3.2. Conventional Group-based VLC Codec System
3.2.3. Group-searching Scheme
3.2.3.
3.2.3.
3.2.3. Group-searching Scheme
An economical group-searching scheme with high operation rate and low complexity determines the performance of a group-based VLC decoder because the decoding procedure is performed after the group information is obtained. We use inter-group symbol memory mapping and Pseudo-Constant-Length-Code (PCLC) in order to achieve such a group-searching scheme. If all codeword lengths are the same , the numerical properties of codewords in a group can be applied to the whole coding table. We apply a procedure, namely PCLC procedure, to equalize codeword lengths by adding redundant binary digits, 00…0, behind VLC codewords. Therefore, PCLC codewords, which have the same length as the longest VLC codeword, can be treated as binary numbers, PCLC_codenums.
Table 3-1 : Example of inter-group symbol memory mapping
Group Valid codelength PCLC_mincode base address
G0 1 8 00100100 0
G1 1 6 00110000 4
G2 1 4 01100000 5
Table 3-2 : Group information for Table 3-1
It is easily to distinguish PCLC codewords and PCLC_codenums from each other because the VLC code is a prefix code. As a result, a PCLC table is established with PCLC_codenums placed in ascending order, i.e. codenum0 < codenum1 < … <
codenumn. This results in ascending PCLC_mincodes as well, i.e. mincode0 <
mincode1 < … < mincoden. Based on the PCLC table, the base addresses have to be assigned in PCLC_mincode order, i.e. base_addr0 < base_addr1 < … < base_addrn, for inter-group symbol memory mapping. An example of the PCLC table and its intra/inter-group symbol memory mapping is shown in Table 3-1, and the group information of this PCLC table is given in Table 3-2, where the valid bit indicates whether the group information is used. We can see in Table 3-1 that G2 is inserted in the middle of G1. This placement is specialized for decoding to save memory space of symbol memory.
According to PCLC tables and symbol memory maps, the conventional decoding group searching scheme is realized by applying numerical properties to bitstream and symbol addresses. Similar to PCLC codewords, a decoded bitstream that has the same length as the PCLC codewords is treated as a binary number, bitstream_num. Because the bitstream is a sequence of concatenated codewords, such as codewordi – codewordj – etc, a relation between the bitstream and the PCLC table can be expressed by PCLC_codenumi ≦ bitstream_num < numerical comparisons. The decoded codeword belongs to group Gx when the hit condition, PCLC_mincodex ≦ bitstream_num < PCLC_mincodex+1, is encountered. Let’s see the process of decoding one symbol from bitstream “001111010110…”
Figure 3-6 : Process of decoding a symbol
Figure 3-7 : Process of encoding a symbol address
According to the relation between PCLC tables and the symbol address, the conventional encoding group searching scheme is realized by applying numerical properties to codewords and symbol addresses. Based on the encoded symbol, the relative symbol address can be fetched. A relation between the symbol address and the PCLC table can be expressed by base_addri ≦ symbol address < numerical comparisons. The decoded codeword belongs to group Gy when the hit condition, base_addry ≦ bitstream_num < base_addry+1, is encountered. Let’s see the process
of encoding one symbol from the symbol address “19 (5’b10011)”
Figure 3-8 : Block diagram of conventional group-based VLC decoder architecture
The conventional VLC codec system is designed for MPEG applications with coding tables up to 256-entry 12-bit symbols and 16-bit codewords. This system performs concurrent encoding and decoding procedures by accessing the same group information and achieves table programmability by loading data into on-chip memories. To complete the VLC codec processes of MPEG videos, this design includes the operations of sign bits and escaped run-levels (escRL) following VLC codewords. By the efficient symbol conversion, the memory requirement is reduced to (25x8 + 28x8 + 28x12 + 32x29) bits for a CBS-LUT, a symbol address memory, a symbol memory, and 32-entry group-information. Block diagram of the conventional VLC codec system is shown in Figure 3-8. It mainly consists of the following components.
The group-based VLC encoder/decoder is composed of group detectors and combinational logic circuits to realize the VLC codec processes.
The input FIFO stores the input bitstream. According to previous decoded
results, the Dec_bitstream selector transmits codewords bitstream to the VLC decoder.
Besides, this selector detects sign bits and escRLs when VLC codewords are decoded.
The Enc_bitstream concatenater adds sign bits or escRL’s behind VLC codewords and concatenates encoded results into a single bitstream. Then, every 32 bits of the encoded bitstream in the concatenater is shifted into the Output FIFO.
The special code detector recognizes special codes, such as escape and EOB, by checking decoded symbol addresses instead of decoded symbols. Without waiting for symbol fetching, this detector can determine the length of the additional bits following a VLC codeword. Hence, the next codeword bitstream can be found by the Dec_bitstream selector immediately and the decoding throughput can be increased.
The Enc_en and Dec_en Ctrls determine the operations of the VLC_encoder and decoder according to the condition of input data and FIFOs.
Both symbol address and symbol memories are the on-chip memory modules for storing symbol information.
The symbol converter performs symbol conversion and detects escaped RLP’s and EOB symbols. On the other hand, the symbol recoverer finds correct runs and signed levels based on decoded results.