BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power controlling method of an optical disc drive, and more particularly, to a power controlling method capable of removing an output power shift outputted by a pickup head of an optical disc drive. 
     2. Description of the Prior Art 
     For a company or a human, it is important to manage or store a document. In the past, because most documents are printed or written of otherwise paper-based, if a number of the documents is huge, the documents are inconvenient to users because of their weights and volumes. Recently, as computer technology progresses, digital data is widely utilized and stored in a computer storage medium. Many types of data storage devices have been invented to assist users by simplifying digital data storage. For example, the compact disc recorder was invented. The above-mentioned compact disc recorder, such as a CD-R drive or a CD-RW drive, can store data into a CD-R disc or a CD-RW disc. Because the optical disc has the advantages of low cost, small volume, and big capacity, the users can preserve data more easily. Before writing data into a CD-R disc, the CD-R drive must perform an optimum power control (OPC) procedure to find out an optimum recording power of the CD-R disc. Furthermore, the compact disc recorder performs a burning operation on the CD-R disc according to the optimum recording power to form corresponding pits to store binary data “0”. And the other flat area is utilized to store binary data “1”. For the above-mentioned CD-RW disc drive, the above-mentioned optimum recording power can be utilized as an erase power of the CD-RW disc drive, where the erase power is utilized to reform a CD-RW disc to re-uniform the surface of a recording layer of the CD-RW disc to be flat. 
     Please refer to  FIG. 1 , which is a block diagram of a power controlling system  10  of a CD-R disc drive according to the prior art. The power controlling system  10  encompasses a controller  12 , a power control unit  14 , a pickup head  16 , and a transformer  18 . The controller  12  is utilized to control operations of the power controlling system  10 . The power control unit  14  is utilized to adjust an output power of the pickup head  16  to write data into a CD-R disc, where the power control unit  14  includes a sample/hold circuit  20 , a digital-to-analog converter (DAC)  22 , and a driving circuit  24 . The sample/hold circuit  20  is utilized to control the pickup head  16  and the power control unit  14  as a close loop or an open loop. The DAC  22  is utilized to transform a digital control signal  26  to an analog control voltage  28 . The driving circuit  24  then outputs a control voltage  30  according to the control voltage  28  in order to drive the pickup head  16  to generate a laser having a predetermined power. Therefore, the pickup head  16  can adjust its output power according to the control voltage  30  generated by the power control unit  14 . Furthermore, when the pickup head  16  writes data into the CD-R disc, the variation of the output power of the pickup head  16  is detected. Furthermore, the pickup head  16  of the power controlling system  10  emits an incident pulse to etch the optical disc. The incident pulse is then reflected by the optical disc to form a reflected pulse. The reflected pulse has a reflected pulse level (also called as B-level in the specification) to represent the etching degree of the optical disc. Next, the transformer  18  generates a corresponding feedback signal  32  to the power control unit  14  according to the reflected pulse level (here, the reflected pulse level can correspond to the current output power of the pickup head  16 ) in order to further adjust the output power of the pickup head  16 . If the control signal  26  corresponds to the optimum recording power then the output power of the pickup head  16  can be adjusted to be close to the optimum recording power through the control of the close-loop of the pickup head  16  and the power control unit  14 . At this point, the optimum recording power can be outputted through the pickup head  16  in a stable fashion. 
