Abstract: A recording and/or reproducing apparatus for recording and/or reproducing data on or from an optical disc, in which the tracking error signal format for a recordable optical disc differs from that for a replay-only optical disc. The recording and/or reproducing apparatus includes a read-out processing unit for reading out data from the optical disc, a write processing unit for writing data on the optical disc and a system controller for managing control to verify the recordable optical disc having data recorded by the write processing unit by a reproducing system conforming to the standard for the replay-only optical disc. The recordable optical disc is verified by the reproducing system for the replay-only optical disc.

Abstract: A disk drive includes a DVD optical head and a CD optical head. An objective lens in the CD optical head moves along a reference radial line passing through the center of rotation of an optical disk. An objective lens in the DVD optical head moves along a transfer line parallel to the reference radial line, and is disposed so that an expression R0<(Rmin+Rmax)/2 is satisfied, where Rmin and Rmax represent distances along the reference radial line between the center of rotation and the positions of the objective lens when it is disposed on the innermost and outermost tracks of the optical disk, respectively, and R0 represents a distance along the reference radial line between the center of rotation and the position of the objective lens when its tracking correction direction is towards the center of rotation of the optical disk.

Abstract: A disc driving apparatus and an information readout method according to the present invention realize optimum servo control of a readout unit in reading out the information from a recording medium having lands and grooves. The disc driving apparatus includes a unit for correcting tracking error signals, a unit for inverting signals corrected by the correcting unit to generate inverted tracking error signals, a unit for detecting which of the lands and the grooves the readout unit is scanning, and a unit for selecting the tracking error signals or the inverted tracking error signals according to the results of detection by the detection unit to tracking-control the readout unit by the selected signals.

Abstract: An apparatus which stabilizes an operation of a disc driver in a mode conversion setting section includes a pickup which reads information from a disc that is inserted into the disc driver, a filter which performs a low-pass filtering of a servo error signal that is generated during an operation of the pickup, a selector which selects one of the servo error signal and the low-pass filtered servo error signal from the filter and outputs the selected servo error signal to drive the pickup using the selected servo error signal, and a control signal generator which generates a signal to control the selector to select the low-pass filtered servo error signal in a mode conversion setting section of the disc driver. Accordingly, a defocus and a detrack can be prevented in the mode conversion setting section of the disc driver.

Abstract: A system, method, and apparatus for coordinating tasks in a control system for an optical disc drive for optical media with a pitted premastered area that cannot be overwritten and a grooved user-writeable area that can be overwritten. The optical disc drive includes a first processor operable to communicate with a second processor, wherein the first processor includes instructions for performing non-time-critical tasks, and the second processor includes instructions for performing time-critical tasks, such as reading from and writing to optical media in the disc drive. The first processor monitors the status of the time critical tasks in the second processor and transmits commands to perform operations in the second processor. The first processor also controls power mode, manages recovery from errors, controls focus, tracking/seeking, spin, physical sector address (PSA), a laser, adjusts gains, and monitor cartridge load/eject.

Abstract: A control system for controlling tracking and seeking in an optical disc drive for optical media with a pitted premastered area that cannot be overwritten and a grooved user-writeable area that can be overwritten. The control system determines whether an optical pickup unit is over the premastered or the writeable areas of the optical media by measuring peak to peak amplitude of a tracking error signal. Gains and calibration values are selected based on whether an optical pickup unit is over the premastered or the writeable areas of the optical media. Reading from and writing to the optical media is disabled while jumping tracks. Control of jumping tracks on the optical media includes controlling: jumping a single track on the optical media; jumping a specified number of tracks every specified number of revolutions of the optical media; auto jumpbacks, and jumping in forward or reverse directions.

Abstract: The present invention is a method and system to provide focus control in a multi-layer optical storage medium. The method comprises generating a beam of light and reflecting the beam of light off one of a first and a second optical layers on a disk. The reflected beam is detected and a servo signal is provided in response to the reflected beam. The light source is directed to move from a first position to a second position based on the servo signal, so as to focus the light source from one of the first or the second optical layer to another one of the first or the second optical layer.

