KR20080066544A - Method for selectiong focus servo process - Google Patents

Abstract

A method for selecting focus servo scheme is provided to automatically select either an AD(Astigmatism Detection) or a DAD(Differential Astigmatism Detection) control scheme so as to perform an optimal focus scheme regardless of disk type. A method for selecting focus servo scheme comprises the steps of measuring the amount of first servo crosstalk by using a first focus servo scheme and calculating a first comparative value representing the first servo crosstalk(S704), measuring the amount of second servo crosstalk by using a second focus servo scheme and calculating a second comparative value representing the second servo crosstalk(S708), and selecting a focus servo scheme corresponding to the scheme having a smaller value by comparing the first servo crosstalk value with the second servo crosstalk value(S713).

Description

How to select focus servo method {Method for selectiong focus servo process}

1 is a graph of servo-crosstalk of the main beam and the focus error signal of the AD system.

Fig. 2 is a graph showing servo error of the focus error signal, the main beam, and the sub beam of the DAD method.

3 is a view showing the main beam M and the sub-beams S1, S2 formed on the disk track and the process of adding three beams.

FIG. 4 is a graph showing a state in which each servocrosstalk is added in FIG.

Fig. 5A is a diagram showing a servocrosstalk amount reduction value when a main beam and a sub beam are normally aligned in a DAD method.

Fig. 5B is a reduction value of the servocrosstalk when the subbeam (subbeam) is rotated about 4 degrees.

Fig. 5C shows the reduction value of the servocrosstalk when the main-sub beam distance is about 10 mu m apart.

6 is a flowchart of an optical pickup head performed by a conventional focus servo method.

7 is a flowchart illustrating an optical pickup head including a method for automatically selecting a focus servo method according to the present invention.

(Explanation of symbols for the main parts of the drawing)

11, 21: main photodetector cell 12, 22: focused main beam

24: sub-photodetector cell 25: focused sub-beam

13: Main beam focus error signal (AD system)

23: main beam focus error signal (DAD method)

26: Focus error signal of two main beams and two sub beams (DAD method)

14: Servocrosstalk graph of main beam (main beam) (AD system)

27: Servocrosstalk graph of sub beam (sub beam) (DAD method)

28: Servocrosstalk graph of main beam (main beam) (DAD method)

29: Servocrosstalk graph of main beam and sub beam (DAD method)

The present invention automatically selects one of the AD method (Astigmatism Detection) and the DAD (Differential Astigmatism Detection) before the focus-on step of the optical pickup device to implement an optimal focus servo. The present invention relates to a focus servo method automatic selection method.

Recently, the optical pickup device of disc driver recognizes and plays various storage media such as DVD ± R, DVD ± RW, DVD ± R DL, DVD RAM disc, etc., and each disc with different thickness, track pitch, data density, etc. In the optical pickup device of the DVD system which can be reproduced by the photo detector, the disc is inserted and the type of disc inserted is determined at the same time. This process is performed at the same time as the focus servo operation that focuses the beam on the disk surface. Usually, various types of disks are discriminated by the time difference between the signal reflected from the data surface of the DVD and the signal reflected from the DVD surface. Will be

Currently, most optical pickup focus control is controlled by AD (non-point aberration method) such as DVD ± R, RW, R DL, and DVD RAM disk by DAD (differential control astigmatism) method.

There is a noise phenomenon inevitably generated in the focus servo control as described above. This is a phenomenon in which the control signal of the left and right tracking servo flows into the control signal of the upper and lower focusing servo as the beam passes between the land and the groove when the beam passes the track surface. This is called servo crosstalk.

The servocrosstalk phenomenon is a factor that deteriorates the characteristics of the recording and reproducing signals by inserting a sinusoidal noise wave centered at a constant offset into the focus servo signal. In the superimposed control method, the amplitude and offset size may be greatly reduced in some cases, and thus may be effectively suppressed.

