The present invention relates to a disk reproduction apparatus and, more particularly, to an improvement in a tracking servo circuit used in a disk reproduction apparatus.
In a digital recording/reproduction system presently known in the field of audio equipment, an audio signal is converted into a digital signal by the PCM (Pulse Coded Modulation) technique, the digital signal is recorded on a recording medium such as a disk or magnetic tape, and the recorded digital signal is reproduced.
CDs in which bit arrays corresponding to digital data are formed on a 12-cm diameter disk and optically read are most popular. In a disk reproduction apparatus, an optical pickup incorporating a semiconductor laser and photoelectric conversion element is linearly moved from the outer to inner peripheral side of the disk, and the same time the CD is rotated at a CLV (Constant Linear Velocity) to read data recorded on the CD. A CD-ROM and DVD-ROM are known in addition to the CD.
FIG. 1 is a block diagram of a conventional disk reproduction apparatus. A disk 11 is driven by a disk motor 9 at the CLV. Data recorded on the disk 11 is read by an optical pickup (PU) 1 and supplied to an RF circuit 12.
The RF circuit 12 extracts focus and tracking error signals from an output from the optical pickup 1 and supplies them to a servo control circuit 10. At the same time, the RF circuit 12 binarizes the reproduced signal and supplies the resultant signal to a PLL circuit 13 as an EFM signal.
The PLL circuit 13 generates a PLL clock (VCOCK) serving as a EFM-synchronized reproduction clock for reading the EFM signal. The center frequency of the clock (VCOCK) is 17.2872 MHz four times the bit rate (4.3218 MHz) of the EFM signal. The PLL circuit 13 supplies the EFM signal and the PLL clock (VCOCK) to a signal processing circuit 14. The signal processing circuit 14 performs EFM demodulation, subcode demodulation, error correction processing, and the like. An output from the signal processing circuit 14 is supplied as digital data to a ROM and a DAC (Digital-to-Analog Converter) 16. An output from the DAC 16 is output as reproduced audio output through a low-pass filter (LPF) 17.
The EFM demodulation circuit in the signal processing circuit 14 generates a PLL frame clock (PFS) from the clock VCOCK and supplies it to a memory (not shown) and a clock circuit 15.
A system controller 18 is controlled by a microprocessor (not shown) and supplies a reproduction rate control signal HS and a clock control signal SW to the clock circuit 15. In addition, the system controller 18 supplies, to the signal processing circuit 14 and the servo control circuit 10, system control signals representing control operations such as control of play, stop, and cue search, and muting ON/OFF control. A control signal SW is output as a high-level ("H") signal from the system controller 18 in reproducing so-called ROM data such as character codes and images to be reproduced at high speed. The signal SW is kept at low level ("L") for audio data because it must be read at a constant clock rate.
The microprocessor can detect the positions of ROM and audio data on the disk in advance by reading a TOC (Table of Contents) table at the inner periphery of the disk. In accessing ROM data recorded at a given position on the disk, the microprocessor outputs, to the system controller 18, control signals, i.e., the signal SW of "H" level and a signal for moving the pickup to the target position.
The clock circuit 15 generates a reference clock (MCK) for the signal processing circuit 14 on the basis of a clock (XCK) from a quartz oscillator or the PLL clock (VCOCK) from the PLL circuit 13 in accordance with the reproduction rate control signal (HS) and the clock control signal (SW). The signal processing circuit 14 frequency-divides the reference clock (MCK) by 2,304 to output a frame clock (MFS) for a signal processing system. The clock (XCK) supplied from the quartz oscillator to the clock circuit 15 is directly output as a reference clock (SCK) for the servo control circuit 10.
FIG. 2 is a block diagram showing a tracking servo circuit in the conventional disk reproduction apparatus.
The tracking servo circuit comprises an A/D converter 3, a tracking digital equalizer 4, and a D/A converter 5 and is formed into a chip 20 made of silicon or the like.
A signal read from the disk by the optical pickup 1 is supplied as a current signal to a head amplifier 2 and amplified and output as a tracking error signal TE of the voltage signal. The tracking error signal TE is converted into digital data by the A/D converter 3. The converted digital data signal is supplied to the tracking digital equalizer (DEQ) 4.
The tracking digital equalizer 4 has a gain for realizing necessary phase compensation processing and gain compensation processing. An output from the tracking digital equalizer 4 is output as a digital data signal having a finite bit length.
