Abstract:
An optical disc drive apparatus includes: a laser beam source; a collector device disposed as facing a disc that can record a signal, which collects a laser beam emitted from the laser beam source onto the disc; a record processing module which records the signal on the disc through the collector device while modulating laser power of the laser beam source; a reflecting light level detecting module which receives the reflecting light when the signal is recorded and detects a level of the reflecting light; a laser power control module which controls the laser power when the signal is recorded based on the detected level of the reflecting light; a tilt control module which controls a tilt angle between the disc and the collector device based on the detected level of the reflecting light; and a switching module which switches between control of the laser power and the tilt angle.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   The present invention contains subject matter related to Japanese Patent Application JP 2005-216629 filed in the Japanese Patent Office on Jul. 27, 2005, the entire contents of which being incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical disc drive apparatus which can record a signal, and a signal recording method of the same. 
   2. Description of the Related Art 
   In recent years, writable optical discs such as CD±R, DVD±R, DVD±RW, and Blu-ray disc are commercially available. In an optical disc apparatus which can meet these optical discs, for example, there is an optical disc apparatus (for example, see Patent Reference 1) which sets a skew target value based on the signal level of an RF playback signal acquired in signal recording to perform skew control (or, it is referred to as tilt control, but hereinafter, it is referred to as tilt because they have the same meanings). However, the RF signal level (the level of a light reflecting from a pit) in the section for detection during recording is varied not only by the change in the tilt angle but also by irregularities in a recording film and temperature change. More specifically, even though the disc is written at the same recording power, the pit is not formed in the same shape because of irregularities in the recording film and temperature change. 
   In order to solve such problems, under present circumstances, a technique is generally used which is called running OPC (Optimum Power. Control) (hereinafter, it is referred to as ROPC) (for example, see Patent Reference 2). In an ROPC system, the laser recording power is controlled in real time by monitoring the level of a reflecting light so that the level of the reflecting light is made constant. More specifically, for example, suppose a disc is written at a certain level of recording power, or a certain level of multi-pulse power. In the midway of the process, when it is detected that the level of the reflecting light is dropped, which is caused by irregularities in the recording film, for example, the ROPC system performs control so that the recording power is reduced because the pit shape is too deep. 
   Patent Reference 1: JP-A-2000-331364 (Paragraph [0054] and  FIG. 4 ) 
   Patent Reference 2: JP-A-2002-288831 (Paragraph [0008]) 
   SUMMARY OF THE INVENTION 
   However, when tilt control is performed based on the level of the reflecting light in the midway of operation of the ROPC system, the following problems occur. For example, as described above, since the level of the reflecting light is varied because of irregularities in the recording film, the level of a reflecting light having a zero tilt angle is sometimes detected even though the tilt angle is not zero. When such a state continues, the tilt angle may not be converged on zero, and thus tilt control may be impossible. In addition, in this case, the recording power is changed to try to compensate for fluctuations in the tilt angle, and thus the overall recording margin is reduced. Furthermore, when tilt control is performed all the time during recording, recording quality is deteriorated and the loads of the system process time become great. 
   It is desirable to provide an optical disc drive apparatus and a signal recording method of the same, which can perform a recording process while tilt control is being performed in an appropriate manner even though the level of the reflecting light fluctuates in recording because of some cause such as irregularities in the recording film. 
   According to an embodiment of the invention, an optical disc drive apparatus includes: a laser beam source; a collector device which is disposed as it faces a disc that can record a signal and which collects a laser beam emitted from the laser beam source onto the disc; a record processing module which records the signal on the disc through the collector device while it is modulating laser power of the laser beam source; a reflecting light level detecting module which receives the reflecting light when the signal is recorded and which detects a level of the reflecting light; a laser power control module which controls the laser power when the signal is recorded based on the detected level of the reflecting light; a tilt control module which controls a tilt angle between the disc and the collector device based on the detected level of the reflecting light; and a switching module which switches between control of the laser power and control of the tilt angle. 
   In an embodiment of the invention, there is provided the switching module which switches between control of the laser power and control of the tilt angle. Therefore, for example, when laser power control is stopped in tilt control, laser power is made constant. From the viewpoint of performing tilt control, the level of the reflecting light from the disc is detected in an appropriate manner, and recording quality can be prevented from being deteriorated. 
