Device for and method of focus control, and information reproduction apparatus having the device

The first detection unit detects a focus search state in which focus search is required. The second detection unit detects one of the plurality of layers currently being accessed, i.e., information is being recorded or reproduced. Then, the focus search unit moves a light beam relative to the plurality of layers to obtain a focused state of the light beam on a desired one of the plurality of layers when the first detection unit detects the focus search state. Importantly, the focus search unit moves the light beam from one side of two surfaces of the storage medium, which is closer to the layer detected by the second detection unit, to the other side of the two surfaces of the storage medium. Therefore, the focus search can be quickly completed, and hence the reproduction of information can also be performed quickly.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an information storage medium such as a 
multi-layer optical disc including Compact Disc, Compact Disc-Recordable, 
DVD, DVD-Recordable, DVD-RAM, etc., on and from which information is 
recorded and reproduced. More specifically, the present invention relates 
to the focus servo control technique for focusing the light beam on the 
respective layers of the multi-layer optical disc (so-called "focus 
search"). 
2. Description of the Prior Art 
Recently, there are known a multi-layer information storage medium having 
multiple recording layers and an information recording/reproducing 
apparatus for recording and/or reproducing information on and/or from the 
multi-layer storage medium. Such information recording/reproducing 
apparatus is provided with an optical pickup generally including a laser 
diode for emitting a light beam, a polarization beam splitter, an 
objective lens and photodetectors. At the time of reproducing information, 
the optical pickup emits the light beam and receives the reflected light 
from the optical disc to reproduce recorded information. At the time of 
recording information, the apparatus modulates the light beam by the 
information to be recorded, and then irradiates the light beam thus 
modulated on the storage medium to record information. 
When irradiating the light beam on the storage medium, it is necessary to 
focus the light beam on the recording surface of the optical disc 
(so-called "focus search") and then constantly maintain the focused state 
of the light beam on the layer of the storage medium (so-called "focus 
servo control"). The conventional information reproduction apparatus 
generally moves the objective lens vertically in relation to the layer of 
the optical disc to execute the focus search and the focus servo control. 
The focus search is also required to restore the focused state of the 
light beam and restart the focus servo control when the focused state is 
lost (i.e., out-of-focus state). 
In recording and/or reproducing information on and/or from the multi-layer 
optical disc, the focus search should be performed to detect a certain 
target layer with which the recording and/or the reproduction is carried 
out. In such case, the conventional apparatus first moves the light beam, 
and accordingly moves the focal point of the light beam, toward the 
position of the pickup beyond the layer of the disc closest to the optical 
pickup, and subsequently starts the focus search from one side of the disc 
near to the pickup to the other side of the disc remote from the pickup. 
Specifically, the focal point is temporarily moved toward the pickup 
beyond the layer of the disc closest to the pickup so that the focal point 
is positioned out of the optical disc. Subsequently, the objective lens is 
moved remotely from the pickup so that the focal point passes through the 
respective layers of the disc and the so-called "S-curves" in the focus 
error signal is detected and counted, thereby permitting the focal point 
to be positioned on the target layer. 
In this method, however, the detection of the target layer may sometimes 
take relatively long period of time. Namely, assuming that the focused 
condition is lost during the reproduction of the fifth layer of the 
6-layered disc, the pickup is temporarily moved to the original position 
and then moved toward the disc until the fifth S-curve, corresponding to 
the fifth layer, is detected. Namely, the focal point of the light beam is 
once moved out of the disc and then moved to traverse four layers before 
reaching the fifth layer. In this way, the movement of the pickup is 
relatively large, particularly for the disc having large number of layers, 
and the time required to restore the focused state is necessarily 
relatively long. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a focus servo control 
device, with which the focus search in the multi-layer storage medium can 
be quickly completed. 
It is another object of the present invention to provide a method for the 
focus servo control, using the above focus servo control device. 
According to one aspect of the present invention, there is provided a focus 
control device for performing focus servo control with respect to an 
information storage medium having a plurality of layers, including: a 
first detection unit for detecting a focus search state in which focus 
search is required; a second detection unit for detecting one of the 
plurality of layers currently being accessed; and a focus search unit for 
moving a light beam relative to the plurality of layers to obtain a 
focused state of the light beam on a desired one of the plurality of 
layers when the first detection unit detects the focus search state, 
wherein the focus search unit moves the light beam from one side of two 
surfaces of the storage medium, which is closer to the layer detected by 
the second detection unit, to the other side of the two surfaces of the 
storage medium. 
