Optical disc device and reproduction method

Due to the axial runout of an optical disc, a frequency of a high-frequency current to be superimposed onto a DC current could not be maintained. In order to solve the problem described above, a drive signal is generated by superimposing a high-frequency signal onto a DC current, the drive signal is applied to a laser beam light source, thereby the light source is driven; and a servo signal at a signal level corresponding to a defocus amount of the laser beam relative to the surface of the optical disc is generated based on a reflected light of the laser beam from the recording surface of the optical disc, and a low-frequency component of the servo signal is extracted, and thereby the frequency of the high-frequency signal to be superimposed onto the DC current in the light source driver is controlled based on the low-frequency component of the servo signal.

This application relates to and claims priority from Japanese Patent Application No. 2007-103829, filed on Apr. 11, 2007, the entire disclosure of which is incorporated herein by reference.

BACKGROUND

Conventionally, laser diodes have been used as the light source in an optical disc drive, and laser beams emitted from the laser diodes have been known for their strong coherence. For this reason, in optical disc devices, a high-frequency superimposition method has been used as a method for reducing laser noise caused by the coherence between an outgoing beam from a laser diode and reflected light from an optical disc (see JP5-89465 A).

The high-frequency superimposition method is a method of, in a reproduction mode, producing a driving current in which a high-frequency current is superimposed on a DC current; and driving the laser diode based on the generated drive signal. Since the superimposition of such a high-frequency current can make spectrum of a laser beam turn into a multimode spectrum, the coherence of the laser beam can be reduced, and so laser noise due to reflected light from an optical disc can be reduced.

Furthermore, with respect to the high-frequency superimposition method, it has been known that the relationship between a light path length L between a light emitting point and an optical disc and the frequency F of high-frequency current at which the laser noise becomes lowest (hereinafter referred to as optimal frequency), is expressed in the below formula where c represents the velocity of light:

[Formula 1]
f=c/(4×1)  (1)
This formula (1) corresponds to pulse-driving the laser diode, in which the laser diode is turned off while the reflected light from an optical disc is re-entering the laser diode.

SUMMARY

The present invention relates generally to an optical disc device and a reproduction method, and is suitable for use in an optical disc device compatible with, for example, BD (Blu-ray Disc).

An optical disc device generally causes runout in axial direction, so called axial runout, of an optical disc during rotational driving of the optical disc. Due to this runout, the light path length between the laser diode light emitting point and the optical disc changes periodically.

However, in conventional optical devices, the axial runout of an optical disc has not been considered, and the frequency of a high-frequency current, which is to be superimposed on a DC current, has been fixed to a value determined during the design period so as to be in accordance with formula (1). Therefore, current optical disc devices have a problem of not being able to maintain the frequency of the high-frequency current at an optimal frequency.

Under the above-described circumstances, since a reflected light from an optical disc re-enters an on-state laser diode, laser noise is caused by the coherence between an outgoing beam from the laser diode and reflected light from an optical disc. As a result, the problem of deterioration in the quality of a reproduction signal may arise.

The current invention has been devised in consideration of the above-described points, and offers an optical disc device and reproduction method that are capable of, effectively preventing deterioration in the quality of a reproduction signal in advance.

In order to solve the problem described above, according to an aspect of the invention, an optical disc device that concentrates a laser beam of predetermined power onto a recording surface of an optical disc and reproduces information recorded on the optical disc based on laser beam light reflected from the recording surface of the optical disc includes: a light source driver generating a drive signal by superimposing a high-frequency signal onto a DC current and applying the drive signal to a laser beam light source to drive the light source; and a servo controller that generates a servo signal at a signal level corresponding to the defocus amount of the laser beam relative to the surface of the optical disc based on laser beam light reflected from the recording surface of the optical disc and extracts a low-frequency component of the servo signal to control the frequency of the high-frequency signal to be superimposed onto the DC current in the light source driver based on the low-frequency component of the servo signal.

