1. Field of the Invention
The present invention relates to an optical disk recording and reproducing apparatus for recording information onto an optical disk and reproducing information recorded on an optical disk, and particularly to an optical disk recording and reproducing apparatus comprising a circuit for preventing a tracking drive signal from being offset.
2. Description of the Prior Art
In an optical disk reproducing apparatus, the tracking servo of an optical pickup is performed in accordance with a tracking error signal, and in more detail, with a track cross signal in which the pulse width is varied in accordance with the amount of shift (amount of shift to the radially inner or outer side of a track on an optical disk) of an objective lens provided in the optical pickup against a thread with the optical pickup mounted thereon.
For example, when the objective lens shifts to the radially inner side of a track on an optical disk, the tracking servo performs drive control based on a current track cross signal in such a manner as to put the objective lens back to its normal position against the track by applying a tracking drive voltage to a tracking actuator so that the objective lens is moved radially outward to be put back its normal position against the track. That is, the tracking error balance is corrected to be zero.
FIG. 3 is a block diagram illustrating the operation of an objective lens and a tracking actuator included in an optical pickup 30 which is provided in an optical disk recording and reproducing apparatus. In FIG. 3, the numeral 36 indicates a light source for emitting laser beam, where a laser beam emitted from the light source 36 passes through a hologram element 35 to be reflected at a mirror 34, and then is condensed by the objective lens 33 to spot-irradiate the recording surface of an optical disk 38. Then, a returning light beam reflected from the recording surface of the optical disk 38 is condensed by the objective lens 33 again to be reflected at the mirror 34, and then is divided into a plurality of light beam groups when passing through the hologram element 35 to enter multi-divided light receiving element groups 37. It is noted that the description about a focusing servo for focusing a laser beam onto the recording surface of the optical disk 38 is omitted here.
In the reflected light from the optical disk 38, which is then condensed by the objective lens 33, is contained +/−1st order light components diffracted by the track groove on the recording surface of the optical disk 38, first and second regions formed in a divided manner perpendicularly to the direction of the track groove on the surface of the hologram element 35 being positioned in such a manner as to correspond to regions modulated, respectively, by +1st and −1st order light components. The returning light beam passes through the first and second regions in the hologram element 35 to be divided in different directions, and then received by the multi-divided light receiving element groups 37 to be photoelectrically converted. Then, a tracking error detection circuit 39 selects suitable signals among signals separately output from the multi-divided light receiving element groups 37, and obtains the difference therebetween to generate a tracking error signal.
Next will be described the shift of the objective lens 33 against a thread (for moving the optical pickup 30 in the radial direction of the optical disk 38) not shown in the figure. A lens holder 32 with the objective lens 33 mounted thereon is supported movably onto the tracking actuator 31 so as to be movable in the radial direction (tracking direction) of the optical disk 38 against the thread. Therefore, the lens holder 32 is driven by the tracking actuator 31, where a tracking drive voltage is supplied to the tracking actuator 31 through a tracking actuator drive circuit 40 based on the tracking error signal output from the tracking error detection circuit 39 to perform tracking servo control.
In the case above, the lens holder 32, i.e., objective lens 33 shifts in the tracking direction with, for example, an eccentricity of the optical disk 38, where the optical disk 38 has a high rotational frequency (eccentricity frequency) of, for example, several to several tens of Hz, while the driving operation for the thread has a low frequency of, for example, 1 Hz. Therefore, in the case of tracking servo, since the objective lens 33 only follows the track groove on the optical disk 38, the objective lens 33 is to shift against the thread due to an eccentricity of the track groove.
FIG. 4 is a block diagram showing a circuit and a low-pass filter connected thereto for canceling the offset of focusing drive signals and tracking drive signals in a system LSI (Large Scale Integration) 1 provided in a conventional optical disk recording and reproducing apparatus.
It is noted that the system LSI 1 comprises, though not shown in the figure, an RF amplifier for amplifying RF signal, i.e., read-out signal from an optical pickup when reproducing an optical disk; a digital signal processing section for performing signal demodulation processing and error correction processing in accordance with the data format of the optical disk after converting an RF signal output from the RF amplifier into digital data and then storing the generated data in a RAM; a stream demultiplexing section for demultiplexing data stream output from the digital signal processing section into audio data, subpicture data and video data; a subpicture decoder for performing predetermined decode processing by inputting the subpicture data output from the stream demultiplexing section; a video decoder for performing predetermined decode processing by inputting the video data output from the stream demultiplexing section; a video processor for synthesizing data output from the video decoder and data output from the subpicture decoder; a video encoder for converting the synthesized data output from the video processor into a video signal for display to display an image on a display device; and a system controller for controlling the above-described components.
