JPH0544099B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a tracking control device used for recording and reproducing recording media such as optical disks and magneto-optical disks. The present invention relates to a tracking control device that performs tracking control based on changes in beam reflected light from columns.

(Background of the Invention) Conventionally, as a tracking control method in an optical disk device or a magneto-optical disk device, in addition to a main beam for recording and reproduction, two sub-beams for tracking are irradiated onto a recording medium, and the reflected light is A so-called two-beam method is known in which tracking control is performed based on changes.

Furthermore, tracking control using the two-beam method can be divided into the following two types.

One method is to provide concentric or spiral track grooves in advance on an optical disk or magneto-optical disk, and control tracking based on reflection information such as reflected light or diffracted light in the two-beam method by the track grooves. It is.

Another method is to write information bits in precise concentric circles or in a spiral during recording without providing track grooves on optical disks or magneto-optical disks to form tracks made up of bit strings, and then to create tracks made up of bit strings during playback. This is a tracking control method based on reflected light information using the two-beam method.

The recording method of the information bit string used for tracking control is that in the case of optical disks, bits (uneven pits) are physically formed by irradiation with laser light, while in the case of magneto-optical disks, bits (uneven pits) are formed on the disk. Bits are formed by vertically magnetizing the perpendicularly magnetized film on the surface upward or downward. Therefore, for optical disks, changes in reflectance are detected by the two-beam method, while for magneto-optical disks, reflection information is detected as rotation of the beam polarization plane (Kerr rotation or Faraday rotation) due to the perpendicular magnetization direction. is obtained.

FIG. 8 schematically shows a disk recording state when an information bit string without track grooves is used as a track. 1 is a recording medium such as an optical disk or a magneto-optical disk;
Bits 2 recorded as concavo-convex pits or in the direction of perpendicular magnetization are formed on the surface, and the direction in which the bits 2 are arranged becomes a track 3 shown by a broken line. In the two-beam method for recording media with such recording bits, in addition to the main beam Bm for recording and reproduction,
Using two sub beams Bs1 and Bs2, the sub beam
Tracking control is performed so that the main beam Bm is located at the track center based on the information reflected from bit 2 by Bs1 and Bs2.

FIG. 9 shows a tracking control circuit using a conventional two-beam method, in which light from a light source 4 passes through a lens 5, a diffraction grating 6, a beam splitter 7, and a reflecting mirror 8, and then passes through an objective lens 9 into three beams. The track 3 of the recording medium 1 is irradiated as a beam. The reflected beam from the bits forming track 3 is polarized at right angles by beam splitter 7 to form the main beam.
The reflected light of Bm is received by the detector Dm and taken out as a reproduced signal. On the other hand, the secondary beam Bs
1 and Bs2 are received by detectors Ds1 and Ds2, and provided to a differential amplifier 11.

If the three beams are now in the state shown in FIG. 8, the sub-beams Bs1 and Bs2 are equally shifted from the bit center, so the outputs of the detectors Ds1 and Ds2 are equal. Here, since bit 2 moves relative to the three beams Bm, Bs1, and Bs2 due to disk rotation, the outputs of detectors Ds1 and Ds2 change depending on the presence or absence of bit 2, and the frequency is, for example, It is about 1MHz to 10MHz.

On the other hand, differential amplifier 11 and servo amplifier 12
And the response frequency of the rotary drive motor 13 is 10 to 20K.
Hz, the presence or absence of bit 2 does not affect high frequency components. Therefore, the secondary beams Bs1 and Bs
2 is used to detect the average value of reflected light due to movement of the bit string.

On the other hand, if a relative positional shift occurs between the track and the irradiation beam (shift in the direction perpendicular to the track) due to disturbance etc. during operation, the difference in the average amount of reflected light detected by the sub-beams Bs1 and Bs2 will be different. The tracking error is detected by the dynamic amplifier 11, and after being amplified by the servo amplifier 12, it is applied to the drive motor 13, and by rotating the reflecting mirror 8, the sub beams Bs1 and Bs2 are generated.
Tracking control is performed so that the difference in the average amount of reflected light detected at is always zero, in other words, so that the main beam Bm is always at the center of the track.

