JPH0423331B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical playback device, and in particular to a focus system for a disc player that uses a laser beam to extract playback signals from a disc on which audio signals, video signals, etc. are recorded in the form of pits. It is most suitable for use in servo systems or tracking servo systems.

In this type of disk player, a focus servo system and a track are used to keep the laser spot in focus with respect to the disk surface in order to accurately follow a spiral track formed by vertical rows of pits with a width of 1 μm or less. The optical system is provided with a tracking servo system that deflects the laser spot in the width direction. The focus servo system is equipped with a moving magnet or moving coil type electromagnetic drive device that controls the objective lens in the optical axis direction. The focus error signal is generated by converting changes in the focusing state of the reflected beam due to changes in the height of the disk reflecting surface into changes in the shape of the image formed by the cylindrical lens, It is detected by a pair of photodiodes.

The tracking servo system is equipped with an electromagnetic drive device that moves the optical axis of the objective lens in the width direction of the track, or a galvanometer mirror device that deflects the light beam incident on the objective lens in the width direction of the track. In a well-known optical system, a reading laser beam is divided by a diffraction grating into three beams whose positions are slightly shifted in each of the longitudinal and width directions of the track to form three spots, and each reflected beam is separated into three beams. It is detected by a photodiode, and a tracking error is also detected based on the difference in the amount of light received by the two outer lights.

In addition to the above-mentioned focus servo and tracking servo, a tangential servo that swings a laser beam in the tangential direction of the track is sometimes used in order to eliminate time axis fluctuations in the reproduced signal.

These servo systems are mainly provided to prevent playback signals from deteriorating due to disc surface runout, eccentricity, and rotational fluctuations of the spindle motor. For example, regarding frequency components
Must have servo gain of 60dB or more.

Furthermore, when reproducing data after accessing a desired position on the disk, a jump pulse signal of a constant width may be applied to the tracking servo system to move the beam in large steps. In this case, each servo system needs to have a response band from DC to 10 KHz in order to stably control the jump and the transient response after the jump and to strengthen the vibration resistance of the player.

A tracking servo system or a focus servo system has a servo device (objective lens or galvano mirror) with a minute mass, and has a structure similar to a speaker (tweeter) consisting of a diaphragm and a voice coil. Therefore, the servo device is several KHz
It vibrates in the audible range, and its mechanical noise can be harsh, especially in the case of acoustic playback. In particular, the low-frequency components of the reproduced audio information (EFM signal or NRZ encoded PCM signal) may be superimposed on the servo error information, or noise caused by scratches or fingerprints on the disk may be superimposed on the servo error information, so the focus device or tracking device may mechanical noise is likely to occur due to unnecessary movement. Also, this type of servo system is generally a second-order delay system, so
For the phase compensation, 1 of the frequency-gain characteristic
The frequencies around ~3KHz are often raised, and mechanical noise is particularly likely to be amplified in this frequency band.

The present invention has been made in view of the above-mentioned problems, and is intended to reduce mechanical noise during reproduction without reducing the required performance of a tracking servo system or a focus servo system.

The optical playback device of the present invention is a disk playback device that reads information recorded on a track of a disk by irradiating a read beam from a pickup so as to follow the track of the disk. tracking control means for controlling a read beam from a pickup to follow a track on a disk by supplying a servo signal based on a tracking error signal generated based on the tracking error signal to a servo device via a phase compensation circuit; and access control means for controlling access of the reading beam from the pickup to a desired position on the disk.

The phase compensation circuit includes a time constant circuit and a changeover switch that change the frequency characteristics of a servo system that controls the tracking state of the reading beam, and the access control means moves the reading beam at high speed to a desired reproduction track. The changeover switch is changed depending on the time and playback, and during playback, the time constant circuit of the phase compensation circuit is controlled so that the gain of the servo system is reduced in a predetermined portion of the audible frequency band from the gain during access. It is characterized in that it is made to look like this.

Next, the present invention will be explained based on examples. FIG. 1 is a block diagram of the optical system and servo system of an optical disc player to which the present invention is applied. On the disk 1, information is recorded as columns of concave or convex topological structures (pits) along a spiral track. The reproduced signal is generated by guiding the light beam formed by the laser 2 in the pickup to a phase structure along the track of the disk 1 via the optical system, and then transmitting the reflected light modulated by this phase structure to the photodiode array 20. It is obtained by converting it into a signal.

The optical system includes mirrors 3 and 4, a diffraction grating 5, a magnifying lens 6, a polarizing beam splitter 7, a λ/4 wavelength plate 8, a mirror 9, a focusing lens 10, and a cylindrical lens 1.
It consists of 1. The beam emitted from the laser 2 passes through mirrors 3 and 4 and enters the diffraction grating 5, where it is divided into three beams. Furthermore, the beam that has been expanded by the magnifying lens 6 and passed through the beam splitter 7 is converted into a circularly polarized beam by the λ/4 wavelength plate 8, and is applied to the focusing lens 10 (or objective lens) from the mirror 9. The focusing lens 10 is attached to a servo device whose position can be controlled in two axes: a tracking direction (track width direction), a focus direction (beam optical axis direction), and a focus direction (beam optical axis direction).

