Abstract:
An apparatus carries out encoding variable length codes (VLCs) represented by variable-length codewords and their lengths, and segmenting variable-length codewords of the VLCs into N-bit segments for the transmission thereof, and, if the length of the last segment of the N-bit segments is shorter than N, inserting bits having the value zero between the last bit of the last segment and a start code with a start code prefix followed by a start code value identifying the type of the start code in order to represent the start of a code sequence such that the first bit of the start code is the most significant bit of a byte. The apparatus comprises a byte alignment unit that operates in response to a byte alignment signal. The byte alignment unit produces a pseudo codeword representative of a set of parallel bits with the value zero having a width equal to the maximum bit length of the variable-length codewords instead of the variable-length codeword, and a pseudo length representative of the number of bits contained between the last bit of the last segment and the start code instead of the length of the variable-length codeword.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus for encoding and segmenting variable length codes(VLCs); and, more particularly, to a VLC encoding and segmenting apparatus which is capable of byte-aligning start codes. 
     DESCRIPTION OF THE PRIOR ART 
     Variable-length coding is a technique often used for lossless data compression. Specifically, this technique is used to convert fixed-length data to variable-length codewords based on the statistical occurrences of the data. Each length of the variable-length codewords is chosen in such a manner that shorter codewords are used to represent more frequently occurring data and longer codewords are selected to represent less frequently occurring data. By properly assigning the variable-length codewords to a library of all possible source data, the average word length of the variable-length codewords becomes shorter than that of the source data, thereby rendering it possible to achieve data compression. 
     In general, the encoding process of the VLCs represented by variable-length codewords and their lengths, and the segmenting process of the variable-length codewords of the VLCs for the transmission thereof can be implemented by using a lookup table in which the variable-length codewords and their length data are stored as the contents thereof. A lookup table based VLC encoding and segmenting apparatus is disclosed in, e.g., Shaw-Min Lei et al., &#34;An Entropy Coding System for Digital HDTV Applications&#34;, IEEE Transactions on Circuits and Systems for Video Technology, 1, No. 1, pp. 147-154(March 1991). Some of the major circuit components of the VLC encoding and segmenting apparatus include a PLA (Programmable Logic Array) and three barrel shifters. The PLA is a lookup table which maps the input data to variable-length codewords to produce the VLCs, i.e., the variable-length codewords and their lengths. A first barrel shifter concatenates these variable-length codewords together based on their lengths codewords to provide a chain of variable-length codewords; and a second barrel shifter segments the chain of variable-length codewords into n-bit words for output; and a third barrel shifter accumulates each length of the variable-length codewords to generate a control signal which indicates whether the output of the VLC encoding and segmenting apparatus is available. Especially, the information on the lengths of the variable-length codewords produced from the PLA is supplied to the first and the third barrel shifters in a decoded form which means the position of the only 1 indicates the length. That is, if the maximum length of a variable-length codeword is sixteen, a set of 16-bit lines must be employed to represent each length of the variable-length codewords. The decoded form of the length of the variable-length codewords enables the third barrel shifter to rapidly produce a control signal of the second barrel shifter. However, since a hardware embodiment of the VLC encoding and segmenting apparatus must be provided with n-bits of data lines of a same width as the largest number of bits that a variable-length codeword can have, as the codeword length in bits desired increases, so does the size of the hardware embodiment, and, consequently, the manufacturing cost thereof as well. The problem becomes especially serious when far longer variable-length codewords need to be processed in their decoded forms. 
     Another lookup table based VLC encoding and segmenting apparatus is disclosed in a copending commonly owned application, U.S. Ser. No. 08/556,530, entitled &#34;APPARATUS FOR ENCODING VARIABLE-LENGTH CODES AND SEGMENTING VARIABLE-LENGTH CODEWORDS THEREOF&#34;, which is incorporated herein by reference. This VLC encoding and segmenting apparatus employs an adder to process the length of the variable-length codewords in an encoded form so as to surmount the aforementioned shortcomings of the VLC encoding and segmenting apparatus of Shaw-Min Lei. 
