Title: Optical disk device with improved track jump features
Abstract: An optical disk device starts movement of a thread in a track-on state, and releases the track-on state when the thread moves by a certain amount, and gives a driving kick signal to an actuator. The movement of the thread and actuator can be placed in sufficiently good balance so that failure of track jump is prevented, thereby improving the reliability of the device body and suppressing the cost-up of the device body.
Patent Number: 6,937,546 Issued on 08/30/2005 to Ono
| Inventors:
|
Ono; Takayuki (Osaka, JP)
|
| Assignee:
|
Funai Electric Co., Ltd. (Osaka, JP)
|
| Appl. No.:
|
059346 |
| Filed:
|
January 31, 2002 |
Foreign Application Priority Data
| Feb 15, 2001[JP] | P. 2001-038947 |
| Current U.S. Class: |
369/44.28; 369/30.16; 369/30.17 |
| Intern'l Class: |
G11B 007/08 |
| Field of Search: |
369/4428,301.6,301.7
|
References Cited [Referenced By]
U.S. Patent Documents
| 5200937 | Apr., 1993 | Hosoya et al.
| |
| 5285432 | Feb., 1994 | Nakane.
| |
| 5612933 | Mar., 1997 | Iso et al.
| |
| 5696646 | Dec., 1997 | Satoh.
| |
| 5699332 | Dec., 1997 | Nakano.
| |
| 5870356 | Feb., 1999 | Ikeda.
| |
| 5875161 | Feb., 1999 | Takegawa.
| |
| 5933397 | Aug., 1999 | Yamashita et al.
| |
| 5956299 | Sep., 1999 | Aoki.
| |
| 5959947 | Sep., 1999 | Inoue et al.
| |
| 6064633 | May., 2000 | Kuwayama et al.
| |
| 6078454 | Jun., 2000 | Takahashi et al.
| |
| 6175465 | Jan., 2001 | Kawachi et al.
| |
| 6249496 | Jun., 2001 | Tsukahara et al.
| |
| 6633521 | Oct., 2003 | Mochizuki et al.
| |
| Foreign Patent Documents |
| 10-320938 | Dec., 1998 | JP.
| |
Primary Examiner: Patel; Gautam R.
Attorney, Agent or Firm: Morgan, Lewis & Bockius LLP
Claims
1. An optical disk device comprising:
a thread on which a pick-up head is placed, said pick-up head serving to read
data recorded on an optical disk by irradiating a track formed on a recording face
of said optical disk with an optical beam focused by a lens and detecting the reflected
light,
wherein said pick-up head includes a holder that holds the lens, the holder being
rotatable with respect to the thread;
a lens moving unit adapted to rotate the holder of said pick-up head relative
to said thread;
a thread moving unit adapted to move said thread as well as said pick-up head
in the radial direction of the optical disk; and
a movement controller adapted to control said thread moving unit to start movement
of said thread while controlling said lens moving unit to perform track-on control
so that the lens of said pick-up head is located on a prescribed track, and thereafter
when it is detected that said thread has moved a prescribed amount on the basis
of a tracking servo signal potential starting the rotation of said holder by said
lens moving unit,
wherein said movement controller also detects whether or not said lens and said
prescribed track are displaced from each other because the tracking servo signal
has exceeded a prescribed potential, and when the prescribed potential is exceeded,
controlling said thread moving unit by terminating an applied drive kick signal,
and controlling said lens moving unit by terminating the tracking servo signal
and applying the drive kick signal to said lens moving unit.
2. An optical disk device comprising:
a thread on which a pick-up head is placed, said pick-up head serving to read
data recorded on an optical disk by irradiating a track formed on a recording face
of said optical disk with an optical beam focused by a lens and detecting the reflected
light,
wherein said pick-up, head includes a holder that holds the lens, the holder
being rotatable with respect to the thread;
a lens moving unit adapted to rotate the holder of said pick-up head relative
to said thread;
a thread moving unit adapted to move said thread as well as said pick-up head
in the radial direction of the optical disk; and
a movement controller adapted to control said thread moving unit to start movement
of said threads and thereafter, when it is detected that said lens has deviated
from said prescribed track by a prescribed amount or more owing to movement of
said thread, starting the rotation of said holder by said lens moving unit.
