JP2840685B2 - Clock signal generator for data reproduction and data reproduction device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a data reproduction clock signal generation device and a data reproduction device, and more particularly to forming an external clock signal using a reference clock signal recorded on a recording medium. The present invention relates to a clock signal generator for data reproduction and a data reproduction device for reproducing data using the external clock signal.

[Summary of the Invention]

The clock signal generator for data reproduction according to the present invention comprises:
Each of the reference clock signals actually recorded on the recording medium is sampled using a plurality of external clock signals having different phases from each other, and the maximum value of the difference between the sample values of each external clock signal is determined. By using the signal as a data reproduction clock signal, an external clock signal having a high window margin and suitable for data reproduction can be obtained.

Further, the data reproducing apparatus according to the present invention samples each of the reference clock signals actually recorded on the recording medium by using a plurality of external clock signals having different phases from each other, and calculates a difference between sample values due to each external clock signal. A maximum value is obtained, an external clock signal corresponding to the maximum value is used as a data reproduction clock signal, and a reproduction signal obtained by reproducing data is sampled using the external clock signal. By reproducing the data by comparing it with about 1/2 of the maximum value of the difference between the sample values of the reproduction signal of the reference clock signal, data reproduction with a high noise margin can be performed.

[Conventional technology]

In magneto-optical recording, as shown in FIG. 4, data is recorded by magnetizing a portion on a disk, which has reached the Curie temperature by laser irradiation, in the direction of an externally applied magnetic field. That is, for example, a laser diode is pulsed to emit light, an external magnetic field is modulated by recording data, and pits (portions magnetized by the external magnetic field) corresponding to the spot size of the laser beam are formed on the recording surface in a time-division manner. By doing so, data is recorded. By the way, if the interval of the pulse for driving the laser light source is made smaller than the spot size of the laser beam in order to increase the recording density, a part of the spot of the laser beam overlaps with a part of the pit formed earlier, and the magnetizing occurs first. The pits to be formed later are overwritten (overwritten) on a part of the pits. That is, the pits are formed one by one in a time-division manner, and when the recording density is increased, overlap occurs. As a result, a so-called edge recording in which a change point (edge) of the magnetization direction has information is obtained.

In addition, a magneto-optical recording medium, for example, a magneto-optical disk is driven to rotate at a constant linear velocity (CLV) or a constant angular velocity (CAV) by a spindle motor so that the pits are formed in a spiral or concentric pit row. It has become. By the way, in the case of constant angular velocity (CAV) driving, the distance (length) that the recording head travels per unit time is smaller on the inner circumference than on the outer circumference. Therefore, as shown in FIG. It becomes larger than the overlap amount on the outer periphery. When data is reproduced from the pits recorded and formed in this way, as shown in FIG. 6, the signal reproduced from the inner pit is temporally advanced (shifted) from the signal reproduced from the outer pit. It takes shape. Therefore, in the so-called external clock system, the inner peripheral pits are reproduced by using the same external clock signal as a synchronized clock signal (hereinafter referred to as an external clock signal) obtained from a magneto-optical disk for reproducing the outer peripheral pits. Then, the change point, that is, the position of the edge cannot be accurately detected.
In other words, the so-called detection window width (window) becomes narrow, and when the modulation method of the external magnetic field is the so-called NRZI modulation, the noise margin with respect to the threshold for judging the presence or absence of an edge at the time of reproduction decreases.

[Problems to be solved by the invention]

As a method of solving the above problem, a method of forming a pit by performing correction in advance at the time of recording based on radius information and surrounding environment (temperature, power of a laser light source, and the like) can be considered. It is difficult to find a proper recording condition. Furthermore, differences in characteristics (individual differences) of the magneto-optical disk and differences between the data recording device and the data reproducing device may occur, so that correction at the time of recording alone is not sufficient to solve the above problem. Therefore, in order to achieve high-density recording, it is necessary to correct, for example, the phase of the external clock signal based on the waveform of a signal reproduced during data reproduction (reproduced signal waveform).

