LOOK-AHEAD SEEK CORRECTION IN HIGH-PERFORMANCE CD-ROM DRIVES

As the performance of the CD-ROM drives continues to increase, special attention should be paid to any element in the system that disturbs t he desired specifications. When very low access times are considered, it will be mandatory to reach the target track within only one seek ac tion, i.e. without any consequent retry or correction seek. Many CD-RO M drives count the tracks crossed during seeking and, by comparing thi s count with the pre-calculated seek length, the exact target location can be detected. This paper considers the error that affects the trac k-counting mechanism due to the rotating disc spiral. It will be shown that, for a given seek length, the number of crossed tracks differs b etween outside-and inside-oriented seeks. The involved equations will be derived and an algorithm will be proposed for determining the corre ct number of tracks to be crossed during a seek action. The proposed a lgorithm, which is optimized for microprocessor implementation, relies on two look-up tables and a learning scheme that allows look-ahead se ek correction for both spiral rotation and system tolerances.