Patent Publication Number: US-2005128902-A1

Title: Optical disc system and associated tilt angle calibration method

Description:
BACKGROUND OF INVENTION  
      1. Field of the Invention  
      The invention relates to an optical disc system, and more particularly, to an optical disc system capable of calibrating the tilt angle between an optical disc and an object lens.  
      2. Description of the Prior Art  
      Optical discs are well suited for storing large amounts of data and have a long lifetime. For these reasons, optical discs are widely used in today&#39;s ever increasingly computerized society. Optical discs do not suffer from magnetic deterioration over time as do magnetic media types. Compact discs (CDs), compact disc read only memories (CD-ROMs), digital versatile discs (DVDs), and various types of writable versions of these formats are all commonly used today. Although differing in physical form and digital format, in general, all optical disc systems project a beam of light on an optical disc through an object lens. This light is reflected off the optical disc and detected by an optical detector. Information is encoded on the optical disc using fine marks also referred to as pits that alter the reflected light. By detecting the different way the light reflects from these pits, the information encoded on the disc can be reproduced by the optical disc system.  
      When reproducing information from an optical disc, it is important to properly align the optical disc and the object lens. The light beam must be focused directly on tracks containing the pit marks, and the reflected light must be centered on the optical sensor. If the incident light beam on the surface of the optical disc is misaligned relative to the disc surface, it may be impossible for the optical sensor to accurately reproduce the encoded data. If the alignment is close but still incorrect, increased data errors and clock jitter in the received data stream will lower the quality of the optical disc system.  
      To ensure proper alignment, optical disc systems typically employ a tilt servo device for adjusting the tilt angle between the optical disc and the object lens. The tilt servo device maintains the incident light beam at an angle perpendicular to the surface of optical disc so that the reflected light is focused directly in the center of the optical receiver. In order to fulfill this task, the tilt servo device needs to include a tilt detection means for detecting a misalignment of the tilt angle so that the tilt servo device can make the appropriate correction. Two commonly used tilt detection means are the tilt sensor and the jitter meter.  
      The tilt sensor is normally used in conjunction with a separate beam of light, referred to herein as the tilt beam, which is similar to the beam of light used to read the data from the optical disc, referred to herein as the read beam. By reflecting the tilt beam off the optical disc onto a separate light sensing tilt sensor, the tilt angle between the optical disc and the object lens can be directly measured. The tilt beam/sensor pair and the read beam/sensor pair must physically be as close to each other as possible so that the tilt angle seen by the tilt sensor will accurately approximate the tilt angle seen by the read sensor. However, it is often difficult to perfectly align these two light beams. Using a separate light beam and sensor pair, which must themselves be properly aligned, to correct an alignment problem is not an optimal solution.  
      Another common solution to this tilt angle calibration problem involves the use of a jitter meter. Because the jitter of the recovered data stream increases as the optical disc and the object lens become misaligned, the tilt angle producing the least amount of jitter can be assumed to be the optimal alignment. A jitter meter can be included in the optical disc system, allowing the optical disc system to monitor the jitter of the received data stream and appropriately control the tilt servo device to adjust the tilt angle producing the least amount of jitter. However, using the jitter meter significantly increases the complexity of the design and requires extra components such as the jitter meter to be added to the optical disc system.  
      Fukumoto et al. in U.S. Pat. No. 6,493,296 disclose an optical disc inclination detecting method, an optical pickup device, and an optical disc system, which use a push pull system to solve the above mentioned tilt angle calibration problem. Their optical disc system uses a dividing unit to divide the read beam and form a main light spot and two side spots on the optical disc. Separate photo detectors are used to measure the light on the main spot and the two side spots. Signal generators are used to detect the tilt angle of the optical disc and control the tilt servo device to calibrate the tilt angle according to the output of the photo detectors. This optical disc system, however, again requires additional hardware components and increased design complexity, increasing the overall cost of the optical disc system. A need remains for a low cost solution to the tilt angle calibration problem.  
