Abstract:
A position control method, which positions the relative position of an optical disk and an optical pickup to a position that provides the optimum value of the jitter value of the detection signal obtained by detecting the reflected light of an optical spot, has a jitter value acquisition procedure, which carries out multiple feed operations within the adjustment range of the relative position and obtains the jitter value at each feed position, an edge position setting procedure which sets as the edge positions relative positions that exceed a prescribed preset jitter threshold value, an optimum position computation procedure, which computes the optimum position that yields the optimum value for the jitter value, and an edge position setting procedure that sets the edge positions on the condition that there are at least two relative positions that yield a jitter value that exceeds the jitter threshold value.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention concerns an optical pickup position control method that is used in an optical disk device that records and/or plays back information by shining an optical spot from an optical pickup onto an optical disk, and that positions the relative position of said optical disk and said optical pickup in a position that gives the optimum value of the jitter value of the detection signal obtained by detecting the reflected light of said optical spot.  
         BACKGROUND OF THE INVENTION  
         [0002]    An optical disk device that records and plays back information on an optical disk such as a CD, DVD, CD-ROM-, DVD-ROM-, CD-R, DVD-R, etc., conventionally has an optical pickup that shines a laser light or other optical spot onto the optical disk and plays back or records information, and a drive device that adjusts the relative position between the optical disk and the optical pickup. In such an optical device, the control system, which includes the drive device, is provided with an automatic adjustment mechanism that adjusts the operation center position of skew, focus, etc. to the optimum position in order to minimize the effect of the jitter that occurs in the playback signal (RF signal) that is detected by the optical pickup, and the skew and focus operation center position is determined by, for example, procedures such as the following.  
           [0003]    (1) If there is a relationship as shown in FIG. 10 between the jitter value and the relative position of the optical disk and the optical pickup, a jitter value that is greater than the minimum jitter value by a standard amount is taken as the jitter threshold value. For this jitter threshold value, the upper limit of the error-correctable jitter value is mainly taken as the standard.  
           [0004]    (2) Taking point A of FIG. 10 as the standard, while varying the relative position of the optical disk and the optical pickup, one obtains the jitter value at each position and determines the relative position of the two that exceeds a set jitter threshold value, that is, the relative position (edge position) of the upper value of the optical disk and the optical pickup at point B.  
           [0005]    (3) When edge position point B is determined in the relative position adjustment of the optical disk and the optical pickup, point C, which is the midpoint between point A and point B, is set as the operation center position in the relative position adjustment of the optical disk and the optical pickup.  
           [0006]    However, a problem with such a method of setting the operation center position in the relative position adjustment of the optical disk and the optical pickup is that, because it is taken as a premise that the effect of any disturbance is not received, if the effect of any disturbance is received, it is impossible to set the accurate operation center position corresponding to the jitter characteristics. That is, if a disturbance acts on the optical disk device, such as damage occurring to the optical disk or vibration during measurement of the jitter value, then, as shown by the dotted line in FIG. 10, the jitter value becomes spotwise large, so a point D that exceeds the jitter threshold value arises before reaching point B in FIG. 10, and therefore if the operation center position is computed based on point D and point A, an incorrect operation center position will be set.  
           [0007]    It is an object of the present invention to provide an optical pickup position control method that makes it possible, in positioning the relative position of an optical disk and an optical pickup, to set the correct operation center position even if there is an effect by a disturbance, etc.  
         SUMMARY OF THE INVENTION  
         [0008]    The optical pickup position control method of the present invention, being an optical pickup position control method that is used in an optical disk device that records and/or plays back information by shining an optical spot from an optical pickup onto an optical disk, and that positions the relative position of said optical disk and said optical pickup in a position that gives the optimum value of the jitter value of the detection signal obtained by detecting the reflected light of said optical spot, has a jitter value acquisition procedure that performs multiple feed operations within the adjustment range of said relative position and obtains the jitter value at each feed position. An edge position setting procedure is provided that, based on the feed positions and jitter values obtained by this jitter value acquisition procedure, sets as the edge positions relative positions that will be greater than a preset prescribed jitter threshold value. An optimum position computation procedure is provided that computes the optimum position that will give the optimum value of said jitter value, based on the edge positions set by the edge position setting procedure, and an optimum position adjustment procedure is provided that adjusts the relative position of said optical disk and said optical pickup to the optimum position computed by the optimum position computation procedure. The edge position setting procedure sets the edge positions on the condition that there are at least two relative positions that will give a jitter value that exceeds said jitter threshold value.  
