Optical disc apparatus switching focus point between layers Number:7,151,722 from the United States Patent and Trademark Office (PTO) owispatent A focus jump technique enables focus control on recording layers of a disc in such a manner that its effect is not absorbed by disturbance or a variation in the movement speed of an objective lens. The technique involves monitoring level of a focus error signal and rejecting noise from the error signal. A speed sensor detects movement speed of an objective lens; and a speed control circuit generates a voltage for controlling the objective lens, based on the detected movement speed. Movement speed of the objective lens is detected during focus jump, a corresponding lens drive signal is generated, and an end position is determined from behavior of the error signal immediately before the end of the jump. A focus control is pulled, from a focus point corresponding to one recording layer, into a focus point corresponding to another recording layer forcibly in a stable manner.<H switching, apparatus, Optical, disc, ap, 7151722.html, Patent, pto, 7151722

Title: Optical disc apparatus switching focus point between layers

Abstract: A focus jump technique enables focus control on recording layers of a disc in such a manner that its effect is not absorbed by disturbance or a variation in the movement speed of an objective lens. The technique involves monitoring level of a focus error signal and rejecting noise from the error signal. A speed sensor detects movement speed of an objective lens; and a speed control circuit generates a voltage for controlling the objective lens, based on the detected movement speed. Movement speed of the objective lens is detected during focus jump, a corresponding lens drive signal is generated, and an end position is determined from behavior of the error signal immediately before the end of the jump. A focus control is pulled, from a focus point corresponding to one recording layer, into a focus point corresponding to another recording layer forcibly in a stable manner.

Patent Number: 7,151,722 Issued on 12/19/2006 to Tada, et al.

Inventors: Tada; Yukinobu (Yokohama, JP), Ishikawa; Yoshinori (Yokohama, JP)
Assignee: Hitachi, Ltd. (Tokyo, JP)

Appl. No.: 10/988,533

Filed: November 16, 2004

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
09935795	Aug., 2001	6865141

Foreign Application Priority Data


Oct 25, 2000 [JP]			2000-332104
May 30, 2001 [JP]			2001-162601

Current U.S. Class: 369/44.28 ; 369/53.28; 369/94

Current International Class: G11B 7/00 (20060101)

References Cited [Referenced By]

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6298012	October 2001	Watanabe et al.
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6298020	October 2001	Kumagami
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6744709	June 2004	Kobayashi
6756574	June 2004	Higuchi

Foreign Patent Documents


63046625	Feb., 1988	JP
02223023	Sep., 1990	JP
05242490	Sep., 1993	JP
09-050630	Feb., 1997	JP
10-188294	Jul., 1998	JP
10-269581	Oct., 1998	JP
10-302272	Nov., 1998	JP
11-345420	Dec., 1999	JP
11-345421	Dec., 1999	JP
2000-155954	Jun., 2000	JP

Other References

Machine translation of JP 05-242490. cited by examiner.

Primary Examiner: Miller; Brian E.
Assistant Examiner: Agustin; Peter Vincent
Attorney, Agent or Firm: McDermott Will & Emery LLP

Parent Case Text

This application is a continuation of application Ser. No. 09/935,795 filed Aug. 24, 2001 now U.S. Pat. No. 6,865,141.

Claims

What is claimed is:

1. An optical disc apparatus having a focus jump function for enabling a focus control on each of a plurality of recording layers of a disc, comprising: an objective lens for focusing laser light on a recording layer of the disc; focus error signal generating means for generating a focus error signal based on reflection light that is obtained through the objective lens; generating means for generating, based on the focus error signal, a focus control signal for controlling the objective lens; drive voltage generating means for outputting a voltage necessary to move the objective lens; moving means for moving the objective lens in a direction approximately perpendicular to the recording layers of the disc in accordance with the output voltage of the drive voltage generating means; speed detecting means for detecting a movement speed of the objective lens, means for rejecting noise from the focus error signal; means for monitoring a level of the focus error signal for detecting an operation of focus point returning; and speed control voltage generating means for generating a voltage for controlling the objective lens based on the movement speed detected by the speed detecting means and the level of the focus error signal monitored by the means for monitoring a level of the focus error signal, wherein a movement speed of the objective lens is detected during a focus jump from a first recording layer to a second recording layer, a lens drive signal corresponding to the detected movement speed is supplied to the moving means, and an end position of the focus jump is determined based on behavior of the focus error signal immediately before an end of the focus jump, whereby a focus control is forcibly pulled, from a focus point corresponding to the first recording layer, into a focus point corresponding to the second recording layer.

2. An optical disc apparatus having a focus jump function for enabling a focus control on each of a plurality of recording layers of a disc, comprising: an objective lens for focusing laser light on a recording layer of the disc; a signal processing circuit for generating a focus error signal based on reflection light that is obtained through the objective lens; a focus control circuit for generating, based on the focus error signal, a focus control signal for controlling the objective lens; a drive voltage generating circuit for outputting a drive voltage necessary to move a focus position of the objective lens between recording layers; an actuator for moving the objective lens in a direction approximately perpendicular to the recording layers of the disc in accordance with the output voltage of the drive voltage generating circuit; a differentiation circuit for detecting a movement speed of the objective lens by differentiating the focus error signal; means for monitoring a level of the focus error signal for detecting an operation of focus point returning; and speed control voltage generating means for generating a voltage for controlling the objective lens based on the movement speed detected by the differential circuit and the level of the focus error signal monitored by the means for monitoring a level of the focus error signal, wherein a movement speed of the objective lens is detected during a focus jump from a first recording layer to a second recording layer, a lens drive signal corresponding to the detected movement speed is supplied to the actuator, and an end position of the focus jump is determined based on behavior of the focus error signal immediately before an end of the focus jump, whereby a focus control is forcibly pulled, from a focus point corresponding to one recording layer, into a focus point corresponding to the second recording layer.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical disc apparatus for optically reproducing a signal from a disc or optically recording and reproducing a signal on and from a disc. In particular, the invention relates to an optical disc apparatus capable of reproducing a signal from a disc having a plurality of recording layers from the disc surface side or recording and reproducing a signal on and from a disc having a plurality of recording layers from the disc surface side.

