Abstract:
An optical disk playing apparatus which performs at least a focusing servo operation for reading light as a servo operation and irradiates the reading light onto one optical disk, receives reflected light from the optical disk by a pickup to obtains a light receiving signal, moves an irradiated position of the reading light onto the optical disk to cross tracks on the disk; generates a tracking error signal in response to the reading signal during the track crossing; compares the tracking error signal with a threshold value; and determines the type of the optical disk in accordance with the number of pulses in the compared result signal within a predetermined period from start of the track crossing.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an optical disk playing apparatus for automatically discriminating one optical disk to be either of two types of optical disks which have different recording densities for recoding information so as to play the one optical disk, and a method for discriminating the type of a disk. 
   2. Description of the Related Art 
   In a disk playing apparatus capable of playing both optical disks, a CD (compact disk) and a DVD (digital versatile disk), which have different recording densities, when an optical disk is set to the playing apparatus, the playing apparatus discriminates the type of the set disk between a CD and a DVD. 
   A typical method for discriminating an optical disk in the playing apparatus utilizes a fact that the CD and DVD have different pit lengths formed on their surfaces. Specifically, when the disk is rotated at a prescribed rotation speed, the maximum or minimum pit length is measured from the disk, and the type of the disk is discriminated in size of the pit length. In another method, after the maximum or minimum pit length has been detected, the rotation speed of a spindle motor is controlled such that a time length of the pit becomes constant, and the type of the disk is discriminated from the controlled rotation speed. Further, in a known method, the type of the disk is discriminated by utilizing that a PLL (phase locked loop) circuit is locked when a pit with a predetermined length is read out. 
   In any method for discriminating the type of the disk by utilizing difference of pit lengths as described above, after a disk has been set to the playing apparatus, the disk must be controlled to be rotated to a predetermined rotation speed such as a prescribed rotation speed. Therefore, there is a problem in that a result of the discrimination of the disk type cannot be necessarily obtained before rotation speed of the disk reaches the predetermined rotation speed. Further, there is another problem in that considerably long time is required for discriminating the type of the disk since control of a tracking servo system and a focusing servo system must be performed progressively to a condition where the tracking and focusing errors are within tolerance so that the pits on the disk are read out by a pickup. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the present invention is to provide an optical disk playing apparatus and a method for discriminating an optical disk, where the type of the optical disk can be discriminated within a comparatively short time. 
   According to the present invention, there is provided an optical disk playing apparatus for automatically discriminating one optical disk to be either of two types of optical disks which have different recording densities so as to play the one optical disk in accordance with the discriminated result, comprising: a pickup unit for irradiating reading light onto the one optical disk, and receiving reflected light from the one optical disk to output a light receiving signal; a servo device for controlling the pickup unit to perform at least a focusing servo operation for the reading light as a servo operation; a moving device for moving an irradiated position of the reading light by the pickup unit onto the one optical disk to cross tracks which are formed on the one optical disk; a tracking error signal generator for generating a tracking error signal in response to the light receiving signal from the pickup unit during the track crossing of the irradiated position; a binarizing device for comparing the tracking error signal with a threshold value; and a determining device for determining the type of the optical disk in accordance with a number of pulses output from the binarizing device within a predetermined period from start of the track crossing of the irradiated position. 
   According to the present invention, there is provided a method for discriminating at least two types of optical disks which have different recording densities, the method comprising: performing at least a focusing servo operation for reading light as a servo operation, irradiating the reading light onto one optical disk, receiving reflected light from the one optical disk to obtain a light receiving signal; moving an irradiated position of the reading light onto the optical disk to cross tracks which are formed on the optical disk; generating a tracking error signal in response to the light receiving signal during the track crossing of the irradiated position; comparing the tracking error signal with a threshold value to generate a binarized signal; and determining the type of the one optical disk in accordance with a number of pulses in the binarized signal within a predetermined period from start of the track crossing of the irradiated position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating an arrangement of an optical disk playing apparatus according to the present invention. 
       FIG. 2  shows an optical system of a pickup of the playing apparatus shown in  FIG. 1 . 
       FIG. 3  shows the light receiving surface of a photodetector of the pickup shown in  FIG. 2 . 
       FIG. 4  is a block diagram specifically illustrating a part of the playing apparatus shown in  FIG. 1 . 
       FIG. 5  is a flowchart illustrating an operation for discriminating the type of a disk performed by a system control circuit. 
       FIGS. 6A and 6B  show respective waveforms of a tracking error signal and a binarized signal when a disk is a CD. 
