Method and apparatus for composite tracking servo system with track offset correction and rotary optical disc having at least one correction mark for correcting track offset

A composite wobbled tracking servo system uses a rotary optical disc which has a header field and a data field alternately arranged along the direction of rotation to provide a sector, and which includes pregrooves formed in at least the data field and at least one pair of wobbled track marks disposed in the header field in a relation wobbled relative to the center of a track. A light spot is directed toward and onto the optical disc to detect a push-pull tracking error signal from the reflection of the light spot diffracted by the pregroove, and a wobbled tracking error signal is detected from the reflection of the light spot traversing the wobbled track marks. The push-pull tracking error signal is corrected on the basis of the wobbled tracking error signal to attain the tracking control with higher accuracy, thereby eliminating an undesirable track offset attributable to tilting or eccentricity of the optical disc.

BACKGROUND OF THE INVENTION 
This invention relates to a tracking servo system for tracking the center 
of a track with a light spot, and more particularly to a composite 
tracking servo system in which a differential diffraction method using 
pregrooves or a so-called push-pull tracking method is combined with a 
wobbled tracking method using wobbled track marks disposed in a relation 
wobbled relative to the center of a track, and which is suitable for 
application to an optical code data memory such as a 
recordable/reproducible adding type optical disc system or an erasable 
type-optical disc system. 
In the push-pull tracking method, an optical disc having guide grooves or 
so-called pregrooves formed previously along the direction of rotation of 
the disc is irradiated with a light spot, and an unbalance of the 
distribution of the reflection of light diffracted from the pregrooves is 
based to detect a track error which is fed back to a servo system. This 
push-pull tracking method is disclosed in, for example, U.S. Pat. No. 
4,363,116. Since this push-pull tracking method utilizes the distribution 
of the reflection of light diffracted from the pregrooves, an offset 
attributable to an eccentricity or tilt of the disc tends to occur, and, 
because of such an offset, the light spot cannot be accurately positioned 
on the center of the track. According to studies conducted by the 
inventors, a tilt of 0.7.degree. or an eccentricity of 100 .mu.m, for 
example, results in an offset of about 0.1 .mu.m. 
On the other hand, a tracking servo system of three spots type is widely 
employed in a playback-only system such as a CD (a compact disc). However, 
this tracking servo system is unfit for application to a combined 
recording/reproduction system. The tracking servo system of three spots 
type is disclosed in, for example, U.S. Pat. No. 3,876,842. 
SUMMARY OF THE INVENTION 
With a view to solve the prior art problem of the offset pointed out above, 
it is a primary object of the present invention to provide an optical 
tracking method and apparatus which can eliminate the undesirable offset 
(the error component) thereby ensuring more accurate tracking. 
The present invention utilizes the so-called track wobbling method in which 
an optical disc is previously formed with one or more sets of wobbled pits 
disposed in a relation wobbled relative to the center of a track, and the 
relative amounts of light reflected after being passed through these pits 
as a result of irradiation with a light spot are compared to detect a 
track error, if any. This track wobbling method is already known per se 
and disclosed in, for example, U.S. Pat. No. 4,223,187. 
According to this track wobbling method, the true position of light spot 
passed through the wobbled pits can be detected. Therefore, a more 
accurate servo system can be provided as compared to a servo system 
utilizing the push-pull method based on the distribution of diffraction by 
the pregrooves. On the other hand, however, the track wobbling method 
requires provision of 1,000 or more wobbled pits per track, resulting in a 
corresponding reduced data efficiency. The track wobbling method has such 
another problem that it is not compatible with the push-pull method. 
In view of the above prior art problems, the present invention provides a 
tracking system of high utility which has such features that (1) the merit 
of the wobbled tracking method is maintained; (2) the data efficiency is 
not degraded; and (3) it is compatible with the push-pull tracking system 
which is a prior art system most widely employed in this field. The 
tracking servo system according to the present invention is a composite of 
the push-pull tracking system using the pregrooves and the wobbled 
tracking system using the wobbled track marks, and has a dual structure so 
that an offset of a low-frequency component from the dc level, which 
offset tends to occur in the push-pull servo system, can be suppressed in 
the wobbling servo system. More precisely, one complete track on an 
optical disc is divided into a plurality of sectors each of which includes 
a header field (or an index field)previously formed with pits and a data 
field on which the user records desired information. In the header field, 
wobbled track marks in the form of one or more pairs of elongate pits are 
previously formed in a relation wobbled relative to the center of the 
track, and pregrooves for tracking purpose are previously formed in at 
least the data field. The word "previously" as used herein means that the 
wobbled track marks and the pregrooves have been provided before the user 
records desired information on the data field. Preferably, the wobbled 
track marks and the pregrooves which have been formed during preparation 
of a mother disc are provided by replication of the mother disc. It is 
also preferable that address information (a track address and a sector 
address) for identifying the specific sector and a sector mark indicating 
the head of the specific sector are previously formed in the header field. 
