On the low-frequency suppression performance of DC-free runlength-limited modulation codes

Basic trade-offs between the rate of combined DC-free runlength-limited (DCRLL) modulation codes and the amount of suppression of low-frequency components are presented. The main results are obtained by means of a numerical study of dependencies between statistical properties of ideal, “maxentropic” DCRLL sequences. The numerical results are mathematically founded by proving the observed behavior of the Shannon capacity of the DCRLL constraints for asymptotically large values of digital sum variation. Presented characteristics of maxentropic DCRLL sequences comply with the corresponding properties of maxentropic pure DC-free sequences, as previously considered by Justesen (1982) and Immink (1991). Knowledge of the maxentropic bounds enables us to evaluate the performances of implemented DCRLL codes with respect to their low-frequency suppression capability. Among the considered codes are the EFM code as applied in the compact disc system, and the EFMPlus code which has been adopted as the coding format of the DVD system