Abstract:
An optical disk device includes pickup head for irradiating an optical disk with an optical beam and receiving a reflected beam from the optical disk to output an RF signal, first equalizing unit which boosts a frequency band of the RF signal from the pickup head, the frequency band corresponding to a minimum recording signal recorded on an optical disk, clock generating unit which generates a clock signal on the basis of the RF signal boosted by the first equalizing unit, second equalizing unit which equalizes the RF signal or the boosted RF signal to a partial response waveform signal on the basis of the clock signal generated by the clock generating unit and an equalization coefficient, and a decoding section which executes maximum likelihood decoding on the partial response waveform signal on the basis of the clock signal generated by the clock generating unit to output a reproduction signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2005-288758, filed Sep. 30, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an optical disk device that uses a PLL circuit to reproduce signals from an optical disk with an increased density, and a method for playing an optical disk.  
         [0004]     2. Description of the Related Art  
         [0005]     With conventional optical disks, binarization based on level slicing is carried out depending on whether a reproduction signal level is higher or lower than a threshold. However, for high definition digital versatile discs (HD DVDs), their increased density tends to reduce the amplitude of reproduction signals. Consequently, the binarization based on level slicing is likely to result in frequent identification errors. Thus, HD DVDs (High Definition Digital Versatile Discs) employ partial response maximum likelihood (PRML) to binarize reproduction signals (Toshiba Review: Vol. 60, No. 1 (2005), P 25 to P 28, “Technique for Processing Reproduction Signals from HD DVDs”, Hiroshi KASHIWARA).  
         [0006]     A reproduction signal processing block for HD DVD is shown in FIG. 1 of Toshiba Review: Vol. 60, No. 1 (2005), P 25 to P 28, “Technique for Processing Reproduction Signals from HD DVDs”, Hiroshi KASHIWARA. In this block, PRML is executed by an equalizer and a Viterbi decoder. An RF signal (reproduction signal) passes through the equalizer. The Viterbi decoder then modifies an actual waveform to an ideal one, which is then used as a reproduction signal. Before the circuit shown in  FIG. 1  can operate normally, a PLL circuit dedicated for PRML must operate normally.  
         [0007]     To execute a PRML process to reproduce signals from an optical disk with an increased density, it is necessary to stabilize PLL operations in order to allow PRML operate stably.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     An aspect of the present invention provides an optical disk device comprises pickup head for irradiating an optical disk with an optical beam and receiving a reflected beam from the optical disk to output an RF signal, first equalizing unit which boosts a frequency band of the RF signal from the pickup head, the frequency band corresponding to a minimum recording signal recorded on an optical disk, clock generating unit which generates a clock signal on the basis of the RF signal boosted by the first equalizing unit, second equalizing unit which equalizes the RF signal or the boosted RF signal to a partial response waveform signal on the basis of the clock signal generated by the clock generating unit and an equalization coefficient, and a decoding section which executes maximum likelihood decoding on the partial response waveform signal on the basis of the clock signal generated by the clock generating unit to output a reproduction signal. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0009]      FIG. 1  is a perspective view schematically showing the structure of an optical disk device in accordance with an embodiment of the present invention;  
         [0010]      FIG. 2  is a block diagram showing the configuration of an optical pickup in the optical disk device shown in  FIG. 1 ;  
         [0011]      FIG. 3  is a diagram showing circuit blocks including a pickup shown in  FIG. 2 ;  
         [0012]      FIG. 4  is a block diagram showing a system that plays HD DVD;  
         [0013]      FIGS. 5A and 5B  are diagrams showing a 2T signal before and after boosting; and  
         [0014]      FIG. 6  is a diagram showing a 3T signal for DVD media and a 2T signal for HD DVD media. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Description will be given of an optical disk device that can play HD DVD (High Definition Digital Versatile Disc) media and non-HD DVD media.  