     According to the MPEG(Moving Pictures Expert Group), there are many established standards on moving picture coding. One of them concerns a byte alignment of start codes as follows. Each of the start codes represents the start of a certain code sequence and consists of a start code prefix followed by a start code value. The start code prefix is a string of twenty three bits having the value zero followed by a single bit with the value one. The start code prefix thus becomes the bit string &#34;0000 0000 0000 0000 0000 0001&#34;. The start code value is an eight bit integer which identifies each type of the start codes. Especially, all start codes are to be byte aligned. This is achieved by inserting bits having the value zero before the start code prefix such that the first bit of the start code prefix is the most significant bit of a byte. 
     Hitherto, in the VLC encoding and segmenting art, there is no known apparatus which has the capability of byte-aligning the start codes. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a primary object of the present invention to provide an improved VLC encoding and segmenting apparatus which is capable of byte-aligning the start codes. 
     In accordance with the present invention, there is provided an apparatus for encoding variable length codes (VLCs) represented by variable-length codewords and their lengths, and segmenting the variable-length codewords of the VLCs into N-bit segments for the transmission thereof, and, if the length of the last segment of the N-bit segments is shorter than N, for inserting bits having the value zero between the last bit of the last segment and a start code consisting of a start code prefix followed by a start code value identifying the type of the start code in order to represent the start of a code sequence such that the first bit of the start code is the most significant bit of a byte, the apparatus comprising: 
     a first register for storing a series of source codes and producing each of the source codes in response to a enable signal which is associated with each input time of the source codes; 
     a lookup table for mapping each of the source codes into each of the VLCs to produce each of the variable-length codewords and its length; 
     a second register for storing each of the variable-length codewords, and, in response to the enable signal, for producing said each stored variable-length codeword; 
     a third register for storing each length of the variable-length codewords, and, in response to the enable signal, producing said each stored length; 
     a byte alignment unit, in response to a byte alignment signal, producing a pseudo codeword representative of a set of parallel bits with the value zero having a width equal to the maximum bit length of the variable-length codewords, and a pseudo length representative of the number of bits to be inserted between the last bit of the last segment and the start code; 
     a first barrel shifter, in response to a first control signal representative of the length of a present input variable-length codeword or the pseudo length, for concatenating the present input variable-length codeword and a concatenated variable-length codeword together or concatenating the concatenated variable-length codeword and the pseudo codeword, to thereby produce a newly concatenated variable-length codeword; 
     a fourth register for storing the concatenated variable-length codeword, and, in response to the enable signal, producing said each concatenated variable-length codeword; 
     a second barrel shifter, in response to a second control signal, for producing a fixed-length segment out of combined inputs of the present input variable-length codeword and the concatenated variable-length codeword; 
     an adder for adding the length of the present input variable-length codeword or the pseudo length and an added length to produce a newly added length; 
     a fifth register for comparing the newly added length with N to store the newly added length if the newly added length does not exceed N and, otherwise, to store a residue representative of the number of exceeding bits after the adding as the added length and to produce an output-available signal which represents the availability of the fixed-length segment of the second barrel shifter; and, in response to the enable signal, producing said each stored added length as the second control signal of the second barrel shifter; and 
     a sixth register for storing the fixed-length segment from the second barrel shifter and, in response to the enable signal, producing said each stored fixed-length segment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which: 
     FIG. 1 represents a block diagram of a VLC encoding and segmenting apparatus in accordance with the present invention; 
     FIG. 2 is a detailed block diagram of the byte alignment unit of FIG. 1; 
     FIGS. 3A and 3B provide an explanatory diagram for illustrating the operation of the VLC encoding and segmenting apparatus shown in FIG. 1; 
     FIG. 4 schematically shows a first barrel shifter for concatenating variable-length codewords; and 
     FIG. 5 schematically illustrates a second barrel shifter for segmenting a chain of concatenated variable-length codewords. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown a block diagram of a VLC encoding and segmenting apparatus in accordance with a preferred embodiment of the present invention. The VLC encoding and segmenting apparatus encodes variable length codes(VLCs), represented by variable-length codewords and their lengths, and segmenting variable-length codewords of the VLCs into fixed-length segments for the transmission thereof. As shown in FIG. 1, a series of source codes is inputted to a first register 10. According to the MPEG, for example, each of the source codes consists of a maximum run-length of 63 and a level ranging from -2047 to 2047. The term `run-length` means the number of successive runs with a zero value, and the non-zero value following the successive zeros is called a level. The first register 10 sequentially stores the source codes and outputs them to a lookup table 20 in response to a enable signal which is associated with each input time of the source codes. 
