Abstract:
This invention implements a variable-length code pipeline decoding process as hardware by providing additional bit processing means, reducing the load on external control, and clarifying encoded data shift means.  
     For this purpose, in order to determine a code length and additional bit length, two different decode processes are executed, the overall process is separated into three stages, i.e., a stage for shifting out a code word of encoded data, a decode processing stage, and a symbol determination &amp; additional bit processing stage, and these stages are executed in a pipeline manner.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a decoding apparatus and method for encoded data of still images and moving images.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventionally, a scheme using an entropy coding technique using a variable-length code is well known as one compression coding technique of still images and moving images. This technique is also adopted in JPEG (Joint Photographic Expert Group) as an international standard. In recent years, especially, many hardware implementation means using a Huffman code table as a variable-length code have been proposed. Such prior art will be explained below.  
           [0003]    A conventional decoding apparatus comprises a shift means which comprises a circuit capable of shifting input encoded data for respective bits, a decoder for obtaining the code length and frequency of generation of a head code output from the shift means by comparing the encoded data output from the shift means and a pre-stored minimum or maximum code word of each code lengths a symbol memory for storing decoded data (symbol data) in the order of frequency of generation, an additional bit processor for executing an additional bit process, and a shift amount select means for controlling a shift amount of the shift means.  
           [0004]    The shift amount select means selects, as the shift amount of the shift means, the code length obtained by the decoder in an odd cycle, and an additional bit length output from the symbol memory in an even cycle. In this example, the throughput of the decoding process is always 0.5 symbols/cycle. In addition, the following technique for improving the throughput of the decoding process is known.  
           [0005]    In this technique, a decoding apparatus comprises two different decoders, and a first decoder executes the same process in the aforementioned decoder. On the other hand, a second decoder pre-stores code words with high frequencies of generation and symbol data corresponding to these code words, and outputs symbol data in one cycle for a code word with a high frequency of generation.  
           [0006]    For other code words, the first decoder generates an address of the symbol memory, and outputs symbol data output from the symbol memory in the next cycle as decoded data. According to this example, a throughput of a maximum of 1 symbol/cycle can be obtained.  
           [0007]    In order to realize a high-speed decoding process, a specific decoding means must be implemented as hardware and the circuit must be operated at high frequency. However, the conventional apparatus suffers the following problems.  
           [0008]    Conventionally, since an additional bit processing means is unknown in the decoding process, it is difficult to attain hardware implementation. Especially, when a process is done at the throughput of a maximum of 1 symbol/cycle, since the additional bit processor must at least cope with this processing rate, a hardware implementation means of the additional bit processor is very important upon realizing a high-speed circuit.  
           [0009]    When two different blocks (second decoder and symbol memory) output decoded data depending on input encoded data, since their latencies (execution times) are different, the input timings of symbol data to a selector deviate from each other. Hence, control that considers this timing deviation is required, resulting in complicated control.  
           [0010]    It is hard to implement a pipeline process. Even if code lengths corresponding to the code words stored in the second decoder are stored in advance and a code length corresponding to a selected code word is shifted out from the shift means in one cycle to solve the aforementioned problem, since the output from the decoding apparatus is one cycle, an arithmetic operation for the decoding process, e.g., an additional bit arithmetic operation must be done in one cycle, and it is difficult to realize a pipeline structure in synchronous circuit design.  
           [0011]    In order to achieve efficient, high-speed processes, the processing unit must be shifted for each encoded data. However, conventionally, the shift amount control of the input means to the decoding apparatus is unknown.  
           [0012]    [0012]FIG. 11 shows the arrangement of a variable-length decoding apparatus as prior art  1 . This decoding apparatus comprises a shift-out means (to be referred to as “shift” hereinafter)  1201  that comprises a circuit capable of shifting input encoded data for respective bits, a decoder  1203  for obtaining the code length and frequency of generation of a head code output from the shift  1201  by comparing the encoded data output from the shift  1201  and a minimum or maximum code word of each code length, which is stored in advance, a symbol memory  1205  for storing decoded data (symbol data RRRR/SSSS) in the order of frequency of generation, an additional bit processor  1202  for executing an additional bit process, and a shift amount select means  1204  for controlling the shift amount of the shift  1201 .  
           [0013]    The shift amount select means  1204  selects, as the shift amount of the shift  1201 , the code length obtained by the decoder  1203  in an odd cycle, and an additional bit length output from the symbol memory  1205  in an even cycle.  
           [0014]    [0014]FIG. 12 shows the arrangement of prior art  2  which can improve the throughput of the decoding process in addition to the decoding apparatus of prior art  1 . In this prior art, a high-speed symbol decoder  1352  executes a high-speed decoding process of a plurality of selected symbols, and a symbol decoder  1353  decodes other symbols. As an example of a select means of symbols to be decoded by the high-speed symbol decoder  1352 , a plurality of symbols in descending order of frequency of generation, or symbols with zero runlength may be selected.  
           [0015]    A characteristic feature of prior art  2  lies in that priority is given to the decoding process of the high-speed symbol decoder  1352 . If input encoded data hits a code word corresponding to a symbol registered in the high-speed symbol decoder  1352 , the decoding result of the high-speed symbol decoder  1352  is preferentially selected as an output.  
           [0016]    [0016]FIG. 13 shows prior art  3  as a technique for further improving the throughput of prior art  2 . A variable-length decoding apparatus according to prior art  3  comprises a 1-code word decoder  1403  for decoding a head code word, and a 2-successive code word decoder  1402  capable of decoding a successive sequence of two code words from the head with high frequency of generation, and is characterized in that priority is given to the 2-successive code word decoder  1402 . If a hit has occurred in the 2-successive code word decoder  1402 , since two code words can be decoded at one time, the throughput can be further improved.  
           [0017]    As the encoded data sizes of still images and moving images increase, the required processing performance for an encoding processing apparatus becomes considerably high. Especially, since the variable-length decoding apparatus must decode a variable-length code, it is very difficult to improve the throughput. For this reason, various solutions have been proposed so far, but the following problems remain unsolved.  
