Abstract:
An optical drive includes a comparator for comparing a first input signal with a second input signal to generate a first output signal at a first node; a signal source coupled to a second node, the signal source outputting a second output signal; and a switch for outputting a third output signal at a third node, the third output signal controlling a laser power of the optical drive, wherein when the switch is switched to the first node, the first output signal is transmitted to the third node and serves as the third output signal, and when the switch is switched to the second node, the second output signal is transmitted to the third node and serves as the third output signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a continuation of U.S. application Ser. No. 10/249,221 entitled, “METHOD FOR CONTROLLING OUTPUT POWER OF A PICK-UP HEAD USING APC LOOP”, which was filed on Mar. 24, 2003 and is included herein by reference. 
     
    
     BACKGROUND  
       [0002]     The present invention relates to an optical drive, more particularly, an optical drive for automatically controlling output power of a pick-up head of an optical drive with an APC loop.  
         [0003]     In recent years, along with the increasing operating capability of the computer system combined with the development of Internet technology, users have widely made use of the computer system as the multi-media audiovisual medium and made use of the computer as a bridge for connecting with a network to access all kinds of information. Due to the increasing need of the data storage quantity, various tools and apparatuses for storing data immediately become popular. Since the optical disk has the advantages of compactness, large storage capacity, and inexpensiveness, related products became very attractive. Recently, the functions of the optical drive (such as a CD-RW drive) have increased, and the reading quality and access speed of the optical drive have been improved continuously. Moreover, in addition to the original specification of CD, the new specification of DVD appears with much larger capacity and the same physical volume with CD. Nowadays, the optical drive has become the standard equipments of the computer system.  
         [0004]     The CD-RW drive access data according to the optical principles, therefore the reading and writing operations depend on a pick-up head, which is usually a laser head. During the reading process, the CD-RW drive will set the output power of the output laser of the pick-up head to a predetermined value to set the wavelength of the output laser to a constant value so that the wavelength of reflected light is equal to a value of a optical signal that a sensor of the CD-RW drive can detect. The optical disk stores the data by the way of pressing or recording some concaves, convexes, or special membranes with various optical characteristics on the surface of the optical disk so that the optical sensor can distinguish a plurality of different wavelengths of reflected light to store the data with the digital form. During the writing process, a CD-RW drive also will set the output power of the output laser of the pick-up head to a predetermined value to set the wavelength of the output laser to a constant value so that the pick-up head can identify the parameters of the membranes on the surface of the optical disk and control the laser to output a plurality of wavelengths continuously according to the digital data to be written onto the optical disk. Therefore, the digital data can be pressed and recorded onto the optical disk.  
         [0005]     Please refer to  FIG. 1 . During the reading process and writing process, in order to make the CD-RW drive maintain the output power of the laser pick-up head at a predetermined value without fluctuating with the changes of the environment such as the temperature, the prior art usually makes use of an APC loop  10  in a CD-RW drive as shown in  FIG. 1  to form a feedback closed loop with a pick-up head  20  for stabilizing the output power. The APC loop  10  comprises a drive circuit  18 , a comparator circuit  14 , a sensor  12 , and a signal source  16 . The drive circuit  18  is electrically connected to the pick-up head  20  for driving the pick-up head  20 . The comparator circuit  14  comprises a first input port, a second input port, and an output port. The comparator circuit  14  compares two signals respectively transmitted from the first input port and the second input port to generate a corresponding comparative signal y. The output port is electrically connected to the drive circuit  18  for outputting the comparative signal y to the drive circuit  18 . The sensor  12  is used to detect the output power of the pick-up head  20  to generate a corresponding detecting signal ε and to input the detecting signal E to the first inputs port of the comparator circuit  14 . The signal source  16  is used to provide a signal γ to the second input port of the comparator circuit  14 , and the signal γ represents the expected value of the output power of the pick-up head  20  of the CD-RW drive. The sensor  12  creates the signal ε which then feedbacks to the comparator circuit  14 , and the APC loop  10  makes use of the comparator circuit  14  to compare the feed-backed signal ε with the signal γ representing the expected value of output power to generate the comparative signal y for controlling the output power of the pick-up head  20 . Making use of the feedback control mechanism and designing the APC loop  10  with proper parameters can maintain the output power of the pick-up head  20  at an anticipant value. The user can insert various signal amplification circuits or power amplification circuits among the components of the APC loop  10  (such as inserting amplification circuits between the output port of the comparator circuit  14  and the drive circuit  18 ) according to practical needs. Moreover, the comparator circuit  14  can be achieved with various circuit configurations, and generally the comparator circuit  14  comprises an operational amplifier  22 , a capacitor  24 , and two resistors  26  and  28  that are connected as shown in  FIG. 1 . The signal source  16  usually is a digital signal-processing (DSP) chip for generating a digital signal that is transformed through a D/A converter.  
