JP2004364340A - Variable length coding method and device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable length coding method capable of constituting a variable length code which can be decoded forwards and backwards, reduces useless bit patterns and improves coding efficiency. <P>SOLUTION: A code word table is configured for storing a plurality of code which can be decoded forward and backward and in which the code of the level of a DCT coefficient after quantization is added to the final bit in correspondence with the last DCT coefficient after quantization and the various combination of runs and levels, and a code word inputted from the code word table corresponding to the last DCT coefficient after quantization and the combination of runs and levels is selected and outputted as coded data. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a variable length coding method and apparatus.

  The variable-length code has a short code length on average by assigning a frequently occurring symbol to a short code length code and a rarely occurring symbol to a long code length code based on the frequency of occurrence of the symbol. The coding system is as described above. Therefore, when the variable length code is used, the data amount can be significantly reduced as compared with the data before encoding. As a method of configuring such a variable length code, Huffman's algorithm that is optimal for a memoryless information source is known.

  As a general problem of the variable length code, when an error is mixed in the coded data due to a transmission line error or other reasons, the effect of the data after the error is mixed is propagated correctly by the decoding device. The point is that decoding cannot be performed. Therefore, when there is a possibility that an error may occur in the transmission path, a method is generally adopted in which a synchronization code is periodically inserted at certain intervals to prevent the propagation of the error. A bit pattern that does not appear in a combination of variable length codes is assigned to the synchronization code. According to this method, even if an error occurs in the encoded data and decoding becomes impossible, by finding the next synchronization code, error propagation can be prevented and decoding can be performed correctly. However, even when the synchronous code is used, as shown in FIG. 39 (a), the encoded data from the point where an error occurs and cannot be decoded correctly to the point where the next synchronous code is found can be decoded. Can not do.

  Therefore, the variable length code is changed from the normal configuration shown in FIG. 40 to that shown in FIG. 41, so that not only the characteristic that can be decoded from the normal forward direction but also the reverse direction as shown in FIG. There is known a method of forming a code that can be decoded. In addition, since such a code can read encoded data in the reverse direction, it can be used for reverse reproduction on a storage medium such as a disk memory for storing encoded data. Such a variable length code that can be decoded not only in the forward direction but also in the reverse direction is referred to as a reversible code here. An example of the reversible code is described in, for example, Japanese Patent Application Laid-Open No. 5-300027 "Reversible Variable Length Coding". In the reversible code of this known example, as shown in FIG. 41, each code word has a longer code length at the end of the code word of the Huffman code which is a variable length code decodable from the forward direction as shown in FIG. This is a variable length code that can be decoded even in the reverse direction by adding bits under conditions that do not match the end of another codeword.

  However, this reversible code adds bits to the end of a codeword of a variable-length code that can be decoded only in the forward direction, so that useless bits increase and the average code length increases. As a result, the coding efficiency is greatly reduced as compared with a variable length code that can be decoded only in the forward direction.

  Another problem of the conventional reversible code is that when the number of information symbols is large, the amount of storage required by the variable-length encoding / decoding device increases, which is not suitable for practical use. was there. An example of a case where the number of information symbols is large is DCT coefficient encoding which is often used in image encoding of a moving image or a still image. Normally, in image coding, DCT (discrete cosine transform) of 8 × 8 is performed, and 8-bit linear quantization is performed on orthogonal transform coefficients obtained as DCT coefficients. Further, zigzag scanning is performed on the quantized DCT transform coefficients from a low frequency band, and variable length coding is performed using a set of zero-run and non-zero coefficients.

  For example, ITU-T DRAFT Recommendation H. In H.263 (1995), the quantization index value is -127 to +127 (the index value 0 is a run, so the number of indexes is 253), the number of zero runs is up to 63, and the last DCT coefficient of the block is a non-zero coefficient. Since the coding for discriminating whether or not is adopted, the number of information symbols is a very large number of 253 × 64 × 2 = 32384. Then, H. In H.263, the DCT coefficient with a high appearance probability is coded with a variable length code, but the DCT coefficient with a low appearance probability is a 1-bit code and a zero run as a code indicating whether the block is the last non-zero coefficient. The fixed-length coding is performed using a total of 15-bit codes including a 6-bit code and an 8-bit code as a quantization index, and a code called an escape code is added to the head of the fixed-length code.

  Encoding / decoding is possible even if all codewords are stored as a codeword table, but with the above-described code configuration, a fixed-length 15-bit code is not a variable-length code. Since encoding / decoding can be performed separately, it is only necessary to prepare a codeword table of DCT coefficients and escape codes having a high appearance probability. Therefore, it is possible to reduce the storage capacity by reducing the codeword table of the variable length code.

  However, as described above, the conventional reversible code cannot add an escape code because it is necessary to add a bit to the end of a variable-length code word that can be decoded only in the forward direction. Therefore, even when the number of information symbols is large, it is necessary to prepare code words corresponding to all information symbols, and the storage amount becomes enormous.

  As described above, a reversible code according to the related art, that is, a variable-length code that can be decoded from both the forward direction and the reverse direction is obtained by adding a bit to the end of a code word of a variable-length code that can be decoded only from the forward direction. Since a reversible code is configured, the number of useless bit patterns increases, the average code length increases, and there is a problem that the coding efficiency is significantly reduced as compared with a variable length code that can be decoded only in the forward direction. In such a variable-length encoding / decoding device using a reversible code, it is necessary to prepare a codeword table having codewords corresponding to all information symbols in some form. When the number is large, there is a problem that a large storage amount is required, and there is a problem that it is not suitable for practical use in any case.

  SUMMARY OF THE INVENTION It is an object of the present invention to provide a variable-length coding method and apparatus which can be decoded in both the forward and reverse directions, and have high practicality.

  More specifically, an object of the present invention is to provide a variable length coding method and apparatus which can be decoded in both forward and reverse directions and do not require a large storage amount even when the number of information symbols is large. And

  In order to solve the above-mentioned problem, the present invention provides a variable-length coding that outputs coded data composed of a variable-length codeword corresponding to an input quantized DCT coefficient. A code that is decodable and has a plurality of codewords obtained by adding a code of the level of the quantized DCT coefficient to the last bit and corresponding to different combinations of the last, run, and level of the quantized DCT coefficient, respectively. A word table is formed, and a code word corresponding to a combination of last, run, and level of the quantized DCT coefficient input from the code word table is selected and output as encoded data. Here, the plurality of codewords stored in the encoding table are configured such that the delimiter of the codeword can be identified by, for example, a predetermined number of “1” or “0”.