     The lookup table 20 maps each of the source codes into a VLC to produce each variable-length codeword and its length corresponding to each of the source codes onto leads 27 and 29, respectively. In the preferred embodiment of the invention, the lookup table 20 is implemented by a programmable logic array(PLA). The lookup table 20 shown in FIG. 1 consists of an uncoded word table AND-plane 22, a codeword table OR-plane 24 and a code length table OR-plane 26. The lookup table 20 is described in Shaw-Min Lei et al., supra. 
     In particular, in the preferred embodiment of the invention, since the length of the variable-length codeword is processed in an encoded form, if a variable-length codeword has a maximum of 16-bit length, the length can be represented using 5 bits because the value of codeword length ranges from 1 to 16. Each of the variable-length codewords from the codeword table 24 and its length from the code length table 26 are provided to a second register 28 and a third register 30 via the lead 27 and the lead 29, respectively. 
     The second and the third registers 28 and 30 sequentially store each of the variable-length codewords from the codeword table 24 and each corresponding length of the variable-length codewords from the length table 26, respectively; and output each of the stored variable-length codewords and each of the stored lengths to a byte alignment unit 200 in response to the above-described enable signal, respectively. 
     The byte alignment unit 200, responsive to a byte alignment signal which is provided via a line 199 from a coding controller(not shown) so as to foretell the occurrence of a start code, selectively produces the variable-length codeword and its length or a pseudo codeword and a pseudo length. The pseudo codeword is a set of parallel bits with the value zero having a width equal to the maximum bit length of the variable-length codewords. And the pseudo length represents the number of bit zero to be inserted between the last bit of the last variable-length codeword and the start code. Details of the byte alignment unit 200 is described with reference to FIG. 2 
     Referring to FIG. 2, the byte alignment signal on the line 199 is supplied as a switching control signal to a first and a second switches 202 and 206. By the operation of the first switch 202, if the byte alignment signal is not provided, each stored 16-bit variable-length codeword from the second register 28 is inputted through the switch 202 to a first and a second barrel shifters 32 and 40 over parallel leads 25A, however, if the byte alignment signal is provided, 16-bits of pseudo codeword having zero value is inputted thereto. By the operation of the second switch 206, if the byte alignment signal is not provided, each stored 5-bit length from the third register 30 is supplied to an adder 36 over parallel leads 31A, however, if the byte alignment signal is provided, 5-bits of pseudo length on a line 205 is inputted thereto. The 5-bits of pseudo length above described is made by the operation of complement of 2 of a newly added length provided via a line 37 from the adder 36. 
     Referring back to FIG. 1, as will be described below, another  16  -bits from a fourth register 34 are also inputted to the first and the second barrel shifters 32 and 40 over parallel lead 35. The first barrel shifter 32 is responsive to a codeword length signal on a lead 31A representative of the length stored in the third register 30 or the pseudo length to form a 16-bit window on its 32-bit inputs. The 16-bit window is shiftable across the 32-bit inputs on leads 25A and 35 and its position is directly determined by the codeword length signal inputted through the lead 31A. The codeword length signal indicates a shift of the 16-bit window between zero and fifteen. If each bit is numbered as 0, 1, . . . , 31 from the leftmost bit, a codeword length signal of &#34;M&#34; indicates that the 16-bit window encompasses the Mth through (M+15)th bit on leads 25A and 35. After forming the window, the first barrel shifter 32 produces a 16-bit string to the fourth register 34 via a lead 33. The fourth register 34 stores the bit-string produced by the first barrel shifter 32 and, in response to the enable signal, provides the stored bit-string to the inputs of the first and the second barrel shifters 32 and 40. 