2. Description of the Related Art

Among the currently standardized digital video discs (or digital versatile discs; hereinafter referred to as "DVDs") are single-surface/single-layer discs, double-surface/single-layer discs, single-surface/double-layer discs, and double-surface/double-layer discs. That is, in contrast to other conventional discs such as compact discs (hereinafter referred to as "CDs") and laser discs (hereinafter referred to as "LDs") that have only one recording layer on one surface, there are DVDs that have two recording layers on one surface to increase the recording capacity.

For example, FIG. 2A shows a single-surface/double-layer disc that is produced by forming a recording layer on each of two 0.6-mm-thick discs, forming a high-reflectance aluminum film and a semitransparent gold film on the respective discs, and bonding together the two discs. FIG. 2B shows a double-surface/double-layer disc that is produced by bonding together two 0.6-mm-thick discs in each of which information is multiplexed in the depth direction.

In the above double-layer discs, information is recorded in each recording layer. When the level of a signal for driving an objective lens is increased gradually as shown in FIG. 2D (it is assumed that the objective lens approaches the disc accordingly), a point (hereinafter referred to as "focus point") where the beam is focused at the lower recording layer (hereinafter referred to as "0th layer") occurs in a focus error signal as shown in FIG. 2C at a certain position of the objective lens. When the objective lens is further elevated, a focus point corresponding to the upper recording layer (hereinafter referred to as "first layer") occurs at a position of the objective lens that is higher than the previous position. In short, in double-layer discs, the beam is focused at each recording layer by moving the objective lens vertically. In CDs and LDs, it is sufficient to focus on the single recording layer on the single surface. On the other hand, in multi-layer discs such as DVDs having two or more planes where information was recorded from one side, unless switching is made from a focus point corresponding to a recording layer at which the beams is currently focused to another focus point corresponding to another recording layer, information stored in the latter recording layer cannot be read out.

The focus point switching between layers (hereinafter referred to as "focus jump") is described in Japanese Unexamined Patent Publication Nos. Hei. 9-50630 and Hei. 11-345420, for example. The method disclosed in the publication No. Hei. 9-50630 is as follows. For example, a focus jump from the 0th layer to the first layer is performed as shown in FIG. 3. For a move from the 0th layer to the first layer, first, the focus servo loop is rendered in an open state or a hold state and the objective lens is accelerated by applying an elevation voltage to the actuator for driving the objective lens. In the interlayer region of the focus position between the 0th layer and the first layer, in a period when the focus error signal is between threshold levels, the voltage application to the actuator is stopped. After the focus error signal has exceeded the upper threshold level, a lowering voltage is applied to the actuator for a prescribed period and the focus servo loop is closed in the vicinity of the focus point corresponding to the first layer to complete the focus jump. This method enables a stable focus jump irrespective of a variation in interlayer distance, noise that is added to the focus error signal, a variation in the sensitivity of the actuator for driving the objective lens, and other factors.

The method disclosed in the publication No. Hei. 11-345420 is as follows. A focus error signal at the end of a deceleration pulse is measured in a focus jump, and the output timing of a deceleration pulse in the next focus jump is corrected by using the measured value. By repeating this operation, optimum output timing of a deceleration pulse in a focus jump is learned through adjustments. This method enables a stable focus jump even in a case where the focus error signal is not balanced properly or has a distorted waveform due to an offset in circuitry, a local variation in disc characteristics, a variation in interlayer distance or reflectance, or a variation in pickup characteristics and maximum acceleration attained by the actuator is small.

SUMMARY OF THE INVENTION

However, the above conventional techniques have the following problems because they do not take into account, in performing a focus jump from one layer at which the beam is focused currently to another layer, a phenomenon called "surface vibration," a noise component that is added to the focus error signal, and disturbance that is introduced during execution of a focus jump. The surface vibration occurs when the disc is not completely flat and is warped or curved or the surface of the disc mounted is not perpendicular to the rotary shaft of a spindle motor because of insufficient mechanical accuracy of a turn table or some other factor.

As shown in FIGS. 4A and 4B, in a state that the servo loop is closed, the voltage of the objective lens drive signal varies according to the surface vibration component to maintain the focused state. Assume that a certain acceleration voltage is applied to perform a focus jump. The speed after the start of acceleration depends on whether the acceleration voltage is added to a valley of the variation of the objective lens drive signal (see FIG. 4A) or a peak portion thereof (see FIG. 4B), for the following reasons. In the case of FIG. 4A, the objective lens moving direction is opposite to the focus jump movement direction and hence the acceleration caused by the applied acceleration voltage is small. On the other hand, in the case of FIG. 4B, the objective lens moving direction is the same as the focus jump movement direction and hence the acceleration caused by the applied acceleration voltage is large.