       FIGS. 7A and 7B  show respective waveforms of a tracking error signal and a binarized signal when a disk is a DVD. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. 
     FIG. 1  shows an arrangement of an optical disk playing apparatus to which the present invention is applied. In the playing apparatus, a pickup  1  irradiates laser light onto an optical disk  2 , receives reflected light of the laser light from the disk  2 , and generates a signal depending on an intensity of the received light. The optical disk  2  is rotationally driven by a spindle motor  4 . Either of a CD and a DVD can be used as the optical disk  2 . Herein, the term “CD” includes any kind of a CD such as CD-R or CD-ROM, and “DVD” includes any kind of a DVD such as DVD-R or DVD-ROM. 
   In an optical system of the pickup  1 , as shown in  FIG. 2 , two semiconductor laser devices  11  and  12  are provided. The first semiconductor laser device  11  emits a first laser beam having a wavelength of 650 nm for a DVD. The second semiconductor laser device  12  emits a second laser beam having a wavelength of 780 nm for a CD. The semiconductor laser devices  11  and  12  are selectively driven by drive circuits  26  and  27  in response to commands from a system control circuit  5  as described later. 
   The first laser beam emitted from the first semiconductor laser device  11  reaches a prism  14  for combining optical paths through a first grating  13 . The first grating  13  is provided for dividing the first laser beam into a plurality of luminous fluxes (zero-order light and ± first-order lights). The zero-order light is a main beam where the incident first laser beam has been passed through the grating directly, and used to perform a focusing servo operation and generate a reading signal. The ± first-order lights are sub beams created due to diffraction of the first laser beam, and used to perform a tracking servo operation with a differential push-pull method together with the zero-order light. The optical path combining prism  14  passes the first laser beam therethrough in the forward direction of the beam. 
   The second laser beam emitted from the semiconductor laser device  12  reaches the optical path combining prism  14  through a second grating  15 . The second grating  15  is provided for dividing the second laser beam into a plurality of luminous fluxes (zero-order light, and ± first-order lights). In the case of the second laser beam similarly as the first laser beam, the zero-order light is a main beam where the incident second laser beam has been passed through the grating directly, and used to perform the focusing servo operation and generate a reading signal. The ± first-order lights are sub beams created due to the diffraction of the second laser beam, and used to perform the tracking servo operation with a differential push-pull method together with the zero-order light. The optical path combining prism  14  reflects the incident second laser beam at substantially right angle to the beam. The forward direction of the reflected laser beam is equal to the forward direction of the first laser beam described above, or the direction of the optical disk  2  as a recording medium. 
   A beam splitter  16 , a collimator lens  17 , and an objective lens  18  are arranged between the optical path combining prism  14  and the optical disk  2 . The beam splitter  16  passes the laser beam from the optical path combining prism  14  directly toward the optical disk  2 . 
   The collimator lens  17  converts the laser beam from the beam splitter  16  into a parallel light and supplies it to the objective lens  18 . The objective lens  18  is a bifocal lens, and converges the laser beam as a parallel light onto a recording surface of the disk  2 . The laser beam reflected at the recording surface of the disk  2  is converted into a parallel light by the objective lens  18  and the collimator lens  17 , and then reflected by the beam splitter  16 . The beam splitter  16  reflects the reflected laser beam at an angle of about 90 degrees to incidence of the beam. A cylindrical lens  19  and a photodetector  20  are arranged in that order in the direction of the laser beam reflected by the splitter  16 . The cylindrical lens  19  is an astigmatic generation element for generating astigmatism. 
   The pickup  1  is further provided with an actuator  10  including a focusing portion for shifting the objective lens  18  in a direction of its optical axis and a tracking portion for shifting the objective lens  18  in a radius direction of the disk perpendicular to the optical axis. 
   The photodetector  20  includes a light receiving unit  21  for receiving the main beam and light receiving units  22  and  23  for receiving the sub beams arranged to sandwich the unit  21  as shown in  FIG. 3 . The light receiving unit  21  has a four-divided surface for receiving light. Two directions of the four-division are corresponding to the line of the radius of the disk and tangential line of the track, respectively. The light receiving unit  21  includes four light receiving elements  21   a–   21   d  corresponding to the respective portions of the four-divided surface. 
   Output signals from the light receiving units  21 – 23  are supplied to a head amplifier  6  and a servo circuit  8 . The head amplifier  6  obtains an RF (radio frequency) signal in accordance with output signals from the light receiving elements  21   a – 21   d  of the light receiving unit  21  as described later, amplifies the signal to be an RF signal Rf, and supplies it to an information data reproducing circuit  9 . 
