JPH0636307A - Method and device for cancelling tracking offset - Google Patents

Abstract

PURPOSE:To easily cancel the tracking offset of an optical disk driving device. CONSTITUTION:A timing generating circuit 48 generates a sampling signal S based on reflected light from a part having no pits of a frame synchronizing pattern on an optical disk, and a sampling and holding circuit 42 samples and holds a tracking error signal TE1 by the sampling signal S, and this sampled and held signal is used to cancel the tracking offset.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tracking offset canceling method and apparatus for an optical disk drive which performs push-pull type tracking servo.

[0002]

2. Description of the Related Art In the push-pull method, one light beam is applied to a pit on an optical disk, and the intensity distribution of light diffracted and reflected by the pit changes depending on the relative position change between the pit and the beam spot. This is a method of generating a tracking error signal by utilizing this. Figure 7
9 shows a change in intensity distribution of return light according to a change in relative position between a pit and a beam spot.

Since the push-pull method uses only one beam, it has the advantages that the optical system can be simplified and the assembly and adjustment can be facilitated.

[0004]

However, in the push-pull method, as shown in FIG. 8A, the optical axis of the objective lens 10, that is, the image forming optical system has two photodiodes PD1 and PD2. The position of the spot on the detector 12 is offset if it is deviated from the optical detector 12 including it, that is, the optical axis of the tracking error signal detection system.
Further, as shown in FIG. 8B, the spot on the detector 12 is distorted due to the skew of the disk 2.

The push-pull method is used not only in an optical disk drive device but also in a magneto-optical (MO) disk drive device and has similar problems.
O format (5 inch type A, 3.5 inch)
Then, as shown in FIG. 9, the offset cancellation is performed using a mirror surface dedicated to the offset cancellation called "ODF". However, there is no such thing on the compact disc (CD) format.

An object of the present invention has been made in view of such a situation, and an object thereof is to make it possible to easily cancel a tracking offset of a drive device for an optical disk such as a compact disk.

[0007]

A tracking offset canceling method of the present invention generates a tracking error signal by a push-pull method and uses a sampling signal based on a reflected light from a pit-free portion in a frame synchronization pattern of an optical disk, The tracking error signal is sampled, and the tracking servo signal is used to cancel the offset of the tracking servo.

The tracking offset canceling apparatus of the present invention is a tracking error signal generating means (for example, a tracking error signal generating means for generating a tracking error signal by a push-pull method).
The first and second photodiodes PD1 and PD2, the amplifiers 14A and 14B, and the subtractor 4 of FIG.
1) and a sampling signal generating means for generating a sampling signal based on the reflected light from the pit-free portion in the frame synchronization pattern of the optical disc (for example, the adder 45, the binarization circuit 46, the PLL 47 and the timing generation in FIG. 4). Circuit 48), sampling means for sampling the tracking error signal by the sampling signal (for example, sample hold circuit 42 in FIG. 4), and offset cancellation for canceling the tracking servo offset using the sampled tracking error signal. Means (for example, the subtractor 44 in FIG. 4).

[0009]

In the tracking offset canceling method and apparatus of the present invention, the tracking error signal is generated by the push-pull method, and the tracking error signal is generated by the sampling signal based on the reflected light from the pit-free portion in the frame synchronization pattern of the optical disk. Is sampled and the tracking servo signal is used to cancel the offset of the tracking servo. In this way, since the tracking offset is canceled by utilizing the portion already existing on the optical disc, the offset can be easily canceled.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a structural example of a tracking servo device for a compact disc using the present invention. The optical disc, that is, the compact disc 2 is rotationally driven by a spindle motor 4. Laser diode (LD)
The laser light emitted from 6 passes through the half mirror 8 and is focused on the disk 2 by the objective lens 10. It is assumed that the objective lens 10 is subject to focus servo (not shown).

The light reflected from the disk 2 passes through the objective lens 10 again, is reflected by the half mirror 8 and is reflected by the photodetector 1 including two photodiodes PD1 and PD2.
Incident on 2. The output signals of the photodiodes PD1 and PD2 are the preamplifiers 14A and 14B, respectively.
Is amplified by and is supplied to the tracking offset cancel circuit 16.

Tracking offset cancel circuit 1
As will be described later in detail, 6 outputs a tracking error signal in which the offset amount is canceled, and this signal is supplied to the phase compensation circuit 18. Phase compensation circuit 18
Performs phase compensation of the tracking error signal in order to stabilize the servo. The tracking error signal output from the phase compensation circuit 18 is amplified by the drive amplifier 20 and drives the tracking actuator 22. As a result, the objective lens 10 moves in the radial direction of the disk 2 to establish tracking servo.