That is, the degree of acceleration caused by the acceleration voltage depends on the direction in which the objective lens is moving when a focus jump is started. The movement speed of the objective lens when switching is made to a focus point corresponding to another recording layer and deceleration is started varies and hence the degree of deceleration by the deceleration voltage also varies. Since the deceleration voltage is constant, depending on the speed at the start of deceleration, excessive deceleration may cause a return to the focus point corresponding to the previous recording layer or insufficient deceleration may cause passage of the focus point corresponding to the target recording layer. This means a problem that it is difficult to perform a focus jump in a stable manner.

The method of the publication No. Hei. 11-345420 solves the above problem in such a manner that an optimum acceleration voltage and deceleration voltage for a position concerned are learned through several focus jumps to enable a satisfactory focus jump. However, focus jumps are unstable and may fail at a strong possibility until an optimum focus jump is learned. There is another problem that when the movement speed of the objective lens is varied by disturbance or the like during a focus jump, the apparatus cannot absorb influence of the disturbance and the focus jump becomes unstable because data obtained by learning provides a constant acceleration voltage and deceleration voltage.

The method of the publication No. Hei. 11-345420 has still another problem that the circuit scale tends to be large because a large amount of learning data needs to be stored to cope with surface vibration and a local variation in disc characteristics.

A first object of the invention is to provide, by solving the above problems, an optical disc apparatus capable of performing a focus jump in a stable manner without requiring a large amount of memory irrespective of influence of surface vibration, a variation in interlayer distance, noise that is added to the focus error signal, a variation in the sensitivity of the actuator for driving the objective lens, disturbance that is introduced during a focus jump, and other factors.

Previously, DVDs that required a focus jump were ones for reproduction only (e.g., DVD-ROM (DVD-read only memory) and DVD-VIDEO) on which large-capacity image data of a movie for example, a program, or the like is recorded in advance. DVDs for recording (e.g., DVD-RAM (DVD-random access memory), DVD-R (DVD-recordable), and DVD-RW (DVD-rewritable)) had only a single recording layer and hence did not require a focus jump.

Incidentally, digital broadcasts of high-resolution digital moving picture data were started recently. To enable long-term recording of such data in usual homes or the like, large-capacity recording media are necessary. The above-mentioned DVD recording media are insufficient in storage capacity and hence multi-layering of recording discs is indispensable to obtain more capacity.

However, although the capacity is increased by multi-layering a recording surface, a move between layers is needed because of random accessibility that is a merit of the medium of disc; it is necessary to perform a focus jump as performed in DVDs for reproduction only. The above-described conventional techniques give no consideration to the focus jump in discs capable of recording data and have the following problems that relate to the focus jump to be performed during recording.

Where a recording medium having multiple recording layers is used for, for example, recording a digital broadcast of high-resolution digital moving picture data as mentioned above, real-time recording is required in which the broadcast is recorded parallel with its reception and hence all-time recording on the recording media should be enabled. Further, in the case of a disc, there may occur a case that the next recording position is distant from the current recording position. Therefore, a move to a target position should be performed instantaneously. This results in a problem that if a move to a target recording layer fails in a focus jump, another attempt should be started from focusing on the original recording layer, which takes time and makes it difficult to enable all-time recording (data is lost during such an attempt). There is another problem that a recorded portion may be erased erroneously unless a move to a target position is performed correctly during recording.

A second object of the invention is to provide, by solving the above problems, an optical disc apparatus capable of performing a focus jump in a stable manner even during recording in such a manner that in performing a focus jump not only does the optical disc apparatus control the actuator so that the focus position deviates from a target recording layer by monitoring whether the level of the focus error signal exceeds a set threshold level, but also it prevents erroneous erasure of data in a recorded portion by decreasing the laser power to such a value that recording cannot be effected.

To attain the first object, the invention provides an optical disc apparatus having a focus jump function for enabling a focus control on each of a plurality of recording layers of a disc, comprising an objective lens for focusing laser light on a recording layer of the disc; focus error signal generating means for generating a focus error signal based on reflection light that is obtained through the objective lens; generating means for generating, based on the focus error signal, a focus control signal for controlling the objective lens; drive voltage generating means for outputting a voltage necessary to move the objective lens; moving means for moving the objective lens in a direction approximately perpendicular to the recording layers of the disc in accordance with the output voltage of the drive voltage generating means; and speed detecting means for detecting a movement speed of the objective lens, wherein a movement speed of the objective lens is detected during a focus jump, a lens drive signal corresponding to the detected movement speed is supplied to the moving means, and an end position of the focus jump is determined based on behavior of the focus error signal immediately before an end of the focus jump, whereby a focus control is pulled, from a focus point corresponding to one recording layer, into a focus point corresponding to another recording layer forcibly in a stable manner.

Further, the invention provides an optical disc apparatus having a focus jump function for enabling a focus control on each of a plurality of recording layers of a disc, comprising an objective lens for focusing laser light on a recording layer of the disc; focus error signal generating means for generating a focus error signal based on reflection light that is obtained through the objective lens; generating means for generating, based on the focus error signal, a focus control signal for controlling the objective lens; drive voltage generating means for outputting a voltage necessary to move the objective lens; moving means for moving the objective lens in a direction approximately perpendicular to the recording layers of the disc in accordance with the output voltage of the drive voltage generating means; and means for monitoring a level of the focus error signal, wherein a movement speed of the objective lens is detected during a focus jump, a lens drive signal corresponding to the detected movement speed is supplied to the moving means, and an end position of the focus jump is determined based on behavior of the focus error signal immediately before an end of the focus jump, whereby a focus control is pulled, from a focus point corresponding to one recording layer, into a focus point corresponding to another recording layer forcibly in a stable manner.