3. The optical disk device according to claim 2, wherein until a center of said
lens deviates from the prescribed track by a prescribed amount or more, said controller
controls said thread moving unit to apply force having a prescribed magnitude to
said thread continuously.
4. The optical disk device according to claim 2, wherein when the center of said
lens deviates from the center of said prescribed track by a prescribed amount or
more, said controller controls said thread moving unit and said lens moving unit
to control the moving speed of the lens at a constant speed.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates an optical disk device for performing read and write
of data for an optical disk such as a CD, MD or DVD, and particularly to control
of track jump of moving a pick-up head to a target track formed on the recording
face of an optical disk.
2. Description of the Related Art
Traditionally, there has been an optical disk for reading the data
recorded on the optical disk such as a CD, MD, DVD, etc. and writing data in the
optical disk. The optical disk includes a plurality of concentric or spiral tracks
formed on the recording face. The optical disk device reads data recorded on the
track by detecting the reflected light of an light beam projected on the track.
Further, the optical disk device writes data in the track by irradiating the light beam.
Where the data recorded on the track is to be read or the data is to be recorded
in the optical disk, in some cases, the optical disk device performs track jump
to move a pick-up head to a target track. The target track refers to a track for
which the read or write of data is performed. The track jump refers to an operation
of moving the irradiating position of an light beam in a radial direction of the
optical disk to locate the position on the target track. For example, JP-A-10-320938
discloses conventional control of the operation relative to track jump in the optical
disk device.
The pick-up head is placed on a thread which is movable in the radial direction
of the optical disk. The thread is adapted to be movable in the radial direction
of the optical disk by a thread motor (hereinafter simply referred to as a motor).
The lens of the pick-up head is adapted to be movable for the thread. The lens
is adapted to be movable in the radial direction of the optical disk by an actuator.
The track jump includes the cases of moving the thread and not moving the thread.
Now, the track jump of moving the thread will be explained below.
First, a kick signal (the kick signal which is first given at the start of
track jump is referred to a driving kick signal) is applied to a motor to start
the movement of the thread. The driving kick signal is given with a prescribed
magnitude and for a prescribed time. After the driving kick signal has been stopped,
the kick signal (hereinafter referred to as an accelerating kick or decelerating
kick) having a magnitude corresponding to the moving speed of the thread is applied
to perform the constant speed control of moving the thread toward the target track
at a constant speed.
The constant speed control employs the wavelength (period) of a tracking error
signal described later. Further, after a lapse of a prescribed time from when the
driving kick signal is applied to the motor, the kick signal is applied to the
actuator so that the moving control of the lens of the thread and actuator is started.
The irradiating position of the light beam successively traverses the tracks
formed on the recording face of the optical disk. A mirror section which totally
reflects the light beam is formed between the adjacent tracks. On the basis of
the amount of light reflected from the optical disk, a sinusoidal tracking error
signal (TE signal) is obtained which represents that the lens center traversed
the track. The tracking error signal is created by a wavelength whenever the lens
center traverses the track. Therefore, by counting the number of the waves of the
tracking error signal, the number of the tracks which the lens center has traversed
is obtained. Further, on the basis of the wavelength (period) of the tracking error
signal, the moving speed of the lens relative to the optical disk is obtained.
Immediately before starting the track jump, the optical disk device computes
the number of tracks over which the lens center must move to reach the target track.
Concretely, the number of tracks to move is computed on the basis of a difference
between the track address representative of the present track position and that
representative of the target track position.
When the lens center approaches the target track to a certain degree, the optical
disk device brakes the motor to stop the thread beforehand. For example, when the
lens center reaches the track located 100 tracks before the target track, the thread
is stopped. Thus, the pick-up head body moves to the vicinity of the target track.
In this case, the optical disk device does not stop the movement of the lens
by
the actuator, but continues the movement of the lens toward the target track. When
the optical disk device judges that the lens center has reached the target track
on the basis of the counted number of waves of the tracking error signal, it causes
the lens center to track on the target track by tracking servo.
Meanwhile, when the driving kick signal is simultaneously applied to the
thread and actuator, since the thread has poorer response than that of the actuator,
the actuator starts to move before the thread starts to move. Therefore, the thread
and the actuator move separately so that the subsequent control cannot be carried
out appropriately. This presents a problem of greatly vibrating the lens and others.
In such a case, there is strong possibility of the track jump ending in failure.