However, the AD method of controlling only the main beam may be more advantageous in low density disks, such as DVD RAM or BD, which have been used only. Therefore, an efficient control method for this part has been required. Regardless of the need, the AD and DAD control methods are automatically selected as necessary to perform the optimal focus servo.

In order to solve the above technical problems, it is necessary to understand the nature and differences of the AD method (Astigmatism Detection) and the DAD method (Differential Astigmatism Detection).

First, astigmatism is one of Seidel's five aberrations in an optical system with a symmetry axis such as a lens. The image of the object point away from the main axis does not become a perfect point but becomes a ring or radial blur. It is the aberration that appears when you have different optical forces for two perpendicular bundles of light.

If the object is not on the optical axis, there will be two different bundles of light, where the plane of light passing through the optical axis is called the tangent plane.

The other plane of light that does not cross the optical axis of the optical system is called the sagittal plane, and aberrations appear here because the light on the tangent plane is closer than the plane of the arrow.

In the photodetector of the optical pickup device, such astigmatism is clearly generated. The reflected light reflected from the disk surface and passed through the objective lens and the grating (diffraction grating) is formed by dividing the photodetector into one main beam and two sub beams.

At this time, the main beam and the sub-beams that are formed in each sensing cell of the photodetector (refer to three detectors on the left side of FIGS. 1 and 2 are divided into four cells each into four cells) are the signals obtained by adding the servocrosstalk amount to the main signal. In this case, ignoring the sub beam and controlling only the main beam is called AD method (Astigmatism Detection: astigmatism), and controlling combining the main beam and the sub beam is DAD (Differential Astigmatism Detection: differential controlled boiling point). Aberration method)

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Referring to FIG. 1, as shown in FIG. 1A, a focus error signal of an AD method that ignores a sub-beam and performs a focus servo only on a main beam when a beam is formed in a photo detector cell is illustrated. The beam bound as indicated by reference numeral 12 in a) generates a focus error signal as shown in reference numeral 13 in FIG. A wave-shaped servo crosstalk signal flowing in a small amount from the tracking signal on the waveform curve of the main beam focus error signal 13 is shown in the graph of Fig. 1 (c) and 14.

In the servo crosstalk signal 14 of FIG. 1 (c), -1 on the x-axis is when the lens is in the land of the disc, 0 is when the lens is in the groove of the disc, +1 is the lens next to the disc. Indicates when it is located in the land. The y-axis represents a ratio with respect to the value when the standard value is set to 1, where the standard value may be the focus error signal of the main beam, that is, the peak-to-peak value of FIG. 1 (b) (13).

The servocrosstalk signal as described above interrupts the characteristics of the signal even after the focus on (the beam formed on the left photodetector cell becomes round) and causes a small wave in the optical signal. Interfere with playback.

2 shows a focus error signal when using a DAD method using sub-beams formed above and below the main beam, and the amplitude is further increased by simply adding the main beam and the sub-beam as shown in FIG. As the focus error signal is increased, it helps the DPP method to perform the focus-on (the photodetector cell on the left side of the main beam side of the main beam side of the drawing). The focus-on operation becomes easy according to the amplitude). However, if the phase of the sub beam and the main beam are in the same phase, the servo overlap is increased due to the wave overlap phenomenon where the servo beams of the main beam and the sub beam are combined. do.

In order to eliminate this, the DPP-Main-Sub method shifts the sub beam toward the track by changing the phase of the wave, and then combines the main beam and the sub beam, which results in the same result as the wave attenuation as shown in FIG. As a result, the total servocrosstalk plus the main beam and the sub beam is greatly reduced.

In the servo crosstalk signals 27 and 28 of Fig. 2 (c), -1 on the x-axis is when the lens is located in the land of the disk, 0 is when the lens is in the groove of the disk, and +1 is the lens of the disk. It shows when it is located in the next land. The y-axis represents a ratio with respect to the value when the standard value is set to 1, where the standard value may be the focus error signal of the main beam, that is, the peak-to-peak value of FIG. 2 (b) (13).

