JPH0547900B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of a three-beam type tracking control device in an optical information recording/reproducing device or an optical information reproducing device, and is particularly effective when a magneto-optical recording medium is used.

First, a conventional tracking control device will be explained with reference to FIGS. 1 and 2. In Figure 1, 1 is an information track, 2 and 3 are tracking beams, and the tracking beams 2 and 3 are irradiated so that half of the information track 1 is on the opposite side in the width direction (disc radial direction). Sometimes, the main beam 4 used for recording and reproduction or only for reproduction is located directly above the information track 1 (the center of the main beam 4 is located directly above the information track 1).
irradiated at a position that coincides with the center of Further, each of the beams 2 to 4 is at a position shifted from the information track 1 by a constant distance in the length direction (in the direction of the arrow). 5, 6, and 7 are photodetectors; photodetector 5
receives the reflected light from the tracking beam 2 on the information track 1, the photodetector 6 receives the reflected light from the tracking beam 3, and the photodetector 7 receives the reflected light from the main beam 4. Convert.
8 and 9 are current-voltage conversion circuits that convert signals input from the photodetectors 5 and 6 into voltage values; 10 is a differential amplifier; the signal from the current-voltage conversion circuit 8 is input to the inverting input terminal of the Signals from the current-voltage conversion circuit 9 are respectively input to the input terminals. 11 is a phase compensating circuit, and 12 is an amplifier for driving the tracking mirror 13.

When reproducing information recorded on the information track 1, a light beam is emitted from a light source (not shown), and each of the beams 2 to 4 is irradiated onto the information track 1 in the state shown in FIG. 1, for example. The reflected light enters the photodetectors 5 to 7 through a known optical path and is photoelectrically converted. The signal photoelectrically converted by the photodetector 7 is amplified by an amplifier (not shown) and taken out as a reproduced signal. In addition, the signals photoelectrically converted by the photodetectors 5 and 6 are converted into voltage values by current-voltage converters 8 and 9, respectively, and differentially amplified by a differential amplifier 10, and the tracking signal obtained thereby is phase-corrected. The signal is sent to a circuit 11 and an amplifier 12, thereby driving a tracking mirror 13 and performing tracking control. In other words, as shown in FIG. 1, when the tracking beams 2 and 3 are half-stretched on opposite sides of the information track 1, the main beam 4 connects a spot directly above the information track 1. , the tracking mirror 13 is driven so that only half of the tracking beams 2 and 3 are placed on the information track 1 (as shown in FIG. 1).

However, in the above-mentioned device, the tracking beam 2 and the tracking beam 3 to be compared are shifted by a certain distance in the direction of the information track 1 (in the direction of the arrow in FIG. 1). When the reflectance of the disk differs in the circumferential direction as shown in FIG. 3, the output of the differential amplifier 10 fluctuates as shown in FIG. 3b (differentiated form of FIG. 3a). Since this fluctuation occurs regardless of the presence or absence of the information track 1, the fluctuation directly becomes noise in the tracking signal, resulting in incomplete tracking control. Particularly in a magneto-optical disk where the signal depending on the presence or absence of the information track 1 is small, the influence of such noise is large.

In order to solve this problem, the output signal of the light receiving means that receives the preceding tracking beam is delayed by an amount sufficient to compensate for the time difference corresponding to the beam interval, and the delayed output signal is The tracking signal may be generated based on the level difference between the output signal of the light receiving means that receives the reflected light of the following beam.

However, when a disk-shaped recording medium is rotated at a constant angular velocity, the linear velocity of the beam relative to the recording medium differs between the inner and outer circumferences of the recording medium, so the time difference is and the outer periphery. Therefore, if the delay time is kept constant, a problem arises in that the time difference cannot be correctly compensated for either the inner circumference or the outer circumference of the recording medium.

An object of the present invention is to provide a tracking control device that can accurately remove noise from a tracking signal when a disk-shaped recording medium is rotated at a constant angular velocity.

To achieve this object, the present invention provides a method for eliminating the time difference between the output signals of the two light receiving means caused by the longitudinal spacing of the information tracks of the two tracking beams. and a delay control means that changes the delay time of the delay means according to the position in the radial direction on the recording medium to which the tracking beam is irradiated, and the tracking signal generating means is provided with a delay means that delays the output signal of the tracking signal. A tracking signal is generated based on the level difference between the output signals of the two light receiving means with no time difference using the output signals of the light receiving means.

Hereinafter, the present invention will be explained in detail based on illustrated embodiments.

FIG. 4 is a block diagram of a tracking control device showing one embodiment of the present invention. The same parts as in FIG. 2 are denoted by the same symbols. 14 is a pulse generator, and the high level time T of the output pulse from the pulse generator 14 is in the disk circumferential direction (in the direction of the arrow in FIG. 1).
is equal to the time difference between tracking beam 2 and tracking beam 3 passing through the same point (on information track 1). 15 is an inverter, 16,1
Reference numeral 7 denotes a sample and hold circuit that samples and holds the signals inputted from the current-voltage converters 8 and 9 at the rising edge of the output pulse from the pulse generator 14 or inverter 15, respectively. The sample and hold circuit 18 samples and holds the output from the sample and hold circuit 16 at the rising edge of the output pulse from the inverter 15.