     In the meantime, each of the lengths stored in the third register 30 or the pseudo length is sequentially inputted to an adder 36 in response to the enable signal. The adder 36 serves to add the length of the present variable-length codeword or the pseudo length to an added length stored in a fifth register 38. A newly added length is provided to the fifth register 38 and the byte-alignment unit 200. The fifth register 38, if a newly added length from the adder 36 does not exceed a predetermined positive integer value which represents the bit number of a segment for transmission, i.e., 16, stores the newly added length at the fifth register 38. Otherwise, the fifth register 38 stores a residue representative of the number of exceeding bits after the adding as the added length; and produces an output-available signal which represents that the fixed-length segment of the second barrel shifter 40 is available and, in response to the enable signal, produces each stored added length to the second barrel shifter 40. 
     The second barrel shifter 40 is responsive to the value of the added length stored in the fifth register 38 on a lead 39, to form a 16-bit window on its 32-bit inputs which are provided by both the leads 25A and 35. The 16-bit window is shiftable across the 32-bit inputs on leads 25A and 35 and its position is directly determined by the added length signal. The added length signal indicates a shift of the 16-bit window between zero and fifteen. According to an added length signal of &#34;N&#34;, the 16 bit-window of the second barrel shifter 40 encompasses the (16-N)th through (31-N)th bit on leads 25A and 35. After forming the window, the second barrel shifter 40 produces a 16-bit string to a sixth register 42 via a lead 41. The sixth register 42 stores the 16-bit window output bit-string from the second barrel shifter 40 and produces the stored bit-string to a video buffer(not shown) in response to the enable signal. When the output-available signal is produced at the fifth register 38, the output bit-string or output segment from the sixth register 42 will be stored in the buffer which stores only available 16-bit segments for the transmission thereof. 
     The operation of the VLC encoding and segmenting apparatus in FIG. 1 may be more readily understood with reference to an example shown in a tabular form in FIGS. 3A and 3B. It is assumed that the maximum length of the variable-length codewords is 8 bits. 
     The first column of FIG. 3A is made for each input order of following registers: the second register 28 for the variable-length codewords from the codeword table 24; the third register 30 for each corresponding length of the variable-length codewords stored in the second register 28; the fourth register 34 for storing the output segment of the first barrel shifter 32; the fifth register 38 for storing an added length from the adder 36; and the sixth register 42 for storing the output segment of the second barrel shifter 40. The second and last columns of FIG. 3A are built for the byte alignment signal and the output-available signal, respectively. 
     As shown in FIG. 3A, the operation of the VLC encoding and segmenting apparatus is explained focusing on the case where the alignment signal has been received. 
     According to the inputting orders &#34;a&#34; and &#34;b&#34; in the first column, two variable-length codewords are sequentially produced as shown in the third column, and each length corresponding to the two variable-length codewords is stored as shown in the fourth column. The marks of &#34;X&#34; contained in the third column represent entirely meaningless bits out of all bits of a segment provided from the codeword table 24. For the brief description, when the alignment signal is received at the inputting order &#34;b&#34;, the pseudo codeword &#34;0000 0000&#34; and the pseudo length &#34;5&#34; are depicted in the third and fourth columns of the inputting order &#34;c&#34;, respectively. Actually, the pseudo codeword and the pseudo length are not stored in the second and the third register 28 and 30, but provided from the byte alignment unit 200. 
