JPH0765508A - External clock generator and data reproducing apparatus - Google Patents

Abstract

PURPOSE:To accurately reproduce data by selecting external clock having the optical phase for sampling of the reproduced signal using the reference signal recorded on a recording medium. CONSTITUTION:An A/D converter 18 executes the sampling using a clock transmitted from a selector 16. During the sampling of the reference information recorded on the recording medium, the selector 16 sequentially selects external clocks CK0 to 3 having different phases for each sampling to send these clocks to the A/D converter 18. An arithmetic circuit 20, FF21 and EXOR22 send a polarity change data of the differential value regarding the current and preceding sampling data to the controller 30. When polarity of differential value is changed, the controller 30 sends a command to the selector 16 to sample the subsequent recorded data with an external clock having currently phase outputted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an external clock generator and data suitable for use in a reproducing device for reproducing recorded data from a recording medium such as a so-called compact disc, magneto-optical disc, magnetic disc, magnetic tape or the like. Reproduction device

[0002]

2. Description of the Related Art In magneto-optical recording, as shown in FIG. 5, data is recorded by magnetizing a portion of the disk, which has reached the Curie temperature by laser irradiation, in the direction of an externally applied magnetic field. is there.

That is, for example, a laser diode is pulse-driven to emit light, an external magnetic field is modulated with recording data,
Data is recorded by forming pits (portions magnetized by an external magnetic field) corresponding to the spot size of the laser beam on the recording surface in a time division manner.

By the way, when the pulse interval for driving the laser light source is made smaller than the spot size of the laser beam in order to increase the recording density, part of the spot of the laser beam overlaps with part of the pit formed earlier, The pits that will be formed later will be overwritten (overwritten) on some of the previously magnetized pits. That is, the pits are formed one by one in a time-divisional manner, and when the recording density is increased, overlap occurs, and as a result, so-called edge recording in which the change point (edge) of the magnetization direction has information is obtained.

Further, for example, a magneto-optical disk is rotationally driven by a spindle motor at a constant linear velocity (CLV) or a constant angular velocity (CAV) so that the pits are formed in a spiral or concentric circle as a pit row. ing.

By the way, in the case of constant angular velocity (CAV) driving, the distance (length) that the recording head advances in a unit time is smaller in the inner circumference than in the outer circumference. Therefore, as shown in FIG. The amount becomes larger than the overlap amount at the outer circumference.

When data is reproduced from the pits thus recorded and formed, as shown in FIG. 7, the signal reproduced from the inner pits is forward (shifted) in time with respect to the signal reproduced from the outer pits. It becomes the shape. Therefore, in the so-called external clock system, for example, when the inner peripheral pits are reproduced by using the same external clock as the synchronized clock signal (hereinafter referred to as an external clock) obtained from the magneto-optical disk for reproducing the outer peripheral pits, 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 external magnetic field modulation method is so-called NRZI modulation, there is less noise margin with respect to the threshold value for judging the presence or absence of an edge during reproduction. Become.

Conventionally, recording is performed on the inner and outer circumferences of the above-mentioned disc by forming pits by making a correction beforehand at the time of recording based on radius information and surrounding environment (temperature, power of laser light source, etc.). The inconvenience associated with data reproduction was prevented.

Further, conventionally, a clock signal is superimposed on a data signal recorded on a magneto-optical disk, the recorded clock signal is extracted at the time of reproduction, and this clock signal is used as a reproduction clock. The above-mentioned inconvenience has been prevented by a so-called self-clock system used as a signal.

[0011]

However, in order to correct the pits at the time of recording in accordance with the radius information, the surrounding environment, etc.,
It is very difficult to obtain detailed recording conditions for performing the correction. In addition, because the characteristics of magneto-optical discs (individual differences) and the data recording device and the data reproducing device may differ, it is possible to correct the data recorded on the inner and outer circumferences of the disc just by correcting the pits during recording. It was not possible to completely prevent the inconvenience associated with reproduction.

If the so-called self-clock system is adopted, it is necessary to use a dedicated modulation system.
There is a problem that a clock pull-in area is required so that clock extraction can be sufficiently performed.

The present invention has been made in view of the above-mentioned problems, and outputs an external clock having an optimum phase without being affected by the difference in characteristics between the inner and outer circumferences of a disk and the devices for reproducing data. And an external clock generation device and a data reproduction device in which data can be reproduced accurately, a data modulation method can be freely selected, and a clock pull-in area is not required. With the goal.