           [0018]    The throughput varies depending on the hit ratio of the high-speed symbol decoder  1352 . Even if all data hit, the performance of a maximum of only one symbol per decoding sequence is obtained. This throughput is insufficient in consideration of the performance required for a variable-length decoding apparatus in the future. When the symbol decoder  1353  can decode in one cycle, an effect obtained upon adopting a parallel arrangement with the high-speed symbol decoder  1532  is lost. Such problem occurs when a table that stores symbol data as a decoding result comprises an asynchronous RAM or hardwired. The recent advance of semiconductor techniques allows to operate at higher clock frequency even if the circuit arrangement remains the same, and a conventional circuit which processes in two cycles can now process in one cycle.  
           [0019]    As in prior art  2 , the throughput varies depending on the hit ratio of the 2-successive code word decoder  1402 , and the performance of a maximum of two code words per decoding sequence can be obtained. However, this technique can be implemented in Huffman coding used in MPEG, but cannot be applied to JPEG. This is because variable-length encoded data in JPEG is made up of a Huffman code word and additional bit. For this reason, the 2-successive code word decoder  1402  cannot decode if it simply compares two successive code words with input encoded data, and must consider an additional bit corresponding to a head code word to be decoded by the 1-code word decoder  1403 .  
         SUMMARY OF THE INVENTION  
         [0020]    It is an object of the present invention to implement a variable-length decoding apparatus that can hardly achieve high-speed operations as a pipeline process consisting of three stages by synchronous design using a synchronous RAM. It is another object of the present invention to allow two different decoders to share an additional bit processing circuit and decoded data storage means, and to improve the throughput while minimizing an increase in circuit scale.  
           [0021]    In order to achieve the above object, a variable-length decoding apparatus of the present invention comprises the following arrangement.  
           [0022]    That is, a variable-length decoding apparatus for decoding encoded data, comprises:  
           [0023]    shift means for shifting out a code word and an additional bit corresponding to the code word of input encoded data for each cycle;  
           [0024]    a symbol memory for storing decoded data corresponding to a plurality of N code words contained in the input encoded data;  
           [0025]    first decode processing means for generating an address of the symbol memory, a code length, and an additional bit length for each of Nt code words fewer than the N code words of the code words input from the shift means;  
           [0026]    second decode processing means for generating a code length and an address of the symbol memory for each of the N code words;  
           [0027]    address select means for selecting one of the two addresses of the symbol memory input from the first and second decode processing means;  
           [0028]    first additional bit processing means for shifting bits of the output from the shift means to the left by the code length input from one of the first and second decode processing means;  
           [0029]    second additional bit processing means for shifting bits of the output from the first additional bit processing means to the right by an amount corresponding to symbol data output from the symbol memory; and  
           [0030]    operation control means for outputting a shift amount to the shift means.  
           [0031]    It is still another object of the present invention to provide a decoding apparatus and method, which can be applied to decoding of JPEG and MPEG encoded data, and can obtain a high throughput.  
           [0032]    In order to achieve the above object, for example, one decoding apparatus of the present invention comprises the following arrangement.  
           [0033]    That is, a decoding apparatus for decoding variable-length encoded data, and outputting symbol data, comprises:  
           [0034]    first shift-out means for shifting out a code word of input encoded data in accordance with shift amount select means, and outputting a head code word and subsequent encoded data;  
           [0035]    first decode means for decoding the head code word output from the first shift-out means, and generating first symbol data and a bit length N (N is an integer) of the code word;  
           [0036]    second shift-out means for further shifting the head code word and subsequent encoded data output from the first shift-out means on the basis of the bit length N output from the first decode means, and outputting a subsequent first code word; and  
           [0037]    second decode means for, when the subsequent first code word output from the second shift-out means belongs to one of a code word group obtained by selecting in advance some of all code words which form the encoded data, generating second symbol data as a decoding result and a bit length M (M is an integer) of the code word,  
           [0038]    wherein the shift amount select means determines, as a shift amount of the first shift-out means, a shift amount by selecting a bit length N+M obtained by adding the bit lengths N and M when the second decode means generates the second symbol data, and by selecting the bit length N in other cases.  
           [0039]    In the above arrangement, a maximum of two symbol data can be output in one cycle, and the throughput can be remarkably improved compared to the prior art.  
           [0040]    Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]    [0041]FIG. 1 is a block diagram of a variable-length decoding apparatus according to an embodiment of the present invention;  
         [0042]    [0042]FIG. 2 is a block diagram showing the internal arrangement of a decode processor  1  ( 103 ) in FIG. 1 in the embodiment of the present invention;  
         [0043]    [0043]FIG. 3 is a timing chart of the embodiment of the present invention;  
         [0044]    [0044]FIG. 4 shows the format of encoded data;  
         [0045]    [0045]FIG. 5 is a block diagram showing the internal arrangement of a decode processor  2  in FIG. 1 in the embodiment of the present invention;  
         [0046]    [0046]FIG. 6 is a flow chart showing the flow of the process in the decode processor  1  in the embodiment of the present invention;  
         [0047]    [0047]FIG. 7 is a flow chart showing the flow of the process in the decode processor  2  in the embodiment of the present invention;  
         [0048]    [0048]FIG. 8 is a flow chart showing the flow of the process in an additional bit processor  1  in the embodiment of the present invention;  
         [0049]    [0049]FIG. 9 is a table comparing the decode processors  1  ( 103 ) and  2  ( 104 );  
         [0050]    [0050]FIG. 10 is a block diagram of a decoding apparatus according to the second embodiment of the present invention;  
         [0051]    [0051]FIG. 11 is a block diagram of a conventional decoding apparatus;  
         [0052]    [0052]FIG. 12 is a block diagram of a conventional decoding apparatus;  
         [0053]    [0053]FIG. 13 is a block diagram of a conventional decoding apparatus;  
         [0054]    [0054]FIG. 14 shows an example of encoded data input to a decoding apparatus of the second embodiment;  
         [0055]    [0055]FIG. 15 is a table showing the relationship between symbols and code words to be decoded by a specific symbol generate decoder  1103  and all-symbol generate decoder  1104 ;  
         [0056]    [0056]FIG. 16 is a table showing the states of respective data for respective cycles in the second embodiment;  
         [0057]    [0057]FIG. 17 is a block diagram of a decoding apparatus according to the third embodiment of the present invention;  
         [0058]    [0058]FIG. 18 is a timing chart showing the sequence of a variable-length decoding apparatus in the third embodiment, and correspondence with three code words registered in a dynamic code word table  1303 ;  
         [0059]    [0059]FIG. 19 is a block diagram showing the internal arrangement of a specific symbol address generator  1304  in FIG. 17;  
         [0060]    [0060]FIG. 20 is a block diagram showing the internal arrangement of an additional bit processor  1302  in FIG. 17;  
         [0061]    [0061]FIG. 21 is a block diagram of a decoding apparatus according to the fourth embodiment of the present invention;  
         [0062]    [0062]FIG. 22 shows a zigzag scan of DCT coefficients;  
         [0063]    [0063]FIG. 23 shows a combination of RRRR/SSSS in the fourth embodiment;  
         [0064]    [0064]FIG. 24 is a block diagram of a decoding apparatus according to the fifth embodiment of the present invention;  
         [0065]    [0065]FIG. 25 is a table showing the relationship between a mask pattern and SSSS stored in a mask pattern table in the third and sixth embodiments;  
         [0066]    [0066]FIG. 26 is a block diagram of a decoding apparatus according to the sixth embodiment of the present invention;  
         [0067]    [0067]FIG. 27 is a block diagram showing the arrangement of a shift-out unit  1201  in FIG. 26;  
         [0068]    [0068]FIG. 28 is a block diagram showing the internal arrangement of a RUN 0 /EOB address generator  2102  in FIG. 26;  
         [0069]    [0069]FIG. 29 is a block diagram showing the internal arrangement of an all-symbol address generator  2103  in FIG. 26;  
         [0070]    [0070]FIG. 30 is a block diagram showing the internal arrangement of an additional bit processor  2108  in FIG. 26; and  
         [0071]    [0071]FIG. 31 is a timing chart showing, as a sequence, an operation example of the variable-length decoding apparatus of the sixth embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0072]    Preferred embodiments of the present invention will be described in detail hereinafter.  