   The information data reproducing circuit  9  restores data recorded on the optical disk  2  by binarizing the RF signal Rf output from the head amplifier  6  and then performing demodulation and error correction of the binarized signal. Further, the circuit  9  reproduces information data (for example, video, audio, and computer data) by performing information data decode for the restored data, and outputs the data as the reproduced information data. 
   The servo circuit  8  has a focusing error signal generating circuit  24  and a tracking error signal generating circuit  25  as shown in  FIG. 4 . The focusing error signal generating circuit  24  generates a focusing error signal FE indicating focusing error of a beam spot on the recording surface of the optical disk  2  in accordance with respective output signals from the light receiving elements  21   a – 21   d  of the light receiving unit  21 . The tracking error signal generating circuit  25  generates a tracking error signal TE indicating error of beam spot on the optical disk  2  from the center position of the track in the radius direction of the disk in accordance with the respective output signal from the light receiving units  22  and  23 . These error signals are supplied to the servo circuit  8 . Also, the tracking error signal generating circuit  25  supplies the tracking error signal TE to a binary circuit  86 . 
   The binary circuit  86  binaraizes the tracking error signal TE. That is, the circuit  86  generates a binarized signal. The binarized signal has a pulse waveform of which a logic level is “1” when a signal level of the tracking error signal TE is equal to or higher than a predetermined level TH, and “0” when it is lower than TH. An output terminal of the binary circuit  86  is connected to an edge counter  87 . The edge counter  87  is a counter that counts a front edge of the binarized signal output from the binary circuit  86 . The edge counter  87  starts counting in response to a command from the system control circuit  5 . The counted value by the edge counter  87  is supplied to the system control circuit  5 . 
   In the servo circuit  8 , as shown in  FIG. 4 , a frequency signal FG as an AC signal indicating a current rotation speed of the spindle motor  4  for rotating the optical disk  2  via a turntable is supplied to a rotation speed detection unit  81 . The unit  81  generates a rotation speed signal indicating a rotation speed of a spindle corresponding to the frequency signal FG, and the rotation speed signal is supplied to the system control circuit  5  and a rotation speed error signal generating unit  82 . The unit  82  generates a rotation speed error signal indicating a difference between the rotation speed signal and a reference rotation speed signal supplied from the system control circuit  5 , and supplies the rotation speed error signal to an equalizer  83 . The equalizer  83  generates a spindle drive signal SPD in accordance with the rotation speed error signal, and supplies the spindle drive signal SPD to the spindle motor  4  through a drive circuit  31  when a switch  84  is turned on. The spindle motor  4  rotationally drives the optical disk  2  at a rotation speed corresponding to the spindle drive signal SPD. The spindle motor  4  has an AC generator (not shown) which supplies the frequency signal FG corresponding to the current rotation speed to the servo circuit  8 . The spindle servo system can rotationally drive the spindle motor  4  at the rotation speed indicated by the reference rotation speed signal supplied from the system control circuit  5 . 
   In the servo circuit  8 , the focusing error signal FE is supplied to an equalizer  74 . The equalizer  74  generates a focusing drive signal FD and supplies the focusing drive signal FD to the drive circuit  29  when a switch  75  is turned on. The drive circuit  29  drives the focusing portion of the actuator  10  mounted on the pickup  1  in response to the focusing drive signal FD, and the focusing portion operates to adjust a focal point of the beam spot irradiated onto the disk  2 . 
   Further, in the servo circuit  8 , the tracking error signal TE is supplied to an equalizer  76 . The equalizer  76  generates a tracking drive signal TD and supplies the tracking drive signal TD to a drive circuit  30  through an adder  79  when a switch  77  is turned on. The drive circuit  30  drives the tracking portion of the actuator  10  mounted on the pickup  1  in response to the tracking driver signal TD, and the tracking portion moves the position of the beam spot irradiated on the disk  2  in the radius direction of the disk by just a distance corresponding to a driving current based on the tracking drive signal TD. 
   Each of the above switches  75 ,  77 ,  84  performs ON-OFF control action in response to the commands from the system control circuit  5 . The switch  75  is turned on when the focusing servo is controllably operated, the switch  77  is turned on when the tracking servo is controllably operated, and the switch  84  is turned on when the spindle servo is controllably operated. 
   Further, while not shown in  FIG. 4 , the servo circuit  8  generates a slider drive signal SD in response to the tracking error signal TE, and supplies the signal SD to a slider  100  through a drive circuit  28 . This causes the slider  100  to shift the pickup  1  in the radius direction of the disk at a rotation speed corresponding to a drive current based on the slider drive signal SD. 
