A Comparison of the Low-Frequency Properties of DC-free Codes Generated by Look-Ahead and Guided Scrambling Techniques

Suppression of the low-frequency (lf) components of the modulated data stream is a system requirement in optical recording which facilitates the usage of servo systems for reading the optical disc. Insufficient suppression of the lf components of the coded spectrum will lead to improper functioning of the servo systems, and/or acoustical noise generated by the servo systems, and/or additional power dissipation in the servo amplifiers. Low-frequency components are avoided in state of the art systems such as CD and DVD by using codes, called dc-free codes, which suppress the energy in the lf range. Clearly, the more lf suppression, the more overhead in coding rate must be spent, and a sound trade-off has to be sought between proper lf suppression and coding efficiency. It has been found that lf suppression can also be improved by coding methods, which use a look-ahead (LA) algorithm that looks ahead p codewords, and evaluates, based on a suitable metric, the full search tree of 2 possible choices of codewords in the tree. Recently, coding schemes, which use Guided Scrambling (GS) have been proposed for suppressing the lf content. It has been shown that GS schemes can achieve a lf suppression which is very close to the theoretical maximum. In this paper, we will report on a performance comparison of coding schemes based on Look-Ahead and Guided Scrambling techniques.