     SUMMARY OF INVENTION  
      It is therefore a primary objective of the claimed invention to provide an optical disc system and method capable of calibrating the tilt angle between an optical disc and an object lens, to solve the above-mentioned tilt angle problem.  
      According to the claimed invention, an optical drive system is disclosed comprising an optical disc, an object lens for focusing light on the optical disc, a tilt servo for adjusting a tilt angle between the optical disc and the object lens, an optical electric integrated circuit (OEIC) for detecting light reflected from the optical disc, a DPD generator for generating a differential phase detection (DPD) signal according to the output of the OEIC, and a tilt search block receiving the DPD signal and being connected to the tilt servo. The tilt search block controls the tilt servo to adjust the tilt angle between the optical disc and the object lens according to the DPD signal.  
      Also according to the claimed invention, a method is disclosed for calibrating the tilt angle between an optical disc and an object lens in an optical storage device. The method comprises the following steps: (a) providing a tilt servo for adjusting the tilt angle between the optical disc and the object lens, (b) providing an optical electric integrated circuit (OEIC) for detecting light reflected from the optical disc, (c) generating a differential phase detection (DPD) signal according to the output of the OEIC, and (d) controlling the tilt servo to adjust the tilt angle between the optical disc and the object lens according to the DPD signal.  
      These and other objectives of the claimed 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 block diagram of an optical disc system according to the first embodiment of the present invention.  
       FIG. 2  is a block diagram showing the OEIC and the DPD signal generator of  FIG. 1 .  
       FIG. 3  is a graph of DPD signal amplitude vs. tilt angle.  
       FIG. 4  is a graph of DPD signal amplitude while searching for the optimal tilt angle using the DPD signal.  
       FIG. 5  is a block diagram of an optical disc system according to the second embodiment of the present invention.  
       FIG. 6  is a flowchart describing a method of calibrating the tilt angle of an optical disc system according to the present invention. 
    
    
     DETAILED DESCRIPTION  
       FIG. 1  shows a block diagram of an optical disc system  100  according to the first embodiment of the present invention. The optical disc system  100  comprises an optical disc  102 , an object lens  104 , an optical electrical integrated circuit (OEIC)  106 , a differential phase detection (DPD) signal generator  108 , a tilt search block  110 , and a tilt servo  112 . Incident light is reflected off the optical disc  102  at a tilt angle controlled by the tilt servo  112 . The tilt servo  112  receives a control signal T DRIVE  from the tilt search block specifying a tilt angleα between the optical disc  102  and the object lens  104 . The tilt servo  112  controls the tilt angleα. Reflected light L passes through the object lens and is detected by the OEIC  106 . The OEIC  106  is an optical sensor that generates electrical signals according to the reflected light L. The DPD signal is a common signal required in optical disc systems and the DPD signal generator  108  generates this DPD signal according to the output of the OEIC  106 . The tilt search block  110  uses the amplitude of the DPD signal to compare the accuracy of different tilt angles and determine an optimal tilt angle having the lowest amplitude DPD signal.  
       FIG. 2  shows a block diagram of the connection between the OEIC  106  and the DPD signal generator  108  of  FIG. 1 . The DPD signal generator  108  comprises first and second adders  204 ,  206 ; first and second amplifiers  208 ,  210 ; first and second equalizers  212 ,  214 ; first and second level comparators  216 ,  218 ; a phase comparator  220 ; first and second low-pass filters  222 ,  224 ; and a subtractor  226 . The OEIC  106  is divided into four sensors A, B, C, D. The first adder  204  adds the output of the OEIC  106  for the A and D sensors. The output of the first adder  204  is amplified by the first amplifier  208 , equalized by the first equalizer  212 , level compared by the first level comparator  216 , and then connected to the phase comparator  220 . Similarly, the second adder  206  adds the output of the OEIC  106  for the C and B sensors. The output of the second adder  206  is amplified by the second amplifier  210 , equalized by the second equalizer  214 , level compared by the second level comparator  218 , and also connected to the phase comparator  220 . The phase comparator  220  compares the phase of the incoming signals and generates two pulse trains which are then low-pass filtered by the first low-pass filter  222  and the second low-pass filter  224  respectively. The subtractor  226  generates the DPD signal according to the difference between the output of the first and second low-pass filters  222 ,  224 . It should be noted that alternative methods of generating the DPD signal are well known in the prior art and can also be used with the present invention. As the DPD signal is well known in the prior art, further description of its generation is hereby omitted.  
