JP2007207283A - Frequency detecting device, frequency detecting method, and optical disk device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a frequency detecting device for performing accurate frequency detection without being affected by interference between codes, when detecting a frequency from a reproduced signal. <P>SOLUTION: In the frequency detecting device 14, a plurality of sampling values of the reproduced signal are compared with a reference value (V0), a first sample point (S0) and a second sample point (S1) being sampling values near a first zero cross point (Z0) being a first cross point on a graph of the reproduced signal and the reference value are specified, a first tilt value (α) defined by these difference is obtained, as the same way, a second tilt value (β) defined by difference of a third sample point and a fourth sample point, a third tilt value (γ) defined by difference of a fifth sample point and a sixth sample point, a time from the first zero cross point to the third zero cross point is corrected based on the first to the third tilt values (α, β, γ), and the frequency of the reproduced signal is detected based on the corrected time. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a frequency detection apparatus and a frequency detection method for detecting a frequency (error) by measuring a SYNC pattern length of a sampled reproduction signal of an optical disk or the like.

As the density of optical discs increases, it is desired to improve the accuracy of waveform detection. In Patent Document 1, a digital RF signal obtained by A / D converting an RF signal reproduced from an optical disk is compared with a plurality of set values, thereby detecting a predetermined waveform from the digital RF signal and measuring the wavelength of the waveform. A technique for performing accurate waveform detection by performing A / D conversion based on this wavelength is described.
JP 2005-243087 A

  However, in Patent Document 1, the zero cross length of the reproduction signal is measured in order to detect an error between the frequency of the reproduction signal and the frequency of the reproduction circuit, but this technique is applied to a high-density recording medium such as HD DVD. When applied, it is difficult to correctly measure the zero cross length due to strong intersymbol interference, and there is a problem that high-precision frequency control cannot be performed.

  It is an object of the present invention to provide a frequency detection apparatus that performs accurate frequency detection that is not affected by intersymbol interference when detecting a frequency from a reproduction signal.

  In one embodiment of the present invention, a plurality of sampling values of the reproduction signal (RF) are obtained, the plurality of sampling values are compared with a reference value (V0), and a first graph on the reproduction signal and the reference value is plotted. The first sample point (S0) and the second sample point (S1), which are the sampling values in the vicinity of the first zero cross point (Z0) that is the intersection, are specified, and the difference between the first sample point and the second sample point And a third sampling point that is the sampling value in the vicinity of the second zero-crossing point (Z1) that is the second intersection on the graph of the reproduction signal and the reference value. (S2) and the fourth sample point (S3) are specified, a second slope value (β) defined by the difference between the third sample point and the fourth sample point is obtained, and the reproduction signal and the reference value are determined. The third intersection on the graph The fifth sample point (S4) and the sixth sample point (S5), which are the sampling values in the vicinity of the zero cross point (Z2), are specified, and are defined by the difference between the fifth sample point and the sixth sample point. A third slope value (γ) is obtained, and the time from the first zero cross point to the third zero cross point is corrected based on the first to third slope values (α, β, γ), and corrected. The frequency detection device detects the frequency of the reproduction signal based on time.

  Provided is a frequency detection device that performs accurate frequency detection from a reproduction signal from an optical disk without being affected by intersymbol interference.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a block diagram showing an example of the configuration of an optical disc apparatus according to an embodiment of the present invention, and FIG. 2 is a block diagram showing an example of the configuration of a frequency detector included in the optical disc apparatus according to an embodiment of the present invention. FIG. 3 is a block diagram showing another example of the configuration of the frequency detector included in the optical disc apparatus according to the embodiment of the present invention. FIG. 4 is an RF signal read by the optical disc apparatus according to the embodiment of the present invention. FIG. 5 is a graph showing an example of the relationship among the zero cross point, sample point, wavelength T, and sample number N of the RF signal read by the optical disc apparatus according to the embodiment of the present invention. 6 is a graph showing an example of the relationship among the zero-cross point, the sample point, the wavelength T, and the number of samples N of the RF signal read by the optical disc apparatus according to the embodiment of the present invention, and FIG. FIG. 8 is a graph showing an example of the relationship among the zero-cross point, sample point, wavelength T, number of samples N1, and time difference e of the RF signal read by the optical disc device according to the embodiment, and FIG. 8 is an optical disc device according to an embodiment of the present invention. FIG. 9 is a flowchart for obtaining the value of the period 3T from the wavelength T2 in the optical disc apparatus according to one embodiment of the present invention.

