Patent Publication Number: US-2005128910-A1

Title: Method for determining a threshold write-in power of a compact disc and related compact disc drive

Description:
BACKGROUND OF INVENTION  
      1. Field of the Invention  
      The present invention relates to a disc drive, and more particularly, to a method for determining a threshold write-in power of a compact disc, so that a pickup of the disc drive can record data onto a program area of the CD by emitting laser beams of a predetermined power smaller than the threshold write-in power.  
      2. Description of the Prior Art  
      In recent years, compact discs (CDs) have been developed to bring a variety of advantages to storage applications, such as compact size, low cost and large data-recording capacity. CDs are becoming one of the most popular data-storing media. Typically, a disc drive is used to record and access data on a CD.  
      Please refer to  FIG. 1 , which is a schematic diagram of a disc drive  10  capable of recording data onto a CD  20  according to the prior art. The CD  20  has a spiral track  22  progressing from the center outward and covered by a photoresist layer. In general, the CD  20  comprises a lead-in area  90 , a program area  92  and a lead-out area  94 . The disc drive  10  comprises a pickup  12  for accessing data of the CD  20 . While the drive  10  writes data onto the CD  20 , the pickup  12  makes the photoresist layer of the track  22  on the CD  20  be intermittently exposed to an on-and-off laser according to the data. The exposed photoresist layer of the track  22  will cause pits to form. On the contrary, the unexposed photoresist layer will be kept as lands. Reflections of the pits and the lands are not similar. In this way, different data (for example, digital “0” or “1”) can be represented by the pits and the lands respectively, and stored in the CD  20 . While reading the data stored in the CD  20 , the drive  10  can receive reflecting laser light from the CD  20  to read the data stored in the CD  20 .  
      Please refer to  FIG. 2 .  FIG. 2  is an enlarged plot according to a dashed line section of the CD  20  shown in  FIG. 1 . For a rewritable CD  20 , its track  22  can be divided into two kinds of tracks, one is a data track  26  for recording data, and the other is a wobble track  28  for recording relative information of each frame on the CD  20 . The data track  26  is an arc along the CD  20  and around the center of the CD  20 , such as the track  22 . Because  FIG. 2  is an enlarged plot of a tiny part of the track  22 , the data track  26  shown in  FIG. 2  is a straight line. However, the wobble track  28  not only follows an arc along the CD  20  and around the center of the CD  20 , as shown in  FIG. 2 , but also appears as sinusoidal with small amplitude along the track  22 . The pickup  12  of the drive  10  can receive reflected light from the wobble track  28  to form a wobble signal. The disc drive  10  can detect which part of data on the CD  20  is being read by the pickup  12  based on the wobble signal.  
      According to the Orange Book regulating the specification of the CD  20 , while the emitted laser power from the pickup  12  has optimal power, the reflected signal measured by the pickup  12  is an AC coupled high frequency (HF) signal with a perfect symmetrical amplitude. Please refer to  FIG. 3  which shows a waveform of the HF signal reflected from the CD  20  while the pickup  12  of the disc drive  10  writes data onto the CD  20  based on an optimal write-in power, where the horizontal axis represents time, the vertical axis represents amplitude, and the place marked as level dc represents a corresponding amplitude of a long-term average of the waveform. If a laser is reflected from a pit, the HF signal shows an upper amplitude A 1  over the level dc. If a laser is reflected from a land, the HF signal shows a lower amplitude A 2  below the level dc. A measurement amplitude parameter β=(A 1 −A 2 )/(A 1 +A 2 ) is for comparing the amplitudes A 1  and A 2 .  
      During writing data into the CD  20 , the disc drive  10  will encode the data, resulting in a total extended length of pits equaling to a total extended length of lands. In other words, a total spent time of the laser reflecting from pits and a total spent time of the laser reflecting from lands are the same, which causes a long-term average level dc of the reflected HF signal to be exactly in the middle of the upper amplitude A 1  and the lower amplitude A 2 , that is β=0. If the laser power emitted from the pickup  12  is lower than the optimal power, if the laser-emitting time is too short or if the laser beam is not normal to the CD  20 , insufficient extended pits result, which makes the waveform of the HF signal move downward and causes A 1  to be less than A 2 , leading to β&lt;0. On the contrary, if the laser power emitted from the pickup  12  is higher than the optimal power or if the laser-emitting time is too long, an over-length of an extended pit is formed, which makes the waveform of the HF signal move upward and causes A 1  to be more than A 2 , leading to β&gt;0. In other words, β represents an amount of the pits matching an amount of the lands during encoding. When β does not equal to 0, it means either the length of the pit or that of the land is incorrect, resulting in errors during encoding.  
