1. Field of the Invention
The present invention generally relates to signal coding and, more particularly, to multi-dimensional coding for high-density storage media applications.
2. Background of the Invention
Even though already a multi-billion dollar industry, the digital recording industry is expected to expand further in the future as an almost insatiable appetite for more storage continues to grow. This increase is partly fueled by the steady move towards digital systems, as has happened for example in the audio industry with the replacement of the analog Long Play (LP) disk by the digital Compact Disk (CD). Digital disk recording systems include magnetic and optical recording, the latter mainly for read only applications. Whether optical or magnetic, one of the main goals of ongoing research is to increase areal density in bits per unit area.
Modulation codes, for most recording systems, focus on increasing linear density through the reduction of Inter-Symbol Interference (ISI). Further increases in storage density are potentially available by reducing track width and increasing track density. However, this results in undesirable Inter-Track Interference (ITI) and a reduction in signal-to-noise ratio (SNR). Consequently, typical magnetic recording systems have linear-to-track density ratios of only 25 to 1. Head misalignment or side reading (cross talk) that occurs between the read head and adjacent track data causes ITI. This has been acknowledged as an important noise source that can be alleviated by employing sophisticated signal processing techniques whilst reading several adjacent tracks simultaneously with a multi-track head. An additional advantage of reading multiple tracks in parallel can be gained by employing two-dimensional run-length limited (d, k) modulation codes. These have attracted much attention in recent years as a means of increasing storage capacity by relaxing the timing constraint, k, along the tracks. Timing recovery is then achieved in a joint manner from information taken across a number of tracks.
Currently, the storage capacity of media such as, for example, a Hard Disk Drive (HDD) or an optical drive is limited by the state of the art of head, media and write technology. Contiguous media provides the best opportunity to increase storage space using the best cost metrics. One of the vexing problems in media storage is the inability of magnetic media to handle transition flux changes in excess of 500-800 Kbpi due to overwrite issues.
Coding and other methods and apparatus have been proposed to reduce the number of such transitions. However, all of such existing approaches suffer from one or more deficiencies. For example, transition detection is performed by peak detection methods that employ Analog-to-Digital Converters (ADCs), thereby increasing power consumption, which is not ideal for Microdrive and other portable media applications. Further, there is significant room for improvement in the current capacities of 1.3 bits per transition.
Multi head and multi track combinations along with the new perpendicular recording techniques have been used to increase the capacity of storage mediums. These approaches are expensive and generally will suffer from reliability issues due to the increased number of heads as well as the characteristics of the new media used in the perpendicular recording system.
Accordingly, it would be desirable and highly advantageous to have a coding method and apparatus for high-density storage media applications that overcome the above-described deficiencies of the prior art.