Furthermore, according to an aspect of the invention, a reproduction method for concentrating a laser beam having a predetermined power onto a recording surface of an optical disc, and reproducing information recorded on the optical disc based on laser beam light reflected from the recording surface of the optical disc, includes: a first step of generating a drive signal by superimposing a high-frequency signal onto a DC current and applying the drive signal to a laser beam light source to drive the light source; and a second step of generating a servo signal at a signal level corresponding to the defocus amount of the laser beam relative to the surface of the optical disc based on laser beam light reflected from the recording surface of the optical disc and extracting a low-frequency component of the servo signal to control the frequency of the high-frequency signal to be superimposed onto the DC current in the light source driver based on the low-frequency component of the servo signal.

DETAILED DESCRIPTION

An embodiment of the invention will be described below with reference to the attached drawings.

(1) Configuration of Optical Disc Device According to Present Embodiment

InFIG. 1, reference numeral1indicates an entire optical disc device according to the embodiment, compatible with, for example, BD. This optical disc device includes an interface unit3for communicating with external devices. Various commands from a host computer2are input to an arithmetic controller4via this interface unit3.

The arithmetic controller4is configured as a microcomputer including a CPU (Computer Processing Unit), a ROM (Read Only Memory) and a RAM (Random Access Memory), and executes required control processing and arithmetic processing based on the commands from the host computer2and a control program previously stored in the ROM.

In practice, for example, when receiving a record command from the host computer2, the arithmetic controller4controls the driving of a spindle motor5so that a loaded optical disc6rotates in a rotary mode suitable for the recording mode of that optical disc6(such as CAV (Constant Angular Velocity) mode, CLV (Constant Linear Velocity) mode or ZCLV (Zoned Constant Linear Velocity) mode, etc.).

Also, at the same time, the arithmetic controller4performs predetermined signal processing such as adding error-correcting code to write data sent from the host computer2along with the recording command, and then sends this write data to a modulation processing unit7.

The modulation processing unit7performs modulation processing for the write data it receives, using a (1, 7) RLL (Run Length Limited) code, a (2, 7) RLL code or a (2, 10) RLL code, and then sends the record data acquired that way to a laser diode driver (LDD: Laser Diode Driver)9in an optical pickup8.

The laser diode driver9blinks a laser diode10in accordance with the record data it receives. As a result, a laser beam L1modulated by the drive signal is emitted from the laser diode10, and the laser beam L1is concentrated onto the recording surface of the optical disc6via an optical system including an objective lens11in the optical pickup8. Consequently, the record data is recorded on the optical disc6based on the laser beam L1.

Furthermore, a reflected light L2of the laser beam L1from the optical disc6is collected on a light-receiving surface of a photodetector12via an optical system in the optical pickup8. Then, the photodetector12photoelectrically converts the reflected light L2incident on the light-receiving surface, and sends an RF (Radio Frequency) signal obtained in that way to a servo controller13.

The servo controller13generates, based on the RF signal it receives, a focus error signal at the signal level corresponding to the shift amount (defocus amount) of the focal position of the laser beam L1from the recording surface of the optical disc6, and generates a tracking error signal at the signal level corresponding to the shift amount (off-track amount) of the scanning position of the laser beam L1from a target track.

Also, the servo controller13generates a focus control signal and a tracking control signal based on the above-generated focus error signal and tracking error signal, and sends these signals to a triaxial actuator (not shown in the drawing) supporting the objective lens11in the optical pickup8.

Thus, the triaxial actuator moves, in accordance with these focus control signal and tracking control signal, the objective lens11in an axial direction or radical direction as necessary. Consequently, the laser beam L1just-in-focuses on the recording surface of the optical disc6, and the focus control and tracking control are performed so that the laser beam L1scans on a corresponding track.

Meanwhile, when receiving a reproduction command from the host computer2, the arithmetic controller4drives the spindle motor5so that the loaded optical disc6rotates in a rotation mode suitable for the recording mode for that optical disc6.