The wobbled track mark may act also as the sector mark. In the recording 
and reproduction of data on and from the data field, a light spot is 
directed to the prewobbling pits disposed in the header field, so as to 
detect an accurate, prewobbling tracking error signal free from any offset 
of the position of the light spot from the center of the track. Then, a 
push-pull tracking error signal including an offset and utilizing the 
distribution of the reflection of light diffracted from the pregrooves is 
corrected on the basis of the prewobbling tracking error signal, thereby 
eliminating the offset attributable to deviation of the diffracted light 
beam on a light detector and ensuring stable and highly accurate tracking 
operation. While tracking in the manner described above, data are recorded 
on or reproduced from one of the pregrooves or a land between the 
pregroove and an adjacent pregroove.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The tracking error signal detecting method according to the present 
invention is the combination of the prewobbling method and the push-pull 
method as described already. This detecting method will be first 
described. 
FIG. 1 shows the structure of one form of an optical system for recording 
and reproducing information on and from an optical disc. Referring to FIG. 
1, semiconductor laser drive signals 1 and 2 are applied to a 
semiconductor laser driver circuit 3 to drive a semiconductor laser 4. 
Light emitted from the semiconductor laser 4 passes through a coupling 
lens 5, a beam splitter 6, a galvana mirror 7, a 1/4 wavelength plate 8 
and an objective lens 9 to form a light spot 11 on a recording surface of 
an optical disc 10 thereby irradiating a track 12. The reflection of light 
diffracted from the track 12 returns to the optical system again, and, 
after being reflected by the beam splitter 6, is received by a two-divided 
light detector 13 (light-receiving sections 131 and 132) to be turned into 
an electrical signal. FIGS. 2a to 2d illustrate various examples of the 
structure of pregrooved tracks of a recordable/reproducible optical disc 
of the present invention adapted to be used in such an optical disc 
apparatus. Referring to FIG. 2a, one complete track 12 is divided into, 
for example, 64 sectors each of which includes a set of a header field 123 
previously formed with pits and a data field 122 on which the user records 
desired information. In the header field 123, a selected number of, or, 
for example, a pair of elongate pits (wobbled track marks) 20-1 and 20-2 
wobbled relative to the track center are previously formed by replication, 
so as to detect a tracking error signal according to the prewobbling 
method. Besides these elongate pits 20-1 and 20-2, pits 14 indicative of a 
track address, a sector address, a synchronizing signal, etc. are formed 
along the track center without being arranged in a wobbled relation. 
However, these pits 14 are not necessarily essentially required in the 
tracking servo system of the present invention and may be omitted as 
required. 
In FIGS. 2a to 2d, the wobbled track marks 20-1 and 20-2 act also as sector 
marks indicating the head of the corresponding sector. However, these 
marks 20-1 and 20-2 may be disposed in any other position, for example, 
between the sector marks and the address signal prepits or in a gap area 
between the address signal prepits and the data field. Preferably, the 
prepits (including the wobbled track marks) previously formed in the 
header field 123 are of phase structure having a depth which is 1/4 or 1/8 
of the wavelength of the laser beam used for recording and reproduction of 
information on and from the data field 122. It is also preferable that at 
least the pregrooves formed in the data field 122 are of phase structure 
having a depth which is 1/8 of the wavelength of the laser beam and are 
concentrically or spirally arranged. 
In FIG. 2b, the track center registers with the center of the pregrooves 
15-1 and 15-2, and data are recorded on these pregrooves. Although, in 
FIG. 2b, no pregrooves are provided in the area where the prepits 14 are 
formed, the pregrooves 15-1 and 15-2 may be formed in this area too. In 
FIGS. 2c and 2d, the track center registers with the centerline extending 
between the adjacent pregrooves 15-1 and 15-2, and data are recorded on a 
land 15-3 defined between the pregrooves 15-1 and 15-2. As shown in FIG. 
