1.  Field of the Invention
The present invention relates generally to multi-level coding techniques, and more specifically to a system and method for coding data using an M=6 (2,4) runlength limited code, such code being particularly useful for storing and/or transmitting multi-level data. 
2.  Related Art
Various techniques are currently in use for increasing the recording density on various digital recording mediums such as magnetic tapes and disks and in other similar media.  One technique utilizes an approach known as run-length-limited coding.  This technique requires that each "1" in a coded bit sequence must be separated by a specified number of "0's".  This number of zeros must be at least a minimum quantity, d, to reduce intersymbol interference, and is limited to a maximum quantity k for self clocking purposes.  Such codes are generally referred to as (d,k) run-length-limited (RLL) codes. 
Because conventional recording techniques use saturation recording to store information, binary recording techniques are often used to mark the recording medium.  As a result, conventional (d,k) RLL codes developed to date are developed and optimized to store binary data. 
Significant advances in data storage materials have lead to the development of a storage medium that provides a linear response characteristic.  One such material providing a linear response characteristic is an electron trapping material such as is disclosed in U.S. Pat.  Nos.  4,864,536, 5,007,037, and 5,142,493, all to Lindmayer. 
Using a material which provides a linear response characteristic yields an advantage over saturation-type media in that it adds an analog dimension to the storage capacity of the medium.  Because the response is linear, the linear-response storage materials provide the ability to encode information in two dimensions--amplitude and phase. 
As a result, the storage medium is no longer confined to storing binary or even tri-level data.  Instead, the concept of M-ary, or non-binary, data coding and storage is provided.  The increased symbol set allowed by such encoding provides the opportunity to dramatically increase the data recording density and transfer rate of the storage device.  For example, the potential storage capacity of a single 51/4 inch disk can be extended to several gigabytes if that disk is implemented using electron trapping materials with M-ary (multi-level) data coding.