JPH0573937A - Detection of tracking error - Google Patents

Abstract

PURPOSE:To enable generation of a tracking error in accordance with the off-tracking amount even if the tracking error exceeding 1/4 of track pitch is in existence and to drastically improve an pull-in performance of the tracking. CONSTITUTION:Reproduced outputs of a pair of wobble pits and clock pit are taken as (a), (c), (b). A prescribed case sorting and the tracking errors of each case are set in a selector 51 based on (a), (c), (b). By the selector 51, the tracking error of the relevant case is outputted by using the inputs coming through a computing element 47, multiplier 49 and the inputs coming through a computing element 48, multiplier 50, based on the outputs of comparators 45, 46.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tracking error detecting method for detecting a tracking error in a case where a tracking error at the time of tracking an information recording medium of a sample servo system is set for each predetermined case. ..

[0002]

2. Description of the Related Art In a magneto-optical disk capable of rewriting data, a sample servo system is used in which tracking information and clock information are obtained from sample servo pits arranged at predetermined intervals in the circumferential direction of the magneto-optical disk. There is a magneto-optical disk. In this sample servo type magneto-optical disk, as shown in FIG. 6, tracks 1 are formed concentrically or spirally, and each track 1 has a sample servo pit SP at a predetermined rotation angle.
Are arranged in a radial pattern. The sample servo pits SP are displaced from the clock pits Pb arranged on the center line of the track 1 by 1/4 track pitch (hereinafter, the track pitch is T) from the center of the track 1 toward the inner circumference and the outer circumference. It is composed of a pair of wobble pits Pa and Pc arranged at positions.

Conventionally, for a sample servo type magneto-optical disk having the above sample format configuration,
For example, when reproducing is performed in the magneto-optical disk device, the tracking error detection circuit detects the tracking error as a-c at the time of tracking by using the reproduction RF outputs a and c of the pair of wobble pits Pa and Pc. It was
FIG. 7 shows the arrangement of the pair of wobble pits Pa, Pc and the clock pit Pb and the reproduction RF output when the beam spot scans the track 1.

FIG. 8 shows the relationship between the reproduction RF outputs a, c and b of the wobble pits Pa and Pc and the clock pit Pb and the tracking error ac. As can be seen from FIG. 8, regarding the tracking errors a to c, the off-track amount may be T / 4 or less, but T / T may be centered around T / 2.
Despite the large off-track in the range of 4 to 3T / 4, only the same value as the tracking error in the range of -T / 4 to T / 4 was detected.

[0005]

In the conventional tracking error detection circuit described above, the tracking error is a-
It was detected as c. However, as for the tracking error a-c, as shown in FIG. 8, although it is largely off-tracked in the range of T / 4 to 3T / 4 centering on T / 2, -T / 4 to T / Only the same value as the tracking error in the range of 4 was detected. Therefore, when the beam spot position exceeds the range of ± T / 4,
There was a problem that the tracking was not pulled in well.

In view of the above-mentioned conventional problems, an object of the present invention is to detect a tracking error which can generate a tracking error according to the amount of off-track and thereby greatly improve the tracking pull-in performance. To provide a method.

[0007]

According to a first aspect of the present invention, there is provided a tracking error detecting method for tracking a sample servo type information recording medium in which sample servo pits including wobble pits and clock pits are arranged in a track direction. In the tracking error detection method for detecting the tracking error of each of the wobble pits and the clock pits, the tracking error for each of the cases is set based on the reproduction output level of the wobble pits and the clock pits, and the tracking error for each case is set. It is characterized in that the tracking error is detected when it is applicable based on the magnitude of the output level.

According to a second aspect of the present invention, there is provided a tracking error detecting method, wherein a pair of wobble pits and clock pits are displaced from each other by T / 4 (where T is the track pitch) inward and outward in the track direction. In a tracking error detecting method for detecting a tracking error at the time of tracking with respect to an information recording medium of the sample servo system in which the sample servo pits are arranged, the reproduction output levels a, c and b of a pair of wobble pits and clock pits are also included. And (1) b <(a + c) / 2 and a <c, E = K {c- (a + c) / 2} -T / 2 (2) b ≧ (a + c) / When 2, E = K {a- (a + c) / 2} (3) b <(a + c) / 2, and when a ≧ c, E = K {c- (a + c) / 2} + T / Here, K is a constant case, and it is necessary to detect the corresponding tracking error based on the magnitudes of the reproduction output levels a, c, and b of the pair of wobble pits and clock pits. Characterize.