2d, the pregrooves 15-1 and 15-2 may be formed at least in the data field 
122. Further, as shown in FIG. 2c, the pregrooves 15-1 and 15-2 may also 
be formed to sandwich the prepits 14 therebetween in the area where the 
prepits 14 are formed in the header field 123. Further, the pregrooves 
15-1 and 15-2 may be formed to sandwich the wobbled track marks 20-1 and 
20-2 therebetween and may extend thoughout the header field 123 and data 
field 122 without any discontinuity. The aforementioned wobbled track 
marks, prepits and pregrooves are previously formed during preparation of 
a mother disc, and a disc substrate obtained by replication of the mother 
disc having the wobbled track marks, prepits and pregrooves previously 
formed thereon is coated with a desired recording film. In the case of, 
for example, ablative recording, a film of a material such as a TeSePb, 
whose principal component is Te, is preferably used as the recording film. 
On the other hand, in the case of magneto-optical recording, a vertical 
magnetized film of a material such as TbFeCo, whose principal component is 
TbFe, is preferably used as the recording film. Also, in the case of phase 
change recording, a film of a material such as an amorphous Te compound is 
preferably used as the recording film. 
FIGS. 3a to 3d illustrate the distribution of diffracted light on the 
two-divided light detector 13 (the light-receiving sections 131 and 132) 
when the light spot 11 directed onto the pregroove 15 of phase structure 
deviates from the center of the pregroove 15. Deviation of the light spot 
11 from the center of the track results in an asymmetrical distribution of 
diffracted light. Therefore, when the diffracted light from the pregroove 
15 is received by the two light receiving sections 131 and 132 disposed in 
parallel to the track to sandwich therebetween the track, and the 
difference between the output signals of the two light receiving sections 
131 and 132 is found, a push-pull tracking error signal 16 can be detected 
as shown in FIG. 3d. FIGS. 3a to 3c show the relation between the light 
spot 11 and the pregroove 15. An interference pattern between light of 
zeroth order and light o first order diffracted by the pregroove 15 
appears on the two-divided light detector 13. In the absence of deviation 
of the light spot 11 from the track, this interference pattern is 
symmetrical with respect to the pregroove 15. On the other hand, in the 
presence of deviation, the symmetry of the interference pattern is lost, 
and the differential output of the two-divided light detector 13 is not 
zero, so that the track error can be detected. This differential output of 
the light detector 13 is fed back to a tracking actuator, for example, the 
galvana mirror 7 to constitute a tracking servo. 
When the light spot 11 is directed toward or positioned on the center of 
the land 15-3 between the adjacent pregrooves 15-1 and 15-2 as shown in 
FIG. 2c or 2d, the polarity of the differential output of the light 
detector 13 should be inverted before being fed back to the tracking 
actuator or galvano mirror 7. That is, when the light spot 11 is directed 
toward or positioned on the land 15-3 between the pregrooves 15-1 and 
15-2, the light spot 11 diverges over the two pregrooves 15-1 and 15-2, 
and the interference pattern due to diffraction appears on the light 
detector 13 as when the light spot 11 is centered on the pregroove 15. It 
is to be noted that the asymmetry of the interference pattern due to 
deviation of the light spot 11 from the track when the light spot 11 is 
centered on the pregroove 15 is inverse in terms of intensity distribution 
to the asymmetry produced when the light spot 11 is centered on the 
inter-pregroove land 15-3. It is therefore necessary to invert the 
polarity of the differential output 16 in the latter case. The light 
detector 13 is in no way limited to that of the two-divided type described 
above and may be of a three-divided type in which another light receiving 
section is interposed between a pair of light receiving sections for 
tracking purpose. The requirement is that the light detector has such a 
structure that at least two light detecting sections (light receiving 
sections) are disposed to sandwich the track therebetween in parallel to 
the extending direction of the track. An example of such a light detector 
detecting the track error, if any, on the basis of diffracted light from a 
pregroove is disclosed in, for example, U.S. Pat. No. 4,525,826. 