     The operation of the power control system  10  of the CD-R disc drive is illustrated as follows. The pickup head  16  can emit lasers to etch the recording layer of the optical disc in order to store data. The characteristics of different optical discs may be different. For example, the recording layers of different types of optical discs may be different because of the materials utilized in their manufacture. In other words, the recording layers of different types of the optical discs may have different absorbing layer characteristics. Therefore, it is not sufficient to simply emit the same power to different optical disc. Emitting the same power may result in unwanted variations of the etching degrees. As mentioned above, when producers manufacture optical discs, the producers will store a needed recording power of a specific optical disc in a lead-in area of the specific optical disc. Furthermore, the power controlling system  10  often utilizes an optimum power control (OPC) process to obtain the recording power P 1  of the optical disc. Once the power controlling system  10  has obtained the recording power P 1 , the recording power P 1  can be utilized to record data in the recording layer of the optical disc. The power controlling system  10  simultaneously obtains the corresponding relationships between the output power of the pickup head  16  and the control signal  26 , according to the specifications of the digital-to-analog converter  22  and the pickup head  16 . For example, when the control signal  26  is DAC 1  (this also means that the control voltage  28  is the voltage V 1 ), the ideal output power of the pickup head  16  is P 1 , however, the real output power of the pickup head is P 2 . The output power P 2  is transformed by the transformer  18  to generate the feedback signal  32  corresponding to a voltage V 2 . Because the pickup head  16  must etch the optical disc to store data (in order to store the binary data “0”), the controller  12  turns on the sample/hold circuit  20  to cause the pickup head  16  and the power control unit  14  to form a close loop. Because the voltage V 2  of the feedback signal  32  is not the same as the voltage V 1  of the control voltage  28 , the driving circuit  24  adjusts the control voltage  30  according to the voltages V 1  and V 2  until the voltages of the feedback signal  32  and the control voltage  28  are the same. Therefore, the pickup head  16  can etch the optical disc by outputting, in a stable fashion, the power P 1 . Therefore, every time the output power of the pickup head  16  is not stable and far from the output power P 1 , the close loop of the power control unit  14  and the pickup head  16  can adjust the output power of the pickup head  16  through the driving circuit to make the output power return the needed optimum recording power P 1 . As a result, the data writing operation can be performed correctly. 
     However, in the actual circuit, because the sample/hold circuit  20  encompasses capacitors and/or other different electronic devices, the sample/hold circuit  20  influences the input signal. In an ideal situation, when the sample/hold circuit  20  is turned on to cause the pickup head  16  and the power control unit to form a close loop, the sample/hold circuit  20  can be utilized to hold the feedback signal  32  inputted by the transformer  18 . Unfortunately, in the actual operation of writing data, the sample/hold circuit  20  performs corresponding on/off operations according to the write-in data. The outputted signal will be affected because the sample/hold circuit  20  is self-influenced by its own operation. In other words, the feedback signal  32  outputted by the sample/hold circuit  20  may vary from the normal. The result is a shift in the output power of the pickup head  16  due to this variation in the feedback signal  32  inputted into the driving circuit  24 . 
     Please refer to  FIG. 2  and  FIG. 3 .  FIG. 2  is a diagram of the output power of the pickup head  16  shown in  FIG. 1 .  FIG. 3  is an operational timing diagram of sample/hold circuit  20  shown in  FIG. 1 . Assume that the CD-R disc drive  10  must output lasers having the power P 1  to etch an optical disc to store the binary data “0” in time T 1 , T 2 , T 3 , and T 4 . The CD-R disc drive  10  must utilize the recording power P 1  to etch an optical disc to store a binary data “0”. This operation requires that in time T 1 , T 2 , T 3 , and T 4 , the sample/hold circuit  20  also be simultaneously turned on such that the pickup head  16  and the power control unit  14  can form a close loop to regulate the output power of the pickup head. However, because the sample/hold circuit  20  comprises capacitors or other electronic devices, frequently turning the sample/hold circuit  20  on and off will result in the above-mentioned operation wrongly influencing the actual output signal of the sample/hold circuit. The result is that the feedback signal  32  inputted into the driving circuit  24  shifts and then influences the output power of the pickup head  16  to move away from the predetermined power P 1 . For example, assume that the sample/hold circuit  20  is always turned on instead of being turned on and off frequently. Also, assume that the value of the input control signal  26  is 10 and the control signal  26  can drive the pickup head  16  to stably output lasers having a power 50 mW. Furthermore, when executing an actual data storing operation, if the CD-R disc drive  10  inputs a control signal  26  of value is 100 into the DAC  22 , the control signal  26  is transformed into the corresponding control voltage  28 , whose voltage is V 1 . However, when the pickup head  16  starts to operate, the pickup head  16  may only output lasers having the power 45 mW because of its characteristics. Therefore, when the pickup head  16  detects the pulse signals reflected from the optical disc, the pulse signals can be transformed into a feedback signal  32 , whose voltage value is V 2 , through the transformer  18 . 