Abstract: A method is provided for outputting a focusing completion signal to realize a focusing condition of an optical pickup head for an optical access apparatus. The method is described as follows. A trace shift of the optical pickup head along a focusing direction is actuated in response to a focus control signal when a focus adjustment operation starts. Then, a focusing error signal and a sub-beam addition signal are generated in response to optical signals from the optical pickup head. A closed-loop enable signal is generated to enable a closed-loop control operation in response to a specified situation of the focusing error signal. The focusing completion signal is outputted when the sub-beam addition signal is at a level greater than a threshold value and when the closed-loop enable signal is generated.

Abstract: The present invention measures first and second detection signals respectively obtained at two focuses during a focus search(by means of an optical head 2. Then, a type of a disk-shaped optical record medium is determined by a microcomputer in a system controller 7, from a comparison between characteristics of the first and second measured detection signals. Moreover, the present invention can measure an amplitude and a frequency of the detection signal obtained at the focus during the focus search to thereby determine the type of the disk by using the measured result. After the parameters are set in accordance with the determined result, the focus servo control is turned on.

Abstract: The present invention provides a low-cost optical recording device that determines an optimum quantity of light for recording and reproducing, and an optimum focus offset value. The present invention also relates to a method of determining such an optimum focus offset value. The optical recording device is equipped with a focus control system for performing a focus control operation to focus light from a light source onto a recording face of an optical recording medium.

Abstract: Methods and systems for controlling a servo are provided. One method includes calculating at least a first switching time to change an amplitude of a command signal to a servo, providing a first amplitude to the servo for the first switching time, and switching the amplitude of the command signal to the servo from the first amplitude to a second amplitude at the first switching time. The second amplitude is the command signal amplitude.

Abstract: A disk apparatus includes a DSP (digital signal processor), and a DSP core of the DSP receives a feedback input from a pick-up, and generates a focus control pulse and a tracking control pulse. That is, a focus servo loop and a tracking servo loop are formed. The DSP core detects a phase difference between a sine wave of a target frequency and a sine wave superposed on an input signal, and adjusts a gain of the servo loop so as to bring a DC component of a correlation signal correlated with the phase difference to a 0 level. More specifically, if the DC component of the correlation signal exceeds a predetermined threshold value, an adjusting amount of the gain is rendered large, and greatly brought closer to the 0 level. On the other hand, if the DC component of the correlation signal is equal to or less than the predetermined threshold value, the adjusting amount of the gain is rendered small, and gradually brought to the 0 level.

Abstract: A focus control system for an optical pickup head is provided. The optical head generates an electronic signal in response to an optical signal. The focus control system comprises a focusing error signal generator, a focus control signal generator and an actuator. The focus control signal generator is electrically connected to the focusing error signal generator, and includes a first, a second and a third compensators with a first, a second and a third gains, respectively, for generating a focus control signal with one of the first, second and third gains in response to the focusing error signal according to a corresponding one of a first, a second and a third operating modes of the optical pickup head. A method for controlling movement of an optical pickup head is also provided.

Abstract: A gain controlling apparatus includes a pre-amplifier for roughly adjusting a gain of a generation signal, which is generated on the basis of a reception signal from an optical disc; a variable amplifier for finely adjusting the gain on the basis of the roughly adjusted gain by the pre-amplifier; a memory for storing a plurality of values for adjusting the gain generated during a previous occasion of gain adjustment in time series; and a controlling device for controlling the pre-amplifier at a current occasion of gain adjustment on the basis of the respective stored values for adjusting the gain and controlling the variable amplifier, wherein the controlling device comprises a predicting device for predicting a current adjustment value, namely an adjusted value for the gain roughly adjusted at the current occasion by the pre-amplifier, and the controlling device controls the pre-amplifier on the basis of the predicted current adjustment value.

Abstract: A method and apparatus for suspending the operation of a rotating optical media player that uses simplified and efficient logic circuitry is described. When an operator initiates a pause operation, for example, by pressing a pause key, the current location of an optical pickup (e.g., a value of the current track ID) is stored. Then, a tracking servo loop is opened and a near zero offset is provided to the input of the tracking actuator. The condition of the optical disk system is then maintained in a “wait” state until the operator decides to end the pause process by selecting an alternate operation, such as “Play”, “Search”, or some other functions. When an alternate operation is received, the tracking servo loop is closed. The system then enters a search mode to reposition the optical head to the position indicated by the track ID, which was previously saved. Finally, the selected function command is activated.