         [0073]    [0073]FIG. 1 shows an embodiment of a variable-length decoding apparatus using the present invention. A shift means or unit  101  shifts out each code word of variable-length encoded data for each cycle using a left bit shift processing circuit. Since the shift unit  101  uses a flip-flop, it latches output data of the left bit shift processing circuit for each cycle. This process is defined as stage  1  of a pipeline process.  
         [0074]    The process of stage  2  will be described in detail below. FIG. 2 shows the internal arrangement of a decode processor  1  ( 103 ). FIG. 6 is a flow chart showing the flow of the process in the decode processor  1 . A table storage means comprises a code word array  402  which comprises a flip-flop, and stores Nt (N≧Nt≧0) of N (N≧1) code words present in advance in a variable-length code table, a code length &amp; code length+additional bit length array  404  for storing two different types of numbers of bits, i.e., Nt code lengths corresponding to the code word array  402 , and Nt code lengths +additional bit lengths M (M≧0), and an address array  403  for storing Nt addresses of a symbol memory  108 . It is checked if code words output from the code word array  402  in the table storage means match the outputs from the shift unit  101  in Nt comparators of a comparator group  408 . In this checking, since the code lengths of code words present in the code word array  402  are known, each of the Nt comparators compares for only a code length to be compared. That is, each comparator in the comparator group  408  compares for the number of bits according to a code length to be compared, which ranges from 1 bit (minimum) to a maximum code length.  
         [0075]    The output signals from these Nt comparators in the comparator group  408  are input to a decoder  401 , which generates a select signal for selecting one of Nt data of the address array  403  and the code length &amp; code length+additional bit length array  404 . The outputs from an MUX  406  are two different signals indicating the code length and the M bits. The signal indicating the M bits is output to an operation controller  105  in FIG. 1, and the code length is output to an additional bit processor  1  ( 102 ). Output data from an MUX  405  is used as the address of the symbol memory  108 .  
         [0076]    On the other hand, the arrangement of a decode processor  2  ( 104 ) is as shown in FIG. 5. The flow of the process in the decode processor  2  is as shown in the flow chart of FIG. 7. In this processor, maximum or minimum code words for respective code lengths are pre-stored in a code word array  502 , and the code length of input encoded data (S 701 ) is obtained using comparators  503  and a priority encoder  504  (S 702 ). If the code word array  502  stores maximum code words, the priority of the priority encoder becomes higher in the order from larger code length; when the array  502  stores minimum code words, the priority becomes lower in the order from smaller code length. Since the number of comparators corresponds the number (N 1 ) of code lengths, the gate scale is constant. An initial data array  505  pre-stores initial data corresponding to maximum or minimum code words for respective code lengths on the code word array  502 . Access to the initial data array is made using a decoder  501  on the basis of the obtained code length (S 703 ). The address of the symbol memory  108  is obtained by adding initial data corresponding to the obtained code length and the input encoded data by an arithmetic device  506  (S 704 ).  
         [0077]    For example, when initial data of minimum code words are stored, the address of the symbol memory is obtained by:  
             ADDR   =                VLCin   -   VLCmin   +   ADDRbase                 =                VLCin   +     (     ADDRbase   -   VLCmin     )                                   
 
         [0078]    where ADDR is the address of the symbol memory, VLCin is lower 8 bits of a code word which is currently shifted out by the shift unit  101 , VLCmin is the minimum code word of an identical code length, and ADDRbase is the address of the minimum code word in the symbol memory. Lower 8 bits obtained by computing (ADDRbase−VLCmin) of the right-handed side correspond to initial data. If the code word, which is currently shifted out by the shift unit  101 , is less than 8 bits, “0” or “0”s is or are padded in the vacant upper bit or bits.  
         [0079]    [0079]FIG. 9 compares the decode processors  1  ( 103 ) and  2  ( 104 ). In a cycle in which encoded data can be decoded by the decode processor  1 , since the variable code length and additional bits can be shifted out in one cycle with respect to the shift unit  101 , successive encoded data can be decoded in the next cycle.  
         [0080]    On the other hand, when decoding is made by the decode processor  2 , the obtained code length is stored in the first cycle, and a shift-out process is done by adding the code length and an additional bit length obtained from the symbol memory  108  in the next cycle. Hence, two cycles are required until the decoding process of encoded data starts.  
