Signal processing for high-density digital magnetic recording

The signal-processing techniques most often used in digital magnetic recording systems are run-length limited (RLL) coding and peak detection (PD). The application of more advanced methods, such as partial-response (PR) signaling and maximum-likelihood sequence detection (MLSD), allows a further increase in the recording densities and reliability presently achieved. After reviewing RLL coding an PD schemes, recording systems using PR signaling and MLSD (PRML) are described. Viterbi decoding and decision-directed algorithms for automatic gain control and timing recovery are discussed for the two types of PR scheme best suited for magnetic recording, class-IV (PR-IV) and extended class-IV (EPR-IV). Results of a performance study on PD and PRML systems which demonstrate the superiority of the PRML approach are presented. For the two PRML systems considered, sensitivity to various parameters and performance in an off-track situation were investigated. The results shown indicate that the two schemes are equally robust and that PR-IV signaling is superior to EPR-IV in off-track situations. A PRML system based on PR-IV signaling is also simpler to realize