Abstract: In an optical disk reproducing device, through a provision of a control circuit, movement of focus point of laser beams is set at positions ahead or back to the focus position causing to enter into an automatic focus control loop, and a retry search signal for searching the focusing position is caused to be generated by the search signal generation means, thereby, a refocusing control in case when an automatic focusing servo loop can not be maintained can be started from the positions ahead or back to the focus position causing to enter into the automatic focus control loop.

Abstract: Non-linearity in a position error signal (PES) for a data storage device is precisely characterized during a calibration phase in the manufacturing process. From this characterization of the PES non-linearity, the data storage device can be programmed with the necessary data to perform accurate, real-time linearization calculations of the PES in order to compensate for non-linearity in the PES during operation. The sequence of calibration steps includes a PES gain calibration, a first servo loop gain calibration, a PES linearization calibration, and an optional second servo loop gain calibration.

Abstract: An optical scanning device includes a radiation source; an optical lens system with an optical axis for focusing a radiation beam supplied, in operation, by the radiation source into a scanning spot on an information carrier, the lens system including a housing, a first lens which is secured in the housing in a fixed position, and a second lens which, viewed in a direction parallel to the optical axis, is elastically suspended in the housing; a first actuator for displacing the lens system in a direction parallel to the optical axis; a first control unit for controlling the first actuator by means of a first control signal; a second actuator for displacing the second lens relative to the first lens in a direction parallel to the optical axis; and a second control unit for controlling the second actuator by means of a second control signal, and in which during operation, the first control unit also controls the second actuator by means of a third control signal which is proportional to the first control signal

Abstract: The present invention provides an optical disc device capable of performing appropriate adjustment of an off-track level. The optical disc device comprises an envelope signal generation unit 104 for generating an envelope signal on the basis of a RF signal from an optical pickup; an off-track signal generation unit 105 for binarizing the envelope signal with the off-track level as a slice level to generate an off-track signal; an off-track measurement unit 106 for measuring high level periods and low level periods of the off-track signal; and an off-track level adjustment unit 109 for adjusting the off-track level which is set on the off-track signal generation unit 105, on the basis of the ratio between the high level periods and the low level periods.

Abstract: An optical disk device that performs a stable seek operation even in the movement from a recorded surface to an unrecorded surface. The optical disk device has gain corrector 30. The gain corrector comprises memory section 22 for storing a gain control value of the AS control system before a rough seek, movement determining section 23 for determining if the rough seek has made a movement from a recorded surface to an unrecorded surface, and computing and setting section 24 for correcting a gain control value of the tracking control system. When it is determined that the rough seek is performed from a recorded surface to an unrecorded surface, the gain corrector corrects the gain control value of the tracking control system according to the rate of change in the gains of the AS control system before and after the rough seek. This correction can prevent saturation of gains of the tracking control system and thus provide a stable rough seek.

Abstract: A layer change control method and an apparatus for an optical medium in which a layer change is accurately performed even where there is a level deviation of a focus actuator error signal (FE). The layer change control method includes measuring a needed time from where a focus point of the optical medium breaks away from a current layer to where the focus point arrives at a desired layer, and controlling a focus driving voltage needed to pull the focus point of the optical medium into the desired layer, and a time to output the focus driving voltage by using the needed time. Therefore, since a layer change can always be accurately performed even where there is a deviation in a FE level, according to the physical characteristic and operational deviation of a focus actuator, a stable disc driving environment can be provided.

Abstract: A tracking error signal compensation device for removing an offset of a tracking error signal to be used for a tracking servo control includes: a low pass filter that extracts a direct current component and a low frequency component whose frequency is lower than a reference frequency from the tracking error signal, and generates a detection signal including the extracted direct current component and the extracted low frequency component; a determination device that determines whether or not a frequency of the tracking error signal is lower than a reference frequency; a holding device that generates a hold signal by holding the detection signal generated by the low pass filter, when the determination device determines that the frequency of the tracking error signal is lower than the reference signal; a detection device that detects the offset of the tracking error signal by using the detection signal and the hold signal; and a removing device that removes the offset detected by the detection device from the tr

Abstract: The present invention aims to obtain a position control device which, even if the range within which a mechanical part can be pulled in for position control is narrow, enables quickly stabilizing the pull-in operation.