The laser beam is controlled by a focusing lens 10 to focus a reading spot on a track on the surface of the disk 1. This reading spot consists of three fine spots along the track direction and slightly shifted in position in the track width direction.

The beam directed to the central fine spot is modulated and reflected by pits in the disk, thereby reversing the direction of rotation of the circularly polarized light. This reflected beam is picked up by a focusing lens 10, follows the optical path in the opposite direction, is converted into linearly polarized light again by a λ/4 wavelength plate 8, is further reflected by a polarizing beam splitter 7, and passes through a cylindrical lens 11. The photodiode array 20 is reached. The outer two of the three fine spots reach the photodiodes 19 and 21 through the same return optical path as described above.

The photodiode array 20 is composed of elements divided into four parts, and adders 12 and 13 add the outputs of two pairs of photodiodes on each opposite side, and adders 14 add the outputs of the adders 12 and 13. By adding the signals, an RF signal as reproduction information can be obtained. This reproduced RF signal is sent from terminal 15 to a demodulation circuit (not shown).

The focus error f is detected by the subtracter 16 calculating the difference between two signals obtained by adding the outputs of each opposing side of the photodiode array 20 by the adders 12 and 13. Further, the tracking error t is detected by calculating the difference between the outputs of the two photodiodes 19 and 21 using the subtracter 17.

The focus error signal f and the tracking error signal t are applied to the biaxial deflection devices 22 and 23 of the focusing lens 10, respectively, so that the focusing lens 10 moves in the focusing direction (direction of arrow A) and the tracking direction (direction of arrow B). direction). Note that the mirror 9 may be a galvanometer mirror and a tracking error signal may be applied to it.

When randomly accessing any track on the disk 1 before starting playback, a jump pulse j is generated from the access control circuit 25, and this jump pulse j is added to the tracking error signal t by the adder 24 to determine the deflection. device 23. As a result, the reading beam is stepped towards the access target position, for example over a width of 100 tracks. Note that the entire pickup optical system is moved by a linear feed motor that moves it, and a separate servo system follows the step feed of the beam.

Next, Figure 2 is a block circuit diagram of the focus servo system or tracking servo system in Figure 1.
Figure 3A shows the frequency-gain characteristics of the servo loop.
FIG. 3B shows the frequency-phase characteristics, respectively.

The frequency-gain characteristics of a servo-based electromechanical transducer have a gain of 60 dB or more at a disk rotation frequency of 3 to 8 Hz, as shown by the solid line in Figure 3A, and a resonance that exists at 10 to 50 Hz. At frequencies above this point, the transmission characteristics of a second-order lag system are exhibited, which are attenuated towards higher frequencies. The phase characteristics are shown in Figure 3B.
As shown by the solid line, it has a band with a phase rotation of -180° or more.

In order to obtain a phase margin and avoid unstable operation at high frequencies, the subtracter 1 in Fig. 1 is used as shown in Fig. 2.
The focus error signal f or the tracking error signal t detected at 6 or 17 is given to the drive circuit 28 through the phase compensation circuit 27. As shown in the frequency-gain characteristic diagram of FIG. 4, the phase compensation circuit 27 improves the frequency characteristic in the 1 to 3 KHz band around the cutoff frequency e (servo system loop gain is 0 dB). As a result, the frequency-gain characteristics of the servo loop are as shown in Figure 3A.
The frequency-phase characteristics are improved as shown by the dotted line in FIG. 3B.

According to the embodiment of the present invention, the constant that determines the gain of the phase compensation circuit 27 shown in FIG. 2 is switched between normal playback (including pause) and access. As the switching signal, a mode signal M generated from the access control circuit 25 in FIG. 1 at the time of access is used.

FIG. 5 shows a specific circuit of the phase compensation circuit 27 of this embodiment, and FIG. 6 shows its frequency-gain characteristics. In FIG. 5, the servo error signal is applied to the non-inverting input of operational amplifier 29 through resistor _R1 , and its output is applied to drive circuit 28 in FIG. The output of the operational amplifier 29 is fed back to the inverting input via a low pass filter consisting of resistors _R2 to _R4 and a capacitor _C1 . As a result, the amount of feedback at frequencies lower than the cutoff frequency (for example, 1 KHz) becomes larger, giving the frequency characteristics shown in FIG. 4.

A resistor R ₅ , a capacitor C ₂ and a switch 30 are connected in series between the non-inverting input of the operational amplifier 29 and the ground, and the switch 30 is turned off by the mode signal M during the access. Therefore, during normal playback or pause, the switch 30 is turned on, and the input of the operational amplifier 29 is connected to a low-pass filter consisting of resistors R ₁ , R ₅ and capacitor C ₂ . This low-pass filter exhibits an attenuation characteristic at a frequency of 1/2πC ₂ (R ₁ +R ₅ ) (for example, 200 Hz) or higher, and reaches a certain amount of attenuation at a frequency of 1/2πC ₂ R ₅ (for example, 1 KHz).