FIG. 4 is a block diagram showing the structure of a push-pull tracking 
servo system based on the principle described with reference to FIGS. 3a 
to 3d, and FIG. 5 is a Bode diagram of the loop transfer function of the 
servo system shown in FIG. 4. Referring to FIG. 4, the servo system 
includes a tracking error detecting element (Kd) 17, a phase compensating 
element (Gc) 18, and a tracking actuator (Ga) 19. According to the 
so-called push-pull tracking system for obtaining the differential signal 
16 indicative of the distribution of the reflection light diffracted from 
the pregroove 15, the galvana mirror 7 is moved so that the light spot 11 
can follow up, for example, eccentricity of the optical disc 10 shown in 
FIG. 1 for the purpose of tracking control. When the mirror 7 is moved, 
the diffracted light distribution on the light detector 13 shifts as shown 
in FIG. 3d. The shifting of the diffracted light distribution results in 
appearance of an offset in the tracking error signal 16. Further, when the 
optical disc 10 tilts, they diffracted light distribution on the two light 
receiving sections 131 and 132 of the two-divided light detector 13 
disposed in parallel to the track 12 is now out of balance, and such a 
phenomenon occurs in which the tracking error signal 16 does not become 
zero even when the light spot 11 is positioned on the track center (the 
center of the pregroove 15 or the center of the land 15-3), that is, a 
track offset occurs. Consequently, the light spot 11 cannot be accurately 
positioned on the track center. Therefore, according to the present 
invention, an offset-free prewobbling tracking error signal detected when 
the light spot 11 irradiates the prewobbling pits 20-1 and 20-2 in the 
header field 123 is utilized for offset correction of the push-pull 
tracking error signal 16 which includes the offset and which is detected 
when the light spot 11 irradiates the pregroove 15 disposed in the data 
field 122. In this manner, while correcting the push-pull tracking error 
signal 16 on the basis of the offset-free prewobbling tracking error 
signal, data pits are accurately recorded and reproducted, with a high S/N 
ratio, on and from the pregroove 15 or inter-pregroove land 15-3. 
How to detect the tracking error signal from the prewobbling pits 20-1 and 
20-2 shown in FIGS. 2b to 2d according to the present invention will now 
be described with reference to FIG. 6. 
FIG. 6 shows the arrangement of at least one pair of elongate pits 20-1 and 
20-2 disposed in a relation wobbled relative to the track center so as to 
detect the prewobbling tracking error signal according to the present 
invention and shows also the relation between the passing position of the 
light spot 11 and the waveform of the light output. When now the light 
spot 11 irradiates the pair of wobbled track marks 20-1 and 20-2 while 
crossing them relative to time, an output signal 21 having a waveform as 
shown appears from the light detector 13 (the sum of the output signals of 
the light receiving sections 131 and 132). It will be seen in FIG. 6 that 
the light output signals relevant to the elongate pits 20-1 and 20-2 
arranged in the relation wobbled relative to the track center have 
opposite polarities depending on the transverse deviation of the center of 
the light spot 11 from the track center. That is, when the light spot 11 
traverses the portion nearer to the pit 20-1, the output signal 21 has a 
waveform as shown by the dotted line, while, when the light spot 11 
traverses the portion nearer to the pit 20-2, the output signal 21 has a 
waveform inverted by 180.degree. in phase, as shown by the solid line. 
Therefore, by detecting the individual peaks 22 (22') and 23 (23') of the 
output signal 21 (the sum output of the two-divided or three-divided light 
detector) appearing when the light spot 11 traverses the pits 20-1 and 
20-2 respectively and then detecting the difference signal 24 
therebetween, this differential signal 24 indicates the amount and 
direction of deviation of the light spot 11 from the track center. 
The method of detecting the prewobbling tracking error signal 24 described 
with reference to FIG. 6 is not based on the diffracted light 
distribution. Therefore, a dc tracking offset attributable to tilting of 
the disk 10 or, for example, movement of the lens 9 or rotation of the 
galvana mirror 7 as a result of tracking control does not occur, so that 
the amount of the true tracking error can be accurately detected. 
The composite wobbled tracking servo system of the present invention can 
attain accurate tracking, since it comprises the combination of the two 
different tracking error detection methods, that is, the method of 
detecting the dc-offset-free tracking error signal 24 obtained from the 
prewobbling pits 20-1 and 20-2 shown in FIG. 6 and the method of detecting 
the dc-offset-including tracking error signal 16 obtained from the 
pregroove 15 shown in FIGS. 3a to 3d. The basic block diagram of the servo 
system based on the pregroove method has been illustrated already in FIG. 
4. Fundamentally, the servo system shown in FIG. 4 includes the tracking 
error detecting element (Kd) 17 converting the amount and direction of 
deviation of the light spot from the track center into an electrical 
signal according to the push-pull method, the phase compensating element 
(Gc) 18, and the tracking actuator (Ga) 19. However, as described already, 
in the case of the push-pull tracking error signal 16 utilizing the 
diffracted light distribution as shown in FIGS. 3a to 3d, an offset occurs 
inevitably in the signal 16 due to, for example, tilting of the disc 10, 
resulting in abnormal tracking. 
