Method and apparatus for encoding data with variable block lengths

Abstract

An encoding system uses a modified 8/9 rate modulation code to encode 8-bit data symbols into 9-bit cells in a conventional manner in accordance with the modified code and 9-bit ECC symbols into 10-bit cells by (i) encoding 8 bits of the symbol into a 9-bit cell in accordance with the modified code, and (ii) inserting into the 9-bit cell the remaining, that is, the non-encoded, bit of the ECC symbol. The system reproduces the 8-bit data symbols by decoding the 9-bit cells in a conventional manner in accordance with the modified code, and the 9-bit ECC symbols by (i) removing from the associated 10-bit cell the bit inserted during encoding, (ii) decoding the remaining 9 bits to reproduce 8 bits of the symbol, and (iii) inserting into the 8 bits the bit that was earlier removed. In an exemplary embodiment, the 8 least significant bits of the ECC symbol are encoded using the modified 8/9 rate code. The 9 bits produced by the code are used essentially as the first 'c' bits and last '10-c' bits of a 10-bit cell. The most significant bit of the ECC symbol is included in the cell as the c+1.sup.st bit. The mapping of 8 bits to 9-bit cells is such that the inclusion of this c+1.sup.st bit does not violate the code's run length limitations, either within the cell or within a modulation code word, which is a concatenation of the cells. The system can similarly encode, using a modified n/m rate code, n-bit and (n+i)-bit symbols, where (n+i)<m, to produce, respectively, m-bit cells and (m+i)-bit cells.