EFM coding: Squeezing the last bits

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 fb = 10 , will be given to illustrate the theory.