Read-back from media with a high permeability saturable keeper over-layer is nonlinear. These nonlinearities are most pronounced at bias levels where the media demagnetization fields are comparable to the head bias field. We propose a simple model for read-back from keepered media. In this model, the medium demagnetizing field modulates the size of the region in which the keeper is saturated and thus modulates head efficiency. This effect distorts the flux waveform. The read voltage, as the derivative of this distorted flux waveform, shows timing asymmetry and shape asymmetry. We demonstrate that distortion of the flux waveform can be treated as a memoryless nonlinear operator over a wide range of recording densities. We demonstrate a method for estimating this memoryless operator and briefly examine strategies for linearizing the read channel.
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