FIG. 7 is a block diagram showing the structure of an embodiment of the 
composite wobbled tracking servo system according to the present 
invention. The tracking servo system of the present invention shown in 
FIG. 7 has a dual servo system structure including a push-pull loop and a 
wobbled loop. In the tracking servo system of the present invention, the 
push-pull loop includes a tracking error detecting element (Kd) 25 
utilizing the pregrooves, phase compensating elements (G.sub.1) 27 and 
(G.sub.2) 29, and a tracking actuator (Ka) 30. The wobbled loop includes a 
tracking error detecting element (Kw) 26 utilizing the wobbled track 
marks, a low-pass filter (Fw) 28, and the common phase compensating 
elements, 27, 29 and the common tracking actuator 30 of the push-pull 
loop. 
In the disc structure employed in the present invention and shown in FIGS. 
2a to 2d, suppose, for example, that the number n of sectors per complete 
track is n=32, and the frequency fo of rotation of the disc 10 is fo=30 
Hz. Then, the prewobbling tracking error signal 24 detected from the 
prewobbling pits 20 included in the header field 123 is sampled at a 
frequency of nfo=32.times.30=960, that is, about 900 Hz. This prewobbling 
tracking error signal 24 is an accurate servo signal free from any offset 
attributable to, for example, tilting of the disc 10. Therefore, this 
prewobbling tracking error signal 24 is suitable for controlling a 
lowfrequency range of the servo system of FIG. 7. 
In the servo block diagram shown in FIG. 7, the prewobbling track error 
signal 24 is passed through the low-pass filter (Fw) 28 having a 
band-limiting filter characteristics as shown in FIG. 8, and a 
band-limited servo signal 31 that can handle the servo action in the 
low-frequency range appears from the low-pass filter (Fw) 28. When, for 
example, n=32 and fo=30 Hz as described above, the cut-off frequency Fc of 
the low-pass filter (Fw) 28 is preferably selected to be Fc=l00 Hz. 
On the other hand, in the case of the push-pull tracking error signal 16 
detected from the pregrooves 15, the method of signal detection is based 
on the diffracted light distribution. Therefore, this signal 16 includes a 
dc offset attributable to, for example, tilting of the disc 10, as 
described already. In the tracking servo system of the present invention, 
the pregroove tracking error signal 16 covers from a low-frequency range 
to a high-frequency range as shown in FIG. 8. The prewobbling tracking 
error signal 31 of the high gain band-limited by the low-pass filter (Fw) 
28 in the manner described above and the push-pull tracking error signal 
16 of low gain are added in an adder circuit 32 as shown in FIG. 7, and 
the resultant tracking error signal 33 is phase-compensated by the phase 
compensating circuits (G.sub.1) 27 and (G.sub.2) 29 and is then applied to 
the tracking actuator (Ka) 30 to consistitute the servo block 
FIG. 9 is a block diagram showing the structure of another embodiment of 
the tracking servo system according to the present invention. The tracking 
servo system shown in FIG. 9 has also a dual servo system structure 
including a push-pull loop and a wobbled loop. The push-pull loop includes 
a tracking error detecting element (Kd) 25 converting the amount and 
direction of deviation of the light spot from the track center into an 
electrical signal on the basis of the distribution of diffracted light 
from the pregrooves, a phase compensating element (K.phi.) 29 and a 
tracking actuator (Ka) 30. The wobbled loop includes a tracking error 
detecting element (Kw) 26 utilizing the wobbled track marks, a sample and 
hold circuit (Gs) 27, a low-pass filter (Fw) 28, and the common phase 
compensating element (K.phi.) 29 and the common tracking actuator (Ka) 30 
of the push-pull loop. 
In the disk structure employed in the present invention, suppose, for 
example, that the number n of sectors per complete track is n=32, and the 
frequency fo of rotation of the disc is fo=30 Hz. Then, the prewobbling 
tracking error signal 24 detected from the prewobbling pits 20 is sampled 
at a frequency of about 900 Hz. This prewobbling tracking error signal 24 
is suitable for controlling a low-frequency range of the servo system. The 
prewobbling tracking error signal 24 is passed through the sample and hold 
circuit 27 and then through the low-pass filter (Fw) 28 having a 
band-limiting filter characteristic as shown in FIG. 8, and a band-limited 
servo signal 31 that can handle the servo action in the low-frequency 
range appears from the low-pass filter (Fw) 28. When, for example, n=32 
and fo=30 Hz as described above, the cut-off frequency Fc of the low-pass 
filter (Fw) 28 is preferably selected to be Fc=l00 Hz. 