FIG. 2 shows the format of one frame of a compact disc. The present invention utilizes the frame sync pattern (ie, periodic pattern) in this format.

FIG. 3 shows the relationship between the frame synchronization pattern FRP of a certain track n and the pits of adjacent tracks. The pit P1 constitutes a half cycle of the frame synchronization pattern, and its length is Tmax = 11T (1T is 4,
The cycle is 3218 MHZ, which is 1T of EFM). The pit P2 is a pit indicating the end of the frame synchronization pattern. Pits P3 and P4 are respectively
It is a pit of adjacent tracks (n + 1) and (n-1). Since the hatched area MR between the pits P1 and P2 in FIG. 3 is a mirror surface (a surface without pits), the output signal of the photodetector 12 when the laser spot LS hits this portion. Is sampled and used for offset cancellation is a feature of the present invention.

The size of MR is about Tmax (2Tp (track pitch) -Pw (pit width)), for example, Tm.
ax = 3.18 μm, Tp = 1.6 μm, Pw = 0.8
If the area is μm, the area of MR = 7.63 μm ² . Further, the diameter of the laser spot LS is D _LS = 2.1 μm
(Airy disk diameter = 1.22 × λ / NA, λ = 7
80 nm, NA = 0.45), so the area of LS = π
(2.1 / 2) ² = 3.46 μm ² . Therefore, MR
> LS, which means that the mirror surface is sufficiently wider than the spot diameter.

FIG. 4 shows a specific configuration example of the offset cancel circuit 16 of the tracking servo device of FIG. 1, that is, the configuration of an embodiment of the tracking offset cancel device of the present invention. The photodetector 12 is divided into two photodiodes PD1 and PD2, and the reflected light from the disc 2 finally strikes the vicinity of the center. This light is called LSD. The light LSD moves to the left and right in FIG. 4 depending on the position of the objective lens 10 in FIG.

The signals output from the photodiodes PD1 and PD2 are amplifiers 14A and 14A, respectively.
It is amplified by B and becomes signals PP1 and PP2. Signal P
1 and PP2 are supplied to the subtractor 41 and
It is supplied to the adder 45.

The subtractor 41 calculates the difference between the signal PP1 and the signal PP2 and outputs a signal TE1 indicating this difference, that is, a tracking error signal, to the sample hold circuit 42. The sample hold circuit 42 samples and holds the signal TE1 by the timing signal SH corresponding to the mirror surface of the above-mentioned area MR output from the timing generation circuit 48 described later, and outputs the sampled signal OF. This signal OF is the offset component of the tracking error signal generated by the movement of the objective lens 10 and the skew of the disk 2 shown in FIG.

Since the signals PP1 and PP2 have a sufficiently high band (RF band), the band of the signal TE1 is also high. The low-pass filter 43 receives the signal TE1 and outputs a signal TE2 which is in the tracking servo band.

The subtractor 44 receives the output signal TE2 of the low pass filter 43 and the output signal OF of the sample hold circuit 42, and receives the offset component OF from the signal TE2.
Is subtracted and the signal TE3 is output. Signal TE3F,
As described above, after the phase is compensated by the circuit 18, the amplifier 2
It is amplified by 0 and drives the tracking actuator 22.

The adder 45 adds the signal PP1 and the signal PP2. The sum of the signal PP1 and the signal PP2 is the total amount of light on the photodetector 12, and thus becomes an RF signal. The binarization circuit 46 outputs a signal DT that binarizes this RF signal. The PLL 47 receives the signal DT and generates a clock signal CK locked to this signal. The timing generation circuit 48 receives this clock signal CK as a system clock and generates a sampling signal SH at a timing corresponding to the mirror surface of the region MR.

FIG. 5 shows a specific example of the structure of the timing generation circuit 48 shown in FIG. 4, and FIG. 6 is a timing chart showing signals at various parts of the circuit shown in FIG. 6 shows the pits on the optical disc 2, but only the portion of the frame synchronization pattern is shown. DT indicates a signal corresponding to the frame synchronization pattern and a signal corresponding to the data, and FIG.
Shows a signal having a length of about 2 frames.

The frame sync pattern detection circuit 50 of FIG.
Judges that this is a frame synchronization pattern when two Tmax (= 11T) are continuously detected,
The frame synchronization detection signal FS in the "H" state is output. Since this circuit is an existing circuit, detailed description is omitted.

The counter 53 counts the clock signal CK while DT = “H”. When the output value of the counter 53 is equal to Tmax (11T), the comparator 54 sets its output signal TMH to "H". DT is "L"
Then, the counter 53 is cleared and the comparator 5
4 sets its output signal TMH to "L". That is,
The signal TMH in the "H" state is generated when a pit having a length of Tmax is detected.