           [0009]    It is desirable to adopt the method of the present invention if one is to perform positioning, in the relative position between an optical disk and an optical pickup, of the tilt position and focus position, etc. between the disk and pickup. The tilt position expresses the inclination (skew) position of the optical pickup with respect to the optical disk, and the tilt position must be adjusted in order to cause the optical spot from the optical pickup to be incident to the information recording surface of the optical disk at right angles. The focus position must be adjusted so that the depth of focus of the optical spot from the optical pickup is fixed; specifically, position adjustment can be achieved by applying a prescribed bias voltage to a focus servo mechanism. The jitter value can be obtained by taking out the high-frequency components of the RF signal that serves as the detection signal output from the optical pickup, and performing the prescribed signal processing. Also, in setting the edge positions, it is set as a condition that there be at least two relative positions of the optical disk and optical pickup that exceed the jitter threshold value, but it is preferable to have three relative positions that exceed the jitter threshold value. By setting the condition that there be at least two relative positions that exceed the jitter threshold value, said edge positions are never recognized as the edge positions uniquely, even if a part occurs that spotwise exceeds the jitter threshold value because of vibration or other disturbance. Therefore the optimum operation center position can be set without being affected by any disturbance, etc.  
           [0010]    In the foregoing, it is desirable that acquisition of the jitter value at each feed position in said jitter value acquisition procedure be accomplished by rotating the optical disk at a prescribed speed, taking in multiple jitter values along the inside circumference of said optical disk, and taking the average of the obtained multiple jitter values excluding the maximum value. By thus taking the average value of the multiple jitter values that are obtained, the effect of damage or other disturbances arising on the surface of the optical disk can be reduced, thus making it possible to even more accurately determine the operation center position of the optical disk and optical pickup. In particular, by calculating the average value excluding the maximum value, then even if a scratch, etc. arises that extends in the radial direction of the disk, this can be excluded when computing the operation center position.  
           [0011]    If the method of the present invention employs a jitter characteristics smoothing procedure that, following said jitter value acquisition procedure, smoothes the obtained jitter characteristics if the jitter characteristics by which the jitter values corresponding to the feed positions are plotted are judged not to conform to the preset jitter characteristics, it is desirable that the edge position setting procedure and optimum position computation procedure be carried out based on this jitter characteristics smoothing procedure. In such a situation, the jitter characteristics smoothing procedure can be carried out if one checks the characteristics of the jitter value with respect to the relative position of the optical disk and optical pickup and the jitter characteristics curve is neither an increasing function, nor a decreasing function, nor convex upward, nor convex downward. Specifically, the jitter characteristics smoothing procedure is carried out if variability arises in the jitter values obtained in the feed positions in the jitter value acquisition procedure due to surface deviation of the optical disk, etc.  
           [0012]    Specifically, with reference to FIG. 7, in the jitter characteristics smoothing procedure, denoting the jitter values obtained at feed position X 0 ≦X≦Xmax by J(X 0 )≦ J(X)≦J(Xmax), the smoothed jitter values Js(X 0 ), Js(X), Js(Xmax) at each feed position can be calculated by the following formulas ((1))-((3)).  
             Js (X 0 )=[3 xJ (X 0 )+J(X 0 +1)]/4   &lt;1&gt; 
             Js (X)=[ J (X−1)+2 xJ (X)+ J (X+1)]/4 . . .   &lt;2&gt; 
             Js (Xmax)=[ J (Xmax−1)+3 xJ (Xmax)]/4   &lt;3&gt; 
           [0013]    In the edge position setting procedure, the edge positions are set as follows based on the smoothed jitter characteristics thus obtained.  
           [0014]    (1) Set as the first edge position the feed position of jitter value Jedge, which is the smaller of Js(X 0 ) and Js(Xmax) (first edge position setting step).  
           [0015]    (2) Compare this Jedge and each Js(X) in sequence from this first edge position in the direction toward the feed position at which another Js(X 0 ) or Js(Xmax) is taken, and take as the second edge position the feed position at which Jedge≦Js(X) (second edge position setting step).  
           [0016]    Also, in the optimum position computation procedure, which is done based on the first and second edge positions set in the edge position setting procedure, specifically it calculates the midpoint of the obtained first and second edge positions and takes this midpoint as the optimum position (operation center position).  
           [0017]    By thus carrying out the jitter characteristics smoothing procedure and then the jitter position setting procedure and the optimum position computation procedure, one can eliminate the variability in the jitter values that is caused by the surface deviation, etc. that occurs on individual optical disks and can compute the optimum position, and thus in the optimum position adjustment procedure one can more accurately perform positioning of the optical disk and optical pickup.  