On the other hand, in the case of the push-pull tracking error signal 16 
detected from the pregrooves 15-1 and 15-2, the method of signal detection 
is based on the diffracted light distribution. Therefore, this signal 16 
includes a dc offset attributable to, for example, tilting of the disc 10, 
as described already. This pregroove tracking error signal 16 covers from 
a low-frequency range to a high-frequency range. The prewobbling tracking 
error signal 31 of high gain bandlimited by the low-pass filter (Fw) 28 
and the push-pull tracking error signal 16 of low gain are added in an 
adder circuit 32. The resultant tracking erro signal 33 is 
phase-compensated by the phase compensating element (K.phi.) 29 and is 
then applied to the tracking actuator (Ka) 30 to constitute the servo 
block. The wobbled loop acts to suppress a low-frequency offset of less 
than 30 Hz attributable to secular variations of the mechanical and 
optical systems and tilting, eccentricity or the like of the disc 10. When 
the proportional sensitivities of the individual elements only are noted 
for convenience of description, the steady-state error .DELTA.x is 
expressed as follows: 
##EQU1## 
where K.sub.F is the sensitivity of the offset. It can be seen from the 
above expression that the proportional sensitivity Kw of the wobbled loop 
detecting the true track center suppresses the offset occuring in the 
push-pull loop. 
FIGS. 8 and 10 are Bode diagrams. It will be seen that the proportional 
sensitivity Kd of the pushpull loop is about 2 dB and is constant in the 
range of from dc to 10 KHz. The proportional sensitivity Kw of the wobbled 
loop is about 14 dB and is low-passed at 30 Hz. As will be apparent from 
the equation described above, (Kw-Kd) represents the amount of suppression 
for all the offsets including the gain offset occurring in the push-pull 
loop and is 12 dB and 9 dB at dc and 30 Hz respectively. Therefore, even 
when a dc offset of, for example, 0.1 .mu.m occurs, the practical track 
error is suppressed to 0.02 .mu.m. The band of the wobbled loop is 
restricted by the period of the wobbled track marks, hence, by the 
sampling period. Therefore, in order to stabilize the operation of the 
servo system, it is necessary to attenuate the gain in the high-frequency 
range by the low-pass filter (Fw) 28. 
FIG. 11 is a block diagram showing the basic structure of the tracking 
error detecting element 26 and sample-hold circuit 27 shown in FIG. 9. 
FIG. 12 shows signal waveforms appearing at various parts of FIG. 11. 
Waveforms shown in (a), (b), (c), (d) and (e) of FIG. 12 correspond to 
signals 40, 41, 42, 43 and 44 shown in FIG. 11 respectively. 
The operation of the blocks shown in FIG. 11 will be described with 
reference to FIG. 12. The relative luminance signal from the light 
detector 13 (the sum signal of the output signals of the light receiving 
sections 131 and 132) is applied as an input signal 40 having a waveform 
as shown in (a) of FIG. 12. This input signal 40 is applied to a binary 
coding circuit 35 where the input signal 40 is clipped at a threshold 
level 47 shown in (a) of FIG. 12 to be turned into a binary signal 41 
having a waveform as shown in (b) of FIG. 12. The binary signal 41 is 
applied to a pit detecting circuit 36 where elongate pits (wobbled track 
marks) carrying track error information are detected. The pit detecting 
circuit 36 applies a first sample signal 42 having a waveform as shown in 
(c) of FIG. 12 to a first sample and hold circuit 37 and applies also a 
second sample signal 43 having a waveform as shown in (d) of FIG. 12 to a 
second sample and hold circuit 38. The information (22 shown in (a) of 
FIG. 12) contained in first elongate pit 20-1 is sampled by the sample and 
hold circuit 37 while the first sample signal 42 is in its high level, and 
is then held in the sample and hold circuit 37 as soon as the sample 
signal 42 is turned into its low level from the high level. Thus, an 
output signal 44 held at a level 22-1 and having a waveform as shown in 
(e) of FIG. 12 appears from the first sample and hold circuit 37. This 
output signal 44 from the sample and hold circuit 37 and the input signal 
40 are applied to a differential circuit 39, and the output signal 24 of 
the differential circuit 39 is applied to the second sample and hold 
circuit 38. The signal 24 is sampled while the second sample signal 43 is 
in its high level and is then held as soon as the signal 43 is turned into 
its low level from the high level. Thus, a tracking error signal 45 having 
a waveform as shown in (f) of FIG. 12 appears from the second sample and 
hold circuit 38. 
