Adaptive cross-parity (AXP) code for a high-density magnetic tape subsystem

This paper describes an error-correction system, called adaptive cross-parity (AXP) code, for the IBM 3480, a new high-density 18-track tape storage subsystem. Redundancy is applied to two interleaved sets of nine tracks in the same proportion as that in the previous IBM 3420 tape machines. The coding structure, however, is simpler, for it avoids the complex computations of Galois fields. The coding structure is based on a concept of interacting vertical and cross-parity checks, where the cross-parity checks span both sets of tracks and are used in either set in an adaptive manner. As a result, the overall error-correcting capability is substantially improved without increasing the redundancy. Decoding, in which simple parity equations are processed, is designed to progress iteratively. By means of adaptive use of redundancy, the new method corrects up to three known erroneous tracks in any one set of nine tracks and up to four known erroneous tracks in the two sets together. The code also identifies the first unknown erroneous track in each of the two sets, and subsequently identifies the second unknown erroneous track in one of the two sets while providing correction for all these tracks. The result is generalized for a system with any number of tracks divided into a multiple number of unequal sets.