Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-12-09
2001-07-24
Edun, Muhammad (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044350, C369S044410, C369S060010
Reexamination Certificate
active
06266303
ABSTRACT:
The present invention relates to a device for reading from and/or writing to optical recording media according to the preamble of claim
1
.
A device of this type is disclosed in U.S. Pat. No. 4,497,048. In this known device, there is provision of a photodetector which is divided into four areas. Tracking information is obtained by combining the signals from two areas in each case to form a summation signal and by ascertaining a phase difference between the first and the second summation signal such that it is not necessary to acquire the amplitude of the reproduced signal. This device functions satisfactorily when the optical recording medium to be read out has a well-defined and constant geometry of the information-carrying layer. In particular, it is necessary that the depressions in the information-carrying layer which serve for information storage, the so-called pits, have a constant depth in relation to the wavelength of the light used for the read-out operation.
What is disadvantageous with the known device is the fact that only slight variations in the geometry of the pits are tolerated, and that larger deviations lead to errors in the formation of the tracking information. The result of this is that high error rates occur during the read-out of the optical recording medium or, in the extreme case, such a read-out becomes completely impossible.
The object of the present invention is to improve the known device to the extent that it can be used without any problems even for optical recording media having a different pit depth or pit geometry.
According to the invention, this is achieved by virtue of the fact that a delay element is assigned to at least one of the areas of the photodetector. This has the advantage that the areas of the photodetector can be temporally adjusted relative to one another. Since the photodetector receives an interference pattern which is caused by the pits arranged on the optical recording medium and may vary depending on the disk geometry and the track geometry (pit depth, etc.), on the type of disk and on manufacturing tolerances and also on the ageing of the laser scanner, the temporal relationship between the detector signals also varies. The temporal adjustment of the detector signals relative to one another which is possible according to the invention enables variations of this type to be compensated for. The temporal adjustment can in this case be carried out by means of a single delay element assigned to an area of the photodetector. However, significantly better adjustment is possible if a delay element is assigned to a plurality, or each, of the areas. The greatest flexibility is made possible if a delay element is assigned to each of the areas. If a delay element is assigned to all but one of the areas, then all of the areas provided with a delay element can be adjusted to that area without a delay element. It is also possible to combine the areas into groups, to assign a delay element to each group and to adjust the groups relative to one another. If an area consists of a plurality of individual areas, then a delay element can be assigned to each of these individual areas.
The delay elements can be set externally, for example during production of the device or when a device service is being carried out.
The invention provides for the presence of control means for setting the delay time of the delay element or elements. This has the advantage that the delay elements can be adjusted at any time. For example, adjustment can be carried out after each insertion of a new optical recording medium, at specific time intervals, for example during brief interruptions in the playback or writing operation of an optical recording medium, or at intervals which are suitably defined in a different way. One adjustment possibility consists in setting fixed delay times which are matched to the type of optical recording medium. The type of recording medium is in this case defined by the pit depth or the pit geometry, the track width or similar characteristic properties. What may be involved here are the known audio CD or new optical recording media having a high recording density (so-called DVD). It is likewise possible to vary the delay times in a type of “try-and-error method” with regard to the initial settings and to retain the new value when the tracking behaviour becomes better. If the tracking behaviour becomes worse, the delay times are reset to the preceding setting.
Provision is furthermore made for providing a phase detection means for detecting the phase angle of the signals emitted by the areas of the photodetector. This has the advantage that the adjustment is possible in an optimum manner since the exact phase angle is determined and, consequently, the delay times of the delay elements can be set exactly.
A further refinement of the invention makes provision of a circuit arrangement for applying a combination of the output signals of the areas to a phase angle detector. This has the advantage that the phase angles of the individual signals are evaluated by a single phase angle detector in that the signals of the individual areas of the photodetector are passed successively to the phase angle detector via the circuit arrangement. The circuit arrangement, for example a multiplexer, is advantageously installed in the device, with the result that the phase angle can be determined internally. Alternatively, this is advantageous for external determination of the phase angle since no unnecessarily high contact-making outlay is required in this case.
The circuit arrangement is advantageously switched by a control means, for example a microcontroller. According to the invention, it is likewise possible for switching to be effected according to a specific timing cycle or a clock triggered in a different way.
An advantageous development of the invention consists in the fact that the phase angle detector is the phase detector of the device. This has the advantage that an additional component is not necessary, and that functional groups of the device which are present in any case are optimally utilized.
In a development of the invention, provision is made for using a microcontroller which controls both the circuit arrangement and the setting of the delay time of the delay elements. This has the advantage that the control is carried out by means of a component which can be integrated in a cost-effective manner and can be adapted flexibly to changed boundary conditions. A logic circuit can also advantageously be used instead of a microcontroller.
An advantageous development of the invention consists in arranging a low-pass filter at the output of the phase detector. This has the advantage that an averaged DC signal is emitted in this way, thereby enabling automatic setting of the delay time even if the light beam scanning the optical recording medium is not following a track. The light pulses impinging on the detector do not in this case have to originate from a data. track; they may equally well be the signals which occur during random crossing of the tracks. In this way, rapid setting of the delay times is possible before the actual playback or recording operation begins. Since the phase detector is in any case not required for tracking during this time, a changeover between the functions of tracking and phase adjustment of the delay elements is also unnecessary.
The adjustment of delay elements of the device according to the invention which are able to be set is advantageously carried out according to the method described in claim
8
. This has the advantage that phase differences caused by component properties are compensated for in a simple and cost-effective manner. After the method has been carried out, the device is provided with optimally adapted settings which are retained until, if appropriate, renewed adjustment is necessary or desirable. Not only can the method described be used during production for the purpose of optimum setting of the device, but it can also be employed for the purpose of adjustment during operation of the said device.
Deutsche Thomson-Brandt GmbH
Edun Muhammad
Kiel Paul P.
Tripoli Joseph S.
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