Patent Publication Number: US-RE42235-E

Title: Compensation method and device for tracking operation of optical storage system

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for compensating a tracking operation of an optical storage system, and more particularly to a method for compensating a tracking operation of a pickup head of an optical storage system. The present invention also relates to a device for compensating a tracking operation of an optical pickup head of an optical storage system. 
     BACKGROUND OF THE INVENTION 
     Optical storage systems record digital data onto the surface of a storage medium, which is typically in the form of a rotating magnetic or optical disc, by altering a surface characteristic of the disc. The digital data serves to modulate the operation of a write transducer (write head), which records binary sequences onto the disc in radially concentric or spiral tracks. When reading this recorded data, a read transducer (read head), positioned in close proximity to the rotating disc, detects the alterations on the medium and generates a sequence of corresponding pulses in an analog read signal. These pulses are then detected and decoded by read channel circuitry in order to reproduce the digital sequence. When the pickup (read/write) head of the computer storage system operates, a light beam emitted by a light source such as a laser diode is focused by an object lens on the rotating disc so as to realize the information carried thereby. 
     Referring to  FIG. 1 , a conventional tracking control device  10  for controlling tracking operation of an optical pickup head (or, an actuator)  11  comprises a pre-amplifier  12 , a compensator  13  and a power amplifier  14 . An optical disc  1  is often rendered eccentric in the manufacturing process. In addition, when the optical disc is loaded into an optical disk drive and then clamped by a spindle motor, the center optical disc might be eccentric from the center of the optical disk drive, resulting in a certain degree of runout R while rotating. During operation, an error signal e between the position P of the optical pickup head  11  relative to the disc  1  and the runout R is processed by the pre-amplifier  12  to generate a tracking error TE. If the tracking error TE is substantially zero, it means the optical pickup head  11  has precisely locked the target track, and will acquire correct data. In order to converge the tracking error TE to zero, the generated tracking error TE is transmitted to the compensator  13 , e.g. a digital signal processor (DSP), to be processed. The compensator  13  operates on the tracking error TE to generate a tracking output signal TRO for the shift control of the optical pickup head  11 . The power amplifier  14  then magnifies the generated tracking output signal TRO for actuating the optical pickup head  11  to move along the current tracking direction. The position information of the optical pickup head  11  relative to the disc  1  is then detected and transmitted to the pre-amplifier  12  again, and the above procedures are repetitively executed until the tracking error TE is lowered to an acceptable level (substantially zero). 
     As is understood by those skilled in the art, the performance of an optical disk drive, including quality and speed, depends largely on the tracking operation of the optical pickup head  11 . For example, the pickup rate of the optical disk drive will be adversely affected if the above-mentioned tracking operation has to repeat a number of times to lock the correct track. 
     In addition to the eccentricities, some parameters such as the gain variations of different power amplifiers and the moving sensitivity variations of different optical pickup heads (both of the variations may depend on manufacturing processes or element degenerations) might also cause the increase of tracking operation time because they are not taken into account in advance in the conventional tracking control method. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and a device for compensating a tracking operation of a computer storage system to simultaneously compensate some operating parameters in addition to runout in order to speed up tracking. 
     It is another object of the present invention to provide a tracking output signal generator for increasing tracking performance of an optical disk drive by compensating some operating parameters in advance. 
     In accordance with an aspect of the present invention, there is provided a method for controlling a tracking operation of an optical storage system. The optical storage system comprises a pickup head for picking up data from a storage medium. Firstly, a runout associated with a relative motion between the pickup head and the storage medium is obtained when the optical pick head is in a status of focus-on but not track-on. A calibration procedure is next performed to find maxima of runout and a tracking output signal, respectively. Thereafter, a calibration factor is defined and calculated by using the derived maxima and the nominal factors of the power amplifier and the optical pickup head, while the calculated calibration factor is stored in the compensator for the use of the optical pickup head. After the pickup head is in a status of track-on, the stored calibration factor is used to provide compensation to the optical disk drive in the following normal operation procedure including data reading or writing operations. 
     In an embodiment, the nominal factor includes a gain of the power amplifier. 
     In an embodiment, the nominal factor includes a sensitivity of the pickup head. 
     In an embodiment, the calibration factor K is defined by the following formula: 
       K   =         C   NOM     ×     D   NOM         C   ×   D           
 
where
 
     C NOM  is a nominal gain of a power amplifier, 
     D NOM  is a nominal sensitivity of the pickup head, 
     C is an actual gain of the power amplifier in operation, and 
     D is an actual sensitivity of the pickup head in operation. 
     In an embodiment, the method of the present invention further comprises a step of filtering out noise from the tracking output signal to obtain a maximum of the tracking output signal. 
     In an embodiment, 
         C   ×   D     ≈       R   MAX       TRO   MAX           
 
