GB1359427A - Data processing systems - Google Patents

Abstract

1359427 Variable length encoders INTERNATIONAL BUSINESS MACHINES CORP 6 Oct 1972 [29 Dec 1971] 46159/72 Heading G4A An input-character (byte) is translated into a minimal redundancy code by sequentially scanning a field of bits representing inner vertices and sinks of a binary tree to generate in a register a path vector to the particular sink corresponding to the input character. In the embodiment described, each input byte to be encoded is assigned a particular value of K from 0 to 5 and corresponds to one of the sinks C, E, F, H, J, K in Fig. 1A. The field representing the tree T=11010010100, where the zeros represent the sinks, is stored in a register. With a 1 initially stored in the right-hand stage of the output shift register P, the bits of field T are scanned sequentially from left to right and P is shifted left (steps 13, 14, 18, 19, 28) until a 0 bit is found (step 14). If K=0 when the first 0 is found this corresponds to sink C and the steps at the left-hand side of Fig. 2 cause shift left of register P until the initial 1 bit is lost and then a final two shifts under control of a counter (step 18) so that the output code 00 is obtained corresponding to the two left-hand branches taken to reach sink C. With higher values of K additional steps 16, 21-23 and 27 are introduced so that path vectors 010 and 011 are generated if the input byte corresponds to sink E and sink F, respectively. If K is still non-zero when sink F is tested, register P is shifted right (step 22) to retrace the path back through inner vertices D (=01) and B (=0) and P is then changed to 1 (step 27) corresponding to vertex G and the tracing process is continued for the right-hand side of the tree until the appropriate sink is reached.