Also, the arithmetic controller4, in conjunction with the above, controls the laser diode driver9in the optical pickup8so as to generate a drive signal made by superimposing a high-frequency current onto a DC current of a predetermined level, and applies the drive signal to the laser diode10. As a result, a laser beam L1of predetermined power is emitted from the laser diode10, and the laser beam L1is concentrated onto the recording surface of the optical disc6via the optical system including the objective lens11in the optical pickup8.

The reflected light L2of the laser beam L1from the optical disc6enters the light-receiving surface of the above-described photodetector12via the optical system in the optical pickup8, and the reflected light L2is photoelectrically converted in the photodetector12. Then, an RF signal obtained in this photoelectric conversion is sent to the servo controller13and a reproduction processor14.

The servo controller13, as is does in the recording mode, generates a focus control signal and a tracking control signal based on the RF signal, and sends these signals to the above-described triaxial actuator in the optical pickup8. Accordingly, the same focus control and tracking control as that in the recording mode are performed based on the focus control signal and tracking control signal.

Also, the reproduction processor14executes, for the RF signal it receives, automatic gain control processing for keeping a signal amplitude constant; waveform equalization processing for correcting an optical spatial frequency degradation; and predetermined reproduction processing such as reduction control processing and binarization processing, etc. Then, it sends the obtained binarization data to a demodulation processor15.

The demodulation processor15performs, for the binarization data it receives, demodulation processing according to the (1, 7) RLL code, (2, 7) RLL code or (2, 10) RLL code, and then sends the obtained demodulation data to the arithmetic controller4.

The arithmetic controller4performs as necessary, for the obtained demodulation data it receives, error-correction processing using a error-correcting code in the demodulation data. Then, it sends the obtained read data to the host computer2via the interface unit3.

(2) High-Frequency Superimposition System According to Present Embodiment

Next, a high-frequency superimposition system employed in the optical disc device6according to the present embodiment will be described below.

As described above, in the high-frequency superimposition system, the optimal frequency of a high-frequency current to be superimposed on a DC current in the reproduction mode is uniquely determined depending on the light path length from the laser diode10light emitting point to the optical disc6.FIG. 2shows the relationship between the optimal frequency of a high-frequency current to be superimposed onto that DC current and the light path length. Moreover, in addition to the above-described relationship between the light path length and the optimal frequency, the optimal frequency also changes periodically due to the axial runout of the optical disc6.FIG. 3shows the relationship between the axial runout amount (the amount of displacement of the recording surface of the optical disc6from a reference position in the axial direction) when the light path length inFIG. 2is 60 [mm] and the optimal frequency.

One of the characteristic features of the optical disc device1according to the present embodiment is that, in order to respond to the changes in the light path length from the laser diode10light emitting point to the optical disc6due to the axial runout of the optical disc, in reproduction mode, the low-frequency component of a focus control signal including the information for the axial runout amount of the optical disc6is extracted in the servo controller13; and the frequency of the high-frequency current to be superimposed onto the DC current in the laser diode driver9is changed based on the extracted low-frequency component of the focus control signal.

FIG. 4shows the specific configuration of the servo controller13in the optical disc device1. As shown inFIG. 4, in the servo controller13, an RF signal sent from the photodetector12in the optical pickup8is input to a servo signal generating circuit20. Then, the servo signal generating circuit20generates the above-described focus error signal and tracking error signal based on the RF signal sent from the photodetector12in the optical pickup8during the reproduction mode, and sends these generated signals to a phase compensation circuit21.

The phase compensation circuit21performs predetermined phase compensation processing in order to stabilize a servo system for the focus error signal and tracking error signal it receives. Then, it sends the post-phase compensation processing focus error signal and tracking error signal to a drive circuit22, as a phase-compensated focus error signal and a phase-compensated tracking error signal respectively.