     When the feedback signal  32  is inputted into the sample/hold circuit  20  and utilized to adjust the output power of the pickup head  16  through the above-mentioned close loop, the sample/hold circuit  20  makes the feedback signal  32  shift to voltage V 1  instead of the original voltage V 2  because the sample/hold circuit  20  is turned on and off frequently. With respect to the driving circuit  24 , the output power of the pickup head  16  is not adjusted through the driving circuit  24  because the outputs of the sample/hold circuit  20  and the DAC  22  are the same. Therefore, the pickup head  16  stably outputs lasers having 45 mW power instead of the required output power 50 mW. Apparently, because of the influence of the sample/hold circuit  20 , an offset Δp between the predetermined output power P 1  and actual output power P 2  of the pickup head  16  is generated (of course, according to the circuit characteristic of the sample/hold circuit  20 , the actual output power P 2  of the pickup head  16  may be greater than the predetermined power P 1 ). In the actual operation of writing data, the CD-R disc drive  10  can utilize the control signal corresponding to the optimum power, generated through the OPC procedure, to drive the pickup head  16 . Unfortunately, the sample/hold circuit  20  causes the actual power of the pickup head  16  to shift from the optimum power; the etching degree of the optical disc and the stored data are both negatively influenced. Therefore, when the optical disc is utilized to read stored data, the data may be read incorrectly because of the incorrect etching degree. 
     For a prior art CD-RW disc drive, the CD-RW disc drive can be utilized to perform a data writing and a data erasing operation on a CD-RW disc. The pickup head of the CD-RW disc drive needs a writing power to etch the optical disc to generate holes on the optical disc. Furthermore, it needs an erasing power to remove the data on the optical disc (that is, the erasing power is utilized to reform the recording layer of the optical disc in order to re-uniform the surface of the optical disc so that the data can be removed). As known by those skilled in the art, the operation and the structure of the CD-RW disc drive when outputting the erasing power is the same as the operation and the structure of the CD-R disc drive  10 . In other words, the erasing power is stabilized through a close loop. That is, the erasing power is utilized to form a land on the optical disc. In addition, the CD-RW disc drive turns on the sample/hold circuit to form the close loop. Therefore, like the CD-R disc drive  10 , the CD-RW disc drive utilizes the control signal corresponding to the predetermined erasing power; however, because the sample/hold circuit  20  may shift the output power of the pickup head, the data writing operation is also negatively influenced. 
     SUMMARY OF THE INVENTION 
     According to an exemplary embodiment of the claimed invention, a power controlling method for controlling a power utilized by an optical disc drive to write data into an optical disc is disclosed. The power controlling method comprises: executing a first predetermined procedure to obtain a first predetermined control signal corresponding to a first predetermined control signal needed by a pickup head to record the data into the optical disc; inputting a test data having a plurality of first bits with a first voltage level and a plurality of second bits with second voltage level; when receiving the first bits with the first voltage level, driving the pickup head to output a first power according to the first predetermined control signal; calculating a difference between the first power and the first predetermined power; and executing a second predetermined procedure to obtain a second predetermined control signal corresponding to a second predetermined power needed by the pickup head to write the data into the optical disc, and adjusting the second predetermined power according to the difference to generate a third predetermined power; wherein a third predetermined control signal corresponds to the third predetermined power, and the first power and a difference between the first power and the first predetermined power is equivalent to a difference between the second predetermined power and the third predetermined power. 
     According to another exemplary embodiment of the claimed invention, a power controlling method for controlling a power utilized by an optical disc drive to write data into an optical disc is disclosed. The power controlling method comprises: executing a first predetermined procedure to obtain a first predetermined control signal corresponding to a first predetermined control signal needed by a pickup head to record the data into the optical disc; inputting a test data comprising a plurality of first bits with a first voltage level and a plurality of second bits with a second voltage level; when receiving the first bits with the first voltage level, driving the pickup head to output a first power according to the first predetermined control signal; calculating a difference between the first power and the first predetermined power; and executing a second predetermined procedure to obtain a second predetermined control signal corresponding to a second predetermined power needed by the pickup head to write the data into the optical disc, and adjusting the second predetermined power according to the difference to generate a third predetermined power; wherein a third predetermined control signal corresponds to the third predetermined power, and the first power and a difference between the first power and the first predetermined power is equivalent to a difference between the second predetermined power and the third predetermined power. 