In addition, a clock signal is superimposed on a data signal recorded on the magneto-optical disk, and the recorded clock signal is extracted at the time of reproduction, and a so-called self-timer using this clock signal as a clock signal for reproduction is used. There is a clock system. In this system, a dedicated modulation system needs to be used, and a clock pull-in area for sufficiently extracting a clock is required.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and particularly when reproducing a magneto-optical disk driven at a constant angular velocity (CAV) by using an external clock method, particularly, a magneto-optical disk recorded at a high density. The purpose of the present invention is to provide a data reproduction clock signal generator for forming an external clock signal capable of preventing a data error caused by a phase shift of a reproduction waveform between an inner circumference and an outer circumference of a disk when reproducing. It is an object of the present invention to provide a clock signal generator for data reproduction that can freely select a modulation method without being affected by differences in characteristics of a medium (individual differences), environments at the time of recording, and differences between data recording devices and data reproduction devices.

Another object of the present invention is to provide a data reproducing apparatus for reproducing data by using an external clock signal from the data reproducing clock signal generating apparatus. It is an object of the present invention to provide a data reproducing apparatus which is not affected by a difference between a recording apparatus and a data reproducing apparatus.

[Means for solving the problem]

The clock signal generator for data reproduction according to the present invention comprises:
A data reproduction clock signal generator for generating a data reproduction clock signal used for reproduction of a recording medium in which a clock pit is provided and a reference clock signal having a frequency equal to the highest recording frequency of data is recorded. External clock signal forming means for forming a plurality of external clock signals having phases different from each other in synchronization with the reproduced signal of the clock pit; and using the external clock signals formed by the external clock signal forming means, the reference clock signal A / D that samples the playback signal of the A / D converter and converts the sampled value to a digital sampled value
Conversion means, and calculation means for calculating a difference between sample values corresponding to the respective external clock signals from the A / D conversion means,
Maximum value detecting means for detecting the maximum value of the difference between the sampling values corresponding to each of the external clock signals based on the arithmetic output from the arithmetic means; and the external value corresponding to the maximum value detected by the maximum value detecting means. The above problem is solved by having external clock signal selecting means for selecting and outputting an external clock signal from the clock signal forming means.

Further, the data reproducing apparatus according to the present invention provides a data reproducing apparatus using a recording medium in which a clock pit having a frequency equal to the highest recording frequency of data is provided and a reference clock signal having a frequency equal to the highest recording frequency of data is recorded. An external clock signal forming means for forming a plurality of external clock signals having phases different from each other in synchronization with a reproduction signal of the clock pit; and an external clock signal forming means for reproducing a reproduction signal reproduced from the recording medium. A / D conversion means for sampling using an external clock signal formed by the A / D conversion means and converting the sampled value into a digital sampling value, and calculating a difference between the sample values of the reproduction signal from the A / D conversion means. Calculating means for calculating the reference clock signal based on the calculation output from the calculating means. Selecting a maximum value detecting means for detecting the maximum value of the difference between the sample values corresponding to the partial clock signal, and an external clock signal from the external clock signal forming means corresponding to the maximum value detected by the maximum value detecting means; Clock signal output means, and a difference between the sampled value of the reproduction signal from the arithmetic means using the external clock selected by the external clock signal selection means and the maximum value from the maximum value detection means. / 2 by comparing with a value of / 2, and having a data reproducing means for reproducing the data,
Solution to the Problems

(Operation)

The clock signal generator for data reproduction according to the present invention comprises:
It is suitable for data reproduction by sampling the reproduction signal of the reference clock signal recorded on the recording medium using external clock signals having different phases from each other and selecting the external clock signal having the phase with the largest difference between the sample values. And generates an external clock signal.

Further, the data reproducing apparatus according to the present invention is characterized in that a difference between a sample value obtained by sampling a data reproducing signal using an external clock signal from the data reproducing clock signal generating apparatus and a reproduced signal of the reference clock signal is obtained. Is reproduced by comparing with a value of about の of the difference between the maximum sampled values when sampling with the external clock signal.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a block circuit diagram of a data reproducing apparatus using a data reproducing clock signal generator according to the present invention. FIG. 2 is a diagram showing a specific recording format of a magneto-optical disk used in the data reproducing apparatus.

First, the recording format of the magneto-optical disk will be described.