         [0081]    An address selector  106  selects the output from the decode processor  1  ( 103 ) in a cycle in which at least one match is detected by the Nt comparators in the decode processor  1  ( 103 ), and selects the output from the decode processor  2  ( 104 ) in other cycles. On the other hand, the additional bit processor  1  ( 102 ) receives the code lengths from the decode processors  1  ( 103 ) and  2  ( 104 ). FIG. 8 is a flow chart of the process in the additional bit processor  1 .  
         [0082]    In the process in the additional bit processor  1 , in a cycle in which at least one match is detected by the Nt comparators in the decode processor  1  ( 103 ), the code length input from the decode processor  1  ( 103 ) is selected, and a left bit shift operation is done (S 802 , S 804 , S 805 ). In other cases, the code length input from the decode processor  2  is selected, and a bit shift operation is made (S 803 ). In this case, the result of the bit shift process is stored in a flip-flop for each cycle (S 806 ). The aforementioned operations are made in stage  2  of the pipeline process.  
         [0083]    The process executed in stage  3  of the pipeline process will be explained below. In stage  3 , the output from the symbol memory  108  and the output from the additional bit processor  1  ( 102 ) are used. An additional bit processor  2  ( 107 ) performs a right bit shift operation of input data from the flip-flop of the additional bit processor  1  ( 102 ) using an additional bit length as a part of symbol data output from the symbol memory  108  as a shift amount.  
         [0084]    The result of this shift operation is used as output additional bit data of this decoding apparatus. Symbol data output from the symbol memory  108  is directly used as output symbol data of this decoding apparatus. If the address output from the decode processor  2  ( 104 ) was used in the previous cycle, the operation controller  105  adds the code length stored in the previous cycle, and an additional bit length as a part of symbol data output from the symbol memory  108 , and outputs the sum to the shift unit  101 .  
         [0085]    [0085]FIG. 3 is a timing chart of the variable-length decoding apparatus of this embodiment, and FIG. 4 shows an example of encoded data input to this decoding apparatus.  
         [0086]    In cycle  1 , variable-length code  1  is shifted out by the shift unit  101  as it is shifted up to the MSB side. The decode processors  1  ( 103 ) and  2  ( 104 ) receive identical encoded data. Variable-length code  1  is not present in the table storage means of the decode processor  1  ( 103 ), and all the outputs from the comparator group  408  are false.  
         [0087]    On the other hand, the decode processor  2  ( 104 ) calculates and outputs the code length of variable-length code  1 , and the address of the symbol memory  108 . The additional bit processor  1  ( 102 ) executes a left bit shift process of output data from the shift unit  101  on the basis of the code length input from the decode processor  2  ( 104 ), and stores the result in the flip-flop. The operation controller  105  stores the code length input from the decode processor  2  ( 104 ) in the flip-flop.  
         [0088]    In cycle  2 , the symbol memory  108  outputs symbol data corresponding to variable-length code  1  as decoded data. The additional bit processor  2  executes a right bit shift process of data input from the additional bit processor  1  ( 102 ) using a part of symbol data as a shift amount, and outputs the result as additional bit data. The operation controller  105  also receives an additional bit length as a part of symbol data, adds the code length stored in the previous cycle and the additional bit length, and outputs the sum as a shift amount to the shift unit  101 .  
         [0089]    In cycles  3  and  4 , the same operations as in cycles  1  and  2  are done, respectively. In cycle  5 , variable-length code  3 , which is currently shifted out by the shift unit  101 , matches one of code words pre-stored in the flip-flop in the decode processor  1  ( 103 ). The additional bit processor  1  ( 102 ) executes a left bit shift process based on the code length input from the decode processor  1  ( 103 ). The operation controller  105  outputs the code length+additional bit length input from the decode processor  1  ( 103 ) to the shift unit  101  as a shift amount.  
         [0090]    That is, in the next cycle  6 , additional bits have already been shifted out, so the next variable-length code  5  is shifted out from the shift unit  101  this time. In this cycle as well, the decode processor  1  ( 103 ) detects a match of a Huffman code word, and the additional bit processor  1  ( 102 ) and operation controller  105  execute the same operations as in cycle  5 . On the other hand, in stage  3  of the pipeline process, the symbol memory  108  outputs symbol data corresponding to variable-length code  3  as decoded data, and the additional bit processor  2  ( 107 ) executes a right bit shift process using a part of symbol data as a shift amount, and outputs the result of the bit shift process as additional bit data.  
         [0091]    The same processes are done in cycles  7  to  9 . In cycle  10 , the decode processor  1  ( 103 ) does not detect any match with code words, and the code length of the decode processor  2  ( 104 ) and the address of the symbol memory are enabled in stage  2  of the pipeline process. In stage  3  of the pipeline process, symbol data corresponding to variable-length code  7  and additional bits are output.  
         [0092]    As described above, according to this embodiment, a variable-length decoding apparatus which can hardly attain high-speed operations can be implemented as a pipeline process consisting of three stages by synchronous design using a synchronous RAM. The additional bit processing circuit and decoded data storage means can be shared by two different types of decoders and, hence, the throughput can be improved while minimizing an increase in circuit scale.  
         [0093]    [Second Embodiment] 
         [0094]    [0094]FIG. 10 is a block diagram of a variable-length decoding apparatus which is applied to JPEG in the second embodiment of the present invention.  
         [0095]    A shift-out unit  1101  receives encoded data made up of variable-length code words and additional bits. The input/output data widths of encoded data of the shift-out unit  1101  have identical numbers of bits, and are equal to or larger than a maximum code word length+maximum additional bit length. The shift-out unit  1101  mainly combines input encoded data and that present in the shift-out unit  1101  in the current cycle, and shifts out the combined data in accordance with a shift amount input from an operation controller  1105 .  
         [0096]    Encoded data output from the shift-out unit  1101  is parallelly input to an all-symbol generate decoder  1104  and left-shift unit  1102 . Note that the left-shift unit  1102  comprises a shift circuit and the like. In this circuit, since the input bitstream is output from its MSB side, “left shift” is used herein.  
         [0097]    The all-symbol generate decoder  1104  decodes symbol data (RRRR/SSSS) for all code words which form encoded data, and outputs the number N of bits (integer) obtained by adding the code length and additional bit length (SSSS).  
         [0098]    On the other hand, the next code word, which follows the code word corresponding to symbol data decoded by the all-symbol generate decoder  1104  and additional bits, is shifted out, since the output data from the left-shift unit  1102  has undergone the left-shift process by N bits input from the all-symbol generate decoder  1104 .  