           [0018]    And by calculating by above formulas &lt;1&gt;-&lt;3&gt; in the jitter characteristics smoothing procedure and carrying out the edge position setting procedure by steps (1) and (2), even if it has jitter characteristics that are asymmetrical with respect to the optimum position (operation center position), the appropriate edge positions can be ascertained, and a more appropriate optimum position can be computed in the optimum position computation procedure.  
           [0019]    It is desirable that the aforesaid optical pickup position control method of the present invention has an implementation decision procedure that decides, according to the type of the optical disk, whether to carry out the jitter value acquisition procedure, the edge position setting procedure, the optimum position computation procedure, and the optimum position adjustment procedure. Deciding according to the type of the optical disk means basically deciding whether to carry out the procedures according to the density of the information recorded on the optical disk; for example, the recording density of the information on a CD or CD-ROM is low and the margin in the relative position adjustment is large, so it suffices to carry it out only for those in which the margin is small, such as DVD or DVD-ROM, without carrying out the position control method for the optical pickup.  
           [0020]    By thus carrying out the position control method only if the margin in relative position adjustment is small, if a CD, etc. is mounted onto the optical disk device, data reading can begin without doing automatic adjustment, so starting of the optical disk device and beginning of reading of the data can be done more quickly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a rough perspective view of an entertainment device of an embodiment of the present invention.  
         [0022]    [0022]FIG. 2 is a block diagram showing the internal structure of the entertainment device.  
         [0023]    [0023]FIG. 3 is a block diagram showing the structure of the optical disk device, and the mechanical controller that controls such device.  
         [0024]    [0024]FIGS. 4A and 4B explain the method of skew adjustment associated with the tilt control of the method of the present invention.  
         [0025]    [0025]FIG. 5 is a flowchart expressing the optical pickup position control method of the present invention.  
         [0026]    [0026]FIG. 6 is a flowchart expressing the jitter value acquisition procedure of the method of the present invention.  
         [0027]    [0027]FIG. 7 is a graph explaining the jitter characteristics smoothing procedure of the invention.  
         [0028]    [0028]FIG. 8 is a flowchart expressing the edge position setting procedure of the invention.  
         [0029]    [0029]FIG. 9 is a graph explaining the edge position setting procedure of the invention.  
         [0030]    [0030]FIG. 10 is a graph explaining the procedure for setting the optimum position in a prior art optical pickup position control method. 
     
    
     EXPLANATION OF THE SYMBOLS  
       [0031]    [0031] 30  optical disk device  
         [0032]    [0032] 32  optical pickup  
         [0033]    [0033] 100  optical disk  
         [0034]    S 5 , S 6  jitter value acquisition procedure  
         [0035]    S 9  jitter characteristics smoothing procedure  
         [0036]    S 10  edge position setting procedure  
         [0037]    S 11  optimum position computation procedure  
         [0038]    S 12  optimum position adjustment procedure  
         [0039]    S 102  first edge position setting step  
         [0040]    S 107  second edge position setting step  
         [0041]    X 1  first edge position  
         [0042]    X 2  second edge position  
         [0043]    VP optimum position  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]    The following detailed description is of the best mode or modes of the invention presently contemplated. Such description is not intended to be understood in a limiting sense, but to be an example of the invention presented solely for illustration thereof, and by reference to which in connection with the following description and the accompanying drawings one skilled in the art may be advised of the advantages and construction of the invention. In the various views of the drawings, like reference characters designate like or similar parts.  
         [0045]    [0045]FIG. 1 shows an entertainment device  1  on which is mounted an optical disk device that implements the optical pickup position control method of the present invention. Entertainment device  1  conventionally executes a game program when a user calls up a game program, etc. recorded on a CD, CD-ROM, DVD, DVD-ROM, or the like and gives instructions by manipulating an operation controller  2 . The output of entertainment device  1  is connected to a television receiver or other display device  3 , and the screens during execution are displayed on this display device  3 . Power is delivered to entertainment device  1  from, for example, the commercial public power generally supplied to homes, and entertainment device  1  is started by turning on a master switch, not pictured, that is provided on the rear surface of the device and pressing a power switch  11  provided on the front surface of the device. Power switch  11  may also function as a reset button when it is pressed during operation of entertainment device  1 .  
         [0046]    Controller slot  12  and card slot  13  are provided on the front surface of entertainment device  1  Operation controller  2  is connected to controller slot  12 , and memory card  4  is connected to card slot  13 . Optical disk device  30  is provided near controller slot  12  and card slot  13 , which are provided on the front surface of the device. This optical disk device  30  is a disk-loading optical disk device in which, by manipulating operation switch  14 , a disk tray advances and retracts from entertainment device  1 .  
