Offset compensation apparatus for tracking error signal of optical recording reproduction apparatus

An offset compensation apparatus for a tracking error signal in an optical recording reproduction apparatus adds or subtracts a signal, which is proportional to the intensity of a read head laser beam, to or from a tracking error signal (which indicates the difference between a track of an optical disk and a a read head laser beam spot on the optical disk) so as to cancel offsets occur in the tracking error signal that are in proportion to the intensity of the read head laser beam and, thus, compensates for the offset in the tracking error signal. Since the offset compensation signal is also indicative of the amount of laser light irradiated to the optical disk, the offset can be compensated easily and accurately with a small scale simple circuit arrangement rather than using the larger-scale and more conventional divider attenuator combination of the prior art.

FIELD OF THE INVENTION 
The present invention relates to an offset compensation apparatus for a 
tracking error signal of an optical recording reproduction apparatus which 
optically records and reproduces information of a magneto-optical disk 
apparatus, etc. 
BACKGROUND OF THE INVENTION 
In order to follow a specified track by a light beam spot when carrying out 
recording and reproducing of information for an optical disk, a tracking 
control for controlling a light beam spot position has been carried out. 
The tracking control detects a tracking error which is a radial difference 
between the track and the light beam spot on the optical disk, and is 
carried out based upon the detected tracking error. In order to detect the 
tracking error, a push-pull method is mostly used for an optical disk, 
especially on which recording will be carried out, from the standpoint of 
efficiency of using a light beam. 
However, the position detection method by the push-pull method has a 
disadvantage. It is that a spot difference is detected by an error 
detector in spite of irradiating the light beam spot in the center of the 
track in the case of a slant of a disk, etc. 
In this way, an offset of .DELTA.V occurs in the tracking error signal when 
the spot difference occurs as shown in FIG. 3(a). A point where the 
tracking error signal becomes 0 is regarded as the center of the track, 
and the tracking control is made so as to control the position of the 
light beam spot. Therefore, in this case, the light beam spot control is 
carried out regarding not O point which shows that the true spot 
difference is equal to 0 but O' point which is off-centered as the center 
of the track, thereby preventing information from being correctly recorded 
and reproduced. 
In addition, the offsets change as reflectance of the disk and the output 
of the laser beam change. For example, if in FIG. 3(b), the laser output 
increases to a times the output shown in FIG. 3(a), positive and negative 
amplitude of the tracking error signal become a times. Since the O point 
which shows the center of the true track is the middle point of the 
positive and negative amplitude, as apparent from FIG. 3(b), the offset 
becomes a times. For this reason, in order to carry out the accurate 
tracking control, the offset should be compensated as required in 
accordance with the disk reflectance and the laser output. 
The following description will discuss a conventional example of the offset 
compensation method for the tracking error signal. 
As to the first conventional example, there exists a method which has been 
disclosed in Japanese Laid-Open Patent Application No. 258232/1989 
(Tokukaihei 1-258232). FIG. 4 shows the arrangement. An optical disk 1 is 
rotated by a drive motor 2. An optical pick-up 3 moves radially on the 
optical disk 1. A tracking error which is a radial difference on the 
optical disk 1 between the track and the light beam spot is detected by an 
optical system (not shown) in the optical pick-up 3, and becomes a 
tracking error signal TE via a tracking error detection circuit 4. The 
tracking error signal TE is sent to an amplifying circuit 5 and supplied 
to an addition circuit 6. An HF signal (information reproducing or 
"readback" signal) which is simultaneously obtained from the optical 
system (not shown) in the optical pick-up 3 is supplied to a head 
amplifier 12 via an HF signal detection circuit 11, and to a signal 
processing circuit 14 after being amplified. The HF signal is branched and 
its DC component is picked out by an LPF (low-pass filter) 13 so as to be 
adjusted for a suitable level by a variable resistor VR for a level 
adjustment. The DC component of the HF signal is added to a tracking error 
signal TE' in the addition circuit 6. 
Since the DC component of the HF signal fluctuates in proportion to 
variations in the laser output and the reflectance of the optical disk 1, 
the offsets of the tracking error signal TE' are compensated by means of 
the DC component of the HF signal in the first conventional example. In 
this way, after the tracking error signal TE' has been compensated in the 
addition circuit 6, a compensation for stabilization of a tracking control 
system is made in a phase compensation circuit 8 by means of an amplifier 
7 for gain adjustment. A tracking coil 10 is driven by a tracking coil 
driving circuit 9 and the tracking control is carried out so that the 
light beam spot follows the track. 