According to a third aspect of the present invention, there is provided a pair of tracking error detection methods, which are offset from each other by T / 4 (where T is a track pitch) inward and outward in the direction of concentric tracks. In a tracking error detection method for detecting a tracking error at the time of tracking on a sample servo type information recording medium in which sample servo pits composed of wobble pits and clock pits are arranged, a reproduction output level a of a pair of wobble pits and clock pits , C, b as a tracking error E, (1) when b <a <c, E = K {c- (a + c) / 2} -T / 2 (2) when a <b <c , E = K {b− (a + c) / 2} −T / 4 (3) a <b and c <b, E = K {a− (a + c) / 2} (4) c <b < a When E = -K {b- (a + c) / 2} + T / 4 (5) b <c <a, E = K {c- (a + c) / 2} + T / 2 where K is a constant The case is characterized in that the tracking error is detected based on the magnitudes of the respective reproduction output levels a, c and b of the pair of wobble pits and clock pits.

[0010]

According to the tracking error detecting method of the present invention, the tracking error is set for each predetermined case and each case is set based on the reproduction output level of the wobble pit and the clock pit. The corresponding tracking error is detected based on the magnitude of each reproduction output level of the clock pit.

In the tracking error detecting method according to the second aspect of the present invention, the tracking error E is defined as (1) b <(a + c) / based on the reproduction output levels a, c and b of the pair of wobble pits and clock pits. 2 and a <c, E = K {c- (a + c) / 2} -T / 2 (2) b ≧ (a + c) / 2, E = K {a- (a + c) / 2} (3) When b <(a + c) / 2 and a ≧ c, E = K {c− (a + c) / 2} + T / 2 Here, K is a constant case, and a pair of wobbles is used. Based on the magnitudes of the reproduction output levels a, c and b of the pits and the clock pits, the corresponding tracking error is detected.

In the tracking error detecting method according to the third aspect of the present invention, the tracking error E is calculated based on the reproduction output levels a, c and b of the pair of wobble pits and clock pits, and (1) b <a <c When E = K {c- (a + c) / 2} -T / 2 (2) a <b <c, E = K {b- (a + c) / 2} -T / 4 (3) a < When b and c <b, E = K {a- (a + c) / 2} (4) When c <b <a, E = -K {b- (a + c) / 2} + T / 4 (5 ) When b <c <a, E = K {c- (a + c) / 2} + T / 2 where K is a constant, and each reproduction output level of a pair of wobble pits and clock pits is divided. Based on the sizes of a, c, and b, the tracking error is detected when applicable.

By doing so, it is possible to generate a tracking error according to the amount of off-track, and it is possible to greatly improve the tracking pull-in performance.

[0014]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 2 is a block diagram showing the configuration of an embodiment of a magneto-optical disk device to which the present invention is applied. Sample servo type magneto-optical disk 12 as information recording medium
Are rotated by the spindle motor 11. The magnetic head 13 applies a magnetic field having a polarity corresponding to a recording signal to the magneto-optical disk 12. Also,
The optical (OP) block 14 irradiates the magneto-optical disk 12 with laser light. The slide motor 15 moves the magnetic head 13 and the optical block 14 in the radial direction of the magneto-optical disk 12. The AGC circuit 16 controls the gain of the signal output from the optical block 14 and outputs the reproduction signal to A
Output to the / D converter 27. The AGC circuit 16 also outputs a part of the reproduced signal to the A / D converter 18. The APC circuit 17 controls, for example, a semiconductor laser built in the optical block 14 and controls the intensity of laser light output from the semiconductor laser.

The A / D converter 18 A / D converts the reproduction signal supplied from the AGC circuit 16, and outputs a servo pattern detection circuit 19, a PLL error detection circuit 20, a tracking error detection circuit 21 and a gray code detection circuit 22. Are supplied to each. The timing generator 23 generates a predetermined timing signal corresponding to the output of the servo pattern detection circuit 19 and outputs it to the PLL error detection circuit 20, the tracking error detection circuit 21, and the gray code detection circuit 22. VCO24
Generates a master clock having a predetermined frequency and phase corresponding to the output of the PLL error detection circuit 20, and supplies the master clock to the timing generator 23, the A / D converter 18, and the delay controller 26.

