Abstract: An optical recording disk comprises a track divided into sectors and having a longitudinal axis. Areas on this track for writing data using a predetermined increment alternate with prewritten marker areas. Each of these comprises at least two patterns having leading and trailing edges. Part of at least one of these patterns is offcenter relative to the track axis and associated with at least one edge of a pair of leading and trailing edges combined with another pair of leading and trailing edges to define along the axis two predetermined distances. Each of these distances is between two consecutive multiples of the data writing increment. The patterns are adapted to be scanned optically in succession by a read spot to detect any track following error, by virtue of interaction of the read spot with at least the offcenter pattern part, and with reference to a timebase synchronization signal resulting from interaction of the read spot with at least two leading edges and at least two trailing edges.

Abstract: A protected optical disk of welded construction comprises a housing the diameter of which is its largest dimension. The housing consists of parts fabricated from a thermoplastic material joined together by at least one circular weld which occupies part of a plane annular joining area perpendicular to the axis of rotation of the disk. An internal chamber is bordered radially by this joining area. An information carrying layer is contained in this chamber. A pair of grooves lies one on either side of the portion of each of the parts included in the weld and there is a circularly disposed sharp edged projection between the grooves of each pair. In this way the grooves and the projection form a weld preform on at least one joining surface of the parts.