A problem arises when the prewobbling elongate pit 20-1 or 20-2 is damaged 
by a scar or dust present on the disc surface. This is because a wrong 
tracking error signal will be detected in such a case. FIG. 13 is an 
enlarged view of part of the recordable/reproducible optical disc 10 
employed in the present invention and illustrates that one of prewobbling 
elongated pits is damaged by a scar. In FIG. 13, there are shown a first 
prewobbling elongate pit 20-1, a second prewobbling elongate pit 20-2, a 
pregroove 15 and a damaged portion 50. In FIG. 13, the length 101 of the 
first elongate pit 20-1, the length 102 of the gap and length 103 of the 
second elongate pit 20-2 have a ratio of 6:4:6. However, the ratio is in 
no way limited to that specified above, when this part is regarded to be a 
servo mark (a wobbled track mark) and the pattern of modulation of an 
index and data to be recorded on the pregroove 15 is, for example, a 
two-to-seven modulation pattern. 
FIG. 14 is a block diagram showing the practical structure of the tracking 
error detecting circuit 26 utilizing the prewobbling pits. This tracking 
error detecting circuit 26 is provided by adding a servo mark detecting 
circuit 47 and a third sample and hold circuit 46 to the basic circuit 
block diagram shown in FIG. 11. FIG. 15 is a time chart of the operation 
of the circuit 26 shown in FIG. 14. In (a) of FIG. 15, there are shown an 
N-th servo mark period 51, an N-th pregroove period 52, an (N+1)-th servo 
mark period 51', and a damaged-portion period 53 due to the presence of a 
scar. Waveforms shown in (a), (b), (c), (d), (e), (f), (g) and (h) of FIG. 
15 correspond to signals 40, 42, 43, 49, 44, 48, 24 and 45 shown in FIG. 
14 respectively. The operation of the circuit 26 shown in FIG. 14 will be 
described with reference to FIG. 15 and part of FIG. 11. 
The relative luminance signal from the light detector 13 is applied as an 
input signal 40 having a waveform as shown in (a) of FIG. 15. This input 
signal 40 is applied to the binary coding circuit 35 where the input 
signal 40 is turned into a binary signal 41. The binary signal 41 is 
applied to the pit detecting circuit 36, and a signal 42 including a first 
sample pulse (s-own in (b) of FIG. 15) indicative of detection of the 
first elongate pit and a signal 43 including a second sample pulse (shown 
in (c) of FIG. 15) indicative of detection of the second elongate pit 
appear from the pit detecting circuit 36. The first and third sample and 
hold circuits 37 and 46 sample and hold the input signal 40. The output 
signal 44 of the first sample and hold circuit 37 having a waveform as 
shown in (e) of FIG. 15 and the output signal 48 of the third sample and 
hold circuit 46 having a waveform as shown in (f) of FIG. 15 are applied 
to the differential circuit 39, and a tracking error signal 24 having a 
waveform as shown in (g) of FIG. 15 appears from the differential circuit 
39. The binary signal 41 is also applied to the servo mark detecting 
circuit 47 which carries out pattern matching and generates a mark 
detection signal 49 having a waveform as shown (d) of FIG. 15 when the 
servo mark is recognized as a result of pattern matching. The second or 
output sample and hold circuit 38 samples the tracking error detection 
signal 24 (H in (d) of FIG. 15) and holds the signal 24 (L in (d) of FIG. 
15), thereby finally generating a tracking error signal 45 based on the 
prewobbling pits and having a waveform as shown in (h) of FIG. 15. The 
above description refers to the normal operation. When a scar 50 as 
illustrated in FIG. 13 is present on the disc surface, the damaged-portion 
period 53 appears in the waveform shown in (a) of FIG. 15. However, in the 
circuit form shown in FIG. 14, the output sampling and hold circuit 38 
holds the previous data, unless the servo mark detecting circuit 47 
mal-operates. When the circuit 47 mal-operates, a pulse 54 as indicated by 
the dotted line in (d) of FIG. 15 appears. Therefore, disturbance due to 
the presence of a scar on the disc surface, which disturbance is 
unavoidable in the case of the structure shown in FIG. 11, would not 
appear in the final output signal 45, and the signal 45 has a solid-line 
waveform free from a damage component 55 shown by the dotted line in (h) 
of FIG. 15. 
