Improved Runlength Control When Using Low-Density Parity-Check Codes

In high-density magnetic recording systems, a runlength constraint (typically (0, k)) is imposed to aid then timing recovery system. However, the traditional encoding structure where the runlength limited (RLL) encoder follows the ECC encoder is not a good fit when using LDPC codes. The main difficulty is that the LDPC decoder requires soft information, which is incompatible the typical RLL decoders which output hard decoding decisions. In this paper, we propose a structure so that soft information can be delivered directly to the LDPC decoder while the runlength constraint is satisfied. By integrating the bit-flipping scheme proposed in (Vasic and Pedagani, IEEE Trans.