<< Optical Disc Device According to One Embodiment of the Present Invention >>
<Configuration of optical disc apparatus>
First, an example of the configuration of an optical disc apparatus that is an embodiment of the present invention will be described. In FIG. 1, an optical disk device 10 is a device that reads information from an optical disk medium D, and includes a pickup head (PUH) 11, an A / D converter (ADC) 12, a phase comparator 13, a frequency detector 14, and a loop. A filter 15, a VCO (Voltage Controlled Oscillator) 16, a transversal filter 17, and a Viterbi decoder 18 are included.

  The pickup head 11 reproduces a signal corresponding to information recorded on the optical disk medium D, and receives a laser light source that irradiates the optical disk medium D with laser light and a laser beam that receives the laser light reflected from the optical disk medium D. (Not shown). The reproduction signal output from the light receiver is amplified by a reproduction amplifier (not shown) to become a reproduction RF signal.

  The A / D converter 12 is an element that performs A / D conversion on the input reproduction RF signal and outputs a digital RF signal (multi-value RF signal). This digital RF signal is a multivalued digital value output at substantially constant time intervals.

  A / D conversion in the A / D converter 12 is controlled by a control signal output from the VCO 16. That is, the A / D conversion cycle (time interval) is determined based on the oscillation frequency of the VCO 16.

  The A / D converter 12 has an adjustment mechanism (a kind of amplifier) for adjusting the offset (zero level / slice level) and amplitude of the input reproduction RF signal, and the reproduction RF signal adjusted by this adjustment mechanism is converted to A. / D conversion. The adjustment of the reproduction RF signal by this adjustment mechanism is performed based on the signal adjustment information from the transversal filter 17.

  The phase comparator 13 is a circuit that compares the phases of the multilevel RF signal output from the A / D converter 12 and the output signal from the VCO 16 and outputs a phase difference.

  The frequency detector 14 detects (measures) the frequency of the multilevel RF signal input from the A / D converter 12 and outputs a frequency error signal representing the difference between this frequency and the frequency of the output signal from the VCO 16. It is. The frequency detector 14 also outputs a frequency error detection signal used as an error information control signal for controlling whether or not the output frequency error signal is used in the loop filter 15. Details of the internal configuration of the frequency detector 14 will be described later.

  The loop filter 15 is a circuit that generates a voltage for controlling the VCO 16 based on the phase error output from the phase comparator 13 and the frequency error output from the frequency detector 14.

  The VCO 16 is an oscillation circuit that oscillates at a frequency corresponding to the control voltage output from the loop filter 15 and functions as a control signal generator.

  The transversal filter 17 is a filter that equalizes the multilevel RF signal into a PR waveform. The transversal filter 17 functions as a waveform equalizer, corrects reproduction distortion, and supplies signal adjustment information for adjusting the offset (zero level / slice level) and amplitude of the reproduction RF signal to the A / D converter 12. Is output.

  The Viterbi decoder 18 decodes the data equalized by the transversal filter 17.

(Frequency detector)
Next, an example of the configuration of a frequency detector capable of performing an operation to avoid the influence of intersymbol interference according to an embodiment of the present invention will be described with reference to the drawings.

Outline FIG. 2 is a diagram for explaining the outline of the function of the frequency detector 14. The frequency detector 14 receives an RF signal and detects, for example, a 13T3T waveform, a 13T3T waveform detector 27, and a 13T3T waveform detector. In response to a plurality of sample points S0, S1, S2, S3, S4, and S5 in the vicinity of the zero-cross point detected by 27, an error correction unit 28 that corrects an error, and a frequency signal corrected from the error correction unit 28 And an arithmetic unit 29 that outputs a frequency error signal d as compared with the reference frequency. That is, the frequency detector 14 detects a 13T3T waveform from the reproduction signal, and further corrects a time error due to intersymbol interference.

Details Next, FIG. 3 is a block diagram showing an example of a more detailed configuration of the frequency detector 14.
In FIG. 3, the frequency detector 14 includes a waveform detection unit 22, a wavelength measurement unit 23, a pattern wavelength ratio evaluation unit 24, a peak hold unit 25, and a relative frequency error calculation unit 26, and inputs a multilevel RF signal, A frequency error signal and a frequency error detection signal are output.

