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BACKGROUND OF THE INVENTION
This invention relates to an optical recording medium in which a data area is constituted by a pit string made up of pits formed along the track center scanned by a laser beam and lands (mirror surface sections) and in which the pit string in the
data area is read with pre-set clocks so as to be reproduced as information signals, a method for reproducing data recorded on the optical recording medium, and a data recording device or laser cutting device employed for producing the optical recording
medium.
An optical recording medium, such as one rotated at CAV (constant angular velocity), referred to herein as an optical disc, has a recording format, such as a recording format for a data area as shown in FIG. 21, in which pits P are formed along a
track center Tc, with a pit width d of 0.5 .mu.m and a pit length per clock of 0.86 .mu.m and at a track pitch Tp of 1.6 .mu.m along the disc radius, and are aligned in their positions along the track direction. The portions devoid of the pits are lands
left as mirror surface regions.
For reproducing information signals from the optical disc having the above-mentioned recording format, the disc is rotated at CAV and a playback laser beam is radiated on the track center Tc for relative scanning with a laser spot BS.
The laser beam reflected from the optical disc is caused to fall on a light receiving element and converted into detected signals as electrical signals having a signal level corresponding to the reflected light volume. The detected signals are
further demodulated by a signal demodulating circuit for producing playback information data.
The reflected light falling on the light receiving element after reflection by the mirror surface region is the light which has undergone substantially total reflection by the mirror surface. Thus the reflected light volume is abundant so that a
detection signal having a high signal level is outputted from the light receiving element. On the other hand, the light volume of the reflected light modulated by the pit is lesser than that of the reflected light from the mirror surface region, so that
a detection signal having a low signal level is outputted from the light receiving element.
In a downstream signal processor, detection signals outputted in series by the light receiving element are sampled with pre-set clock signals and thereby converted to bi-level data having a pulse amplitude corresponding to the signal level. The
bi-level data is processed for decoding error-correction codes, such as parity codes or interleaving, for producing playback information data.
Since data corresponding to the pit P is a logical "1" and data corresponding to the mirror surface region is a logical "0", data having a long concatenation of "1"s or "0"s suffers from increased deviation in the dc balance. That is, the
digital sum value (DSV) is offset t,o the (-) side or to the (+) side, thus producing an unstable state of the servo control system.
In addition, such data recording has a drawback that the data length is substantially increased, which is not meritorious in increasing the data recording density.
Another known recording method is shown in FIG. 22 in which recording is made so that the boundary between the pit P and the mirror surface region M is logically "1" and the pit P as well as the mirror surface region M other than the boundary is
logically "0". Data reproduction is made in a similar manner. Such recording method is meritorious for increasing data recording density since it is unnecessary to increase the data length in distinction from the firstly stated system.
Consequently, the conventional practice has been to record pits on the optical disc after 8-bit data is converted to 14-bit data in accordance with the eight-to-fourteen modulation (EFM). Playback information data are produced after decoding EFM
codes in addition to decoding of the error correction codes, as stated hereinabove.
However, the EFM system is not meritorious for high-density recording because it is 14-bit data converted from 8-bit data that is to be recorded. Although it is desirable to employ a direct data recording system for high density recording, the
above-mentioned problem caused by increased dc balance offset is raised.
Besides, with the conventional optical disc, since the recording data are implemented by a bit string pattern consisting of mirror surface sections M and pits P formed on the track center Tc, the number of pits P and a range in which the pits P
are formed differ from track to track depending on recording data. That is, the proportion between the number or size of the pits P and the number or size of the lands M in a data area per track is not equal and differs from track to track.
This leads to such a situation in the optical disc fabrication that, when a recording pattern on a stamper (a pit array pattern consisting of pits P and lands M) is to be transcribed onto a resin substrate by an injection molding method using the
stamper, the flow rate of the molten resin into cavities is not uniform due to the differential density of the protrusions and recesses on the stamper resulting in fluctuations in the state of adhesion of the molten resin to the stamper. The result is
that the contour of the pits P of the completed optical disc is locally deviated from the prescribed shape, while molding defects such as interruptions in the mirror surface sections M are produced.
Such molding defects are produced in particular in servo areas of the optical disc constructed in accordance with the sampled servo system. That is, the servo area is usually separated from the data area by a mirror area constituted solely by
mirror surface sections M. Thus the mirror area is continuous along the radius of the optical disc. The result is that molten resin flows quickly through the mirror area towards the outer periphery of the cavity, so that a so-called ghost, that is
broken edges of servo pits caused by the radially continuous mirror area, tends to be produced.
In addition, with the above-described sampled servo type optical disc, the following problem arises during tracking servo control during reproduction.
That is, the tracking servo control in the conventional sampled servo system is carried out using a pair of wobbled pits Pa and Pb pre-formed with a shift of one-fourth of a track pitch in opposite directions from the track center Tc, as shown in
FIG. 23.