   The system control circuit  5 , including a microcomputer, controls the playing apparatus, and generates various control signals depending on operator inputs from an operation unit (not shown) by user and current working condition of the playing apparatus. The servo circuit  8  performs servo control operations in response to the various control signals generated in the system control circuit  5 . 
   In the above arrangement, when the drive circuit  26  drives the semiconductor laser device  11  in response to a drive command from the system control circuit  5 , the first semiconductor laser device  11  emits the first laser beam for a DVD, the first laser beam is divided into a plurality of luminous fluxes (zero-order light and ± first-order lights) by the first grating  13  as described before, and then reaches the collimator lens  17  through the optical path combining prism  14  and the beam splitter  16 . On the other hand, when the drive circuit  27  drives the second semiconductor laser device  12  in response to the drive command from the system control circuit  5 , the second semiconductor laser device  12  emits the second laser beam for a CD, the second laser beam is divided into a plurality of luminous fluxes (zero-order light and ± first-order lights) by the second grating  15 , and then reflected toward the disk  2  by the optical path combining prism  14 . The reflected laser beam reaches the collimator lens  17  through the beam splitter  16 . 
   When either of the first or the second laser beam is emitted, the laser beam is collimated by the collimator lens  17  and reaches the objective lens  18 . The objective lens  18  focuses the laser beam on the recording surface of the disk  2  and an oval light spot is formed on the recording surface. 
   The laser beam, after being modulated and reflected by the information data pit on the recording surface of the disk  2 , passes through the objective lens  18  and the collimator lens  17  and returns to the beam splitter  16  which diverges the beam from the optical path started from the semiconductor laser device, and enters respective light receiving surfaces of the light receiving units  21 – 23  of the photodetector  20  through the cylindrical lens  19 . The reflected light of zero-order light from the disk  2  reaches the light receiving unit  21 , and the reflected lights as ± first-order lights from the disk  2  reach the light receiving units  22  and  23 , respectively. 
   In the photodetector  20 , a circular light spot is formed on each of the light receiving surfaces of the light receiving units  21 - 23 . For example, as shown in  FIG. 3 , a circular light spot MS of the zero-order light in the intersection of defining lines of the light receiving surfaces of the light receiving elements  21   a ,  21   b ,  21   c ,  21   d  is formed in the light receiving unit  21 , and circular light spots SS 1  and SS 2  of the ± first-order lights are formed in the centers of light receiving units  22  and  23 , respectively. 
   An RF signal Rf and a focusing error signal FE are generated in response to respective output signals from the light receiving elements  21   a – 21   d  of the photodetector  20 . In addition, a tracking error signal TE is generated in response to respective output signals from the light receiving units  22  and  23 . Assuming that respective output signals of the light receiving elements  21   a – 21   d  are referred as Aa–Ad in that order, and respective output signals of the light receiving units  22  and  23  are referred as B and C in that order, the RF signal Rf is given in the head amplifier  6  by:
 
 Rf=Aa+Ab+Ac+Ad 
 
the focusing error signal FE is given in the focusing error signal generation circuit  24  by:
 
 FE =( Aa+Ac )−( Ab+Ad )
 
and the tracking error signal TE is given in the tracking error signal generating circuit  25  by:
 
 TE=B−C 
 
   The focusing error signal FE is supplied to the drive circuit  29  through the equalizer  74  as a focusing drive signal FD when the switch  75  is turned on. The drive circuit  29  shifts the objective lens  18  in the optical axis direction to the focusing portion of the actuator  10  in response to the focusing drive signal FD and adjusts the focal point of the beam spot irradiated onto the disk  2 . 
   The tracking error signal TE is supplied to the drive circuit  30  through the equalizer  76  as a tracking drive signal TD when the switch  77  is turned on. The drive circuit  30  shifts the objective lens  18  in the radius direction of the disk to the tracking portion of the actuator  10  in response to the tracking drive signal TD and deviates the position of the beam spot irradiated onto the disk  2  thereby. 
   When an optical disk  2  is set on a turntable (not shown) in the playing apparatus, the system control circuit  5  performs the operation for discriminating the type of the disk  2 . 
   In discrimination of the type of the disk, first the system control circuit  5 , as shown in  FIG. 5 , sends an emission drive command to a drive circuit  27  (step S 11 ). When the emission drive command is supplied to the drive circuit  27 , the drive circuit  27  supplies a drive current to the second semiconductor laser device  12  in response to the command, causing the second semiconductor laser device  12  to emit the second laser beam for a CD. 