Abstract: A method of controlling an optical drive to perform a braking process in a layer jump process. The optical drive has a vertically movable pickup head, a preamplifier, a controller, and a low pass filter. The controller receives a focus error signal produced by the preamplifier to produce a focus control signal, and sends the focus control signal to the low pass filter to produce a layer distance balancing signal, so that the pickup head is controlled by the layer distance balancing signal to perform the layer jump process. The method of the present invention comprises the steps of: performing the braking process in accordance with a braking signal and the layer distance balancing signal when the focus error signal reaches a braking start point; and performing a closed-loop focusing control process when the focus error signal reaches a closed-loop focusing control point.

Abstract: A recording and/or reproducing apparatus for recording and/or reproducing data on or from an optical disc, in which the tracking error signal format for a recordable optical disc differs from that for a replay-only optical disc. The recording and/or reproducing apparatus includes a read-out processing unit for reading out data from the optical disc, a write processing unit for writing data on the optical disc and a system controller for managing control to verify the recordable optical disc having data recorded by the write processing unit by a reproducing system conforming to the standard for the replay-only optical disc. The recordable optical disc is verified by the reproducing system for the replay-only optical disc.

Abstract: An optical pickup including a light detection element to divide light reflected from an optical recording medium into first through fourth light beam portions in a radial direction of the optical recording medium. From detection signals from the light detection element, a reproduction signal detection portion detects an information signal, and a tracking error signal detection portion detects a tracking error signal. The light source emits light that is focused as a single light spot on a main track of the optical recording medium. The tracking error signal detection portion gain controls and sums push-pull signals from the detection signals to detect, during a recording operation, a tracking error signal with reduced push-pull offset without erasing existing signals on adjacent tracks. The reproduction signal detection portion sums and gain controls the detection signals to detect an information signal containing reduced crosstalk from adjacent tracks.

Abstract: There is provided a track jump control device and method for an optical disc drive. A track jump control device of the present invention for jumping an optical spot from one track to another track in an optical disc drive, comprising an optical spot position command generator for generating predetermined optical spot position command data to designate the position of an optical spot every predetermined sampling period by commencing operation in response to a track jump command provided from an external source and for generating switching control polarity data for switching control, an adder for subtracting tracking error data from the optical spot position command data; and a switching unit for generating the optical spot position command data with a polarity which is left intact or reversed in response to the switching control polarity data.

Abstract: A method for generating a center error signal in an optical storage system, as well as an optical storage system, is disclosed. Through the use of the method and system in accordance with the present invention, a decrease in manufacturing costs is achieved since very precise and expensive mirrors and/or photo detectors are not needed. A first embodiment of the invention includes a method for generating a center error signal in an optical storage system, the optical storage system comprising a tracking coil and an optical pick up unit, the optical pick up unit including a light beam. The method comprises the steps of sensing a voltage by a tracking coil, providing the voltage to a center error generation circuit, generating a center error signal from the center error generation circuit based on the voltage and utilizing the center error signal to center the light beam. A second embodiment of the invention includes an optical storage system.

Abstract: Disclosed is a method of tracking control for an optical disc recorder and reproducer, which includes the steps of detecting a land/groove switching position, storing tracking error signals for a predetermined time if the land/groove switching position is detected, obtaining a mean value of the stored tracking error signals, and judging whether the mean value is larger than a predetermined permissible tracking level, and if it is judged that the mean value is larger than the predetermined tracking level, determining a direction of a track center, correcting the track center, and compensating for a DC offset.

Abstract: The present invention relates to an apparatus of measuring how much an eccentricity affects each phase of a disk and controlling disk tracking based on the each effect rate. The present apparatus with disk phase identifying means stores information on dense/sparse state of a tracking error (TE) signal in connection with characteristic of an output signal from said means while a tracking servo is inactivated, and, in data reproduction mode, it identifies a current disk phase based on characteristic of the output signal from said means, and adjusts tracking characteristic of the tracking servo based on the stored information in connection with signal characteristic in order to compensate effect rate of eccentricity on the identified disk phase. Through minute adjustment of servo characteristic for each disk phase, error range of TE signal to track in real time is reduced, whereby, tracking for an eccentric disk becomes easier and more reliable.

Abstract: A servo-control apparatus is provided for use with an optical disc player for reading and/or recording information. The apparatus includes an adjustment section for performing coarse and fine adjustment of a servo loop gain in accordance with a storage medium loaded in the player; a memory for storing a gain adjustment value; a discriminator for determining, upon one of reading and recording information, that the storage medium has already been subjected to the coarse and fine adjustment; and a controller. The controller allows the adjustment section to perform the fine adjustment of the servo loop gain in accordance with the stored adjustment value when it is determined that the storage medium has already been subjected to the coarse and fine adjustment.

