Optical disc apparatus having improved read accuracy with non-linear servo signal gain during track access and focus initialization

A head signal process circuit 18 generates a focus error signal representing the distance between an optical disc 11 and an objective lens 20 using a detection signal detected by an optical detector of an optical head 14. The focus error signal is sent to a non-linear amplifying circuit 21. The level of the focus error signal in the vicinity of a servo point is amplified. The level of the focus error signal apart from the servo point is suppressed. The output of the non-linear amplifying circuit 21 is sent to an A/D converter 22. Thus, a servo digital signal is generated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical disc apparatus for optically 
processing information over an information storage medium such as an 
optical disc. 
2. Description of the Related Art 
In recent years, as optical disc apparatuses which process information over 
an information storage medium, optical disc apparatuses have been actively 
used. On an optical disc as an information storage medium, spiral-shaped 
or concentric-shaped information tracks named grooves are formed. In the 
optical disc apparatus, by forming physical holes, changing magnetic 
characteristics, or changing metal state at an information track or 
between information tracks, information is written (recorded) or read 
(reproduced). 
Generally, in the optical disc apparatus, as an optical disc is rotated, a 
signal surface on which an information signal is recorded vertically moves 
due to machining accuracy, rotation accuracy, and so forth. To precisely 
read the information signal, the optical disc apparatus is generally 
provided with a focusing function for keeping the distance between an 
objective lens (which focuses a light beam of a light source on a target) 
and an information storage medium constant. In other words, a focus servo 
system is used which drives the objective lens corresponding to the 
vertical movement of the signal surface so that the objective lens focuses 
a laser beam irradiated by an optical head on the signal surface of the 
information storage medium. 
As described in the Japanese Patent Application Laid-open No. 
HEI4(1992)-49530, a light beam reflected from the information storage 
medium is detected by an optical head. The optical head outputs a 
detection signal. The detection signal is converted into a digital signal 
by an A/D converter. Corresponding to the resultant signal, the objective 
lens is driven by a digital servo system. 
The output range of the detection signal is limited. Thus, when the optical 
disc apparatus is operated, the objective lens should be aligned in such a 
range. To do that, the optical disc apparatus is provided with a circuit 
which reciprocally moves the objective lens over the optical disc using 
triangular pulses or the like so as to seek a focus position. 
In addition, the optical disc apparatus is provided with a function for 
causing the focused light spot to target track on the information storage 
medium. This function is referred to as the tracking function. The 
tracking function is accomplished in the following manner. A detection 
signal (representing the distance between a target track on the 
information storage medium and the light spot) is generated corresponding 
to the light beam reflected from the information storage medium. The 
detection signal is converted into a digital signal by an A/D converter. 
Corresponding to the digital signal, the objective lens is driven by a 
digital servo system. 
As described above, in the conventional optical disc apparatus, both the 
detection signal for use in the focus servo system and the detection 
signal for use in the tracking servo system are converted into digital 
signals by the A/D converters. 
However, in the optical disc apparatus, an optical disc can be replaced 
with another one. Thus, the level of signal varies corresponding to the 
reflection ratio, the shape of guide grooves, and emboss data (sector mark 
and so forth). In addition, when the focus servo operation or tracking 
servo operation is performed, a small variation of signal should be read 
in the vicinity of a servo point. Thus, the A/D converters should have 
wide dynamic range and high accuracy. Moreover, when one digit deviates in 
the A/D converters, the focus servo system is adversely affected. As a 
result, the servo function may not correctly work. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an optical disc apparatus 
for improving the reading accuracy of a signal in the vicinity of a servo 
point, suppressing the peak level of the signal, and for stably 
controlling focus servo function and tracking servo function against 
variations of external conditions (such as temperature) and replacement of 
a disc without necessity of A/D converters having high accuracy and wide 
dynamic range. 
In the optical disc apparatus of the present invention, a non-linear 
amplifying circuit amplifies a signal with a larger gain in the vicinity 
of a control target than at its peak. This amplifying circuit amplifies a 
focus signal and/or a tracking error signal. Corresponding this amplified 
signal, a drive control signal for driving an objective lens in focus 
direction and tracking direction is generated. 