  According to the present invention, it is possible to provide a variable-length encoding method and apparatus which can be decoded in both the forward and reverse directions, and which are practical in terms of encoding efficiency and storage capacity required for the apparatus. .

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram illustrating a configuration of a variable-length encoding / decoding device according to an embodiment of the present invention. The variable-length encoding / decoding device according to the present embodiment is roughly divided into a codeword table creation unit 101, an encoding unit 114, a transmission or accumulation system 104, and a decoding unit 115. First, the function of each of these units will be briefly described. The codeword table creating unit 101 creates a codeword table based on the occurrence probabilities of information symbols. To the forward codeword table 110 and the backward codeword table 112. Encoding section 114 encodes the information symbol into a variable-length code, and outputs the variable-length code to transmission system or storage system 104 as encoded data. Decoding section 115 decodes the coded data input via transmission system or storage system 104 to reproduce the original information symbols.

  Next, a detailed configuration and operation of each unit of the present embodiment will be described. In the encoding unit 114, the input information symbols are input to the encoder 103. On the other hand, the codeword table 102 stores information symbols created in advance by the codeword table creation unit 101 and codewords of variable-length codes in association with each other. The encoder 103 selects a codeword corresponding to the input information symbol from the codewords stored in the codeword table 102, and outputs the selected codeword as encoded data. This encoded data is sent to the decoding unit 115 through the transmission system or the storage system 104. At this time, a synchronization code is inserted into the encoded data at regular intervals.

  In the decoding unit 115, a synchronization code is detected by the synchronization code detection unit 105 from the encoded data input from the transmission system or the storage system 104, and the encoded data between the synchronization codes is stored in the buffer 106. The forward decoder 110 starts decoding from the head of the encoded data stored in the buffer 106 based on the forward decoding tree from the forward decoding tree creating unit 111, and the backward decoder 108 The decoding is started from the end of the encoded data stored in the buffer 106 based on the backward decoding tree from the backward decoding tree creating unit 113.

  The decoded value judging section 109 calculates a decoded value from a decoded result (referred to as a forward decoded value) obtained by the forward decoder 107 and a decoded result (referred to as a backward decoded value) obtained by the backward decoder 108. And outputs the final decoding result. That is, if there is an error in the encoded data, the decoded value determining unit 109 can detect the presence of the error because a bit pattern that does not appear in the decoding tree is generated. Therefore, as shown in FIG. To determine the decoded value. FIG. 2 shows a method of determining a decoded value between synchronization codes.

  First, as shown in FIG. 2 (a), when the position of a codeword where an error is detected in the forward decoding result and the backward decoding result (error detection position) does not intersect, only the decoding result in which no error is detected Are used as decoding values, and the decoding results at the two error detection positions are not used as decoding values, and are discarded.

  When the error detection positions of the forward decoding result and the backward decoding result intersect as shown in FIG. 2B, the decoding result in which no error is detected in both cases is used as the decoding result value. In this case, the decoding result between the codewords at the two error detection positions is not used as a decoded value and is discarded.

  Also, as shown in FIG. 2C, when an error is detected only in a one-way decoding result of the forward decoding result and the backward decoding result (in this example, an error is detected only in the forward decoding result. ) Discards the decoded value for the codeword at the error detection position, and uses the decoded result in the reverse direction for the decoded values for the subsequent codewords.

  Further, as shown in FIG. 2D, when an error is detected in both the forward decoding result and the backward decoding result for the same code word, the code word at the error detection position is similar to FIG. 2C. Are discarded, and the decoding results in the reverse direction are used as decoding values for the subsequent codewords.

  The codeword table creation unit 101 creates a codeword table of codes that can be decoded in both forward and reverse directions based on the occurrence probability of information symbols. The codeword table created by the codeword table creation unit 101 is sent to the codeword table 102 in the encoding unit 114 and to the forward codeword table 110 and the backward codeword table 112 in the decoding unit 115. The forward decoding tree creating unit 111 creates a forward decoding tree from the forward codeword table 110. Further, the backward decoding tree creating unit 113 creates a backward decoding tree from the backward codeword table 112.

  FIG. 3 is a block diagram illustrating a configuration of the codeword table creation unit 101. The code selection unit 21 receives information on the occurrence probabilities of the information symbols as input, selects a code system having the smallest average code length from among code systems that can be selected based on this information, and sends the selection result to the codeword configuration unit 22. send. The codeword configuration unit 22 configures the codeword of the code selected by the codeword selection unit 21 and creates a codeword table. The codeword table created by the codeword construction unit 22 is sent to the codeword table 102 in the encoding unit 114 and the forward codeword table 110 in the decoding unit 115. A codeword table described in a direction opposite to the forward direction is sent to the backward codeword table 112 in the decoding unit 115.

The code selection unit 21 rearranges the input information symbols in descending order of the occurrence probabilities to S = {S1, S2,..., Sn}, and the occurrence probabilities P = {p1, p2,. pn}, based on a set C of configurable codes,

Is selected. Here, Li is a code length that can be calculated by the weight of the code word given from the code word forming unit 22. The code word weight in this case refers to the number of “1” or “0” in the code word.

Hereinafter, a method of forming a code word in the code word forming unit 22 will be described.
FIG. 4 shows an example of how to create a binary code sequence having a constant weight serving as a basis for a variable length code according to the present embodiment. As shown in FIG. 4A, a grid-like directed graph from the start point to the end point is formed. When following the route from the start point to the end point in this directed graph, “0” is selected when the left route is selected, and “1” is selected when the right route is selected. Is generated. In this example, a binary code sequence having a code length of 5 bits and a weight (the number of “1”) of 2 is generated. In general, there are as many binary code sequences having a code length of n bits and a weight of w as the number of combinations that select w from n. Here, there will be 10 binary code sequences, the number of combinations of selecting two out of five.