Specifically, the amount of reflected light when the laser beam spot BS traverses the wobbled pits Pa and Pb is sampled, and a tracking error signal is found based upon the difference between these signals. The spot BS is moved radially of the
optical disc until the signal level, for example, of the tracking error signal becomes equal to zero, by way of performing tracking servo control.
On the other hand, the so-called track jump, which is the movement of the spot to a neighboring or other track, is performed by opening the tracking servo control loop, moving the spot to near a target track and subsequently closing the tracking
servo control loop for capturing the spot BS to the target track.
During such track jump, that is when the laser beam spot BS scans the track obliquely, the tracking error signal in the sampled servo system is a sine wave signal, as shown in FIG. 24, such that it is not unequivocally set with respect to a
displacement x of the beam spot BS from the track center.
It is when the spot BS is within a range 201, shown by hatched lines, that the tracking error signal is determined unequivocally with respect to the displacement x. That is, it is when the spot BS is within the range 201 that the spot BS can be
captured without fail with respect to the track center Tc.
On the other hand, if the displacement x is larger and is outside the range 201, that is within the range 202, tracking servo control becomes unstable. Such unstable state tends to be produced when the laser beam spot BS is moved with an
elevated speed along the radius of the optical disc as during the track jump.
There is produced an error in the distance the beam spot BS is moved during track jump with the tracking servo loop being turned off. If the tracking servo control loop is turned on outside the range 201, such as within the range 202 the
possibility is high that the beam spot WW be captured to a track other than the target a track. In such case, track jump needs to be performed a second time. Thus the conventional tracking servo has a drawback that the track jump cannot be preformed
stably.
SUMMARY OF THE INVENTION
In view of the above-described status of the art, it is an object of the present invention to provide an optical recording medium in which, even if data includes a succession of continuous logical "1"s or "0"s, dc balance may be optimized, that
is the DSV may be drawn closer to zero, without the necessity of performing modulation, such as EFM, which might otherwise produce increased data lengths, and in which stabilization in servo control and high density in the recording data may be achieved
simultaneously.
It is another object of the present invention to provide an optical recording medium in which the proportion of the size of pits and that of the lands in the data area per track may be rendered equal to each other, so that, when transcribing a
recording pattern (pit string pattern consisting of pits and lands) on the stamper onto the resin substrate by, for example, resin injection molding, the flow velocity of the molten resin into the stamper cavities may be rendered uniform for the cavities
in their entirety, thereby eliminating the molding defects during fabrication of the optical recording media. It is a further object of the present invention to provide an optical recording medium in which the servo area may be detected easily when the
optical recording medium is of the sampled servo type.
According to the present invention, there is provided an optical recording medium comprising a first pit string having a succession of pits and mirror surface sections and being formed on one side of a track center as a reference, and a second
pit string having pits and mirror sections, logically inverted in their array from the pits and the mirror surface sections of the first pit string. The laser beam is radiated onto the track center for accessing information signals represented by the
pits and the mirror surface sections.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a), 1(b) and 1(c) are diagrammatic illustrating a recording format of a sampled servo read-only optical recording disc according to a first embodiment of the present invention and particularly illustrating a data segment, an address
segment and an address/data segment, respectively.
FIGS. 1(a), 1(b) and 2(c) are diagrammatic views for illustrating the types of sectors of the optical disc of the first embodiment, and in particular show a 512-byte sector, a 1024-byte sector and a 2048-byte sector, respectively.
FIG. 3 schematically shows an example of a recording format in a servo area and a near-by data area in the optical disc of the first embodiment.
FIGS. 4(a), 4(b), 4(c) and 4(d) show essential parts of the optical disc of the first embodiment, especially showing its servo region and its vicinity, wherein FIG. 4(a) schematically shows an example of a recording format for the servo area and
its vicinity, 4(b) shows a push-pull signal waveform produced on reproducing the servo area and its vicinity, FIG. 4(c) shows an RF signal waveform produced on reproducing the servo area and its vicinity, and FIG. 4(d) shows waveforms of a mirror region
detecting gating signal, a data area detecting gating signal, a servo area detecting gating signal, a clock detecting gating signal, an address data region detecting gating signal and clock pulses.
FIGS. 5(a), 5(b) and 5(c) shows essential portions, above all, data, of the optical disc according to the first embodiment, and in particular show an example of the recording format in the recording data section of the data region, a push-pull
signal waveform produced on reproducing the data region and an RF signal waveform produced on reproducing the data region, respectively.
FIG. 6 is a diagrammatic view illustrating bit allocation to address data in an address data section in an address segment in the optical disc according to the first embodiment.
FIG. 7 is a schematic view showing the contents of the gray code represented by pits formed in an address data section in an address segment in the optical disc according to the first embodiment.
FIG. 8 is a block diagram showing an arrangement of a playback system of a disc reproducing device according to an embodiment of the present invention.
FIG. 9 shows an arrangement of an optical pickup in the playback system of a disc reproducing device according to an embodiment of the present invention.
FIGS. 1O(a) to (d) show the intensity distribution of the reflect,ed light, on radiation of a laser beam to the optical disc according to the first embodiment.