Further, the invention provides an optical disc apparatus having a focus jump function for enabling a focus control on each of a plurality of recording layers of a disc, comprising an objective lens for focusing laser light on a recording layer of the disc; focus error signal generating means for generating a focus error signal based on reflection light that is obtained through the objective lens; generating means for generating, based on the focus error signal, a focus control signal for controlling the objective lens; drive voltage generating means for outputting a voltage necessary to move the objective lens; moving means for moving the objective lens in a direction approximately perpendicular to the recording layers of the disc in accordance with the output voltage of the drive voltage generating means; means for monitoring a level of the focus error signal, speed detecting means for detecting a movement speed of the objective lens; and speed control voltage generating means for generating a voltage for controlling the objective lens based on the movement speed detected by the speed detecting means, wherein a movement speed of the objective lens is detected during a focus jump, a lens drive signal corresponding to the detected movement speed is supplied to the moving means, and an end position of the focus jump is determined based on behavior of the focus error signal immediately before an end of the focus jump, whereby a focus control is pulled, from a focus point corresponding to one recording layer, into a focus point corresponding to another recording layer forcibly in a stable manner.

Furthermore, the invention provides an optical disc apparatus having a focus jump function for enabling a focus control on each of a plurality of recording layers of a disc, comprising an objective lens for focusing laser light on a recording layer of the disc; focus error signal generating means for generating a focus error signal based on reflection light that is obtained through the objective lens; generating means for generating, based on the focus error signal, a focus control signal for controlling the objective lens; drive voltage generating means for outputting a voltage necessary to move the objective lens; moving means for moving the objective lens in a direction approximately perpendicular to the recording layers of the disc in accordance with the output voltage of the drive voltage generating means; means for rejecting noise from the focus error signal; means for monitoring a level of a signal obtained by rejecting the noise form the focus error signal; speed detecting means for detecting a movement speed of the objective lens; and speed control voltage generating means for generating a voltage for controlling the objective lens based on the movement speed detected by the speed detecting means, wherein a movement speed of the objective lens is detected during a focus jump, a lens drive signal corresponding to the detected movement speed is supplied to the moving means, and an end position of the focus jump is determined based on behavior of the focus error signal immediately before an end of the focus jump, whereby a focus control is pulled, from a focus point corresponding to one recording layer, into a focus point corresponding to another recording layer forcibly in a stable manner.

To attain the second object, the invention provides an optical disc apparatus having a focus jump function for enabling a focus control on each of a plurality of recording layers of a disc on and from which data can be recorded and reproduced, comprising an objective lens for focusing laser light on a recording layer of the disc; focus error signal generating means for generating a focus error signal based on reflection light that is obtained through the objective lens; generating means for generating, based on the focus error signal, a focus control signal for controlling the objective lens; drive voltage generating means for outputting a voltage necessary to move the objective lens; moving means for moving the objective lens in a direction approximately perpendicular to the recording layers of the disc in accordance with the output voltage of the drive voltage generating means; means for controlling power of a laser that is used for recording and reproducing data on and from the disk; means for detecting whether a focus position of the objective lens will deviate from a recording layer; and control means for starting, when a focus jump becomes necessary during data recording, the focus jump after switching laser light power that is currently made high to enable the data recording to a low power for reproduction, wherein when a focus jump is performed, whether a focus position of the objective lens will deviate from a destination recording layer is detected and deviation from the destination recording layer is prevented by controlling the moving means, whereby the focus jump can be performed stably during data recording.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an optical disc apparatus and a focus jump method according to an embodiment of the present invention;

FIGS. 2A 2D show the structures of two-layer discs and outline a focus jump that is performed in those discs;

FIG. 3 shows an example of an objective lens drive signal for a focus jump in a conventional optical disc apparatus;

FIGS. 4A and 4B show examples of timing, with respect to a surface vibration component, of applying a voltage for a focus jump in a conventional optical disc apparatus;

FIG. 5 shows a configuration of a pickup shown in FIG. 1 and a specific example of a signal processing circuit for processing a focus error signal;

FIG. 6 is a graph showing how a focus error signal varies depending on a disc deviation;

FIG. 7 is a timing chart showing a specific example of how individual circuits operate in a focus jump from a 0th-layer focus point to a first-layer focus point in the optical disc apparatus of FIG. 1;

FIG. 8 is a timing chart showing another specific example of how the individual circuits operate in a focus jump from a 0th-layer focus point to a first-layer focus point in the optical disc apparatus of FIG. 1;

FIG. 9 is a timing chart showing a specific example of how the individual circuits operate in a focus jump from a first-layer focus point to a 0th-layer focus point in the optical disc apparatus of FIG. 1;

FIG. 10 is a timing chart showing another specific example of how the individual circuits operate in a focus jump from a first-layer focus point to a 0th-layer focus point in the optical disc apparatus of FIG. 1;

FIG. 11 shows a specific method for detecting a minimum value of a focus error signal by means of a value holding circuit shown in FIG. 1;

FIG. 12 shows a specific method for detecting a maximum value of a focus error signal by means of the value holding circuit shown in FIG. 1;

FIG. 13 is a flowchart showing a specific example of an algorithm of focus jump controls that are performed by a microcomputer in the optical disc apparatus of FIG. 1;

FIG. 14 is a block diagram showing an optical disc apparatus and a focus jump method according to another embodiment of the invention;