As shown in Figs. 3 and 4, the DAD method having the characteristic of adding and attenuating waves having opposite phases to each other by placing the main beam and the sub beam in the groove and the land (Fig. 3 (a)) is shown in Fig. 4A. This results in adding the servo crosstalk of the phase-shifted main beam and the sub-beams, reducing overall servocrosstalk (Fig. 4 (b)) and adding an offset adjust to remove the offset component through the traverse signal. If performed, the servocrosstalk will be extremely reduced.

FIG. 3 (a) shows a mechanism in which the sub beams S1 and S2 located on the land side and the main beam M located on the groove side are added to each other by being out of phase with each other in the DAD method. In this case, the role of offsetting the sub beam by half (TP / 2) of the track pitch is DPP-Main-Sub (Differential Push-Pull: Differential Push-Pull Control). Subbeam) method or the like.

Since the DC voltage and AC amplitude of the main beam and the sub beam are different from each other, the Ks value is added as a correction operator to the sub beam in the DAD method as shown in FIG. 3 (b). Use the same Ks value as the correction operator in.

As shown in FIG. 4, when Offset Adjust is performed through a Traverse signal when a signal having a small amplitude and a small offset amount is generated, a good optical signal having better recording and reproduction characteristics is obtained.

In the servo crosstalk signal 29 in Fig. 4, -1, 0, +1 on the x-axis indicate when the lens is located on the land, groove, and next land of the disc, respectively. The y-axis represents a ratio with respect to the value when the standard value is set to 1, where the standard value may be the focus error signal of the main beam, that is, the peak-to-peak value of FIGS.

In general, when a high density disc such as a DVD RAM disc is played, the DAD method using the main beam and the sub beam is advantageous, but the above DAD method has the following problems, but due to the optical characteristics of the DAD method, the optical pickup system Even if only a slight change occurs in the first half (for example, referring to FIG. 5, there may be a case in which the sub-beam is rotated in the R direction or the distance between the main beam and the sub-beam is not stable). There is a case that the characteristics are bad.

As a result, as shown in FIG. 5A, if the beam can be aligned ideally, the servo crosstalk generated by the AD method using only the main beam is 6%. In the DAD method using the sub beam, the amount of servocrosstalk can be reduced to 0.2%. there was.

However, when the sub-beam is rotated only 4 ° from the angle of alignment with the main beam due to a certain unpredictable cause as shown in FIG. 5B, even if the DAD method cannot be reduced to less than 1%, especially as shown in FIG. 5C. If the distance between the main and sub beams is only about 10 μm, which is an error that can be sufficiently generated mechanically, it can be reduced by only 3% even if the DAD method is used, so that the utility of using a more complicated DAD method than the AD method is questioned. A decrease in efficiency occurred.

For the above reason, the present invention is characterized in that the optical pickup device can selectively switch between the AD method and the DAD method depending on the case when the focus servo is performed. To do more than once.

Referring to FIG. 7, the focus error signal A1 is measured while performing an AD disk recognition operation using only the main beam at the time of focus search by the lens up-down operation (S701). The focus on (focusing) is completed by the up-down operation of the lens (S702). The servo crosstalk signal B1 generated when the focus is turned on is measured (S703). The first comparison value V1 is calculated and stored based on the measured focus error signal A1 and the servo crosstalk signal B1 (S704).

The first comparison value V1 is a value representing the amount of servo crosstalk after focus on, and may be calculated by, for example, the following equation.

V1 = B1 _{p_p} / A1 _{p_p} × 100 (%) (Equation 1)

Where A1 _{p_p} is the focus error signal a1, i.e., the peak to peak value of Figs. 1 (b) and 13, and B1 _{p_p} is the servo crosstalk signal B1, i.e. Is the peak-to-peak value.

Then, the focus search is performed again by the up-down operation of the lens. In this case, the focus error signal A2 of the main beam and the focus error of the sub-beams are performed while performing a DAD disk recognition operation using the sub-beams to the main beam. The focus error signal A3 of the signal and the signal obtained by adding the main beam and the sub beam are measured (S705). The focus on is completed by the up-down operation of the lens (S706). The servo crosstalk signal B2 generated when the focus is turned on is measured (S707). The second comparison value V2 is calculated and stored based on the measured main beam error signal A1 and the servo crosstalk signal B2 (S705).