         [0016]      FIG. 1  is a diagram schematically showing the structure of an optical disk device in accordance with a first embodiment of the present invention.  FIG. 2  is a block diagram showing the configuration of an optical pickup integrated into the optical disk device.  
         [0017]     As shown in  FIG. 1 , the optical disk device in accordance with the present embodiment is mainly composed of a disk motor  2  that rotatively drives the disk  1 , a pickup  3  that reads signals recorded on the disk  1 , a feed motor  4  that moves the pickup  3  in a radial direction of the disk  1 , and a main board on which a microcomputer, a signal processing circuit, and the like are mounted. The feed motor  4  is provided with sensor for sensing the rotational conditions of the motor such as the rotating frequency, speed, and rotating direction of the motor. A feed motor driving signal circuit uses an output signal from the sensing signal to control the feed motor  4  during track search.  
         [0018]     As shown in  FIG. 2 , the pickup  3  is mainly provided with light emitting diode  5  that outputs a laser beam, an objective lens  6  supported by a wire or blade spring (not shown) and which focuses the laser beam emitted by the light emitting diode  5  on a surface of the disk  1 , which is thus irradiated with the laser beam, a focusing coil  7  and a tracking coil  8  which controllably moves the objective lens  6  in a focus direction and a tracking direction, respectively, a photodetector  9  that receives the reflected beam from the disk  1 , and a monitor detector  10  which monitors the reflected beam from the disk  1  and which feeds back data to the light emitting diode  5 .  
         [0019]      FIG. 3  is a diagram showing circuit blocks including the pickup  3 . The circuit has an analog signal processing section  20  to which a signal read by the pickup  3  is supplied and DSP (Digital Signal Processor)  12  to which a signal amplified by the analog signal processing section  20  is supplied.  
         [0020]     As shown in  FIG. 3 , the pickup  3 , connected to the analog signal processing section  20 , has a 4-divided photodetector  9  and sub-beam detectors  13  and  14  that create track error signals for CD. The pickup  3  further has a light emitting diode  5  that emits a laser beam with which the optical disk  1  is irradiated, a splitter  15  that separates the generated laser beam from the reflected beam from the optical disk  1 , the objective lens  6 , and a condensing lens  16  placed in front of the 4-divided photodetector  9 .  
         [0021]     The optical disk  1  is irradiated, via the splitter  15  and objective lens  6 , with an optical beam emitted by the light emitting diode  5 . A reflected beam from the optical disk  1  is guided to the 4-divided photodetector  9  via the objective lens  6 , splitter  15 , and condensing lens  16 .  
         [0022]     The 4-divided photodetector  9  consists of a four-piece light receiving element including photo-detecting cells  9   a,    9   b,    9   c,  and  9   d.    
         [0023]     Outputs from the photo-detecting cells  9   a,    9   b,    9   c,  and  9   d  and sub-beam detectors  13  and  14  are input to the analog signal processing section  20 . The analog signal processing section  20  then amplifies and subjects the signals to an addition and a subtraction to output a tacking error signal (TE), a focus error signal (FE), an RF signal, and a MIRR signal.  
         [0024]     The tracking error (TE) signal and focus error (FE) signal are servo signals from the optical disk  1  which are used to perform servo operations of tracking and focusing the objective lens  6 . The RF signal is a read reproduction data signal. The MIRR signal indicates the envelope of the RF signal.  
         [0025]     An RF amplifier  21  adds together and amplifies output signals from the photo-detecting cells  9   a,    9   b,    9   c,  and  9   d  of the photodetector  9  to output an RF signal.  
         [0026]     That is, when the outputs from the photo-detecting cells  9   a,    9   b,    9   c,  and  9   d  are defined as A, B, C, and D, the RF amplifier  21  uses a signal RF=A+B+C+D to generate a high frequency signal RF.  
         [0027]     Similarly, the analog signal processing section  20  uses a signal FE=(A+C)−(B+D) to generate a focus error signal FE. The RF amplifier  20  also uses a signal TE=(A+B)−(C+D) to generate a tracking error signal TE.  