In order to obviate such an inconvenience, a method has been proposed which applies
the driving kick signal to the actuator after a lapse of a prescribed time after
the driving kick signal has been applied to the thread as described above.
However, the time taken from when the driving kick signal is applied to
the motor to when the thread starts to move is affected by the friction force generated
when the thread is moved. The friction force varies according to the elements which
constitute the thread. Therefore, the conventional method which applies the kick
signal to the actuator after a lapse of a prescribed time provides some devices
in which the thread has already moved excessively and some devices in which the
thread does not almost move (there are unevenness among the devices). Thus, in
order to control the thread and actuator in good balance, the above prescribed
time from when the driving kick signal is applied to the motor to when the kick
signal is applied to the actuator must be set for each device. This presents a
problem of complicating the process of manufacturing the device body and hence cost-up.
SUMMARY OF THE INVENTION
This invention has been made under the above circumstances, and therefore an
object of this invention is to provide an optical disk which can perform track
jump involved with movement of a thread appropriately and also can restrain cost-up.
In order to achieve the above object, an optical disk according to this invention
has the following configurations in order to solve the above problem.
(1) The optical disk device comprises:
a thread on which a pick-up head is placed, said pick-up head serving to read
data
recorded on an optical disk by irradiating a track formed on a recording face of
said optical disk with an optical beam focused by a lens and detecting the reflected light;
a lens moving means for moving the lens of said pick-up head relative to said
thread
in a radial direction of said optical disk;
a thread moving means for moving said thread as well as said pick-up head in
the
radial direction of the optical disk; and
a movement control means for controlling said thread moving means to start movement
of said thread, and thereafter when it is detected that said lens has deviated
from said prescribed track by a prescribed amount or more owing to movement of
said thread, starting the movement of said lens by said lens moving means.
(2) The above movement control means, until said lens deviates from the prescribed
track by a prescribed amount or more, controls said thread moving means to apply
force having a prescribed magnitude to said thread continuously.
(3) The above movement control means, when the center of the lens has deviated
from the center of the predetermined track, controls said thread moving means and
said lens moving means to control the moving speed of the lens at a constant speed.
In this configuration, in a "track-on" state where the center of the lens of
the
pick-up head is located on the center of a predetermined track, the movement of
the thread is started. Therefore, when the thread does not yet move, or has moved
a little, the center of the lens is situated on the center of the predetermined
track. When the thread starts to move, the body of the pick-up head will gradually
deviate from the center of the predetermined track with movement of the thread.
However, because of the track-on state, control is made so that the lens is situated
on the center of a predetermined track by the lens moving means. Thus, the force
taken for the lens moving means to situate the lens on the center of the predetermined
track, i.e. "tracking servo signal" increases gradually. By detecting the magnitude
of the tracking servo signal, the amount of movement of the thread can be detected.
When the thread has moved by a suitable amount, i.e. the tracking servo-signal
has increased by a predetermined amount or more, the track-on state is released
and the movement of the lens is controlled at a constant speed by the thread moving
means and the lens moving means.
Thus, without being affected by unevenness in the components constituting the
thread, whenever the thread moves by a suitable amount, the movement of the lens
can be started by the lens moving means. Thus, during the track jump, the movement
of the thread and movement of the lens by the lens moving means can be stabilized.
This prevents the failure of track jump, thereby improving the reliability of the
device body and also suppressing the increase of the production cost of the device body.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing the configuration of the main part of an optical disk
device which is an embodiment of this invention;
FIG. 2 is a view showing the thread and holder on which the pick-up head of
the optical disk device which is an embodiment of this invention;
FIG. 3 is a view showing a section of the optical disk;
FIG. 4 is a flowchart showing the processing of track jump of the optical disk
device which is an embodiment of this invention;
FIG. 5 is a timing chart showing the control of a thread and an actuator during
track jump in the optical disk device which is an embodiment of this invention;
FIG. 6 is a timing chart showing the control of a thread and an actuator during
track jump in the optical disk device which is an embodiment of this invention;
FIG. 7 is a timing chart showing the control of a thread and an actuator during
track jump in the optical disk device which is an embodiment of this invention; and
FIG. 8 is views showing the movement of the thread and actuator of the optical
disk device which is an embodiment of this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now, a description will be given in more detail of preferred embodiments of
this invention with reference to the accompanying drawings.