         [0006]     However, the APC loop  10  has a very serious drawback when the output power of the pick-up head  20  is to be changed in the CD-RW drive. That is, the APC loop  10  needs to take a period of time to reach steady state. Please refer to  FIG. 2 .  FIG. 2  is a schematic diagram showing how the signal γ, the comparative signal y, the detecting signal ε (as shown in  FIG. 1 ), and the voltage drop Vc of the capacitor  24  vary with the time dimension t. Please notice that regarding the parameters of the components of the APC loop  10 , when the signal γ is set as γ 1  and the APC loop  10  reaches the steady state, the comparative signal y can be set as y 1 , the detecting signal ε can be set as γ 1 , and the voltage drop Vc is (γ 1 -y 1 ). When the signal γ is γ 2  and the APC loop  10  reaches the steady state, the comparative signal y is y 2 , the detecting signal ε is γ 2 , and the voltage drop Vc becomes (γ 2 -y 2 ). When that CD-RW drive wants to raise the output power of the pick-up head  20  from a lower value to a higher value, the signal γ will be switched from γ 1  to γ 2  at time t 1 . At this time, the voltages of all nodes in the APC loop  10  will be shifted from original steady-state values to new steady-state values. However, due to the effect of capacitance in the APC loop  10  (such as the capacitor  24  that provides most of the effect of capacitance in the APC loop  10  as shown in  FIG. 1 ), the new steady state will be reached after the effective capacitor is charged. As shown in  FIG. 2 , the voltage drop Vc is (γ 1 -y 1 ) at time t 1 , and then at time t 2  the voltage drop Vc enters a steady-state value (γ 2 -y 2 ) after charging process. Similarly, the comparative signal y is y 1 +(γ 2 -γ 1 ) at time t 1 , and at time t 2  the comparative signal y reaches a steady-state value y 2  after charging process. The detecting signal ε is γ 2 ′ at time t 1 , and then enters a steady-state value γ 2  at time t 2 . When that CD-RW drive wants to adjust the output power of the pick-up head  20  from a higher value to a lower value, the signal γ will be shifted from γ 2  to γ 1  at time t 3 . At this time, the voltages of all nodes in the APC loop  10  will be shifted from original steady-state values to new steady-state values. However, due to the effect of capacitance in the APC loop  10 , the new steady state will be reached after the effective capacitor is discharged. As shown in  FIG. 2 , the voltage drop Vc is (γ 2 -y 2 ) at time t 3 , and then enters a steady-state value (γ 1 -y 1 ) at time t 4  after discharging process. Similarly, the comparative signal y is y 2 -(γ 2 -γ 1 ) at time t 3 , and then enter a steady-state value y 1  at time t 4  after discharging process. The detecting signal ε is γ 1 ′ at time t 3 , and then reaches a steady-state value γ 1  at time t 4  after discharging process.  
         [0007]     The above-mentioned effect of capacitance in the APC loop resulting from the charging/discharging process toward the effective capacitor will cause a period of time of unsteady state, and the unsteady state will do harm to the operations of the CD-RW drive. During the writing process, when the reading speed of a buffer is higher than the writing speed, the CD-RW drive must stop recording until the register enters the idle status. Because the long period of time of unsteady state leads to the destabilization of the output power, bug data are easily generated in the connecting point. During the reading process, the long period of time of unsteady state easily leads to the servo failure. For example, the tracking servo or the focusing servo may be out of control during the reading process.  
       SUMMARY  
       [0008]     It is therefore an objective of the claimed invention to provide an optical drive to solve the above-mentioned problems of the prior art.  
         [0009]     According an embodiment of the claimed invention, an optical drive is disclosed. The optical drive comprises: a comparator for comparing a first input signal with a second input signal to generate a first output signal at a first node; a signal source coupled to a second node, the signal source outputting a second output signal; and a switch for outputting a third output signal at a third node, the third output signal controlling a laser power of the optical drive, wherein when the switch is switched to the first node, the first output signal is transmitted to the third node and serves as the third output signal, and when the switch is switched to the second node, the second output signal is transmitted to the third node and serves as the third output signal.  