In FIG. 2, one sector (or block) as an information recording unit is composed of a predetermined number of segments (or frames). One segment is composed of a servo area 1 and a data area 2 (a header area 3 in which information such as an address is recorded in a head segment of a sector). In the servo area 1, a so-called pair of wobble pits for obtaining a so-called tracking error signal and a clock pit for synchronizing a clock are provided in advance. Data (information) is stored in the data area 2 using the magneto-optical effect.
Is recorded or reproduced. In order to solve the above-mentioned problem caused by the phase shift of the data reproduction signal between the inner circumference and the outer circumference of the magneto-optical disk, for example, the reference clock signal and the fixed level signal are stored in the data area 2 of the second segment. Is recorded. That is, for example, as shown in FIG. 3, when the data area 2 of the second segment is reproduced, the amplitude and frequency of the reproduced signal are equal to the highest frequency of the data reproduced signal (in general, the highest frequency is equal to the minimum amplitude due to frequency characteristics. The reference clock signal is recorded in the first half of the data area 2 of the second segment so as to be equal to the following. Further, in the remaining portion of the data area 2 of the second segment, a signal is recorded such that the reproduced signal at the time of reproduction has a constant level.

When reproducing a recording medium having the above-described recording format, for example, a magneto-optical disk, a synchronized clock signal (hereinafter referred to as an external clock signal) formed from the clock pits is used as a clock signal for reproducing data. Can be In the present invention, the phase of the external clock signal is corrected using the reference clock signal as described later. Thus, in the present embodiment, a magneto-optical disk having the above-described recording format is used as a recording medium on which a reference clock signal is recorded.

Next, a data reproduction clock signal generator will be described.

In FIG. 1, an RF (high-frequency) signal reproduced from a magneto-optical disk that is rotated at a constant angular velocity (CAV) by a spindle motor is supplied to a terminal 10 via an amplifier, an equalizer, and the like. ). The terminal 11 is supplied with a signal obtained by reproducing a clock pit in the servo area 1 of the magneto-optical disk.

External clock generating circuit 12 is constituted by, for example, by a circuit such as a PLL, to form the external clock signal CK ₀ from a signal reproduced from the clock pits. The external clock signal CK ₀ is, for example, three delay circuits 1 connected in series.
Delayed at 3, 14, and 15, respectively. Thus, a plurality of external clock signals CK ₀ , CK ₁ , CK
External clock signal forming means for forming a _2, CK ₃ is the external clock generating circuit 12, a delay circuit 13-15.

Selector 16, a control signal from the counter 17 is supplied from the external clock signal CK ₀ ~CK ₃ one of the external clock signal, for example, sequentially selects the A / D converter 18. The reproduction signal described above is supplied to the A / D converter 18 via the terminal 10, and the A / D converter 18 uses external clock signals CK _{0 to} CK ₃ from the selector 16. For example, as shown in FIG. 3, the reproduced signal of the reference clock signal is sampled, and the sampled value is converted into a digital sample value S. This sample value S is supplied to the register 19 and the arithmetic circuit 20. Thus, in the present embodiment, the A / D converter 18 controls the external clock signals CK ₀ to CK ₀ to
Used as a sampling means for sampling the reproduced signal of the reference clock signal using the CK _3.

The register 19 delays the sample value S by one sampling time, and stores the delayed sample value S in the arithmetic circuit 20.
To supply. The arithmetic circuit 20 outputs each external clock signal CK ₀
Sample values S _N corresponding to ~CK _{3 (N} represents the number of the external clock signal) calculates the absolute value D _N of the difference. For example, as shown in FIG. 3, the absolute value D _N of the difference between the sample values S _N corresponding respectively to each external clock signal _{CK 0 ~CK 3, D N =} | S N1 -S N2 | (N external (Representing the number of the clock signal). Each external clock signal CK ₀
Absolute value D _N of the difference between the sample values corresponding to ～CK ₃ register 2
1 and to the comparator 22. The register 21, the absolute value D _N of the difference between the sample value, for example by one sample time delay, the comparator absolute value D _N of the difference between the delayed sample values
Supply 22. The comparator 22 compares the absolute value D _N of the difference between the current sample value from the absolute value D _N and the arithmetic circuit 20 of the difference between the delayed sample values, and supplies the comparison result to the controller 30. For example, as shown in FIG.
When the absolute value D ₁ is greater than the difference between the absolute value D ₂ is delayed sample values of the difference between the current sample value, to store the absolute value D ₂ of the difference between the current sample value in the register 21 To control the register 21. That is, the register 21, the reproduction signal of the reference clock signals recorded on the magneto-optical digital maximum value of the absolute value D _N of the difference between the sample value when sampled with the external clock signal CK ₀ ~CK ₃ respectively Is stored.

