EFM CODING - SQUEEZING THE LAST BITS

Runlength-limited (RLL) codes have found widespread usage in optical a nd magnetic recording products. Specifically, the RLL codes EFM and it s successor, EFMPlus, are used in the Compact Disc (CD) and the Digita l Versatile Disc (DVD), respectively. EFMPlus offers a 6% increase in storage capacity with respect to EFM. The work reports on the feasibil ity and limits of EFM-like codes that offer an even larger capacity. T o this end, we will provide an overview of the various limiting factor s, 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 o f 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), wi ll be given to illustrate the theory.