FIG. 11 is a diagrammatic view showing the plan configuration of a light-receiving surface of a photodetector in the optical pickup shown in FIG. 9.
FIGS. 12(a) through 12(d) show a far-field pattern of the reflected light on the light-receiving surface of the photodetector.
FIGS. 13(a) through 13(e) are timing charts showing the operation of the tracking servo in the playback system in the disc reproducing device according to the embodiment shown in FIG. 8.
FIG. 14 is a circuit diagram of an example of a tracking error signal generating circuit built into the reproducing system, above all, the servo error signal generating circuit, according to the embodiment shown in FIG. 8.
FIG. 15 is a circuit diagram showing another embodiment of the tracking error signal generating circuit.
FIG. 16 is a timing chart showing the operation of the tracking servo, above all, track jump, in the reproducing system of the disc reproducing device according to the embodiment shown in FIG. 8.
FIG. 17 is a block diagram showing an arrangement of a laser cutting device employed for producing the optical disc according to the first embodiment.
FIG. 18 is a schematic view showing essential portions of an optical disc according to a second embodiment, above all, another embodiment of a recording format in its servo region and vicinity.
FIG. 19 is a schematic view showing essential portions of an optical disc according to a third embodiment, above all, still another embodiment of a recording format in its servo region and its vicinity.
FIG. 20 is a schematic view showing essential portions of an optical disc according to a fourth embodiment, above all, a further embodiment of a recording format in its servo region and its vicinity.
FIG. 21 is a schematic view showing a recording format of a conventional optical disc.
FIG. 22 is a schematic view showing the playback logic for the conventional optical disc.
FIG. 23 is a schematic view showing the format for the servo pits of the conventional optical disc.
FIG. 24 is a waveform diagram showing tracking error signals produced on reproducing the conventional optical disc.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1 to 20, preferred illustrative embodiments of the optical recording medium, as applied to a read-only optical disc of the sampled servo system, referred to herein as an optical disc, are explained in detail.
The optical disc is of the type rotated at a constant angular velocity (CAV) and has a recording format in which each track is divided into plural sectors each of which is made up of plural segments. Referring to FIGS. 1(a) to 1(c), each segment
is made up of a 3-byte servo region Zs having servo pits and a 34-byte data region Zd for recording data per se. By the format constitution of the data region Zd, the segments are classified into three types of segments.
The first type segment is a data segment constituted by the three-byte servo region Zs followed by a recording data region 2d made up of a 1-byte clamp area Zc and a 33-byte recording data region Zw, as shown in FIG. 1(a). The second type
segment is an address segment constituted by the three-byte servo region Zs followed by a data region Zd made up of a 1-byte clamp area Zc, a 14-byte address data area Za, a 2-byte automatic power control (APC) data and a 17-byte blank Zb, as shown in
FIG. 1b. The third type segment is an address/data segment constituted by the three-byte servo region Zs followed by a data region Zd made up of a 1-byte clamp area Zc, a 14-byte address data area Za, a 2-byte APC data and a 17-byte data region Zw, as
shown in FIG. 1c.
There are also three types of sectors, namely a 512-byte sector, a 1024-byte sector and a 2048-byte sector.
The 512-byte sector comprises a series array of a first segment string made up of the leading address/data segment followed by nine data segments and a second segment string made up of the leading address segment followed by the nine data
segments, as shown in FIG. 2(a). The actual recording capacity of the recording data is 611 bytes, as shown by the following formula (1):
The 1024-byte sector comprises a series array of two of the above-mentioned first segment strings and two of the above-mentioned second segment strings, arranged in alternation with one another, as shown in FIG. 2(b). The actual recording
capacity of the recording data is 1222 bytes, as shown by the following formula (2):
The 2048-byte sector comprises a series array of four of the above-mentioned first segment strings and four of the above-mentioned second segment strings, arranged in alternation with one another, as shown in FIG. 2(c). The actual recording
capacity of the recording data is 2444 bytes, as shown by the following formula (3):
A first embodiment of the optical disc having the above-mentioned format constitution, above all, its recording format, is explained by referring to FIGS. 3 to 5. The recording format of the optical disc according to the first embodiment has the
servo region Zs made up of a mirror area Zm constituted solely by a mirror surface and a servo area Zss constituted by an array of servo pits, with the servo area Zss being separated by the mirror area Zm from the data region Zd, as shown in FIGS. 3 and
4. That is, the servo region Zs is made up of the servo area Zss at a center and two mirror areas Zm on either sides of the servo area Zss.
The recording format of the data region Zd, above all, the recording data area Zw, of the optical disc, is such that a pit string made up of pits P and lands (mirror surface sections) M formed on the radially inner peripheral side of the track
center Tc, and an inverted pit string made up of the pits P and the lands M formed on the radially outer peripheral side of the track center Tc, with the second sequence of the pits and the lands being the inverse of the firstly mentioned pit string, as
shown in FIG. 5(a). The effect is such that wherever there is a pit P on the radially inner peripheral side of the track center T.sub.c, there is a land M on the radially outer peripheral side of the track center T.sub.c, and vice versa. One-track data
is constituted by the pit string and the inverted pit string, arranged on either sides of the track center.