   For the laser beam, for example, a laser beam having a wavelength of about 650 nm is used. However, it is not limited thereto. A laser beam of green, blue or violet having a wavelength shorter than 650 nm, or a laser beam having a wavelength longer than 650 nm may be used. In addition, lights other than visible lights may be used. 
   The collector device means an objective lens or an optical system including the objective lens, which may be any elements which can collect a laser beam on a disc. 
   In an embodiment of the invention, more specifically, the switching module includes: a first acquiring module which acquires a first variation that is a variation in the detected level of the reflecting light; and a first control module which stops control done by the laser power control module and starts control done by the tilt control module when the first variation is equal to or greater than a first threshold. In addition, the switching module includes: a second acquiring module which acquires a second variation that is a variation in the laser power; and a second control module which stops control done by the laser power control module and starts control done by the tilt control module when the first variation is below the threshold and the second variation is equal to or greater than a second threshold. As described above, when the first variation is abnormal, the switching module switches to tilt control without determination whether the second variation is abnormal that is the variation in laser power. Thus, the process is simplified. 
   The variation is a difference between the level at a certain timing and the level at a certain timing after that. In this case, as described later, it may be a difference between the mean value of levels within a certain predetermined period and the level after the predetermined period. Alternatively, the variation may be the variation in time base, that is, it may be a slope. 
   In an embodiment of the invention, the first acquiring module includes: a first computing module which computes a mean value of the levels of the reflecting light within a predetermined period; and a second computing module which computes a difference between the computed mean value and the detected level of the reflecting light after the predetermined period as the first variation. Alternatively, the second acquiring module includes: a first computing module which computes a mean value of the laser power within a predetermined period; and a second computing module which computes a difference between the computed mean value and the detected laser power after the predetermined period as the second variation. The predetermined period can be set freely. “To compute a mean value of the levels of the reflecting light within a predetermined period” also includes a meaning that “sampling is made for a predetermined number of times (for example, twice or greater) to compute a mean value”. 
   In an embodiment of the invention, the record processing module can record the signal on the disc at the laser power including a recording level and a playback level that is a level in a playback process of the signal and in a multi-pulse mode; and the tilt control module performs control within a period that the laser power is turned to the playback level by the record processing module. Alternatively, the record processing module can record the signal on the disc at the laser power including a recording level and an erase level that is a level in an erase process of the signal and in a multi-pulse mode; and the tilt control module performs control within a period that the laser power is turned to the erase level by the record processing module. In the multi-pulse mode, in recording by multiple pulses, the level of the reflecting light is unstable. In the space section such as the erase section and the playback section, the level of the reflecting light becomes stable. Thus, the level of the reflecting light is sampled in the space section to detect a stable, appropriate level of the reflecting light more than in recording by multiple pulses. 
   In an embodiment of the invention, the record processing module includes a determination module which determines whether an erase section length for recording at an erase level is equal to or greater than a threshold; and the tilt control module performs control within an erase section equal to or greater than the threshold when the determination module determines that the erase section length is equal to or greater than the threshold. Alternatively, the record processing module includes a determination module which determines whether a playback level section length for recording at a playback level is equal to or greater than a threshold; and the tilt control module performs control within a playback level section equal to or greater than the threshold when the determination module determines that the playback level section length is equal to or greater than the threshold. There are various space section lengths such as the erase section length and the playback level section length. Therefore, as in an embodiment of the invention, tilt control is performed when the space section length is equal to or greater than the threshold, and thus a stable, appropriate level of the reflecting light is detected. The threshold can be set freely. 
   In an embodiment of the invention, the record processing module includes a first modifying module which changes recording speed; and the tilt control module has a second modifying module which changes the threshold in response to a factor of recording speed to be changed. When the recording speed is faster, the space section length becomes shorter as well. Therefore, the threshold is changed so that a relatively long space section is selected, and thus a stable, appropriate level of the reflecting light is detected. 