[0014]

An external clock generator according to the present invention uses an external clock phase for reproducing recorded data recorded on a recording medium by using reference information recorded on the recording medium. An external clock generating device for performing compensation and outputting, wherein external clock generating means for generating a plurality of external clocks having mutually different phases and a plurality of external clocks from the external clock generating means are selected and output. External clock selecting means, sampling means for sampling a reproduction signal using the external clock selected by the external clock selecting means, and delay means for outputting a sample output from the sampling means with a predetermined delay. A sample output of the reference information from the sample means and a sample delayed by the delay means. Polarity difference detecting means for detecting the difference in the polarity of the difference value and the polarity detecting means for detecting the change in the polarity of the difference value, and the polarity detecting means detects the difference in the polarity of the difference value. Control means for selectively controlling the external clock selection means so as to select and output the external clock used when obtaining the sample output as the external clock for reproducing the recording data recorded in the recording medium. The above-mentioned problems are solved by having the following features.

Further, the data reproducing apparatus according to the present invention uses the reference information recorded on the recording medium to perform the phase compensation of the external clock for reproducing the recorded data recorded on the recording medium. A data reproducing device for reproducing recorded data recorded on the recording medium by using a phase-compensated external clock, and an external clock generating means for generating a plurality of external clocks having mutually different phases,
External clock selecting means for selecting and outputting a plurality of external clocks from the external clock generating means one by one; sampling means for sampling a reproduction signal using the external clock selected by the external clock selecting means; A delay means for outputting a sample output from the sampling means with a predetermined delay and a difference between the sample output of the reference information from the sample means and the sample output delayed by the predetermined delay by the delay means are detected. Polarity detection means for detecting a change in polarity of the difference value, and when the polarity change of the difference value is detected by the polarity detection means, used to obtain a sample output of the difference value with the changed polarity. Control means for selecting and controlling the external clock selection means so as to select and output an external clock, The recording means samples and reproduces the record data recorded on the recording medium by using the external clock selected by the external clock selecting means by the selection control of the control means, and solves the above-mentioned problems. To do.

[0016]

In the external clock generating device according to the present invention, the external clock generating means generates a plurality of external clocks having different phases for reproducing the record data recorded on the recording medium. Each of these external clocks is supplied to the external clock selection means. The external clock selecting means selects one of the plurality of external clocks for each sampling period of the sampling means described below, for example, and supplies the selected external clock to the sampling means. The sampling means samples the reproduction signal of the reference information recorded on the recording medium by using the external clocks having different phases, which are supplied every one sampling period. This sample output is supplied to the delay means and the polarity detection / detection means. The delay means delays the sample output by, for example, one sample and supplies the delayed sample output to the polarity detection means.

The polarity detecting means detects the difference between the sample output sampled by the current external clock and the sample output sampled by the external clock one sample before, and also detects the polarity of the sample output, For example, it is stored once. Then, a change in polarity is detected by comparing the polarity of the next detected difference value with the stored polarity of the difference value one sample before, and data indicating this change in polarity is supplied to the control means. .

The fact that the polarity of the difference value has changed means that the external clock has the optimum phase for sampling the edge position of the reproduction signal of the recorded data. Therefore, when the polarity detection unit detects a change in the polarity of the difference value, the control unit uses the external clock used when the sample output of the difference value with the changed polarity is obtained as the recording medium. The external clock selecting means is selectively controlled so as to select and output the recorded data recorded in (1) as an external clock for reproducing.

As a result, it is possible to output an external clock having a phase most suitable for reproducing recorded data.

Next, in the data reproducing apparatus according to the present invention, the external clock generating means generates a plurality of external clocks having different phases for reproducing the record data recorded on the recording medium. Each of these external clocks is supplied to the external clock selection means. The external clock selecting means selects one of the plurality of external clocks for each sampling period of the sampling means described below, for example.
This is supplied to the sampling means. The sampling means samples the reproduction signal of the reference information recorded on the recording medium by using the external clocks having different phases, which are supplied every one sampling period. This sample output is supplied to the delay means and the polarity detection / detection means. The delay means delays the sample output by, for example, one sample and supplies the delayed sample output to the polarity detection means.

The polarity detecting means detects the difference between the sample output sampled by the current external clock and the sample output sampled by the external clock one sample before, and also detects the polarity of the sample output, For example, it is stored once. Then, a change in polarity is detected by comparing the polarity of the next detected difference value with the stored polarity of the difference value one sample before, and data indicating this change in polarity is supplied to the control means. .