Specifically, if the diameter of the reproducing laser beam on the recording surface is set to 1.5 to 1.6 .mu.m, similarly to that used for a conventional compact disc reproducing device, a number of pits P, each having a pit width d of 0.5
.mu.m, are formed on the radially inner and outer peripheral sides of the track center Tc, as in the case of the conventional compact disc.
The distance between the track centers Tc, that is the track pitch, is selected to be 1.6 .mu.m and, as shown in FIG. 4(a), the distance t.sub.a between the inner side pit string in the data region Zd of a given track T.sub.2, such as the first
track T.sub.1, and the outer side inverted pit string in the data region Zd of the neighboring track, is selected to be equal to the distance t.sub.b between the inner side pit string and the outer side inverted pit string for the track T.sub.1.
That, is, with the optical disc of the present first embodiment, the inner side pit string is formed at a distance corresponding to one-fourth of a track pitch on the inner side of the track center Tc, while the outer side pit string is formed at
a distance corresponding to one-fourth of the track pitch on the outer side of the track center Tc. That is, pit strings are arranged along the track direction at a pitch equal to one-half of the track pitch along the radius of the optical disc.
The spot BS of the playback laser beam scans the track center Tc, and is of such a size that both the inner side pits P and the outer side pits P are encompassed within the beam spot BS.
The relation of pit strings on the inner and outer sides of the track center Tc, as seen from the scanning spot BS, is such that, if there is the pit P on the inner side, it is faced by the land M on the outer side, whereas, if there is the land
M on the inner side, it is faced by the pit P on the outer side.
Consequently, as long as the recording data area Zw of the data region Zd of the disc is scanned by the spot BS, there necessarily exists one of the pits within the beam spot BS.
For reproducing the optical disc of the present first embodiment, clock signals are generated during reproduction from a clock mark Mc formed in the servo area Zss of the servo region Zs, as later explained, and the pit string and the inverted
pit string of the data region Zd are reproduced based upon output timings of the generated clock signals. In FIGS.4(a) and 5(a), vertical ruled lines schematically indicate the output timings of the clock signals.
It is thus seen that the leading and trailing ends of the pits P in both the pit string and the inverted pit string in the recording data area Zw are formed in synchronism with the output timings of the clock signals, as shown in FIG. 5(a). On
the other hand, the logical data reproduced from the pit string has a constitution in which the boundary between the pit P and the land M, that is the above-mentioned leading or trailing end, is logically "1", while the portions of the pit P and the
lands M other than the boundary are logically "0".
The servo area Zss in the servo region Zs is sandwiched between mirror areas (lands) Zm formed on either sides of the track, as shown in FIG. 4(a). The leading end of the servo area Zss is arranged at a position displaced 1.5 clocks from the
trailing end of the data region Zd, while the trailing end of the servo area Zss is arranged at a position displaced 3.5 clocks from the leading end of the data region Zd.
That is, the data region Zd and the servo area Zss are spaced apart from each other by the mirror area corresponding to 1.5 clocks, that is an integer and a fractional number of clocks, instead of an integer number of clocks, thereby enabling the
distinction to be made between the data region Zd and the servo region Zs.
Besides, the same arraying pattern of the pits (servo pits) P constituting the servo area Zss is repeated at an interval of three tracks. Specifically, if the servo area Zss is divided at an interval of two clocks into three zones, beginning
from the leading end, with the first, second and third zones being termed regions A, B and C, as shown in FIG. 4(a), the combinations of pit arrays contained in the three zones are of three different patterns.
In the illustrated example, pits P are formed in the zones A and C on the inner side of the first track T.sub.1, while a sole pit P is formed on the outer side thereof across the zones A and B. That is, the pits P are formed on both the inner and
outer sides of the zone A, whereas the pits P are formed on the outer side of the zone B and on the inner side of the zone C. With the first track T.sub.1, the pits in the zone A are used as clock marks Mc for clock detection, while the pits P in the
zones B and C are used as a servo mark Ms for tracking error detection.
On the other hand, a pit P is formed across the zones B and C on the inner side of the second track T.sub.2, while pits P are formed on the outer side thereof in the zones A and C. That is, the pits P are formed on both the inner and outer sides
of the zone C, whereas the pits P are formed on the outer side of the zone A and on the inner side of the zone B. With the second track T.sub.2, the pits in the zone C are used as clock marks Mc for clock detection, while the pits P in the zones A and B
are used as a servo mark Ms for tracking error detection.
On the other hand, a pit P is formed across the zones A and B on the inner side of the third track T.sub.3, while a sole pit P is formed on the outer side thereof in the zones A and C. That is, the pits P are formed across the inner side and the
outer side of zone B, whereas the pits P are formed on the inner side of the zone A and on the outer side of the zone C. With the third track T.sub.3, the pits P in the zone B are used as clock marks Mc for clock detection, while the pits P in the zones
A and C are used as a servo mark Ms for tracking error detection.