The seek and tracking servo controller 25 controls the optical block 14 and the slide motor 15 in response to the outputs of the tracking error detection circuit 21 and the gray code detection circuit 22.

The delay controller 26 is a VCO 24.
The input master clock is delayed by the time corresponding to the control signal supplied from the drive controller 32, and is output to the A / D converter 27 and the encoder 28 as a data read clock and a data write clock, respectively. The equalizer 29 is the A / D converter 27.
The reproduced data outputted by the above is subjected to a predetermined equalizing process and then outputted to the data detection circuit 30. The data detection circuit 30 detects data from the output of the equalizer 29 and outputs it to the decoder 31. The decoder 31 decodes the data from the data detection circuit 30 and outputs it as read data.

Next, the operation will be described. The drive controller 32 controls the spindle motor 11 to rotate the magneto-optical disk 12 at a predetermined speed when, for example, a recording command is issued from a unit (not shown). The encoder 28 encodes the write data supplied from a circuit (not shown) at the timing of the data write clock, and supplies the encoded data to the magnetic head 13. Magnetic head 13
Changes the magnetic pole applied to the magneto-optical disk 12 in accordance with the write data supplied from the encoder 28. At this time, the magneto-optical disk 1 from the optical block 14
Since the laser beam is radiated to No. 2, predetermined data is written in the magneto-optical disk 12.

On the other hand, when the reproduction mode is instructed, the optical block 1 is controlled by the drive controller 32.
4 makes the intensity of the laser beam applied to the magneto-optical disk 12 weaker than that at the time of recording. Then, the information recorded on the magneto-optical disk 12 is reproduced. The reproduction signal output from the optical block 14 is supplied to the AGC circuit 16, the gain thereof is adjusted, and then the A / D converter 2
7 is supplied. The A / D converter 27 A / D-converts the reproduction signal in response to the data read clock input from the delay controller 26. The reproduction data output from the A / D converter 27 is equalized to a predetermined characteristic by the equalizer 29 and then supplied to the data detection circuit 30. The data detection circuit 30 detects desired data from various data. The data detected by the data detection circuit 30 is supplied to the decoder 31, decoded and then output as read data to a circuit (not shown).

On the other hand, a part of the reproduction signal output from the AGC circuit 16 (for example, each reproduction RF signal of the wobble pits Pa, Pc and the clock pit Pb) is supplied to the A / D converter 1.
8 and A / D converted. The data output from the A / D converter 18 is the servo pattern detection circuit 19, P
It is supplied to the LL error detection circuit 20, the tracking error detection circuit 21, and the gray code detection circuit 22, respectively. The servo pattern detection circuit 19 detects a servo pattern from the input data and outputs the detection result to the timing generator 23. The timing generator 23 generates a timing signal at a predetermined timing corresponding to the output from the servo pattern detection circuit 19, and outputs it to the PLL error detection circuit 20, the tracking error detection circuit 21, and the gray code detection circuit 22, respectively. There is.

The PLL error detection circuit 20 detects a PLL error from the output of the A / D converter 18. VCO24
Controls the frequency and phase of the output clock in accordance with the PLL error signal output by the PLL error detection circuit 20. The master clock output from the VCO 24 is supplied to the timing generator 23, the A / D converter 18, and the delay controller 26.

The tracking error detection circuit 21 calculates (detects) a tracking error signal based on the output data of the A / D converter 18. The tracking error detection circuit 21 is according to the present invention, and its details will be described later with reference to FIG.

The tracking error signal from the tracking error detection circuit 21 is supplied to the seek and tracking servo controller 25, and the seek and tracking servo controller 25 receives the input tracking error detection circuit 21 and gray code detection circuit 2.
The optical block 14 and the slide motor 15 are controlled according to the output of 2. The slide motor 15 moves the magnetic head 13 and the optical block 14 in the radial direction of the magneto-optical disk 12 under the control of the seek and tracking servo controller 25.

