Abstract:
A laser beam is focused at positions on an optical axis of the optical pickup by a focusing device, corresponding to kinds of the discs which are different in thickness. The focusing device is moved along the optical axis. A photodetector is provided for receiving a laser beam reflected from the disc and for producing a focus error signal. A detector is provided for detecting the difference between the focus error signals, caused by the difference in thickness of the discs and the difference in focal point of the laser beam, thereby discriminating the kind of the disc.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an optical disc discriminating system in an optical disc player for discriminating various kinds of optical discs such as CD and DVD (Digital Video Disc or Digital Versatile Disc).  
           [0002]    A compatible single optical disc player which is capable of reproducing a CD (compact disc) and a LD (laser disc) has been popular. Since the thickness of the CD and the LD from the rear surface to the recording surface is the same (1.2 mm), it is possible to use an optical pickup in which a laser beam is focused on a focal point for reproducing both of the CD and the LD.  
           [0003]    In order to discriminate the kind of the disc mounted on the player, the diameter of the disc is detected for discriminating the CD or the video disc.  
           [0004]    Recently, the DVD having a high recording density has been developed. Although the thickness of the CD from the surface to the recording surface is 1.2 mm, the thickness of the DVD is about 0.6 mm. However, the principle of a reading system of recording pits of the DVD is the same as that of the CD. Therefore, a compatible single optical disc player which is capable of reproducing the CD and the DVD has greatly been proposed.  
           [0005]    In such a compatible CD/DVD player, in order to properly focus the beam for reading the information on the recording surface of the disc, two types of lenses are provided, each having a focal length suitable for a thickness of a corresponding disc As another type of the compatible CD/DVD player, a double focus lens using a hologram element is provided.  
           [0006]    [0006]FIG. 8 a  shows an optical pickup having a double focus lens using a hologram element (diffraction grating).  
           [0007]    The double focus lens comprises a diffraction grating  13 , an objective lens  11  disposed on the same optical path, and a collimator lens  14 . A beam of light is paralleled by the collimator lens  14  and divided into three spectral beams of 0 order spectrum (having a 90 diffraction angle) and ±1 order spectrum by the diffraction grating  13  (−1 order spectrum is not shown). Since focal lengths of the 0 order spectrum and the ±1 order spectrum are different from each other, each of the 0 order spectrum (hereinafter called zero-order spectrum) and the ±1 order spectrum (hereinafter called first-order spectrum) is focused on a different position of the same line.  
           [0008]    Concretely, the zero-order spectrum is properly focused on the recording surface of the DVD, while the first-order spectrum is properly focused on the recording surface of the CD.  
           [0009]    In operation, when the pickup is gradually moved away from the disc, the recording surface is irradiated with the zero-order spectrum first. A four-divided photodetector provided in the pickup (not shown) produces an S-shaped signal as a focus error signal. When the pickup is further moved away, the first-order spectrum reflected from the disc passes an optical path of the zero-order spectrum, thereby forming a pseudo spectrum. Thus, a pseudo S-shaped signal of the pseudo spectrum is produced. Finally, an S-shaped signal of the first-order spectrum is produced as a focus error signal.  
           [0010]    [0010]FIG. 8 b  shows S shaped signals of the corresponding zero-order spectrum, pseudo spectrum, and first-order spectrum with respect to the movements of the double focus lens of FIG. 8 a . The spectral ratio of the zero-order spectrum and the first-order spectrum is set to 1:1.  
           [0011]    As aforementioned, the zero-order spectrum achieves the optimum focus on the DVD, while the first-order spectrum achieves the optimum focus on the CD. If the amount of the S-shaped signal by the zero-order spectrum on the DVD is 100%, the amount of the signal corresponding to the CD is small, less than 100%. Similarly, if the amount of the S-shaped signal by the first-order spectrum on the CD is 100%, the amount of the signal corresponding to the DVD is small, less than 100%. Thus, in the double focus lens, the amplitudes of the respective focus error signals are different from each other. The ratio of zero-order spectrum to first-order spectrum of the DVD, namely zero-order spectrum/first-order spectrum is larger than the ratio of zero-order spectrum to first-order spectrum of the CD, that is zero-order spectrum/first-order spectrum.  