On the other hand, the register 31 is a counter for controlling the selector 16.
17 and receives the control signal from the register 21 as described above.
Each time is updated with a larger value, the latest control signal is stored. That is, the register 31 contains
Control signal when the absolute value D _N of the difference between the sample value of the reproduced signal of the above-mentioned reference clock signal selects the external clock signal becomes maximum at the selector 16 is finally stored.

The controller 30 receives the control signal stored in the register 31 and receives the external clock signal as an external clock signal for data reproduction described later.
The counter 17 is controlled to select and output. For example, in the example shown in FIG. 3, the external clock signal CK ₂ is determined as a clock signal for data reproduction and output.
Further, the controller 30 controls the maximum value (S _MAX −S _MIN ) of the difference between the sample values stored in the register 21 at this time to be stored in the register 23. Thus, in this embodiment, the maximum value detecting means for detecting the maximum value (S _MAX -S _MIN ) of the difference between the sample values corresponding to each of the external clock signals CK _{0 to} CK ₃ includes the selector 16, counter 17, register 19, 21,
It comprises an arithmetic circuit 20, a comparator 22, and a controller 30,
Maximum value detected by the maximum value detection means (S _MAX −S _MIN )
The external clock signal selecting means for selecting and outputting the external clock signal from the external clock signal forming means corresponding to the above comprises the selector 16, counter 17, controller 30, and register 31.

As described above, in the data reproduction clock signal generator according to the present invention, the phase correction of the external clock signal is performed based on the reproduction signal of the reference clock signal actually recorded on the magneto-optical disk, so that the magneto-optical A data reproduction clock that can perform data reproduction without being affected by differences in pit density between the outer and inner rims of the disc, environmental conditions during recording, differences in characteristics between recording and playback devices, and individual differences between discs. A signal can be obtained. Further, by using the reference clock signal, that is, the repetitive pattern, the intersymbol interference between the patterns becomes uniform, and no shift of the external clock signal occurs. Furthermore, if a part of the area where the reference clock signal is recorded has a defect or the like, and the level of the reproduced signal in this part decreases, the absolute value of the sample value becomes small. The clock signal is not determined as a reproduction clock signal. Even if there is a defect, the maximum value of the sample value difference can be accurately detected by performing the above-described operation of detecting the maximum value of the sample value difference a plurality of times.

 Next, the operation of the data reproducing apparatus will be described.

As described above, in the data reproduction clock signal generator, the external clock signal whose phase has been corrected using the reference clock signal recorded in the data area 2 of the second segment shown in FIG. Is determined to have the largest difference between sample values, and the selector
Using the external clock signal selected in step 16, a sample of a reproduction signal of the above-described reference clock signal is obtained from a reproduction signal obtained by reproducing a signal of a constant level following the reference clock signal in the data area 2 of the second segment. The maximum amplitude of the noise is obtained by the same operation as when the maximum value difference is obtained.

Specifically, the controller 30, for example as shown in FIG. 2, by using the external clock signal CK ₂ the difference between the sample value of the reproduced signal of the reference clock signal described above is maximized,
The reproduction signal of the constant level signal is sampled, and the maximum value of the difference between these sample values is stored in the register 21. That is, the register 21 stores the maximum amplitude of noise (N) when a signal of a certain level following the reference clock signal is reproduced.
_MAX− N _MIN ) is stored. The controller 30 stores the maximum amplitude of the noise (N _MAX −N _MIN ) in the register 24.

The arithmetic circuit 25 calculates, for example, the so-called NRZI from the maximum amplitude (S _MAX −S _MIN ) of the reproduction signal of the reference clock signal from the register 23 and the maximum amplitude (N _MAX −N _MIN ) of the noise from the register 24.
A threshold value TH for determining the presence or absence of an edge necessary for reproducing data recorded by the modulation method is calculated and supplied to the comparator 26. The threshold value TH for concrete The edge _{detection, TH = (| S MAX -S} MIN | - | N MAX -N MIN |) / 2 calculated by the formula represented by the supplies to the comparator 26.

The comparator 26 compares the difference between the sample value of the data reproduction signal recorded in the data area 2 of the third and subsequent segments of the selector from the arithmetic circuit 20 with the threshold value TH to determine the presence or absence of an edge. To detect. For example, when the difference between the sample values of the data reproduction signal is larger than the threshold value TH, it is determined that there is an edge, and when it is smaller than the threshold value TH, it is determined that there is no edge. A signal indicating the presence or absence of the edge is taken out via the terminal 27.