         [0028]     The MIRR signal is produced by sensing the peak and bottom of the RF waveform to execute the calculation {(peak)−(bottom)}. When a lens jump occurs, that is, when the driving coil  11  is used to move the objective lens  6  a distance corresponding to a plurality of tracks in the tracking direction, the MIRR signal is used to check the actual number of tracks corresponding to the distance the lens has moved.  
         [0029]     The track error (TE) signal for CD playing is produced by calculating the difference (E−F) between an output current E from the sub-beam detector  13  and an output current F from the sub-beam detector  14 .  
         [0030]     DSP  12  is connected to CPU  17  and operates on the basis of instructions from CPU  17 .  
         [0031]     Now, adjustment of RF amplitude will be described. RF amplitude adjustment in optical disk equipment is intended to achieve a target RF amplitude value on the basis of the MIRR signal. Specifically, an A/D converter in DSP  12  reads the current MIRR signal level and compares it with a preset target value. On the basis of the comparison, the A/D converter then adjusts the RF amplitude of the RF amplifier  20 .  
         [0032]     An RF signal amplified by the RF amplifier  20  is supplied via a switching unit  22  to an HD DVD equalization circuit  23  corresponding to HD DVD media or a CD/DVD equalization circuit  24  corresponding to CD media and DVD media. When an optical disk is inserted into the device and the media is then determined, the switching unit  22  determines to which of the equalization circuits  23  or  24  the signal is supplied.  
         [0033]     The CD/DVD equalization circuit  24  optimizes boost amount and the like so as to, for example, minimize the level of jitter or to optimize the slice level of the RF signal. In other words, the boost amount and the like are set so as to maximize read rate.  
         [0034]     If the media is HD DVD, the HD DVD equalization circuit  23  processes the RF signal so as to ensure the operation of the PLL circuit  50 . The processed RF signal is supplied to the PLL circuit  50  and A/D converter  30 . The A/D converter  30  digitally converts the signal and supplies the resultant signal to a PRML processing section  40 . The PRML processing section  40  executes a PRML process on the signal and supplies the resultant signal to DSP  12 .  
         [0035]     The configurations of the PLL circuit and PRML processing section will be described with reference to  FIG. 4 .  
         [0036]     In the PLL circuit  50 , the RF signal is input to a phase comparator  51 . The phase comparator  51  compares the RF signal with a comparison signal output by a voltage control oscillator (VCO)  53 . The phase comparator  51  then outputs a phase difference component as a pulse-like phase difference signal. The phase difference signal has its high frequency component blocked by a loop filter (integration circuit/low pass filter)  52 . The phase difference signal is thus converted into a DC signal, which is then input to the voltage control oscillator  53 . The voltage control oscillator  53  has a specified free-running frequency to vary oscillation frequency depending on the phase difference signal. On the basis of the input signal, the voltage control oscillator  53  adjusts the oscillation frequency to output a clock signal. The clock signal is supplied to the phase comparator  51 ; the clock signal corresponds to a comparison signal. The clock signal output by the voltage control oscillator  53  is supplied to the A/D converter  30  and PRML processing section  40 .  
         [0037]     The RF signal supplied by the HD DVD equalization circuit  23  is digitally converted by the A/D converter  30 . The RF signal is then supplied to the equalizer  41  and a delay unit  47  in the PRML processing section  40 . On the basis of an equalization coefficient calculated by an equalization coefficient calculating section  45 , the equalizer  41  equalizes the RF signal to obtain a partial response waveform. Here, the target partial response waveform is a PR value (1, 2, 2, 2, 1) or (3, 4, 4, 3). The PR value may have another pattern. The equalizer is driven by a clock signal supplied by the PLL circuit  50 .  