Thus, in the vicinity of control target point (servo point), the signal can 
be precisely read. In contrast, in the vicinity of signal peak, the signal 
can be coarsely read. Thus, the reading accuracy of the signal in the 
vicinity of the servo point can be improved. In addition, the peak amount 
of signals can be suppressed. Without necessity of an A/D converter having 
high accuracy and wide dynamic range, focusing servo function and tracking 
servo function can be precisely controlled against variations of external 
conditions (such as temperature) and replacement of a disc. 
These and other objects, features and advantages of the present invention 
will become more apparent in light of the following detailed description 
of a best mode embodiment thereof, as illustrated in the accompanying 
drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Next, with reference to the accompanying drawings, embodiments of the 
present invention will be described. 
FIG. 1 shows the construction of principal portions of an optical disc 
apparatus of the present invention. In the figure, an optical disc 11 is 
rotated by a spindle motor 13 at for example a constant speed. The motor 
13 is controlled by a motor control circuit 12. 
On the optical disc 11, spirally-shaped or concentrically-shaped grooves 
(information tracks) are formed. On each information track, a plurality of 
sector marks (header information) are recorded at predetermined intervals. 
A sector mark is formed of a combination of a plurality of physical holes. 
A sector represents an address of information record area (for example, a 
track number or a sector number). On the optical disc 11, a light spot is 
irradiated by a known optical head (pick-up) 14. By irradiating a light 
spot on the optical disc 11, information can be written (recorded) thereto 
or read (reproduced) therefrom. The optical head 14 comprises a laser 
diode which irradiates a laser beam and an optical detector which detects 
a light beam reflected from the optical disc 11. The optical head 14 is 
moved in the radial direction of the optical disc 11 by a linear motor 15. 
The laser diode of the optical head 14 is controlled by an optical disc 
controller 16 through a laser control circuit 17. A detection signal 
detected by the optical detector of the optical head 14 is processed by a 
head signal process circuit 18 in a particular manner. 
FIG. 2 shows the construction of the head signal process circuit 18. In the 
figure, reference numeral 80 is the optical detector of the optical head 
14. The optical detector 80 has four detecting areas 80a, 80b, 80c, and 
80d, each of which detects the amount of light. 
An output signal of the detecting area 80a is sent to adders 82a and 82c 
through an amplifier 81a. An output signal of the detecting area 80b is 
sent to adders 82b and 82c through an amplifier 81b. An output signal of 
the detecting area 80c is sent to adders 82b and 82c through an amplifier 
81c. 
An output signal of the adder 82c is sent to both a non-inverted input side 
of a differential amplifier 83 and an adder 84. An output signal of the 
adder 82b is sent to both an inverted-input side of the differential 
amplifier 83 and the adder 84. Thus, an output signal S1 of the 
differential amplifier 83 is a subtraction signal where the output signal 
of the adder 82a is subtracted from the output signal of the adder 82b. An 
output signal S2 of the adder 84 is a sum signal where the output signal 
of the adder 82a is added to the output signal of the adder 82b. The 
output signals S1 and S2 are used for performing a tracking control and an 
access control. 
An output signal of the adder 82c is sent to an inverted-input side of a 
differential amplifier 85. An output signal of the adder 82d is sent to a 
non-inverted input side of the differential amplifier 85. Thus, an output 
signal S3 of the differential amplifier 85 is a subtraction signal where 
the output signal of the adder 82d is subtracted from the output signal of 
the adder 82c. The output signal S3 is a focus error signal which 
represents the distance between the optical disc 11 and the objective lens 
20. 
As shown in FIG. 1, the focus error signal is sent to a non-linear 
amplifying circuit 21. In the non-linear amplifying circuit 21, the focus 
error signal in the vicinity of a servo point is amplified, whereas the 
signal apart therefrom is suppressed. The output of the non-linear 
amplifying circuit 21 is sent to an A/D converter 22. In the A/D converter 
22, a servo digital sinal is generated. 
