Using pit-depth modulation to increase capacity and data transfer rate in optical discs

Pit depth modulation (PDM), a technique in which an individual pit on an optical disc can take on one of multiple available depths, offers considerable improvement to disc capacity. We have used commercially-available mastering equipment and standard polycarbonate injection-molding techniques to manufacture compact discs of standard pitch but with individually varying pit depths. The pits have identical lengths and are written end-to-end to form a continuous groove whose depth varies over an /spl sim//spl lambda//4 range. These PDM discs can be read with standard optical reader heads using either push-pull or 3-beam tracking. We present experimental results that demonstrate our ability to manufacture multi-level pits of 0.6 /spl mu/m length, undo significant intersymbol interference, and recover the original data signal after readout by a conventional CD reader head. Additionally, results indicate that PDM continues satisfactory operation when subjected to perturbations such as defocus and disc tilt as allowed by CD specifications. Work is underway to apply PDM to optical discs with pits of DVD dimensions.