The drive circuit22generates a focus control signal based on the phase-compensated focus error signal it receives, and generates a tracking control signal based on the phase-compensated tracking error signal it receives. Then, it sends these signals to the triaxial actuator in the optical pickup8in the above-described manner. It sends the focus control signal also to a low-pass filter circuit23as well.

The low-pass filter circuit23extracts the low-frequency component, which includes the basic rotation frequency component of the optical disc6, from the focus control signal it receives. Then, the low-pass filter circuit23sends the extracted low-frequency component to a gain control amplifier (GCA: Gain Control Amplifier)24.

The gain control amplifier24amplifies the focus low-frequency component signal it receives by a gain determined in the LDD/VCO characteristic learning function described below, and then sends the amplified focus low-frequency component signal to the laser diode driver9in the optical pickup8as an oscillation frequency control signal.

The laser diode driver9has a configuration, for example, like that shown inFIG. 5, and inputs the record data it receives from the modulation processing unit7and the control signal it receives from the arithmetic controller4to the drive circuit35during the reproduction mode.

The drive circuit35is configured from a current-to-current conversion circuit having, for example, a hundredfold to two-hundredfold gain. The drive circuit35generates a drive signal by amplifying the record data it receives using a gain corresponding to the current value of the control signal, and applies the drive signal to the laser diode10via an adder circuit36. Thus, the laser diode10blinks based on the drive signal, and the record data is optically recorded on the optical disc6.

Meanwhile, the laser diode driver9, in the reproduction mode, inputs an oscillation frequency control signal sent from the servo controller13to a voltage controlled oscillator (VCO)30, which is a high-frequency current source.

The voltage controlled oscillator30has an oscillation characteristic, for example, as shown inFIG. 6, and generates a high-frequency current having a frequency corresponding to the voltage level of the oscillation frequency control signal it receives. Thus, the high-frequency current having a frequency corresponding to the axial runout of the optical disc6at that time is generated from the voltage controlled oscillator30. Then, the voltage controlled oscillator30sends the high-frequency current to a binarization circuit31and an adder circuit36.

The binarization circuit31binarizes the high-frequency current it receives, and sends a high-frequency digital current acquired that way to a frequency divider circuit32. Then, the frequency divider circuit32generates a pulsed monitor signal of a relatively low frequency by frequency-dividing the high-frequency digital signal, and sends the pulsed monitor signal to a frequency counter circuit33.

The frequency counter circuit33operates based on a reference clock CLK sent from the modulation processing unit7(FIG. 1), and measures the frequency of the monitor signal it receives. More specifically, the frequency counter circuit33adds up pulse numbers (the number of pulses forming the monitor signal) input from the frequency divider circuit32during one cycle of the reference clock CLK. Then, the frequency counter circuit33stores the result of the addition of the pulse numbers acquired that way in a register34. Regarding the result of addition, the register34stores the result for at least one rotation period of an optical disc, so that it can be used for the LDD/VCO characteristic learning function described below.

Meanwhile, in the reproduction mode, the adder circuit36receives a temperature-compensated DC current having a constant voltage from the arithmetic controller4via an amplifier37. Then, the adder circuit36adds up (superimposes) this DC current and a high-frequency current provided from the voltage controlled oscillator30, and applies the drive signal acquired that way to the laser diode10as described above. Consequently, the laser diode10lights-up based on this drive signal, and a laser beam of constant power is emitted onto the optical disc6.

Next, the LDD/VCO characteristic learning function installed in the optical disc device1will be described below.

As described above, in the high-frequency superimposition system according to the present embodiment, the frequency of a high-frequency current superimposed onto a DC current is changed based on the low-frequency component of the focus control signal. Consequently, in the optical disc device1according to the present embodiment that employs such a high-frequency superimposition system, the voltage controlled oscillator30should follow the change of the axial runout amount of the optical disc6, and generate a high-frequency current having a frequency corresponding to the axial runout amount at that time.

