Runlength-limited (RLL) codes have found widespread usage in optical and magnetic recording products. Specifically, the RLL codes EFM and its successor, EFMPlus, are used in the Compact Disc (CD) and the Digital Versatile Disc (DVD), respectively. EFMPlus offers a 6% increase in storage capacity with respect to EFM. The work reports on the feasibility and limits of EFM-like codes that offer an even larger capacity. To this end, we will provide an overview of the various limiting factors, such as runlength constraint, dc-content, and code complexity, and outline their relative effect on the code rate. In the second part of the article we will show how the performance predicted by the tenets of information theory can be realized in practice. A worked example of a code whose rate is 7.5% larger than EFMPlus, namely a rate 256/476, (d =2#k =15) code, showing a 13 dB attenuation at f b =10 ;3 , will be given to illustrate the theory. I.

Introduction The technique of enumerative coding [1] makes it possible to translate source words into codewords and vice versa by invoking an algorithmic procedure rather than performing the translation with a look-up table. The usage of long codewords makes it possible to approach a code rate which is arbitrarily close to Shannon's noiseless capacity of the constrained channel. The risk of extreme error propagation precluded its usage in practical systems. Single channel bit errors may result in error propagation that could corrupt the entire data in the decoded word, and, of course, the longer the codeword the greater the number of data symbols affected. This article will evaluate the effects of error propagation of enumerative coding, where it is assumed that the constrained code is used in the conventional code configuration. It will be shown that when certain measures are taken, the average error propagation can be controlled to a level which is quite acceptable for many

Introduction In channel coding schemes we are usually faced with the problem of translating a given source word into another codeword and vice versa that satisfies some prescribed constraints. In the absence of an algorithmic rule defining the relationship between the source word and codeword, the translation operation will be simple look-up tables. As hardware grows with the number of codewords used, i.e. exponentially with the codeword length, there is a technological limit to the length of the words that can be translated using such a simple look-up table. A preferable alternative technique, called enumerative coding, makes it possible to perform the translation byinvoking an algorithmic procedure [1]. Essentially,enumerative decoding is accomplished by forming the weighted sum of the codeword received. The integer-valued weights used in forming the sum are a function of the channel constraints in force. Encoding is done by a method which is

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