(Problems to be Solved by the Invention) However, when the bit string during recording is used as a track without providing a track groove, the bit string formed by uneven pits on the optical disk and the perpendicular magnetization on the magneto-optical disk In both bit rows, the size of each bit in the track direction and the vertical direction (width direction) is not constant, and the average amount of reflected light changes depending on the size of the bit.

In other words, if bits are pre-recorded, such as on playback-only video discs or compact discs, there is no problem if you control the power of the laser beam during recording to make the bit widths the same. In order to control the bit width to keep it constant, recording devices become complicated, and even for re-writable optical discs and magneto-optical discs, the device configuration to keep the bit width constant during re-recording becomes complicated. and the cost of recording and reproducing equipment becomes expensive.

Next, we will discuss the influence of changing the bit size on tracking control using the two-beam method in the 10th section.
This will be explained with reference to the figures.

FIG. 10 shows the relationship between tracking error signals when the bit is large (a in the figure) and when the bit is small (b in the figure).

As is well known, when the recording frequency is low, the irradiation time of the recording beam onto the recording medium is long, and conversely, when the recording frequency is high, the irradiation time of the recording beam onto the recording medium is short.

On the other hand, recording media generally have a heat storage effect, and if the recording beam is irradiated for a long time, the bits will become larger in both the length and width directions due to the accumulation of heat. The recording characteristic is that the bits become smaller in both the length and width directions due to less heat accumulation.

Therefore, when the frequency of the recorded bits is low, the width of the recorded bits becomes large as shown in Figure 10a, and as the frequency increases, the width of the recorded bits becomes smaller as shown in Figure 10b. . Here, since the tracking error signal obtained by the conventional two-beam method is obtained as the difference in the average amount of reflected light between the sub-beams Bs1 and Bs2, the interval Ws between the sub-beams Bs1 and Bs2 in the direction perpendicular to the track is kept constant. In this case, as shown in Figure 10c, there is a relationship of TEl>TEh between the tracking error signal TEl (solid line) when the frequency is low and the tracking error signal TEh (dashed line) when the frequency is high, and the tracking error Detection sensitivity decreases as the frequency increases, resulting in the problem that tracking errors cannot be detected with constant sensitivity.

Furthermore, in the case of magneto-optical disks, because the bit array is formed by vertical magnetization upward or downward, there is little difference in reflectance depending on the presence or absence of bits, and the S/N ratio of the tracking error signal is said to be poor. There were also problems.

(Object of the Invention) The present invention has been made in view of these conventional problems, and provides a tracking method that allows tracking errors to be detected with constant sensitivity regardless of the state of recorded bits. The purpose is to provide a control device.

(Summary of the invention) In order to achieve this object, the present invention comprises two steps.
In a tracking control device that performs tracking control based on changes in reflected light from a bit string of recorded information on a recording medium using the beam method, the amplitude of the alternating current component of the beam reflected light generated when the beam passes through bits one after another is detected ( The tracking error is detected as the difference between the amplitude detection signals of the two sub-beams (envelope detection), and the tracking error is normalized by dividing this tracking error by the amplitude detection signal of the main beam or the amplitude addition signal of the sub-beams. The tracking error detection sensitivity is always kept constant regardless of the state of the bit.

(Embodiment) FIG. 1 is a circuit block diagram showing an embodiment of the present invention.

First, to explain the configuration, Dm is a detector that converts the reflected light of the main beam Bm into an electrical signal, Ds1, Ds
2 is a detector that converts the reflected light of the two sub-beams Bs1 and Bs2 into electrical signals. 14a, 14b,
14c is an amplifier, and each detector Ds1, Dm and
The detection signal representing the presence or absence of a bit from Ds2 is amplified. High-pass filters 15a, 15b, and 15c extract only alternating current components depending on the presence or absence of bits contained in each output signal from the amplifiers 14a to 14c. Rectifier circuits 16a, 16b, and 16c output a signal obtained by envelope detection of the alternating current component, which is an amplitude change in the alternating current component depending on the presence or absence of a bit output from each of the high-pass filters 15a to 15c. Reference numeral 11 denotes an operating amplifier, and rectifier circuits 16a and 1 output an amplitude detection signal of the sub beam.
The output of 6c is connected as an input, and a tracking error signal is detected as the difference between the amplitude detection signals of the two sub-beams. 17 is a divider, and operating amplifier 11
The output of the rectifier circuit 16b is connected to the numerator input terminal A, and the output of the rectifier circuit 16b which outputs the amplitude detection signal of the main beam is connected to the denominator input terminal B. Therefore, the divider 17 generates a division output of A/B, and the tracking error signal from the operational amplifier 11 is normalized by this division.