FIG. 15 is a timing chart showing a specific example of how individual circuits operate in a focus jump from a 0th-layer focus point to a first-layer focus point in the optical disc apparatus of FIG. 14;

FIG. 16 is a timing chart showing another specific example of how the individual circuits operate in a focus jump from a 0th-layer focus point to a first-layer focus point in the optical disc apparatus of FIG. 14;

FIG. 17 is a timing chart showing a specific example of how the individual circuits operate in a focus jump from a first-layer focus point to a 0th-layer focus point in the optical disc apparatus of FIG. 14;

FIG. 18 is a timing chart showing another specific example of how the individual circuits operate in a focus jump from a first-layer focus point to a 0th-layer focus point in the optical disc apparatus of FIG. 14; and

FIG. 19 is a flowchart showing a specific example of a focus jump control algorithm of a microcomputer in the optical disc apparatus of FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, main symbols used in the drawing denote the following:

1 . . . Optical disc; 3 . . . Objective lens; 4 . . . Pickup; 7 . . . Signal processing circuit; 8 . . . Focus control circuit; 9 . . . Tracking control circuit; 12 . . . Differentiation circuit; 13 . . . Microcomputer; 14 . . . Low-pass filter; 15 . . . Previous value holding circuit; 16 . . . Elevation voltage value; 16a . . . Elevation voltage value A; 16b . . . Elevation voltage value B; 17 . . . Lowering voltage value; 17a . . . Lowering voltage value A; 17b . . . Lowering voltage value B; 18, 18a, 18b . . . Adder; 19a 19c, 19e . . . Changeover switch; 19d . . . On/off switch; 20 . . . Gain factor; 21 . . . Multiplier; 22, 22a, 22b . . . Signal level comparison circuit; 23a . . . Threshold level A; 23b . . . Threshold level B; 23c . . . Threshold level C; 24 . . . Offset value; 25 . . . Value holding circuit; 26 . . . Deviation-from-layer preventing elevation voltage; 27 . . . Deviation-from-layer preventing lowering voltage; 28a . . . Threshold level A; 28b . . . Threshold level B; 28c . . . Threshold level C; 28d . . . Threshold level D; and 29 . . . Laser power control circuit.

Embodiments of the present invention will be hereinafter described with reference to the accompanying drawings.

FIG. 1 is a block diagram showing an optical disc apparatus and a focus jump method according to an embodiment of the invention. Reference symbol 1 denotes a disc having two or more recording layers on one side; 2a, a clamper; 2b, a turn table; 3 . . . an objective lens; 4, a pickup; 5, a sled motor; 6, a spindle motor; 7, a signal processing circuit; 8, a focus control circuit; 9, a tracking control circuit; 10, a sled control circuit; 11, a spindle control circuit; 12, a differentiation circuit; 13, a microcomputer; 14, a low-pass filter (hereinafter referred to as "LPF"); 15, a previous value holding circuit; 16a, an elevation voltage value A; 16b, an elevation voltage value B; 17a, a lowering voltage value A; 17b, a lowering voltage value B; 18a and 18b, adders; 19a 19c, changeover switches; 19d, an on/off switch; 19e, a changeover switch; 20, a gain factor; 21, a multiplier; 22a and 22b, signal level comparison circuits; 23a, a threshold level A; 23b, a threshold level B; 23c, a threshold level C; 24, an offset value; and 25, a value holding circuit.

FIG. 7 shows, with the horizontal axis as a time axis, schematic waveforms of an objective lens displacement, a focus error signal, a focus error differentiation signal, an objective lens drive signal, and an objective lens movement speed and operations of the switches 19a 19d.

As shown in FIG. 1, the disc 1 that has been set on the turn table 2b is fixed to the turn table 2b by the damper 2a. The disc 1 is rotated as the spindle motor 6 is rotated.

To read out information on the disc 1, the microcomputer 13 supplies an emission control signal to a semiconductor laser that is incorporated in the pickup 4.

FIG. 5 shows a configuration of the pickup 4 including the semiconductor laser and an optical system as well as a configuration for focus error signal detection of the signal processing circuit 7. Reference numeral 1 denotes the disc; 3, the objective lens; 51, a half prism; 52, the semiconductor laser; 53, a condenser lens; 54, a photodetector; and 55, an error calculation unit (error amplifier).

In FIG. 1, a light beam emitted from the semiconductor laser 52 passes through the half prism 51, is given converging action by the objective lens 3, and forms a beam spot on the disc 1. After being reflected by the disc 1, the laser beam again passes through the objective lens 3, is reflected by the half prism 51, passes through the condenser lens 53, and forms a spot on the photodetector 54.

A specific structure of the photodetector 54 for detecting a focus error signal will be described below.

The photodetector 54 consists of four areas A D in which areas on each diagonal line are paired with and connected electrically to each other. The photodetector 54 is located at such a position that when the objective lens 3 is focused on the disc 1, the beam spot on the photodetector 54 assumes a circle and hence an output of the error amplifier 55 obtained by amplifying the difference between the sums of outputs of the diagonal areas of the photodetector 54 becomes zero. If the disc 1 is deviated vertically from the focus position of the objective lens 3, the beam spot on the photodetector 54 assumes an ellipse that is longer in the vertical direction or horizontal direction. Based on this phenomenon, a focus error signal (FE signal) as shown in FIG. 6 that reflects the amount and direction of a deviation from the focus position is obtained from the error amplifier 55 (what is called the astigmatism method).