   A signal recording method according to an embodiment of the invention is a signal recording method including the steps of: collecting a laser beam emitted from a laser beam source through a collector device which is disposed as it faces a disc that can record a signal and recording the signal on the disc while laser power of the laser beam source is being modulated; receiving the reflecting light when the signal is recorded and detecting a level of the reflecting light; controlling the laser power when the signal is recorded based on the detected level of the reflecting light; controlling a tilt angle between the disc and the collector device based on the detected level of the reflecting light; and switching between control of the laser power and control of the tilt angle. 
   As described above according to an embodiment of the invention, even though the level of the reflecting light fluctuates in recording because of some cause such as irregularities in the recording film, recording can be processed in an appropriate manner while tilt control is being performed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram depicting the configuration of an optical disc apparatus according to an embodiment of the invention; 
       FIG. 2  shows a block diagram depicting the configuration of a laser control part and a tilt control part; 
       FIG. 3  shows a graph depicting the relationship between the tilt angle controlled by the tilt control part and the RF signal level in signal recording; 
       FIG. 4  shows a flow chart depicting the operation of a recording process in the optical disc apparatus; 
       FIGS. 5A to 5C  show diagrams depicting various signals in recording according to an embodiment; 
       FIGS. 6A and 6B  show diagrams depicting the timing to switch between tilt control and laser power control; 
       FIGS. 7A to 7D  show diagrams depicting various signals in recording according to another embodiment of the invention; and 
       FIG. 8  shows a block diagram depicting the configuration of a laser control part and a tilt control part according to still another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, embodiments of the invention will be described with reference to the drawings. 
     FIG. 1  shows a block diagram depicting the configuration of an optical disc apparatus according to an embodiment of the invention. 
   An optical disc apparatus  100  has a spindle motor  8 , an optical pickup  6 , an RF amplifier  9 , a three-axis actuator  7 , and a servo control part  17 . 
   The spindle motor  8  rotates and drives an optical disc  2  such as a DVD±R/RW, a CD-R/RW, and a Blu-ray disc. The optical pickup  6  has a laser beam source  5 , an objective lens  3  which collects the laser beam emitted from the laser beam source  5  onto the optical disc  2 , a photodetector (PD)  4  which detects a light reflected and returning from the disc  2 , and the other elements. For the laser beam source  5 , for example, solid laser, particularly to a laser diode (LD) is used, but it is not limited thereto. In addition to this, the optical pickup  6  has an optical system and other elements, not shown, which leads the laser beam emitted from the laser beam source  5  to the objective lens  3 . The RF amplifier  9  creates a focus error signal, a tracking error signal, an RF signal and other signals based on various signals outputted from the PD  4  of the optical pickup  6 . The three-axis actuator  7  particularly moves the portion of the objective lens  3  of the optical pickup  6  in the tracking direction, the focusing direction and the tilt direction. The servo control part  17  outputs various servo signals to the three-axis actuator  7  and the spindle motor  8  based on the focus error signal, the tracking error signal and the RF signal. 
   In addition, the optical disc apparatus  100  has a thread motor, not shown, which moves the optical pickup  6  in the radial direction of the disc  2 . The servo control part  17  also outputs servo signals to the thread motor. 
   The optical disc apparatus  100  has a system controller  15 , a laser control part  16 , a synchronization detector &amp; A/D converter  10 , a signal modulator, demodulator &amp; ECC (Error Correction Code) part  11 , a buffer memory  12 , a video sound processing part  13 , a D/A converter  14 , and an interface  18 . 
   The system controller  15  receives and outputs various signals to comprehensively control the overall optical disc apparatus  100 . The laser control part  16  receives a modulated signal  42  from the signal modulator, demodulator &amp; ECC part  11  to modulate the laser power of the laser beam source  5  in order to write the signal on the disc  2 , or to control laser power based on an RF signal. The synchronization detector &amp; A/D converter  10  creates clocks based on a synchronized signal recorded on the optical disc  2  at a predetermined interval, and converts an analog signal to a digital signal. The signal modulator, demodulator &amp; ECC part  11  modulates and demodulates signals, adds ECCs, and performs an error correcting process based on ECCs. The buffer memory  12  temporarily stores data when the data is processed by the signal modulator, demodulator &amp; ECC part  11 . The video sound processing part  13  performs a necessary video process and a sound process to output video and sounds in analog forms through the D/A converter  14 . The interface  18  is an interface to connect an external computer and a video sound source, not shown. 