       FIG. 3  shows a graph of DPD signal amplitude vs. tilt angle. As the tilt angle increases from a minimum angle of A MIN  to a maximum angle of A MAX , the amplitude of the DPD signal dips to a minimum level DPD MIN  at the optimal tilt angle A OPT . This property can be used to determine the optimal tilt angle directly using the DPD signal. A measurement such of the amplitude of the DPD signal such as a peak-to-peak measurement or the DPD signal envelope  300  shown in  FIG. 3  can be used to track the amplitude of the DPD signal. Using the DPD signal to determine the optimal tilt angle is more accurate than the tilt sensor used in the prior art because the DPD signal directly corresponds to the light received at the OEIC  106 . As the output of the OEIC is used to decode the data, it is therefore highly beneficial to calibrate the tilt angle directly using the output of the OEIC  106 . Additionally, determining the optimal tilt angle directly using the DPD signal, which is already required in the prior art, means that very few (if any) additional hardware components need to be included in the optical disc system.  
       FIG. 4  shows a graph of DPD signal amplitude while searching for the optimal tilt angle using the DPD signal.  FIG. 4  includes an example tilt servo control signal T DRIVE  and the positive-half of the resulting DPD signal. For illustrative purposes, the example diagram tilt servo control signal T indicates a voltage used to drive the tilt DRIVE servo  112  directly corresponding to the associated tilt angle. In  FIG. 4 , the tilt angle produced by the tilt servo  112  directly corresponds to the voltage value of the T DRIVE  signal. For instance, a T DRIVE  value equal to 2V indicates a 2 deg tilt angle, a T DRIVE  value equal to 1V indicates a 1 deg tilt angle, etc. This property is for illustrative purposes in this example only and is not a requirement of the present invention. To illustrate a searching process according to the present invention, assume that the ideal tilt angle providing the best detection of the reflected light L is 0.7 deg. To begin calibration, the tilt search block  110  controls the tilt servo  112  to scan a first plurality  400  of five tilt angles starting at 2 deg and having an angle separation of 1 deg. The amplitudes of the DPD signal are compared for each tilt angle in the first plurality  400  of tilt angles and the tilt angle of 1 deg is found to have the lowest amplitude DPD signal. The tilt search block  110  then controls the tilt servo  112  to scan a second plurality  402  of five tilt angles starting at 0.4 deg and having an angle separation of 0.3 deg. The amplitudes of the DPD signal are compared for each tilt angle in the second plurality  402  of tilt angles and, because the tilt angle of 0.7 deg has the lowest amplitude DPD signal, 0.7 deg is used as the tilt angle by the optical disc system.  
