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Coding Techniques For The Noisy Magnetic Recording Channel: A State-Of-The-Art Report

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Abstract

Coding techniques for improving the reliability of information storage on noisy magnetic recording channels are considered. It is assumed that the Lorentzian channel model applies and that the retrieved signal is perturbed with additive white Gaussian noise. The immunity against additive noise of state-of-the-art codes such as DC-free, runlength-limited, and trellis codes are assessed
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Conference Paper
The saturation recording channel with fixed-amplitude bipolar inputs, a linear Lorentzian read-back transition response, and additive Gaussian noise is modeled. Linear density increase on such a channel can be achieved only by increasing the clock rate of the written data signal. The authors define a measure of coding gain for the recording channel that compares a coded system with an uncoded system, each at a different clock rate, but with the same linear density (or data rate). This measure can be decomposed into the sum of an equalization gain and a fundamental coding gain. It is shown that good equalization gain can be achieved by choosing multidimensional codewords such that their power spectra are `matched' to the channel pulse response spectrum, and they have increased minimum distance at the channel output
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In the construction of modulation codes, a natural selection process occurs which chooses from a set of sequences which satisfy an underlying system constraint to produce a practical code. A method of selection which increases detectability on a magnetic recording channel using maximum likelihood sequence estimation is analyzed. It is shown that significant coding gains are achievable through the judicious selection of code sequences. In other cases, despite moderate costs in terms of decreased code rate, system reliability is improved
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The sound from a Compact Disc system encoded into data bits and modulated into channel bits is sent along the 'transmission channel' consisting of write laser - master disk - user disk - optical pick-up. The maximum information density on the disk is determined by the diameter d of the laser light spot on the disk and the 'number of data bits per light spot'. The effect of making d smaller is to greatly reduce the manufacturing tolerances for the player and the disk. The compromise adopted is d approximately equals 1 mu m, giving very small tolerances for objective disk tilt, disk thickness and defocusing.
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Maximum-likelihood sequence estimation of binary coded and uncoded information, stored on an optical disc, corrupted with additive Gaussian noise is considered. We assume the presence of inter-symbol interference and channel/receiver mismatch. The performance of the maximum-likelihood detection of runlength-limited sequences is compared against both uncoded information and information encoded by Hamming-distance-increasing convolutional codes.
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Thesis
Channel codes, sometimes called transmission or line codes, are applied in storage systems such as magnetic tape or disc and optical discs. Applications are also found in transmission systems over fiber or metallic cable. A channel code converts the digital source information to a form suitable for a specific transmission medium. For example DC-free codes are designed in such a way that the encoded signal has suppressed frequency components in the region around zero frequency. These codes are for example applied in transmission systems having insufficient response in the low-frequency range. Another requirement imposed on a channel code originates from the fact that the maximum distance between transitions in the encoded signal, the maximum 'run length', should be limited to enable a simple system clock recovery in the receiver. This thesis deals with systematic methods of designing DC-free and run-length-limited codes. Procedures are given for a simple enumerative encoding and decoding of the codewords. Also described are several properties of channel codes such as spectral and run length distributions. Criteria derived from information theory are used to compare the channel codes.
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