     FIG. 2  shows a block diagram depicting the configurations of the laser control part  16  and a tilt control part and an RF signal level variation acquiring part included in the servo control part  17  shown in  FIG. 1 . 
   The laser control part  16  has an ROPC part  21 , an LD driver  22 , and a laser power variation acquiring part  40 . The laser power variation acquiring part  40  has a sample hold (S/H) circuit  23 , a subtractor  24 , a threshold setting part  25 , and a comparator  26 . 
   The LD driver  22  outputs a drive signal  41  to drive the laser beam source  5 . The ROPC part  21  receives an RF signal  36  which is a signal of the reflecting light in recording, controls the drive of the LD driver  22  based thereon, and controls laser power in order to prevent a detrimental effect because of irregularities in the recording film of the disc  2 . 
   For the ROPC part  21 , a general ROPC system can be used. More specifically, the ROPC part  21  compares the level of the reflecting light from a pit at the optimum recording power in OPC (in calibration prior to signal recording) with the level of the reflecting light from the pit in signal recording. Then, based on the compared result, it executes a recording process while it is correcting optimum recording power determined in OPC whenever necessary. Alternatively, the ROPC part  21  may store a table expressing the relationship between the RF signal level and laser power for processing based on the table. For the process executed by the ROPC part  21 , various image processing methods can be considered in addition to this. 
   The laser power variation acquiring part  40  monitors the variation in the recording level of laser power, for example, and acquires the variation. More specifically, the S/H circuit  23  receives the signal  41  at the level of laser power outputted from the ROPC part  21 , and holds the laser power  41  in response to a timing signal  37  outputted from the system controller  15 . The subtractor  24  computes a difference between the held laser power and the laser power currently outputted, and outputs the difference to the comparator  26 . The comparator  26  compares the inputted differential signal with a threshold freely set at the threshold setting part  25 , binarizes the compared result, for example, and outputs it to the system controller  15 . 
   A tilt control part  20  has a tilt driver  32 , and a target value setting part  27 . An RF signal level variation acquiring part  50  has a S/H circuit  28 , a subtractor  29 , a threshold setting part  30 , and a comparator  31 . 
   The tilt driver  32  receives an error signal between the RF signal  36  and a target value set at the target value setting part  27  as the RF signal  36  outputted from the RF amplifier  9  is used as the amount to be controlled. Then, the tilt driver  32  outputs a drive signal  43  to the three-axis actuator  7  so that the error signal is turned to zero. The RF signal level variation acquiring part  50  monitors the RF signal level outputted from the RF amplifier  9 , and acquires the variation. More specifically, the S/H circuit  28  receives the RF signal  36  outputted from the RF amplifier  9 , and holds the RF signal  36  in response to a timing signal  38  outputted from the system controller  15 . The subtractor  24  computes a difference between the held RF signal and the RF signal currently outputted, and outputs the difference to the comparator  31 . The comparator  31  compares the inputted differential signal with a threshold freely set at the threshold setting part  30 , binarizes the compared result, for example, and outputs it to the system controller  15 . 
     FIG. 3  shows a graph depicting the relationship between the tilt angle controlled at the tilt control part  20  and the RF signal level in signal recording. The target value of the tilt angle is almost zero. For example, the target value set at the target value setting part  27  is the RF signal level (point C) corresponding to the tilt angle of zero. It is needless to say that the tilt angle is a relative angle between the recording surface of the disc  2  and the objective lens  3 . When the tilt angle is zero, the optical axis of the laser beam between the objective lens  3  and the disc  2  is vertical with respect to the recording surface. When laser power is constant, the depth of the pit formed in the disc  2  is shallower as the tilt angle is greater in recording, and the light quantity of the reflected and reflecting light is greater. Thus, the RF signal level becomes great. Accordingly, the relationship is as depicted in the graph shown in  FIG. 3 . 
   In addition, in the signal playback process, the relationship is a graph that the graph shown in  FIG. 3  is reversed as the horizontal direction is used as the axis. This is because a shift of the reflected and reflecting light becomes greater as the tilt angle is greater, and in association with this, the received light quantity at the optical pickup  6  becomes small. 