The fact that the polarity of the difference value has changed means that the external clock has the optimum phase for sampling the edge position of the reproduction signal of the recorded data. Therefore, when the polarity detection unit detects a change in the polarity of the difference value, the control unit uses the external clock used when the sample output of the difference value with the changed polarity is obtained as the recording medium. The external clock selecting means is selectively controlled so as to select and output the recorded data recorded in (1) as an external clock for reproducing. The external clock selected by the external clock selection means is supplied to the sampling means.

The sampling means uses the external clock to sample and reproduce the recording data recorded on the recording medium. As described above, the external clock for sampling the recording data has the optimum phase for sampling the edge position of the reproduction signal of the recording data. Therefore, the recorded data can be accurately reproduced.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of an external clock generator and a data recovery device according to the present invention will be described below with reference to the drawings.

The external clock generating device and the data reproducing device according to the present invention can be applied to a disc reproducing device for reproducing recorded data from a magneto-optical disc which is a recording medium, for example.

The main part (data reproducing system) of the disk reproducing apparatus to which the present invention is applied is, as shown in FIG. 1, an external clock for generating an external clock for reproducing the recorded data recorded on the magneto-optical disk. It has a generation circuit 12 and first to third delay circuits 13 to 15 which apply a predetermined delay to the external clock. The external clock generating circuit 12 and the first to third delay circuits 13 to 15 constitute external clock generating means.

Further, the data reproducing system includes the external clock generating circuit 12 and the first to third delay circuits 13 to 15
Selector 16 (external clock selecting means) for selecting and outputting each external clock supplied from each of them, and phase compensation of the reproduction signal of the recording data and the external clock supplied by using the external clock from the selector 16. In order to do so, it has an A / D (analog / digital) converter 18 (sampling means) for sampling the reproduction signal of the reference pattern (reference information) recorded together with the recording data.

The data reproducing system includes a register 19 (delay means) for delaying the sampling data from the A / D converter 18 by one sample, for example, and sampling data one sample before from the register 19. And detecting the difference between the current sampling data from the A / D converter 18 and detecting the polarity of this difference value,
An arithmetic circuit 20 that outputs polarity data indicating the polarity of the difference value, a flip-flop 21 that delays the polarity data from the arithmetic circuit 20 by one sample, and polarity data that is one sample before from the flip-flop 21. And an exclusive OR gate (EXOR gate) 22 for comparing the polarity data from the arithmetic circuit 20 and detecting a change in the polarity of the polarity data.

The arithmetic circuit 20, the flip-flop 21 and the EXOR gate 22 constitute a polarity detecting means.

Further, the data reproducing system outputs the count data for controlling the external clock selecting operation of the selector 16 and the count data of the counter 17 based on the output from the EXOR gate 22. The controller 30 (control means) for controlling the output of the counter and the register 31 for storing the count data from the counter 17 under the control of the controller 30.

The magneto-optical disk is rotationally driven, for example, at a constant angular velocity (CAV) to record recording data. This recording data is recorded for each sector (or block) that is an information recording unit as shown in FIG. 2, for example.

One sector is composed of a predetermined number of segments (or frames), and one segment includes a servo area 1 and a data area 2 (a header area in which information such as an address is recorded in the first segment of the sector). 3).

In the servo area 1, a pair of wobble pits for obtaining a so-called tracking error signal,
Clock pits for synchronizing clocks are provided in advance. Further, record data is recorded in the data area 2 by utilizing the magneto-optical effect.

In the data area 2 of the second segment, for example, as shown in FIG. 3, the frequency of the reproduced signal when the data area 2 of the second segment is reproduced is the highest in the data reproduced signal. A reference pattern for compensating the phase of the external clock so as to be equal to the frequency is recorded.

In the disk reproducing apparatus having the data reproducing system having such a structure, the magneto-optical disk is rotationally driven at the same angular velocity as at the time of recording the recording data, and the recording data recorded on the magneto-optical disk is recorded. Is played. This reproduction signal is supplied to the A / D converter 18 via the input terminal 10.

A signal obtained by reproducing the clock pits in the servo area 1 of the magneto-optical disk is supplied to the input terminal 11. External clock generation circuit 12
Is composed of, for example, a PLL (Phase Locked Loop) circuit, and forms the external clock CK ₀ from the signal reproduced from the clock pit. This external clock CK ₀ is supplied to the selector 16 and
Is supplied to the delay circuit 13.