           [0012]    In the compatible CD/DVD player, a focus servo control system is provided for each disc. In a focus servo control, the pickup is vertically moved against the disc, and a focus error (FE) signal is produced for detecting a proper focal point for focusing in. However, in the aforementioned player using the double focus lens, a plurality of focus error signals (S-shaped signals by zero-order spectrum, pseudo spectrum and first-order spectrum) are obtained. Accordingly, a problem that focusing in is achieved by an S-shaped signal detected first arises. In other words, there may be occur an erroneous focusing by an improper spectrum.  
         SUMMARY OF THE INVENTION  
         [0013]    An object of the present invention is to provide an optical disc discriminating system for an optical disc player which may properly discriminate a CD and a DVD, and select an optimum S-shaped signal in a focus error signal for focusing in.  
           [0014]    According to the present invention, there is provided a system for discriminating various kinds of optical discs which are different in thickness, in an optical disc player having an optical pickup, comprising focusing means for focusing a laser beam at positions on an optical axis of the optical pickup, the number of the positions being corresponded to the number of kinds of the discs, moving means for moving the focusing means along the optical axis, photodetector means for receiving a laser beam reflected from the disc and for producing a focus error signal, detector means for detecting the difference between the focus error signals, caused by the difference in thickness of the discs and the difference in focal point of the laser beam, and for discriminating the kind of the disc.  
           [0015]    The focusing means comprises a diffraction grating and an objective lens.  
           [0016]    The detector means detects the difference in level of the focus error signal.  
           [0017]    In another aspect of the invention, the detector means detects the difference in timing of production of the focus error signal.  
           [0018]    In a further aspect, the detector means detects the difference in position of the focusing means when the focus error signal is produced.  
           [0019]    These and other objects and features of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0020]    [0020]FIG. 1 is a block diagram showing an optical disc discriminating system according to the present invention;  
         [0021]    [0021]FIGS. 2 a  to  2   g  are time charts showing signals for explaining an operation of the system;  
         [0022]    [0022]FIG. 3 is a flowchart showing an operation of the system;  
         [0023]    [0023]FIG. 4 is a block diagram showing an optical disc discriminating system employed for second and third embodiments of the present invention;  
         [0024]    [0024]FIGS. 5 a  and  5   b  are time charts showing signals for explaining operations of the second and third embodiments;  
         [0025]    [0025]FIG. 6 is a flowchart showing an operation of the second embodiment;  
         [0026]    [0026]FIG. 7 is a flowchart showing an operation of the third embodiment; and  
         [0027]    [0027]FIGS. 8 a  and  8   b  are diagrams showing the principle of the system using a hologram element as double focus lens. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    Referring to FIG. 1 showing a system of a CD/DVD player according to the present invention, an optical disc  20  to be reproduced is mounted on the player and rotated by a spindle motor  21 . An optical pickup  22  has a double focus lens which has the same principle as FIG. 8 a . The pickup  22  emits a laser beam, and pits formed on a recording surface of the disc  20  is irradiated with the beam for reading information recorded with the pits.  
         [0029]    An output signal of the pickup  22  is fed to a focus error (FE) producing circuit  23 . The FE producing circuit  23  produces a focus error signal. After removing an unnecessary signal of a high frequency range from the focus error signal, the focus error signal is applied to a peak holding circuit  24 . The peak holding circuit  24  holds the maximum amplitude voltage of the positive pole side of the focus error signal as a peak voltage at a substantially constant value. The peak holding circuit  24  applies the peak voltage to a microcomputer  25 .  
         [0030]    The microcomputer  25  produces a reset (RST) signal which is applied to the peak holding circuit  24  for resetting the maximum voltage held in the circuit. When a next focus error signal is applied to the peak holding circuit  24 , a maximum amplitude voltage of the positive pole side of the next focus error signal is held as the maximum voltage.  