Thus, in the present embodiment, the external clock signal forming means for forming a plurality of external clock signals CK _{0 to} CK ₃ having different phases from each other includes the external clock generating circuit 12 and the delay circuit 13.
And the A / D converter 18 is used as sampling means for sampling a reproduction signal reproduced from a recording medium using external clock signals CK _{0 to} CK ₃ formed by external clock signal forming means. A / D converter 1
The arithmetic means for calculating the difference between the sample values of the reproduced signal from the reference signal 8 and the external clock signal CK ₀ of the reproduced signal of the reference clock signal based on the arithmetic output from the arithmetic means comprises the register 19 and the arithmetic circuit 20. Maximum value detecting means for detecting the maximum value of the difference between the sample values corresponding to .about.CK ₃ comprises the register 21, the comparator 22, and the controller 30, and the external value corresponding to the maximum value detected by the maximum value detecting means. External clock signal selecting means for selecting and outputting an external clock signal from the clock signal forming means is configured by the selector 16, the counter 17, the controller 30, and the register 31,
The difference between the sampled value of the reproduced signal from the calculating means using the external clock signal selected by the external clock signal selecting means and the value of about 1/2 of the maximum value from the maximum value detecting means is compared with the data. The data reproducing means for reproducing
3, 24, an arithmetic circuit 25, a comparator 26, and a controller 30.

As described above, in the data reproducing apparatus according to the present invention, by using the external clock signal whose phase has been corrected based on the reproduction signal of the reference clock signal actually recorded on the magneto-optical disk, Edge detection that is not affected by differences in pit density between the outer and inner rims, environmental conditions during recording, differences in characteristics between the data recording device and data reproducing device, individual differences between disks, etc. it can. Also, since the threshold value for performing edge detection is determined based on the reproduction signal of the reference clock signal actually recorded on the magneto-optical disk, differences in characteristics between the data recording device and the data reproduction device, Data can be reproduced without being affected by individual differences. Further, by including the maximum value of noise obtained from a reproduction signal of a signal of a certain level recorded on the magneto-optical disk in the threshold value for performing edge detection, it is possible to further increase the noise margin.

The present invention is not limited to the above-described embodiment. For example, the frequency of the reference clock signal recorded on the magneto-optical disk may be a frequency other than the highest recording frequency of data.

Further, in consideration of a case where an external clock signal cannot be reproduced in one selector due to a defect of a reference clock signal recorded on a magneto-optical disk, for example, the above-described external clock signal is used by using a plurality of selector clock signals. The operation of forming a signal may be performed a plurality of times.

Further, for example, the threshold value TH used for detecting the edge of the reproduction signal of the NRZI modulation method may be set to TH = | S _MAX −S _MIN | / 2.

In the above embodiment, the external clock signal whose phase has been corrected is used for edge detection. However, the present invention is not limited to edge detection. For example, when data is reproduced from a NRZ-modulated reproduction signal, the above-described data reproduction clock is used. The presence / absence of a mark may be determined directly from the reproduced signal in synchronization with the external clock signal from the signal generation signal.

As is apparent from the above description, according to the present invention, the reference clock signal for detecting the phase information is recorded at the head of the recording data, and when reproducing the data, the reference clock signal is reproduced and the reference clock signal is reproduced. By correcting the phase of the external clock signal based on the clock signal, an external clock signal having a large window margin, that is, an external clock signal suitable for data reproduction can be obtained. Further, by recording data at a high density on a magneto-optical disk which is driven to rotate at a constant angular velocity (CAV), and even when pits to be formed overlap, by performing data reproduction using the external clock signal, Data reproduction with a higher noise margin than before can be performed. In addition, by correcting the phase of the external clock signal based on the reproduction signal of the reference clock signal actually recorded on the magneto-optical disk, environmental conditions during recording, differences in characteristics between the recording device and the reproduction device, Data can be reproduced without being affected by individual differences. Further, since an external clock method can be used, a modulation method can be freely selected.