         [0099]    A specific symbol generate decoder  1103  checks if one of code words, which are registered in advance, matches encoded data output from the left-shift unit  1102 . If the two data match, the decoder  1103  asserts a hit signal (=“1”), and outputs the corresponding symbol data and M (integer) bits obtained by adding its code word length and additional bit length. For example, a plurality of symbols in descending order of frequency of generation are decoded by the specific symbol generate decoder  1103 .  
         [0100]    The operation controller  1105  controls (determines) the shift amount of the shift-out unit  1101 . The shift amount is N+M bits if a hit signal is asserted, or is N bits if a miss has occurred.  
         [0101]    In this way, the all-symbol generate decoder  1104  always decodes and outputs symbol data independently of hits in the specific symbol generate decoder  1103 , and if a hit has occurred, two symbol data are output at the same time.  
         [0102]    &lt;Description of Operation&gt; 
         [0103]    The operation of the variable-length decoding apparatus in the second embodiment will be explained below. For the sake of simplicity, encoded data input to the variable-length decoding apparatus consists of eight different symbol data. In this case, the relationship between symbol data and code words decoded by the specific symbol generate decoder  1103  and all-symbol generate decoder  1104  is as shown in FIG. 15. The all-symbol generate decoder  1104  decodes all of eight different symbol data, and the specific symbol generate decoder  1103  decodes top three different symbol data with higher frequency of generation.  
         [0104]    [0104]FIG. 14 shows a bit pattern of the input encoded data. In FIG. 14, A indicates additional bits, which form an arbitrary bit pattern of 0s or 1s. Also, the input/output encoded data width of the shift-out unit  1101  is 16 bits.  
         [0105]    [0105]FIG. 16 shows the states of respective data in respective cycles, and the data states will be explained below.  
         [0106]    In cycle  0 , the shift-out unit  1101  shifts out code word “00” (which is 2-bit data as can be seen from FIG. 15). This code word “00” is decoded by the all-symbol generate decoder  1104 , and  0 / 1  (RRRR/SSSS) is output as symbol data. Also, N=3 bits as the sum of 2 bits (code word length) and 1 bit (additional bit length) is output to the left-shift unit  1102 . The left-shift unit  1102  left-shifts encoded data input from the shift-out unit  1101  by N=3 bits, and outputs the shifted data to the specific symbol generate decoder. “Zeros” in ( ) in FIG. 16 are padded by the left-shift unit  1101 . The specific symbol generate decoder  1103  checks if the encoded data input from the left-shift unit  1102  matches one of three code words “00”, “01”, and “100” which are registered in advance. In the output from the left-shift unit  1102  in cycle  0 , since code word “01” is shifted out, the specific symbol generate decoder  1103  asserts a hit signal, and outputs symbol data  0 / 2  and M=4 bits to the operation controller  1105 .  
         [0107]    On the other hand, the operation controller generates a shift amount for the shift-out unit  1101  under the following condition. Note that L bits represent the data size and S bits represent the shift amount in the shift-out unit  1101  in the current cycle.  
         [0108]    If hit occurs:  
                                                                                                                                                                           IF (L ≧ N) THEN                IF ((L − N) ≧ M) THEN                S = N + M                ELSE                S = N                END IF                ELSE                S = 0                END IF           If miss occurs:           IF (L ≧ N) THEN                S = N                ELSE                S = 0                END IF                      
 
         [0109]    In cycle  0 , since the code word hits the specific symbol generate decoder  1103  and (L−N)≧M, N+M=7 bits is output as the shift amount. Also, both a symbol data  2  enable signal indicating that symbol data output from the specific symbol generate decoder  1103  is enabled, and a symbol data  1  enable signal indicating that symbol data output from the all-symbol generate decoder  1104  is enabled are asserted (=“1”)  
         [0110]    In cycle  1 , the shift-out unit  1101  shifts out code word “1011”, and the all-symbol generate decoder  1104  outputs symbol data  0 / 4  and N =8 bits. In the left-shift unit  1102 , although code word “11011” is shifted out, since this code word is not registered in the specific symbol generate decoder  1103 , a miss occurs, and a hit signal is deasserted (=“0”). Hence, the shift amount S=8 bits is output to deassert the symbol data  2  enable signal and assert the symbol data  1  enable signal. The same operations are repeated for cycles  2  and  3 .  
         [0111]    In cycle  4 , the data size of only 8 bits is present in the shift-out unit  1101 . The all-symbol generate decoder  1104  decodes shifted-out code word “00” and outputs symbol data  0 / 1  and N=3 bits. In the output of the left-shift unit  1102 , code word “100” is shifted out, and the specific symbol generate decoder  1103  generates symbol data  0 / 3 . However, since category SSSS= 3 , the additional bit length is 3 bits, and M=6 bits is output. However, the number of effective bits input from the left-shift unit  1102  to the specific symbol generate decoder  1103  in the current cycle is L−N=8−3=5 bits, which are smaller than M bits. Therefore, although a hit has occurred upon a code word, the shift-out unit  1101  does not execute a shift-out process, and a symbol data  2  enable instruction signal is deasserted. Hence, in cycle  4 , the variable-length decoding apparatus outputs only one symbol data.  
         [0112]    In cycle  5 , code word “100”, decoding of which was tried by the specific symbol generate decoder  1103  in the previous cycle, is decoded again by the all-symbol generate decoder  1104 . Also, the specific symbol generate decoder  1103  decodes code word “00”, thus outputting two symbol data.  
         [0113]    With the aforementioned arrangement and operation, the throughput of the variable-length decoding apparatus can be improved, although such improvement is hardly attained in the prior art. Furthermore, even in variable-length encoded data which is made up of Huffman code words and additional bits used in JPEG, two symbol data can be output at the same time in a single decoding sequence.  
         [0114]    [Third Embodiment] 
         [0115]    [0115]FIG. 17 is a block diagram of a variable-length decoding apparatus according to the third embodiment of the present invention.  