      Besides β, a block error rate (BLER) and signal jitter in the duration of data-reading can also be used to judge a correction of data-writing. If there is something wrong when the CD  20  is written to, even between identical bits, the last times of signal-reading (that is, the extended length of the pits or the lands) are not the same, which increases the signal jitter. If a BLER generated by a processor  18  to calculate data read by the pickup  12  is larger than a threshold BLER equivalent to a data-decoding capability of the processor  18 , the disc drive  10  can determine that the pickup  12  probably read incorrect data.  
      Please refer to  FIG. 1  again. The disc drive  10  further comprises an absolute time in pregroove decoder (ATIP decoder)  14  for decoding the absolute time code acquired from the pickup  12 , an eight-to-fourteen modulator (EFM)  16  for modulating the data into EFM data, and the processor  18  for calculating the BLER of data read by the pickup  12 .  
      In general, an optimal power calibration (OPC) is processed on the lead-in area  90  of the CD  20  to calculate the optimal power P opt , regardless of whether the CD  20  is a CD-RW or a DVD-RW. The optimal power P opt  increases according to the outward tracking of the pickup  12 . The optimal power P opt  ensures that the data recorded onto an inner circle of the CD  20  have a better quality. However, when the pickup  12  records data onto an outer circle of the CD  20 , since an optimal power corresponding to the outer circle has a power level higher than that of the optimal power P opt  of the inner circle, the CD  20  can become burned out due to the diversity of dye coated onto the CD  20  and improper writing strategy.  
     SUMMARY OF INVENTION  
      It is therefore a primary objective of the claimed invention to provide a method for determining a threshold write-in power of a CD. A disc drive can therefore control a pickup to emit laser beams of a power less than the threshold write-in power onto the CD, so that data recorded onto a program area of the CD can be identified correctly by a processor without the possibility of burning out the CD.  
      According to the claimed invention, the method includes the following steps: (a) recording M test data onto M sectors of an outer area of the CD with a pickup by emitting laser beams of a variety of distinct test powers, (b) reading the M test data of the M sectors with the pickup and calculating M corresponding error rates of the M test data, and (c) comparing the M error rates and therefore calculating the threshold write-in power, which is smaller than a smallest test power in an upper-bound test power set consisting of a plurality of test powers whose corresponding error rates are all larger than a threshold error rate, and is larger than a largest test power in a lower-bound test power set consisting of a plurality of test powers whose corresponding error rates are all smaller than the threshold error rate.  
      According to the preferred embodiment, the threshold error rate relates a data-decoding capability of the processor. The stronger that the data-decoding capability of the processor is, the larger the threshold error rate becomes. Similarly so for the power of laser beams emitted by the pickup while recording data onto the program area of the CD.  
      According to the preferred embodiment, the outer area is located at a lead-out area of the CD. However, the outer area can be located at the end of the program area of the CD.  
      It is an advantage of the claimed invention that a method recording data onto the lead-out area of the CD and calculating the threshold write-in power before recording any data onto the program area of the CD can protect the CD from being burned out by laser beams of too great a power, without severely impacting the quality of data recorded onto the CD.  
      These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a schematic diagram of a disc drive accessing data of a CD according to the prior art.  
       FIG. 2  is an enlarged plot according to a dashed line section of the CD shown in  FIG. 1 .  
       FIG. 3  is a waveform of a high frequency signal HF reflected from the CD when a pickup of the disc drive records data onto the CD by emitting laser beams of an optimal power according to the prior art.  
       FIG. 4  is a schematic diagram of a disc drive accessing data of a CD of the preferred embodiment according to the present invention.  
       FIG. 5  is a flowchart of a method of the preferred embodiment according to the present invention.  
       FIG. 6  shows a relation between test power and corresponding BLER according to the present invention.  
       FIG. 7  shows a relation between test power and corresponding DC jitter value according to the present invention.  
       FIG. 8  is a schematic diagram showing a curve-fit of a plurality of data based on a multi-degree polynomial (P test =2.3196*BLER 2 −749.2*BLER+60325) according to method of  FIG. 5 . 