In accordance with the device thus configured, the first detection unit 
detects a focus search state in which focus search is required. The second 
detection unit detects one of the plurality of layers currently being 
accessed, i.e., information is being recorded or reproduced. Then, the 
focus search unit moves a light beam relative to the plurality of layers 
to obtain a focused state of the light beam on a desired one of the 
plurality of layers when the first detection unit detects the focus search 
state. Importantly, the focus search unit moves the light beam from one 
side of two surfaces of the storage medium, which is closer to the layer 
detected by the second detection unit, to the other side of the two 
surfaces of the storage medium. Therefore, the focus search can be quickly 
completed because the distance of the pickup movement is shortened, and 
hence the reproduction of information can also be performed quickly. 
The focus search unit may include: an S-curve detection unit for detecting 
an S-curve in a focus error signal; and a determination unit for 
determining the focused state when the S-curve detection unit detects the 
S-curve corresponding to the desired one of the layers. By this, the focus 
search can be readily performed by detecting the S-curves in the focus 
error signal, and the system configuration can also be simplified. 
The focus search unit may further include a counter for counting a number 
of the S-curves to detect the S-curve corresponding to the desired one of 
the layer. Further, the focus search unit may include: a memory for 
storing a total number of the layers of the storage medium and an 
identification information of the layer detected by the second detection 
unit; and a controller for controlling the movement of the light beam 
relative to the layers based on the total number and the identification 
information stored in the memory. Still further, the focus search unit may 
further include a unit for moving the light beam to traverse the layers to 
detect a total number of the layers of the storage medium prior to 
recording and/or reproduction of information on and/from the storage 
medium. 
In one embodiment, the first detection unit may detect the focus search 
state when the light beam becomes out-of-focus with respect to the layer. 
In another embodiment, the first detection unit may detect the focus 
search state when a focus search instruction is inputted. 
According to another aspect of the present invention, there is provided an 
information reproduction apparatus including: the focus servo device 
described above; a beam irradiation unit for irradiating a light beam on 
the storage medium; a beam receiving unit for receiving the light beam 
reflected by the storage medium to produce a light reception signal; and a 
reproduction unit for applying a signal processing onto the light 
reception signal to output a reproduction signal. 
According to still another aspect of the present invention, there is 
provided a focus control method for performing focus servo control with 
respect to an information storage medium having a plurality of layers, 
including the steps of: detecting a focus search state in which focus 
search is required; detecting one of the plurality of layers currently 
being accessed; and moving a light beam relative to the plurality of 
layers to obtain a focused state of the light beam on a desired one of the 
plurality of layers when the focus search state is detected, wherein the 
moving step moves the light beam from one side of two surfaces of the 
storage medium, which is closer to the layer detected by the second 
detection unit, to the other side of the two surfaces of the storage 
medium. 
The nature, utility, and further features of this invention will be more 
clearly apparent from the following detailed description with respect to 
preferred embodiment of the invention when read in conjunction with the 
accompanying drawings briefly described below.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the present invention will now be described 
below with reference to the attached drawings. 
First, the description will be given of the configuration of the 
information reproduction apparatus according to the present invention with 
reference to FIG. 1. As shown in FIG. 1, the information reproduction 
apparatus SP includes the optical pickup 2, the RF amplifier 3, the 
decoder 4, the CPU 5, the serve IC 6, the input unit 7, the memory 8, the 
focus actuator 9 and the signal processor unit 10. The optical pickup 2 
includes an objective lens R (shown in FIG. 2) for converging the light 
beam L onto one of the layers of the optical disc 1 and a cylindrical lens 
(not shown) for giving an astigmatism to the reflected light from the 
optical disc 1. The optical pickup 2 irradiates the light beam L onto the 
optical disc 1 and receives the reflected version of the light beam L from 
the optical disc 1 to convert it to an electric signal S. The RF amplifier 
3 produces the focus error signal Sfe and the RF signal Srf from the 
electric signal S supplied from the optical pickup 2, and then amplifies 
those signals. The decoder 4 decodes the amplified RF signal Srf to 
produce the reproduction signal Sp corresponding to the recorded 
information, and supplies it to the signal processor unit 10. The servo IC 
6 produces the focus drive signal Sfd and carries out the focus servo 
control for the light beam L. The CPU 5 communicates with the servo IC 6 
using the control signal Sc to control the focus servo operation by the 
servo IC 6. In addition, the CPU 5 performs total control of the 
information reproduction apparatus SP. The input unit 7 is used to input 
necessary information to the CPU 5, and the memory 8 temporarily stores 
certain information necessary for the control processing, including the 
focus servo control by the CPU 5. The focus actuator 9 drives the 
objective lens in the optical pickup 2 in the vertical direction towards 
the optical disc 1 based on the focus drive signal Sfd. The signal 
processor unit 10 applies necessary processing onto the reproduction 
signal Sp and outputs the resultant signal to an external unit. 