         [0116]    In the variable-length decoding apparatus in the third embodiment, symbol data as decoding results are stored in a single symbol memory  1311  as a synchronous RAM. Address generation of that memory is implemented by two means, i.e., a specific symbol address generator  1304  and all-symbol address generator  1305 . The specific symbol address generator  1304  obtains the number of bits for a code word length+additional bit length for some limited symbols of all symbols present in encoded data, and the all-symbol address generator  1305  obtains a code word length for each of all symbols. The latency required for the variable-length decoding apparatus to decode one symbol data is two cycles. When the shift amount shifted out by a shift-out unit  1301  hits in the specific symbol address generator  1304 , since the shift-out unit  1301  shifts out a code word and additional bits in one cycle, the next symbol can be decoded in the next cycle, and a throughput of maximum of 1 symbol/cycle can be obtained. That is, how to select symbols to be decoded by the specific symbol address generator  1304  largely influences the throughput. Hence, in the third embodiment, the frequency of generation of symbol data output from the symbol memory  1311  is measured, and a plurality of symbols in descending order of frequency of generation are decoded by the specific symbol address generator  1304 . Furthermore, these symbols to be selected are dynamically replaced in turn.  
         [0117]    &lt;Detailed Description of Operation&gt; 
         [0118]    The operation of the variable-length decoding apparatus of the third embodiment will be described below. For the sake of simplicity, assume that there are  26  different code words A to Z in ascending order of absolute value as variable-length codes.  
         [0119]    The number of symbols to be decoded by the specific symbol address generator  1304  is three.  
         [0120]    [0120]FIG. 18 is a timing chart of the variable-length decoding apparatus and also shows three code words registered in a dynamic code word table  1303 , and a sequence. If symbols have the same frequency of generation, a symbol corresponding to a code word having a smaller absolute value is preferentially selected.  
         [0121]    In cycle  0 , the shift-out unit  1301  shifts out code word A. Since this cycle is the first cycle of the decoding process, no code words are registered in the dynamic code word table. Hence, a miss has occurred in the specific symbol address generator  1304 , and the address generated by the all-symbol address generator  1305  is selected by a selector  1310  as the address of the symbol memory  1311 .  
         [0122]    The internal arrangement of the specific symbol address generator  1304  is as shown in FIG. 19. The specific symbol address generator  1304  operates as follows in this cycle. That is, a comparator group  1501  determines based on three different code words and code lengths input from the dynamic code word table  1301  that the input encoded data does not match any of these code words, and the hit signal remains deasserted (=“0”).  
         [0123]    On the other hand, the all-symbol address generator  1305  obtains a code length by comparing with a maximum code word as a maximum absolute value present in each code length as in the conventional method. This maximum code word is input from a static maximum code word table  1306 . In this case, a setup of maximum code words in the static maximum code word table  1306  must be completed before the beginning of decoding of the variable-length decoding apparatus, and table entry values remain unchanged during decoding like the dynamic code word table  1303 .  
         [0124]    In cycle  1 , the symbol memory  1311  outputs symbol data corresponding to the address generated by the all-symbol address generator  1305  in the previous cycle, and this data is used as the output of this variable-length decoding apparatus. Since the shift-out unit  1301  shifted out encoded data by the code word length in the previous cycle, the additional bits of this symbol data have already been shifted out by the shift-out unit  1301 .  
         [0125]    Furthermore, an additional bit length can be obtained from symbol data by a known method, and is output after a right shift process. The internal arrangement of an additional bit processor  1302  is as shown in FIG. 20 (the contents of a mask pattern table  1605  in FIG. 20 are as shown in FIG. 25). If a hit has occurred in the specific symbol address generator  1304 , since the code word +additional bits are shifted out from the shift-out unit  1301  in one cycle, a flip-flop  1602  temporarily latches data which has been shifted to the left by the code word length, and a right shift process is executed in the next cycle.  
         [0126]    A static code word table  1309  pre-stores code words, code lengths, code lengths+additional bit lengths, and symbol memory addresses of all symbols or a plurality of symbols with higher frequency of generation. In the static code word table  1309 , a setup of entries must be completed before the beginning of decoding of the variable-length decoding apparatus, like in the static maximum code word table  1306 , and entry values remain unchanged.  
         [0127]    A generation frequency histogram  1308  counts the frequency of generation of symbol data present in the static code word table  1309  in accordance with the output from the symbol memory  1311  to select top three symbol data with higher frequency of generation, which are supplied to the dynamic code word table  1303 .  
         [0128]    When the operations in cycles  0  and  1  are repeated up to cycle  5 , three different data, i.e., code words A, B, and C are selected in the dynamic code word table  1303 .  
         [0129]    In cycle  6 , the shift-out unit  1301  shifts out code word C. Since code word C is present in the dynamic code word table  1303 , a hit is determined in the specific symbol address generator  1304 . Hence, the shift-out unit  1301  simultaneously shifts out code word C and additional bits during cycle  6 , and shifts out the next code word in the next cycle  7 .  
         [0130]    As for cycle  7 , a symbol with high frequency of generation can always be processed by the specific symbol address generator  1304  by counting the frequency of generation based on the output from the symbol memory  1311 , thus improving the throughput.  
         [0131]    In the third embodiment, only three different symbols are selected in the dynamic code word table  1303 , but all symbols in maximum may be selected by a trade-off with the gate scale. Also, the types of code words are limited to code words A to Z for the sake of simplicity, but this embodiment can be applied to any Huffman codes.  
         [0132]    [Fourth Embodiment] 
         [0133]    The fourth embodiment will be described in detail below. FIG. 21 is a block diagram of a variable-length decoding apparatus in the fourth embodiment.  
         [0134]    In general, in JPEG, an 8×8 block that has undergone a DCT arithmetic process is quantized, and the quantization coefficients undergo an entropy coding process. FIG. 22 shows the state of zigzag transformation (zigzag scan) of a DCT block. The order DCT coefficients are input to a variable-length encoding apparatus is that after zigzag transformation. As for DC components, a one-dimensional entropy coding process is done to have a difference value from the previous DCT block as SSSS by a method called DCPM. After that, 63 successive DCT coefficients undergo a two-dimensional entropy coding process of two-dimensional RRRR/SSSS. FIG. 23 shows combinations of RRRR/SSSS. In case of around {fraction (1/10)} as a normal compression ratio in an image compressed by JPEG, combinations of RRRR/SSSS generated are offset depending on the positions (scan count values) of DCT coefficients in the zigzag order.  
