Loose composite constraint codes and their application in DVD

Constrained coding is used in recording systems to translate an arbitrary s equence of input data to a channel sequence with special properties require d by the physics of the medium, Very often, more than one constraint is imp osed on a recorded sequence; typically, a run-length constraint is combined with a spectral-null constraint. In this paper, we introduce a low-complex ity encoder structure for composite constraints, based on loose multimode c odes. The first channel constraint is imposed strictly, and the second cons traint is imposed in a probabilistic fashion. Relaxing the second constrain t is beneficial because it enables higher code rates and simplifies the enc oder. To control the second constraint, a multimode encoder is used. We rep resent a set of multimode coded sequences by a weighted trellis and propose using a limited trellis search to select optimal output. Using this method , we modify the EFM+ code used in digital versatile disk (DVD), We combine EFM+'s run-length constraint with the first- and second-order spectral-null constraints. The resulting EFM++ code gives more than 10-dB improvement in suppression of low-frequency spectral content in the servo bandwidth over the original EFM+ code with the same complexity,