  The waveform detector 22 receives the multilevel RF signal output from the A / D converter 12 and detects whether or not a predetermined waveform is included. This waveform detection is performed in order to extract a sync pattern (synchronization signal waveform) from the multilevel RF signal and control the time interval of A / D conversion in the A / D converter 12 based on this sync pattern. .

  As will be described later, the waveform detection unit 22 stores the first, second, and third threshold values Vth, Vtc, and Vtl, and three threshold values, and uses these three threshold values in the multilevel RF signal. First and second waveforms W1 and W2 are detected.

  The wavelength measurement unit 23 measures the wavelength of the predetermined waveform detected by the waveform detection unit 22. At this time, when the predetermined waveform includes a plurality of wavelengths, the ratio is calculated.

  The “wavelength” here is an amount corresponding to the time interval required for the multilevel RF signal to cross the slice level, and the multilevel RF signal changes from the slice level (zero point) to the peak (positive extreme value). Corresponds to the time interval from the slice level (zero point) to the valley (negative extreme value) and back to the slice level again. That is, “wavelength” is the time required for a mountain or valley to appear and disappear.

  The case where the predetermined waveform includes a plurality of wavelengths is, for example, a case where the sync pattern is a 13T or 3T mountain valley. At this time, the ratio of the wavelength 1313 (= 4.33) between the wavelength at the peak (time interval until returning from the slice level) 13T and the wavelength 3T at the valley is the wavelength ratio.

  As can be seen from the above, the “wavelength” can be measured in units of time, for example, seconds. Instead, the number of multi-level RF signals output during that time (the number of A / D conversions in the A / D converter 12) can be measured as a unit, and this is sufficient. This is because, in order to control the time interval of A / D conversion, it is sufficient to know the relative time determined by the number of conversions, etc., even if the absolute time is not known.

  The pattern wavelength ratio evaluation unit 24 determines whether or not the detected waveform may be a sync pattern based on the wavelength ratio calculated by the wavelength measurement unit 23. For example, when the sync pattern is 13T or 3T mountain valley, if the ratio of the detected wavelengths is close to 4.33, the waveform may be a sync pattern.

  The peak hold unit 25 holds the maximum value of the wavelength in the waveform determined by the pattern wavelength ratio evaluation unit 24 as having the possibility of the sync pattern (peak hold). This is because the sync pattern is set to have the longest wavelength (longest wavelength) in the multilevel RF signal, and the sync pattern is detected more reliably.

  The peak hold in the peak hold unit 25 is performed at a certain cycle, and a frequency error detection signal for notifying whether or not a frequency error has been detected is output at each end of the cycle.

  At this time, the peak-held wavelength is limited to a wavelength range determined corresponding to the longest wavelength expected value. For example, when a wavelength in a range of ± 10% of the longest wavelength expected value is input to the peak hold unit 25, the wavelength at that time is held by the peak hold unit 25. When a wavelength of a value larger than the value of the wavelength held within the upper and lower limits (for example, ± 10% of the longest wavelength expected value) is input, the held wavelength value is updated. The If a wavelength having a value outside this range is input, the process is performed as if a value (upper limit value or lower limit value) within the range where the held wavelength value is closest to the input value is input.

  The longest wavelength expected value is the longest wavelength that can be generated in the system and is stored in a storage unit (for example, a memory or a register). As described above, this corresponds to the fact that the sync pattern is set to have the longest wavelength (longest wavelength) in the multilevel RF signal. For example, when the sync pattern is 13T or 3T, the longest wavelength expected value is 16T.

  The relative frequency error calculation unit 26 subtracts the longest wavelength expected value that is the longest wavelength that can be generated in the system from the longest wavelength obtained by the peak hold unit 25, and outputs a frequency error based on the calculated wavelength difference.

(Frequency detection method)
Next, a frequency detection method mainly by the wavelength measuring unit 23 will be described in detail below using the flowcharts of FIGS.

  A mark / space pattern is detected from the input RF signal, and the wavelength of the pattern is measured. When the wavelength ratio evaluation unit 24 evaluates that the measured wavelength falls within a certain ratio, it is determined as a sync pattern, and the sync pattern length is obtained. A value obtained by subtracting the expected value from the obtained sync pattern length is output as a frequency error.