FIG. 2 shows the vertical section of the optical disc 1 shown in FIG. 1. 
The optical disc 1 includes the first layer LY.sub.1 and the second layer 
LY.sub.2, each carrying different information in the form of the recording 
pits formed thereon. The optical disc 1 is irradiated with the light beam 
L for reproducing the recorded information from the lower surface side of 
the disc 1. The focus actuator 9 controls the position of the objective 
lens R relative to the optical disc 1 such that the light beam L is 
converged on the first layer LY.sub.1 to reproduce information therefrom. 
Similarly, the focus actuator 9 controls the position of the objective 
lens R relative to the optical disc 1 such that the light beam L is 
converged on the second layer LY.sub.2 to reproduce information therefrom. 
In the following description, the surface of the disc on the side of the 
pickup (i.e., the lower surface of the optical disc 1 in this example) on 
which the light beam is irradiated will be referred to as "beam 
irradiation surface", and the opposite surface (i.e., the upper surface of 
the optical disc 1 in this example) on which the light beam is not 
irradiated will be referred to as "beam non-irradiation surface". 
Next, the focus servo control according to the present invention will be 
described with reference to FIGS. 3, 4A and 4B. Here, it is assumed that 
the total number of the layers of the optical disc 1 ("2" in the example 
of the optical disc shown in FIG. 2) is inputted, in advance, from the 
input unit 7 to make the CPU 5 know the total number. Alternatively, the 
total number of layers in the optical disc 1 may be recorded on the 
optical disc 1 at the innermost circumferential area thereof so that the 
CPU 5 can read it to know the total number of the layers of the disc prior 
to the start of the reproduction. Also, it is noted that the CPU 5 always 
recognizes which one of the plural layers is being accessed (i.e., read or 
written), and memorizes such information in the memory 8, so as to control 
the servo IC 6. 
In the focus servo control, first of all, it is determined whether or not 
the out-of-focus state takes place (step S1). Here, the "out-of-focus 
state" means such a situation that the focal point of the light beam L is 
displaced from the surface of the layers of the optical disc due to the 
external vibration or the like. If No, the reproduction of the recorded 
information is continued. If the out-of-focus state takes place (step S1; 
Yes), then it is determined whether or not the layer being accessed prior 
to the detection of the out-of-focus state is nearer to the beam 
irradiation surface than to the beam non-irradiation surface (step S2). In 
step S2, the CPU 5 makes determination based on the total number of the 
layer of the optical disc 1 and the identification of the layer from which 
the recorded information is currently read out. It is noted that the CPU 5 
always recognizes which one of the layers is being accessed. Specifically, 
if the out-of-focus state takes place during the reproduction of the first 
layer LY.sub.1, the CPU 5 determines that the layer currently being 
reproduced is near to the beam non-irradiation surface. On the contrary, 
if the out-of-focus state takes place during the reproduction of the 
second layer LY.sub.2, the CPU 5 determines that the layer currently being 
reproduced is near to the beam irradiation surface. 
If step S2 results in Yes, the objective lens R is temporarily moved 
downwardly to the lower surface side of the optical disc 1 to the position 
below the second layer LY.sub.2 (step S5). Then, the servo IC performs, 
under the control by the CPU 5, the focus search upwardly from the 
position (i.e., in the direction toward the beam non-irradiation surface) 
on the basis of the total number of the layer and the identification of 
the layer currently being accessed (step S6, see. FIG. 4A). This focus 
search counts the number of the S-curves (the S-shape portions in the 
focus error signal Sfe observed when the focal point of the light beam L 
passes through the layers), which appear in the focus error signal Sfe 
during the upward movement of the light beam L, to detect the position of 
the layer to which the light beam to be focused. In this embodiment, if 
the out-of-focus state takes place during the reproduction of the second 
layer LY.sub.2, the focus search in step S6 is terminated when the initial 
S-curve corresponding to the layer LY.sub.2 is detected. After the layer 
to be focused is detected in step S6, the focus servo control is resumed 
from the detected position (step S7). Subsequently, it is determined 
whether or not the reproduction is completed (step S8). If Yes, the 
process ends. If No, the process returns to step S1 to repeat the process. 
