| 摘要 |
A demodulator for digital-versatile disk (DVD) optical disks converts 16-bit codewords stored on the disk into 8-bit symbols or user bytes that are sent to the host after error correction. Rather than use the modulation tables in the DVD specification in reverse, the entries in the modulation table are sorted and combined. The four states stored in the DVD modulation table are reduced to two states or conditions. All entries from states 1 and 4 are sorted into unique tables that have unique mappings of codewords to symbols. Since the unique mappings are not sequence or state dependent, no state information is stored in the unique tables. Entries from states 2 and 3 are sorted into duplicates tables that have duplicate mappings, where a codeword can map to two different symbols, depending on the state sequence. One of the two symbols is chosen based on bits in the following codeword, which is the next state. The next state is stored with each entry in the duplicates tables, while all current state information is deleted from all tables. The main modulation table is reduced from 1024 entries to 541 in the main unique and duplicates demodulation tables, while a substitute modulation table is reduced from 348 to 209 entries in the substitute unique and duplicates tables. Table size is reduced by 45%.
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