Perhaps the most significant breakthrough in the field of optical data storage technology has been the shift from analog to digital recording. Many benefits such as error correction and data compression result from the introduction of digital schemes. It must be recognized, however, that by introducing the techniques of digital recording, delicate features in the recorded patterns of information on the media have been lost. To eliminate the gap between digital and analog schemes and to increase recording capacity, various multi-level recording methods have been investigated.
The tradition in digital storage is to try to record “perfect marks” on the storage medium (disk, tape, etc.). An ideal mark is one that is fully saturated in one or the other state (e.g., amorphous or crystalline), fills the width of the track uniformly, has smooth boundaries, and has the correct length and center position. Writing ideal (or near-ideal) marks entails substantial efforts on the part of the drive/media designer. As higher recording density is required, obtaining ideal or nearly ideal recording marks becomes more difficult. What is needed is an alternative approach to optical recording that will enable higher recording densities to be achieved.