FIG. 3 shows an arrangement example of the sample servo pits on the magneto-optical disk 12 of the sample servo system according to the present invention and the reproduction RF output. In FIG. 3, the pair of wobble pits Pa and Pc are arranged at positions displaced by 1 / 4T from the center of the track 1 toward the inner circumference and the outer circumference, and the clock pit Pb is arranged on the center of the track 1. ing. The reproduction RF outputs a, c and b by the pair of wobble pits Pa and Pc and the clock pit Pb when the beam spot scans the center of the track 1 are
As shown in the figure.

When the beam spot tracks in the radius (r) direction, a pair of wobble pits Pa, P
c, reproduction RF output a obtained from clock pit Pb,
For c and b, the position of a certain track 1 is used as a reference (0).
When illustrated as, it becomes as shown in FIG. In FIG. 4, reproduction RF outputs a, c, b obtained from the pair of wobble pits Pa, Pc and clock pit Pb are given by the following equation. a = αsin (2πΔr) + β ・ ・ ・ ・ ・ ・ ・ ・ (1) b = αcos (2πΔr) + β ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (( 2) c = -αsin (2πΔr) + β (3) where α is the peak value, β is the average value (offset), and β = (a + c ) / 2.

In FIG. 4, the reproduction RF outputs a, b, and c for one track pitch up to the adjacent track 1 on the outer peripheral side and the inner peripheral side in the radial direction with the position of a certain track 1 as a reference (0). Shows. Therefore, when the beam spot position deviates from the track 1, the reproduction RF outputs a, b,
c takes a value shown in the figure corresponding to a position displaced from time 0 with reference to the position 0 of a certain track 1.

FIG. 1 is a block diagram showing an embodiment of the tracking error detecting method according to the present invention, and is a block diagram showing an embodiment of the tracking error detecting circuit 21 of FIG. In FIG. 1, the tracking error detection circuit 21 includes a latch circuit 41 to 43 and an arithmetic unit 44, 4.
7, 48, comparators 45 and 46, multiplier 49,
50 and a selector 51. The A / D converter 18 supplies the reproduction RF signal corresponding to each wobble pit to the latch circuits 41 to 43. The latch circuit 41 sequentially operates the A / D converter 1 at the timing of the timing signal Ta.
The reproduction RF output a of the wobble pit Pa from 8 is latched and output. The latch circuit 42 latches the reproduction RF output b of the clock pit Pb from the A / D converter 18 one after another at the timing of the timing signal Tb and outputs it. The latch circuit 43 latches the reproduction RF output c of the wobble pit Pc from the A / D converter 18 one after another at the timing of the timing signal Tc, and outputs it.

The arithmetic unit 44 calculates the sum of the output a of the latch circuit 41 and the output c of the latch circuit 43, further calculates 1/2 of the sum, and outputs it to the comparator 45.
The comparator 45 outputs the output (a + c) from the arithmetic unit 44.
/ 2 is compared with the output b of the latch circuit 42, and the result is output to the selector 51 as logic "1" or "0". Here, for example, the logical "1" is set when b <(a + c) / 2, and the logical "0" is set when b≥ (a + c) / 2.

The comparator 46 compares the output a of the latch circuit 41 with the output c of the latch circuit 43, and outputs the result to the selector 51 as a logic "1" or "0". Here, for example, when a <c, a logical "1" is set,
When a ≧ c, the logical value is “0”. The arithmetic unit 47 calculates the difference a− between the output a of the latch circuit 41 and the output c of the latch circuit 43.
c is calculated, and 1/2 is calculated, and the result (ac) / 2 is supplied to the multiplier 49. Multiplier 49
Outputs the output of the computing unit 47 to the selector 51 by multiplying it by K. Further, the calculator 48 calculates the difference c−a between the output c of the latch circuit 43 and the output a of the latch circuit 41, and further calculates 1/2 of the difference c−a, and the result (c−a) / 2 is obtained. It is supplied to the multiplier 50. The multiplier 50 outputs the output of the calculator 48 as K
It is multiplied and output to the selector 51.

The selector 51 outputs the output K (c) of the multiplier 50 when the output from the comparator 45 is logic "1" and the output from the comparator 46 is logic "0".
Output obtained by adding a value (-T / 2) prepared in advance in the selector 51 to -a) / 2, that is, {K (c-a) / 2}.
-T / 2 [= K {c- (a + c) / 2} -T / 2],
It is sent as a tracking error.

