| 摘要 |
PURPOSE:To eliminate the effect onto a compression rate when a reference dictionary is increased by selecting a code representing a reference dictionary unit as a variable length code at a shorter length for a reference dictionary unit with high frequency of occurrence than the length for a reference dictionary unit with low frequency of occurrence. CONSTITUTION:A reference dictionary 5 is divided by plural reference dictionary units 1-16 and a code word is represented by a number of reference dictionary units 1-6 and an index 19 representing a registration position in the reference dictionary units 1-16. In this case, number of codes is increased by the code represented by the reference dictionary units 1-16 but a variable length code is adopted for the code representing the reference dictionary units 1-16, and the code of the reference dictionary unit with high frequency of occurrence is represented in shorter bit number and the code of the reference dictionary unit with low frequency of occurrence is represented in longer bit number. Thus, the increase in the length of the code word is suppressed and the compression rate in matching with the increase in the reference dictionary is sufficiently improved. |