  FIG. 5 shows a first method of forming a codeword of a variable length code (hereinafter, referred to as a reversible code) that can be decoded in the forward direction and the backward direction in the codeword forming unit 22. First, as shown on the left side of FIG. 5, the weight (the number of “1”) is constant (the number of “1”) by the method of FIG. In this case, one binary code sequence is created. Next, as shown on the right side of FIG. 5, a code word of the reversible code is formed by adding "1" to the beginning and end of each of the binary code sequences and assigning "0" to the shortest code. This reversible code always starts with the code “0” for the information symbol A and “1” for the other information symbols B to J, and when three “1” s appear in all three. This is a code configuration in which the code ends, that is, a configuration where the code delimiter (code length) is known.

  The necessary and sufficient condition for the variable-length code to be a reversible code, that is, a code that can be decoded in both the forward and reverse directions, is that all codewords are assigned to the leaves of the forward and backward decoding trees. is there. The leaf of the decoding tree is the end of the decoding tree, that is, a place where there is nothing earlier. For example, in the variable-length code of FIG. 5, the codewords corresponding to all the information symbols A to J are the leaves of the forward decoding tree shown in FIG. 6A and the leaves of the backward decoding tree of FIG. Is also assigned, it can be seen that decoding is possible in both the forward and reverse directions.

  The parameters of the variable length code codeword configuration method in FIG. 5 correspond to the occurrence probabilities of various information symbols by setting the number of “1” in the codeword as a weight and setting the weight to a natural number w of 2 or more. be able to. Of course, the same applies to the case where the bit is inverted and the number of “0” is considered as the weight and discussed.

  FIG. 7 shows a second method of forming a codeword of a reversible code in the codeword forming unit 22. First, as shown on the left side of FIG. 7, the weight (the number of “1”) is constant (one in this case) by the method of FIG. Create a hexadecimal code sequence. Next, as shown in the center of FIG. 7, "1" is added to the beginning and end of each of the binary code sequences, and further, as shown on the right side of FIG. Add words.

  In this variable length code, the code length can be determined by counting the number of symbols at the beginning of each code. In the example of FIG. 7, the code configuration ends when four symbols at the beginning of each code appear, that is, the code configuration in which a code segment (code length) is known.

  In the variable-length code of FIG. 7, the codewords corresponding to all the information symbols A to J may be the leaves of the forward decoding tree shown in FIG. 8A or the leaves of the backward decoding tree shown in FIG. It can be seen from the assignment that decoding is possible in both the forward and reverse directions.

  FIG. 9 shows a third method of forming a codeword of a reversible code in the codeword forming unit 22. The reversible code has a code configuration in which the number of “0” and “1” are the same, so that the code length of the code word can be known. That is, consider the lattice-like directed graph shown in FIG. 9A, and when starting from the start point and selecting the left-side route, consider "0" when selecting the right-side route and "1" when selecting the right-side route. Correspondingly, a route that reaches a diagonal black dot passing through the start point is generated as a codeword.

  In this case, the codewords corresponding to all the information symbols A to J are assigned to the leaves of the forward decoding tree shown in FIG. 10A and the leaves of the backward decoding tree shown in FIG. It can be seen that decoding is possible in both directions.

  Next, a method for shortening a reversible code according to the present invention will be described. Since the number of information symbols, that is, the number of codes of the variable length code is limited, it is possible to shorten a part of the variable length code. Here, shortening refers to reducing the code length of some codes without increasing the code length of other codes. FIG. 11 shows an example of a method for shortening a reversible code.

  For example, the reversible code shown in FIG. 7 can be decoded from either the forward direction or the reverse direction even if the last bit of the codeword corresponding to the information symbols G, H, I, and J is deleted. . By utilizing this, in FIG. 11, the code lengths of the four codewords corresponding to the information symbols G, H, I, and J are reduced.

  In the variable-length code shown in FIG. 11, the codewords corresponding to all the information symbols A to J also appear on the leaves of the forward decoding tree shown in FIG. Since it is also assigned to the leaves of the tree, it can be seen that decoding can be performed in both the forward and reverse directions.

Next, a method of extending a reversible code according to the present invention will be described with reference to FIG. Extension of a reversible code can be realized by adding an equal-length code to at least one of the beginning and end of each code to increase the number of codewords having the same code length. In the example of FIG. 13, a 2-bit equal-length code is added to the end of each of the reversible codes shown on the left side. In this case, the code length of each code of the reversible code is increased by 2 bits as a whole, but the number of codewords having the same code length is quadrupled. Generally, when an n-bit equal-length code is added, the code length increases by n bits as a whole, but the number of codewords having the same code length can be increased by 2 ⁿ times. Since the isometric code is obviously a reversible code, even if the isometric code is added to the beginning or end of the reversible code, it is still a reversible code.

  FIG. 14 shows a comparison between a reversible code obtained by performing variable length coding on an information symbol, which is an English alphabet, according to the present invention, and a reversible code disclosed in a known example (Japanese Patent Laid-Open No. 5-300027). It is shown. The reversible code according to the present invention shown in FIG. 14 is a code obtained by expanding the code obtained by adding a 2-bit equal length code to the end of the code when the weight of the code word is set to 0 in the method shown in FIG. It is.

  Although the reversible code according to the present invention is inferior to the Huffman code, which is an optimal code of a variable length code that can be decoded only in the forward direction, the average code length is shorter and superior to the reversible code disclosed in the known example. You can see that it has performance. This is because the known reversible code is a variable-length code that can be decoded only in the forward direction with a bit added to the end of the Huffman code, whereas the present invention does not add an extra bit, This is because the reversible code is formed from the beginning, so that there is little useless bit pattern.

Next, another embodiment of the present invention will be described.
FIG. 15 is a block diagram showing a configuration of a variable length encoding / decoding device according to another embodiment of the present invention. The variable-length encoding / decoding device according to the present embodiment is roughly divided into a codeword table creation unit 201, an encoding unit 213, a transmission or accumulation system 205, and a decoding unit 214.

  The function of each of these units will be briefly described. The codeword table creation unit 201 creates a codeword table based on the occurrence probabilities of information symbols, sends the codeword table to the codeword table 202 in the encoding unit 213, and further creates the codeword table. The parameters of the codewords in the table are sent to the decoded graph / decoded value table creation unit 208 in the decoding unit 214. The encoding unit 213 encodes the information symbol into a variable length code, and outputs the variable length code to the transmission system or the storage system 205 as encoded data. Decoding section 214 decodes the encoded data input via transmission system or storage system 205 to reproduce the original information symbols.