    
    
     DETAILED DESCRIPTION  
      After recording test data onto a lead-out area of a CD and determining a threshold write-in power of the CD, the method according to the present invention records data onto a program area of the CD with a pickup by emitting laser beams of a power less than the threshold write-in power, so as to protect the CD from being burned out.  
      Please refer to  FIG. 4 , which is a schematic diagram demonstrating a disc drive  50  of the preferred embodiment accessing data of a CD  60  according to the present invention. The CD  60  can be a CD-R, a CD-RW, a DVD-RW, a DVD+R or a CD of any other type. The disc drive  50  comprises a pickup  52  for accessing data of the CD  60  by emitting/receiving laser beams onto/from the CD  60 , an ATIP decoder  54  for decoding data read from the pickup  52 , an EFM  56  for modulating data ready to be recorded onto the CD  60 , and a processor  58  for processing data read from the pickup  52  and for calculating an error rate corresponding to the processed data. The processor  58  is capable of processing two types of error rates: one is a BLER, which defines how many parity inner code (PI) errors are contained in every eight consecutive ECCs of a CD, and the other is a DC jitter value, which defines a standard deviation between pits and lands.  
      Please refer to  FIG. 5 , which is a flowchart of a method  100  of the preferred embodiment for determining a threshold write-in power P th  of the CD  60  according to the present invention. The method  100  comprises the following steps:  
      Step  102 : Start;  
      (The CD  60  is placed on the disc drive  50 .)  
      Step  104 : Execute the OPC process on a lead-in area of the CD  60  and calculate an optimal power P opt ; 
          Step  106 : Record M test data onto M distinct sectors of a lead-out area of the CD  60  with the pickup  52  by emitting laser beams of a plurality of test powers P test , each of which is greater than the optimal power P opt ; (According to the preferred embodiment, the M test data are the same. However, any test data can be different from the remaining M−1 test data. Moreover, the pickup  52  can record the M test data onto the end of a program area instead of the lead-out area of the CD  60 . Lastly, the M distinct test powers, which correspond to the M test data and are formed according to the optimal power P opt , can have values ranging from (1−25%)*P opt  to (1+25%)*P opt . For example, if M is equal to 15 and the optimal power P opt  is equal to 100 mW, a largest (the first) test power of the M 1  test powers is equal to (1+25%)*100 mW=125 mW, a smallest (the last) test power of the M test powers is equal to (1−25%)*100 mW=75 mW and an n th  test power of the M test powers is equal to  
             (     n   -   1     )       (     15   -   1     )       *   50   ⁢           ⁢   mW     +     75   ⁢           ⁢     mW   .     )             
       

      Step  108 : Read the M test data recorded onto the M sectors of the CD  60  with the pickup  52  and calculate M BLERs corresponding to the M test data with the processor  58 .  
      (The M BLERs corresponding to the M test data are used to demonstrate the method  100  of the present invention. Of course, the processor  58  can calculate M DC jitter values of the M test data instead of the M BLERs.)  
      Step  110 : Compare the M BLERs and calculate the threshold write-in power P th  with the processor  58 ; and  
      (The threshold write-in power is smaller than a smallest test power P Ltest  in an upper-bound test power set consisting of a plurality of test powers whose corresponding BLERs are all larger than a threshold BLER th , and is larger than a largest test power P Stest  in a lower-bound test power set consisting of a plurality of test powers whose corresponding error rates are all smaller than the threshold BLER th . According to the preferred embodiment, the BLER th  is equal to 100.)  
      Step  112 : End.  
      (Therefore, the pickup  52  can be controlled to record data onto the CD  60  by emitting laser beams of predetermined powers, each of which is less than the threshold write-in power P th , so as to protect the CD  60  from being burned out.)  