         [0135]    In the fourth embodiment, a specific symbol address generator  1704  selects a symbol (RRRR/SSSS) to be decoded in accordance with a scan count value in consideration of the above phenomenon, thereby improving the throughput.  
         [0136]    &lt;Detailed Description of Operation&gt; 
         [0137]    The operation of the decoding process is substantially the same as in the third embodiment, except for entry of code words to a dynamic code word table  1703 . In the fourth embodiment, a plurality of tables that select symbols with high frequency of generation corresponding to the scan count values are prepared before the beginning of the decoding process. For example, in the fourth embodiment, three different tables are prepared in advance in correspondence with an initial scan (scan count values 1 to 23), middle scan ( 24  to  40 ), and last scan ( 41  to  63 ). The number of symbol entries in each table is a trade-off with the gate scale. If the system of the third embodiment is used, three different symbols are set in each table. In this way, as the scan count value is counted up, the specific symbol address generator  1704  selects a symbol to be processed at high speed, thus improving the hit rate and the throughput.  
         [0138]    [Fifth Embodiment] 
         [0139]    The fifth embodiment will be described below. FIG. 24 is a block diagram showing an apparatus in the fifth embodiment.  
         [0140]    &lt;Outline of Arrangement&gt; 
         [0141]    In general, in encoded data with low compression ratio, the frequency of generation of symbols with a small runlength value is high. Conversely, in encoded data with high compression ratio, the frequency of generation of symbols with a large runlength value is high.  
         [0142]    Hence, in the fifth embodiment, an optimal one of a plurality of tables, which are prepared in advance, is selected in accordance with the compression ratio of encoded data to be decoded, so as to select a symbol (RRRR/SSSS) to be decoded by a specific symbol address generator  1804 , thereby improving the throughput. ps &lt;Detailed Description of Operation&gt; 
         [0143]    The arrangement and operation of this embodiment are substantially the same as those in the third embodiment except for a supply unit of code words, code word lengths, code word lengths+additional bit lengths, and symbol memory addresses to the specific symbol address generator  1804 .  
         [0144]    Before the beginning of decoding, the compression ratio of encoded data to be decoded is set from an apparatus outside the variable-length decoding apparatus. A selector  1808  selects one of tables, which are prepared in advance, in accordance with that compression ratio, and inputs the selected table to the specific symbol address generator  1804 . In this embodiment, the selected table remains unchanged during decoding.  
         [0145]    As described above, according to the third to fifth embodiments, by adaptively selecting symbols which are to undergo a high-speed decoding process, the throughput can be improved compared to the prior art.  
         [0146]    [Sixth Embodiment] 
         [0147]    The sixth embodiment will be described below.  
         [0148]    &lt;Apparatus Arrangement&gt; 
         [0149]    [0149]FIG. 26 is a block diagram of a decoding apparatus of the sixth embodiment.  
         [0150]    The arrangement of the decoding apparatus will be explained first. Encoded data input to this decoding apparatus is input to a shift-out unit  2101 . FIG. 27 shows the arrangement of the shift-out unit  2101 . The input encoded data is shifted by a right-shift unit  2301  to be coupled to the final effective bit of encoded data output from a left-shift unit  2302 . On the other hand, a flip-flop  2304  outputs the shifted-out encoded data to a RUN 0 /EOB address generator  2101  and all-symbol address generator  2103 , and supplies it to the left-shift unit  2302 . The left-shift unit  2302  shits bits corresponding to the shift amount input from an operation controller  2107  to the left.  
         [0151]    On the other hand, an input apparatus to this variable-length decoding apparatus inputs encoded data to the variable-length decoding apparatus if the data size to be input to the variable-length decoding apparatus in the current cycle is equal to or smaller than a value obtained by subtracting the data size from the data bus width of encoded data.  
         [0152]    [0152]FIG. 28 shows the internal arrangement of the RUN 0 /EOB address generator  2102 . When the runlength is “0” and a code word corresponding to an EOB symbol is shifted out, the RUN 0 /EOB address generator  2102  outputs an address of a symbol memory and a shift amount. These data are respectively stored in a code word length+additional bit length table  2404  and symbol memory address table  2405  as the code word length+additional bit length corresponding to zero runlength and EOB symbol, and the address of the symbol memory.  
         [0153]    Comparators of a comparator group  2401  receive code words corresponding to zero runlength and EOB, and check if they match. If at least one of the comparators of the comparator group  2401  matches a code word, data corresponding to that code word are selected from two tables, i.e., the code word length+additional bit length table  2404  and symbol memory address table  2405  and are output. At the same time, a hit signal is asserted.  
         [0154]    [0154]FIG. 29 shows the internal arrangement of the all-symbol address generator  2103 . The all-symbol address generator  2103  outputs at least a code word and an address of a symbol memory  2105  corresponding to a symbol which is not registered in the RUN 0 /EOB address generator  2102 . An implementation means of the all-symbol address generator  2103  uses known prior art. Encoded data is compared with maximum code words for respective code word lengths in a comparator group  2501 . The outputs from comparators are supplied to a priority encoder  2502  which is given higher priority in ascending order of code length.  
         [0155]    In this case, a minimum one of code word lengths from the comparators which determined that the encoded data value is equal to or smaller than the maximum code word length is used as a code word length of the currently shifted-out code word. The address of the symbol memory assumes a value obtained by subtracting the difference from the maximum code word of the currently shifted-out code word from the value of a corresponding code length selected from a symbol memory address table  2506  that stores the addresses of maximum code words of respective code word lengths on the symbol memory.  
         [0156]    [0156]FIG. 30 shows the internal arrangement of an additional bit processor  2108 . The processing sequence of additional bits varies depending on whether or not a hit has occurred in the RUN 0 /EOB address generator  2102 . If a hit has occurred, encoded data is shifted to the left by a code length, and is then delayed by one clock by a flip-flop  2602 . A hit signal is delayed by one cycle by a flip-flop  2603  to be used as a select signal of a selector  2604 , and if a hit has occurred, the output from the flip-flop  2602  is selected. The output from the selector  2604  is logically ANDed with a bit pattern selected from a mask pattern table  2605  in accordance with symbol SSSS. The relationship between the mask pattern and SSSS is the same as that shown in FIG. 25. The AND signal undergoes a right shift process for the number of bits obtained by subtracting the value of symbol SSSS from 11, and the shift process result is output as additional bits.  
