Optical disc having focus offset area

摘要

A record carrier is for recording information by writing marks in a track on a recording layer. The shortest mark used for recording the information has a length of a predefined minimum number d of channel bit lengths. The record carrier ( 11 ) has a pregroove that is modulated by a carrier pattern containing long marks ( 18,19 ) that provides a focus area ( 12 ) at a predefined location on the recording layer. The long marks have lengths of at least two times the length of the shortest mark for being substantially longer than the effective diameter of the scanning spot. A scanning device locates the focus area and determines the best focus offset by detecting the maximum read signal amplitude while scanning the carrier pattern.