Patent Application
20100074542
This application claims the priority of Korean Patent Application No. 2008-89481 filed on Sep. 10, 2008, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an apparatus for decoding a context adaptive variable length code of the moving picture compression standard, H.264, and more particularly, to an apparatus for decoding a minimum memory access-based context adaptive variable length code (CAVLC), and a table search method for decoding a context adaptive variable length code.
2. Description of the Related Art
Moving picture compression standard, H.264, is a new standard for moving picture compression that has a narrower scope than MPEG-4 Visual and is designed primarily to support transporting rectangular video frames of efficient and robust coding. The H.264 compression standard has been estimated as one of the next-generation moving picture compression technologies all over the world since it is possible for the H.264 compression standard to enable significantly better compression and improved support for reliable transmission. With the incorporation of the H.264 compression standard with next-generation services such as satellite digital multimedia broadcasting (DMB) service, it is particularly expected that the H.264 compression standard is widely used in the field of its applications such as high-quality image compressions, image transfers on internet or cable modem, digital data broadcasting, next-generation mobile phone, etc.
H.264 encoding systems are mainly divided into a context adaptive variable length coding (hereinafter, referred to as ‘CAVLC’) system, and a context-based adaptive binary arithmetic coding (hereinafter, referred to as ‘CABAC’) system. In these years, the CAVLC system has been used in the H.264 baseline profile, and both of the CAVLC and CABAC systems have been used in the main profile.
A CAVLC decoding process is carried out dividedly in 3 steps: one step of calculating a ‘TrailingOnes’ value and a ‘TotalCoefficient’ value for a 4×4 block, another step of calculating a ‘Total_Zero’ value and the other step of computing a ‘Run-Before’ value to restore 4×4 block data that are present before the encoding process. This decoding process is basically based on the table search. That is to say, the code values of ‘TrailingOnes,’ ‘TotalCoefficient,’ ‘Total_Zero’ and ‘Run_Before’ are stored in the form of a variable length code (hereinafter, referred to as ‘VLC’) table as defined in the H.264 compression standard. In each step of the decoding process, a decoder reads bits that are encoded as much as length of a code word stored in the VLC table, and then reads the code values that have been stored in a pair with the length of the code word in the VLC Table to compare the read code values with the previously read, encoded bit value, followed by repeating these steps until the decoder finds a code word whose code value is exactly equivalent to the length of the code word.
In general, the CAVLC table search method includes: storing lengths and code values of bit strings of a CAVLC table in each array, and when the arrays for the lengths and code values of the bit strings are completely formed, determining a decoding coefficient by referring to lengths and code values, in the array, corresponding respectively to the bit strings included in the CAVLC table, and comparing the length and code value with those of an input bit stream.
However, the conventional decoding methods have problems in that a decoding speed may be slow due to the repeated memory accesses since the lengths and code values stored in the arrays are repeatedly referred until the decoder finds a bit string that is equivalent to the input bit stream.
The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide an apparatus and table search method for decoding an adaptive variable length code of new first decoding information (TrailingOnes (T1s)) and second decoding information (TotalCoefficient (Tc)), which may minimize memory accesses to rapidly decode an adaptive variable length code.
Also, it is another object of the present invention to provide an apparatus and table search method for decoding an adaptive variable length code capable of decreasing a table size of the CAVLC table and reducing memory accesses to 2 cycles of the memory accesses by reconstructing the CAVLC table of the first decoding information (TrailingOnes (T1s)) and the second decoding information (TotalCoefficient (Tc)) according to the number of leading zero and the remaining code values except for the bit strings up to the leading zero.
According to an aspect of the present invention, there is provided a table search method for decoding a context adaptive variable length code. Here, the table search method includes: reconstructing a context adaptive variable length code table of first decoding information (TrailingOnes) and second decoding information (TotalCoefficient) into 2-step tables and storing the reconstructed 2-step tables in advance; and performing a table search to decode the first decoding information and the second decoding information, by using the information stored in the 2-step tables, depending on whether the remaining bits except for the number of leading zero are present in the inputted bit stream.
According to another aspect of the present invention, there is provided an apparatus for decoding a context adaptive variable length code including a controller reconstructing a context adaptive variable length code table of first decoding information (TrailingOnes) and second decoding information (TotalCoefficient) into 2-step tables; a shifter activated by the controller to perform a table search to decode the first decoding information and the second decoding information, by using the information stored in the 2-step tables, depending on whether the remaining bits except for the number of leading zero are present in the inputted bit stream; and a memory storing the context adaptive variable length code table and the reconstructed 2-step tables.
The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
Hereinafter, exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. For the exemplary embodiments of the present invention, detailed descriptions of known functions and constructions that are related to the present invention are omitted for clarity when they are proven to make the gist of the present invention unnecessarily confusing.
In accordance with one exemplary embodiment of the present invention, a method for decoding a first decoding information (TrailingOnes (T1s)) and a second decoding information (TotalCoefficient (Tc)) in each of luma and chroma is described in detail, as follows. First of all, examples of a TrailingOnes and TotalCoefficient CAVLC table of the H.264 standard is described in brief for convenience’ sake.
The TrailingOnes and TotalCoefficient CAVLC table of the H.264 standard may be, for example, represented as in the following Table 1, and bit strings of luma (0≦nC<2, 2nC<4, 4≦nC<8) and chroma (nC==−1) may be set to correspond to TrailingOnes and TotalCoefficient values (code values). Here, the TrailingOnes and TotalCoefficient CAVLC table, which is reconstructed when the requirement of nC==−1 is satisfied, is for example represented as in the following Table 2.