Further, the selector 51 includes a comparator 45.
When the output from is a logical "0", the comparator 4
6 output K (a-
c) / 2 [= K {a- (a + c) / 2}] is sent as a tracking error.

The selector 51 outputs the output K (c) of the multiplier 50 when the output from the comparator 45 is logic "1" and the output from the comparator 46 is logic "0".
An output obtained by adding a value (T / 2) prepared in advance in the selector 51 to −a) / 2, that is, {K (c−a) / 2} +
T / 2 [= K {c- (a + c) / 2} + T / 2] is sent as a tracking error.

In summary, the tracking error E
Is as follows: (B) When b <(a + c) / 2 and a <c, E = K {c- (a + c) / 2} -T / 2 (4) (b) b ≧ (a + c) ) / 2, E = K {a- (a + c) / 2} (5) (c) b <(a + c) / 2, and a ≧ c, E = K {c- (a + c) / 2} + T / 2 (6) where K is a constant

The above is the case where the tracking error is divided into three cases (three divisions).
The present invention is not limited to this, and the tracking error can be divided into five cases. In this case, for example, case classification and tracking error in each case are set as follows. (A) When b <a <c, E = K {c- (a + c) / 2} -T / 2 (7) (b) When a <b <c, E = K {B- (a + c) / 2} -T / 4 (8) (c) When a <b and c <b, E = K {a- (a + c) / 2} ... (9) (d) When c <b <a, E = -K {b- (a + c) / 2} + T / 4 (10) (e) b When <c <a, E = K {c− (a + c) / 2} + T / 2 (11) where K is a constant, and among these preset cases, a pair of wobbles is used. Based on the sizes of the reproduction output levels a, c, and b of the pits Pa, Pc and the clock pit Pb, the tracking error in the case of corresponding is calculated (detected).

The seek and tracking servo controller 25 in FIG. 2 applies the tracking servo based on the tracking error calculated (detected) as described above.

When the tracking error calculation (detection) is divided into five cases as described above (in the case of five divisions)
The tracking error detection circuit 21 of FIG. 2 for carrying out the tracking error detection method according to the present invention can be configured in the same manner as in FIG. In this case, for example, reproduction RF
Three latch circuits for respectively latching outputs a, b, and c, the size of the reproduction RF output a and the reproduction RF output b, and b
And c, the magnitudes of c and a are compared, and three comparators that output the results are compared, and (ac) / 2,
Four calculators for performing respective calculations of (c−a) / 2, (b−a) / 2, and (b−c) / 2, and output (b−a) / 2 and output (b−c) A tracking error detection circuit using an adder that adds / 2, four multipliers that multiply a constant K, and a selector that determines which of the above settings is applicable and outputs the tracking error. 21 can be configured.

The tracking error E in the case of the above five divisions can be said as follows from FIG. 4 and the above equations (7) to (11). That is, in the case of the above (a), -T / in FIG.
It is a section of 2 to -3T / 8, and in this section, the tracking error E changes according to the reproduction RF output c of the thick line portion of FIG. In the case of (b), it is a section from −3T / 8 to −T / 8 in FIG. 4, and in this section, the tracking error E is calculated from the above equation (8).
It changes depending on the reproduction RF output b of the thick line portion of the line. In the case of (c) above, it is a section from -T / 8 to T / 8 in FIG. 4, and in this section, the tracking error E is the above (9).
According to the equation, it varies depending on the reproduction RF output a of the thick line portion in FIG. In the case of (d) above, T / 8 to 3T / 8 in FIG.
In this section, the tracking error E is calculated from the above expression (10) by the reproduction RF output b of the thick line portion in FIG. The symbol is added)) Change. In the case of the above (e), it is a section of 3T / 8 to T / 2 in FIG. 4, and in this section, the tracking error E is (1)
According to the expression (1), it changes depending on the reproduction RF output c of the thick line portion in FIG.

Next, the tracking error obtained by the present invention (in case of 3 divisions and in case of 5 divisions) will be described with reference to FIG.

In the conventional case, when the beam spot position (off-track position) exceeds T / 4, the tracking error becomes small as shown by the chain line B, and it becomes difficult to return to the original track. On the other hand, in the present invention, the tracking error amount does not decrease even when the beam spot position (off-track position) exceeds T / 4, and conversely, the solid line B (in the case of 3 divisions) and the halftone line C (in the case of 5 divisions). In the case where the beam spot position (off-track position) exceeds T / 4, the original track can be restored until T / 2. Therefore, in the present invention, the pull-in range of tracking is significantly wider than in the conventional case. Therefore, the tracking pull-in performance is significantly improved. The present invention is particularly effective when the tracking servo is turned on immediately after the seek ends.