         [0031]    The output signal from the pickup  22  is further applied to an RFOK producing circuit  37 . The RFOK producing circuit  37  receives an added signal of a four-divided photodetector. When the added focus signal exceeds a predetermined threshold value, the RFOK producing circuit  37  produces an RFOK signal which is applied to the microcomputer  25 . Namely, the production of the RFOK signal means the laser beam is located at a position near focused position.  
         [0032]    The focus error signal from the FE producing circuit  23  is further applied to a focus zero cross (FZC) producing circuit  26 . The FZC producing circuit  26  is a zero cross comparator comprising an operational amplifier and produces a focus zero cross detecting signal in the form of pulses. The production of the FZC signal means that the spectrum is focused. The FZC signal is applied to the microcomputer  25 .  
         [0033]    The focus error signal from the FE producing circuit  23  is also applied to a fixed contact of a loop switch  27 . The loop switch  27  is operated by an FE shunt signal from the microcomputer  25 .  
         [0034]    A movable contact of the loop switch  27  is connected to a focus servo control circuit  28  which is connected to the microcomputer  25 . The focus servo control circuit  28  is further connected to a focus servo coil drive circuit  29  which is connected to an actuator coil  30 .  
         [0035]    When the loop switch is closed (ON-state), a focus servo loop comprising the FE producing circuit  23 , microcomputer  25 , focus servo control circuit  28 , focus servo coil drive circuit  29  and actuator coil  30  is closed. Thus, the focus error signal is applied to the focus servo controller  28  for automatically controlling the focus servo.  
         [0036]    When the loop switch  27  is opened (OFF-state), the microcomputer  25  operates the focus servo coil drive circuit  29  to produce the drive voltage for driving the actuator coil  30 . Thus, the lens in the pickup is moved downward and upward at a predetermined speed for discriminating the disc.  
         [0037]    The microcomputer  25  is further connected to a RAM  32 , a ROM  33  and a motor drive circuit  31 . The RAM  32  stores data of signals (time and voltage) detected by circuits, and set values obtained by the detected signals The ROM  33  stores data of set values necessary for the system. The motor drive circuit  31  produces a drive signal for driving the spindle motor  21 .  
         [0038]    [0038]FIGS. 2 a  to  2   g  show time charts of signals for explaining the operation of the system. As aforementioned, the double focus lens in the pickup  22  has the same principle as FIG. 8 a , and the spectral ratio of the zero-order spectrum and the first-order spectrum is 1:1. Thus, when the lens is vertically moved, the relationship between the maximum amplitudes of the S-shaped signals of zero-order spectrum and first-order spectrum is represented as follows.  
         ( FED 0/ FED 1)&gt;( FEC 0/ FEC 1)  
         [0039]    where  
         [0040]    FED0: maximum amplitude voltage of the S-shaped signal of the DVD by the zero-order spectrum  
         [0041]    FED1: maximum amplitude voltage of the S-shaped signal of the DVD by the first-order spectrum  
         [0042]    FEC0: maximum amplitude voltage of the S-shaped signal of the CD by the zero-order spectrum  
         [0043]    FEC1: maximum amplitude voltage of the S-shaped signal of the CD by the first-order spectrum  
         [0044]    [0044]FIG. 2 a  shows a focus drive voltage SFD for representing a position voltage of the lens. A mark UH is the highest position of the lens (the closest position of the lens to the optical disc  20 ), and a mark UL is the lowest position of the lens (the farthest position of the lens from the disc).  
         [0045]    [0045]FIGS. 2 b  and  2   c  show voltages of the focus error signals (FE) having S shape for the CD and the DVD, respectively, obtained during the upward and downward movements of the lens. A mark TH 1  is a threshold of the voltage stored in the ROM  33  for detecting the S-shaped signal. The TH 1  is set at least smaller than the maximum amplitude voltages obtained by the zero-order spectrum of the CD and DVD. The FZC signal is produced by comparing with the TH 1 . A mark TH 2  is a threshold set to a value between the maximum amplitude voltage of the S-shaped signal of the CD by the first-order spectrum FEC 1  and the maximum amplitude voltage of the S-shaped signal of the DVD by the first-order spectrum FED 1  (FEC1&gt;TH2&gt;FED1). The TH 2  is stored in the ROM  33  or obtained by multiplying the S-shaped signal by the zero-order spectrum and a coefficient together. In this state, influence of the reflectance of the disc is prevented.  