A switch 19 is turned on at the timing when the target tracking position is accessed, and supplies the normalized tracking error signal from the divider 17 to the servo amplifier 12 so that the tracking error signal becomes zero, that is, the main The rotation of the reflecting mirror is controlled by a motor so that the beam coincides with the center of the track consisting of the bit string. On the other hand, the circuit section consisting of the peak detection circuit 18, the AND gate 20, the inverter 22, and the flip-flop 21 is a circuit that determines the tracking start timing when the switch 19 is turned on, and the main beam reaches the target track when accessing the target track position. Since the output of the rectifier circuit 16b becomes maximum when the center position of the rectifier circuit 16b is reached, the peak value of the rectifier circuit 16b is detected by the peak detection circuit 18, and the tracking on/off signal 23 becomes H level due to tracking on. Switch 1 with flip-flop 21 set on the condition that
9 is turned on to start tracking control. Note that the output of the amplifier 14b that amplifies the detection signal of the main beam is taken out to the outside as a reproduced signal.

Next, the tracking control operation according to the embodiment shown in FIG. 1 will be explained with reference to the timing chart shown in FIG.

Assume that the bit string recorded on the recording medium is now in a recording state as shown in FIG. 2a, changing from large bits to small bits and then back to large bits as shown. A main beam Bm and sub-beams Bs1 and Bs2 are irradiated by the two-beam method to the bit string of such a recording medium, and the main beam Bm deviates from the track center by δ, causing 2
It is assumed that the sub-beam Bs1 of the sub-beams of the book is close to the center of the track. For convenience of explanation, the main beam Bm and the sub beams Bs1 and Bs2
The light intensities of are assumed to be equal.

If the bit row moves with respect to the beam due to rotation of the disk in the beam irradiation state shown in FIG. 2a, then the main beam and sub beam detectors Ds1, Ds1, Dm and Ds
Two detection outputs are obtained. That is, since the amplitude of the detection output of the main beam Bm in Fig. 2c is the largest and the sub-beam Bs1 is closer to the track center side due to the track deviation δ, the detection output of the sub-beam Bs1 in Fig. 2b is next. is large, and the output of the sub-beam Bs2 in FIG. 2d, which has deviated from the track, is the smallest. Furthermore, the detection output of each beam becomes an alternating current component with a frequency depending on the presence or absence of a bit, and where the bit is large, the alternating current frequency is low and the amplitude is large, while when the bit is small, the frequency is high and the amplitude is small. The reason why such a detection signal of the beam reflected light is obtained is, as shown in Figure 10, due to the difference in the bit width during recording and the frequency characteristics of the optical system (MTF effect), which is known from the well-known chemical theory.
It is obtained as a result of synthesis with Furthermore, the 10th
In the explanation of the figure, we pointed out the problem that the tracking error detection sensitivity decreases as the bit width becomes narrower, but in the case of Figure 2, in addition to the narrower bit width, the bit length also decreases. This also indicates that the performance of the optical system deteriorates due to the smaller size, that is, the MTF effect emphasizes amplitude fluctuations in the reproduced signal due to differences in bit recording frequencies.

In this way, the reproduced signals of the reflected beams detected by the detectors Ds1, Dm, and Ds2 are given to the high-pass filters 15a to 15c, and AC components depending on the presence or absence of bits are taken out, and the rectifier circuit 16 of the next stage
Detecting the amplitude change of the AC component in a to 16c,
That is, envelope detection is performed, and amplitude detection signals shown in FIG. 2e, f, and g are obtained.

Here, the amplitude detection signals shown in FIG. 2 e to g are as follows:
The signal level differs due to the difference in the irradiation area of each beam with respect to the bit.
Since the beam irradiation areas and shapes of the main beams Bm, Bs1, and Bs2 are the same, and the MTF effect of the optical system is also the same, the rate of signal change due to the bit size, which depends on the bit recording frequency, is the same for each amplitude detection signal. be equal.