         [0047]    As shown in the block diagram of FIG. 2, the device main body inside entertainment device  1  has system main body  21  as the main board on which the CPU is mounted, and I/O port  22 , which is connected to system main body  21 , and connected to I/O port  22 , via bus line  23 , are controller slot  12 , card slot  13 , and USB port  15 , etc., as well as mechanical controller  40  and system controller  50 . System main body  21  performs control of the device as a whole and computation processing of game programs and other software Through I/O port  22 , main body  21  performs operation control of operation controller  2  and other external devices connected to controller slot  12 , card slot  13 , and USB port  15 , etc. and of mechanical controller  40  and system controller  50 , and it processes signals output from external devices, etc. connected to I/O port  22 . Mechanical controller  40  performs operation control of optical disk device  30  as described in greater detail below, and optical disk device  30  is connected to bus line  23  via mechanical controller  40 . System controller  50  controls power source circuit  60  and manages the power supply state of entertainment device  1  as a whole based on operation signals from power switch  11  and signals from sensors (not pictured) that monitor the temperature state of the CPU that constitutes system main body  21 ; power source circuit  60  is connected to bus line  23  via this system controller  50 . Mechanical controller  40  and system controller  50  also have a direct port connection without going through bus line  23 , and are constituted so that system controller  50  outputs signals of two levels, low and high, to mechanical controller  40  according to the operation signal of power switch  11 . For example, it outputs Low if power switch  11  is pressed at initial startup, and it outputs High if power switch  11  is pressed during operation of entertainment device  1  and a restart command is given.  
         [0048]    As shown in FIG. 3, optical disk device  30  has spindle motor  31  and optical pickup  32 , spindle servo unit  33 , tracking adjustment unit  34 , focus adjustment unit  35 , slide feed adjustment unit  36 , and skew adjustment unit  37 . Although not pictured, adjustment units  34 - 37  are constituted so as to have a feed mechanism that adjusts the attitude of optical pickup  32  with respect to optical disk  100 , and a stepping motor that serves as the drive source of this feed mechanism. Also, although not pictured in FIG. 3, provided on this optical disk device  30  are a disk tray loading mechanism for extending and retracting the disk tray from entertainment device  1 , and a raising and lowering mechanism that raises and lowers spindle motor  31  and optical pickup  32  so that they do not interfere with the disk tray.  
         [0049]    Spindle motor  31  rotates optical disk  100  mounted in the disk tray Provided on the end of the rotation shaft of said spindle motor  31  is a chucking member that engages with the center hole of optical disk  100 . Spindle motor  31  is rotation-controlled by spindle servo unit  33  in such a way that information can be detected by optical pickup  32  regardless of the radial-direction position of optical pickup  32  with respect to optical disk  100 . Tracking adjustment unit  34  has a two-axis device that adjusts the objective lenses that constitute optical pickup  32  so that the optical spot from optical pickup  32  is shined accurately onto the tracks off optical disk  100 , an actuator that imparts driving force to such two-axis device, and a driver IC that drives this actuator. Focus adjustment unit  35  performs position adjustment of optical pickup  32  so that the depth of focus of the optical spot from optical pickup  32  is fixed with respect to the information recording surface of optical disk  100 , and has an advance-and-retract mechanism that position-adjusts optical pickup  32  with respect to the information recording surface of optical disk  100  in the direction of approaching and withdrawing from it, a stepping motor that imparts driving force to this advance-and-retreat mechanism, and a driver IC that operates this stepping motor. Slide feed adjustment unit  36  adjusts the position, in the radial direction of optical disk  100 , of optical pickup  32  by track jumping, etc.; although not pictured in the diagram, it has a thread feed mechanism that consists of a rack that extends along the radial direction of optical disk  100  and a gear that engages with this rack, a stepping motor that causes this gear to rotate, and a driver IC that drives this stepping motor. Optical pickup  32  is attached on the rack, and when the gear turns by the stepping motor, optical pickup  32  moves by means of the rack along the radial direction of optical disk  100 . If track jumping is to be done, first, by the two-axis device of tracking adjustment unit  34 , the center of the optical axis of the optical spot is aligned on the track to which the jump is to be made, then optical pickup  32  is slid by the slide feed mechanism. Skew adjustment unit  37  adjusts the tilt of optical pickup  32  with respect to optical disk  100  so that the optical spot shined from optical pickup  32  is incident perpendicular to optical disk  100 .  