In the second conventional example, a method shown in FIG. 5 is adopted. 
Here, for convenience of explanation, those members of the second 
conventional example that have the same arrangement and function, and that 
are mentioned in the aforementioned first conventional example are 
indicated by the same reference numerals and description thereof is 
omitted. 
In FIG. 5, the total amount of a reflected light from the optical disk 1 is 
detected by the optical system (not shown) in the optical pick-up 3, and 
becomes a total signal TS via a total signal detection circuit 15. The 
total signal TS is amplified in the head amplifier 16 and supplied to a 
divider 17. As shown in FIGS. 3(a) and 3(b), the amplitude and offset of 
the tracking error signal TE increase or decrease as the laser power or 
the reflectance of the optical disk 1 increases or decreases. At the same 
time, the total signal TS also increases or decreases accordingly. For 
this reason, when the tracking error signal TE' is standardized by 
dividing it by the total signal TS' in the divider 17, the amplitude and 
offsets of the tracking error signal TE' is always kept constant, and the 
standardized tracking error signal TE' is supplied to the addition circuit 
6. Moreover, a constant offset compensation voltage E is applied to the 
addition circuit 6 via the variable resistor VR. 
In this way, when the tracking error signal TE' is divided by the total 
signal TS', the amplitude of the tracking error signal TE' becomes 
constant regardless of a fluctuation in the laser output or the 
reflectance. As a result, since a constant amount of the offset occurs, a 
constant level of the offset compensation voltage is applied to the 
addition circuit 6. 
However, the above conventional arrangement has the following problem. 
Namely, since the offset compensation cannot be carried out at the time of 
recording by the art of the first conventional example (FIG. 4), the art 
cannot be applied to the optical disk 1 where information is recorded. 
This problem is clarified by considering that the HF signal is outputted 
only at the time of reproducing, it is not clear whether or not the HF 
signal is outputted at the time of recording, and if outputted, it is not 
guaranteed that the DC component reflects the reflectance of the optical 
disk 1 and the laser output. 
The art of the second conventional example (FIG. 5) is applicable to the 
optical disk apparatus where information is recorded, but has the problem 
that the divider 17 is necessary. 
In general, since the divider 17 is expensive, it is an undesirable unit 
from a viewpoint of lowering the cost of the optical disk apparatus. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to make the offset compensation 
possible with a simple arrangement at the time of recording and 
reproducing without using expensive parts. 
In order to accomplish the above object, an offset compensation apparatus 
for a tracking error signal in an optical recording reproduction apparatus 
according to the present invention, includes: 
creation means for creating a signal which is proportional to intensity of 
a light beam to be irradiated to an optical recording medium; 
detection means for detecting the tracking error signal; and 
compensation means for compensating for an offset of the tracking error 
signal based upon the signal created in proportion to the intensity of the 
light beam. 
With the above arrangement, since the offset of the tracking error signal 
is compensated based upon the signal which is proportional to the 
intensity of the irradiation light beam, the offset of the tracking error 
signal is compensated easily and accurately based upon the laser output 
which changes even in the optical recording reproduction apparatus, where 
the laser output changes frequently, such as a magneto-optical disk 
apparatus. 
In the above arrangement, the compensation means is desirable to include: 
means for specifying an amount and a polarity of the on occurring offset 
based upon the detected tracking error signal so as to output a 
corresponding multiplication data; 
multiplication means for multiplying the signal created in proportion to 
the intensity of the light beam by the multiplication data which has been 
inputted; and 
addition means for adding the multiplication result and the tracking error 
signal which have been inputted. 
With the above arrangement, the tracking error signal and the 
multiplication result are added, and a compensation is made so that 
positive and negative peak values of the tracking error signal become 
equal. Accordingly, the offset which occurs in the tracking error signal 
in proportion to the intensity of the light beam is cancelled. The amount 
and the polarity of the offset are found based upon the additional result. 
For a fuller understanding of the nature and advantages of the invention, 
reference should be made to the ensuing detailed description taken in 
conjunction with the accompanying drawings.