Among the amplitude detection signals of each beam obtained by the rectifier circuits 16a to 16c, the outputs of the rectifier circuits 16a and 16c corresponding to the sub beams are given to the operational amplifier 11, and the tracking shown in FIG. 2h is performed as the difference between the two signals. An error signal is output from the differential amplifier 11. This tracking error signal includes a level change of the error signal depending on the bit size, and if the tracking error signal from the operational amplifier 11 is used as it is for tracking control, the tracking error will change depending on the bit size. The detection sensitivity will be different.

Therefore, in the present invention, the tracking error signal detected by the differential amplifier 11 is input to the numerator input terminal A of the divider 17, and the main beam from the rectifier circuit 16b is input to the denominator input terminal B of the divider 17. The amplitude detection signal is input, and the tracking error signal is normalized by dividing the tracking error signal by the amplitude detection signal of the main beam. That is, regarding the tracking error signal obtained by the operational amplifier 11 shown in FIG. 2h, the level change of the tracking error signal when the bit is large and when the bit is small is shown in FIG. 2f.
It is the same as the rate of change in level of the main beam amplitude detection signal shown in Figure 2, and by dividing the tracking error signal by the main beam amplitude detection signal, it is normalized to have a constant signal level Ve shown in Figure 2i. The tracking error signal can be obtained as the output of the divider 17. That is, the output of the divider 17 (A/
B) is independent of the bit frequency and this divider 17
The output Ve is proportional to the tracking error δ shown in FIG. 2a.

Next, the operation for starting the tracking control shown in the timing chart of FIG. 2 will be explained.

Usually, a large number of bit sequences, ie, tracks, are recorded on a recording medium, and when tracking control is not performed, the irradiation beam crosses the tracks one after another due to disturbances or the like.

FIG. 3 shows signal waveforms at various points when the irradiation beam crosses three tracks. That is, Figure 3a
is the detection signal of the sub beam Bs1, b is the main beam
Detection signal of Bm, c in the same figure is a detection signal of sub beam Bs2, d in the same figure is the output signal of the operational amplifier 11, e in the same figure
shows an input signal to the denominator input terminal B of the divider 17, and f shows an output signal of the divider 17.

As shown in the timing chart of Fig. 3, when the irradiation beam crosses three tracks before tracking control starts, the sub beams Bs1 and Bs2 become the main beam Bm as shown in Fig. 10. Since they are symmetrically separated by an interval Ws, each beam Bs1, Bm,
The amplitude detection signal (envelope detection signal) based on Bs2 has a constant phase difference as shown in FIGS. 3a to 3c. Among these amplitude detection signals (envelope detection signals), the peak of the detection signal by the main beam is the main beam.
This shows that Bm coincides with the center of the track, and the peak detection circuit 18 in the embodiment of FIG.
By detecting the peak of the detection signal from the main beam and activating tracking control, it is possible to stably shift to tracking control from a state where the tracking error is zero.

This tracking control starting operation is performed by the peak detection circuit 18, the AND gate 20, the inverter 22, and the flip-flop 2 in the embodiment of FIG.
It is done in 1. That is, before tracking control is started, the tracking on/off signal 23 is at the L level and the reset terminal of the flip-flop 21 is at the H level by the inverter 22, so the Q output goes to the L level and the switch 19 is open and tracking control is released.
Next, when the tracking on/off signal 23 becomes H level to start tracking control, the output of the inverter 22 is inverted to L level and the reset of the flip-flop 21 is released. In this state, when the peak detection circuit 18 detects the peak value of the amplitude detection signal (envelope detection signal) from the rectifier circuit 16b accompanying the track movement of the irradiation beam, an H level output is generated and the AND gate 20 enters the permissive state. Set flip-flop 21 with
= Turn on switch 19 with H level output,
Tracking control is started by supplying the output of the divider 17 to the servo amplifier 12.

Next, a bias circuit consisting of diodes D1 and D2 and a specified voltage source Vf is provided on the denominator input terminal B side of the divider 17 in the embodiment shown in FIG. It is as follows.