Next, a detailed configuration and operation of each unit of the present embodiment will be described.
In the encoding unit 213, the input information symbols are input to the encoder 203. The codeword table 202 stores information symbols created in advance by the codeword table creation unit 201 and codewords of variable-length codes in association with each other. However, the codeword table 202 does not store codewords corresponding to all of the information symbols that can be input, and stores only codewords corresponding to some information symbols having a relatively high appearance frequency. It is assumed that The encoder 203 selects a codeword corresponding to the input information symbol from the codewords stored in the codeword table 202 and outputs the selected codeword as encoded data.

  On the other hand, when an information symbol for which no codeword corresponding to the codeword table 202 exists, that is, an information symbol having a relatively low appearance frequency, is input to the encoding unit 213, the fixed-length code corresponding to the information symbol is fixed-length. The code encoding unit 204 creates the code word by adding an escape code in the code word table 202 to the beginning and end of the fixed-length code, and outputs this as encoded data.

  The encoded data output from the encoder 203 in the encoding unit 213 in this way is sent to the decoding unit 214 through the transmission system or the storage system 205. At this time, a synchronization code is inserted into the encoded data at regular intervals.

  The decoding unit 214 includes a synchronization code detection unit 206, a buffer 207, a decoding graph / decoding value table creation unit 208, a forward decoding unit 209, a fixed length decoding unit 210, a backward decoding unit 211, and a decoding value determination unit. The synchronization code detecting unit 206 detects a synchronization code from encoded data input from the transmission system or the storage system 205, and stores the encoded data between the synchronization codes in the buffer 207. The forward decoder 209 starts decoding from the beginning of the encoded data stored in the buffer 207, and the backward decoder 211 starts decoding from the end of the encoded data stored in the buffer 207. The decoded value determination unit 212 calculates a decoded value from a decoded result (referred to as a forward decoded value) obtained by the forward decoder 209 and a decoded result (referred to as a backward decoded value) obtained by the backward decoder 211. And outputs the final decoding result.

  FIG. 16 is a diagram illustrating a method of forming a codeword by adding an escape code to the beginning and end of a fixed-length code in the codeword table creation unit 201 in FIG. As shown in FIG. 16A, when the information symbol is other than “A”, the code word created by the code word table creation unit 201 has a code delimiter by the appearance of three “1” s. It is based on reversible codes that can be understood. This reversible code is referred to as code (1).

  On the other hand, the code (2) shown in FIG. 16B is a code word “1011” corresponding to the information symbol “C” of the code (1) as an escape code for indicating the beginning and end of the fixed-length code. The code word formed by adding the bits to the beginning and end of the fixed length code is newly added to the code (1). However, in the reversible code portion corresponding to the code (1) in the code (2), the code word “1011” used as the escape code in the code (2) is not used, and information smaller by one by the code (1) is used. Only code words corresponding to symbols A to I are used. As is clear from comparison of FIGS. 16A and 16B, the number of information symbols that can be encoded for code (2) has increased from “10” of code (1) to “17”.

  In order to decode such a code (2), the code (1) is always decoded once, both in the case of decoding in the forward direction and in the case of decoding in the reverse direction, as can be seen from FIGS. That is, since the fixed-length code can be decoded in both the forward and reverse directions, if the code (1) can be decoded in both the forward and reverse directions, the code (2) is also decoded in the forward direction. Decoding is also possible from the opposite direction.

  In the case of a conventional reversible code, it is necessary to prepare all codewords corresponding to information symbols that can be input in the encoding unit 213 and the decoding unit 214 as a codeword table. On the other hand, according to the code (2), the fixed-length code portion can be separately prepared as a 3-bit binary code as the fixed-length code encoding unit 204 and the fixed-length code decoding unit 210. Only the codeword of code (1) needs to be stored in the table 202. Therefore, compared with the conventional reversible code, the storage amount of the variable length encoding / decoding device can be significantly reduced.

  The decoding graph / decoding value table creator 208 creates a decoding graph and a decoding value table based on the Sharkwijk algorithm, and the forward decoder 209 and the backward decoder 211 decode these decoding graph and decoding data. Perform decoding based on the value table.

  FIG. 17 shows a method of counting binary code sequences having a constant weight. JPMShalkwijk: "An algorithm for source coding", IEEE Trans. Infrom Theory, vol. IT-18, no. 3, pp. 395-399, May 1972, as an algorithm for counting binary code sequences having a constant weight. Is well known as the Sharkwijk algorithm. In the Sharkwijk algorithm, as shown in FIG. 17A, a grid-like directed graph from a START point to an END point is formed based on a “Pascal triangle”. The value indicated by a numeral at each node is the sum of the values of the nodes at the starting points of the two arrows entering each node, with the value of the node at the START point being 1. That is, the value of each node is determined by the Pascal's triangle.

  Then, in the directed graph of FIG. 17A, when following the route from the START point to the END point according to the input information symbol, the codeword “0” is selected when the left route (arrow) is selected, and the right route is selected. When (arrow) is selected, a binary code sequence having a weight of 2 can be generated by associating the codeword “1” with each other. Here, the value obtained by subtracting the value of the node at the start of the arrow from the value of the node at the end of the arrow when “0” is selected is the code word of the same code length of the binary code sequence. Is the ordinal value of For example, if the input information symbol is “01001”, the sum of the differences between the node values for three “0” s is (1-1) + (3-2) + (4-3) = 2 is the order value. Here, the sequential value is a value for ordering different codewords having the same code length. FIG. 17B shows the relationship between information symbols and order values in this case.

  An example in which the Sharkwijk algorithm is applied to encoding is known. In this case, encoding is performed by converting a value corresponding to the above-described order value into a binary code. In contrast, in the present embodiment, this algorithm is used for decoding as described below.

  That is, the decoding graph / decoding value table creation unit 208 creates a directed graph as a decoding graph based on the above-mentioned Sharkwijk algorithm. That is, this decoded graph is a directed graph in which values determined by Pascal's triangle are set as node values and codewords "1" and "0" are arrows, and an example is shown in FIG. FIG. 18B shows an example of a method of calculating a decoded value in the forward decoder 208 and the backward decoder 211 using the decoded graph.

  Code (1) shown in FIG. 16 (a) is a reversible code in which when the head is other than “0”, a break can be recognized at a point where three “1” appear. When calculating the decoded value of the reversible code of the code (1), if the head of the reversible code is “0” in the forward direction, the decoded value is set to 0; Is deleted, and the order value is calculated by the Sharkwijk algorithm. Since the number of codewords having a code length shorter than the codeword to be decoded is the sum of the values of the diagonal right nodes at the END point in FIG. 18A, the sum is added to the order value. Is the decoded value.