In FIG. 4, the system LSI 1 comprises: an A/D port 2 as an input port for receiving and analog/digital-converting a focusing drive signal, which is output to a focusing actuator drive circuit not shown in the figure, as a feedback signal; a reference voltage value storing means 4 for storing the digital voltage value of a first focusing drive signal input through the A/D port 2 as a reference voltage value; a reference voltage value storage control means 3 adapted to control in such a manner as to store the digital voltage value of the first focusing drive signal in the reference voltage value storing means 4 as a reference voltage value; a comparing means 5 for comparing the reference voltage value and the digital voltage value of a focusing drive signal input through the A/D port 2; a D/A conversion and offset cancel circuit 10 (including a circuit for performing the same processing as follows also for tracking drive signals) for digital/analog-converting a digital focusing drive signal generated based on a focusing error signal, which is input from the digital signal processing section not shown in the figure, into an analog focusing drive signal and for adjusting the analog focusing drive signal so as not to be offset in accordance with a comparison result of the comparing means 5 and then to be output; and a focusing output port 11 for outputting the analog focusing drive signal.
The system LSI 1 also comprises: an AMD port 6 as an input port for receiving and analog/digital-converting a tracking drive signal, which is output to the tracking actuator drive circuit 40 (refer to FIG. 3), as a feedback signal; a reference voltage value storing means 9 for storing the digital voltage value of a first tracking drive signal input through the A/D port 6 as a reference voltage value; a reference voltage value storage control means 8 adapted to control in such a manner as to store the digital voltage value of the first tracking drive signal in the reference voltage value storing means 9 as a reference voltage value; a comparing means 7 for comparing the reference voltage value and the digital voltage value of a tracking drive signal input through the A/D port 6; a D/A conversion and offset cancel circuit 10 for digital/analog-converting a digital tracking drive signal generated based on a tracking error signal, which is input from the digital signal processing section not shown in the figure, into an analog tracking drive signal and for adjusting the analog tracking drive signal so as not to be offset in accordance with a comparison result of the comparing means 7 and then to be output; and a tracking output port 12 for outputting the analog tracking drive signal.
The optical disk recording and reproducing apparatus also comprises a low-pass filter 13 for allowing only a low-frequency component of the analog focusing drive signal output from the focusing output port 11 to pass therethrough to be output as a focusing drive signal for the focusing actuator drive circuit and also to be fed back to the A/D port 2; and a low-pass filter 14 for allowing only a low-frequency component of the analog tracking drive signal output from the tracking output port 12 to pass therethrough to be output as a tracking drive signal for the tracking actuator drive circuit 40 (refer to FIG. 3) and also to be fed back to the A/D port 6.
Meanwhile, an output terminal of the above-mentioned low-pass filter 14 is connected with the tracking actuator drive circuit 40 and the A/D port 6 to output tracking drive signals, where when the temperature of the tracking actuator drive circuit 40 is increased due to its operation, tracking drive signals can be offset.
When the temperature of the tracking actuator drive circuit 40 is increased, the resistance of the circuit is also increased due to the temperature characteristics of resistors, transistors, etc. that constitutes the tracking actuator drive circuit 40, and thereby the coil current for the tracking actuator 31 (refer to FIG. 3) is reduced, resulting in a slowdown in the operation of the tracking actuator 31. In order to compensate for the reduction, a servo control section not shown in the figure functions in such a manner as to raise the voltage of tracking drive signals for the tracking actuator drive circuit 40.
However, since the currently set resistance of a feedback line L1 for tracking drive signals is high, that is, the resistance of a resistor R2 (e.g., 22KΩ) in the low-pass filter 14 is high, the reference voltage value of the reference voltage value storing means 9 that is set by a first tracking drive signal is lower than the original voltage value, whereby a tracking drive signal with a lower voltage value than that of the tracking drive signal that should originally be output to the tracking actuator drive circuit 40 is to be output from the low-pass filter 14.
Therefore as mentioned above, when the temperature of the tracking actuator drive circuit 40 is increased, the resistance of the circuit is also increased due to the temperature characteristics of resistors, transistors, etc. that constitutes the tracking actuator drive circuit 40, and thereby the coil current for the tracking actuator 31 (refer to FIG. 3) is reduced, resulting in a slowdown in the operation of the tracking actuator 31, and in order to compensate for the reduction, the servo control section not shown in the figure functions in such a manner as to raise the voltage of tracking drive signals for the tracking actuator drive circuit 40. Consequently, the voltage of tracking drive signals output from the low-pass-filter 14 is raised. However as shown in FIG.5, a tracking drive signal TD is to be offset as represented by OS, where performing recording and reproducing operations as it is suffers from troubles.
The prior art disclosed in Japanese Patent Laid-Open Publication No. Hei 8-138255 is adapted to switch the connection of resistors in accordance with pit regions or group regions on an optical disk to adjust the gain of focusing error signals or tracking error signals, but not adapted to prevent tracking drive signals from being offset when the temperature of the tracking actuator drive circuit is increased.
The prior art disclosed in Japanese Patent Laid-Open Publication No. Hei 5-182206 relates to focusing control, but not adapted to prevent tracking drive signals from being offset when the temperature of the tracking actuator drive circuit is increased.
The prior art disclosed in Japanese Patent Laid-Open Publication No. Sho 61-11944 is adapted to be capable of correcting an error in a focus deviation detection signal even if generated due to the temperature change in the apparatus, but not adapted to prevent tracking drive signals from being offset when the temperature of the tracking actuator drive circuit is increased.