In FIG. 6, the horizontal axis represents the distance between the objective lens 3 and the disc 1 and the vertical axis represents the level of a focus error signal. The focus error signal has a feature that its S-shaped curve crosses the zero level when the objective lens 3 is focused on a disc recording layer. The S-shaped curve may have opposite polarities depending on how the outputs of the photodetector 54 are connected to the error amplifier 55. In the case of a system in which the S-shaped curve has opposite polarities, naturally it is proper to consider that the relationship between the signal level and the disc displacement is opposite.

The focus error signal generated by the error amplifier 55 is supplied to the focus control circuit 8 (see FIG. 1), which generates, by using a delay compensator, a lead compensator, etc., and outputs a drive signal for an actuator (not shown) for moving the objective lens 3, to enable a feedback control in the vicinity of the zero-cross point of an S-shaped curve of the focus error signal. The output signal is supplied to the changeover switch 19b. In a steady state, the changeover switch 19b is switched to the G-side according to an instruction from the microcomputer 13 and thereby supplies a focus control signal to the pickup 4 as a drive signal. The vertical position of the objective lens 3 is controlled according to this drive signal and a feedback loop focus control is realized, whereby the disc 1 always stays at a focus position.

On the other hand, a tracking error signal (TE signal) generated by the signal processing circuit 7 is supplied to the tracking error control circuit 9, which generates a drive signal for moving the objective lens 3 in the tracking direction by a feedback control. This drive signal is supplied to the pickup 4. The position of the objective lens 3 is controlled in the tracking direction according to the drive signal that is supplied to the inside of the pickup 4, and a feedback loop tracking control is thereby realized. The beam spot is always located on pits that are formed in a recording layer of the disc 1. The drive signal that is output from the tracking control circuit 9 is also supplied to the sled control circuit 10, which generates a drive signal for controlling the sled motor 5 in accordance with the deviation of the objective lens 3 in the tracking direction. The sled motor 5 is driven according to the drive signal received, whereby a sled (base) of the pickup 4, that is, the pickup 4 itself, is moved.

Further, the signal processing circuit 7 supplies the spindle control circuit 11 with rotation cycle information that is read from the disc 1. The spindle control circuit 11 generates a signal for driving the spindle motor 6 based on the received rotation cycle information and supplies it to the spindle motor 6.

In a steady state, the focus, tracking, spindle, and sled controls are performed in the above-described manners with the objective lens 3 focused on a recording layer.

Where the disc 1 is a DVD having two recording layers on one side, there may occur a case that it is necessary to switch from a focus point corresponding to a recording layer at which the beam is focused to another focus point corresponding to the other recording layer. A description will be made below of an exemplary case that the objective lens 3 is located at such a position as to be focused on the 0th recording layer and it is desired to move the focus position of the objective lens 3 to the first recording layer (i.e., it is desired to jump from a focus point corresponding to the lower (0th) recording layer to a focus point corresponding to the upper (first) recording layer).

In a steady state, a drive signal for driving the objective lens 3 that is output from the focus control circuit 8 being in a state that a focus point corresponding to the 0th recording layer is established is supplied to the on/off switch 19d. Since the on/off switch 19d is closed in this steady state, the drive signal is supplied to the previous value holding circuit 15 as it is. The previous value holding circuit 15 continues to hold, as it is, a value that has been held so far and supplies it to the LPF 14 until the value of the drive signal changes. The LPF 14 has such a frequency characteristic as to reject high-frequency components (noise components) of the signal for driving the objective lens 3 and not to reject low-frequency components such as a surface vibration component that is caused by rotation of the disc 1 having a warp or the like. As such, the LPF 14 rejects mainly noise components from the drive signal and supplies a resulting signal to the adder 18a. In a steady state, the components to the LPF 14 always operate.

To cause a focus jump to a focus point corresponding to the first recording layer, the microcomputer 13 sets, in drive voltage generation circuits, initial values of drive voltages necessary for the focus jump, that is, a constant elevation voltage value A (16a) and a constant elevation voltage value B (16b) that are acceleration voltage values and a constant lowering voltage value A (17a) and a constant lowering voltage value B (17b) that are deceleration voltage values necessary to decelerate and stop, after acceleration, the objective lens 3 at a position corresponding to a first-layer focus point. Further, the microcomputer 13 sets, in their setting circuits, initial values of a threshold level A (23a), a threshold level B (23b), a threshold value C (23c), an offset value 24, and a gain factor 20. After setting those initial values, the microcomputer 13 switches the changeover switches 19b and 19c to the H-side and the B-side, respectively, and opens the on/off switch 19d. As a result of the switching of the changeover switch 19b and the on/off switch 19d, the feedback loop by which the objective lens 3 has been controlled so far is made an open loop and the feedback control is stopped.

Then, the microcomputer 13 issues, to the changeover switch 19a, an instruction to switch to the C-side, whereby the elevation voltage value A (16a) is supplied to the adder 18a. The adder 18a adds the elevation voltage value A (16a) to a signal that is free of high-frequency noise components (rejected by the LPF 14), and supplies a resulting addition signal to the changeover switch 19c. Since the changeover switch 19c is switched to the B-side, the addition signal is supplied from the changeover switch 19c to the changeover switch 19b as it is. Since the changeover switch 19b is switched to the H-side, the addition signal is supplied to the pickup 4 via the changeover switch 19b. Since the elevation voltage value A (16a) is applied to the actuator, the objective lens 3 starts to go up.