     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 THE DRAWINGS 
         FIG. 1  is a block diagram of a power controlling system of a CD-R disc drive according to the prior art. 
         FIG. 2  is a diagram of the output power of the pickup head shown in  FIG. 1 . 
         FIG. 3  is an operational timing diagram of sample/hold circuit shown in  FIG. 1 . 
         FIG. 4  is a flow chart of the power controlling method according to the present invention. 
         FIG. 5  is a characteristic diagram of the pickup head utilizing the present invention power controlling method. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 4  and  FIG. 5  in conjunction with  FIG. 1 .  FIG. 4  is a flow chart of the power controlling method according to the present invention.  FIG. 5  is a characteristic diagram of the pickup head  16  utilizing the present invention power controlling method. The present invention power controlling method is to turn on the sample/hold circuit  20  in a first test time (step  102 ) to make the pickup head  16  and the power control unit  14  form a close loop and to cause the output end of the sample/hold circuit  20  hold the signal of the input end. Additionally, the control signal  26  (shown as DAC 1  in  FIG. 5 ) is inputted into the DAC  22  and is transformed into an analog control voltage  28  by the DAC  22  (step  104 ). Next, the analog control voltage  28  is inputted into the driving circuit  24 . The driving circuit  24  generates a control voltage  30  to drive the pickup head  16  (step  106 ), and the pickup head  16  also detects the output power (step  108 ), which is shown as the power P 1  in  FIG. 5 . However, in the preferred embodiment, the method further makes the sample/hold circuit  20  frequently turning on and off according to a test data (step  110 ). For example, the present invention power controlling method is to input an eight-to-fourteen modulation (EFM) data into the CD-R disc drive  10 , wherein the EFM data is a bit stream comprising a plurality of digital bits “0” and “1”. For the power controlling system  10 , when the digital bit “1” is being stored, the sample/hold circuit is turned off, and when the digital bit “0” is being stored, the sample/hold circuit is turned on. Therefore, in the second testing time, the sample/hold circuit  20  is turned on and off according to the EFM data. When the sample/hold circuit  20  is turned on, the control signal  26  (shown as the signal DAC 1  in  FIG. 5 ) is inputted into the DAC  22  (step  112 ). The DAC  22  transforms the digital control signal  26  into an analog control voltage  28 . Here, the analog control voltage  28  is then inputted into the driving circuit  24  where the driving circuit  24  generates a control voltage  30  to drive the pickup head (step  114 ). Furthermore, the pickup head  16  also detects the output power (step  116 ), which is shown as the power P 1 ′ in  FIG. 5 . 
     As mentioned above, when the sample/hold circuit  20  is frequently turned on/off according to the data, the actual output power P 1 ′ of the pickup head  16  shifts because of the influence of the sample/hold circuit  20 . In  FIG. 5 , the predetermined power is P 1 , and the offset between the actual output power P 1 ′ and the predetermined power P 1  is ΔP (step  118 ). Therefore, the influence of the sample/hold circuit  20  on the output power of the pickup head  16  is thus obtained. Obviously, when the power controlling system  10  actually writes data into the recording layer of an optical disc, the offset ΔP can be utilized to adjust the output power of a control signal (step  120 ). 
     For example, when the power controlling system  10  performs the above-mentioned power adjustment in a power calibration area (PCA) of an optical disc the following events occur. The sample/hold circuit  20  is turned on to make the pickup head  16  and the power control unit  14  form a close loop. Next, the pickup head of the power controlling system  10  reads an indicative optimum recording power from a lead-in area, wherein the indicative recording power is the optimum recording power indicated by the manufacturers of the disc. In addition, the pickup head  16  performs a write testing operation on a continuous sequence of test blocks of the test area on the optical disc by utilizing seven writing powers that are less than the optimum recording power, the optimum recording power, and seven recording powers that are larger than the optimum recording power. If the optimum recording power of the OPC is P 2  then the corresponding control signal  26  is DAC 2  according to  FIG. 5 . Unfortunately, in the actual operation of writing data, the sample/hold circuit  20  usually influences the output power P 2 . Therefore, in this embodiment, the offset ΔP is utilized to calculate the actual output power P 2 ′ of the pickup head  16  (i.e. P 2 −ΔP). 