  T1 and T2 are obtained by setting T1 as the first wavelength and T2 as the second wavelength in the continuous mark + space or space + mark. This is obtained by interpolation / extrapolation using the sampled necessary data and the measurement result of the number of data. T represents a channel clock period.

  A data string obtained by sampling an RF signal, which is a reproduction signal from the disk, with PSCK is input data.

  Here, consider a case where an nT-length mark / space reproduction RF signal is asynchronously sampled with PSCK having a certain frequency. If the channel rate and the PSCK frequency are the same, the number of data corresponding to the nT long mark / space is n. If the PSCK frequency is higher than the channel rate, the number of data corresponding to nT long marks / spaces is greater than n. Conversely, if the PSCK frequency is lower than the channel rate, the number of data becomes smaller than n. One mark / space section can be detected by crossing a certain signal level. Thus, the difference between the channel rate and the PSCK frequency can be known by counting the number of samples in the known mark / space section.

  Since the next-generation DVD is ultra-high-density recording, the DC level of the relatively short T-length reproduction RF signal is strongly influenced by the preceding and following mark / space patterns (T-length). For this reason, it is not possible to correctly detect one mark / space section simply by detecting the crossing of signal levels in the vicinity of the average DC level.

  Therefore, as shown in FIG. 4, three reference levels (Vth> Vtc> Vtl) are given, and the crossover time is interpolated / extrapolated using neighboring data (S0 to S5) crossing each reference level. Thus, the detection accuracy is improved.

  If it is determined that the mark to be detected + space or space + mark, data before and after crossing the three reference levels are sampled. Also, the number of data between each crossover is counted.

  As shown in FIGS. 4 and 5, sample points detected before and after the first rising zero-cross point Z0 are sample points S0 and S1, and sample points detected before and after the falling zero-cross point Z1 are sample points S2, The sample points detected before and after the subsequent rising zero-cross point Z2 are set as sample points S4 and S5 (step ST11).

  In practice, as shown in FIG. 4, after straddling while rising Vth, straddling while falling Vtc, and when straddling while striking Vtl, it is regarded as a measurement object, and samples straddling while rising Vth are S0, S1, Samples that straddle Vtc while falling are S2, S3, and samples that straddle Vtl are S4 and S5 (step S21).

  In this example, it is assumed that the waveform has a characteristic close to PR12221 as the PR class.

  By interpolation / extrapolation prediction, as shown in FIG. 6, if the differences between the sample points S0, S2, S4 and the actual zero cross points Z0, Z1, Z2 are time difference e0, time difference e2, and time difference e4, respectively, (Step ST12, step ST22).

e0 = S0 / (S1-S0)
e2 = S2 / (S3-S2)
e4 = S4 / (S5-S4)
Next, the number of samples N1 and N2 is set to N1: the number of samples included between S0 and S3. N2: The number of samples included between S2 and S5 (steps ST13 and ST23).

  Further, the inclination α is inclination information between samples of the 13T3T leading zero cross point, and is given by the following equation (step ST14).

α = | S1-S0 |
Here, the inclination β is the inclination information between samples of the zero cross point (Z1) in the middle of 13T3T, and is given by the following equation (steps ST15 and ST24).

β = | S3-S2 |
Here, the inclination γ is inclination information between samples of the zero cross point (Z2) at the end of 13T3T, and is given by the following equation (step ST25).

γ = | S5-S4 |
Using the above, as shown in FIG. 5 and FIG. 6, the wavelength T1 between the zero-cross points Z0 and Z1 after the interpolation / extrapolation prediction, and the wavelength T2 between the zero-cross points Z1 and Z2 are obtained by the following equations.

T1 = N1 + e0−e2
T2 = N2 + e2-e4
Here, if TA represents the length of the entire sync pattern including T1 and T2, the sync pattern length after the interpolation / extrapolation prediction is expressed by the following equation.

TA = T1 + T2 = N1 + N2 + e0-e4
This is the sync pattern length measurement method using only interpolation / extrapolation prediction without considering intersymbol interference.

  Next, a method for measuring the sync pattern length including correction in consideration of intersymbol interference will be described with reference to FIGS.

  An equation for measuring the sync pattern length T1 including correction in consideration of intersymbol interference is as follows (step ST16).

T1 = N1 + e0-e2 + (α / β-1)
Further, the equation for measuring the sync pattern length T2 including the correction considering the intersymbol interference is as follows (step ST26).