         [0038]     The signal equalized by the equalizer  41  is supplied to a Viterbi decoder  42  and a delay unit  46 . The Viterbi decoder  42  is driven by the clock signal supplied by the PLL circuit  50 . The Viterbi decoder  42  executes maximum likelihood decoding on the partial response waveform to obtain a reproduction signal. The Viterbi decoder  42  then supplies the reproduction signal to DSP  12  and an ideal waveform calculating section  43 . The ideal waveform calculating section  43  converts the reproduction signal into an ideal waveform. The ideal waveform obtained is supplied to an equalization error detector  44 .  
         [0039]     The equalization error detector  44  compares the partial response waveform supplied via the delay unit  46  with an ideal waveform to detect an equalization error. The detected equalization error is supplied to the equalization coefficient calculating section  45 . The delay unit  46  adjusts the partial response waveform and the ideal waveform supplied by the ideal waveform calculating section  43  so that the waveforms are input to the equalization error detector  44  at the same time.  
         [0040]     The equalization coefficient calculating section  45  calculates an equalization coefficient from the equalization error generated by the equalization error detector  44  and a signal supplied by the A/D converter  30  via the delay unit  47 . The equalization coefficient calculating section  45  supplies the calculated equalization coefficient to the equalizer  41 .  
         [0041]     To operate the PRML processing section  40 , it is essential to allow the PLL circuit  50  to operate stably. To allow the PLL circuit  50  to operate stably, signals including minimum signals 2T, which is not present in DVD, must be reliably used as PLL lock signals. The 2T signals account for about 30 percents of all the signals. Accordingly, 2T signal detecting performance relates greatly to reproducing performance. The RF signal equalization circuit is conventionally adjusted mainly to improve the capability of reproducing RF signals. However, the present device adjusts the HD DVD equalization circuit  23  taking into account improvement of PLL lock performance.  
         [0042]     A clock signal supplied by the PLL circuit  50  is required for the PRML processing circuit  40  to execute a PRML process. All the signals including the minimum signals 2T are required for the PLL circuit  50  to lock PLL. Thus, to lock stable PLL, the HD DVD equalization circuit  23  executes a process of boosting the minimum signal (2T signal). By boosting the 2T signal shown in  FIG. 5 ( a ), it is possible to amplify the signal amplitude of the 2T signal to improve the PLL lock performance as shown in  FIG. 5 ( b ).  
         [0043]     Specifically, boost setting is carried out as shown in  FIG. 6 . Normally, a boost amount of about 3 dB is specified for DVD, and a boost amount of about 6 dB is specified for HD DVD. However, experiments indicate that, for HD DVD, a boost amount of at least 12 dB and at most 24 dB is required to obtain sufficient PLL lock performance. This is partly due to degradation of actual signals. Experiments indicate that, owing to its high frequency, the 2T signal component may become smaller than it originally is if the optical band of the pickup is insufficient. Moreover, various signal degradation factors are present in a signal transmission path. The transmission path itself may function as a low pass filter. In this case, the 2T signal, having a high frequency, has its signal amplitude reduced below its original value. Thus, to operate the PLL circuit  50  stably, it is essential to boost the 2T signal.  
         [0044]     The 2T signal is thus boosted in order to allow the PLL circuit  50  to operate stably. Accordingly, in the above embodiment, the boosted RF signal is supplied to the PRML processing section  40  as well as the PLL circuit  50 , but the following is also possible. The boosted RF signal may be supplied to the PLL circuit  50 , whereas a non-boosted RF signal may be supplied to the PRML processing section  40 .  
         [0045]     As described above, to play HD DVD, the minimum signal (2T signal) is selectively boosted to allow the PLL circuit  50  to operate stably. This allows a clock signal to be stably supplied to the PRML processing section  40 . As a result, the PRML processing section  40  operates stably to suppress the occurrence of errors.  
         [0046]     Even if the PRML technique is used for a reproduction system for DVD media, amplifying minimum signals (3T signals) for DVD allows the PLL circuit  50  to operate stably. This also allows the PRML processing section  40  to operate stably to suppress the occurrence of errors.  
         [0047]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.