FIG. 1 is a block diagram showing the construction of the main portion of an
optical disk device which is an embodiment of this invention. An optical disk device
1 according to this embodiment serves to read the data recorded in the optical
disk
10 such as a CD, MD, DVD, etc. or write the data in the optical disk
10. Reference numeral
2 denotes a control section for controlling
the operation of the device body. Reference numeral
3 denotes a spindle
motor for rotating the optical disk
10 loaded in the device body. Reference
numeral
4 denotes a pick-up head for irradiating the optical disk with a
light beam and detecting the light reflected from the optical disk
10. The
pick-up head
4 includes a light-projecting unit, a four-divided photo-diode
and an objective lens
4a.
As shown in FIG. 2, the pick-up head
4 is placed on a thread
11.
The thread
11 is adapted to be movable in the radial direction (direction
indicated by an arrow in FIG. 2) of the optical disk
10 by a thread motor
(not shown). The objective lens
4a of the pick-up head
4 is
attached to a holder
12. The holder
12 is rotatably attached to a
rotary shaft provided in the vicinity of the center of the thread
11. The
holder
12 rotates in the direction indicated by arrows in FIG.
2.
The holder
12 is rotated by the actuator (not shown). The rotation of the
holder
12 moves the lens
4a in the radial direction of the
optical disk
10. Incidentally, it should be noted that the holder
12
is adapted to be movable also in the direction of the rotary shaft of the spindle
motor
3 for rotating the optical disk
10.
Reference numeral
5 denotes a tracking driver for applying a control
signal (kick signal, brake signal, tracking servo signal (TS signal), etc.) to
the above actuator to rotate the holder
12 relative to the thread
11.
The rotation of the holder
12 moves the lens
4a in the radial
direction of the optical disk
10.
Reference numeral
6 denotes a thread driver for applying a control
signal (kick signal, brake signal, etc.) to the above thread motor to move the
thread
11 in the radial direction of the optical disk
10. With the
movement of the thread
11, the pick-up head
4 moves in the radial
direction of the optical disk
10.
Reference numeral
7 denotes a servo-processor for applying a signal
to the tracking driver
5 and thread driver
6. Reference numeral
8
denotes an R-F amplifier for generating a tracking error signal described later.
With the aid of a split photo-diode, the pick-up head
4 detects the reflected
light of a light beam irradiating the optical disk
10 from a projecting
section, and supplies the output from the split photo-diode to the RF amplifier
8. The RF amplifier
8 creates a tracking error signal on the basis
of the signal (output from the split photo-diode) supplied from the pick-up head
4. The tracking error signal thus created is supplied to the servo-processor
7.
Incidentally, it should be noted that during reproducing, the RF amplifier
8 creates a reproduction signal on the basis of the output from the split
photo-diode. The reproduction signal is produced from an output section (not shown).
During recording, the control section
2 controls the projecting section
of the pick-up head
4 to write the data in the optical disk
10. The
details of the operation during the reproducing and recording will not be explained here.
The tracking error signal represents a deviation of the position irradiated with
the light beam from the center of a track formed on the optical disk
10.
The irradiated position is a position opposite to the center of the lens
4a.
The servo-processor
7 exerts tracking servo in a track-on state. Exerting
the tracking servo refers to apply the tracking servo signal created on the basis
of the tracking servo signal to the tracking driver
5. The tracking driver
5 drives the actuator on the basis of the tracking servo signal to locate
the center of the lens
4a, i.e. position irradiated with the light
beam to the center of the track.
An explanation will be given of the operation of track jump in the optical disk
device
1 according to this embodiment. As well known, the optical disk
10
includes a plurality of tracks
21 spirally or concentrically formed on the
recording face. A mirror portion
22 is formed between the adjacent tracks
21 (FIG.
3). The mirror portion
22 is a total reflection region.
Incidentally, FIG. 3 shows a section of the optical disk
10 with the lower
face being the recording face. It should be noted that the optical disk
10
actually includes the tracks
21 formed more densely.
The operation of track jump will be explained taking an example from the track
jump from track A to track C shown in FIG.
3. The optical disk device
1
has recognized the positions of the track A and the track C. It is assumed that
the track A and the track C are apart from each other by several hundreds of tracks.
In FIG. 3, it is assumed that track B is located between the track A and track
C, and apart from the track C by 100 tracks.