   Next, the operation of the recording process in the optical disc apparatus  100  will be described.  FIG. 4  shows a flowchart depicting the operation.  FIG. 5A  shows a modulated signal created at the signal modulator, demodulator &amp; ECC part  11  in the recording process, and  FIG. 5B  shows a drive signal of the LD driver  22  in the recording process. In addition,  FIG. 5C  shows the waveform of the RF signal of the reflected and reflecting light when a signal is recorded by the drive signal. In  FIG. 5A , the modulated signal is depicted in the NRZI (Non Return to Zero Inverted) mode, but this is merely an example. For example, it may be the NRZ mode, or may be the combination of these with RLL (Run Length Limited) mode, or modes such as 8/16 modulation. 
   As shown in  FIG. 5B , for the RF signal, when the laser power is first applied to the disc  2  at the recording level, pits are started to form. When pits are started to form, the intensity of the reflecting light is varied by the pits in turn being formed. Thus, the level of the RF signal  36  suddenly drops. More specifically, the initial state is a high reflectance state in which no pits are formed in a recording layer, and the pits are started to form to reduce the reflectance even though the recording power is constant. When the recording film of the disc  2  has irregularities, the signal wanders up and down, as shown in an arrow on the RF signal  36  shown in  FIG. 5C . When the laser power and the tilt angle are controlled at the same time, it is difficult to tell that the fluctuations are caused by fluctuations in the tilt angle or caused by irregularities in the recording film, or caused by both. Then, the optical disc apparatus  100  is operated as described below. 
   First, while the ROPC part  21  controls laser power based on the RF signal  36  (Step  401 ), the LD driver  22  modulates laser power based on the modulated signal, and thus the recording process is executed. At this time, the tilt control part  20  does not perform tilt control. When the system controller  15  outputs a sampling pulse  37  at a given timing, for example, the S/H circuit  23  of the laser power variation acquiring part  40  holds a laser power signal  41   a  (Pw 1 ) in recording outputted from the LD driver  22  (Step  402 ). In addition, when the system controller  15  outputs a sampling pulse  38  at a given timing, the S/H circuit  28  of the RF signal level variation acquiring part  50  holds an RF signal  36  (Lv 1 ) (Step  402 ). 
   It is fine that the sampling pulses  37  and  38  are outputted when the laser power  41  is at the recording level  41   a,  for example, that is, they are outputted within a mark section  42   a  of a modulated signal  42 . However, not necessarily in the mark section  42   a,  they may be outputted when the laser power  41  is at a playback level  41   b,  that is, the sampling pulses  37  and  38  may be outputted within the space section  42   b  of the modulated signal  42 . 
   In the RF signal level variation acquiring part  50 , the RF signal  36  is held, after a predetermined time period, the S/H circuit  28  outputs the held RF signal level (Lv 1 ) to the subtractor  29 , and the difference is taken between it and the RF signal level (Lv 2 ) currently inputted to the subtractor  29 . More specifically, a variation (Lv 2 −Lv 1 ) is acquired (Step  403 ). Similarly, in the laser power variation acquiring part  40 , the laser power  41  is held, after a predetermined time period, the S/H circuit  23  outputs the held laser power (Pw 1 ) to the subtractor  24 , and the difference is taken between it and the laser power (Pw 2 ) currently inputted to the subtractor  24 . More specifically, the variation (Pw 2 −Pw 1 ) is acquired (Step  404 ). A predetermined time period can be set freely. Any orders of doing Step  403  and Step  404  are fine, or these steps may be done at the same time. 
   When this is done, the comparator  31  compares the variation with a threshold TH 1 . When Lv 2 −Lv 1 ≧TH 1  (YES at Step  405 ), the comparator  31  outputs a H-logic signal to the system controller  15 . Based on this, the system controller  15  outputs a stop signal to the ROPC part  21  to stop laser power control, whereas it outputs a start signal to the tilt driver  32  to start tilt control. More specifically, laser power control is switched to tilt control. Thus, the ROPC part  21  stops control (Step  407 ), and the tilt driver  32  start tilt control (Step  408 ). When the ROPC part  21  stops control, the LD driver  22  does not receive the control signal from the ROPC part  21 , that is, the laser power is not finely controlled, and the recording process is continued based only on the modulated signal. 