In FIG. 7, time point A is a start point of the focus jump. The elevation voltage value A (16a; also denoted by "Vup1" in FIG. 7) is applied, as it is, as an objective lens drive signal, to the actuator for driving the objective lens 3.

Referring to FIG. 1, a focus error signal that is output from the signal processing circuit 7 is supplied to the differentiation circuit 12. The differentiation circuit 12 differentiates the received focus error signal. The differentiation circuit 12 may be a high-pass filter (HPF) that performs differentiation with respect to time in a prescribed band.

The focus error signal that is output from the signal processing circuit 7 is also supplied to the comparison circuit 22a, the adder 18b, and the value holding circuit 25.

FIG. 7 shows a focus error signal and a focus error differentiation signal (hereinafter abbreviated as "differentiation signal") that occur when a focus jump from a focus point corresponding to the 0th recording layer to a focus point corresponding to the first recording layer is performed. Those signals will be described below in detail for each of sections that are defined by time points A I.

When the objective lens 3 goes up after the focus jump was started at time point A, the focus error signal rises gradually from a level close to the middle level until time point B. In this section from time point A to B, the differentiation signal rises gradually from a level close to the middle level, reaches a maximum value, then gradually decreases, and finally returns to the middle level (zero) at time point B when the focus error signal has a maximum value. As the objective lens 3 goes up further, at time point D the focus position enters the interlayer region between the 0th recording layer and the first recording layer. The focus error signal decreases gradually from the maximum value and reaches the middle level (zero). In the section from time point B to D, the differentiation signal decreases from the middle level (zero), reaches a minimum value, then increases gradually, and finally reaches the middle level (zero) again. In the section from time point D to E that corresponds to the interlayer region, both of the focus error signal and the differentiation signal are at the middle level (zero). As the objective lens 3 goes up further, the focus position enters the first layer region and hence the focus error signal falls gradually from a level close to the middle level until time point G. In the section from time point E to G, the difference signal falls gradually from a level close to the middle level, reaches a minimum value, then gradually increases, and finally reaches the middle level (zero) at time point G when the focus error signal has a minimum value. As the objective lens 3 goes up further, a first-layer focus point is established at time point I. The focus error signal gradually increases from the minimum value and reaches the middle level (zero). In the section from time point G to I, the differentiation signal increases from the middle level (zero), reaches a maximum value, then decreases gradually, and finally reaches the middle level (zero). At time point I when the first-layer focus point is established, the focus error signal and the differentiation signal reach the respective middle levels (zero).

By using the differentiation signal, more specifically, by detecting a time point (zero-cross point) when the differentiation signal crosses the middle level (zero), a position of the objective lens 3 corresponding to time point B can be detected easily and reliably. Although time point B can also be detected by monitoring the level of the focus error signal, the detection is not reliable because the amplitude of the focus error signal varies depending on the disc, for example, and hence is not uniform.

Therefore, the differentiation signal that is output from the differentiation circuit 12 is supplied to the microcomputer 13. The microcomputer 13 detects that the objective lens 3 has passed a position corresponding to time point B by detecting a time point (zero-cross point) when the differentiation signal received reaches the middle level (zero).

When detecting, first time, the position corresponding to time point B, the microcomputer 13 issues, to the changeover switch 19e, an instruction to switch to the K-side, whereby the value of the threshold level A (23a) is supplied to the comparison circuit 22a. The comparison circuit 22a compares the focus error signal that is supplied from the signal processing circuit 7 with the threshold level A (23a) and supplies a comparison result to the microcomputer 13. As the objective lens 3 goes up further, the level of the focus error signal becomes lower than the threshold level A (23a) at time point C, whereupon the comparison circuit 22a supplies a comparison detection signal to the microcomputer 13. When receiving the comparison detection signal, the microcomputer 13 switches the changeover switch 19a to the D-side, whereby the elevation voltage value B (16b) is supplied to the adder 18a.

The adder 18a adds the elevation voltage value B (16b) to a signal that is free of high-frequency noise components (rejected by the LPF 14), and supplies a resulting addition signal to the changeover switch 19c. Since the changeover switch 19c is switched to the B-side, the addition signal is supplied from the changeover switch 19c to the changeover switch 19b as it is. Since the changeover switch 19b is switched to the H-side, the addition signal is supplied to the pickup 4 via the changeover switch 19b. Since the elevation voltage value B (16b) is applied to the actuator, the objective lens 3 continues to go up.

As shown in FIG. 7, after the elevation voltage value switching time point C, the elevation voltage value B (16b; also denoted by "Vup2") is supplied, as it is, to the actuator for driving the objective lens 3. The elevation voltage value B (16b) is set smaller than the elevation voltage value A (16a). Therefore, the elevation speed of the objective lens 3 is lower than when the elevation voltage value A (16a) was applied to the actuator. After time point C has been passed, the microcomputer 13 issues, to the changeover switch 19e, an instruction to switch to the L-side. The changeover switch 19e supplies the threshold level B (23b) to the signal level comparison circuit 22a. The signal level comparison circuit 22a compares the level of the focus error signal that is supplied from the signal processing circuit 7 with the threshold level B (23b). When the level of the focus error signal becomes lower than the threshold level B (23b) (time point F in FIG. 7), the signal level comparison circuit 22a supplies a signal to that effect to the microcomputer 13. When detecting that time point F has been passed, to apply a voltage value for decelerating the objective lens 3 that has continued to go up, the microcomputer 13 issues, to the changeover switch 19c, an instruction to switch to the A-side.