The method for tracking servo control by the above-described pit array of the servo region Zs will be explained in detail subsequently.
The recording format of the data region Zd, especially that of the address segment or the address/data segment, of the optical disc of the first embodiment, comprises a recorded series array of an access code and a sector code, each representing
the Gray code by a pit string and an inverted pit string, as shown in FIGS. 3 and 6. In FIG. 3, only the access code is shown.
Taking an example of the address segment, the access code is made up of an upper order address area (AH, AM.sub.H) indicating an upper order address of the track address and a lower order address area (AM.sub.L, AL) indicating the lower order
address thereof, with the upper address area being divided into two address areas AH and AM.sub.H and with the lower address area being divided into two address areas AM.sub.L and AL, as shown in FIG. 6. Each of the address areas AH, AM.sub.H, AM.sub.L
and AL is formed by four bits and allocated to a 14-clock area based upon output timings of the clock signals. With each of the address areas AH, AM.sub.H, AM.sub.L and AL, the Gray code is represented by the pit array formed at the central 12-clock
area, as shown in FIG. 7.
A sector code is made up of an upper address area S.sub.H and a lower address area S.sub.L, indicating the upper order and lower order addresses of the sector address, and inverted address areas S.sub.L and S.sub.H inverted from these data.
Similarly to the access code, the sector code has an address area arranged to represent the Gray code by the pit array formed in a central 12-clock area.
Turning to the pit string representing the Gray code, the inner peripheral side pit string is constituted by a leading pit Pt and a trailing end pit Pr, each having a pit length corresponding to the relevant Gray code, and a land M having a
length corresponding to the relevant Gray code, while the outer peripheral side inverted pit string is constituted by a leading end land M1 and a trailing end land M2, each having a pit length corresponding to the relevant Gray code, and a central pit P
having a length corresponding to the relevant Gray code, as shown in FIG. 7.
The leading end pit Pt and the trailing end pit Pr in the inner peripheral side pit string are overlapped by two clocks with the central pit Pc in the outer peripheral side inverted pit string, such that the Gray code recorded in each address
areas AH, AM.sub.H, AM.sub.L, AL, S.sub.H and S.sub.L may be read by detecting the overlapped portions by the playback laser beam.
In the above pit string, it is the pit string portion and the inverted pit string portion corresponding to the central 12 clocks that are read as the Gray code. The 1-clock pits on either ends play the part of negative polarity marks used for
rendering the push-pull signal into negative polarity signal.
The clamp area Zc comprises a staggered array of clamp pits P.sub.CLMP, each having a pit length corresponding to one clock, as shown in FIGS. 3 and 4(a). In the outer peripheral side pit string, three lands M, each having one-clock length, are
arrayed with the clamp pits P.sub.CLMP in-between, whereas, in the inner peripheral side pit string, three clamp pits P.sub.CLMP are arrayed with the lands M, each having a length corresponding to one clock, in-between.
The clamp operation by the pit array of the clamp area Zc is explained subsequently.
The reproducing method for reproducing information signals from the above-described optical disc is now explained by referring to the reproducing system of the reproducing device shown in FIG. 8.
The reproducing system of the disc reproducing device is made up of an optical pickup 2 for radiating a laser beam L on an optical disc 1 of the first embodiment and detecting the volume of light reflected from the recording surface, and a signal
processor 3 for reproducing data from the playback signals from the optical pickup 2,
The optical pickup 2 is arranged on the same side of the optical disc 1 as a spindle motor 4 rotationally driving the optical disc 1 and is substantially of the same construction as the conventional optical pickup employed in a compact disc
reproducing device, as shown in FIG. 9. That is, the optical pickup 2 includes a laser light source 11, a collimator lens 12 for collimating the light outgoing from the laser light source 11 into a parallel beam, and an objective lens 13 condensing the
collimated light from the collimator lens 12 for radiating the condensed light on the recording surface 1a of the optical disc 1. The optical pickup 2 also comprises a beam splitter 14 arranged on an optical path between the collimator lens 12 and the
objective lens 13 for splitting the light reflected from the recording surface 1a of the optical disc 1, and a photodetector 15 for detecting the light volume of the reflected light Lr split by the beam splitter 14. The optical pickup unit finally
includes a converging lens 16 arranged on an optical path between the beam splitter 14 and the photodetector 15 for converging the reflected by the beam splitter 14.
The objective lens 13 is slightly moved by a two-dimensional actuator 17 in a direction towards and away from the optical disc 1 and in the radial direction of the optical disc 1. The two-dimensional actuator 17 has a magnetic circuit comprising
a focusing coil, a tracking coil and a magnet, all not shown.
The optical pickup 2 is tracking servo controlled so that the track center Tc of the optical disc 1 is scanned by the center of the spot BS of the laser beam L radiated from the laser light source 11.