DESCRIPTION OF THE EMBODIMENT 
Referring to FIG. 1, the following description will discuss an embodiment 
of the present invention. Here, for convenience of explanation, those 
members of the present embodiment that have the same arrangement and 
function, and that are mentioned in the aforementioned conventional 
example are indicated by the same reference numerals and description 
thereof is omitted. 
An optical disk apparatus (optical recording reproduction apparatus) of the 
present embodiment compensates for an offset of a tracking error signal TE 
based upon a signal V.sub.L which fluctuates in proportion to a laser 
output (light beam output). For example, a laser output monitor voltage 
which has been obtained by photoelectrically converting the laser output 
by means of a photoelectrical transducer (not shown) is given as the 
signal V.sub.L which fluctuates in proportion to the laser output, but the 
signal V.sub.L is not limited to this, and any signals which fluctuate in 
accordance with a change in intensity of an light beam to be irradiated to 
an optical disk are applicable. 
After being amplified by a head amplifier 19, the signal V.sub.L is 
supplied to a multiplying D/A (digital/analog) converter 20. The 
multiplying D/A converter 20 recognizes digital data (multiplication data) 
supplied by a microcomputer 22 as a coefficient, and multiplies an input 
signal by the coefficient so as to output the multiplied result. In other 
words, the multiplying D/A converter 20 functions as a sort of a variable 
resister capable of digitally changing an output, so adjusts the level of 
the signal V.sub.L which fluctuates in proportion to the laser output. 
The adjusted signal is supplied to an addition circuit 6 and added to a 
tracking error signal TE' so that its offset is compensated. A 
compensation means mainly includes the multiplying D/A converter 20, the 
microcomputer 22, the additional circuit 6 which will be described later, 
and an A/D converter 21 which will be described later. 
In the optical disk apparatus shown in FIG. 1, the compensation for the 
offset of the tracking error signal TE' is performed through the following 
procedure. 
First, at the time of turning on the power source of the apparatus and 
replacing the optical disk, only a focus control for controlling an 
optical system (not shown) in an optical pick-up 3 is carried out so that 
a light beam spot is focused on the surface of an optical disk 1 (optical 
recording medium), but a tracking control for making the light beam spot 
follow a specified track is not carried out. 
At this time, if absolute values .linevert split.V+.linevert split. and 
.linevert split.V-.linevert split. of positive and negative peaks of the 
tracking error signal TE are not equal, an offset .DELTA.V=(.linevert 
split.V+.linevert split.-.linevert split.V-.linevert split.)/2 occurs (see 
FIG. 3(a)). 
In this state, the tracking error signal TE is first transmitted directly 
to the addition circuit 6, and passes through the D/A converter 21. 
Thereafter, the tracking error signal TE is fetched by the microcomputer 
22 so that the positive and negative peak values of the tracking error 
signal TE are obtained. Then, an amount of the offset and its polarity are 
calculated in the microcomputer 22 in accordance with the above formula. 
Next, certain preset initial data (temporarily referred as .alpha.) are 
transmitted from the microcomputer 22 to the multiplying D/A converter 20. 
Then, the signal V.sub.L is multiplied by .alpha. in the multiplying D/A 
converter 20. The signal .alpha..V.sub.L which has been multiplied by 
.alpha. is added to the tracking error signal TE' in the addition circuit 
6. The output of the addition circuit 6 is again monitored by the A/D 
converter 21 and the offset .DELTA.V' is recalculated by using the above 
method. New data .alpha.' are transmitted to the multiplying D/A converter 
20 so that the .DELTA.V' becomes 0 based upon the obtained .DELTA.V'. 
-.alpha.'.V.sub.L =.DELTA.V is held by executing so-called feedback control 
which repeats the above procedure. In other words, data .alpha.', which 
make tracking error signal TE' become 0 in the output of the addition 
circuit 6, is determined. 
As a result, the adjustment is made so that the tracking error signal TE is 
symmetric with respect to zero, that is, the positive and negative peak 
values become equal. 
Since the signal V.sub.L fluctuates in proportion to the laser output, if 
the laser output fluctuates to a times and the offset becomes a times 
accordingly, the signal V.sub.L fluctuates to a times (see FIG. 3(b)). 
This always ensures the most suitable offset compensation by using the 
above-mentioned method at the time of turning on the power source of the 
apparatus and replacing the optical disk even if the laser output 
fluctuates. 