         [0010]     According to another embodiment of the present invention, a circuit is disclosed. The circuit comprises: a comparator for comparing a first input signal with a second input signal to generate a first output signal at a first node; a first signal source coupled to a second node, for outputting a second output signal; a first switch coupled to a third node, wherein when the first switch is switched to the first node, the first output signal is transmitted to the third node, and when the first switch is switched to the second node, the second output signal is transmitted to the third node; and a switch controller for controlling the switching operation of the first switch in response to the first output signal and the second output signal.  
         [0011]     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  
       [0012]      FIG. 1  is a functional block diagram of an APC loop of the prior art.  
         [0013]      FIG. 2  is a schematic diagram showing all signals in the APC loop as shown in  FIG. 1  varying with time.  
         [0014]      FIG. 3  is a functional block diagram of an APC loop of the present invention.  
         [0015]      FIG. 4  is a schematic diagram showing all signals in the APC loop as shown in  FIG. 3  varying with time. 
     
    
     DETAILED DESCRIPTION  
       [0016]     Please refer to  FIG. 3 .  FIG. 3  is a functional block diagram of an APC loop  30  of the present invention. The APC loop  30  comprises a sensor  32 , a drive circuit  38 , a comparator circuit  34 , a first switch  54 , a second switch  56 , a first signal source  50 , a switch controller  58 , and a second signal source  36 . The sensor  32  is used to detect output power of a pick-up head  40  to generate a corresponding detecting signal ε. The comparator circuit  34  comprises a first input port, a second input port, and an output port. The comparator circuit  34  is used to compare two signals inputted from the first input and the second input port to generate a corresponding comparative signal y, and the output port is used to output the comparative signal y. The first switch  54  is used to select either an output signal of a power supply  52  or the detecting signal ε of the sensor  32  to input the selected signal to the first input port of the comparator circuit  34 . The first signal source  50  is used to provide a first signal. The drive circuit  38  is electrically connected to the pick-up head  40  for driving the pick-up head  40 . The second switch  56  is used to select either the first signal or the comparative signal y′ to output the selected signal to the drive circuit  38 . The second signal source  36  is used to provide a second signal y to the second input port of the comparator circuit  34 , and the second signal γ represents the expected value of the output power of the pick-up head  40  in the CD-RW drive. The switch controller  58  is used to control the first switch  54  and the second switch  56  according to at least one node signal value in the APC loop  30 . The user can insert various signal amplification circuits or power amplification circuits among the components of the APC loop  30  (such as inserting amplification circuits between the output port of the comparator circuit  34  and the drive circuit  38 ) according to practical needs. The sensor  32  is usually a photodiode for detecting the output power of the pick-up head  40 . The comparator circuit  34  can be achieved with various circuit configurations, and generally the comparator circuit  34  comprises an operational amplifier  42 , a capacitor  44 , and two resistors  46  and  48  that are connected as shown in  FIG. 1 . The second signal source  36  usually is a digital signal-processing (DSP) chip for generating a digital signal that is transformed through a D/A converter. In addition, the power supply  52  can be a voltage source to provide a plurality of output voltage signals as shown in the following embodiment, and the power supply  52  also can be a current source to provide a plurality of output current signals. In an embodiment of the present invention, the first signal source  50  is generated by a set of comparative signals y according to the initial calibration process from a closed loop status to the steady state of the APC loop  30  in the CD-RW drive. The power supply comprises a system voltage Vcc and a voltage source  52  of a ground potential GND. The switch controller  58  controls the first switch  54  and the second switch  56  according to the comparative signal y of the comparator circuit  34  and the first signal. The operating principles are described as follows. When the second signal γ generates a step transition, the switch controller  58  will utilize the first switch  54  from the detecting signal ε to the voltage source  52 , and utilize the second switch  56  from the signal y to the first signal. When number of times equals to two at which the comparative signal y is equivalent to the signal, the switch controller  58  will return the first switch  54  from the voltage source  52  to the detecting signal ε. At the same, the switch controller  58  will return the second switch  56  from the first signal to the signal y. The APC loop  30  of the present invention is described in detail in  FIG. 4 .  