         [0046]    [0046]FIG. 2 d  shows the zero cross detecting pulse signal FZC for each of CD and DVD obtained by the FZC producing circuit  26  when the zero cross of the S-shaped signal is detected.  
         [0047]    [0047]FIG. 2 e  shows the reset pulse signal RST for each of CD and DVD applied from the microcomputer  25 . FIG. 2 f  shows output signals of the peak voltages from the peak holding circuit  24  for CD and DVD. A mark V 1  is a waveform and a measured point of the maximum amplitude voltage of the S-shaped signal by the zero-order spectrum A mark V 2  is a waveform and a measured point of the maximum amplitude voltage of the S-shaped signal by the first-order spectrum.  
         [0048]    In FIGS. 2 e  and  2   f , marks T 1  and T 2  are times measured by a timer provided in the microcomputer  25 . As shown in Fig,  2   e , the time T 1  represents the time when the peak voltage is reset after the peak voltage of the zero-order spectrum is measured by the FZC formed by the S-shaped signal of the zero-order spectrum. For example, if the pseudo S-shaped signal is large, and if the time is set to the middle between the pseudo spectrum and the first-order spectrum after the peak voltage is measured to pass the pseudo S-shaped signal, the pseudo S-shaped signal is masked. The time T 2  represents the time passing the first-order spectrum from the FZC and measuring the peak voltage of first-order spectrum, or the time until the lens reaches the lowest position UL.  
         [0049]    [0049]FIG. 2 g  shows the RFOK signal of each of CD and DVD, which will be described hereinafter.  
         [0050]    The operation of the system will be described with reference to the flowchart of FIG. 3.  
         [0051]    At a step S 1 , the optical disc  20  is loaded on the disc player. At a step S 2 , the loop switch  27  is opened (OFF-state), and the double focus lens in the pickup  22  is moved up to the highest position UH. At a step S 3 , the peak voltage is reset, and the lens is moved down from the highest position UH to the lowest position UL at a predetermined set speed. During the downward movement of the lens, the pickup  22  produces the focus error signal.  
         [0052]    At a step S 4 , the maximum voltage of the FE of the S-shaped signal is monitored by the FZC signal if the FE exceeds the threshold TH 1 . When FE≧TH 1 , the timer is set at a step S 5 . At a step S 6 , the peak voltage of the positive pole side of the S-shaped signal obtained first by the peak holding circuit  24  for a predetermined time is stored in the RAM  32  as the peak voltage V 1 . At a step S 7 , a time T measured by the timer is compared with the set time T 1  stored in the ROM  33 . If T≧T 1 , the microcomputer  25  applies the reset signal RST to the peak holding circuit  24  for discharging the peak voltage V 1  held therein to be zero at a step S 8 . If not at the step S 7 , the timer is set until T≧T 1  is obtained.  
         [0053]    At a step S 9 , the measured time T is compared with the set time T 2  stored in the ROM  33 . If T≧T 2 , when the timer counts the time T 2  at a step S 10 , the peak voltage held in the peak holding circuit  24  is stored in the RAM  32  as the peak voltage V 2 . AT a step  11 , the peak voltage V 2  is compared with the threshold TH 2  stored in the ROM  33 . If V 2 ≧TH 2 , it is determined that the disc  20  is CD at a step S 13 . If not, it is determined that the disc  20  is DVD at a step S 12 .  
         [0054]    At a step S 14 , the position of the lens is monitored until SFD≧UL. When SFD≧UL, the timer is reset at a step S 15 . At a step S 16 , the lens is moved up. At a step S 17 , the position of the lens is monitored until SFD≧UH. When SFD≧UH, the lens is moved down at a step S 18 . At a step S 19 , the S-shaped signal obtained first is monitored by the FZC until the FE exceeds the threshold THI. When FE≧TH 1 , and when it is determined that the disc determined at the step  11  is CD at a step S 20 , the timer is set at a step S 21 . If the disc is DVD at the step S 20 , the program goes to a step S 23 .  