       FIG. 5  shows a block diagram of an optical disc system  500  according to the second embodiment of the present invention. The optical disc system  500  according to the second embodiment includes the same basic components connected in the same fashion as the first embodiment shown in  FIG. 1 . The reflected light L is received by the OEIC  106  and converted to the DPD signal by the DPD signal generator  108  in the same way as the first embodiment shown in  FIG. 1 . However, in the second embodiment, the tilt search block  110  further comprises an amplifier  502 , a multiplexer  504 , an amplitude detector  505  (or bypass), an analog to digital converter (ADC)  506 , and a central processing unit (CPU)  508 . Because the DPD signal is not designed to indicate the tilt angle, it may be required to amplify the signal in order to more easily measure differences between the amplitude of the DPD signal of different tilt angles. The amplifier  502  is provided for this function. The amplitude detector  505  can be used to assist in detecting the amplitude of the amplified DPD signal or if this function is not built-in, block  505  can be bypassed and software executed by the CPU  508  can be used to compute the amplitude. To allow the CPU  508  to search for the optimal tilt angle, the DPD signal needs to be converted to a digital format usable by the CPU. Because most optical disc systems already include an ADC  506 , the multiplexer  504  is included to allow the reuse of the already existing ADC  506  to digitize the DPD signal. In some optical disc systems, this multiplexer may itself already exist for allowing the reuse of the ADC converter.  FIG. 5  shows an optical disc system having multiple signals (Sig 1 , . . . , SigN) already being multiplexed by the multiplexer  504  and the DPD signal has been added as one of the signals multiplexed by the multiplexer  504 . During tilt angle calibration, the multiplexer  504  is controlled by the CPU  508  to pass the DPD signal to the ADC  506 . A digital DPD signal output by the ADC  506  is received by the CPU  508 . Using a search algorithm, such as the search algorithm shown in  FIG. 4 , the CPU  508  scans a plurality of tilt angles to determine the optimal tilt angle having the lowest amplitude DPD signal. When calibration is complete and the optimal tilt angle has been set, the CPU controls the multiplexer  504  to pass the normal-operation signal(s) to the ADC  506 .  
       FIG. 6  is a flowchart  600  describing a method of calibrating the tilt angle of an optical disc system according to the present invention. The flowchart  600  includes the following steps:  
      Step  602 : Provide a controllable tilt servo for physically adjusting the tilt angle between the optical disc and an object lens. The tilt servo could be implemented with an actuator driver or another device allowing the relative tilt angle between the optical disc and the object lens to be altered. Proceed to step  604 .  
      Step  604 : Generate a differential phase detection (DPD) signal corresponding to the output of an optical electrical integrated circuit (OEIC) as also required in the prior art. Proceed to step  606 .  
      Step  606 : Amplify the DPD signal to enhance the differences of amplitudes of DPD signal corresponding to different tilt angles and proceed to step  608 .  
      Step  608 : Set a default angle spacing and a default center angle. The angle spacing refers to the angle difference between different angles scanned during tilt angle calibration and the center angle refers to the angle in the center of the plurality of angles. Proceed to step  610 .  
      Step  610 : Scan a plurality of tilt angles differing from one another by the angle spacing and centered at the center angle, and simultaneously sample the amplitudes of the DPD signal for computing peak-to-peak amplitudes corresponding to the scanned tilt angles. Proceed to Step  612  .Step  612 : Search the plurality of tilt angles scanned in step  612  to determine lowest digitized DPD amplitude. Proceed to step  614 .  
      Step  614 : Has an angle spacing limit been reached? If the minimum spacing between tilt angles allowable by the tilt servo has been reached then proceed to step  618 , otherwise proceed to step  616 .  
      Step  616 : Reduce the angle spacing parameter and set the center angle parameter to the angle having the lowest digitized DPD amplitude determined in step  612 . Proceed to step  610  to scan a subset of the plurality tilt angles having lower DPD amplitudes.  
      Step  618 : Set the tilt servo to the tilt angle having the lowest amplitude DPD signal in the plurality of tilt angles scanned in step  614 . Tilt angle calibration is finished.  
      In contrast to the prior art, the present invention uses the DPD signal to calibrate the tilt angle of an optical disc system so that the optimal tilt angle is used resulting in the best signal to noise ratio when recovering data encoded on the optical disc. Because a DPD generator in the prior art already generates the DPD signal, the present invention is a cost effective and efficient solution avoiding the use of extra components such as a tilt sensor or a jitter meter. By iteratively scanning pluralities of tilt angles, each plurality of angles having decreasing angle differences and being centered on a tilt angle having the lowest amplitude DPD signal in the previous plurality, an optimal tilt angle having the lowest amplitude DPD signal can be found.  
      Those skilled in the art will readily observe that numerous modifications and alterations of the device 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.