〔The invention's effect〕

According to the clock signal generator for data reproduction according to the present invention, the reference clock signal for detecting the phase information is recorded at the head of the recording data, and when reproducing the data, the reproduction signal of the reference clock signal is also recorded. By correcting the phase of the external clock signal at this time, it is possible to obtain an external clock signal having a large window margin, that is, suitable for data reproduction. Further, data is recorded at a high density on a magneto-optical disk which is driven to rotate at a constant angular velocity (CAV), and an external clock signal capable of accurately reproducing data is obtained even when pits to be formed overlap. be able to. In addition, by correcting the phase of the external clock signal based on the reproduction signal of the reference clock signal actually recorded on the magneto-optical disk, environmental conditions during recording, differences in characteristics between the recording device and the reproduction device, It is possible to obtain an external clock signal capable of performing data reproduction that is not affected by individual differences. Further, since an external clock method can be used, a modulation method can be freely selected.

According to the data reproducing apparatus of the present invention, the reference clock signal for detecting the phase information is recorded at the head of the recording data, and the data is reproduced based on the reproduced signal of the reference clock signal. By using the external clock signal whose phase has been corrected and determining the threshold value for edge detection based on the maximum amplitude of the reproduced signal of the reference clock signal, the magneto-optical disk rotated at a constant angular velocity (CAV) is determined. Data can be accurately reproduced even if the pits to be formed overlap each other. In addition, by correcting the phase of the external clock signal based on the reproduction signal of the reference clock signal actually recorded on the magneto-optical disk, environmental conditions during recording, differences in characteristics between the recording device and the reproduction device, Data can be reproduced without being affected by individual differences.

[Brief description of the drawings]

FIG. 1 is a block circuit diagram of a data reproducing apparatus using a data reproducing clock signal generator according to the present invention, and FIG. 2 is a diagram showing a recording format of a magneto-optical disk used in the data reproducing apparatus. FIG. 3 is a diagram showing the relationship between a reference clock signal and a reproduction signal of a constant level signal and an external clock signal. FIG. 4 is a diagram for explaining the recording principle of a magneto-optical recording medium, and FIG. FIG. 6 is a diagram showing pits formed on a magneto-optical disk of a constant angular velocity (CAV) drive system, and FIG. 6 is a diagram expressing the pits shown in FIG. 5 on a time axis. 12 External clock generation circuit 13-15 Delay circuit 16 Selector 17 Counter 18 A / D converter 19, 21, 23, 24, 31 Register 20, 25 Operation circuit 22 , 26 ... Comparator 30 ... Controller

Claims (2)

(57) [Claims] 1. A data reproducing clock signal generator for generating a data reproducing clock signal used for reproducing a recording medium on which a reference clock signal having a frequency equal to the highest recording frequency of data is recorded, provided with a clock pit. An external clock signal forming means for forming a plurality of external clock signals having phases different from each other in synchronization with the reproduction signal of the clock pit, and using each of the external clock signals formed by the external clock signal forming means. A / D conversion means for sampling the reproduction signal of the reference clock signal and converting the sampled value into a digital sampling value; and outputting the sample value corresponding to each of the external clock signals from the A / D conversion means. Calculating means for calculating a difference; and calculating the difference between the external clock signals based on a calculation output from the calculating means. A maximum value detecting means for detecting a maximum value of a difference between corresponding sample values; and an external circuit for selecting and outputting an external clock signal from the external clock signal forming means corresponding to the maximum value detected by the maximum value detecting means. A clock signal generator for data reproduction, comprising: a clock signal selector. 2. A data reproducing apparatus which uses a recording medium provided with a clock bit and on which a reference clock signal having a frequency equal to the highest recording frequency of data is recorded. External clock signal forming means for forming a plurality of external clock signals having different phases from each other; and a reproduction signal reproduced from the recording medium is sampled using an external clock signal formed by the external clock signal forming means. A / D conversion means for converting a sampling value into a digital sampling value, calculation means for calculating a difference between the sample values of the reproduction signal from the A / D conversion means, and a calculation output from the calculation means A method for detecting a maximum value of a difference between sample values of the reproduction signal of the reference clock signal corresponding to the external clock signals. Large value detecting means, external clock signal selecting means for selecting and outputting an external clock signal from the external clock signal forming means corresponding to the maximum value detected by the maximum value detecting means, and external clock signal selecting means Data reproduction for reproducing data by comparing the difference between the sample value of the reproduction signal from the arithmetic means using the external clock selected in step 1 and the value about 1/2 of the maximum value from the maximum value detection means. And a data reproducing apparatus.