         [0055]    At a step S 22 , the measured time T is monitored until T≧T 1 . When T≧T 1 , the microcomputer  25  is operated to close the loop switch  27  at the step S 23 . At a step S 24 , when the zero cross signal FZC is produced, it is determined that the laser beam is focused. At a step S 25 , a series operation for disc discrimination is terminated.  
         [0056]    In the step S 11 , although the disc is discriminated by V 2 ≧TH 2 , there is another method for discriminating the disc.  
         [0057]    As aforementioned, since (FEC0/FEC1)&lt;(FED0/FED1), the disc may be discriminated by the peak voltages V 1  and V 2  of the peak holding circuit  24 . If a certain value β is set to (FEC0/FEC1)&lt;β&lt;(FED0/FED1), and the disc is discriminated with the value β, the disc can be discriminated without influenced by the reflectance of the disc.  
         [0058]    Namely, as shown in FIG. 2 f , if the peak voltage of the S-shaped signal by the zero-order spectrum is V and the peak voltage of the S-shaped signal by the first-order spectrum is V 2 , it is determined whether V 2 × 62  &gt;V 1  at the step S 11 . When V 2 ×β&gt;V 1 , it is determined the disc is CD at the step S 13 . If not, it is determined the disc is DVD at the step S 12 .  
         [0059]    [0059]FIG. 2 g  shows the RFOK signal of each of CD and DVD.  
         [0060]    In the embodiment, timings for starting the measurement of times T 1  and T 2  may be determined by the RFOK signals, not the FZC of the zero-order spectrum. Furthermore, it is possible to determine the timings when the peak voltage output exceeds the TH 1 .  
         [0061]    The lens may be moved from the lowest position to the highest position. In this method, although the S-shaped signal becomes the order of first-order spectrum, pseudo spectrum and zero-order spectrum, it is possible to discriminate the disc in the same manner.  
         [0062]    In the above described embodiment, it is possible that when the maximum voltage V 2  is detected, it is determined that the lens is positioned at the lowermost position, or may be moved to the uppermost position.  
         [0063]    Although the timing for resetting the focus is set to the middle between the pseudo S-shaped signal and the S-shaped signal by the first-order spectrum, it is possible to reset after the V 1  is measured if the pseudo S-shaped signal is small.  
         [0064]    In order to prevent the influence of the recording surface of the disc, the disc may be rotated at a predetermined speed at the step S 2 .  
         [0065]    [0065]FIG. 4 shows a system employed for second and third embodiments. In the system, an equalizer (EQ)  34  for correcting the drive voltage applied to the actuator coil  30 , and a comparator  36  having a reference voltage source  35  as a reference voltage for detecting voltage are provided, and the peak holding circuit  24  of the first embodiment is omitted. Other structures are the same as those of the first embodiment, and the same parts are identified with the reference numerals as FIG. 1, and the descriptions thereof are omitted.  
         [0066]    The EQ  34  is operated for linearly holding a relationship between the drive voltage of the actuator coil  30  and the position of the lens. An output voltage of the EQ  34  is applied to an input terminal of the comparator  36  and the other input terminal is connected to the reference voltage source  35 .  
         [0067]    [0067]FIGS. 5 a  and  5   b  show signals for operating the system for second and third embodiments. A mark FD is a focus drive voltage of the drive voltage of the actuator coil  30  converted by the EQ  34 . The FD is linearly changed with respect to the position voltage SFD.  
         [0068]    In FIG. 5 a , if the lens is further lowered from the position where the reference voltage E is obtained, times when the S-shaped signals are produced first by the pickup  22  by the respective DVD and CD are different from each other. Therefore, a mark TD 1  is a detecting time of the DVD and a mark TC 1  is a detecting time of the CD. The detecting times TD 1  and TC 1  are stored in the ROM  33  as set values.  