Abstract: With an arrangement having functions of detecting an unrecorded area by comparing a reproduction signal level obtained by conducting reproduction of a disk-shaped recording medium with a threshold, the setting of this threshold is performed by performing reproduction regarding an adjustment area serving as an unrecorded area on the disk-shaped recording medium while varying the adjusting threshold. Then, the threshold to be actually used for detection of unrecorded areas is set based on the comparison results between the adjusting threshold and the reproduction RF signal. Thus, a threshold appropriate for each device is automatically set for example, regardless of conditions such as differences in noise levels superimposed on reproduction signals, from one device to another. Thus, an appropriate threshold for blank checks can be set regardless of irregularities in parts from one device to another (i.e.

Abstract: An optical system of an optical recording apparatus forms a main spot and sub-spots preceding and succeeding the main spot on a signal recording medium. The optical system is set such that the sub-spot is shifted from the main spot in the direction orthogonal to a signal track by a distance expressed by an expression (2k−1) P/4 (k=natural number), in which P denotes a distance between adjacent signal tracks on the signal recording medium. Thus, the sub-spot succeeding the main spot can be used to effectively read the state of a signal recorded on the signal recording medium by the main spot.

Abstract: In order to perform stable focusing jump in accordance with the wobble of a disk, an optical pickup is moved radially inward toward a central area of the disk where the amount of wobble is small. A focusing error signal in accordance with wobble is feed-forwarded during focusing jump so as to also follow the wobble during the focusing jump. Furthermore, the position of an objective lens is maintained while waiting for the signal recording surface of the disk to reach a focus position due to wobble. In addition, the rotation speed of the disk is controlled in accordance with the amount of wobble.

Abstract: Disclosed is an optical disk barcode forming method wherein, as information to be barcoded, position information for piracy prevention, which is a form of ID, is coded as a barcode and is recorded by laser trimming on a reflective film in a PCA area of an optical disk. When playing back the thus manufactured optical disk on a reproduction apparatus, the barcode data can be played back using the same optical pickup.

Abstract: An information recording and reproducing apparatus that maintains high reliability in tracking offset control during recording and reproduction, which is insufficient in methods of the conventional art due to dissimilar characteristics of high density optical disks versus the amount of track offset occurring during radial tilt as well as crosstalk and cross-erase characteristics. The apparatus has a track offset adjustment circuit for adjusting the track offset during recording and does not adjust the track offset during reproduction. Turning the offset adjustment circuit on during recording prevents increased cross-erasure, while turning the offset adjustment circuit off during reproducing expands the margin versus crosstalk.

Abstract: There is provided an optical disk drive for playing or playing and recording an optical disk, and a method for controlling the optical disk drive, wherein, while the rpm of a spindle motor (11) is changed, the spindle motor (11) is controlled according to FG servo by a second spindle motor controller (23), and tracking servo by a tracking controller (18) is turned off.

Abstract: To correct for run-out in an optical disk drive, a run-out reducing gain factor is selected. A run-out correction signal is produced by applying the run-out reducing gain factor to a center error signal. A servo drive signal is refined with the run-out correction signal to produce an adjusted servo drive signal. A tracking actuator is driven with the adjusted servo drive signal.

Abstract: An optical disk device in which data is written to or reproduced from an optical disk having an area which contains a header section and another area which does not contain a header section. When a track deviation of an optical pickup occurs, it is detected whether or not a signal representing the header section is contained in the return light signal from the optical disk. When a signal representing the header section is contained in the return light signal, it is determined that the optical pickup is located in an area which contains a header section and tracking is controlled through a push-pull method etc. On the other hand, when a signal representing the header section is not contained in the return light signal from the optical pickup, it is determined that the optical pickup is located in an area which does not contain a header section and tracking is controlled through differential phase detection etc.

Abstract: An optical disc player executes a focus pull-in operation while avoiding an inconvenience that an objective lens comes into contact with an optical disc, or the like. The objective lens is forcedly moved gradually from a position that is away from the surface of the optical disc and outside a capture range of a focus servo toward the surface of the optical disc. When the objective lens reaches the capture range of the focusing servo loop by the moving operation and a distance between the objective lens and the disc surface is at the minimum distance or is changing in a widening direction, the moving operation of the objective lens is stopped.