  On the other hand, in the reverse direction, as in the case of the above-described forward direction, first, if the head of the reversible code is "0", the decoded value is set to 0; otherwise, the leading and terminal "1" are deleted. After inverting the remaining part, the order value is calculated by the Sharkwijk algorithm. Then, the sum of the order value and the sum of the nodes on the upper right of the END point plus 1 is defined as a decoded value.

  More specifically, with reference to FIGS. 18A and 18B, for example, if the codeword to be decoded for the reversible code is “10101”, the leading and trailing “1” s are first deleted and “010” is added. I do. When an order value is obtained for this “010” by the Sharkwijk algorithm, (1-1) + (3-2) = 1 for two “0” s. In this case, since the sum of the values of the nodes at the diagonally upper right of the END point is 1 + 2 = 3, this value + 1 (= 4) is added to the order value (= 1), and the decoded value is obtained as 5.

  When decoding is performed using such a decoding graph, the amount of storage can be significantly reduced. FIGS. 19A and 19B are diagrams showing a difference in storage amount required between a conventional decoding method using a decoding tree and a decoding method using a decoding graph according to the present embodiment. Comparing the two with the number of nodes, the number of arrows coming out of each node is the same in both cases, but the number of arrows coming in each node is always one in the case of the decoding tree in FIG. . On the other hand, in the case of the decoded graph of FIG. 19B, since two arrows enter each node, the total number of nodes required by the same codeword decreases accordingly. In this example, the decoding tree of FIG. 19A requires 15 nodes, whereas the decoding graph of FIG. 19B requires only 8 nodes. Therefore, the decoding method of the present embodiment using the decoding graph has an advantage that the required storage amount is smaller than that of the conventional method using the decoding tree.

  In this embodiment, in order to decode the code (2) to which the escape code shown in FIG. 16B is added, the decoded value table of the code (1) shown in FIG. ) Are prepared, and when the escape code is decoded in the decoded value table of code (1), the decoded value table of the fixed length code is read. For example, when the codeword is “10110011011”, if the leading “1011”, that is, the escape code is decoded, the next three bits “001” are regarded as a fixed-length code and decoded by the fixed-length code decoding unit 210. I do. In this case, since 1 is obtained as the decoded value of “001”, the decoded result is the information symbol “K” from the fixed-length code decoded value table of FIG.

  Next, an embodiment in which the present invention is applied to a moving image encoder / decoder will be described. FIG. 21 is a block diagram showing a schematic configuration of a moving image encoder / decoder incorporating the above-described variable length encoding / decoding device.

  In the video encoder 709 shown in FIG. 21A, the data encoded by the information source encoder 702 is subjected to variable-length encoding, communication channel encoding, multiplexing, and the like by a video multiplexer 703. Then, after the transmission speed is smoothed by the transmission buffer 704, the data is sent to the transmission system or the storage system 705 as encoded data. The encoding control unit 701 controls the information source encoder 702 and the video multiplexing unit 703 in consideration of the buffer amount of the transmission buffer 704.

  On the other hand, in the moving picture decoder 710 shown in FIG. 21B, the coded data from the transmission system or the storage system 705 is temporarily stored in the reception buffer 706, and the moving picture multiplexing / demultiplexing unit 707 converts the coded data into After demultiplexing, channel coding, and variable-length code decoding are performed, the data is sent to an information source decoder 708, and a moving image is finally decoded.

  FIG. 22 shows the syntax of the moving picture coding method in the moving picture coder 709 and the moving picture multiplexing / demultiplexing unit 707 in FIG. Among the picture layers shown in FIG. 22A, information other than DCT coefficients such as macroblock mode information and motion vector information and INTRA DC is arranged in the upper layer, and DCT coefficient information is arranged in the lower layer. Thus, the reversible code according to the present invention is applied to the lower layer part.

  FIGS. 23A and 23B are block diagrams showing a more detailed configuration of the video multiplexing unit 703 and the video multiplexing / demultiplexing unit 707 in FIG. In the moving image multiplexing unit 703 shown in FIG. 23A, among the coded data from the information source coder 702 in FIG. The information other than the upper layer is subjected to normal variable length coding in the upper layer variable length encoder 901 as an upper layer, and further, the upper layer channel encoder 902 performs error correction with a high degree of redundancy but a high correction capability. The channel is coded by the detection code and sent to the multiplexing unit 905.

  On the other hand, among the encoded data from the information source encoder 702, the DCT coefficients are encoded into a reversible code by the lower layer variable length encoder 903, and the lower layer channel encoder 904 generates an error with less redundancy. After being channel-coded by the correction detection code, it is sent to the multiplexing unit 905. The multiplexing unit 905 multiplexes the encoded data of the upper layer and the encoded data of the lower layer, and sends the multiplexed data to the transmission buffer 704.

  In the moving image multiplex / separation unit 707 shown in FIG. 23B, first, the coded data from the reception buffer 706 is separated by the multiplex / separation unit 906 into an upper layer and a lower layer. The encoded data of the upper layer is decoded by the upper layer channel decoder 907, and the decoding result is sent to the upper layer variable length decoder 909. The lower layer encoded data is decoded by the lower layer channel decoder 908, and the decoding result is sent to the lower layer variable length decoder 910.

  Lower layer variable length decoder 910 decodes the reversible code, and sends the decoding result to information source decoder 708 and upper layer variable length decoder 909. The upper layer variable length decoder 909 decodes the variable length code which is the encoded data of the upper layer, and rewrites the encoding result based on the decoding result of the lower layer variable length decoder 910.

  Here, the lower layer variable length encoder 903 in FIG. 23A is used for the encoding unit 213 in FIG. 15, and the lower layer variable length decoder 910 in FIG. 23B is used for the decoding in FIG. Respectively corresponding to the conversion units 214.

  As seen in the video encoder / decoder of the present embodiment, when the encoding scheme has the syntax as shown in FIG. 22, not only can the variable length code itself be decoded from both directions, In terms of syntax, it is necessary to be able to decode from both directions. In the present embodiment, this request is realized by using a codeword table described below.

  FIGS. 24 to 28 show examples of codeword tables of variable length codes of DCT coefficients used in the lower layer variable length encoder 903. FIG. FIG. 29 shows a codeword table of escape codes.