T2 = N2 + e2-e4 + (γ / β-1)
As a result, the frequency TA to be obtained is expressed by the following equation.

TA = T1 + T2 = N1 + N2 + e0-e4 + (α / β-1) + (γ / β-1)
(Reason why the intersymbol interference is eliminated by the correction process)
Next, the reason why correction based on tilt information is effective will be described. If there is strong inter-code interference, as shown by the line 13T3T2T in FIG. 7, a difference occurs between the value measured by the interval between T2 to be actually measured and the zero cross point (Z2).

  Since the mark length or space length following 13T3T is different, an error occurs depending on whether the mark length is short or long. For example, when a short pattern continues, it may be measured longer than actual.

  When a short pattern continues, generally, the amplitude of the portion becomes small due to intersymbol interference, and the inclination near the zero cross point becomes small.

  Considering the correction amount using this shape as a model, when the inclination is 2, the correction amount is 0T, and when the inclination is 1, the correction amount is -0.5T. In addition, there is no guarantee that the clock frequency and the channel frequency of the waveform data match, and since the correct slope value is unknown, some reference value is required for the correction amount. As this reference value, it is reasonable to use the middle zero cross point portion of 13T3T in which the pattern lengths before and after are defined. In other words, when β and γ are equal, there is no need for correction, but when γ is smaller than β, correction may be made so that the measurement result is shortened.

If expressed in a linear form so that -0.5T when γ / β is 1/2,
Correction amount ΔT = γ / β−1
It becomes. Therefore, it can be seen that the intersymbol interference is eliminated by this correction amount.

  By using the method as described above, it is possible to perform frequency measurement with higher accuracy by a simple method. However, such accuracy is required when the frequency is driven from the time when the clock frequency becomes close to the channel frequency to the range of the PLL capture range sufficient for phase acquisition. Thus, such an assumption does not hold at the time when the frequencies are greatly different. For this reason, it is preferable to perform the correction when the SYNC pattern length falls within a predetermined range from 15T to 17T.

  In addition, when the frequency measurement error is large due to large intersymbol interference, it may be difficult to drive the frequency up to the PLL capture range, but the problem can be avoided by using the above method.

  In addition, high-precision frequency detection can be performed even on a high-density recording optical disk typified by HD DVD.

  With the various embodiments described above, those skilled in the art can realize the present invention. However, it is easy for those skilled in the art to come up with various modifications of these embodiments, and have the inventive ability. It is possible to apply to various embodiments at least. Therefore, the present invention covers a wide range consistent with the disclosed principle and novel features, and is not limited to the above-described embodiments.

1 is a block diagram showing an example of the configuration of an optical disc device according to an embodiment of the present invention. The block diagram which shows an example of a structure of the frequency detector which the optical disc device which concerns on one Embodiment of this invention has. The block diagram which shows another example of a structure of the frequency detector which the optical disc device which concerns on one Embodiment of this invention has. The graph which shows an example of the change of RF signal which the optical disc device concerning one embodiment of the present invention read. 6 is a graph showing an example of a relationship among a zero cross point, a sample point, a wavelength T, and a sample number N of an RF signal read by the optical disc apparatus according to the embodiment of the present invention. 6 is a graph showing an example of a relationship among a zero cross point, a sample point, a wavelength T, and a sample number N of an RF signal read by the optical disc apparatus according to the embodiment of the present invention. The graph which shows an example of the relationship between the zero crossing point of RF signal which the optical disk device based on one Embodiment of this invention read, the sample point, the wavelength T, the sample number N1, and the time difference e. The flowchart for calculating | requiring the value of the period 13T from wavelength T1 in the optical disk apparatus which concerns on one Embodiment of this invention. 6 is a flowchart for obtaining a value of a period 3T from a wavelength T2 in the optical disc apparatus according to an embodiment of the present invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS D ... Optical disk, 11 ... Optical pick-up, 12 ... A / D converter, 13 ... Phase comparator, 14 ... Frequency detector, 15 ... Loop filter, 16 ... VCO, 17 ... Transversal filter, 18 ... Viterbi decoder

Claims (13)