         [0157]    The operation of the operation controller  2107  will be described below.  
         [0158]    The operation controller  2107  compares the data size present in the shift-out unit  2101  in the current cycle with the code word length+additional bit length input from the RUN 0 /EOB address generator  2102  if a hit has occurred, or with the code word length input from the all-symbol address generator  2103  if a miss has occurred. If the data size is smaller than the input value, a selector  2106  selects zero shift amount until a cycle in which the data size becomes equal to or larger than the input value. The shift amount is the code word length+additional bit length if a hit has occurred in the RUN 0 /EOB address generator  2102 , or is the code word length in the first cycle and the additional bit length in the next cycle if a miss has occurred. If two code words have successively been missed, a RUN 1  gambling execution process for outputting information indicating a runlength=“1” to the subsequent blocks is executed in the first cycle. A miss that has occurred in the RUN 0 /EOB address generator  2102  means a runlength=“1” or more. In this manner, the subsequent blocks of the variable-length decoding apparatus can execute a variable-length decoding process without lowering the throughput even when the all-symbol address generator  2103  that requires two processing cycles executes processes.  
         [0159]    &lt;Description of Operation&gt; 
         [0160]    [0160]FIG. 31 is a timing chart showing an operation example of the variable-length decoding apparatus of the sixth embodiment.  
         [0161]    Cycle  0  indicates that encoded data output from the shift-out unit  2101  is parallelly processed by the RUN 0 /EOB address generator  2102  and all-symbol address generator  2103  and, consequently, a miss has occurred in the RUN 0 /EOB address generator  2102 . Hence, the selector  2104  selects the symbol memory address input from the all-symbol address generator  2103 , and the selector  2106  selects a code word length as a shift amount in the operation controller. In case of a processing cycle for DC components, even when a miss has occurred in the RUN 0 /EOB address generator  2102 , no RUN 1  gambling execution is made. In the sixth embodiment, a synchronous RAM is assumed as the symbol memory  2105 .  
         [0162]    In cycle  1 , the shift-out unit  2101  shifts out additional bits corresponding to code word  0 .  
         [0163]    The additional bit lengths are the value of symbol SSSS output from the symbol memory  2105 . The additional bit generator  2108  generates additional bits on the basis of this symbol SSSS, and outputs these bits to subsequent blocks together with symbol data output from the symbol memory  2105 . At this time, the operation controller  2107  asserts an effective data instruction signal (=“1”) to inform the subsequent blocks that the variable-length decoding apparatus outputs effective symbol and additional bit data in the current cycle  1 . Also, the operation controller  2107  selects the additional bit length as a shift amount.  
         [0164]    In cycle  2 , the shift-out unit  2101  shifts out code word  1  of AC components. Since this shifted-out code word  1  matches a code word registered in advance in a RUN 0 /EOB code word table  2403  in the RUN 0 /EOB address generator  2102 , a hit signal is asserted (=“1”). Hence, as the address of the symbol memory, the selector  2104  selects the output of the RUN 0 /EOB address generator  2102 , and the selector  2106  selects the code word length+additional bit length as a shift amount.  
         [0165]    In the next cycle  3 , the symbol memory  2105  outputs symbol data corresponding to code word  1 . In this state, the shift-out unit  2101  has already shifted out additional bits corresponding to code word  1 . For this reason, the additional bit processor  2108  latches data obtained by shifting out code word  1  to shift out additional bits in the flip-flop  2602  in cycle  2 . In this way, additional bits can be generated based on symbol SSSS output from the symbol memory in cycle  3 .  
         [0166]    In cycles  3 ,  4 , and  5 , the same operations as in cycles  0 ,  1 , and  2  are repeated.  
         [0167]    In cycle  6 , code word  4  shifted out by the shift-out unit  2101  is not registered in the RUN 0 /EOB code word table  2403  in the RUN 0 /EOB address generator  2102 , and a miss occurs. Since a similar miss occurred in cycle  4 , two code words successively shifted out by the shift-out unit  2101  are missed. In this case, symbol data which is effective in terms of pipeline operations in the arrangement of the variable-length decoding apparatus cannot be output, and a bubble cycle is generated. Hence, to avoid a decrease in throughput, a signal indicating RUN 1  gambling execution as information indicating a runlength=“1” is asserted (=“1”) in cycle  6 .  
         [0168]    In the subsequent cycle  7 , symbol data and additional bits corresponding to code word  4  are output.  
         [0169]    In cycle  8  as well, since a miss has occurred in the RUN 0 /EOB address generator  2102 , RUN 1  gambling execution is made as in cycle  6 . In the next cycle  9 , symbol data and additional bits corresponding to code word  5  are output.  
         [0170]    As described above, according to the sixth embodiment, the RUN 0 /EOB address generator  2102  is arranged parallel to the all-symbol address generator  2103  that uses prior art, so as to improve the throughput compared to the prior art while suppressing an increase in the number of gates. In order to allow operations at high-speed clock operation frequency, the identical processing latency of the RUN 0 /EOB address generator  2102  is set equal to the all-symbol address generator  2103  to implement a pipeline process. In order to prevent a bubble cycle generated in the pipeline process, RUN 1  gambling execution is made to further improve the throughput.  
         [0171]    When the all-symbol address generator  2103  executes decoding corresponding to RUN 1  symbol data, RUN 1  gambling execution works very effectively in the subsequent block for converting symbol data into orthogonal coefficients, thus further improving the throughput. This is because RUN 1  symbol data are obtained by encoding two orthogonal coefficients, i.e., an insignificant coefficient (orthogonal coefficient value=0) and nonzero significant coefficient and, in such case, a bubble cycle can be prevented in the subsequent blocks.  
         [0172]    In the above embodiments, a hardware decoding apparatus has been explained. However, it is easy for those who are skilled in the art that a memory (table) in the apparatus arrangement of each embodiment comprises a RAM, and other processors can be implemented by a program. Therefore, the present invention can be applied not only to the decoding apparatus but also to a decoding method, a computer program, and a computer readable storage medium that stores the program (a storage medium which is used to install a program in a computer; for example, a floppy disk, CD-ROM, or the like).  
         [0173]    As described above, the present invention can be applied to decoding of both JPEG and MPEG encoded data, and can obtain a high throughput.  
         [0174]    As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.