System for providing reversible variable length codes

摘要

A variable length code system which is instantaneously decodable in both forward direction and backward direction is generated either in asymmetrical form or symmetrical form. The code thus generated has high transmission efficiency, and capable of backward reconstruction of video signals even when some bits are lost during transmission. The symbols to be encoded are prepared (1) with occurrence probability, and they are provisionally encoded to a non-reversible variable length code (2). Then, the provisional code is transformed to an asymmetrical reversible code (4) through equivalent transform process (9), reverse symbol tree process (10), and bit supplement process (12) when suffix condition is not satisfied. The provisional code is also transformed to a symmetrical reversible code (3) by assigning a symmetrical code word to each symbol on a symbol tree (FIG. 5). At least one of symmetrical code or a asymmetrical code is selected (5), and is provided as a final resultant code.