Modeling and signal processing for the nonlinear thin film recording channel

The implementation of high-density partial response channels in disk drives has been hampered by nonlinear interactions between transitions occurring in the write process. The degree of nonlinearity is not characterized by the density relative to the isolated transition pulsewidth, but rather by the transition spacing in relation to the transition width. For this reason the authors ignore the read process entirely and focus on the written magnetization, or write channel output. The origins of the nonlinearities are discussed. A simple generalization of the linear superposition model which is valid at higher densities is introduced. The Euclidean distance of suspected minimum distance paths is plotted versus density to estimate the performance roll-off of the nonlinear channel when maximum likelihood sequence detection is used.>