Therefore, the overall frequency-gain characteristic of the phase compensation circuit 27 during reproduction is as shown by the dashed-dotted line in FIG. That is, the audible band component of the servo error signal is attenuated. As a result, the overall frequency characteristic of the servo loop is such that the gain in the audible band of 200 Hz or higher decreases, as shown by the dashed line in Figure 3A.
This makes it possible to attenuate the harsh mechanical vibration noise of the servo device.

Since the switch 30 is turned off by the mode signal M during access, the frequency characteristics of the phase compensation circuit 27 are as shown by the solid line in FIG. 6, and the necessary response frequency band is as shown by the dotted line in FIG. 3A. Secured.

During playback, the servo gain in the low range (near the disk rotation period) is not changed and is maintained at 60 dB or more, so there is no effect on the focusing and tracking performance for disk surface runout and eccentricity.

Next, FIG. 7 shows a modification of the phase compensation circuit shown in FIG. 5, and FIG. 8 shows its frequency-gain characteristics.
In this modification, in order to further reduce the high frequency gain of the low pass filter operated by the switch 30, a capacitor _C3 is connected in parallel with the resistor _R5 .
As a result, the high-frequency gain of the phase compensation circuit 27 during normal reproduction is further reduced as shown by the dashed line in FIG. This reduces audible vibrations of the servo device at higher frequencies (eg, 5KHz or higher).

FIG. 9 shows yet another modification of the phase compensation circuit, and FIG. 10 shows its frequency-gain characteristics.
In this modification, the phase characteristics are improved during reproduction (when the switch 30 is turned on). In the phase compensation circuit shown in FIG. 5 or 7, when the switch 30 is turned on, the cutoff frequency changes as shown in FIG. 3A.
c′, and the phase margin at this point decreases. For this reason, in FIG. 9, the switches 30 are connected in two so that when the switch is turned on, a new capacitor _C4 is connected in parallel to the capacitor _C1 .

The frequency-gain characteristic of the phase compensation circuit in FIG. 9 shows that the frequency of the gain enhancement part for phase compensation is the same as that of the capacitor C ₄ as shown by the dashed line in FIG. 10.
It is shifted to the lower frequency side by the amount of . As a result, the peak of the phase characteristic of the servo system moves to a lower frequency range as shown by the dashed line in FIG. 3B, and a phase margin is secured at the cutoff frequency c' in FIG. The stability of the servo system is improved when turned on.

In the embodiments described above, since the level of vibration noise differs depending on each servo device, it is preferable that the amount of attenuation of the servo gain in the audible band is determined according to the noise level. In some cases, the present invention can be applied to only either the focus servo system or the tracking servo system to obtain a noise reduction effect, but it may be applied to both.

Furthermore, in each of the embodiments shown in FIGS. 5, 7, and 9, the servo gain switching switch 30 and the time constant circuit (C ₂ , R _5, etc.) may be inserted into the feedback circuit of the operational amplifier 29. Or operational amplifier 29
It may be provided on the output side.

As described above, the present invention switches the gain in the audible band of the servo system that controls the reading beam between playback and access, so that the gain is reduced during playback. Therefore, according to the present invention, the unpleasant audible vibration noise of the servo device during playback can be reduced, and the required frequency response band can be secured during access.

[Brief explanation of drawings]

FIG. 1 is a block diagram of the optical system and servo system of an optical disc player to which the present invention is applied, and FIG.
The figure is a block circuit diagram of the optical servo system, Figure 3A is a frequency-gain characteristic diagram of the servo system, Figure 3B is a frequency-phase characteristic diagram, and Figure 4 is the frequency of the phase compensation circuit of Figure 2. - gain characteristic diagram, FIG. 5 is a circuit diagram of a phase compensation circuit showing an embodiment of the present invention, FIG. 6 is a frequency-gain characteristic diagram of FIG. 5, and FIGS. 8 and 10 are frequency-gain characteristic diagrams corresponding to FIG. 7 and FIG. 9, respectively. In addition, in the symbols used in the drawings, 1...disk, 2...laser, 10...focusing lens, 19,2
DESCRIPTION OF SYMBOLS 1... Photodiode, 20... Photodiode array, 22, 23... Biaxial deflection device, 25... Access control circuit, 30... Switch.

Claims (1)

[Scope of Claims] 1. A disc playback device that reads information recorded on a track of a disc by irradiating a read beam from a pick-up so as to follow the track of the disc, which reads information recorded on a track of the disc based on the read beam from the pick-up. tracking control means for controlling a read beam from a pickup to follow a track on a disk by supplying a servo signal based on a tracking error signal generated by the above to a servo device via a phase compensation circuit; access control means for controlling access of the reading beam from the pickup to a desired position on the disk, and the phase compensation circuit includes a time constant circuit and switching means for switching the frequency characteristics of a servo system that controls the tracking state of the reading beam. The access control means switches the changeover switch during access and playback to move the read beam at high speed to a desired playback track, and controls the time constant circuit of the phase compensation circuit during playback. An optical playback device characterized in that the gain of the servo system is reduced in a predetermined portion of the audio frequency band compared to the gain at the time of access.