However, the sensitivity of the gain control amplifier24(FIG. 4) varies from optical disc device to optical disc device, and not all voltage controlled oscillators30in all the optical devices always generate the same frequency for the high-frequency current in accordance with the axial runout of the optical discs6.

Therefore, as shown inFIG. 7, the optical disc device1according to the present embodiment has an LDD/VCO characteristic learning function for automatically adjusting the gain of the gain control amplifier24, so that the oscillation frequency “f” of the voltage controlled oscillator30is kept within a predetermined range (HF1≦f≦HF2) for the axial runout of the optical discs6(hereinafter referred to as gain adjustment optical discs) having well-known axial runout widths (width of runout in axial direction of the optical disc6).

Furthermore, inFIG. 7, HF1and HF2respectively indicate the minimum oscillation frequencies for the voltage controlled oscillator30and the maximum oscillation frequency of the voltage controlled oscillator30, which are expected when reproducing a gain adjustment optical disc6. More specifically, when the axial runout amount of the gain adjustment optical disc is ±0.25 [mm], the oscillation frequency expected for the voltage controlled oscillator30is 1.25 [GHz] ±5 [MHz], therefore, HF1and HF2at that time are 1.245 [GHz] and 1.255 [GHz], respectively.

FIG. 8shows the specific content of the processing performed by the arithmetic controller4according to the LDD/VCO characteristic learning function. When a command to perform the LDD/VCO characteristic learning processing (hereinafter referred to as an LDD/VCO characteristic learning processing execution command) is provided by the host computer2in accordance with user operation, the arithmetic controller4starts the LDD/VCO characteristic learning processing by changing the operation mode to LDD/VCO characteristic learning mode. First, it performs reproduction processing for reproducing data from a gain adjustment optical disc6(SP1).

Following the above, the arithmetic controller4searches the register34(FIG. 5) in the laser diode driver9for the maximum value (hereinafter referred to as measured maximum frequency) and minimum value (hereinafter referred to as measured minimum frequency) of the frequency of high-frequency currents, and reads out these values (SP2).

Then, the arithmetic controller4judges whether or not the range extending from the measured maximum frequency to the measured minimum frequency (hereinafter referred to as measured oscillation frequency range) retrieved from the register34in step SP2is within an acceptable range (hereinafter referred to as acceptable oscillation frequency range) for the oscillation frequency generated by the voltage controlled oscillator30, which is determined in advance so as to include both HF1and HF2inFIG. 7(SP3).

When a negative result is obtained in this judgment, the arithmetic controller4judges whether or not the measured oscillation frequency range exceeds the acceptable oscillation frequency range (in other words, whether or not the measured maximum frequency is larger than the maximum value in the acceptable oscillation frequency range) (SP4). Then, the arithmetic controller4controls the gain control amplifier24so that the gain control amplifier raises the gain when a negative result is obtained in this judgment (SP5); or reduces it when a positive result is obtained in this judgment (SP6).

Next, the arithmetic controller4repeats the same steps (SP1-SP6-SP1), and when a positive result is finally obtained in step3, it terminates this LDD/VCO characteristic learning processing.

(4) Advantageous Effects of Present Embodiment

As described above, in the optical disc device1according to the present embodiment, because the oscillation frequency of the voltage controlled oscillator30in the laser diode driver9is controlled in accordance with the low-frequency component of the tracking error signal (tracking control signal), it is always possible to superimpose, on the DC current, a high-frequency current with the frequency constantly optimal for the distance between the laser diode10light emitting point and the optical disc6, which changes according to the axial runout of the optical disc6. Consequently, laser noise due to the coherence between an outgoing beam from the laser diode10and reflected light from the optical disc6can be reduced, and deterioration in the quality of a reproduction signal can be effectively prevented in advance.

(5) Another Embodiment of the Invention

In the foregoing embodiment, the case where the invention is applied to the BD-compatible optical disc device1is described. However, the invention is not limited to this example, and is capable of being applied in other various optical disc devices compatible with different kinds of optical discs other than BD.