         [0017]      FIG. 4  is a schematic diagram showing how the second signal γ, the output signal χ of the first switch, the comparative signal y, the detecting signal ε, voltage drop Vc of the capacitor  44 , and the output signal y′ of the second switch vary along with the time t. Please notice that regarding the parameters of the components of the APC loop  30 , when the signal γ is set as γ 1  and the APC loop  30  reaches the steady state, the comparative signal y can be set as y 1 , the detecting signal ε can be set as γ 1  , and the voltage drop Vc is (γ 1 -y 1 ). When the signal γ is γ 2  and the APC loop  30  reaches the steady state, the comparative signal y is y 2 , the detecting signal ε is γ 2 , and the voltage drop Vc becomes (γ 2 -y 2 ). When that CD-RW drive wants to raise the output power of the pick-up head  40  from a lower value to a higher value, the signal γ will be switched from γ 1  to γ 2  at time t 1 . At this time, if y&gt;y′, the switch controller  58  will utilize the first switch  54  from the detecting signal ε to the system voltage Vcc of the voltage source  52  (as shown in  FIG. 4 , the output signal χ jumps to Vcc at time t 1 ). Afterwards, the system voltage Vcc will quickly charge the capacitor  44  to make the voltage drop Vc from the value of (γ 1 -y 1  ) at time t 1  enter the steady-state value (γ 2 -y 2 ) at time t 2  in a very short time, and the switch controller  58  also will utilize the second switch  56  from the comparative signal y to the first signal. Since the first signal is generated through the initial calibration, the first signal is almost equal to the steady-state value y 2  of the signal y. Hence the first signal can replace the comparative signal y to be inputted into the drive circuit  38  before the comparative signal y reaches the steady-state value y 2  for generating an output power of the pick-up head  40  that approximates to the expected output power. After the comparative signal y reaches the steady-state value y 2 , the switch controller  58  utilizes those two switches and returns the APC loop  30  to a closed loop for providing stabilization of the output power. When the CD-RW drive wants to raise the output power of the pick-up head  20  from a higher value to a lower value, the signal γ will be switched from γ 2  to γ 1  at time t 3 . If y&lt;y′, the switch controller  58  will utilize the first switch  54  from the detecting signal ε to the GROUND POTENTIAL GND of the voltage source  52  (as shown in  FIG. 4 , the output signal χ decreases to ground potential GND at time t 3 ). Afterwards, the ground potential GND will quickly discharge the capacitor  44  to make the voltage drop Vc from the value of (γ 2 -y 2 ) at time t 3  enter the steady-state value (γ 1 -y 1  ) at time t 2  in a very short time, and the switch controller  58  also will utilize the second switch  56  from the comparative signal y to the first signal. Since the first signal is generated through the initial calibration, the first signal is almost equal to the steady-state value y 1  of the signal y. Hence the first signal can replace the comparative signal y to be inputted into the drive circuit  38  before the comparative signal y reaches the steady-state value y 2  for generating an output power of the pick-up head  40  that approximates to the expected output power. After the comparative signal y reaches the steady-state value y 1 , the switch controller  58  utilizes those two switches and returns the APC loop  30  to a closed loop for providing stabilization of the output power. Comparing the present invention as shown in  FIG. 2  to the prior art as shown in  FIG. 2 , the capacitor  44  of the present invention has much shorter charging time (t 2 -t 1 ) and the discharging time (t 4 -t 3 ) than the charging time (t 2 -t 1 ) and the discharging time (t 4 -t 3 ) of the capacitor  24  in the prior art. That is, compared with the prior art, the comparative signal y of the present invention arrives at the steady state sooner since the present invention makes use of a relative large voltage drop to charge/discharge the capacitor  44 . In addition, the output signal y of the second switch for controlling the output power of the pick-up head  40  is almost maintained to a steady-state value since the present invention makes use of the first signal that approximates to the steady-state value of the comparative signal y to replace the comparative signal y to be inputted into the drive circuit  38  before the comparative signal y reaches the steady-state value y 2  for generating an output power of the pick-up head  40  that approximates to the expected output power. Therefore, the APC loop  30  of the present invention can quickly stabilize the output power of the pick-up head  40 .  
         [0018]     In contrast to the prior art, the method of the present invention makes use of a first switch to charge/discharge the effective capacitor of the APC loop and a second switch to provide a signal that approximates the steady-state value of the comparative signal to replace the comparative signal to be inputted into the drive circuit  38  for controlling the output power of the pick-up head. Therefore, the APC loop of the present invention can quickly stabilize the output power of the pick-up head. Moreover, in addition to the CD-RW drive, the APC loop of the present invention can also be applied to various rewritable optical drives, including DVD-RW, DVD+RW, DVD-RAM, and so on.  
         [0019]     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.