Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-12-08
2001-02-27
Huber, Paul W. (Department: 2753)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044350, C369S110040
Reexamination Certificate
active
06195318
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for the correction of a tracking error signal during optical scanning of a recording medium in the form of a disc.
BACKGROUND OF THE INVENTION
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.
SUMMARY OF THE INVENTION
The invention is based on the object of specifying a method for the correction of a tracking error signal during optical scanning of a recording medium in the form of a disc, in order to allow evaluation of the tracking error signal despite a superimposed noise element.
In the case of a method according to the precharacterizing clause of claim
1
, this object is achieved by the features specified in the characterizing part.
The invention is based on the following knowledge and idea: Birefringence effects do not normally change the intensity of the reflected light, but only the polarization plane. Using a vectorial representation of the polarization components and light intensities, it was found that the noise element can be ascribed to just one polarization component. By rewriting the formulae which describe the vectorial polarization components and the light intensities, it was possible to calculate the noise element in order to obtain, as the result, exclusively the value of the desired ATIP signal. Thus, the noise which is caused by a polarization shift is compensated for, for a continuously processing solution, by linking the electrical output signals of the scanner arrangement.
According to a development, both amplitude levels of the electrical output signals are evolved during the process of linking the electrical output signals of the scanner arrangement. Both the sum and the difference are then formed from both evolved output signals, after which the results of the sum and difference formation are each squared and, finally, the results of the squaring are added with different weighting. These measures correspond to the computation steps in order to recover the original ATIP signal again from the ATIP signal which is subject to the noise component.
The weighting expediently corresponds to the factor which was used to attenuate that polarization component having the greater magnitude, for the purpose of reinforcing the Kerr effect relative to the polarization component having the smaller magnitude. Mathematically exact compensation for the noise element is then possible.
In the case of a practical implementation of the invention, the magnitudes of the intensities are first normalized, and linearized evolution and squaring functions are then carried out using the normalized values. In this way, it is possible to simplify the computation operations, in order to reduce the computation effort for the data rate to be processed.
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patent: 3923329 A1 (1991-01-01), None
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German Search Report.
Rapp Stefan
Schroeder Heinz-Jörg
Zucker Friedhelm
Deutsche Thomson-Brandt GmbH
Huber Paul W.
Kiel Paul P.
Tripoli Joseph S.
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