The second comparison value V2 is a value representing the amount of servo crosstalk and may be calculated by, for example, the following equation.

V2 = B2 _{p_p} / A2 _{p_p} × 100 (%) (Equation 2)

Wherein A2 _{p_p} is a peak tupikeu value of the focus error signal (A2), i.e., FIG. 2 (b) (23) of the main beam, B2 _{p_p} ie servo crosstalk signal (B2), which is generated in the two focus-on state is also It is the peak-to-peak value of 4 (b) (29).

Alternatively, the second comparison value V2 may be calculated using the focus error signal A3 and the servo crosstalk signal B2 of the sum of the main beam and the sub beam.

That is, the second comparison value V2 may be calculated by the following equation.

V2 = B2 _{p_p} / A3 _{p_p} × 100 (%) (Equation 3)

Wherein A3 _{p_p} is a peak tupikeu value of the main beam and the sub focus error signal of a signal summing beam (A3), i.e., 2 (b) (26) also, B2 _{p_p} servo crosstalk generated in the two focus-on state Signal B2, i.e., the peak-to-peak value of Fig. 4B (29).

Next, the first comparison value and the second comparison value are compared (S709). If the first comparison value of the two values is smaller at this time, the AD method is more than the DAD method, which is a method of universally increasing accuracy. Since it is concluded that it is accurate and efficient, at this time, the focus error signal of the sub-beam is removed (S710), and the focus is turned on by the AD method (S711), and then the tracking balance adjustment (S714), tracking on (S715), and focus deviation adjustment ( S716) and the like to perform a series of control operations.

However, if the first comparison value is larger than the second comparison value because the normal beam is aligned in the above case, the focus signal of the sub-beam is maintained by the DAD method (S713) and the focus is turned on by the DAD method (S712). Thereafter, a series of control operations such as tracking balance adjustment (S714), tracking on (S715), and focus deviation control (S716) are performed.

On the other hand, in the step (S601 ~ S606) of Figure 6 is shown a conventional optical pickup head control step that is not equipped with the present invention, here, whether the AD method or the DAD method is selected, as shown in FIG. There is no series of steps to select smaller values after volume comparison.

In the step of FIG. 7, the first step of FIG. 7 may be started by the DAD method, if the disc that was used during the previous playback or recording was performed by the focus servo by the DAD method. The first comparison value and the second comparison value may be interchanged. In other words, the entire work step (sub beam focus signal removal / maintain, focus on) from the comparison part to the tracking balance control step in the flowchart is changed left and right.

Similarly, in the above case, the servocrosstalk amount of the first stage will be the servocrosstalk amount by the DAD method, and the next step will be the servocrosstalk amount by the AD method.

The specific embodiments of the present invention have been described in detail above with reference to the drawings. However, the technical idea of the present invention is not limited to the above-described embodiment, and the first servo-crosstalk value is obtained by performing the focus servo using the first method, which is the technical idea of the present invention. It is evident that it is substantially within the scope of the technical idea of the present invention that the focus servo is performed in the second method to obtain a second servocrosstalk value, and the two are compared to select a more efficient method. And another embodiment due to the cooking selection can be implemented any number from the above embodiment of the present invention.

As described above, the method of automatically selecting a focus servo method according to the present invention has the following advantages.

First, since the servo-crosstalk obtains a small stable focus error signal, it is possible to minimize an error (for example, when a CD-R disc is misjudged as a CD-RW) when the inserted disc is recognized.

Second, stable focus control can be maintained during the recording or reproducing process according to the introduction of a stable focus error signal.

Third, regardless of the type of disc, as the servo servo is selected as the smaller focus servo, it can be applied to all disc drives (CD-R / RW, DVD recording and playback system, HD compatible BD, etc.). The compatibility of the mounted optical pickup head itself can be greatly increased.