Also in the foregoing embodiment, the case where, in the LDD/VCO characteristic learning mode, the output of the gain control amplifier24is applied to the voltage controlled oscillator30and the gain of the gain control amplifier24is adjusted in accordance with the oscillation frequency of the voltage controlled oscillator30at that time is described. It is limited to this example, however, and other various adjustment means can be employed in the invention, as means for adjusting the gain of the gain control amplifier24.

For example, the gain of the gain control amplifier24may be adjusted by means of, without directly applying the output of the gain control amplifier24when reproducing the forgoing gain adjustment optical disc to the voltage controlled oscillator30, but storing the maximum value and the minimum value of the output of the gain control amplifier24at that time in the gain control amplifier24; applying only these maximum and minimum values of the output of the gain control amplifier24to the voltage controlled oscillator30; and controlling the gain of the gain control amplifier24based on the oscillation frequency of the voltage controlled oscillator30at that time.

Furthermore, as described in the foregoing embodiment, the servo controller13is configured as shown inFIG. 4, however, in the invention, the configuration of the servo controller13is not limited to this example, and the servo controller13may be configured, for example, as shown inFIG. 9, where parts corresponding to those inFIG. 4have the same reference numerals.

The servo controller40shown inFIG. 9differs from the servo controller13inFIG. 4in having a gain control amplifier40consisting of the gain control amplifier24and a switch circuit41, in place of the gain control amplifier24. With a servo controller having this configuration, when the connection terminal inside the switch circuit41is connected to a first switch end41A, the same LDD/VCO characteristic learning processing as that in the foregoing embodiment can be performed; and when the connection terminal inside the switching circuit41is connected to a second switch end41B and the maximum value and the minimum value of the oscillation frequency control signal, which are expected to be acquired when reproducing the gain adjustment optical disc6, are applied from the arithmetic controller4to the second switch end, the same LDD/VCO characteristic learning processing as that in the foregoing embodiment can be performed without using the gain adjustment optical disc6.

Moreover, as described in the foregoing embodiment, the laser diode driver9, as a light source driver, is configured as shown inFIG. 5, however, in the invention, the configuration of the laser diode driver9is not limited to this example, and the laser diode driver9may be configured as, for example, shown inFIG. 10, where parts corresponding to those inFIG. 5have the same reference numerals.

The laser diode driver50shown in thisFIG. 10differs from the laser diode driver9inFIG. 5in having a write strategy circuit51and an interface circuit52, in place of the drive circuit35(FIG. 5).

The write strategy circuit51is a circuit for performing multi-pulse modulation processing for record data sent from the modulation processing unit7based on a clock also sent from the relevant modulation processing unit7. Also, the interface circuit52is a circuit serving, not only as an interface for the arithmetic controller4, but also as a register for storing data, such as strategy execution time information and power information for multi-pulse processing performed in the strategy circuit52. In addition, the interface circuit52contains a digital/analog circuit, so that the arithmetic controller4can apply the oscillation frequency control signal expected to be acquired when reproducing the gain adjustment optical disc to the voltage controlled oscillator30via interface circuit52. Thus, with the laser diode driver50having a configuration like the above, the same LDD/VCO characteristic learning processing as that in the foregoing embodiment can be performed even when an oscillation frequency control signal cannot be obtained from the servo controller13; accordingly, for example, it is possible to have the optical disc device1automatically perform factory-set LDD/VCO characteristic learning processing at a predetermined time, such as when the optical disc device1is powered-on.

According to the invention, even when an axial runout occurs on an optical disc, the frequency of a high-frequency signal to be superimposed on a DC current can be maintained at an optimal frequency at all times, so laser noise due to the coherence between an outgoing beam from a laser diode and reflected light from an optical disc can be prevented. Consequently, an optical disc device and a reproduction method that are capable of effectively preventing deterioration in the quality of a reproduction signal in advance can be achieved.