The laser light source 11 of the optical pickup 2 comprises a semiconductor laser radiating the laser light beam L having the same wavelength as that radiated by the conventional optical pickup. The collimator lens 12 collimates the laser beam L
from the laser light source into a parallel beam which is caused to be incident on the beam splitter 14.
The beam splitter 14 comprises a half mirror for transmitting the outgoing light L from the laser light source 11 through the objective lens 13. The objective lens 13 has a numerical aperture NA equivalent to that of the conventional optical
pickup and converges the outgoing light L from the laser light source 11 transmitted through the beam splitter 14 in order to radiate it on the recording surface 1a of the optical disc 1.
On the recording surface 1a of the optical disc 1, as shown in FIGS. 4a and 5a are arrayed a data region Zd and a servo region Zs, made up of the servo area Zss and the land Zm, in spatial separation from the data region Zd. Above all, in the
data region Zd, a pit string corresponding to the recording data is formed on the inner peripheral side of the data region Zd, and corresponding inverted pit string is formed on the outer peripheral side thereof relative to the track center Tc.
Consequently, the reflected light Lr, reflected by the land Zm constituted solely by the mirror surface, has a light intensity distribution which becomes transversely symmetrical on the drawing sheet, as shown in FIG. 10a. If there exists in the
data region Zd the pit P on the inner peripheral side, while there lacks the pit P on the outer peripheral side, the reflected light Lr has such light intensity distribution that, due to the diffraction by the pit P on the inner peripheral side, the
reflected light deflected towards right on the drawing sheet becomes predominant, as shown in FIG. 10b.
Conversely, if there is the pit P on the outer peripheral side of the data region Zd, while there is no pit P on the inner peripheral side, the reflected light distribution is deflected towards the left more strongly on the drawing sheet, as
shown in FIG. 10c. If there exist pits P on both the inner and outer peripheral sides of the servo area Zss of the servo region Zs, the reflected light Lr has a light intensity distribution which is transversely symmetrical on the drawing sheet, due to
the diffraction by both of the side pits P, as shown in FIG. 10d). However, the reflected light intensity becomes lower than when there is no pit P.
The reflected light Lr, having the light intensity distribution which depends upon the presence or absence of the pits P, is collimated by the objective lens 13 into a parallel beam which is again caused to fall on the beam splitter 14 whereby
part of the reflected light Lr is split by reflection.
The converging lens 16 is constituted by a cylindrical lens for producing a focusing error signal by, for example, the astigmatic method. By the converging lens 16, the reflected light Lr is converged on the light receiving surface of the
photodetector 15.
The photodetector 15 has its light receiving region divided into four sections 15a, 15b, 15c and 15d, as shown in FIG. 11. The photodetector 15 has a far-field pattern on the light receiving surface in which, if there is no pit P on the inner
peripheral side of the track center Tc and there is the pit P on the outer peripheral side thereof, the sections 15a and 15d become lighter, while the sections 15b and 15c become darker due to light diffraction at the pit P, as indicated by hatched lines
in FIG. 12a.
Conversely, if there is the pit P on the inner peripheral side and there is no pit P on the outer peripheral side, as shown in FIG. 12b, the sections 15a, 15d become darker, while the sections 15b, 15c become lighter. If there exist the pits P
on both the inner and outer peripheries, as shown in FIG. 12c, all of the sections 15a to 15d become darker. If there exist no pits P on the inner or the outer peripheries, as shown in FIG. 12d, all of the sections 15a to 15d become lighter.
Consequently, there are three signal levels, that is a high level (H), a mid level (M) and a low level (L), as the signal levels of the RF signal obtained by summing the detection signals, produced by photo-electric conversion by the four regions
15a to 15d, in association with three cases in which there are no pits P on the inner or outer peripheries (FIGS. 10a and 12d), there exists one pit P on the inner or outer peripheral side (FIGS. 10b, 10c and 12a, 12b) and there exist pits P on both the
inner and outer peripheries (FIGS. 10d and 12c).
The signal processor 3 comprises a head amplifier 21 to which a detection signal Si from the photodetector 15 enters and which performs pre-set signal waveshaping based upon the input detection signal Si. The head amplifier 21 has plural
differential amplifiers built therein and outputs four kinds of output signals. The first output signal S1 is a sum signal of detection signals produced by photo-electric conversion by the sections 15a, 15d of the photodetector 15 on which fall the
reflected lights reflected by the inner peripheral sections of the track relative to the track center Tc. The second output signal S1 is a sum signal of detection signals produced by photoelectric conversion by the sections 15b, 15c of the photodetector
15 on which fall the reflected lights reflected by the outer peripheral sections of the track relative to the track center Tc.
The third output signal is a sum signal of detection signals produced by photo-electric conversion by the four sections 15a to 15d of the photodetector 15, that is an RF signal SRF. The fourth output signal is a pulse signal Sp outputted by a
bottom detection circuit, not shown, which is built into the head amplifier 21. The pulse signal Sp, produced by the bottom detection circuit, is a signal which rises at an output timing of the lowermost level signal detected by the bottom detection
circuit from the RF signal.