     In addition, the present invention can also obtain the characteristic curve  40  according to values DAC 1  and DAC 2  of the control signal  26  and actual output powers P 1 ′ and P 2 ′ when the pickup head  16  is influenced by the sample/hold circuit  20 . This represents the function relationship between the output power and the control signal of the pickup head  16  when the CD-R disc drive  10  actually writes data onto an optical disc. Therefore, from the characteristic curve  40  shown in  FIG. 5 , when the recording power is P 2  the control signal  26  is DAC 3 . Generally speaking, within the same CD-R disc drive  10 , the sample/hold circuit  20  has almost the same influence (the above-mentioned offset ΔP) on the pickup head. Therefore, in this embodiment, the present invention can utilize two testing results, the coordinates (DAC 1 , P 1 ′) and (DAC 2 , P 2 ′), to quickly establish a characteristic curve  40 . The sample/hold circuit  20  is first turned on and remains on while a characteristic curve of the CD-R disc drive  10  is obtained through a test operation. The offset ΔP is utilized to shift the characteristic curve that is the result obtained through the above-mentioned test operation. Finally, the actual characteristic curve  40  is obtained. For example, the coordinates (DAC 1 , P 1 ) and (DAC 2 , P 2 ) or many other testing results may be utilized to obtain a reference characteristic curve (such as the curve  60  shown in  FIG. 5 ) through the prior art polynomial curve fitting method. Next, the offset ΔP is utilized to shift the reference characteristic curve to obtain the characteristic curve  40 . This modification obeys the spirit of the present invention. Please note that the characteristic curve  40  can be also generated by utilizing a prior art polynomial curve fitting method. 
     Similarly, for the CD-RW disc drive, the control of the erasing power is the same as the control of the recording power of the CD-R disc drive  10 . When the CD-RW disc drive erases the binary data “0”, it will turn on the sample/hold circuit  20  to form a close loop. When the CD-RW disc drive stores a binary data “1”, the sample/hold circuit  20  is turned off. Therefore, for a specific binary data, the sample/hold circuit  20  also has to be turned on and off according to the specific binary data. As known by those skilled in the art, the recording power is generated according to the erasing power. Therefore, the present invention power controlling method can be utilized in the CD-RW disc drive to remove the influence of the sample/hold circuit on the erasing power when the data of the optical disc is erased. Furthermore, a characteristic curve of an adjusted erasing power and a corresponding control signal can be obtained according to the above-mentioned power controlling method. Regardless of a CD-R or CD-RW disc drive, or any other equivalent changes, these modifications all obey the spirit of the present invention. 
     In contrast to the prior art, the present invention power controlling method first turns on a sample/hold circuit of an optical disc drive and outputs a control signal to drive a pickup head. At the same time, the present invention detects a corresponding predetermined output power, and then outputs a test data into the optical disc drive, and drives the optical disc drive to simulate an actual recording operation according to the control signal. For a CD-R disc drive, the control signal corresponds to a recording power or an erasing power. Therefore, the sample/hold circuit is turned on and off according to the test data. Simultaneously, the pickup head detects an actual output power, and utilizes a predetermined output power and the actual output power to calculate a power offset. The sample/hold circuit utilizes the power offset to adjust the pickup head. Therefore, the present invention power controlling method can utilize the power offset to adjust the output power of the pickup head to make the pickup head to output an actual output power. At the same time, the present invention can utilize the actual output power of the pickup head and the control signal to obtain an actual characteristic curve. Therefore, through the actual characteristic curve, the optical disc drive can quickly determine the needed control signals according to required writing power (in the application of the CD-R disc drive) and required erasing power (in the application of the CD-RW disc drive). As a result, the data storage efficiency is improved. 
     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.
A power controlling method for eliminating output power offset of a pick-up head in an optical disc drive. The power control method includes measuring a first predetermined power related to a first predetermined control signal in a predetermined procedure, inputting a test data containing a plurality of first bits and a plurality of second bits, activating a sample/hold circuit to form a close loop so that the first predetermined control signal can be received and utilized for driving a pick-up head to output a first power when the plurality of first bits are received, and calculating an offset power based on the first predetermined power and the first power for rectifying output power of the pick-up head.