  The information source encoder 702 scans the block of the 8 × 8 DCT coefficient after quantization and performs LAST (0: non-zero coefficient not at the end of the block, 1: 1: non-zero coefficient at the end of the block. A coefficient), RUN (the number of zero runs to a non-zero coefficient) and LEVEL (absolute value of the coefficient) are obtained and sent to the video encoder 709.

  The lower layer variable length encoder 903 in the video encoder 709 is a non-LAST coefficient of INTRA and INTER shown in FIGS. A code word table (first code word table) and a code word table (LAST code) of LAST coefficients in which LAST coefficients of INTRA and INTER shown in FIGS. 26 and 27 correspond to reversible codes (VLC_CODE) for RUN and LEVEL. Codeword table). Then, based on the mode information, the coding is performed by selecting the code word table of the non-LAST coefficient and the LAST coefficient of the INTRA at the time of INTRA and the code word table of the non-LAST coefficient and the LAST coefficient of the INTER at the time of INTER. In FIGS. 26 and 27, "S" of the last bit of VLC_CODE indicates the sign of LEVEL, and when "S" is "0", the sign of LEVEL is positive, and when "1", it is negative.

  For the coefficients that do not exist in this codeword table, the absolute values of RUN and LEVEL are encoded into fixed-length codes as shown in FIG. 29, and escape codes are added to the beginning and end of the fixed-length codes, and The last bit is encoded so that the LAST coefficient and the LEVEL can be distinguished from positive or negative. FIG. 28 is a codeword table of this escape code. The last bit “t” of VLC_CODE used as an escape code indicates whether or not it is a LAST coefficient when added to the beginning of a fixed-length code. When this is "0", it is a non-LAST coefficient, and when it is "1", it is a LAST coefficient. “T” represents the LEVEL code when added to the end of the fixed-length code. When this is “0”, the LEVEL code is positive, and when it is “1”, it is negative.

  FIG. 30 shows an example of encoded data in the present embodiment. As shown in the figure, when decoding is performed in the lower layer from the forward direction, since the code of the LAST coefficient always exists at the end of the block of the DCT coefficient of 8 × 8 pixels, the end of the block may be determined. it can. On the other hand, when decoding is performed in the reverse direction, the beginning of the block can be determined by the appearance of the code of the LAST coefficient of the block preceding by one address. Since this code has a LAST coefficient common to the INTRA mode and the INTER mode, the head of the block can be determined even if a mode exists for each macro block.

  For the first block of the lower layer, a dummy LAST coefficient is coded at the beginning of the lower layer in the lower layer variable length encoder 903, or the next frame is encoded in the lower layer variable length decoder 910. The number of bits in the lower layer is calculated in advance from the number of bits up to the synchronization code of the lower layer, and the calculated number is compared with the number of bits decoded. Alternatively, a buffer in the lower layer variable length decoder 910 adds By adopting a method of inserting a dummy LAST coefficient, it is possible to determine the head of the lower layer when decoding in the reverse direction.

  The reversible code stored in the code word tables shown in FIGS. 24 to 27 is read in the forward direction until the first one bit is “0”, until two “0” appear, and when it is “1”. For example, it is understood that two "1" s are read until the next one bit becomes the last bit indicating the positive or negative of LEVEL. In the reverse direction, the first one bit indicates the sign of LEVEL, and if the next one bit is “0”, it is read until two “0” appear, and if “1”, it is “1”. Please read until two appear.

  FIG. 31 shows a decoding graph for decoding a reversible code applied to the variable length code of the DCT coefficient used in the lower layer variable length encoder 903 in the present embodiment. A decoded value can be calculated from the decoded graph of the encoded data excluding the first two bits and the last two bits of the codeword when viewed from the forward direction.

  In this decoded graph, if the first bit is "0", the right arrow is advanced when "0" appears, and the left arrow is advanced if "1" appears. If the first bit is "1", the right arrow is advanced if "1" appears, and the left arrow is advanced if "0" appears.

  In order to obtain the order value for a binary code sequence having the same code length, the value of the starting node is subtracted from the value of the ending node of the arrow when the left arrow is selected, according to the Sharkwijk algorithm described above. The sum of the values may be used as the order value at the code length.

  On the other hand, in order to obtain the decoded value, if the leading bit is “0”, a value obtained by adding twice the sum of the values of the nodes at the upper right corner of the terminal node to the ordinal value may be used as the decoded value. . If the first bit is "1", the decoded value may be twice the sum of the values of the nodes diagonally to the upper right of the terminal node plus the order value to the value of the terminal node.

  For example, if the code string to be decoded is “0110101”, it is first set to “1101” except for the first one bit and the last two bits when viewed from the forward direction. Since the first bit is "0", if "0" appears in the decoding graph, the right arrow is advanced, and if "1" appears, the left arrow is advanced. In this case, since the difference between the nodes when the left arrow is advanced is (1-1) + (1-1) + (4-3) = 1, the order value is 1. Since the most significant bit is “0”, the order 1 is added to twice (1 + 2 + 3) × 2 = 12 of the total of the nodes diagonally right above the terminal node, and the result is 13; Will be done.

  FIGS. 32 to 34 show an example of a decoded value table used in the lower layer variable length decoder 910. FIGS. 32 to 33 show non-LAST coefficients of INTRA and INTER, and RUN and LEVEL in which decoded values are associated with decoded values. FIG. 34 (a) and (b) are decoded value tables of LAST coefficients in which LAST coefficients of INTRA and INTER are associated with decoded values of RUN and LEVEL, respectively. FIG. 35 is a decoded value table of the escape code.

  In this example, for example, as can be seen in FIG. 34A, the decoded value 13 is a LAST coefficient, the number of RUNs is 3, the absolute value of LEVEL is 1, and the least significant bit is “1”. This indicates that LEVEL is negative. Further, as can be seen from FIG. 35, when the decoded value 41 is decoded, it is determined that the escape code has been decoded. The last bit of the escape code is used to determine whether it is a LAST coefficient or a non-LAST coefficient, and the fixed-length code decoder 210 decodes the absolute values of RUN and LEVEL for the subsequent 13 bits, and then decodes the escape code again. , The last bit determines the sign of LEVEL.