         [0050]    As shown in FIGS. 4A and 4B, skew adjustment unit  37  has a skew angle adjustment mechanism that adjusts the angle of optical pickup  32  with respect to optical disk  100  in the range from the base position BP to the maximum feed position MP, as well as a stepping motor (not pictured) that serves as the drive source for this skew angle adjustment mechanism and a driver IC (not pictured) that drives this stepping motor. By operating skew adjustment unit  37 , optical pickup  32  is positioned in center position CP so as to lie exactly opposite the information recording surface of optical disk  100 . In addition, center position CP is set during the manufacturing stage of entertainment device  1  as the operation center position to be set using a standard disk. Furthermore, skew adjustment unit  37  makes adjustments in radial skew direction RS, which is the radial direction of optical disk  100 ; it does not make any adjustments for the tangential skew direction TS, which is the direction of the tangent to the circle.  
         [0051]    Returning to FIG. 3, mechanical controller  40  has RF amplifier  41 , demodulation/data extraction unit  42 , control unit  43 , and RAM  44 , and is connected directly to system controller  50  by the aforesaid port connection, and it is connected to E 2 PROM  45 , which is mounted on the circuit board that includes mechanical controller  40 . RF amplifier  41  amplifies the RF signal input from optical pickup  32  of optical disk device  30  and outputs it to demodulation/data extraction unit  42 . Demodulation/data extraction unit  42  demodulates this RF signal and extracts the necessary data, and the extracted data is output via bus line  23  and I/O port  22  to system main body  21  and is processed by system main body  21 . Control unit  43  outputs control commands to spindle servo unit  33 , tracking adjustment unit  34 , focus adjustment unit  35 , slide feed adjustment unit  36 , and skew adjustment unit  37  of optical disk device  30  and generates control commands as necessary, storing data in RAM  44  and calling out information recorded on E 2 PROM  45 . Included as information recorded on E 2 PROM  45  are the maximum feed amount in initial operation of skew adjustment unit  37 , the number of steps that yields the center position feed amount, information for deciding whether to cause skew adjustment unit  37  to perform automatic adjustment, information concerning the jitter minimum value measured using a standard disk in the manufacturing stage, and the +α value for adding the prescribed jitter amount to the jitter minimum value and generating the jitter threshold value. Also, the high-frequency component (HF) taken from the RF signal of optical pickup  32  that is amplified by RF amplifier  41  is input into control unit  43 , the jitter value is obtained by control unit  43  based on this high-frequency component, and based on this, control commands are generated for adjustment units  34 - 37 .  
         [0052]    Next, with reference to the flowchart in FIG. 5, the position control procedure for optical pickup  32  by skew adjustment unit  37  and control unit  43  in entertainment device  1  will be described.  
         [0053]    (1) When the master switch on the back of entertainment device  1  is turned on and power switch  11  on the front of the device is pressed, electric power is supplied via power circuit  60  to system main body  21 , mechanical controller  40 , and optical disk device  30 , and entertainment device  1  starts (processing S 1 ).  
         [0054]    (2) Control unit  43  loads into RAM 44 the information recorded in E 2 PROM  45  (processing S 2 ). The loaded information includes information concerning the initial operation of aforesaid skew adjustment unit  37 , and information concerning whether to perform automatic adjustment of the tilt position.  
         [0055]    (3) Based on the information concerning initial operation loaded into RAM  44 , control unit  43  outputs a control command that causes skew adjustment unit  37  to execute an initial operation (processing S 3 ). This initial operation has, specifically, a procedure that changes the inclination position of optical pickup  32  with respect to optical disk  100  in the direction from base position BP toward maximum feed position MP, a procedure that then changes the inclination position by the maximum feed amount in the direction toward base position BP, and a procedure that further changes it from base position BP by the center position feed amount, and following the initial operation, optical pickup  32  is positioned in center position CP, which is set in the manufacturing stage.  
         [0056]    (4) If the user presses operation switch  14  and mounts optical disk  100  inside disk device  30 , control unit  43  monitors the detection signal from RF amplifier  41  and decides whether mounted optical disk  100  is a DVD-ROM or other high-density information recording medium (processing S 4 : implementation decision procedure).  
         [0057]    (5) If it is decided that optical disk  100  is a DVD-ROM or other high-density information recording medium, control unit  43  outputs a control command to skew adjustment unit  37 , adjusts the tilt position of optical pickup  32  with respect to optical disk  100 , measures the jitter value from the high-frequency component of the RF signal from RF amplifier  41  (processing S 5 ), and stores in RAM  44  the number of steps of the feed position and the measured jitter value at each feed position (processing S 6 ). Acquisition of jitter values by this jitter value acquisition procedure S 5  and S 6  is repeated until a tilt position of optical disk  100  and optical pickup  32  that is larger than the jitter threshold value becomes three points in a row (processing S 7 ).  