         [0069]    In FIG. 5 b , the FD produced at the detecting time TD 1  or TC 1  is stored in the ROM  33  as a set value. Marks E 1  and E 2  are voltages thereof.  
         [0070]    The operation of the system of the second embodiment will be described with reference to FIG. 6.  
         [0071]    At a step S 1 , the disc  20  is loaded on the disc player. At a step S 2 , the lens is moved up to the highest position UH. At a step S 3 , the lens is moved down from the highest position UH to the lowest position UL at a predetermined set speed.  
         [0072]    At a step S 4 , the FD is monitored, and when FD≧E, the timer is set at a step S 5 . At a step S 6 , an S-shaped signal detected first is monitored by the FZC. When EF≧TH 1  the timer is stopped at a step S 7 . At a step S 8 , the time during the timer is set is stored in the RAM  32 . At a step S 9 , the time T is compared to the set time TD 1  stored in the ROM  33 . The time T is set to TC 1 &lt;T&lt;TD 1 . If T≦TD 1 , it is determined that the disc is DVD at a step S 10 . If not, it is determined that the disc is CD at a step S 11 .  
         [0073]    Programs in steps S 12  to S 23  are the same as those in the steps S 14  to S 25  of FIG. 3 of the first embodiment. Thus, the descriptions thereof are omitted.  
         [0074]    The operation of the system of the third embodiment will be described with reference to FIG. 7.  
         [0075]    At a step S 1 , the disc  20  is loaded on the disc player. At a step S 2 , the lens is moved up to the highest position UH. At a step S 3 , the lens is moved down from the highest position UH to the lowest position UL at a predetermined set speed. At a step S 4 , the S-shaped signal obtained first is monitored by the FZC until the FE exceeds the threshold TH 1 . When FE≧TH 1 , the FD is stored in the RAM  32  at a step S 5 .  
         [0076]    At a step S 6 , the FD (E 1  or E 2 ) stored in the RAM  32  is compared with a reference voltage E 0  stored in the ROM  33 . The reference voltage E 0  is set to the value to meet E 1 &lt;E 0 &lt;E 2 . When E 0 ≧E 1 , it is determined that the disc is CD at a step S 7 . If not, it is determined that the disc is DVD at a step S 8 .  
         [0077]    Programs in steps S 9  to S 20  are the same as those in the steps S 14  to S 25  of FIG. 3 of the first embodiment. Thus, the descriptions thereof are omitted.  
         [0078]    In the second and third embodiments, the FZC of the zero-order spectrum for monitoring the programs may be effective by the RFOK signal shown in FIG. 2 g  of the first embodiment, or by the peak voltage output exceeds the TH 1 .  
         [0079]    As aforementioned, the lens may be moved from the lowest position to the highest position.  
         [0080]    The EQ  34  is provided for removing noises of the high frequency range of the focus drive signal. If there is no noises, the EQ  34  may be omitted.  
         [0081]    In order to prevent influences of inclination and vibration of the disc, the disc may be rotated at a predetermined speed at the step S 2 .  
         [0082]    In accordance with the present invention, in the first embodiment, the objective lens for focusing the beam on the disc is moved from the predetermined position, thereby obtain the focus error signal. When the first focus error signal appears, the time is measured. When the time becomes the predetermined time, the level of the focus error signal at the predetermined time is compared with a set reference level. Thus, it is possible to discriminate CD and DVD, and to focus in the proper S-shaped signal in accordance with the level of the focus error signal and the measured time.  
         [0083]    Furthermore, in the second and third embodiments, when the first focus error signal appears, the focus drive voltage is detected. The detected focus drive voltage is compared with the reference voltage and the time for detecting the voltage is compared with a reference time. Thus, it is possible to discriminate CD and DVD, and to focus in the proper S-shaped signal in accordance with the focus drive voltage and the measured time.  
         [0084]    While the invention has been described in conjunction with preferred specific embodiment thereof, it will be understood that this description is intended to illustrate and not limit the scope of the invention, which is defined by the following claims.