An output from the split diode, which is the reflected light of the light beam
with which the recording face of the optical disk is irradiated is detected, is
supplied to the RF amplifier
8. On the basis of the supplied output from
the split photodiode, the RF amplifier
8 creates a tracking error signal.
FIG. 4 is a flowchart showing the operation of track jump. FIGS. 5 to
7
are timing charts which show the control of track jump. In the optical disk
1,
the number of tracks from the track A to the track C, i.e. the number of jumping
tracks is computed (s
1). It is decided whether the number of tracks computed
in s
1 is not smaller than a prescribed number of tracks (hereinafter referred
to as "prescribed number of tracks") (s
2). Now it is assumed that the prescribed
number of tracks is 101.
If it is decided in s
2 that the computed number of tracks is smaller than
the prescribed number of tracks, the tracking servo is turned off (s
3),
and a driving kick signal is applied to the actuator. Thereafter, the processing
jumps to s
13 described later.
On the other hand, if it is decided in s
2 that the computed number of
tracks
is not smaller than the prescribed number of tracks, the processing of s
5
to s
12 described below is executed. Thereafter, the processing of s
13
et seq. will be executed.
A driving knock signal having a prescribed magnitude is applied to the thread
motor
(s
5) (T
1 in FIG.
5). At this time, the tracking servo is still
active in the track-on state. Therefore, the actuator is controlled by the tacking
driver S so that the center of the lens
4a is located on the track
A. Although not shown in FIG. 5, the tracking error signal is at a substantially
constant level.
The thread
11 starts to move toward a target track C after a short time
from when the driving kick signal has been applied (T
2 in FIG.
5).
Therefore, the tracking sever signal applied to the actuator by the tracking driver
5 increases gradually. This is because the pick-up head
4 as well
as the thread
11 moves toward the target track C when the thread
11
starts to move the target track C. Specifically, in a state shown in FIG. 8A, when
the thread
11 starts to move toward the target track C as a result of application
of the driving kick signal to the thread
11, since the tracking servo is
active, the actuator rotates the holder
12 according to the amount of movement
of the thread to locate the center of the lens
4a to the track A
(FIG.
8B). Thus, the tracking servo signal applied to the actuator gradually increases.
When the tracking servo signal reaches a prescribed level (s
6) (T
3
in FIG.
5), the tracking servo is turned off and the driving kick signal
is applied to the actuator (s
7, s
8). Further, application of the
driving kick signal to the thread motor is stopped (s
9). It should be noted
that the prescribed driving signal having a predetermined magnitude is applied
to the actuator for a prescribed time.
Thereafter (after T
4 shown in FIG.
5), the accelerating
kick and deceleration kick are applied to the thread
11 and actuator (s
10,
s
11) to perform constant speed control until the center of the lens
4a
reach the track B (s
10, s
11). The control is made so that the
actuator is located at a reference position relative to the thread
11 (FIG.
8C), and the speed of the lens
4a is constant.
Now referring to FIG. 6, an explanation will be given of the constant speed control
of the thread
11. The servo processor
7 detects can detect the moving
speed of the thread
11 by detecting the period of the tracking error signal
supplied from the RF amplifier
8. As described above, since the tracking
error signal is a sinusoidal signal produced whenever the center of the lens
4a
traverses each of the tracks
21, the period of the tracking error signal
varies according to the moving speed of the thread
11. Specifically, as
the moving speed of the lens
4a becomes high, the period of the tracking
error signal becomes short. Inversely, as the moving speed of the lens
4a
becomes low, the period of the tracking error signal becomes long.
The servo processor
7 applies the accelerating kick or decelerating kick
to the tracking driver
5 and the thread driver
6 as a servo signal
so that the period (T
11-T
19 in FIG. 6) of the tracking error signal
is equal to a prescribed period. The tracking driver
5 and the thread driver
6 control the moving speed of the lens
4a on the basis of
the accelerating kick or decelerating kick thus applied.
Incidentally, by counting the number of waves of the tracking error
signal, the number of tracks which the pick-up head
4 (lens
4) has
crossed can be detected.
As described above, at the timing when the thread
11 moved by a prescribed
amount the tracking servo is turned off, and the moving control of the actuator
is started. For this reason, the actuator can be moved at the timing when the thread
has moved by the prescribed amount without being affected by unevenness in the
components constituting the thread and in the friction force produced when the
thread
11 has been moved. Thus, the thread
11 and actuator can be
always situated in good balance.