         [0058]    Jitter value acquisition procedure S 5  and S 6  is described as follows in greater detail, with reference to the flowchart shown in FIG. 6.  
         [0059]    (5-1) Control unit  43  loads into RAM  44  the jitter minimum value recorded in E 2 PROM  45 , and the +α t value (processing S 51 ). Thereupon, the control unit  43  generates and outputs to skew adjustment unit  37  a feed operation command of the prescribed distance in the direction from the current center position CP (see FIG. 4B) toward maximum feed position MP (the MP direction) (processing S 52 ). This feed operation command is output as a number of steps of the stepping motor that constitutes skew adjustment unit  37 .  
         [0060]    (5-2) When the position of optical pickup  32  moves pursuant to said feed operation command, control unit  43  measures the jitter value based on the high-frequency component of the RF signal output from RF amplifier  41  (processing S 53 ). Measurement of this jitter value is done 10 times at a take-in interval of 4 ms at the innermost-circumference radius position of optical disk  100  (disk radial direction 24 mm position).  
         [0061]    (5-3) It is decided whether the jitter value measured by processing S 53  is the minimum value (processing S 54 ), and if it is, the minimum value of the jitter value in RAM  44  is updated (processing S 55 ). Then control unit  43  takes the average of 9 of the 10 jitter values, excluding the maximum value, and stores it in RAM  44  as the typical value of the jitter values in said tilt position (processing S 56 ).  
         [0062]    (5-4) Control unit  43  decides, based on the current feed position and the measured jitter values, whether to terminate the feed operation (processing S 57 ). The decision to terminate is made by deciding whether the current feed position is near maximum feed position MP (in this example, it is set to 3 steps before the maximum feed position) or whether the measured jitter value has been, for three times in a row, greater than the jitter value resulting when the +α value is added to the minimum jitter value. If control unit  43  decides not to terminate the operation, processing S 52  through processing S 57  are repeated until a decision to terminate results.  
         [0063]    (5-5) When the feed operation in the MP direction terminates, optical pickup  32  is returned to center position CP (processing S 58 ). The return operation is done by the same procedure as the procedure in the aforesaid initial operation.  
         [0064]    (5-6) Upon the return to center position CP, control unit  43  generates and outputs a feed operation command concerning the direction toward base position BP (the BP direction) in the same way as in the case of the MP direction (processing S 59 ), and performs measurement of the jitter value at said feed position (processing S 60 ), judgment of the minimum value (processing S 61 ), updating of the minimum value (processing S 62 ), storage into RAM  44  (processing S 63 ), and decision of whether to terminate the feed operation (processing S 64 ).  
         [0065]    (6) Control unit  43  decides whether a tilt position that exceeds the jitter threshold value has been obtained for three points in a row (processing S 7 ). If it is decided that three tilt positions have not been obtained, jitter value acquisition procedure S 5  and S 6  is repeated. If it is decided that three tilt positions have been obtained for three points in a row, control unit  43  terminates jitter value acquisition and decides whether the jitter characteristics curve that is constituted by which one plots the feed positions and jitter values shows a uniform change as a whole processing S 8 ). If said jitter characteristics curve is neither an increasing function, nor a decreasing function, nor convex upward, nor convex downward, the decision is made that it does not show a uniform change. If it is decided that as a whole the jitter characteristics curve shows a uniform change, one proceeds to the following processing, without doing smoothing processing.  
         [0066]    (7) If it is decided that as a whole the jitter characteristics curve does not show a uniform change, smoothing processing is done on the measured jitter values (processing S 9 : jitter characteristics smoothing processing). This smoothing processing S 9  is done specifically by the following procedure.  
         [0067]    (7-1) Now, we assume that a plot of feed positions X and jitter values J(X) obtained by jitter value acquisition procedure S 5  and S 6  has characteristics G 1  as shown in FIG. 7. In FIG. 7, feed position X is set so as to grow larger in the direction from base position BP to maximum feed position MP in FIG. 4B. These jitter characteristics G 1 , which show variability in the plot values, as a whole do not constitute a uniform change, nor are they recognized to be either an increasing function, a decreasing function, convex upward, or convex downward, so smoothing processing is done so that they can be ascertained as a uniform change as a whole.  
         [0068]    (7-2) Denoting by Js(X) the smoothed jitter value at feed position X, smoothing processing is done by taking the average value while weighting the jitter value J(X) of given feed positions X at which the jitter value is measured and the jitter values J(X−1 ) and J(X+1) at feed positions X-−1 and X+1 before and after this feed position; in this embodiment it is determined based on the following formula &lt;4&gt;.  