Next, the operation will be explained using FIGS. 5a, b, and c. Since the two tracking beams 2 and 3 irradiated onto the disk have a certain interval (with respect to the circumferential direction of the disk), this reflected light is photoelectrically converted by photodetectors 5 and 6. When converted into voltage values by the current-voltage conversion circuits 8 and 9, the respective voltages are shown in Figure 5a (here, the voltage in the area without the information track 1 is shown, and this voltage is used for actual tracking). Furthermore, the voltage corresponding to the information track is added), so that the result is as shown in A and B, and there is a time difference t ₀ . For example, if the interval between the two tracking beams 2 and 3 (on information track 1) is 200 μm and the disk rotation speed is 1800 rpm, then on information track 1 with a radius of 150 mm, this time difference t _{0 is 7 μsec, and on information track 1 with a radius of 50 mm, the time difference t 0} is 7 μsec. 1, it is 21 μsec, which is inversely proportional to the scanning radius. Therefore, when the angular velocity is constant, a signal representing the scanning radius is sent from an optical head (not shown) to the pulse generator 14, and the pulse generator 14 thereby controls the duty ratio of the output pulse.

The signal from the current-voltage conversion circuit 8 (A in FIG. 5A) is output to the sample-and-hold circuit 16, and at this time, the signal from the pulse generator 14 to the sample-and-hold circuit 16 as shown in C in FIG. Therefore, the signal is sampled and held by the sample and hold circuit 16 at the rising edge of the output pulse from the pulse generator 14, as shown in FIG. 5c.
The voltage becomes a voltage as shown in FIG. 5c, and is further sent to the sample and hold circuit 18, where it is sampled and held at the rising edge of the output pulse from the inverter 15, resulting in a voltage as shown in FIG. 5c. On the other hand, the signal from the current-voltage conversion circuit 9 ((b) in FIG. 5a) is output to the sample-and-hold circuit 17, and at this time, the signal from the inverter 15 to the sample-and-hold circuit 17 as shown in FIG. If the output pulse is input,
Therefore, the signal is sampled and held by the sample and hold circuit 17 at the rising edge of the output pulse from the inverter 15, and becomes a voltage as shown in FIG. 5C. Therefore, voltages with no time difference t ₀ are input to the differential amplifier 10.
In other words, the state is the same as when the reflected lights of the two tracking beams 2 and 3 are received at the same point on the information track 1 and photoelectrically converted, so even if fluctuation occurs in the disk circumferential direction, the tracking beam A signal indicating whether or not 2 and 3 are on the information track 1 can be obtained correctly, and tracking control can be performed using a tracking signal with a good S/N ratio.

In this embodiment, the photodetectors 5 and 6 correspond to the light receiving means of the present invention, the differential amplifier 10 corresponds to the tracking signal generating means, the sample and hold circuits 16 and 18 correspond to the delay means, and the pulse generator 1
4 corresponds to delay control means.

In this embodiment, sample and hold circuits 16, 1
7 and 18 to eliminate the time difference t ₀ between the outputs obtained from the two tracking beams 2 and 3;
It is also easy to delay one output using a delay means such as .

As explained above, according to the present invention, in order to eliminate the time difference between the output signals of the two light receiving means caused by the interval in the length direction of the information tracks of the two tracking beams, one of the two light receiving means and a delay control means that changes the delay time of the delay means according to the radial position on the recording medium to which the tracking beam is irradiated. Since the tracking signal is generated by the level difference between the output signals of two light receiving means with no time difference using the output signals of
When the disk-shaped recording medium is rotated at a constant angular velocity, noise in the tracking signal can be accurately removed.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the relationship between a general information track and three light beam spots, Fig. 2 is a block diagram of a conventional tracking control circuit, and Fig. 3 is a diagram showing the uneven reflectance of the disk and the fluctuation of the tracking signal caused by this. FIG. 4 is a block diagram of a tracking control circuit showing one embodiment of the present invention, and FIG. 5 is a time chart showing inputs, control inputs, and outputs of each sample and hold circuit. 1... Information track, 2, 3... Tracking beam, 4... Main beam, 5, 6, 7... Photodetector, 10... Differential amplifier, 12... Amplifier, 1
3... Tracking mirror, 14... Pulse generator, 15... Inverter, 16, 17, 18...
Sample and hold circuit.

Claims (1)

[Scope of Claims] 1. When an information track on a disk-shaped recording medium rotating at a constant angular velocity is irradiated with a main beam and two tracking beams at a constant interval in the length direction of the information track. A tracking control device comprising two light receiving means for detecting reflected light generated in the two tracking beams, and a tracking signal generating means for generating a tracking signal based on a level difference between the output signals of the two light receiving means. In order to eliminate the time difference between the output signals of the light receiving means caused by the lengthwise spacing of the information tracks, a delay means for delaying the output signal of either one of the two light receiving means, and the tracking beam is irradiated. and a delay control means for changing the delay time by the delay means according to the position in the radial direction on the recording medium, and the tracking signal generation means uses the output signal of the delay means to detect the two received lights without a time difference. A tracking control device characterized in that a tracking signal is generated based on a level difference between output signals of the means.