Abstract: At a timing when a spot M of a main beam enters a defective area N of an optical disc, the tracking servo control is stopped, and the position the spot M of the main beam is held to the current position with respect to a radial direction of the optical disc, and, after a spot S2 of a rear sub-beam then exits the defective area N, the tracking servo control is resumed.

Abstract: The disclosed invention provides a tracking method of an optical storage. When proceeding with tracking, the optical pickup head can be moved constantly and the TE signal can be detected. Then differentiation of the TE signal is obtained. If the differentiation is less than 0, then the TE signal is masked; otherwise, the TE signal is not masked. Therefore, the TE Hysteresis signal can be determined by the TE signal and its differentiation. The TE Hysteresis signal of 0 represents that the laser beam is in the Pit, and then the tracking procedure is finished. Otherwise, the aforementioned steps are repeated until the end of the tracking procedure.

Abstract: A control device for a focusing system of a compact disk (CD) reader is provided. The control device uses fuzzy logic incorporated to the audio data processing system of the CD reader which is adapted to detect and segregate a light beam reflected by the surface of the compact disk from an incident light beam to the surface. The fuzzy logic control device receives a focus error signal and a derivative of the focus error. It then calculates, using appropriate membership functions, output signals to provide to a focusing servo-system of the CD reader to adjust the distance of the focal plane from the light beam detecting circuitry.

Abstract: A disc-like record carrier has a machine-readable information track in which is provided record carrier control information including the actual physical diameter of the carrier.

Abstract: A servo control apparatus of an optical pickup in which an offset and a gain can be adjustable for each servo control apparatus, and influences caused by changes in environmental conditions and changes with time are reduced without a decrease in operational speed, and increase in size and an increase in cost is disclosed. The servo control apparatus includes a nonvolatile memory for storing compensatory values which correspond to differences of servo characteristics of each servo apparatus and which are measured with the assistance of external measurement instruments, and a compensation portion for compensating an optical pick-up servo signal according to the compensatory values stored in the nonvolatile memory.

Abstract: A disk apparatus includes a DSP core. When turning on power to the apparatus main body or exchanging the MO disk, setting processing of focus offset (offset) values is performed. The DSP core extracts fine clock mark (FCM) signals out of the reproduced signal through tracking on the land/groove. Specifically, in a range that mean value of FCM signal levels is equal to or greater than a predetermined level, FCM signal levels are detected at positions shifted left and right by a predetermined amount from a present offset value. In this manner, scanning is made throughout the range while being shifted by a predetermined amount, each time of which a difference is detected. An offset value where the difference assumes a minimum is determined as an optimal offset value. The optimal offset values are determined respectively for the land and the groove, which are switched according to a track (land/groove) upon reproduction.

Abstract: A calibration of a loop gain in a digital servo system is presented. The loop gain can be calibrated measuring the response in a control signal generated by the digital servo system when the control signal is replaced by a sum of the control signal and a sinusoidal disturbance. The loop gain can be set to provide a desired gain, for example unity, between the control signal and the sinusoidal disturbance at a cross-over frequency.

Abstract: A method and apparatus provide positive lens position control when reading information from a disk storing data using a first and a second encoding format. When reading the data encoded in the second encoding format, the lens is maintained in a substantially stationary radial position with respect to the disk. When the data encoded in the second encoding format has an error, the lens position is changed to find a better position for reading the data.

Abstract: A gain calibration device and method for differential push-pull (DPP) tracking error signals in an optical storage system is provided. The gain calibration method processes the synthesized gain (SPPG) of the sub beam in the DPP tracking error signal components with respect to the main beam. The calibration theorem resides in controlling the objective lens of the pick-up head to form a lens-shift or controlling the tilt of the objective lens relative to the optical disc to make the synthesized DPP tracking error signals generate a correspondingly signal variation owing to the optical path deviation. The synthesized gain is calibrated to make the signal variation a minimum value, and the calibrated synthesized gain is the optimum value.

Abstract: According to the method, the focusing servo is activated at the zero-crossing point of a focusing error signal FE immediately after the focusing error signal FE has exceeded the first threshold th1 or has dropped below the second threshold th2. The absolute value of the first threshold th1 and the second threshold th2 is reduced each time a predetermined time t is elapsed until the focusing servo is activated.