   The comparator  31  of the RF signal level variation acquiring part  50  determines that there is no abnormality when the level is not Lv 2 −Lv 1 ≧TH 1  (NO at Step  405 ), and outputs a L-logic signal to the system controller  15 . On the other hand, the comparator  26  compares the variation (Pw 2 −Pw 1 ) with the threshold. When the power is Pw 2 −Pw 1 ≧TH 2  (YES at Step  406 ), it outputs a H-logic signal to the system controller  15 . When the system controller  15  thus receives the H-logic signal from the comparator  31  and also receives the L-logic signal from the comparator  26 , as similar to the description above, the process goes to Step  407  and to steps after that. On the other hand, when the system controller  15  receives the L-logic signals from the comparators  26  and  31  (NO at Step  405  and NO at Step  406 ), the process returns to Step  401 , and control is continued by the ROPC part  21  with no change. 
   In the tilt control at Step  408 , the tilt driver  32  outputs a drive signal to the three-axis actuator  7  based on the RF signal  36  so that the tilt angle is nearly zero. For example, it is fine for the tilt driver  32  to do feedback as the RF signal is the amount to be controlled at the timing when the sampling pulse is inputted within the mark section  42   a  or within the space section  42   b  of the modulated signal from the system controller  15 . Thus, the RF signal level is monitored at the position at which the RF signal is stable as much as possible, whereby an appropriate RF signal level can be obtained, and tilt control can be performed highly accurately. 
   For the timing to stop tilt control (Step  409 ), this time period can be considered that the tilt servo is started to the time after a predetermined time period, for example. Alternatively, this scheme may be done in which the laser power  41  or the RF signal  36  in recording is kept monitoring in the midway of tilt control, and tilt control is stopped when the RF signal level variation or laser power variation in recording is smaller than the threshold. At this time, of course, control done by the ROPC part  21  is being stopped. When the system controller  15  stops tilt control, the process returns to Step  401 , and control is switched to laser power control by the ROPC part  21 . 
   As described above, in the embodiment, as shown in FIGS.  6 A and  6 B, the recording process is executed as tilt control ( FIG. 6A ) and laser power control ( FIG. 6B ) are switched alternately. Therefore, for example, since laser power control is stopped while tilt control is being performed, laser power is constant. From the viewpoint of performing tilt control, the level of the reflecting light from the disc  2  is detected in an appropriate manner, and recording quality can be prevented from being deteriorated. 
   In the embodiment, when the variation in the RF signal level (Lv 2 −Lv 1 ) is normal at Step  405 , that is, the level is Lv 2 −Lv 1 &lt;TH 1  and the level is abnormal, control is switched to tilt control without determination whether the variation in the laser power (Pw 2 −Pw 1 ) is abnormal, and thus the process is simplified. 
     FIGS. 7A to 7D  show diagrams depicting various signals in recording according to another embodiment of the invention. In the description below, the same function and operation of an optical disc apparatus  100  according to the embodiment are simplified or omitted for description, and different points will be mainly described. 
   In the embodiment, as shown in  FIG. 7B , for example, laser power is processed for recording under control of three values including an erase level. In addition, the multi-pulse mode is used. As described above, when the multi-pulse mode is used, for example, in the tilt control at Step  408  shown in  FIG. 4 , the system controller  15  outputs a sampling pulse a to a tilt driver  32  when the modulated signal is in the space section, that is, in an erase process (see  FIG. 7D ). Thus, a stable, appropriate RF signal level can be detected more than the case in which the level is sampled in the mark section of multiple pulses. Therefore, highly accurate tilt control is possible. In addition, in the embodiment, the timing to sample the RF signal level can be set freely as the amount to be controlled in the ROPC part  21 , but also in this case, sampling may be done within the space section  42   b.    
   In the tilt control in the embodiment, for example, the system controller  15  determines whether the erase section length is longer than a threshold. When it is determined that the erase section length is equal to greater than the threshold, the RF signal may be sampled within the erase section. In the DVD standards, the mode is the 8/16 +NRZI modulation mode, and the signal length, or the erase section length is 3 T to 11 T (T is a clock cycle). In the case of the normal recording speed (recording at the normal speed), it is fine that the threshold for the erase section length is any one of 7 T to 1 T, for example. More specifically, the erase section as long as possible is selected to detect a more stable, appropriate RF signal level. 