The reason the lowermost level signal among the input RF signals SRF is detected by the bottom detection circuit is that, as described previously, the constructive pattern of the clock mark Mc in the servo area Zss is such pattern in which there
exist pits P on both the inner and outer peripheries with respect to the track center Tc and, if this portion is scanned by the beam spot BS, the reflected light Lr becomes weakest in light intensity due to light diffraction by the inner and outer
peripheral pits P, with the signal waveform of the RF signal SRF produced at this time being of the lowest signal level. Consequently, generation of the pulse signal Sp which rises at the stage the lowest level signal SRF is supplied is equivalent to
detection of the clock mark Mc.
The pulse signal Sp is routed to a next stage PLL circuit 22. The PLL circuit 22 generates a clock signal Sc, which gives the playback timing, based upon the pulse signal Sp from the head amplifier 21 and a clock detection gate signal Gc from a
unique pattern detection circuit 26 as later explained. The clock signal Sc from the PLL circuit 22 is supplied to a next stage timing generator (T6) 23 and to an A/D converter 24 as later explained.
The timing generator 23 generates plural kinds of timing signals, as required by various circuits, based upon the clock signals Sc supplied by the PLL circuit 22. In the present embodiment, these timing signals are a reference signal Sb supplied
to a laser driving circuit 18, as required for laser excitation of the laser light source 11 in the optical pickup 2, a servo clock signal Ss as required for servo control and rotation control of the optical disc 1 and a reference clock Su for detecting
pre-set unique patterns, herein the patterns of the mirror area Zm, data region Zd, servo region Zs, clock mark Mc and the address data area Za.
The servo clock signal Ss and the reference clock signal Su are of the same type of signals and represent double signals having a period one-half the period of the clock signal Sc outputted by the PLL circuit 22 and are supplied to the unique
pattern detector 27.
The signal processor 3 includes a clamp circuit 25 downstream of the head amplifier 21. The clamp circuit 25 clamps the levels of the first output signal S.sub.1, second output signal S.sub.2 and the RF signal S.sub.RF at the reference levels
and eliminates noise components caused by fluctuations in reflectance of the playback laser beam based upon the input of the clamp pulse Pc from the unique pattern detection circuit 27 as later explained. Since the clamp area Zc in the recording format
is allocated immediately after the servo region Zs in each segment, as shown in FIGS. 3 and 6, the clamp operation is performed on the segment basis. The clamp area may be detected by forming pits necessarily on the inner peripheral side right after of
the mirror area Zm. The signal processor 3 also includes the e.g. 8-bit A/D converter 24 for converting the first and second output signals S.sub.1, S.sub.2 and the RF signals S.sub.RF entering the A/D converter via the clamp circuit 25, based upon the
output timings of the clock signals Sc from the PLL circuit 22, and a digital equalizer 26 for equalizing the digital signals D.sub.1, D.sub.2 and D.sub.RF from the A/D converter 24 by, for example, a 3-tap digital filter.
The digital equalizer 26 optimizes the input digital signals D.sub.1, D.sub.2 and D.sub.RF, depending on the respective densities, as the equalization coefficients, for providing the optimum error rates, in order to produce optimized digital
signals D.sub.1 ', D.sub.2 ' and D.sub.RF ', and calculates the difference between the digital signals D.sub.1 ' and D.sub.2 ' associated with the first and second output signals S.sub.1 and S.sub.2 in order to produce a push-pull signal D.sub.PP.
The signal waveforms of the push-pull signal Dpp prepared by the digital equalizer 26 and the RF signal S.sub.RF from the head amplifier 21 are shown in FIGS. 4b, 4c and in FIGS. 5b, 5c, respectively. Although the push-pull signal Dpp is
formulated in the present embodiment as digital signals, it is described herein as an analog signal for facilitating waveform comparison with the analog RF signal S.sub.RF.
The push-pull signal waveform and the RF signal waveform, shown in FIGS. 5b add 5c, are waveforms obtained on reproducing the pit string and the inverted pit string of the recording data area Zw in the data region Zd shown in FIG. 5a. Referring
to FIG. 5, since there necessarily exists the mirror surface M on the outer peripheral side in the recording data area Zw with respect to the track center Tc, if there exists the pit P on the inner peripheral side, whereas, if the inner peripheral side
is the mirror surface M, there exists the pit P on the outer peripheral side, so that the signal level of the RF signal S.sub.RF is the mid level (M).
On the other hand, the push-pull signal Dpp is of a signal waveform which becomes zero at a boundary between the pit P and the mirror surface M in the pit string or in the inverted pit string and which is deviated in the (-) direction and in the
(+) direction when there is the pit P in the inner peripheral side and in the outer peripheral side, respectively.
The push-pull signal waveform and the RF signal waveform, shown in FIGS. 4b and 4c , are the waveforms obtained on reproducing the pit string and the inverted pit string in the servo region Zs and the near-by region and, above all, the playback
waveforms obtained on scanning the third track T.sub.3 by the beam spot BS. It is seen from these figures that, since the mirror area Zm is constituted solely by the mirror surface M, and there exists no pit P on the inner or outer peripheral side of
the track center Tc, the RF signal S.sub.RF for the mirror area Zm has a high signal level (H).
