively. The INTER MBs table (see below) is the largest of the five and has 26 codewords; the other four tables are subsets. The codewords of the INTER MBs table have the symmetrical form 0 . . . 01 . . . 10 . . . 0 for coded MBs, and for a non-coded MB the codeword is simply 1. Thus both the first and last bits of the codeword can be interpreted as the COD field. The codewords vary from one to eleven bits, with 1 the smallest and 01111111110 the largest. Thus indexing by the codeword itself would spread the codewords out in a block of 1023 memory addresses. Partitioning into classes according to the number of initial 0s does not help much because the codewords starting with a single 0 run from 010 to 01111111110, a span of 1022.  
         [0055]    The preferred embodiment Index for the INTER MBs table again is a hash type function of the codeword:  
         [0056]    (1) For the codeword starting with 1, take the Index equal to 0.  
         [0057]    (2) For codewords starting with 0, form an index as:  
         [0058]    Index=10*(number of initial 0&#39;s minus 1)+(number of 1&#39;s).  
         [0059]    Thus this index runs from 0 to 41. The index running from 0 to 41 implies a 42-word block of memory can contain the table. Indeed, the 26 Index values are 0-9, 11-17, 21-25, 31-33, and 41.  
                                                                             Codeword       Number       MB type   CBPC   COD + MCBPC   Index   of bits                                        1   0   1       0 (INTER)   00   010   1   3       0   10   00100   11   5       0   01   011110   4   6       0   11   0011100   13   7       1(INTER+Q)   00   01110   3   5       1   10   00011000   22   8       1   01   011111110   7   9       1   11   01111111110   9   11       2(INTER4V)   00   0110   2   4       2   10   01111110   6   8       2   01   00111100   14   8       2   11   000010000   31   9       3(INTRA)   00   001100   12   6       3   11   0001000   21   7       3   10   001111100   15   9       3   01   000111000   23   9       4(INTRA+Q)   00   0111110   5   7       4   11   0011111100   16   10       4   10   0001111000   24   10       4   01   0000110000   32   10       5(INTER4V+Q)   00   00111111100   17   11       5   01   00011111000   25   11       5   10   00001110000   33   11       5   11   00000100000   41   11       Stuffing       0111111110   8   10                  
 
         [0060]    Another preferred embodiment folds the codeword table over onto itself to fill in the unused spaces. That is, the Index has an unused gap between 9 and 11, so the Index values 41 can be mapped into this gap; and the unused gap between 17 and 21 can be filled with the Index values 31-33. This requires a little more computation:  
         [0061]    if (initial bit is 1) Index=0  
         [0062]    else Index=10*(number of initial 0&#39;s minus 1)+(number of 1&#39;).  
         [0063]    if (Index&gt;=25) Index=51−Index.  
         [0064]    That is, the folded codeword table has 26 entries, but the index for the folded table runs from 0 to 25 and thus just fills the 26 word memory block with addresses corresponding to Index.  
         [0065]    More generally, the various RVLC tables of H.263++ all have symmetric codewords of the type 0 . . . 01 . . . 10 . . . 0 plus the codeword 1, and indexes of the form:  
         [0066]    if (initial bit is 1) Index=0  
         [0067]    else Index=N*(number of initial 0&#39;s minus 1)+(number of 1&#39;s).  
         [0068]    can be used for any N such that N&gt;=(maximum number of 1&#39;s). Thus for INTRA MBs use N&gt;=5, for INTER MBs use N&gt;=9, for B MBs use N&gt;=7, for EP MBs use N&gt;=6, and for El MBs use N&gt;=8. Of course, a larger N implies a larger stretch of memory but also more opportunity for folding.  
         [0069]    For example, the foregoing table (the table for INTER MBs) lists the Index with N=10 where the maximum Index=41 for codeword 00000100000; with the Index based on N=9 this codeword would have the maximum Index=37. But the N=10 Index can fold the Index=41 codeword into the gap between Index=9 (codeword 01111111110) and Index=11 (codeword 00100) by using Index=51−Index if Index&gt;=25. If instead the Index had been based on N=9, then this gap closes because codeword 01111111110 would still have Index=9, but codeword 00100 would now have Index=10. Thus the maximum folding would be to the first gap at Index=17 and would use Index=54−Index when Index&gt;=28. With this folding the maximum Index then decreases from 37 to 26 (for codeword 000010000), which implies a memory block of 27 words and all addresses correspond to Index values except for Index=18.