The first delay circuit 13 delays the external clock CK ₀ by a predetermined amount and external clock CK ₁
To supply the selector 16 with the second
To the delay circuit 14. The second delay circuit 14
The external clock CK ₁ is delayed by a predetermined amount to form the external clock CK ₂ , which is supplied to the selector 16 and the third delay circuit 15. Third above
The delay circuit 15 delays the external clock CK ₂ by a predetermined amount to form the external clock CK ₃ , and supplies the external clock CK ₃ to the selector 16.

On the other hand, the controller 30 first controls the counter 17 so that the count data output from the counter 17 becomes 0, and then the sampling operation in the A / D converter 18 described below. The counter 17 is controlled so that the count data of "1", "2", and "3" are sequentially output each time.

For example, when the count data from the counter 17 is 0, the selector 16 selects and outputs the external clock CK ₀ , and when the count data is 1, it selects the external clock CK _1. When the count data is 2, the external clock CK ₂ is selected and output, and when the count data is 3, the external clock CK ₃ is selected and output.

Therefore, the selector 16 is connected to the A / D
The external clocks CK _{0 to} CK ₃ having different phases are selected and output for each sampling operation of the converter 18. These external clocks CK _{0 to} CK ₃
Are supplied to the A / D converter 18, respectively.

The A / D converter 18 first detects the external
Clock CK₀To support the playback signal of the above recorded data.
And then the external clock as shown in FIG.
CK ₁Is used to sample the above reproduced signal.
As described above, external clocks CK having different phases₀~
CK₃Is used to sample the reproduction signal. So
Then, the sampling data S shown in FIG.₁₁, S₁₂，
S_{twenty one}, S_{twenty two}, S₃₁, S₃₂Register 19 and arithmetic circuit 2
Supply to 0.

The register 19 is used for the A / D converter 1
The sampling data supplied from No. 8 is held until the A / D converter 18 outputs the next sampling data to delay the sampling by one sample, and the sampling data is supplied to the arithmetic circuit 20.

The arithmetic circuit 20 detects the difference value between the sampling data one sample before and the current sampling data. Specifically, as shown in FIG. 3, the difference value D between the sampling data S ₁₁ and the sampling data S _12
1 is detected, the difference value D ₂ between the sampling data S ₂₁ and the sampling data S ₂₂ is detected, and the difference value D ₃ between the sampling data S ₃₁ and the sampling data S ₃₂ is detected. Then, the polarity of this difference value is detected. Specifically, as shown in FIG. 3, since the difference between the low-level sampling data S ₁₁ and the high-level sampling data S ₁₂ is detected, the difference value D ₁ is smaller than 0 (D ₁ <0). And its polarity becomes-. Since the sampling data S ₂₁ and the sampling data S ₂₂ are at the same level, the difference value D ₂ is 0 (D ₂ = 0) and the polarity thereof is +. Further, since the difference between the high level sampling data S ₃₁ and the low level sampling data S ₃₂ is detected, the difference value D
₃ becomes a value larger than 0 (D ₃ > 0) and its polarity becomes +.

The polarity data indicating the polarity of this difference value is supplied to the flip-flop 21 and the EXOR gate 22.

The flip-flop 21 delays the polarity data by one sample, and adds this to the EXOR.
Supply to the gate 22. As a result, the EXOR gate 22 is supplied with the current polarity data and the polarity data one sample before.

The EXOR gate 22 compares the current polarity data with the polarity data one sample before, and is active when the polarity inversion occurs between the current polarity data and the polarity data one sample before. Becomes

The fact that the output of the EXOR gate 22 is active means that the polarity of the difference value obtained by sampling the reproduction signal of the recording data has changed. That is, the position where the polarity is changed is the position where the absolute value of the difference is the minimum (zero cross point) as shown in FIG. 4, and it indicates that the sampling position matches the edge position of the reproduction signal ( External clock CK _{2 in} FIG. 4).

Therefore, the controller 30 controls the register 31 so as to store the count data when the output of the EXOR gate 22 becomes active. As a result, the count data capable of outputting the external clock having the optimum phase is stored in the register 31.

When the controller 30 stores the count data in the register 31 in this manner, the controller 30 controls the counter 17 to output the stored count data. As a result, the count data with which the external clock having the optimum phase can be selected is selected by the selector 16
Is supplied to.