The present invention is not limited to this embodiment, and various applications and modifications are conceivable without departing from the gist of the present invention.

[0041]

As described above, according to the present invention, it is possible to generate a tracking error according to the off-track amount. For this reason, even if the beam spot position exceeds T / 4, the tracking error amount does not decrease, but on the contrary, it significantly increases. Therefore, even if the beam spot position exceeds T / 4, the original track is restored until T / 2. Can be returned. Therefore, the present invention can significantly widen the pull-in range of tracking as compared with the related art. Therefore, the tracking pull-in performance can be significantly improved. The present invention is effective when the tracking servo is turned on immediately after the seek ends.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a tracking error detection method according to the present invention.

FIG. 2 is a block diagram showing the configuration of an embodiment of a magneto-optical disk device to which the present invention is applied.

FIG. 3 is an explanatory diagram showing an arrangement example of sample servo pits and a reproduction RF output on a sample servo type magneto-optical disk according to the present invention.

FIG. 4 is an operation explanatory diagram of another embodiment of the tracking error detection method according to the present invention.

FIG. 5 is a comparison diagram showing a tracking error according to an embodiment of the present invention and a conventional tracking error.

FIG. 6 is an explanatory diagram of sample servo pits.

FIG. 7 is an explanatory diagram showing the relationship between the arrangement of sample servo pits and the reproduction RF output.

FIG. 8 is an explanatory diagram showing an example of a conventional tracking error.

[Explanation of symbols]

 12 magneto-optical disk 18 A / D converter 21 tracking error detection circuit 23 timing generator 41-43 latch circuit 44 calculator 45, 46 comparator 47, 48 calculator 49, 50 multiplier 51 selector Pa, Pc wobble pit Pb clock pit T track pitch

Claims (3)

[Claims] 1. A tracking error detection method for detecting a tracking error at the time of tracking on a sample servo type information recording medium in which sample servo pits composed of wobble pits and clock pits are arranged in a track direction. Based on the reproduction output levels of the clock pit and the clock pits, a predetermined case division and a tracking error for each case are set, and it is applicable based on the magnitudes of the reproduction output levels of the wobble pits and the clock pits. The tracking error detecting method is characterized by detecting the tracking error in the case. 2. A sample servo pit composed of a pair of wobble pits and clock pits, which are offset from each other by T / 4 (where T is a track pitch) on the inner and outer circumference sides in the track direction. In a tracking error detecting method for detecting a tracking error at the time of tracking with respect to an information recording medium of a sample servo system, a tracking error E based on each reproduction output level a, c, b of the pair of wobble pits and the clock pit.
When (1) b <(a + c) / 2 and a <c, E = K {c− (a + c) / 2} −T / 2 (2) b ≧ (a + c) / 2, E = K {a- (a + c) / 2} (3) When b <(a + c) / 2 and a≥c, E = K {c- (a + c) / 2} + T / 2 where K is a constant The tracking error is detected based on the magnitudes of the respective reproduction output levels a, c and b of the pair of wobble pits and the clock pit. Error detection method. 3. A sample servo pit composed of a pair of wobble pits and clock pits, which are offset from each other by T / 4 (where T is the track pitch) on the inner and outer circumference sides in the track direction. In a tracking error detecting method for detecting a tracking error at the time of tracking with respect to an information recording medium of a sample servo system, a tracking error E based on each reproduction output level a, c, b of the pair of wobble pits and the clock pit.
(1) When b <a <c, E = K {c- (a + c) / 2} -T / 2 (2) When a <b <c, E = K {b- (a + c) / 2} -T / 4 (3) When a <b and c <b, E = K {a- (a + c) / 2} (4) When c <b <a, E = -K {b- (A + c) / 2} + T / 4 (5) When b <c <a, E = K {c- (a + c) / 2} + T / 2 where K is a constant, and A tracking error detecting method, wherein the tracking error is detected when it corresponds to each of the reproduction output levels a, c and b of a pair of wobble pits and the clock pit.