On the other hand, if step S2 results in No, the objective lens R is 
temporarily moved upwardly to the upper surface side of the optical disc 1 
to the position above the first layer LY.sub.1 (step S3). Then, the servo 
IC performs, similarly to step S6, the focus search downwardly from the 
position on the basis of the total number of the layer and the 
identification of the layer being accessed (step S4, see. FIG. 4B). This 
focus search counts the number of the S-curves, which appear in the focus 
error signal Sfe during the downward movement of the light beam L, to 
detect the position of the layer to which the light beam to be focused. In 
this embodiment, if the out-of-focus state takes place during the 
reproduction of the first layer LY.sub.1, the focus search in step S4 is 
terminated when the initial S-curve corresponding to the layer LY.sub.1 is 
detected. When the layer to be focused is detected in step S4, the focus 
servo is resumed from the detected position (step S7). Thereafter, step S8 
described above is executed. 
As described above, according to information reproduction apparatus of the 
present invention, the focus search is started from the side of one of the 
beam irradiation surface and the beam non-irradiation surface, which is 
nearer to the layer being currently accessed. Therefore, the time period 
necessary for the completion of the focus search may be shortened. For 
example, in the case of DVD recently broadly known, the focus search for 
one layer requires approximately 1.5 msec. Therefore, as compared with the 
case where the focus search is made from the remote surface side, the 
focus search time is reduced for 1.5 msec. Further, since the focus search 
for each layer is started when the out-of-focus state takes place, it is 
possible to quickly complete the focus search to resume focus servo 
control. Still further, since the focus search is made by moving the 
objective lens R in the vertical direction relative to the layers, the 
focus search can be performed with simple configuration. Still further, as 
a result of the acceleration of the focus search, information reproduction 
may be quickly made. 
In the above described embodiment, the CPU 5 recognizes the total number of 
the layers of the optical disc 1 through the input to the input unit 7 by 
the operator. Alternatively, the total number of the layer may be checked, 
in advance of the actual recording and/or reproduction of the optical disc 
1, by moving the objective lens R in the vertical direction to detect the 
number of the layers as the number of the S-curves. By this, the total 
number of the layers may be accurately recognized by actually detecting 
the number in the form of the S-curves, and hence erroneous input by human 
being may be avoided. 
In the above embodiment, the focus search is made when the out-of-focus 
state takes place. However, the focus search according to the present 
invention may be performed upon the arbitrary instruction for changing the 
layer, inputted to the input unit 7 as the signal Sin, by the operator. 
Further, the focus search may be performed at the time of the necessary 
movement of the information recording and/or reading position from one 
layer to another during the reproduction. Still further, the focus search 
may be performed at the time of recording information onto the optical 
disc. Namely, the information recording apparatus employing the focus 
search function of the present invention may detect the appropriate 
recording position on one of the plural layers of the optical disc to 
record information thereon. In this case, the pickup irradiates the light 
beam L which is modulated by the information to be recorded. 
The present invention is applicable to the recording and/or reproduction of 
the optical disc having more than two layers. For example, if the 
out-of-focus state takes place during the access to the fourth layer (from 
the beam irradiation surface side) of the optical disc having five layers 
in total, the focal point of the light beam is moved beyond the layer 
closest to the beam non-irradiation surface to a certain position 
(including the position out of the optical disc), and then the focus 
search is started therefrom to toward the beam irradiation surface, 
thereby returning to the fourth layer of the disc. On the other hand, if 
the out-of-focus state takes place at the second layer (from the beam 
irradiation surface side) of the optical disc having five layers in total, 
the focal point of the light beam is moved beyond the layer closest to the 
beam irradiation surface to a certain position (including the position out 
of the optical disc), and then the focus search is started therefrom to 
toward the beam non-irradiation surface, thereby returning to the second 
layer of the disc. In this way, the more layers the optical disc has, the 
greater the reduction in time of the focus search may be. 
The above described embodiment is directed to recording and/or reproduction 
of information on and/or from the optical disc. However, the present 
invention is applicable to an information storage medium of other type, 
such as a tape-shaped storage medium, on which information 
recording/reproduction is made using the light beam. 
The invention may be embodied on other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments therefore to be considered in all respects as illustrative and 
not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description and all changes 
which come within the meaning an range of equivalency of the claims are 
therefore intended to embraced therein.