The selector 16 selects the external clock (in this case, the external clock CK ₂ shown in FIG. 4) based on the count data as described above, and supplies it to the A / D converter 18. To do.

As a result, in the A / D converter 18, the reproduction data of the recording data can be sampled by the external clock having the optimum phase, and the reproduction can be accurately performed. The recorded data to be reproduced is output to the output terminal 23.
Is output to the outside via.

As is apparent from the above description, the external clock generating device and the data reproducing device according to the present invention detect the difference between the sampling data sampled by the external clocks having different phases, and change the polarity of the difference. By using the external clock according to the above as the external clock for reproducing the recorded data, it is possible to output the external clock having the phase most suitable for reproducing the recorded data.

Therefore, for example, by recording high-density recording data on the magneto-optical disk at a constant angular velocity, even if the overlap amount of the recording data on the inner circumference side and the recording data on the outer circumference side are different, the recording on the inner circumference side is performed. An external clock having a phase suitable for reproducing the data and the recorded data on the outer circumference side can be output, and the recorded data can be reproduced accurately.

Since the external clock having the phase suitable for the situation can be output, it is not affected by the environmental condition at the time of recording, the characteristic difference between the recording device and the reproducing device, and the individual difference of the disc. An external clock for data reproduction can be output, and the recorded data can be reproduced accurately without being affected by the environmental conditions at the time of recording.

Further, since it is possible to use the external clock system, it is not necessary to provide a clock pull-in area, and the selection of the modulation system of the recording data applied to the disc reproducing apparatus can be liberalized. .

Further, the external clock generating circuit 12, the first
The third delay circuits 13 to 15, the selector 16, the A / D converter 18, the register 19, the arithmetic circuit 20, the flip-flop 21, the EXOR gate 22, the counter 17, the register 31, and the controller 30 are included in the comparison. vessel,
Since no adder or the like is provided, the cost can be reduced by simplifying and downsizing the circuit configuration, and the external clock generating device can be integrated into a circuit.

In the above description of the embodiment, the external clock generator and the data recovery device according to the present invention are CAV.
However, the present invention is an apparatus for reproducing data using an external clock, which uses a reference pattern and is a system in which the frequency of the external clock and the data does not fluctuate, or an external clock. If the system is such that the frequency fluctuations of the data match,
Of course, it can be applied to, for example, communication.

[0058]

The external clock generator according to the present invention is
An external clock having a phase most suitable for reproducing recorded data can be output. Therefore, for example, by using an optical disc as a recording medium and recording the recording data at a high angular density on the optical disc at a constant angular velocity, even if the overlap amount of the recording data on the inner peripheral side differs from that on the inner peripheral side, It is possible to output an external clock having a phase suitable for reproducing each of the recorded data and the recorded data on the outer peripheral side, which can contribute to the accurate reproduction of the recorded data in this case.

Further, since the external clock having the phase suitable for the situation can be output, it is not affected by the environmental condition at the time of recording, the characteristic difference between the recording device and the reproducing device, the individual difference of the disc and the like. , It is possible to output an external clock for data reproduction.

Further, since it is possible to use the external clock system, it is not necessary to provide a clock pull-in area, and the recording data modulation system of the equipment in which the external clock generator is installed can be freely selected. Can be possible.

Further, since it can be constituted by the external clock generation means, the external clock selection means, the sampling means, the delay means, the polarity detection means and the control means, the cost can be reduced by simplifying the circuit structure and downsizing. In addition, the external clock generator can be integrated into a circuit.

Next, since the data reproducing apparatus according to the present invention can sample the reproduction signal of the recording data recorded on the recording medium by using the external clock having the most suitable phase, the recording data can be accurately recorded. Can be played. Therefore, for example, when an optical disc is used as a recording medium and the optical disc is rotationally driven at a constant angular velocity to record the recorded data at a high density, even if the overlap amount of the recorded data on the inner peripheral side and the outer peripheral side is different, Since the recorded data can be sampled by an external clock having a phase suitable for reproducing the recorded data on the inner circumference side and the recorded data on the outer circumference side, accurate recording can be performed regardless of the inner circumference side and the outer circumference side. The data can be played back.

Further, since the recording data can be sampled by the external clock having the phase suitable for the situation, it is influenced by the environmental condition at the time of recording, the characteristic difference between the recording device and the reproducing device, the individual difference of the recording medium and the like. The recorded data can be accurately reproduced without any trouble.

