Off-track bit shift compensation apparatus

Abstract

Many of the data read errors encountered in an optical disk system are due to reading off track. These errors are the result of bit shift, which is the shifting of the data transition relative to the center of the detection timing window. This bit shift is the result of the rounded edges of the data marks on the recording medium. A normal data signal is read off the recording medium while the read apparatus is centeed on the data storage track of the recording medium. If the read beam of the read apparatus moves off track radially to either side of the center of the data storage track, the detected data transitions move toward each other due to the rounded edges of the recorded data on the data storage track. This makes the data mark appear shorter and results in bit shift and increased error rate. The off-track bit shift compensation apparatus compensates for this misalignment and the resultant bit shift by dynamically adjusting the analog detection threshold of the read apparatus. The modification of the detection threshold makes the data bit written on the data storage track appear longer. Thus, if the detection threshold is lowered slightly, the detected data transitions will be shifted back to the zero position associated with the read apparatus being centered on the data storage track of the optical disk. This allows the use of both edges of the recorded mark to contain information, thereby doubling the capacity over systems which use the entire mark as a data transition.