Since pits of Tmax are also generated in the data area (other than the frame synchronization pattern), the main counter 51 and the comparator 52 are selected in order to select the one corresponding to the frame synchronization pattern from the signal TMH in the "H" state. Generates a window pulse for T _W after T _F time when the frame synchronization detection signal FS is generated. The main counter 51 is cleared when the signal FS becomes “H”, and increases the count value each time it receives the clock signal CK when the signal FS becomes “L”. In the comparator 52, the output value of the counter 51 is T _F and (T _F +
T _W ), the window signal WD which becomes “H” is output.

The AND gate 55 outputs an AND pulse in the "H" state if the signal TMH in the "H" state is included in the window signal WD in the "H" state. The shift register 56 has a sampling signal S near the center of the region MR.
In order to generate H, an AND pulse, for example, 5T (5
It is delayed by the clock) and output as the sampling signal SH. As described above, the sample hold circuit 4 of FIG.
2 samples and holds the signal TE1 at the timing of the sampling signal SH.

[0027]

According to the tracking offset canceling method and apparatus of the present invention, the tracking error signal is generated by the push-pull method, and the sampling signal based on the reflected light from the pit-free portion in the frame synchronization pattern of the optical disc is used. Since the tracking error signal is sampled and the tracking error signal thus sampled is used to cancel the offset of the tracking servo, the offset can be easily canceled. By canceling the tracking offset, the amount of detrack is reduced, and as a result, the error rate of data is reduced, and stable recording / reproducing can be performed.

The push-pull method does not require the grating required in the three-spot method, and as a result, the design of the optical system can be simplified, the assembly and adjustment can be facilitated, and the cost can be reduced.

Further, when considering the compatibility of MO and CD-ROM, the present invention provides the possibility of using an optical system common to both.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration example of a tracking servo device for a compact disc using the present invention.

FIG. 2 is an explanatory diagram showing a format of one frame of a compact disc.

FIG. 3 is a diagram showing a relationship between a frame synchronization pattern of a track on a compact disc and pits of adjacent tracks.

4 is a block diagram showing a specific configuration example of an offset cancel circuit 16 of the tracking servo device of FIG. 1, that is, a configuration of an embodiment of a tracking offset cancel device of the present invention.

5 is a block diagram showing a specific configuration example of the timing generation circuit 48 of FIG.

FIG. 6 is a timing diagram showing signals of various parts of the circuit of FIG.

FIG. 7 is a diagram showing a change in intensity distribution of return light according to a relative position change between a pit and a beam spot.

FIG. 8 is a diagram showing a cause of an offset of a tracking error signal.

FIG. 9 is a diagram showing an ISO standard MO format.

[Explanation of symbols]

 2 compact disk 12 photodetector 14A, 14B preamplifier 16 offset cancel circuit 41 subtractor 42 sample hold circuit 43 low-pass filter 44 subtractor 45 adder 46 binarization circuit 47 PLL 48 timing generation circuit 50 frame synchronization pattern detection circuit 51 main Counter 52 Comparator 53 Counter 54 Comparator 55 AND Gate 56 Shift Register

Claims (4)

[Claims] 1. A tracking offset canceling method for an optical disk drive apparatus which performs push-pull tracking servo, wherein a tracking error signal is generated by the push-pull method, and reflected light from a portion without a pit in a frame synchronization pattern of the optical disk. A tracking offset canceling method, wherein the tracking error signal is sampled by a sampling signal based on, and the tracking servo signal is canceled by using the sampled tracking error signal. 2. A pit detection signal is output when a pit corresponding to the pit length of the frame synchronization pattern is detected, and a window signal having a predetermined time length including the time at which the frame synchronization pattern should be detected is generated. 2. The tracking offset canceling method according to claim 1, wherein the sampling signal is generated based on the pit detection signal output while the window signal is being generated. 3. A tracking offset cancel device of an optical disk drive device for performing push-pull type tracking servo, and tracking error signal generating means for generating a tracking error signal by the push-pull method, and a pit in a frame synchronization pattern of the optical disk. Sampling signal generating means for generating a sampling signal based on reflected light from a non-existing portion, sampling means for sampling the tracking error signal by the sampling signal, and tracking servo control using the sampled tracking error signal. A tracking offset cancel device, comprising: an offset cancel unit that cancels an offset. 4. The pit detection means for outputting a pit detection signal when the sampling signal generation means detects a pit corresponding to the pit length of the frame synchronization pattern, and the frame synchronization pattern should be detected. And a window means for generating a window signal having a predetermined time length including time, and the sampling signal is generated based on the pit detection signal output when the window signal is being generated. The tracking offset cancel device according to claim 3.