A plurality of sampling values of the reproduction signal are obtained, the plurality of sampling values and the reference value are compared, and the sampling value in the vicinity of the first zero cross point that is the first intersection on the graph of the reproduction signal and the reference value is used. Identifying a first sample point and a second sample point, and determining a first slope value defined by a difference between the first sample point and the second sample point;
The third sampling point and the fourth sampling point that are the sampling values in the vicinity of the second zero crossing point that is the second intersection point on the graph of the reproduction signal and the reference value are specified, and the third sampling point and the second sampling point are specified. Find the second slope value defined by the difference with the 4 sample points,
The fifth sample point and the sixth sample point that are the sampling values in the vicinity of the third zero cross point that is the third intersection on the graph of the reproduction signal and the reference value are specified, and the fifth sample point and the fifth sample point are specified. Find the third slope value defined by the disparity with 6 sample points,
The time from the first zero cross point to the third zero cross point is corrected based on the first to third inclination values, and the frequency of the reproduction signal is detected based on the corrected time. Frequency detection device.   The time from the first zero cross point to the second zero cross point is corrected by a value obtained by subtracting 1 from a value obtained by dividing the first slope value by the second slope value. Frequency detection device.   The time from the second zero cross point to the third zero cross point is corrected by a value obtained by subtracting 1 from a value obtained by dividing the third slope value by the second slope value. Frequency detection device.   The frequency detection apparatus according to claim 1, wherein the first slope value is a slope value between zero cross samples at the beginning of 13T3T.   The frequency detection apparatus according to claim 1, wherein the second inclination value is an inclination value between zero cross samples in the middle of 13T3T.   The frequency detection apparatus according to claim 1, wherein the third inclination value is an inclination value between zero-cross samples at the end of 13T3T. Obtain multiple sampling values of the playback signal,
Compare these sampling values with the reference value,
Identifying the first sample point and the second sample point that are the sampling values in the vicinity of the first zero cross point that is the first intersection on the graph of the reproduction signal and the reference value;
Determining a first slope value defined by the difference between the first sample point and the second sample point;
The third sampling point and the fourth sampling point that are the sampling values in the vicinity of the second zero crossing point that is the second intersection point on the graph of the reproduction signal and the reference value are specified, and the third sampling point and the second sampling point are specified. Find the second slope value defined by the difference with the 4 sample points,
The fifth sample point and the sixth sample point that are the sampling values in the vicinity of the third zero cross point that is the third intersection on the graph of the reproduction signal and the reference value are specified, and the fifth sample point and the fifth sample point are specified. Find the third slope value defined by the disparity with 6 sample points,
The time from the first zero cross point to the third zero cross point is corrected based on the first to third inclination values, and the frequency of the reproduction signal is detected based on the corrected time. Frequency detection method.   8. The time from the first zero cross point to the second zero cross point is corrected by a value obtained by subtracting 1 from a value obtained by dividing the first slope value by the second slope value. Frequency detection method.   8. The time from the second zero cross point to the third zero cross point is corrected by a value obtained by subtracting 1 from a value obtained by dividing the third slope value by the second slope value. Frequency detection method.   The frequency detection method according to claim 7, wherein the first slope value is a slope value between zero cross samples at the head of 13T3T.   The frequency detection method according to claim 7, wherein the second slope value is a slope value between zero cross samples in the middle of 13T3T.   The frequency detection method according to claim 7, wherein the third slope value is a slope value between zero-cross samples at the end of 13T3T. A pickup head for reading a reproduction signal from an optical disc;
An A / D converter for A / D converting the reproduction signal read by the pickup head into a plurality of sampling values;
The plurality of sampling values are obtained, the plurality of sampling values are compared with a reference value, and the sampling value in the vicinity of a first zero cross point that is a first intersection on the graph of the reproduction signal and the reference value is obtained. Identifying a first sample point and a second sample point, and determining a first slope value defined by a difference between the first sample point and the second sample point;
The third sampling point and the fourth sampling point that are the sampling values in the vicinity of the second zero crossing point that is the second intersection point on the graph of the reproduction signal and the reference value are specified, and the third sampling point and the second sampling point are specified. Find the second slope value defined by the difference with the 4 sample points,
The fifth sample point and the sixth sample point that are the sampling values in the vicinity of the third zero cross point that is the third intersection on the graph of the reproduction signal and the reference value are specified, and the fifth sample point and the fifth sample point are specified. Find the third slope value defined by the disparity with 6 sample points,
A detector that corrects a time from the first zero cross point to the third zero cross point based on the first to third inclination values and detects a frequency of the reproduction signal based on the corrected time; An optical disc apparatus comprising:

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