In the portion of the servo area Zss where there is the servo mark Ms, since there is the pit P on the inner or outer peripheral side, the signal level of the RF signal S.sub.RF for the servo mark Ms is the mid level (M). In the portion of the
servo area Zss where there is the clock mark Mc, since there exist the pits P on the inner and the outer peripheral sides, the signal level of the RF signal S.sub.RF for the clock mark Mc is the low level (L).
On the other hand, the push-pull signal Dpp is of a signal waveform which becomes zero at the portions of the track registering with the mirror area Zm and the clock mark Mc and which is deviated in the (-) direction and in the (+) direction at
the portions of the servo area Zss registering with the servo mark Ms if there is the pit P in the inner peripheral side and in the outer peripheral side, respectively.
Downstream of the digital equalizer 26 are connected the above-mentioned unique pattern detection circuit 27, a servo error signal generator 28, a threshold value calculating circuit 29 and a partial response PR (1, 1) detection circuit 30.
The push-pull signal Dpp and the digital RF signal D.sub.RF from the digital equalizer 26 enter the unique pattern detection circuit 27 which, based upon the push-pull signal Dpp, digital RF signal D.sub.RF and the reference clock signal Su from
the timing generator 23, formulate and output five different kinds of unique pattern detection signals Gm, Gd, Gs, Gc, and Ga and the clamp pulse Pc.
If assumed that the third track T.sub.3 is scanned by the beam spot BS of the playback laser beam, the unique pattern detection gate signal, which is generated at the unique pattern detection circuit 27 is made up of a mirror area detecting
gating signal Gm, which goes high only for the portion of the track registering with the mirror area Zm, a data region detecting gating signal Gd which goes high only for the portion of the track registering with the data region Zd, a servo region
detecting gating signal Gs which goes high only for the portion of the track registering with the servo region Zs, a clock detecting gating signal Gc which goes high only for the portion of the servo region Zs of the track registering with the clock mark
Mc and an address data detecting gating signal Ga which goes high only for the portion of the data region Zd of the track registering with the address data area Za, as shown in FIG. 4d.
These detection gate signals Gm, Gd, Gs, Gc and Ga are prepared by the unique pattern detection circuit 27 in the following manner. That is, the unique pattern detection circuit 27 first detects the clock mark Mc in the servo area Zss of the
servo region Zs and, based upon the detection of the clock mark Mc, formulates the clock detecting gating signals Gc, which then is outputted to the PLL circuit 22. The PLL circuit 22 formulates and outputs reference clock signals for the system
operation based upon the input clock detecting gating signal Gc and the pulse signal Sp indicating the clock mark Mc from the head amplifier 21. The clock signal Sc is converted in the timing generator 23 to the reference clock signal Su (and Ss) which
is routed to the unique pattern detection circuit 27.
Based upon the above-mentioned servo region detecting gating signal Gs and the reference clock signal Su from the timing generator 23, the unique pattern detection circuit 27 formulates the remaining components of the unique pattern detecting
gating signal, namely the mirror area detecting gating signal Gm, data region detecting gating signal Gd and the address data area detecting gating signal Ga. The data region detecting gating signal Gd is produced for all of the segments, whereas the
address data area detecting gating signal Ga is produced only for the address and address/data segments.
Of these detection gate signals Gm, Gd, Gs, Gc and Ga, the mirror area detecting gating signal Gm, servo region detecting gating signal Gs and the clock detecting gating signal Gc are supplied to the servo error signal generator 28, whereas the
data region detecting gating signal Gd and the address data area detecting gating signal Ga are supplied to an arithmetic-logical unit 31 as later explained.
The unique pattern detection circuit 27 also generates, from the push-pull signal produced on reproducing the clamp pits P.sub.CLMP making up the clamp area Zc, a clock pulse Pc which rises during zero detection as the push-pull signal is changed
from the (-) polarity to the (+) polarity, and outputs the generated clock pulse Pc.
Referring to the recording format shown in FIGS. 3 and 6, three clamp pits P.sub.CLMP are formed in the inner peripheral pit string with the mirror surfaces or lands M in-between, and three mirror surfaces or lands M are formed with clamp pits
P.sub.CLMP in-between, so that there exist two timings at, which the push-pull signal is changed in polarity from the (-) polarity to the (+) polarity and hence two clamp pulses Pc are outputted per segment.
The clamp pulses Pc, outputted from the unique pattern detection circuit 27, are routed to the clamp circuit 25, as mentioned previously.
A defect indicating flag is logically allocated in a memory built in the inside of the unique pattern detection circuit 27. The defect indicating flag is constructed of at least one bit and is set to "1" via an inner control unit when any one of
the unique patterns results in failure. If detection of all of the unique patterns is achieved successfully, the flag is set to "0". The bit information Df of the defect indicating flag i | | |