           Js ( X )=[ J ( X− 1)+2 xJ ( X )+ J ( X+ 1)]/4   &lt; 4 &gt; 
         [0069]    The coefficient 2 of J(X) in this formula &lt;4&gt; is the weighting value, and the denominator 4 is the number of data points.  
         [0070]    On the other hand, the smoothed jitter values Js(X 0 ) and Js(Xmax) at edge feed positions X 0  and Xmax are determined based on the following formulas &lt;5&gt; and&lt;6&gt;, because for these the jitter measured value on one side is lacking.  
           Js ( X 0)=[3 xJ ( X 0)+ J ( X 0+1)]/4   &lt; 5 &gt; 
           Js (Xmax)=[ J (Xmax−1)+3 xJ (Xmax)]/4   &lt;6&gt; 
         [0071]    The coefficient 3 of J(X 0 ) and J(Xmax) in these formulas &lt;5&gt; and &lt;6&gt; is the weighting value as in formula&lt;4&gt;, and the number of the denominator is the number of data points.  
         [0072]    (7-3) If we replot the smoothed jitter values Js(X) at feed positions X obtained by the above fonnulas &lt;4&gt;-&lt;6&gt;, we can obtain jitter characteristics curve G 2 , from which we can ascertain a uniform change as a whole.  
         [0073]    (8) When the desired jitter characteristics curve is obtained by the above procedure, the edge positions are set based on this jitter characteristics curve (processing S 10 : edge position setting procedure). Specifically, edge position setting procedure S 10  is carried out based on the flowchart given in FIG. 8. In the following explanation, we assume that smoothing processing S 9  is carried out, but if in branch processing S 8  it is decided that there is no need to carry out smoothing processing S 9 , the same processing as the following is carried out based on the actual measured jitter values J(X).  
         [0074]    (8-1) In jitter characteristics curve G 2  shown in FIG. 9 in which feed positions X and jitter values Js(X) are plotted, the size of jitter values Js(X 0 ) and Js(Xmax) at edge feed positions X 0  and Xmax are compared (processing S 101 ), and the feed position of the smaller jitter value Jedge is set as first edge position X 1  (processing S 102 : first edge position setting step). In FIG. 9, Js(X 0 )−Js(Xmax), so feed position X 0  is taken as first edge position X 1 .  
         [0075]    (8-2) When first edge position X 1  is determined, taking this feed position X 0  as the starting point (processing S 103 ), one compares the jitter value Js(X) at each feed position with Jedge at first edge position X 1  (processing S 104 ), and one makes a comparison decision on whether jitter value Js(X) at each feed position X is greater than Jedge at first edge position X 1  (processing S 105 ). If jitter value Js(X) is less than Jedge, one updates feed position X to the next feed position X+1 (processing S 106 ), compares Js(X) and Jedge, and searches for a feed position X for which Jedge≦Js(X).  
         [0076]    (8-3) Once a feed position X where Jedge&lt;Js(X) is found, said feed position is set as second edge position X 2  (processing S 107 : second edge position setting step). Next, one makes a comparison decision on whether feed position X for which this condition is satisfied has reached feed position Xmax (processing S 108 ), and if it is decided that feed position X that has been set as second edge position X 2  has not yet reached feed position Xmax, one updates feed position X to the next feed position X+1 (processing S 109 ), and repeats the comparison between the jitter value at said feed position and Jedge.  
         [0077]    (8-4) If it is decided that feed position X has reached Xmax, one stores first edge position X 1  and second edge position X 2  in RAM  44  and terminates edge position setting procedure S 10 .  
         [0078]    (9) Based on first edge position X 1  and second edge position X 2  obtained in said edge position setting procedure S 10 , one computes the optimum position (processing S 11 : optimum position computation procedure). Specifically, optimum position VP is set as the midpoint of first edge position X 1  and second edge position X 2 , and is determined by the following formula&lt;7&gt;.  
           VP= ( X   1 + X   2 )/2   &lt;7&gt; 
         [0079]    (10) Optimum position VP having been computed by the optimum position computation procedure, control unit  43  generates a control command based on this optimum position VP, outputs it to skew adjustment unit  37 , and adjusts the positioning of optical pickup  32  (processing S 12 : optimum position adjustment procedure).  
         [0080]    (11) When the tilt position adjustment of optical pickup  32  has come to an end, then, so as to maintain said tilt position, control unit  43  outputs to skew adjustment unit  37  a control command to perform intermittent operation (processing S 13 ) and initiates playback of the information recorded on optical disk  100  by optical pickup  32  (processing S 14 ). If it is decided by aforesaid implementation decision procedure S 4  that optical disk  100  is not a high-density information recording medium, then playback of the information by optical pickup  32  is initiated at center position CP, which was set in the manufacturing stage, without performing any of the series of processing from processing S 5  to processing S 12 .  