  As an example, consider a case where the code string to be coded is “110000010000111000101111000010”. In this case, since the last bit of the leading escape code “11000010” is “0”, it is a non-LAST coefficient, and the subsequent 13-bit fixed-length code is calculated. Of these 13 bits, the upper 6 bits represent RUN, so that "000111" represents RUN, and the lower 7 bits represent the absolute value of LEVEL. Therefore, "0001011", the absolute value of LEVEL is 12, and the end escape code " Since the last bit of “11000010” is “0”, LEVEL can be decoded as positive.

  The decoded value determination unit 212 compares the decoded result obtained by the forward decoder 209 (referred to as a forward decoded result) with the decoded result obtained by the backward decoder 211 (referred to as a backward decoded value). ) To determine the decoded value and output the final decoded result. That is, if there is an error in the encoded data, the decoded value determination unit 212 recognizes the presence of the error because the decoding result of the lower layer channel decoder 908 or a bit pattern that did not exist as a codeword occurs. Therefore, a decoded value is determined from the forward decoding result and the backward decoding result as shown in FIG. FIG. 36 shows a method of determining a decoded value of a lower layer.

  First, as shown in FIG. 36 (a), when the position of a macroblock where an error is detected in the forward decoding result and the backward decoding result (error detection position) does not intersect, the macroblock in which no error is detected Only the decoding result is used as a decoding value, and the decoding results at two error detection positions are not used as decoding values. Then, based on the decoding result of the mode information of the upper layer, the decoding result of the upper layer is displayed such that the previous frame is displayed as it is for the INTRA macroblock and the motion compensation is displayed only for the INTER macroblock from the previous frame. Is rewritten.

  In addition, as shown in FIG. 36 (b), when an error detection position intersects between a forward decoding result and a backward decoding result, a decoding result in which no error is detected in both is used as a decoding result value. In this case, the decoding result between the codewords at the two error detection positions is not used as a decoding value, and the previous frame of the INTRA macroblock is displayed as it is based on the decoding result of the mode information of the upper layer. As described above, the decoding result of the upper layer is rewritten so that the INTER macroblock is displayed only by the motion compensation from the previous frame.

  In addition, as shown in FIG. 36 (c), when an error is detected only in a one-way decoding result out of a forward decoding result and a backward decoding result (in this example, only an error is detected in the forward decoding result. Does not use the macroblock at the error detection position as a decoded value, and displays the previous frame for the INTRA macroblock as it is based on the decoding result of the mode information of the upper layer. The decoding result of the upper layer is rewritten so as to display only the motion compensation from the frame. The decoding value for the subsequent macroblock uses the decoding result in the reverse direction.

  Further, when an error is detected in both the forward decoding result and the backward decoding result in the same macroblock as shown in FIG. 36 (d), the decoding value of the macroblock at the error detection position is discarded and decoded. It is not used as a value, and based on the decoding result of the mode information of the upper layer, the previous frame is displayed as it is for the INTRA macroblock, and the upper frame is displayed only for the motion compensation for the INTER macroblock from the previous frame. The decoding result of the hierarchy is rewritten, and the decoding value for the subsequent macroblock uses the decoding result in the reverse direction.

  The order of encoding / decoding of macroblocks may be a method other than the above-described embodiment. For example, when an error occurs, a more important central part of a decoded image is rescued in FIG. The order may be as shown. In other words, assuming that decoding is performed from both directions, the probability that the first part and the last part of the encoded data between the synchronization codes are correctly decoded is high. The order is determined so that is in the center.

In the above embodiment, the present invention is applied to variable-length coding of DCT coefficients. However, it goes without saying that the same variable-length coding can be applied to other information symbols.
Further, in the above-described embodiment, only the binary code has been discussed, but it is easy to extend to a multiple code, and the same effect can be obtained.

  Finally, as an application example of the present invention, an embodiment of an image transmitting / receiving apparatus to which the variable length encoding / decoding apparatus according to the present invention is applied will be described with reference to FIG. An image signal input from a camera 1002 provided in a personal computer (PC) 1001 is encoded by a variable length encoding device incorporated in the PC 1001. The information symbol in this case is not limited to this. For example, the input image signal or a prediction error signal which is a difference between the input image signal and the prediction image signal is subjected to discrete cosine transform by a DCT circuit, and further quantized by a quantization circuit. DCT coefficient data obtained by the conversion. The coded data output from the variable-length coding device is multiplexed with other voice and data information, transmitted by radio by the wireless device 1003, and received by the other wireless device 1004. The signal received by the wireless device 1004 is decomposed into encoded data of an image signal and information of voice and data. Among these, the coded data of the image signal is decoded by the variable length code decoding device incorporated in the workstation (EWS) 1005, and is displayed on the display of the EWS 1005.

  On the other hand, the image signal input from the camera 1006 provided in the EWS 1005 is encoded in the same manner as described above using the variable length encoding device incorporated in the EWS 1006. The encoded data is multiplexed with other voice or data information, transmitted wirelessly by the wireless device 1004, and received by the wireless device 1003. The signal received by the wireless device 1003 is decomposed into coded data of an image signal and voice and data information. Among these, the coded data of the image signal is decoded by the variable length code decoding device incorporated in the PC 1001, and is displayed on the display of the PC 1001.

  The variable-length encoding / decoding device according to the present invention has a code length corresponding to the frequency of appearance of information symbols, has less wasteful bit patterns and is more efficient than in the past, and has a forward and a reverse direction. Since a reversible variable-length code that can be decoded can also be configured, the present invention is particularly effective in a transmission / reception apparatus using a transmission line having many errors, such as the wireless transmission line shown in FIG.

  As described in the above embodiment, the variable-length encoding / decoding device of the present invention can decode with a smaller storage amount as compared with the related art, and the information symbol that cannot be conventionally applied. It can be applied to coding with a large number.

  Specifically, the present invention can be applied to a video encoding / decoding device, and can provide a video encoding / decoding device that is resistant to errors.