When the center of the lens
4a reaches (ta in FIG.
6),
the thread motor is braked down (s
12). Thereafter, the constant speed is
performed by the actuator until the lens
4a reaches the track C (s
13, s
14).
In this case also, the RF amplifier
8 applies the tracking error signal
created on the basis of the input from the pick-up head
4 to the servo processor
7. The servo-processor
7 performs the constant speed control of moving
the lens
4a at a prescribed constant speed with the aid of the actuator
until the center of the lens
4a reaches ½ track before the track C.
As seen from FIG. 7, the constant speed control in s
13 is to apply the
accelerating kick or decelerating kick to the tracking driver
5 and the
thread driver
6 so that the period (T
21-T
27) of the tracking
error signal supplied from the RF amplifier
8 is equal to a prescribed period.
When the center of the lens
4a reaches ½ track before the
track C (tb in FIG.
7), the tracking driver
5 is braked to stop the
movement of the lens
4a by the actuator (s
15).
In s
15, the servo processor
7 controls the amount of braking according
to the moving speed of the lens
4a when it has crossed the immediately
preceding track. Specifically, the braking amount is controlled to the period of
the sinusoidal wave produced when the lens
4a has crossed the immediately
preceding track in the tracking error signal supplied from the RF amplifier
8,
i.e. the amount according to T
27 in FIG.
7. Incidentally, the control
of the amount of braking may be made in terms of either or both of the time t of
braking and value of the braking.
When the servo processor
7 completes application of the braking signal,
it waits for the level of the tracking error signal supplied from the RF amplifier
8 to fall to a prescribed threshold SH or lower (s
16).
Incidentally, it should be noted that the lens
4a does
not stop completely, but is moving at a speed approximately zero toward the track C.
When the level of the tracking error signal falls to the threshold SH, the servo-processor
7 exerts the tracking servo (s
17).
As shown in FIG. 7, the threshold SH is set at a level where the center of the
lens
4a is located on the track C. The servo-processor
7 does
not apply the accelerating kick or decelerating kick to the tracking driver
5
during the period between the timing tc when the braking has been completed and
the timing td when the level of the tracking error signal has fallen to the threshold
SH or lower.
Therefore, the servo-processor
7 exerts the tracking servo when
the center of the lens
4a is located on the track (track C), thereby
realizing the track-on.
In the above description, the tracking servo is exerted when the level of the
tracking error signal has fallen to the threshold SH or lower, thereby realizing
the track-on. However, the tracking servo maybe exerted while the level of the
tracking error signal is within a prescribed range and has fallen, thereby realizing
the track-on.
Further, when the servo-processor
7 exerts the tracking servo, it
also produces a focus-servo signal to make focus adjustment of the lens
4a.
The focus adjustment is carried out in such a way that the lens is driven by an
actuator for focusing (not shown).
Thus, without being affected by unevenness in the components constituting the
thread and in the friction force produced when the thread
11 has been moved,
the optical disk device according to this invention can stabilize the balance between
the movement of the thread
11 when the driving kick signal therefor has
stopped and that of the lens
4a by the actuator. Therefore, since
the processing in and subsequent to step s
8 and is stably performed in FIG.
4 described above, the failure of track jump can be prevented, thereby improving
there liability of the device body and suppressing the increase of the production
cost of the device body.
Further, in order to stop the center of the lens
4a on the
target track, the amount of braking applied to the actuator is adjusted to the
magnitude corresponding to the immediately preceding moving speed of the lens
4a,
and the tracking servo is not exerted for track-on immediately after the braking
has been completed, but the track-on is realized after the center of the lens
4a
is located on the target track. For this reason, when the center of the lens
4a is located on the mirror portion
22, the tracking servo
is not exerted to realize the track-on and occurrence of track slippage is prevented.
Thus, the operation of track jump permits the center of the lens to be located
on the target track, thereby improving the reliability of the device body.
As described above, in accordance with this invention, without being affected
by unevenness in the components constituting the thread, whenever the thread moves
by a suitable amount, the movement of the lens can be started by the lens moving
means. Thus, during the track jump, the movement of the thread and movement of
the lens by the lens moving means can be stabilized. This prevents the failure
of track jump, thereby improving the reliability of the device body and also suppressing
the increase of the production cost of the device body.
*