At this time, the focus error signal that is output from the signal processing circuit 7 is supplied to the differentiation circuit 12. A differentiated version of the focus error signal generated by the differentiation circuit 12 is supplied to the multiplier 21. The multiplier 21 supplies the changeover switch 19c with a result obtained by multiplying the output (differentiation signal) of the differentiation circuit 12 by the gain factor 20. Since the changeover switch 19c is switched to the A-side, the multiplication result is supplied to the changeover switch 19b as it is. The signal obtained by multiplying the differentiation signal by the gain factor 20 is supplied, as a deceleration signal, to the actuator via the changeover switch 19b.

In the section from time point F to G, the focus error signal reflects the displacement of the objective lens 3 (it decreases monotonously). Since in general speed is obtained by differentiating displacement with respect to time, a signal obtained by differentiating the focus error signal represents the speed of the objective lens 3. For example, if a high elevation voltage has been applied to the actuator and hence an elevation speed of the objective lens 3 when switching is made to the deceleration voltage is high, the focus error signal falls steeply from time point F to G. Therefore, a signal obtained by differentiating such a focus error signal has a large value, that is, the deceleration voltage has a large value, which means that the force of decreasing the elevation speed of the objective lens 3 is strong. Conversely, if a low elevation voltage has been applied to the actuator and hence an elevation speed of the objective lens 3 when switching is made to the deceleration voltage is low, the focus error signal falls gently from time point F to G. Therefore, a signal obtained by differentiating such a focus error signal has a small value, that is, the deceleration voltage has a small value, which means that the force of decreasing the elevation speed of the objective lens 3 is weak.

A deceleration voltage value corresponding to an elevation speed of the objective lens 3 can be obtained and the elevation speed of the objective lens 3 can be decreased by using a signal obtained by differentiating the focus error signal from time F to G in the above-described manner. The gain factor 20 is used for adjusting the amplitude of the deceleration voltage that is obtained by differentiating the focus error signal.

The objective lens 3 continues to go up even after the application of the deceleration voltage to the actuator. After time point F has been passed, the microcomputer 13 monitors the focus error signal and detects its minimum value.

A method for detecting a minimum value will be described below.

A focus error signal that is output from the signal processing circuit 7 is supplied to the adder 18b and the value holding circuit 25. The adder 18b adds together the offset value 24 and the focus error signal that is supplied from the signal processing circuit 7, and supplies a resulting signal to the comparison circuit 22b. The offset value is used for preventing erroneous detection of a minimum value when the focus error signal is influenced by noise or the like. According to an instruction from the microcomputer 13, the comparison circuit 22b compares the offset-value-added output of the adder 18b with the output of the value holding circuit 25. If the output value of the adder 18b is smaller than the output value of the value holding circuit 25, the comparison circuit 22b supplies a comparison result signal to the value holding circuit 25. In response to the comparison result signal, the value holding circuit 25 updates the value that has been held so far to the value of the focus error signal.

FIG. 11 schematically shows how a minimum value is detected.

In FIG. 11, the solid line represents a focus error signal and a dotted line represents a signal obtained by adding an offset value to the focus error signal. Where no offset value is added to the focus error signal, that is, the offset value is zero, the solid line representing the focus error signal coincides with the dotted line representing the offset-value-added signal. In this case, first, a value of point 1 is held by the value holding circuit 25 as a minimum value. Then, this minimum value is compared with a value of point 2. Since the value of point 2 is smaller than the minimum value, the former is employed as a new minimum value. Then, the new minimum value is compared with a value of point 3. Since the value of point 3 is larger than the minimum value, the minimum value is not updated; the value of point 2 is finally judged as a minimum value of the focus error signal.

A description will now be made of a case where an offset signal is added to a focus error signal.

In the case of detecting a minimum value, the offset value is made a negative value. As a result, a signal obtained by adding the offset value to the original signal (in this case, the focus error signal) has smaller values than the original signal. To detect a minimum value, first, a value of point 1 is held by the value holding circuit 25 as a minimum value. Then, the minimum value is compared with a value of point B obtained by adding the offset value to a value of point 2. Since the value of point B is smaller than the minimum value, the value of point 2 is employed as a new minimum value. Then, the new minimum value is compared with a value of point C that is obtained by adding the offset value to a value of point 3. Since the value of point C is smaller than the minimum value, the value of point 3 is employed as a new minimum value. Then, the new minimum value is compared with a value of point D that is obtained by adding the offset value to a value of point 4. Since the value of point D is smaller than the minimum value, the value of point 4 is employed as a new minimum value. Then, the new minimum value is compared with a value of point E that is obtained by adding the offset value to a value of point 5. Since the value of point E is smaller than the minimum value, the value of point 5 is employed as a new minimum value. Then, the new minimum value is compared with a value of point F that is obtained by adding the offset value to a value of point 6. Since the value of point F is smaller than the minimum value, the value of point 6 is employed as a new minimum value. Then, the new minimum value is compared with a value of point G that is obtained by adding the offset value to a value of point 7. Since the value of point G is larger than the minimum value, the minimum value is not updated; the value of point 6 is finally judged as a minimum value of the focus error signal.

In the example of FIG. 11, the actual minimum value of the focus error signal is the value of point 5. Where the offset value is added to the focus error signal, the value of point 6 is finally judged as a minimum value of the focus error signal. However, if no offset signal is added to the focus error signal, the value of point 2 is finally judged as a minimum value of the focus error signal.

By using a signal obtained by adding an offset value to a focus error signal, a minimum value can be detected without being influenced by noise or disturbance whose amplitude is smaller than t