Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2000-06-02
2004-03-16
Dinh, Tan (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
Reexamination Certificate
active
06707772
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the directing of an incident optical beam to a track of information on a dynamic medium, and, more particularly, to the control and determination of the positioning error of the incident beam with respect to the track. The invention applies advantageously but not limitingly to digital discs, especially compact discs, e.g. Compact Disc Read Only Memory (CDROM), and most particularly to multifunction digital discs such as the Digital Versatile Disc (DVD) storing image data in a compressed manner, for example.
BACKGROUND OF THE INVENTION
A digital disc includes a single spiral track whose relief is representative of the binary information stored on the track of the disc. The track of the disc is illuminated by an incident optical beam, for example a laser spot, and several photodetectors, for example four, detect the reflection of the light beam on the disc. The optical pickup formed by the photodetectors then delivers four elementary signals delivered respectively by the four photodetectors, as well as an overall signal, or useful signal, equal to the sum of the four elementary signals, and from which the binary information read from the track is extracted.
The directing of the optical beam to the track of the rotating disc is performed exclusively on the basis of the four elementary signals delivered by the photodetectors. In an analog approach: the signals are summed in pairs so as to form two signals which are equalized in an analog equalizer before being shaped, by comparison with a threshold, in two comparators. The two signals thus shaped are mutually phase-shifted if the laser spot is not situated on the track. The phase difference between these two signals is then detected, which phase difference corresponds to the positioning error of the beam with respect to the track. This positioning error is then used conventionally in a servo-control loop to modify the incident optical system and direct the optical beam back to the track.
Generally, an analog approach of this type has the drawback of requiring a considerable number of analog components which results in a larger, bulkier system. Moreover, this number of components is even larger when the useful band of the signals contains high frequencies, thus leading in particular to high consumption. Moreover, as the technology advances, the modification and production of new components of the device require considerable design and production time.
Another known approach includes using a digital approach to sample the signals emanating from the photodetectors before digital processing which includes, e.g., the calculation of the phase shifts. In order to avoid spectral aliasing during sampling, the sampling frequency must be at least twice as high as the maximum frequency of the frequency band of the useful signal containing the information. When the sampling frequency is twice the maximum frequency of the useful band, the number of samples does not make it possible to obtain, through straightforward interpolation between these samples, a correct value of phase shift in the high frequency ranges close to the frequency maximum. The approach includes digitally reconstructing all the signals from the samples. However, this requires digital processing operations which are complex and expensive to implement, e.g. Viterbi decoding processing.
SUMMARY OF THE INVENTION
An object of the invention is avoid the drawbacks of the conventional approaches discussed above. This and other objects are achieved by providing a simple digital solution using straightforward interpolation between samples to calculate the positioning error of the beam with respect to the track of the rotating disc; and, more specifically, when the maximum frequency of the useful band of the information is equal to half the sampling frequency.
Such a configuration occurs in certain applications when the speed of rotation of the dynamic medium is very high, for example 12× (a speed of rotation of 1× corresponding to 4 m/s). Maximum frequencies of around 60 MHz are then obtained. Moreover, with the present semiconductor technologies, for example, 0.25 micron technology, certain components cannot work correctly at frequencies above 120 MHz. The sampling frequency is therefore limited to this frequency.
The invention therefore provides a process for directing an incident optical beam to a track of information on a dynamic medium. According to a general characteristic of the invention, the beam reflected by the medium (for example, the DVD disc) is picked up by an optical pickup comprising several photodetectors (at least two, and preferably four). The elementary signals respectively delivered by the photodetectors are used to formulate two secondary signals, sampled and filtered by a low-pass filter having a cutoff frequency at most equal to a quarter of the sampling frequency. The mutual phase shift between these two sampled and filtered secondary signals is representative of the positioning error of the beam with respect to the track. Moreover, the determination of a value of the mutual phase shift includes the selecting, for each secondary signal, of at least one pair of samples situated outside a predetermined amplitude range around a predetermined threshold (for example the value zero). These two pairs of samples make it possible to tag, respectively for the two secondary signals, two transitions of these secondary signals with respect to the threshold and corresponding to one and the same direction of crossing of the threshold. Additionally, two transitions are determined by interpolation from the selected samples, and the time gap between the two transitions is also determined.
The use of a low-pass filter with a cutoff frequency which is at most equal to a quarter of the sampling frequency makes it possible to obtain a sufficient number of samples to calculate the phase shift by straightforward interpolation. This being so, the low-pass filter eliminates the high frequencies from the frequency spectrum. This therefore results in a theoretical deleting of these frequencies, and in practice, an appreciable local attenuation of the amplitude of the signals. In practice, the sampled and filtered signals are noisy and, by tagging, for each secondary signal, a transition of this signal with respect to a threshold (for example the value zero), using at least one pair of samples situated outside a predetermined amplitude range around this threshold, it is possible to ignore samples whose levels or amplitudes might be situated inside this predetermined range and which if not ignored might lead to erroneous obtainings of transition due to the presence of the noise.
Of course, the person skilled in the art can choose a cutoff frequency for the low-pass filter of lower than a quarter of the sampling frequency. The adjusting of this cutoff frequency of the low-pass filter will be performed by the person skilled in the art as a function of the application and of the accuracy which are desired. In practice, it is preferable to choose a cutoff frequency for the low-pass filter which is not less than a fifth of the maximum frequency of the useful band, i.e. a tenth of the sampling frequency, so as not to eliminate too large a number of samples, which would then lead to a degradation in the accuracy of the phase-shift calculation.
Likewise, the person skilled in the art will be able to adjust the value of the amplitude of the range around the predetermined threshold as a function of the application and in particular of the noise level of the signals. By way of indication, an experimental way of determining the value of the amplitude of the range includes, when calibrating the system, in examining the changes in the phase shift (Positioning error) in open loop. Indeed, the person skilled in the art is aware that by reason of the eccentricity of the disc, the theoretical curve of the phase shift, i.e. of the positioning error, exhibits a sawtooth configuration in open loop. Too small an amplitude value for the range then lead
Lebowsky Fritz
Marrec Sonia
Jorgenson Lisa K.
Le Kimlien
STMicroelectronics S.A.
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