1 is a block diagram illustrating a configuration of a variable-length encoding / decoding device according to an embodiment of the present invention. FIG. 4 is a diagram for explaining the operation of the decoded value determination unit in FIG. FIG. 1 is a block diagram of a codeword table creation unit in FIG. FIG. 4 is a diagram showing a method for creating a binary code sequence having a constant weight as a basis of a variable length code according to the present invention. FIG. 3 is a diagram showing a first codeword configuration method in a codeword configuration unit in FIG. 3. FIG. 3 is a diagram showing forward and backward decoding trees created from codewords configured by a first code configuration method. FIG. 3 is a diagram showing a second codeword configuration method in the codeword configuration unit in FIG. 3. FIG. 7 is a diagram showing forward and backward decoding trees created from codewords configured by a second code configuration method. FIG. 3 is a diagram showing a third code configuration method in the codeword configuration unit in FIG. FIG. 10 is a diagram showing forward and backward decoding trees created from codewords configured by a third code configuration method. FIG. 3 is a diagram showing a code shortening method in the code word forming unit in FIG. Diagram showing forward and backward decoding trees created from codewords shortened by a code shortening method FIG. 3 is a diagram showing a code enlarging method in the code word forming unit in FIG. FIG. 6 is a diagram showing a comparison between the characteristics of the variable length code configured in the present embodiment and the variable length code of the known example. FIG. 9 is a block diagram showing a configuration of a variable-length encoding / decoding device according to another embodiment of the present invention. The figure which shows the code | cord | chord structure method in the codeword structure part in FIG. Diagram showing how to count binary sequences with constant weight FIG. 15 is a diagram showing operations of the forward decoder and the backward decoder in FIG. FIG. 7 is a diagram showing a comparison between a conventional decoding tree and a decoding graph in the present embodiment. FIG. 7 is a diagram for explaining a decoded value table according to the present embodiment. FIG. 1 is a block diagram showing a schematic configuration of a moving image encoder / decoder incorporating a variable length encoding / decoding device according to the present invention. FIG. 3 is a diagram illustrating the syntax of encoded data in a video encoder / decoder according to the embodiment. FIG. 21 is a block diagram showing a configuration of a video multiplexing unit and a video multiplexing / demultiplexing unit in FIG. The figure which shows a part of codeword table of the non-LAST coefficient of INTRA and INTER in the embodiment. FIG. 14 is a diagram showing another part of the codeword table of the non-LAST coefficients of INTRA and INTER in the embodiment. FIG. 7 is a diagram illustrating a part of a codeword table of LAST coefficients of INTRA and INTER in the embodiment. FIG. 10 is a diagram illustrating another part of the code word table of the LAST coefficient of INTRA and INTER in the embodiment. FIG. 3 is a diagram illustrating a codeword table of an escape code according to the first embodiment. The figure which shows the encoding method of the codeword which is not in a codeword table in the embodiment. FIG. 3 is a diagram illustrating an example of encoded data according to the first embodiment. The figure which shows the decoding graph in the embodiment. The figure which shows a part of decoding value table of the non-LAST coefficient of INTRA and INTER in the embodiment. FIG. 14 is a diagram showing another part of the decoded value table of the non-LAST coefficients of INTRA and INTER in the embodiment. FIG. 7 is a diagram showing a decoded value table of LAST coefficients of INTRA and INTER in the embodiment. FIG. 7 is a diagram showing a decoded value table of an escape code in the embodiment. FIG. 15 is a diagram for explaining the operation of the decoded value determination unit in FIG. FIG. 3 is a diagram illustrating an encoding / decoding order of macroblocks in the embodiment. FIG. 2 is a diagram showing an example of an apparatus in which a variable length encoding / decoding apparatus according to the present invention is incorporated. Diagram showing a general decoding method for reversible codes Illustration of ordinary variable length code Illustration of conventional reversible code

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 101 ... Code word table preparation part 102 ... Code word table 103 ... Encoder 104 ... Transmission system or storage system 105 ... Synchronization code detection part 106 ... Buffer 107 ... Forward decoder 108 ... Reverse decoder 109 ... Decoding Value determination unit 110: forward decoded word table 111: forward decoding tree creation unit 112: backward codeword table 113 ... backward decoding tree creation unit 114: encoding unit 115: decoding unit 21: code selection unit 22 ... Codeword configuration unit 201 Codeword table creation unit 202 Codeword table 203 Encoder 204 Fixed length encoding unit 205 Transmission system or storage system 206 Synchronous code detection unit 207 Buffer 208 Decoding graph / decoding Value table creation unit 209: forward decoder 220: fixed-length code decoding unit 211: backward decoder 212: decoded value Judgment unit 213 Encoding unit 214 Decoding unit 701 Encoding control unit 702 Information source encoder 703 Moving image multiplexing unit 704 Transmission buffer 705 Transmission or storage system 706 Reception buffer 707 Video Image demultiplexing unit 708 ... source decoder 709 ... video encoder 710 ... video decoder 901 ... lower-layer variable-length decoder 902 ... upper-layer channel encoder 903 ... lower-layer variable-length Encoder 904: Lower layer channel encoder 905: Multiplexer 906: Demultiplexer 907: Upper layer channel decoder 908 ... Lower layer channel decoder 909: Upper layer variable length decoder 910: lower layer variable length decoder 1001 ... personal computer 1002 ... workstation 1003 ... radio 1004 ... radio 1005 ... camera 10 06 ... Camera

Claims (4)

In a variable-length encoding method for outputting encoded data including a variable-length codeword corresponding to an input quantized DCT coefficient,
A plurality of codewords which can be decoded in both forward and reverse directions and in which the code of the level of the quantized DCT coefficient is added to the last bit are different combinations of the last, run and level of the quantized DCT coefficient. Constitute a codeword table stored in correspondence with
A variable length encoding method, wherein a codeword corresponding to a combination of the last, run, and level of the input DCT coefficient after quantization is selected from the codeword table and output as encoded data.   2. The variable according to claim 1, wherein the plurality of codewords stored in the encoding table are configured so that a delimiter of the codeword can be identified by a predetermined number of "1" or "0". Long encoding method. In a variable-length encoding device that outputs encoded data including a variable-length codeword corresponding to an input quantized DCT coefficient,
A plurality of codewords which can be decoded in both forward and reverse directions and in which the code of the level of the quantized DCT coefficient is added to the last bit are different combinations of the last, run and level of the quantized DCT coefficient. And a codeword table stored in correspondence with
Encoding means for selecting a code word corresponding to the combination of the last, run, and level of the input DCT coefficient after quantization from the code word table and outputting the selected code word as encoded data. Variable length coding device.   4. The variable according to claim 3, wherein the plurality of codewords stored in the encoding table are configured such that a delimiter of the codeword can be identified by a predetermined number of "1" or "0". Long encoding device.

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