   The system control circuit  5 , after the step S 11  has been completed, switches off the spindle servo, switches on the focusing servo, and switches off the tracking servo (step S 12 ). That is, the switch  75  is turned on, and switches  77  and  84  are turned off. Since only the focusing servo control is operated, the focal point of the beam spot by the second laser beam is adjusted on the recording surface of the disk  2 . 
   The system control circuit  5  sends a command for vibrating the objective lens to a voltage application circuit  80  in the servo circuit  8  (step S 13 ), and sends a count-start-command to the edge counter  87  (step S 14 ). The system control circuit  5 , after the step S 14  has been completed, determines whether the counted value by the edge counter  87  is more than a prescribed value before a predetermined period will elapse (step S 15 ). 
   The voltage application circuit  80  supplies a vibration drive voltage, varying up and down, to the drive circuit  30  through the adder  79  in response to the command for vibrating the objective lens. This causes the drive circuit  30  to drive the objective lens  18  through the tracking portion of the actuator  10 , and shift the objective lens  18  in the radius direction of the disk within its movable range. That is, the irradiated position of the laser beam moves back and forth in the radius direction on the disk  2  and crosses tracks. 
   In the case where the second laser beam for a CD is emitted, when the disk  2  is a CD, the tracking error signal TE passes through the above predetermined level TH every time the irradiated position of the laser beam crosses a track of the disk  2 . Therefore, when the irradiated position of the laser beam moves in the radius direction on the disk  2 , the tracking error signal TE crossing the predetermined level TH is generated as shown in  FIG. 6A , the binary circuit  86  generates a binarized signal repeatedly showing 0 and 1 as shown in  FIG. 6B , and the edge counter  87  counts the front edge of the binarized signal (pulse) in response to the count-start-command generated at step  14 . The counted value by the edge counter  87  increases and then exceeds the determined value within the predetermined period. 
   On the other hand, when the disk  2  is a DVD, the second laser beam for a CD is emitted. Even if the irradiated position of the laser beam crosses a track of the disk  2 , the tracking error signal TE varies slightly, and the level of the variation of the tracking error signal TE does not reach the predetermined level TH as shown in  FIG. 7A . Therefore, even if the irradiated position of the laser beam moves in the radius direction on the disk  2 , the binary circuit  86  outputs a binarized signal showing its level kept 0 as shown in  FIG. 7B . Further, even if the edge counter  87  is in operation condition where it counts a front edge of the binarized signal in response to the count-start-command, the counted value by the counter  87  remains an initial value. 
   When the counted value by the edge counter  87  is discriminated to be equal to or more than the prescribed value at step S 15 , the system control circuit  5  determines the type of the disk  2  to be a CD (step S 16 ), and starts playing operation for the CD (step S 17 ). On the other hand, when the counted value by the edge counter  87  does not reach the prescribed value, the circuit  5  determines the type of the disk to be a DVD (step S 18 ), and starts playing operation for the DVD (step S 19 ). 
   When starting the playing operation for a CD, the emission of the second laser beam from the semiconductor laser device  12  and the focusing servo are kept on, the switch  77  is turned on and the tracking servo is switched on thereby, the switch  84  is turned on and the spindle servo is switched on thereby, and thus a slider servo (not shown) is switched on. Moreover, an operation mode of the information data reproduction circuit  9  is set to a reproduction mode of audio data. When starting the playing operation for a DVD, the emission of laser beam is switched to the first laser beam from the semiconductor laser device  11 , the focusing servo is kept ON, the switch  77  is turned on and the tracking servo is switched on thereby, the switch  84  is turned on and the spindle servo is switched on thereby, and thus the slider servo (not shown) is switched on. Moreover, the operation mode of the information data reproduction circuit  9  is set to a reproduction mode of both audio and video data. 
   Although the focusing servo is performed with the astigmatism method and the tracking servo is performed with the three-beam method in the above described embodiments, any other method known in the art can be used without limiting to those. In addition, the methods for playing a CD and a DVD may be different from each other. 
   Although the objective lens is vibrated in the radius direction of the disk as the motion for crossing the track, the objective lens can be moved only in one way, or the pickup  1  can be moved in the radius direction of the disk with a slider  100 . 
   As described hereinbefore, according to the invention, the type of the optical disk can be discriminated independently of the rotation speed set for the optical disk, or without waiting arrival of the optical disk to the predetermined rotation speed, and the type of the optical disk can be discriminated with the optical disk being not rotated, consequently the type of the disk can be discriminated within a comparatively short time. 
   This application is based on a Japanese Patent Application No. 2001-107154 which is hereby incorporated by reference.