where
 
     R MAX  is the maximum runout, and 
     TRO MAX  is the maximum of the tracking output signal. 
     In an embodiment, the nominal gain of the power amplifier and the nominal sensitivity of the optical pickup head are constant values. 
     In accordance with another aspect of the present invention, there is provided a control device embedded in an optical storage system. The disclosed control device basically includes a pickup head, a tracking error signal generator, a tracking output signal generator, a power amplifier, a band-pass filter, and a maximum detector. In a calibration procedure, the pickup head, tracking error signal generator, tracking output signal generator, band-pass filter and the maximum detector form a close loop for the purpose of deriving a calibration factor for the computer storage system. Maxima of the runout and the tracking output signal are first derived, while the calibration factor is defined and calculated by using the derived maxima and the nominal factors of the power amplifier and the pickup head. In a normal operation procedure, the pickup head, tracking error signal generator, tracking output signal generator, power amplifier form another close loop to read/record data from/to a disc under the compensation provided by the derived calibration factor. 
     In an embodiment, the tracking error signal generator is a pre-amplifier. 
     In an embodiment, the tracking output signal generator includes a compensator. Preferably, the compensator is a digital signal processor (DSP). 
     In an embodiment, the nominal factor includes the gain of a power amplifier. 
     In another embodiment, the nominal factor further includes the sensitivity of the pickup head. 
     The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram illustrating a conventional tracking control system for an optical pickup head; 
         FIG. 2  is a functional block diagram illustrating a tracking control system for an optical pickup head according to a preferred embodiment of the present invention; and 
         FIG. 3  is a flowchart illustrating a process for controlling a tracking operation of an optical disc drive according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Please refer to  FIG. 2 , which illustrates an optical storage system according to a preferred embodiment of the present invention. The optical storage system of  FIG. 2  is a control device of an optical disc drive, which includes a pre-amplifier  22 , a compensator  23 , a power amplifier  24 , an optical pickup head  25 , a band-pass filter  26 , and a maximum detector  27 . The optical pickup head  25  picks up data from an optical disc  1 . During operation, an error signal e between the position P of the optical pickup head  25  relative to the disc and the runout R is processed by the pre-amplifier  22  to generate a tracking error TE. The tracking error TE is transmitted to the compensator  23 , e.g. a digital signal processor (DSP), to be processed into a tracking output signal TRO, which is next delivered into two separate paths for further processing. In a normal operation procedure, the pre-amplifier  22 , compensator  23 , power amplifier  24  and the optical pickup head  25  form a close loop, so that the TRO signal will sequentially pass through the power amplifier  24  and optical pickup head  25  to derive the position P of the optical pickup head. The error signal e between runout R and the position P is then fed back to the pre-amplifier  22 . The optical pickup head  25  may read data from the optical disc  1  or record data onto the surface of the optical disc  1  in the normal operation procedure. On the other hand, the pre-amplifier  22 , compensator  23 , band-pass filter  26  and the maximum detector  27  form another close loop in a calibration procedure. The TRO signal will thus pass through a series-connected band-pass filter  26  and maximum detector  27  to derive the maximum of the TRO signal (“TRO MAX ” as for short hereinafter) before feeding back into the compensator  23 . Please note that the path established by series-connected band-pass filter  26  and maximum detector  27  is enabled for calibration purpose when the optical disk drive is turned on, while this path is disabled under the normal operation procedure since the calibration factor K (relative deductive steps are described later) has been derived and stored in the compensator  23  for the use of the optical disk drive already. 
     More detailed descriptions regarding the calibration procedure are given as follows firstly. As mentioned in the background, the variations of the power amplifier gain and the optical pickup head sensitivity are not taken into account in the conventional approach. The preferred embodiment introduces a calibration factor K associated with element parameters into the compensator  23  in advance so as to make the tracking output signal TRO independent from the above gain and sensitivity variations, thereby improving the tracking efficiency. In other words, the position P of the optical pickup head  25 , which incorporates therein the calibration factor K, is expressed by:
 
P=TRO×K×C×D   (eq. 1) 
 
where
 
     C is the actual gain of the power amplifier  24  in operation, and 
     D is the actual sensitivity of the optical pickup head  25  in operation. 
     When the optical pickup head  25  is in a status of focus-on but not track-on, the calibration procedure starts and sets the calibration factor K to be 1 (one) at first. Therefore, the product of C and D in (eq. 1) can be shown as: 
               C   ×   D     =     P   TRO             (eq.  2)             
 