With reference to the servo digital signal, the focusing servo operation is 
performed so that the value of the servo digital signal matches the servo 
point. Practically, a signal generated by a servo controller 23 is sent to 
a D/A converter 24. The D/A converter 24 converts the input signal into an 
analog signal. The analog signal is sent to an objective lens actuator 26 
through a driver circuit 25. Thus, the objective lens actuator 26 performs 
the focusing servo operation. 
FIG. 3 shows an example of the non-linear amplifying circuit 21. The 
non-linear amplifying circuit 21 comprises an operational amplifier 50 
(which is a conventional inverting amplifying circuit), diodes 51 and 52, 
feed-back resisters 53 and 54, an input resister 55 (which designates 
gain), and an input resister 56 (which designates change point). 
FIG. 4 shows input/output characteristics of the non-linear amplifying 
circuit 21. As shown in the figure, in the vicinity of the center point of 
the signal, the diodes are in off state. Thus, the non-linear amplifying 
circuit 21 operates as a normal inverting amplifier. Since a current 
corresponding to the value of the input signal flows in the resisters 55 
and 53, the gain of the output signal becomes--Rf1/Ri1. When the output 
value exceeds (1+Rf2/Ri2) VD (where VD is the diode forward voltage), the 
diodes are turned on. 
When the diodes 51 and 52 are ideal diodes and a voltage exceeding the 
diode forward voltage VD is applied to both terminals of the diodes 51 and 
52, the resistances of the diodes become 0. Thus, a current flows in the 
diodes 51 and 52 and thereby they are turned on. Since the minus terminal 
of the operational amplifier 50 is treated as imaginary short, it is 
grounded (namely, the voltage of the minus terminal is the same as that of 
the plus terminal). A voltage where the output signal is divided by the 
resisters 56 and 54 (namely, a voltage where Ri2/(Ri2+Rf2) is multiplied 
by the output signal) is applied to the cathode of the diode 51 and the 
anode of the diode 52. Thus, when the level of the output signal exceeds 
(1+Rf2/Ri2) VD in comparison with the center point of the signal (ground), 
the diode 51 or the diode 52 is turned on. When the diode 51 or 52 is 
turned on, the feed-back resistance of the inverting amplifier of the 
operational amplifier 50 becomes the resistance of the parallel connection 
of the resisters 53 and 54 (namely, Rf1.times. Rf2/(Rf1+Rf2)). Thus, the 
gain of the amplifying circuit becomes -(Rf1/Ri1).times.{Rf2/(Rf1+Rf2)} 
which is smaller than in the vicinity of the center of the signal. Since 
the relation of {Rf2/(Rf1+Rf2)&lt;1} is satisfied, the relation of 
{Rf1/Ri1&lt;(Rf1/Ri1).times.{Rf2/(Rf1+Rf2)} is obtained. Thus, the gain in 
the diode on state is smaller than the gain in the diode off state. 
A signal which is input by the resister 56 is equal to the voltage of the 
plus terminal of the operational amplifier 50. Thus, the input signal of 
the resister 56 does not affect the output signal of the non-linear 
amplifying circuit 21. 
In FIG. 5, the focus error signal which passes through the non-linear 
amplifying circuit is denoted by a dotted line. The servo point is at the 
center of operation of the non-linear amplifying circuit (namely, in the 
vicinity of the voltage of the plus terminal of the operational amplifier 
50). Corresponding to the distance between the optical disc 11 and the 
objective lens 20, the focus error signal which is denoted by a solid line 
is generated. The focus error signal is nearly symmetrical with respect to 
the center of the servo point. When this signal passes through the 
non-linear amplifying circuit, the signal in the vicinity of a servo point 
is amplified with a large gain, whereas the signal in the vicinity of the 
peak of the focus error signal is amplified with a small gain. Thus, after 
the signal is sent to the A/D converter 22, in the vicinity of the servo 
point, the signal is precisely read. In the vicinity of the signal peak, 
the signal is coarsely read. 