         [0081]    An embodiment such as described above has effects such as the following.  
         [0082]    In edge position setting procedure S 10 , measurement of jitter values is terminated and the first edge position X 1  and second edge position X 2  are set under the condition that there are three tilt positions in a row that yield a jitter value that exceeds the jitter threshold value, so even if jitter values are measured in a part where the jitter values are spotwise bad, by making a comparison decision between jitter values Js(X) and Jedge across the entire measurement range of jitter values, one can avoid erroneously stopping the processing of taking in jitter values because of a disturbance, etc.  
         [0083]    And in processing S 53 , a representative value for the jitter value at a prescribed tilt position is set by taking the average of 9 values, namely the 10 jitter values obtained at the innermost-circumference radius position of optical disk  100  excluding their maximum jitter value, thus making it possible to reduce the effect of disturbances such as scratches on the surface of optical disk  100 , and by further reducing the effect of disturbances, etc., the optimum operation center position VP of the tilt position can be determined with greater accuracy.  
         [0084]    In addition, because one obtains the jitter value at the innermost-circumference radius position, where information is sure to be recorded, the jitter value can be obtained by optical pickup  32  regardless of the size of optical disk  100  and the quantity of information recorded on it. Furthermore, by performing smoothing processing (jitter characteristics smoothing procedure) S 9  followed by edge position setting procedure S 10  and optimum position computation procedure S 11 , the optimum position VP can be computed while eliminating the variability in the jitter values caused by the surface deviation, etc. that occurs on individual optical disks  100 , so optimum position adjustment procedure S 12  makes it possible to more accurately position optical disk  100  and optical pickup  32 .  
         [0085]    In addition, by calculating smoothed jitter values Js(X) by above formulas &lt;4&gt;-&lt;6&gt; in smoothing processing S 9  and performing edge position setting procedure S 10  in the steps from processing S 101  to processing S 109 , even if they have jitter characteristics that are asymmetrical with respect to optimum position VP, the optimum edge positions can be ascertained, and a more appropriate optimum position VP can be computed by optimum position computation procedure S 10 .  
         [0086]    In addition, having implementation decision procedure S 4  makes it possible to perform jitter value acquisition procedure S 6 , jitter characteristics smoothing procedure S 9 , edge position setting procedure S 10 , optimum position computation procedure S 11 , and optimum position adjustment procedure S 12  only if optical disk  100  is a DVD-ROM or other high-density information recording medium, thus making it possible to dispense with these procedures and begin playback by optical pickup  32  more quickly for a CD-ROM or other recording medium whose recording density is relatively low.  
         [0087]    This invention is not limited to the above described embodiment but also includes variations such as, but not limited to the following.  
         [0088]    In the above described embodiment, the optical pickup position control method of this invention is applied to the tilt position control of optical disk  100  and optical pickup  32 . However, the method may also be applied to focus bias adjustment in focus adjustment unit  35  of optical pickup  32 , providing the same operation and effects as described in the above embodiment. In addition, while in the above embodiment, the optical pickup position control method of this invention is applied in optical disk device  30  mounted on entertainment device  1 , it may also be applied to an optical disk device used in an ordinary personal computer.  
         [0089]    In addition, while in the above described embodiment, smoothing processing S 10  is performed by determining the average value of measured jitter values by formulas &lt;4&gt;-&lt;6&gt;, in the formula for smoothing the measured jitter values, the determination may be made by recursive calculation by the least-squares method.  
         [0090]    In addition, while in the above described embodiment, computation of optimum position VP in optimum position computation procedure S 11  is determined by formula&lt;7&gt;, the computation formula may be suitably modified according to the shape of jitter characteristics curve G 2 .  
         [0091]    Also, for the specific structure and shape, etc. in implementing this invention, another structure, etc. may be adopted insofar as it is within the scope that allows the purpose of this invention to be achieved.  
         [0092]    With the above-described optical pickup position control method of this invention, because in setting the edge positions the condition is imposed that there be at least two relative positions that yield a jitter value that exceeds the jitter threshold value, the effect is achieved that the optimum operation center position can be set without being affected by disturbances, etc.  
         [0093]    While the present invention has been described at some length and with some particularity with respect to the several described embodiments, it is not intended that it should be limited to any such particulars or embodiments or any particular embodiment, but it is to be construed with references to the appended claims so as to provide the broadest possible interpretation of such claims in view of the prior art and, therefore, to effectively encompass the intended scope of the invention.