The signal V.sub.L does not fluctuate in accordance with the fluctuation in 
an amount of an offset caused by a peculiar difference of reflectance in 
the optical disk 1, but in the optical disk apparatus for recording and 
reproducing information, since a control is made so that the laser output 
changes several times to ten odd times at the time of recording and 
reproducing, the fluctuation in an offset amount caused by a change in the 
laser output is overwhelmingly greater than the fluctuation in the 
reflectance in the optical disk 1. For this reason, even the compensation 
method based upon the signal V.sub.L which is mentioned in the present 
embodiment can be used effectively. 
In addition, since the signal V.sub.L is outputted regardless of recording 
and reproducing, the above method makes it possible to always compensate 
for the offset. 
Here, referring to FIGS. 2(a) and 2(b), the following description will 
discuss a difference between in the case where the compensation for an 
offset amount .DELTA.V of the tracking error signal TE is made based upon 
a total signal TS, and in the case where the compensation is made based 
upon the signal V.sub.L. Incidentally, the total signal TS is a signal 
which shows the total of an amount of a reflected light from the optical 
disk 1, and which changes in accordance with the reflectance of the 
optical disk 1. 
FIGS. 2(a) and 2(b) is a drawing showing how each waveform of the tracking 
error signal TE and the total signal TS change with a change in the spot 
position on the optical disk 1. 
In the case where the compensation for the offset amount .DELTA.V of the 
tracking error signal TE is made based upon the total signal TS with no 
tracking control carried out, after all, the compensation for the offset 
amount .DELTA.V which is a constant value should be made based upon the 
total signal TS which fluctuates in accordance with the position of the 
light beam spot. 
However, as to a normal optical disk, the tracking error signal TE 
fluctuates as the light beam spot traverses the track on the disk (see 
FIG. 2(a)), and the total signal TS also fluctuates with a fluctuation in 
the tracking error signal TE (see FIG. 2(b)). Meanwhile, as shown in FIG. 
2(a), the offset amount .DELTA.V of the tracking error signal TE does not 
fluctuate and is kept constant as far as the laser output or reflectance 
of the optical disk does not fluctuate. 
In contrast, according to the present embodiment, since the signal V.sub.L 
is a signal which is directly proportional to an irradiation amount of the 
laser beam, the level of the signal V.sub.L does not fluctuate and is kept 
constant as far as the irradiating light amount does not fluctuate. In 
other words, the constant offset amount .DELTA.V is compensated based upon 
the signal V.sub.L of the constant level. Therefore, the compensation for 
the offset amount can be made much more easily and accurately based upon 
the signal V.sub.L of the constant level as described in the embodiment 
than based upon the total signal TS which fluctuates as mentioned above. 
In addition, since the signal V.sub.L is outputted regardless of recording 
or reproducing, in the present embodiment, it is possible to always 
compensate for an offset easily and accurately with a simple arrangement 
in which an expensive divider, an attenuator with a large-scale circuit, 
etc. are not utilized. 
As mentioned above, the optical disk apparatus according to the present 
embodiment is arranged so as to include: 
the detection means (for example, the photoelectrical transducer) for 
detecting the signal V.sub.L representing the amount of the laser beam to 
be irradiated to the optical disk 1; and 
the compensation means (addition circuit 6) for cancelling the offset by 
adding or subtracting the signal detected by the detection means to or 
from the tracking error signal TE representing the difference between the 
track and the laser beam spot on the optical disk 1. The signal V.sub.L 
representing the laser beam amount is a signal which is proportional to 
the intensity of the light beam. 
With the above arrangement, since the signal which is proportional to 
intensity of the laser beam is added to or subtracted from the tracking 
error signal TE representing the difference between the track of the 
optical disk 1 and the laser beam spot on the optical disk 1, the offset 
which occurs in the tracking error signal TE in proportion to the 
intensity of the laser beam, is cancelled, so the offset of the tracking 
error signal TE is compensated. 
In other words, since the signal V.sub.L, which has been obtained by 
directly detecting the laser beam amount, is used as the signal which is 
proportional to the intensity of the laser beam, even in the case where 
the laser output changes, the compensation for the offset of the tracking 
error signal TE is always made accurately according to the change in the 
laser output. Therefore, the optical disk apparatus, where the 
compensation method of the present embodiment for the offset of the 
tracking error signal TE is adopted, is realizable with a simple 
arrangement. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.