When the servo operation is performed, the level of the focus error signal 
is not in the vicinity of peak. Only when the focusing operation is 
performed (namely, the apparatus is turned on), the level of the focus 
error signal should be in the vicinity of peak. Thus, when the level of 
the focus error signal is in the vicinity of peak, it is not precisely 
read. In other words, when the focusing operation is performed, with a 
signal received from the CPU 30, the objective lens 20 is driven so that 
it is most separated from or approached to the optical disc 20. Next, the 
objective lens 20 is gradually approached to the optical disc 11 (or 
separated therefrom) so as to detect the focus error signal. When the 
level of the focus error signal exceeds its peak, the focusing servo loop 
is turned on. Corresponding to the level of the focus error signal at this 
time, the optical head is moved to the servo point. Thus, when the level 
of the focus error signal is in the vicinity of peak, it can be coarsely 
read. 
The non-linear amplifying circuit 21 may be any amplifying circuit where 
the gain of the signal lowers as it is apart from the center (for example, 
an amplifying circuit with an amplitude limiter or a logarithmic amplifier 
with a feed-back diode). 
As described above, in the head signal process circuit 18 shown in FIG. 1, 
the signal (S1) for use in the tracking operation and the signal (S2) for 
detecting the moving direction of the optical head are generated. These 
signals are sent to the servo controller 23 through the filter circuit 27, 
the binary circuit 28, and the A/D converter 22. In the servo controller 
23, control signals are generated corresponding to these signals. The 
servo controller 24 drives the objective lens actuator 26 and the linear 
motor 15 through the D/A converter 24 and the driver circuit 25 by using 
the control signals. 
In FIG. 1, reference numeral 30 is a CPU which generally controls the 
apparatus. Reference numeral 31 is a memory. Reference numeral 32 is an 
interface circuit which connects the apparatus to an external unit. 
In this embodiment, since the focus error signal for use in the focus servo 
operation is processed by the non-linear amplifying circuit 21, the 
accuracy of the focus error signal in the vicinity of the servo point can 
be improved and the amount of peak of the signal can be suppressed. Thus, 
without necessity of an A/D converter having high accuracy and wide 
dynamic range, the focus control can be stably performed against 
variations of external conditions (such as temperature) and replacement of 
a disc. 
Next, with reference to FIG. 6, another embodiment of the present invention 
will be described. 
In this embodiment, the tracking error signal (above-described signal S1) 
which is generated by the head signal process circuit 18 corresponding to 
the signal detected by the optical detector in the optical head 14 is sent 
to a non-linear amplifying circuit 40. The tracking error signal 
represents the relation of a particular guide groove on the optical disc 
11 and a light spot. 
The construction of the non-linear amplifying circuit 40 is similar to that 
of the non-linear amplifying circuit 21 descried in the first embodiment. 
FIG. 7 shows an example of the non-linear amplifying circuit 40. The 
non-linear amplifying circuit 40 comprises an operational amplifier 90 
(which is a conventional inverting amplifying circuit), diodes 91 and 92, 
feed-back resisters 93 and 94, an input resister 95 (which designates 
gain), and an input resister 96 (which designates change point). 
FIG. 8 shows input/output characteristics of the non-linear amplifying 
circuit 40. As shown in the figure, in the vicinity of the center point of 
the signal, the diodes are in off state. Thus, the non-linear amplifying 
circuit 40 operates as a normal inverting amplifier. Since a current 
corresponding to the value of the input signal flows in the resisters 95 
and 93, the gain of the output signal becomes Rf1/Ri1. When the output 
value exceeds (1+Rf2/Ri2) VD (where VD is the diode forward voltage), the 
diodes are turned on. 
