Optical disc having focus offset area

Device for scanning a track on a record carrier (11) via a beam of radiation (24), the track comprising marks on a recording layer, the beam entering through an entrance face of the record carrier and constituting a scanning spot having an effective diameter on the track, the marks having lengths corresponding to an integer number of channel bit lengths T and the shortest marks having a length of a predefined minimum number d of channel bit lengths T for being detectable via the scanning spot having said effective diameter, the recording layer comprising a pregroove for indicating the track, the pregroove exhibiting a wobble constituted by displacements of the pregroove in a direction transverse to the longitudinal direction of the track, and the pregroove comprising a pregroove modulation of the depth and/or width of pregroove areas for constituting a carrier pattern containing long marks, the long marks having lengths of at least two times the predefined minimum number d of channel bit lengths T for being substantially longer than the effective diameter of the scanning spot, and the carrier pattern constituting a focus area at a predefined location on the recording layer,

• the device comprising
• a head (22) for providing the beam,
• focus servo means (25) for focusing the beam on the track for constituting said scanning spot,
• a front-end unit (31) for generating a scanning signal (33) for detecting marks in the track, and
• a focus adjustment unit (32) for locating the focus area and for adjusting the focus servo means in dependence on an amplitude of the scanning signal due to the carrier pattern during scanning the focus area.