   Alternatively, when the recording process is performed at double speed, not at the normal recording speed, the system controller  15  may properly change the threshold of the erase section length in accordance with the recording speed. When the recording speed becomes faster, the erase section length becomes shorter. Thus, the threshold is changed to detect a stable, appropriate RF signal level all the time. For example, the threshold can be changed freely in such a way that the threshold is 8 T and 9 T at double speed, and 10 T or greater at quadruple speed, 8× speed and so on. 
   The selection process for the erase section based on the threshold described above may be adapted to tilt control in the single pulse mode as shown in  FIGS. 5A to 5C . 
     FIG. 8  shows a block diagram depicting the configurations of a laser control part, a tilt control part and an RF signal level variation acquiring part according to still another embodiment of the invention. In the embodiment, as compared with the form shown in  FIG. 2 , the configurations of a laser power variation acquiring part  140  and an RF signal level variation acquiring part  150  are different. 
   The laser power variation acquiring part  140  of a laser control part  116  has a mean value computing part  47  which computes the mean value of laser power sampled at a S/H circuit  23  in a multiple number of times, for example. More specifically, laser power  41  sampled at the S/H circuit  23  in a multiple number of times is digitally converted by an A/D converter  45 . A mean value computing part  47  takes in the digital data, computes a mean value, and outputs the mean value to a D/A converter  49 . The D/A converter  49  converts the mean value in an analog manner, and outputs it to a subtractor  24 . The subtractor  24  outputs a differential signal between the current laser power  41  by an LD driver  22  and the mean value to a comparator  26 . 
   Similarly, also in the RF signal level variation acquiring part  150 , an A/D converter  55  digitally converts an RF signal level  36  sampled in a multiple number of times at a S/H circuit  28 . A mean value computing part  57  takes in the digital data, computes a mean value, and outputs the mean value to a D/A converter  59 . The D/A converter  59  converts the mean value in an analog manner, and outputs it to a subtractor  29 . The subtractor  29  outputs a differential signal between the current RF signal level of  36  and the mean value to a comparator  31 . 
   With the configurations of the laser power variation acquiring part  140  and the RF signal level variation acquiring part  150 , laser power control and tilt control can be switched as well. 
   The invention is not limited to the embodiments described above, which can be modified variously. 
   For example, in the graph shown in  FIG. 3 , in the initial stage of the tilt control operation, it is unknown whether the tilt angle corresponding to a certain RF signal level is positive or negative, that is, the slope direction is unknown. Then, for example, prior to the signal recording process, it is possible to check the slope direction caused by the warpage or deformation of a disc. In this case, it is fine that the optical disc apparatus  100  is operated as described below. First, while a disc  2  is being rotated on the outer radius side of the disc, the RF signal level is acquired for a predetermined period or a predetermined number of times, and the maximum value of the tilt angle is stored. Subsequently, while the disc is being rotated on the inner radius side of the disc, the RF signal level is acquired for a predetermined period or a predetermined number of times, and the maximum value of the tilt angle is stored. The stored maximum values of the disc on the outer and inner radius sides are compared to tell in which direction the slope of the disc is oriented. 
   As described above, it may be fine to do measurements at two points between the outer radius side and the inner radius side, or may be at three points among the outer radius side, the center and the inner radius side, or may be four points or greater. Based on measurement information, an interpolation line or curve of radial position information and the slope of the disc  2  may be created. Then, when tilt control is performed based on the stored items of information, the tilt angle direction is known from the initial stage of tilt control, and thus tilt control can be facilitated. In addition, in this case, the amount to be controlled is not necessarily the RF signal, and it may be a tracking error signal. 
   The variation in the RF signal level at Step  403  shown in  FIG. 4  is the differential signal, but instead of this, it may be the variation that a value is differentiated within a predetermined period. 
   In the embodiment shown in  FIGS. 5A to 5C , the laser power is a binary value of the recording level and the playback level. However, it is not limited thereto, and control may be done by a ternary value including the erase level. 
   In  FIG. 7D , the embodiment is described in which the sampling pulse a in tilt control is generated within the space section. However, it is not necessarily in the erase process. The sampling pulse may be generated within the mark section. 
   It should be understood by those skilled in the art that various modifications combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.