First, as shown in the timing chart of FIG. 3, when tracking control is not applied, the amplitude detection signal (envelope detection signal) of the main beam from the rectifier circuit 16b becomes almost zero between tracks ( Figure 3 b). Therefore, divider 1
Since the denominator input signal level of 7 becomes zero in the track movement state of the irradiation beam, the divider 17 becomes saturated or, in the worst case, falls into an oscillation state. Therefore, the tracking error signal (FIG. 3d) when the irradiation beam crosses the track is disturbed, and the tracking jump during access control performed while monitoring this tracking error signal becomes unstable.

Therefore, in the embodiment shown in FIG. 1, the main beam amplitude detection signal (envelope detection signal) shown in FIG. Even if it becomes lower than Vf, divider 1
A signal as shown in Fig. 3e is input to the denominator input terminal B of 7, and as a result, the 3rd
The normalized tracking error signal shown in Figure f has better linearity when crossing the track center than the conventional tracking error signal shown by the broken line (signal in Figure 3D), and the sensitivity of the tracking error signal is widened. A secondary effect is obtained that the value can be approximated to a constant value over a range.

To explain in more detail, the tracking error signal before passing through the divider 17 is the signal shown in FIG. The slope of the error signal, in other words, the error detection sensitivity is different before and after the match. Therefore, the gain of the servo circuit changes depending on the operating point of the tracking servo, resulting in unstable operation. This drawback is not a problem if the servo circuit is operating steadily, but if the servo circuit cannot necessarily be started in ideal conditions,
For example, when the servo circuit is turned on immediately after a track jump, the difference in tracking error detection sensitivity causes a difference in the settling time of the servo system, resulting in a problem that the time required for the track jump becomes longer.

On the other hand, in the present invention using the divider 17 for normalizing the tracking error, as shown in FIG.
As shown in Figure 2, linearity is achieved over a wide range before and after the signal level of zero when the main beam matches the track, compared to when the divider shown by the broken line is not used, and as a result, the tracking error detection sensitivity remains constant. To shorten the time required for a track jump without causing a difference in the settling time for making the main beam match the track, even if a transient response occurs when the servo circuit is turned on immediately after the track jump when the main beam approaches the track jump. I can do it.

FIG. 4 is a circuit block diagram showing another embodiment of the present invention. The difference from the embodiment of FIG. 1 is that the high-pass filter 15b and rectifier circuit 16b are omitted in the main beam signal processing system; A new adder 24 is provided, and the adder 24 connects the rectifier circuits 16a and 16.
c is obtained and the denominator input terminal B of the divider 17 is obtained.
The point is that it is applied to

FIG. 5 shows the rectifier circuit 16 in the embodiment shown in FIG.
a, 16c and the output of the adder 24,
The output of the adder 24 shown in FIG. 5c, which adds the amplitude detection signals (envelope detection signals) for the two sub-beams, is the amplitude detection signal (envelope detection signal) of the main beam Bm shown in FIG. 3b. ), it becomes maximum when the main beam Bm coincides with the track center.

Therefore, similarly to the embodiment shown in FIG. 1, the peak detection circuit 18 detects the peak value of the addition output of the adder 24 and turns on the switch 19, thereby stably starting the tracking control. be able to.

However, compared to the signal in Figure 3b, the addition signal in Figure 5c has a gentle slope near the peak, so it has little effect on improving the linearity of the tracking error signal as shown in Figure 3f. However, there is no problem in practical use. Rather, although the adder 24 is newly required, the high-pass filter and rectifier circuit can be omitted, resulting in the practical effect of reducing the circuit scale.

Note that the diodes D1 and D2 and the constant voltage source Vf shown in the embodiments of FIGS. It is not necessarily necessary to provide this when the maximum gain is determined by the device itself. Further, there are two types of rectifier circuits that perform envelope detection: a half-wave rectifier circuit and a full-wave rectifier circuit, and in the present invention, either rectifier circuit may be used. Further, in the embodiments shown in FIGS. 1 and 4, one divider 17 is used, but in other embodiments, two dividers 17 are used as shown in FIGS. 6 and 7. A similar circuit function can be achieved using dividers 17a and 17b.

