Media noise, nonlinear distortions, and thermal stability in high density recording

Abstract

Low media noise and high thermal stability are required to achieve high density magnetic recording. Reduction of magnetic grain size is required in order to minimize transition jitter. However, very small grains become unstable and spontaneously reverse their magnetization due to thermal fluctuations. Future high density (>20 Gbit/in 2 ) recording systems are likely to operate in a media noise dominated environment and could exhibit a certain amount of signal decay. Recording performance degradation caused by media noise and thermal decay is evaluated. Density-dependent amplitude decay is caused by the presence of demagnetization fields and results in increasing level of nonlinear distortions and overwrite degradation. Magnetization decay is accompanied by changes in media noise in an unstable media as determined by the relative contributions of the transition and particulate noises. Experimental measurements suggest, that channel bit error rate degradation in an unstable medium is determined by signal and noise evolution, as well as by pattern-dependent distortions.

Adaptive Decomposition of Noise Sources in Digital Recording Systems With Media Noise

“…In magnetic recording applications, position jitter is considered to be the dominant media noise effect [25], yet also other types of media noise arise at high densities, like nonlinear transition shifts and width variations [26]. Furthermore, at the output of the channel, electronics noise is added to the signal.…”

“…Based on the adaptation rules (25), the variances are given by the following expressions: (26) where . To simplify these expressions, the expectation values of different signals need to calculated: (27) The latter equation is derived by applying (4) and the formula of Isserlis [33].…”

“…To simplify these expressions, the expectation values of different signals need to calculated: (27) The latter equation is derived by applying (4) and the formula of Isserlis [33]. By applying (27), the expressions given by (26) can be simplified: (28) In normal conditions, the adaptation constants are chosen such that and . Based on these considerations, the expressions can be approximated by (29) ACKNOWLEDGMENT This work was supported by the European Union under the "TwoDOS" IST Project IST-2001-34168.…”

Joint design of optimum partial response target and equalizer for recording channels with jitter noise

“…Since the recording channel can be assumed to be band-limited at high densities, we use bit-rate sampled readback signal in this paper. The resulting readback signal samples with only electronics noise can be written as (1) where is the recorded bit sequence with , is the corresponding transition sequence with and , and is white Gaussian electronics noise with variance .…”

“…At high recording densities, however, the detection performance of PRML detectors suffer seriously due to media noise [1], which is the dominant disturbance in high-density magnetic recording channels. Unlike the most commonly used noise model of additive white Gaussian and data-independent electronics noise (AWGN), the media noise is correlated, data-dependent, nonstationary, and nonadditive in nature.…”