      Please refer to  FIG. 6  and  FIG. 7 .  FIG. 6  shows a relation between test power and corresponding BLER according to the present invention, where the abscissa represents the test power, while the ordinate represents the BLER.  FIG. 7  shows a relation between test power and corresponding DC jitter value according to the present invention, where the abscissa represents the test power, while the ordinate represents the DC jitter value. According to the  FIG. 6  as well as  FIG. 7 , experiments show that as the test power increases, the BLER decreases below the threshold BLER th  in the beginning and increases above the threshold BLER th  gradually, and the DC jitter value decreases below a threshold DC jitter value JV th  at first and increases above the threshold DC jitter value JV th  gradually. Referring to  FIG. 6 , test powers P test  from A to C correspond to BLERs from A to G, all of which are smaller than the threshold BLER th . The lower-bound test power set consists of the test powers from A to C. The smallest test power P Ltest  is the test power C. On the contrary test powers P test  from H to J correspond to BLERs from H to J, all of which are larger than the threshold BLER th . The upper-bound test power set consists of the test powers from H to J. The largest test power P Stest  is the test power H. The threshold write-in power P th  is between the largest test power P Stest  and the smallest test power P Ltest . Of course, the threshold write-in power P th  can be obtained by curve-fitting the M test powers based on a multi-degree polynomial consisting of test powers from A to J, whose independent variable is the BLER and whose dependent variable is the test power, the threshold write-in power P th  equal to the dependent variable while the independent variable is equal to the threshold BLER th . This is the reason why the plurality of test powers P test  in step  106  of the method  100  are formed according to the optimal power P opt *(1±25%). If a smallest test power of the test powers P test  is equal to the threshold write-in power P opt , a BLER corresponding to the smallest test power is probably larger than the threshold BLER th . All of the BLERs corresponding to the test power P test  will stay above the threshold BLER th , and the threshold BLER th  corresponding to a certain data outside of a region consisting of a plurality of data and calculated by curve-fitting a multi-degree polynomial consisting of the plurality of data is probably wrong.  
      Please refer to  FIG. 8 , which is a schematic diagram showing a curve-fit of a plurality of data based on a multi-degree polynomial (P test =2.3196*BLER 2 −749.2*BLER+60325) with EXCEL according to the present invention. From left to right, the plurality of data are (P test , BLER)=(136, 1403), (142, 694), (148, 41), (154, 0), (160, 9), (166, 0), (172, 4), (178, 242) and (184, 1138).  
      Since a BLER corresponding to a recorded data is not smaller than the threshold BLER th  unless the laser power for the pickup  52  to record the data onto an outer region (the lead-out area of the preferred embodiment) of the CD  60  is higher than the optimal power P opt , the M test data recorded onto the lead-out area of the CD  60  according to the plurality of test powers P test  starting from the optimal power P opt  (step  106 ) comprises at least some BLERs corresponding to some initial data (from A to E in  FIG. 6 ) larger than the threshold BLER th , the test powers from A to E hereby ignored in calculating the threshold write-in power P th .  
      Since the lead-out area for the M test data to be recorded onto is located on an outer region of the CD  60 , and the laser power for the pickup  52  to record data onto the outer region of the CD  60  is higher than that for the pickup  52  to recorded data onto an inner region of the CD  60 , a laser power emitted by the pickup  52  onto the program area, an inner region in contrast to the outer region, will not burn out the CD  60  if the laser power is not higher than the threshold write-in power P th .  
       FIG. 6  and  FIG. 7  show that the BLER is more sensitive to the test power P test  than the DC jitter value JV.  
      The BLER th , as well as the JV th , the method  100  selects relates to the quality of data recorded onto the CD  60  and the data-encoding capability of the processor  58 . In detail, if the processor  58  has a data-encoding capability good enough to encode the data recorded onto the CD  60  correctly, the BLER th  that the data recorded onto the CD  60  can endure can have a higher value, and the laser beams projected onto the CD  60  can have a greater power level accordingly; On the contrary, if the processor  58  has a poor data-encoding capability, laser beams of a little power have a larger chance of burning out the CD  60 , and the processor  58  therefore cannot encode the data recorded onto the CD  58  correctly.  
      In step  106  of the method  100 , the pickup  52  records the M test data onto the M sectors of the lead-out area of the CD  60  by emitting laser beams of a variety of power levels based on the optimal power P opt  calculated in step  104 . However, the method  100  can have step  104  omitted. In detail, the pickup  52  in step  106  can record the M test data onto the M sectors of the lead-out area of the CD  60  by emitting a variety of test powers not relating the optimal power P opt . For example, if M is equal to 15, a smallest test power of the test powers can be set to 60 mW, and a difference between any two consecutive test powers can be set to 6 mW according to an empirical rule.  
      In contrast to the prior art, the present invention can provide a method for determining a threshold write-in power by recording test data onto a lead-out area of a CD. A pickup can then record data onto a program area of the CD by emitting laser beams of a power less than the threshold write-in power reducing the chance of burning out the CD.  
      Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.