Nonlinear intersymbol destruction versus linear intersymbol interference on high density disk drives

Abstract
The nonlinear behavior of the hard disk recording channel is described using a simple theoretical model. According to this model, linear superposition applies only when the transition spacing is greater than about 3.5a, where a is the transition width parameter. Two methods to increase density are examined. First, the system variables determining the transition width are identified. Of these the magnetic moment is unique in that its value is not fixed by the level of technology; that is, it can be chosen to optimize channel characteristics. Second, an advanced model is described that abandons linearity in favor of a simple nonlinear channel model. This model applies when the transition spacing is greater than 2.5a. Both of these techniques result in more severe signal roll-off than a hypothetical linear superposition channel.