Claims (13)

Calculating a first comparison value representing the first servocrosstalk amount by measuring the first servocrosstalk amount by a first focus servo method; Calculating a second comparison value representing the second servocrosstalk amount by measuring the second servocrosstalk amount by a second focus servo method; And Comparing the first servocrosstalk value with the second servocrosstalk value and selecting a focus method corresponding to a value having a small size; Focus servo method selection method comprising a. The method of claim 1, Computing the first comparison value, Focusing on the first focus servo method; Measuring a focus error signal of a main beam in the focus on step; Measuring a servocrosstalk signal after the focus on; And Calculating a ratio of the peak to peak value of the focus error signal and the peak to peak value of the servocrosstalk signal; Focus servo method selection method comprising a. The method of claim 1, The first focus method is any one of a focus search method using an AD method (Astigmatism Detection) and a focus search method using a DAD method (Differential Astigmatism Detection) and a second focus method. The method is a focus servo method selection method characterized in that the focus method of the method not selected in the first focus method of the AD method and the DAD method. The method of claim 3, Computing the second comparison value, Focusing on the second focus servo method; Measuring a focus error signal of a main beam in the focus on step; Measuring a servocrosstalk signal after the focus on; And Calculating a ratio of a peak to peak value of the focus error signal of the main beam and the peak to peak value of the servocrosstalk signal; Focus servo method selection method comprising a. The method of claim 3, Computing the second comparison value, Focusing on the second focus servo method; Measuring a focus error signal of a signal obtained by adding a main beam and a sub beam in the focus on step; Measuring a servocrosstalk signal after the focus on; And Calculating a ratio of a peak to peak value of a focus error signal of the sum of the main beam and the sub beam and a peak to peak value of the servocrosstalk signal; Focus servo method selection method comprising a. The method of claim 1, And performing a tracking balance control, a tracking on control, and a focus deviation control according to the focus servo method. The method of claim 6, And removing the focus signal of the sub-beam prior to the focus-on process if the selected servo-crosstalk amount is measured only in the main beam. In performing focus servo, Measuring a first servo crosstalk amount by a focus servo method most recently performed to calculate a first comparison value representing the first servo crosstalk amount; Calculating a second comparison value representing the second servo crosstalk amount by measuring a second servo crosstalk amount by a focus servo method different from the most recently performed method; Comparing the first comparison value with the second comparison value and selecting a smaller one; And Selecting a focus servo method corresponding to the selected servo crosstalk amount; Focus servo method selection method comprising a. The method of claim 8, The most recently performed focus servo method is AD (Astigmatism Detection), Computing the first comparison value, Focusing on the AD method; Measuring a focus error signal of a main beam in the focus on step; Measuring a servocrosstalk signal after the focus on; And Calculating a ratio of the peak to peak value of the focus error signal and the peak to peak value of the servocrosstalk signal; Focus servo method selection method comprising a. The method of claim 8, The other focus servo method is a DAD method (Differential Astigmatism Detection). Computing the second comparison value, Focusing on the DAD method; Measuring a focus error signal of a main beam in the focus on step; Measuring a servocrosstalk signal after the focus on; And Calculating a ratio of a peak to peak value of the focus error signal of the main beam and the peak to peak value of the servocrosstalk signal; Focus servo method selection method comprising a. The method of claim 8, The other focus servo method is a DAD method (Differential Astigmatism Detection). Computing the second comparison value, Focusing on the DAD method; Measuring a focus error signal of a signal obtained by adding a main beam and a sub beam in the focus on step; Measuring a servocrosstalk signal after the focus on; And Calculating a ratio of a peak to peak value of a focus error signal of the sum of the main beam and the sub beam and a peak to peak value of the servocrosstalk signal; Focus servo method selection method comprising a. The method of claim 8, And performing a tracking balance control, a tracking on control, and a focus deviation control according to the focus servo method. The method of claim 12, And removing the focus signal of the sub-beam prior to the focus-on process if the selected servo-crosstalk amount is measured only in the main beam.

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