Output data from the tracking digital equalizer 4 is supplied to the D/A converter 5 and converted into an analog signal. This analog signal is supplied to a driver unit 6. The driver unit 6 is driven by this analog signal to drive the actuator of the optical pickup 1. The tracking servo circuit in the reproduction apparatus performs the above series of operations to so control as to stably read a signal from the disk.
Two different characteristics are set in the tracking digital equalizer 4. In disk reproduction, the tracking digital equalizer 4 uses normal mode characteristics in normal play (i.e., a small offset from the center of servo). When the deviation from the center of servo is large in track catching at the end of search, upon reception of shock, and the like, gain-up mode characteristics for increasing the gain as compared with the normal mode characteristics are used. In this manner, the tracking digital equalizer 4 operates while switching the modes. The "deviation from the center of servo" is defined as a deviation from an electrical reference level corresponding to the position of data to be recorded on a track. The "search" means moving the head from a given track position to a target position (track).
Since the gain is normally set relatively low in the normal mode characteristics, an operation error rarely occurs even in the presence of a scratch on the disk. However, when the deviation from the center of servo is large in track catching at the end of search, upon reception of shock, and the like, convergence of tracking servo degrades. Since the gain is set relatively high in the gain-up mode characteristics, convergence of tracking servo is excellent even when the deviation from the center of servo is large in track catching at the end of search, upon reception of shock, and the like. However, an operation error readily occurs due to a scratch on the disk in the gain-up mode characteristics. Therefore, conventionally, the two different characteristics are selectively used.
FIG. 3 shows the waveform of the tracking error signal TE actually output from the head amplifier 2. This indicates the normal play state (the deviation from the center of servo is small), and the state in which the deviation from the center of servo is large in track catching at the end of search, upon reception of shock, and the like.
The maximum amplitude of the tracking error signal TE output from the conventional head amplifier 2, which is obtained when the recorded data position greatly deviates from the center of servo in track catching at the end of search, upon reception of shock, and the like, is set not to exceed a preset A/D (Analog/Digital) range.
The tracking error signal TE obtained when the recorded data position greatly deviates from the center of servo in track catching at the end of search, upon reception of shock, and the like is converted into digital data by the A/D converter 3. This digital data is input to the tracking digital equalizer 4 and output from the tracking digital equalizer 4 as digital data effectively using a finite bit length.
The tracking error signal TE in the normal play mode represents a small positional deviation of data recorded on a track from the center of servo. This tracking error signal TE has a small amplitude. Upon A/D conversion, the tracking digital equalizer 4 outputs a signal using only the lower bits of the finite bit length, i.e., an output signal having poor accuracy. Hence, during the play, the reproduction performance of the disk may suffer.
The characteristics of the tracking digital equalizer 4 at this time are generally as shown in FIG. 4. The gain in the intermediate frequency range is low. In particular, when the disk has a scratch, the signal in the intermediate frequency range leads to data loss in the arithmetic processing operation in the tracking digital equalizer 4. The reproduction performance may degrade due to the scratch of the disk.
To the contrary, in the normal play mode (the deviation from the center of servo is small) in which the gain of the tracking digital equalizer 4 is set higher than the conventional gain and the gain of the driver unit 6 is decreased to optimize the gain of the overall control system, loss of lower bit data can be prevented in the arithmetic processing operation in the tracking digital equalizer 4, thereby improving the reproduction performance.
A tracking error signal obtained when the deviation from the center of servo is large, e.g., when the optical pickup 1 catches recorded data on a track at the end of search in the above state or when a shock acts on the track, is A/D-converted. The resultant digital data is supplied to the tracking digital equalizer 4. At this time, since the tracking digital equalizer 4 uses the characteristics (gain-up mode gain) having a gain higher than that of the normal mode characteristics, an overflow occurs in the arithmetic processing operation, and the reproduction performance may deteriorate.
In the prior art, the reproduction performance in the normal play mode (the deviation from the center of servo is small) and the reproduction performance in the presence of a scratch of the disk are not good. Assume that the reproduction performance in the normal play mode (the deviation from the center of servo is small) and the reproduction performance in the presence of a scratch of the disk are improved. In this case, the tracking error signal TE obtained when the deviation from the center of servo is large in track catching at the end of search, upon reception of shock, and the like is A/D-converted. When the resultant digital data is supplied to the tracking digital equalizer 4, the reproduction performance suffers. It is, therefore, difficult to improve both the reproduction performance in the normal play mode and the reproduction performance achieved when the deviation from the center of servo is large in track catching at the end of search, upon reception of vibration and/or impact, and the like.