In addition to read-only recording media in the form of discs, recording media are also known which can be written to once or a plurality of times. It is desirable to use a single, universal optical scanner both for reading such media and for writing to such media as can be written to, such as Write Once discs, which are called WO discs, as well as magneto-optical discs, which are called MO discs. Since, in general, the Kerr rotation angle of an MO disc is very small, so-called enhancement is used for processing, with the consequence that the detectors for the tracking error signal also receive polarized light. This leads to birefringence effects which are thermally induced on the WO disc during the writing process producing a noise level on the tracking error signal.
In general, this would have no disturbing effect. According to one industry standard relating to recording media, the tracking error signal is, however, modulated with a small additional oscillation which represents a signal which is called the ATIP signal, or Absolute Time In Pregroove signal. This ATIP signal, which is intended to be used to derive the position of the scanner and the disc speed, is interfered with by the noise level. The noise level makes it impossible to determine the position of the optical scanner during the writing process. The interference can even become so great that it is no longer possible to control the speed of the disc. However, both information items are very important for making it possible to use normal CD players to replay discs recorded in such a way. In addition, the industry standard, which is described in the Orange Book, specifies position monitoring.
One already known solution provides for use of special WO-optical scanners, which operate with circular polarized light. However, an additional scanner is then required for MO. It is also possible to use MO scanners which manage without enhancement. However, it is very difficult to adjust such scanners, so that their use is not feasible for a mass market.