FIG. 16 is a block diagram of another form of the tracking error detecting 
circuit 26 utilizing the prewobbling pits for tracking error detection 
according to the present invention. The first form of the tracking error 
detecting circuit described above includes still the possibility of 
unstable tracking when a plurality of damaged servo marks are present in 
the same track. The form shown in FIG. 16 can deal with the presence of 
plurality of damaged servo marks. Parts enclosed by the dotted line 76 
have the same structure and operate in the some way as the equivalent 
parts of the circuit shown in FIG. 14, and such parts will not be 
especially referred to in the following description. 
FIG. 17 shows signal waveforms appearing at various parts of FIG. 16, and 
waveforms shown in (a), (b), (c), (d), (e), (f), (g), (h), (i) and (j) of 
FIG. 17 correspond to signals 40, 42, 43, 49, 66, 69, 70, 24, 72 and 68 
shown in FIG. 16 respectively. As described above, explanation of the 
tracking error detecting circuit part 76 enclosed by the dotted line is 
unnecessary. The circuit shown in FIG. 16 is based on the fact that the 
tracking error signal 16 detected from the pregrooves is apparently offset 
by the prewobbling tracking error signal 24 and is featured by the 
provision of a circuit, which stores the pregroove tracking error signal 
16 when the servo marks are normally detected, but replaces the pregroove 
tracking error signal by the stored prewobbling tracking error signal when 
the servo marks are dropped out. Such a circuit is added to the track 
error detecting circuit part 76 shown in FIG. 14. 
When the mark detection signal 49 shown in (d) of FIG. 17 is normally 
generated, a flip-flop 60 is reset, and a Q output signal 69 of low level 
appears from the flip-flop 60. As a result, an analog switch 63 conducts 
at its normally-closed contact 64, and the output signal 45 (shown in (h) 
of FIG. 17) of the output sample and hold circuit 38 appears as the final 
output signal shown in (j) of FIG. 17. An analog inverted signal of a 
push-pull signal 75 indicative of pregroove track error detection is 
quantized by an A/D converter 59, and the resultant data 71 is written in 
a memory 61. When a mark drop-out as shown by 54 in (d) of FIG. 17 occurs 
then in the mark detection signal 49, a mark interpolating circuit 56 
operates to apply a mark drop-out detection signal 66 as shown in (e) of 
FIG. 17 to the flip-flop 60 to set the flip-flop 60 as shown in (f) of 
FIG. 17. The analog switch 63 conducts now at its normally-open contact 
65, and, at the same time, the data 72 of the inverted push-pull signal 
written in the memory 61 during normal operation is now read out from the 
memory 61. An output signal 73 as shown in (i) of FIG. 17 appears from a 
D/A converter 62 and is applied to the contact 65 of the analog switch 63, 
so that a corrected final output signal, that is, a prewobbling tracking 
error signal 68 as shown in (j) of FIG. 17 is obtained. A counter 58 is 
reset by a rotation synchronous signal 74 generated every complete 
rotation of the optical disc 10. The counter 58 is incremented by a 
leading edge of an output signal 67 of an OR circuit 57 to which the mark 
detection signal 49 and the mark drop-out detection signal 66 are applied. 
The resultant output signal 70 (shown in (g) of FIG. 17) of the counter 58 
provides read/write address information applied to the memory 61. 
Thus, the embodiments of the tracking servo system of the present invention 
shown in FIGS. 7 and 9 ensure tracking with high accuracy without giving 
rise to an undesirable offset. 
It will be understood from the foregoing detailed description of the 
composite wobbled tracking servo system of the present invention that 
information can be accurately recorded with a high recording density on an 
optical disc, and the information thus recorded with the high recording 
density can be accurately read out from the optical disc. Therefore, the 
present invention can stimulate a great progress when applied to the field 
of digital discs, digital audio discs and the like. The present invention 
can obviate the defect of the prior art manner of information recording on 
pregrooves, that is, the defect of giving rise to a great tracking offset 
attributable to, for example, tilting of an optical disc. The present 
invention can also prevent mal-operation due to defective servo marks or 
sector marks on an optical disc, thereby eliminating the necessity for 
re-writing servo marks and/or sector marks and the necessity for 
replacement of the optical disc. Therefore, the present invention can 
improve the productivity of optical discs and can also reduce the 
manufacturing costs of optical discs.