When the diodes 91 and 92 are ideal diodes and a voltage exceeding the 
diode forward voltage VD is applied to both terminals of the diodes 91 and 
92, the resistances of the diodes become 0. Thus, a current flows in the 
diodes 91 and 92 and thereby they are turned on. Since the minus terminal 
of the operational amplifier 90 is treated as imaginary short, it is 
grounded (namely, the voltage of the minus terminal is the same as that of 
the plus terminal). A voltage where the output signal is divided by the 
resistors 96 and 94 (namely, a voltage where Ri2/(Ri2+Rf2) is multiplied 
by the output signal) is applied to the cathode of the diode 91 and the 
anode of the diode 92. Thus, when the level of the output signal exceeds 
(1+Rf2/Ri2) VD in comparison with the center point of the signal (ground), 
the diode 91 or the diode 92 is turned on. When the diode 91 or 92 is 
turned on, the feed-back resistance of the inverting amplifier of the 
operational amplifier 90 becomes the resistance of the parallel connection 
of the resisters 93 and 94 (namely, Rf1.times.Rf2/(Rf1+Rf2)). Thus, the 
gain of the amplifying circuit becomes -(Rf1/Ri1).times.{Rf2/(Rf1+Rf2)} 
which is smaller than in the vicinity of the center of the signal. Since 
the relation of {Rf2/(Rf1+Rf2)&lt;1} is satisfied, the relation of 
{Rf1/Ri1&lt;(Rf1/Ri1).times.{Rf2/(Rf1+Rf2)} is obtained. Thus, the gain in 
the diode on state is smaller than the gain in the diode off state. 
A signal which is input by the resister 96 is equal to the voltage of the 
plus terminal of the operational amplifier 90. Thus, the input signal of 
the resister 96 does not affect the output signal of the non-linear 
amplifying circuit 40. 
The non-linear amplifying circuit 40 amplifies the tracking error signal in 
the vicinity of the servo point. In contrast, the non-linear amplifying 
circuit 40 suppresses the tracking error signal which is apart from the 
servo point. The output of the non-linear amplifying circuit is sent to 
both the A/D converter 22 and the binary circuit 28. Thus, a servo digital 
signal is generated. 
With reference to the servo digital signal, the focusing servo operation is 
performed so that the value of the servo digital signal matches the servo 
point. Practically, the signal generated by the servo controller 23 is 
converted into an analog signal by the D/A converter 24. Corresponding to 
the analog signal, the objective lens actuator 26 is driven in the 
tracking direction through the driver circuit 25 so as to perform the 
focusing servo operation. 
When the servo operation is performed, the level of the tracking error 
signal is not in the vicinity of peak. The tracking error signal in the 
vicinity of peak is used when the number of tracks is counted. Next, the 
track count operation will be briefly described. 
When the access operation is performed, the tracking servo operation is 
turned off. When the optical head 14 is moved by the linear motor 15, as 
the optical spot traverses a guide groove of the optical disc 11, the 
tracking error signal varies by one cycle. Thus, by digitizing this signal 
at the center thereof, a digital signal which varies by one pulse 
corresponding to one guide groove is generated. Thus, by counting the 
number of guide grooves that the optical head 14 traverses, the distance 
that the optical head 14 moves over the optical disc 11 can be determined. 
By converting the frequency of this pulse to corresponding voltage, the 
speed of the optical head 14 can be detected. Corresponding to the 
information of the distance and the speed, the servo controller 23 
generates a control signal of the linear motor 15. With the control 
signal, the linear motor 15 is controlled and thereby the optical disc 11 
is accessed. When the level of the tracking error signal is in the 
vicinity of peak, it is coarsely read. 
As described above, in the second embodiment, since the tracking error 
signal for use in the tracking servo operation is processed by the 
non-linear amplifying circuit 40, the accuracy of the tracking servo error 
signal can be improved in the vicinity of the servo point and the peak 
amount of the signal can be suppressed. Thus, without necessity of an A/D 
converter having high accuracy and wide dynamic range, the tracking 
control operation can be stably performed against variations of external 
conditions (such as temperature) and replacement of a disc. 
Although the present invention has been shown and described with respect to 
best mode embodiments thereof, it should be understood by those skilled in 
the art that the foregoing and various other changes, omissions, and 
additions in the form and detail thereof may be made therein without 
departing from the spirit and scope of the present invention.