     According to the inputting orders &#34;b&#34;, &#34;c&#34; and &#34;d&#34;, in the fifth column of FIG. 3A, there are depicted three number of output segments of the first barrel shifter 32 which concatenates the variable-length codewords by removing the meaningless bits thereof. Referring to FIG. 4, there is schematically shown the operation of the first barrel shifter 32 for concatenating variable-length codewords, for example, in case of input order &#34;b&#34;. As shown in FIG. 4, if a first 8-bit segment &#34;111X XXXX&#34; from the byte alignment unit 200 and a second 8-bit segment &#34;0000 1111&#34; from the fourth register 34 are inputted over parallel leads 25A and 35, respectively, the first barrel shifter 32 is responsive to the codeword length signal &#34;M&#34;, i.e., M=3, on the lead 31A, to form an 8-bit window on its 16-bit inputs. The position of the 8-bit window is determined by shifting the window by M bits from the lefthand side of the 16-bit inputs. That is, the selection of M bits out of the first segment &#34;111X XXXX&#34; from the left-hand side and another selection of (8-M) bits out of the second segment &#34;0000 1111&#34; from the right-hand side remove the five meaningless bits of the first segment &#34;111X XXXX&#34;, thereby to concatenate the variable-length codewords. After forming the window, the barrel shifter 32 produces an 8-bit window output segment &#34;0111 1111&#34; to the fourth register 34 via the lead 33. 
     In the sixth column of FIG. 3A, the fifth register 38 stores an added length or a residue of an added length subtracted by eight. As described above, the adder 36 serves to add the pseudo length or the length of the present variable-length codeword to the previously added length stored in the fifth register 38. Thus, for instance, data &#34;3&#34; in the input order &#34;c&#34; of the fifth register 38 of FIG. 3A is a residue after summing and subtracting operations of two data &#34;3&#34; and &#34;0&#34; in the input order &#34;b&#34; of the third and the fifth registers 30 and 38: the data &#34;3&#34; is the length of the present variable-length codeword stored in the third register 30 and the data &#34;0&#34; is the residue stored in the fifth register 38. 
     In the seventh column of FIG. 3A, there are depicted three number of output segments of the second barrel shifter 40 which serves to segment a bit-string which consists of the concatenated variable-length codewords from the first barrel shifter 32 and the present variable-length codeword from the second register 28. Referring to FIG. 5, there is schematically shown the operation of the second barrel shifter 40 for segmenting the bit-string, for example, in case of input order &#34;d&#34;. As shown in FIG. 5, if a first 8-bit segment &#34;0000 0000&#34; from the byte alignment unit 200 and a second 8-bit segment &#34;0111 1111&#34; from the fourth register 34 are inputted over parallel leads 25A and 35, the second barrel shifter 40 is responsive to the added length or residue signal &#34;N&#34;, i.e., N=3, on the lead 39 stored in the fifth register 38, to form an 8-bit window on its 16-bit inputs. The position of the 8-bit window is determined by shifting the window by (8-N) bits from the lefthand of the 16-bit inputs. That is, the selection of N bits out of the second segment &#34;0111 1111&#34; from the right-hand side and another selection of (8-N) bits out of the first segment &#34;0000 0000&#34; from the lefthand side produce an output segment &#34;1110 0000&#34; to be stored in the sixth register 42 via the lead 41. 
     In the last column of FIG. 3A, the enable signal of &#34;1&#34; is produced whenever the added length from the adder 36 exceeds 8, thereby to indicate whether the bit-string stored in the sixth register 42 is an available n-bit word. Referring to FIG. 3B, there is shown an exemplary bit stream 300 based on the operation of FIG. 3A. The bit stream 300 has two segments 301 and 302 followed by a start code: a start code prefix 304 and a start code value 306. The two segments 301 and 302 are the available words or segments of the seventh column of FIG. 3A. Especially, the five bits with zero value in the segment 302 is inserted between the variable-length codeword &#34;111&#34; and the start code prefix 304. 
     While the present invention has been shown and described in connection with the preferred embodiments only, it will be readily apparent to those of ordinary skill in the art that many changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.