Further, since the recorded data is reproduced by using the external clock system, it is not necessary to provide the clock pull-in area, and the modulation system of the recorded data used in the data reproducing apparatus can be freely selected. it can.

Further, since it can be constituted by the external clock generation means, the external clock selection means, the sampling means, the delay means, the polarity detection means and the control means, the cost can be reduced by simplifying the circuit structure and downsizing. In addition, the data reproducing device can be integrated into a circuit.

[Brief description of drawings]

FIG. 1 is a block diagram showing a main part of a disc reproducing device in an embodiment in which an external clock generating device and a data reproducing device according to the present invention are applied to the disc reproducing device.

FIG. 2 is a diagram showing a recording format of a magneto-optical disk used in the disk reproducing device.

FIG. 3 is a diagram showing a relationship between a reproduction signal of a reference pattern recorded on the magneto-optical disk and an external clock.

FIG. 4 is a diagram showing a relationship between the reproduction signal and each external clock for explaining an external clock selecting operation of the disc reproducing apparatus according to the embodiment.

FIG. 5 is a diagram showing pits formed on a magneto-optical disk by being rotationally driven at a constant angular velocity.

FIG. 6 is a diagram for explaining a difference between pits formed on an inner peripheral side and an outer peripheral side of the magneto-optical disk.

FIG. 7 is a diagram for explaining an inconvenience that occurs when the pits formed on the inner peripheral side and the outer peripheral side of the magneto-optical disk are reproduced using an external clock of the same phase.

[Explanation of symbols]

10 ・ ・ ・ ・ ・ ・ ・ ・ ・ Recorded data playback signal input terminal 11 ・ ・ ・ ・ ・ ・ ・ ・ ・ Clock pit playback signal input terminal 12 ・ ・ ・・ ・ ・ ・ ・ ・ ・ ・ ・ External clock generation circuit 13〜15 ・ ・ ・ ・ ・ ・ First to third delay circuit 16 ・ ・ ・ ・ ・ ・・ ・ Selector 17 ・ ・ ・ ・ ・ ・ ・ ・ ・ Counter 18 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ A / D converter 19, 31 ・ ・ ・・ ・ ・ Register 20 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ Operational circuit 21 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ Flip-flop 22 ・ ・ ・ ・ ・ ・ ・ ・・ ・ ・ EXOR gate 30 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ Controller

Claims (2)

[Claims] 1. An external clock generator for performing phase compensation of an external clock for reproducing recorded data recorded on a recording medium by using reference information recorded on the recording medium, and outputting the same. External clock generating means for generating a plurality of external clocks having different phases, external clock selecting means for selecting and outputting a plurality of external clocks from the external clock generating means, and selection by the external clock selecting means Sampling means for sampling a reproduction signal by using the external clock generated by the sampling means, delay means for outputting a sample output from the sampling means with a predetermined delay, and sample output and delay means for the reference information from the sample means. Detects the difference between the sample outputs delayed by a predetermined amount, and changes the polarity of this difference value. The polarity detecting means for outputting and the external clock used when a sample output of the difference value with the changed polarity is obtained when the change in the polarity of the difference value is detected by the polarity detecting means. An external clock generation device comprising: a control unit that selectively controls the external clock selection unit so as to select and output the external clock for reproducing the recorded data recorded in the above. 2. The reference information recorded on the recording medium is used to perform phase compensation of an external clock for reproducing the recording data recorded on the recording medium, and the phase-compensated external clock is used. A data reproducing apparatus for reproducing recorded data recorded on the recording medium, comprising: an external clock generating means for generating a plurality of external clocks having mutually different phases; and a plurality of external clocks from the external clock generating means. External clock selecting means for selecting and outputting each one, sampling means for sampling a reproduction signal using the external clock selected by the external clock selecting means, and applying a predetermined delay to the sample output from the sampling means. The delay means for outputting and the sample output of the reference information from the sample means and the delay means A polarity detection means for detecting a difference between the sample outputs subjected to a constant delay and detecting a change in the polarity of the difference value; and a case where a change in the polarity of the difference value is detected by the polarity detection means, And a control means for selectively controlling the external clock selection means so as to select and output the external clock used when the sample output of the difference value with changed polarity is obtained, and the sampling means is the control means. The data reproducing apparatus is characterized in that the recording data recorded on the recording medium is sampled and reproduced by using the external clock selected by the external clock selecting means by the selection control.