Lengths and code values of the bit strings included in the CAVLC table may be stored as in the matrixes of ‘lentab[4] [17]’ and ‘codtab[4] [17]’ of
Then, the decoding apparatus according to one exemplary embodiment of the present invention and the method for decoding a context adaptive variable length code using the same are described in more detail. Hereinafter, only a method for decoding TrailingOnes and TotalCoefficient values is described in the case of nC==−1 table (i.e. chroma) in accordance with one exemplary embodiment of the present invention. However, the TrailingOnes and TotalCoefficient values may be decoded in the case of 0≦nC<2, 2≦nC<4, 4≦nC<8 8≦nC table (i.e. luma) in the same manner as in the chroma. Therefore, detailed description of the luma decoding method is omitted for clarity in one exemplary embodiment of the present invention.
First, a configuration of the decoding apparatus for decoding a context adaptive variable length code is described in more detail with reference to the accompanying drawings.
Referring to
The controller 110 activates the shifter 120 to control a table search operation, stores input data required in the shifter 120, and controls the buffer 130 to output a bit stream of the data stored in the buffer 130. Also, the controller 110 reconstructs the table as pre-set in Table 2, depending on the number of leading zero, and then sets 2-step tables, that is, an index table (T1_Tc_chroma_index-table) and an address table (T1_Tc_chroma_vlc_table) in advance before a decoding process. These 2-step tables may be set by a program pre-set in the controller 110, but be set by allowing a user to directly set the 2-step tables and store the 2-step tables in a memory 140. Hereinafter, the setting the 2-step tables is operated in the controller 110.
The shifter 120 counts the number of 0, that is, the number of leading zero (Count Leading Zero: CLZ) until the shifter 120 meets the first code value of ‘1’ in the bit stream of the inputted data, transmits the number of the counted leading zero (0 bit) and data after the shifter 120 meets the first code value ‘1’, makes access to the memory 140 to perform 2 cycles of table searches, and decodes TrailingOnes and TotalCoefficient values by the 2 cycles of table searches.
The memory 140 stores the TrailingOnes and TotalCoefficient CAVLC table and the reconstructed 2-step tables (index table and address table).
The index table reconstructed in advance and stored by the controller 110 is shown as in
The index value represents the number of leading zero, the Mapping_Flag is a flag that is composed of 1 bit to decode the leftmost description composed of 8 bits. The description includes TrailingOnes and TotalCoefficient values of a corresponding bit stream so as to complete the decoding of the corresponding bit stream by only one access to the memory 140. Here, the number of leading zero represents the continuous number of ‘0’ bit from the starting ‘0’ bit of the input bit stream. When the input bit stream is ‘0001 11,’ a CLZ value is 3, and when the input bit stream is ‘0000 000,’ a CLZ value is 7.
In order to construct the index table, the controller 110 sets an index according to the number of leading zero in the bit stream using the table as listed in Table 2. For example, when the input bit stream is ‘0001 11,’ the number of leading zero is allotted to 3, and therefore an index value becomes 3. Then, the controller 110 determines whether the continuous number (‘11’) is present in the bit stream corresponding to the index, and then sets a Mapping_Flag value to 0 when there is a continuous number in the bit stream and sets a Mapping_Flag value to 1 there is no continuous number in the bit stream. Therefore, the controller 110 sets a Mapping_Flag value to 0 when the input bit stream is ‘0001 11.’ Also, the controller 110 sets a description having 2 kinds of encoding information. Here, the set description uses first encoding (8-bit) information, as shown in
In the above-mentioned decoding apparatus, the shifter 120 should search the reconstructed 2-step table (index table and address table) to decode a context adaptive variable length code for decoding new TrailingOnes (T1s) and TotalCoefficient (Tc) for minimizing cycles of memory accesses using the reconstructed index table. Before this table search, the decoding apparatus reconstructs the 2-step tables and stores the reconstructed 2-step tables in the memory 140, as described above.
Then, the table search operation is described in more detail with reference to
Referring to
Next, the shifter 120 determines whether the Mapping_Flag value is 1 by checking a Mapping_Flag value of the index table (Operation 303). As a result, when the Mapping_Flag value is 1, the shifter 120 checks values stored in a description and outputs the stored values into TrailingOnes and TotalCoefficient values (Operation 304). For example, since the number of leading zero is 2 when the inputted bit stream is ‘001,’ the shifter 120 accesses index 2 of the index table, and therefore the Mapping_Flag value corresponding to the index 2 becomes 1. As a result, the shifter 120 outputs values, ‘2|2’, of kinds of the information stored in the first encoding information into the TrailingOnes and TotalCoefficient values, respectively.
On the contrary, when the Mapping_Flag value is 0, the shifter 120 calculates the final address value of the address table by adding the information stored in the description, that is, a code value and information (T1_Tc_vlc_table_index) required for computing an address value (Operation 305). Then, the shifter 120 makes another access to the memory 140 to output the stored values, which correspond to the computed address value in the address table as shown in
As described above, even when there are any of bit streams, the decoding apparatus may decode a corresponding token, that is, TrailingOnes and TotalCoefficient CAVLCs, by performing a table search only by 2 cycles of memory accesses (worst case). As a result, the decoding apparatus may reduce a memory size by 69% or more, compared to the conventional decoding apparatuses.
In accordance with the exemplary embodiment of the present invention, the apparatus and table search method for decoding a context adaptive variable length code may be useful to improve an overall decoding speed, reduce power consumption, and decrease a memory size when compared to the conventional decoding apparatuses since the repeated memory accesses in the conventional table searches may be reduced to 2 cycles of memory accesses by reconstructing 2-step tables to perform a table search.
While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2008-0089481 | Sep 2008 | KR | national |