That is, FIG. 6 shows an example in which two dividers 17a and 17b are used in the embodiment shown in FIG. , the output of the rectifier circuit 16b is commonly connected to the denominator input terminal B of each divider 17a, 17b. In this way, the sub-beam amplitude detection signal (envelope detection signal) before obtaining the tracking error signal is normalized by the dividers 17a and 17b and then input to the operational amplifier 11, and the normalized tracking error signal is detected. do.

FIG. 7 shows an example in which two dividers 17a and 17b are used in the embodiment shown in FIG. 4, and as in the case of FIG. 17b, and the output of the adder 24 is commonly connected to the denominator input terminal B, and after normalizing the amplitude detection signal (envelope detection signal) of the sub beam with the dividers 17a and 17b. The tracking error signal is input to the operational amplifier 11 to detect the tracking error signal.

(Effects of the Invention) As described above, according to the present invention, in a tracking control device using a two-beam method for recording bit strings, the amplitude detection signal obtained by envelope detection of two sub-beams is Since the tracking error signal given by the difference is normalized with the envelope detection signal from the main beam or the sum signal of the envelope detection signals from the two sub beams, the size of the bit and the frequency characteristics of the optical system due to the difference in recording frequency are normalized. It is possible to obtain a tracking error signal with constant sensitivity regardless of (MTF). Furthermore, the sensitivity of the tracking error signal can be kept constant even if the intensity of the light source changes not only due to the size of the bit string but also due to active means or disturbance during operation of the apparatus. Further, although the tracking control device of the present invention takes as an example a two-beam method that uses a recorded information bit string as a track, it can also be applied to tracking control of a pre-group recording medium in which track grooves are formed in advance. Effective tracking control can be performed even if the That is, in a two-beam tracking control device using a known pre-group recording medium, changes in apparent reflectance due to group diffraction are utilized, but in this case, the formation of Further, the reflectance varies depending on the bits formed, and if the change in reflectance due to the formation of the bits is large, a good tracking error signal and performance for activating tracking control cannot be obtained.
Therefore, the conventional tracking control device for a pre-group recording medium and the tracking control device of the present invention are used in parallel, and in the unrecorded area, the conventional tracking control is performed for the pre-group recording medium. By switching to perform the tracking control according to the present invention, it is possible to eliminate the drawbacks of the conventional tracking control device using the two-beam method for pre-group recording media.
Of course, when used in combination with a conventional device, the circuit parts common to the present invention may be shared, and only the high-pass filter and rectifier circuit of the present invention, which the conventional device does not have, may be switched.

[Brief explanation of the drawing]

FIG. 1 is a circuit block diagram showing an embodiment of the present invention, FIG. 2 is a timing chart showing the operation of FIG. 1, FIG. 3 is a timing chart showing the operation before tracking control starts, and FIG. The figure is a circuit block diagram showing another embodiment of the present invention, FIG. 5 is a timing chart showing the operation of FIG. 4, and FIGS. 6 and 7 are circuit block diagrams showing other embodiments of the present invention. , Fig. 8 is an explanatory diagram showing the principle of the two-beam method, Fig. 9 is an explanatory diagram of the circuit of the conventional device, and Fig. 10 is an explanatory diagram showing how the tracking error detection sensitivity changes depending on the size of the bit. be. 11: Differential amplifier, 12: Servo amplifier, 14
a, 14b, 14c: amplifier, 15a, 15b,
15c: High pass filter, 16a, 16b, 1
6c: Rectifier circuit, 17, 17a, 17b: Divider, 18: Peak detection circuit, 19: Switch, 2
0: AND gate, 21: Flip-flop, 2
2: Inverter.

Claims (1)

[Claims] 1. Tracking in which a recording medium is irradiated with two sub-beams in addition to the main beam for recording and reproduction, and tracking is controlled based on changes in beam reflected light due to bit strings of information recorded on the recording medium. In the control device, a detection means for detecting the amplitude of an alternating current component of the beam reflected light generated when the beam sequentially passes through the bits for each of the main beam and the sub beam, and a detection means for detecting the amplitude detection signal of the sub beam. tracking error detection means for detecting a tracking error signal according to the tracking error signal; and normalization means for normalizing the tracking error signal by dividing it by the amplitude detection signal of the main beam or the amplitude addition signal of the sub beam. A tracking control device characterized by always keeping the detection sensitivity constant.