     On the other hand, the position P of the optical pickup head  25  can be indicated by the following formula based on the well-known control theory: 
             P   =     R   ×     H     (     1   +   H     )                 (     eq   .           ⁢   3     )             
 
and
 
H=A×B×C×D   (eq. 4) 
 
where
 
     A is the actual gain of the preamplifier  22  in operation, 
     B is the actual gain of the compensator  23  in operation, 
     C is the gain of the power amplifier  24  in operation, 
     D is the sensitivity of the optical pickup head  25  in operation, and 
     R is the runout indicating eccentricity of the optical pickup head  21 . 
     since the calculated H ranges from 50 to 1000, therefore
 
P≈R   (eq. 5) 
 
and the product of C and D in (eq. 2) will be: 
               C   ×   D     ≈     R   TRO             (     eq   .           ⁢   6     )             
 
     Since the tracking output signal TRO and also the runout R are both sinusoids, and their maxima will be obtained substantially simultaneously (relative phase delays between these two signals are ignored in the preferred embodiment), maximum of C×D can be derived by using the equation defined below: 
               C   ×   D     =         P   MAX       TRO   MAX       ≈       R   MAX       TRO   MAX                 (     eq   .           ⁢   7     )             
 
where
 
     R MAX  is the maximum runout, and 
     TRO MAX  is the maximum of the tracking output signal TRO. 
     Therefore, the product C×D can be easily calculated by using TRO MAX  and R MAX  according to (eq. 7) after TRO signal passes through the band-pass filter  26  and the maximum detector  27 . In the embodiment, the band-pass filter  26  is provided downstream of the compensator  23  to generate a TRO BPF  signal for the purpose of filtering out noise from the tracking output signal TRO. A maximum detector  27 , such as a peak hold circuit, is then employed to catch a peak value TRO MAX  from the TRO BPF  signal. The obtained TRO MAX  signal is next fed back to the compensator  23  for calculating the product C×D, which is then stored in the compensator  23  temporarily. Please note that the gain of the power amplifier  24  and the sensitivity of the optical pickup head  25  have nominal values that can be accessed from specifications made by manufacturers or by using the values obtained from detecting candidate drives practically. Moreover, the TRO MAX  can be an average of several peak hold values from the filtered TRO signal. Any person having ordinary skills in the art may obtain these parameters as requirements or applications, but any similar modification or rearrangement within the scope of the preferred embodiment should be included in the appended claims. Therefore, after the optical pickup head is in a status of track-on, the calibration factor K according to the above embodiment of the present invention can be defined by the following formula: 
             K   =           C   NOM     ×     D   NOM         C   ×   D       =           C   NOM     ×     D   NOM           R   MAX       TRO   MAX         .               (     eq   .           ⁢   8     )             
 
where
 
     C NOM  is a nominal value regarding a gain of a power amplifier coupled with said pickup head, 
     D NOM  is a nominal value regarding a sensitivity of said pickup head, 
     Since the effects that the gain and sensitivity variations affecting the tracking operation time have been accumulated into a constant value (i.e. the collecting factor K), which indicates that the preferred embodiment should upgrade the performance of the optical storage system based on the above compensation mechanism. By introducing a calibration factor K associated with element parameters into the DSP operation of the compensator  23  in advance, the tracking output signal TRO would be independent from the above gain and sensitivity variations, thereby improving the tracking efficiency. 
     In order to illustrate the process for controlling a tracking operation of an optical disc drive in more details, a flowchart according to a preferred embodiment of the present invention is shown in FIG.  3 . In Step S 10 , a runout R associated with a relative motion between an optical pickup head and a disc, and the position P of the optical pickup head relative to the disc are detected when the optical pickup head is in status of focus-on but not track-on. Then, the maxima of TRO signal and runout R are determined in Step S 20  (both defined as TRO MAX  and R respectively). Thereafter, a calibration factor K is defined and calculated according to (eq. 8), as shown in Step S 30 . Please note that TRO MAX , R MAX , and the nominal values of the power amplifier and the optical pickup head are used to calculate the calibration factor K. This calibration factor K is then stored in the compensator  23  in Step S 40  for compensating the variations of the power amplifier gain and the sensitivity of the optical pickup head. Finally, the optical disk drive will operate with compensation provided by the calibration factor K whatever data reading or recording operations are performed (Step S 50 ). 
     Since the element parameters can be previously compensated in accordance with the present invention, the elements parameters would not have to be compensated for each cycle during the tracking operation, and thus the tracking operation time is considerably reduced. Furthermore, the present invention is illustrated by referring to any kind of optical storage system or optical disk drive, such as a compact disk-read only memory (CD-ROM) drive and a digital versatile disk-read only memory (DVD-ROM) drive. Nevertheless, the present invention can be applied to an optical storage system, for example, a compact disk-recordable (CD-R) drive, a compact disk-rewritable (CD-RW) drive, a digital versatile disk-recordable (DVD-R) drive, a digital versatile disk-rewritable (DVD-RW) drive and a digital versatile disk-random access memory (DVD-RAM) drive. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.