Playing apparatus and playing method

A set of first signal light and reference light with a phase difference of almost 0 degree, a set of second signal light and reference light with a phase difference of almost 180 degrees, a set of third signal light and reference light with a phase difference of almost 90 degrees, and a set of fourth signal light and reference light with a phase difference of almost 270 degrees are generated. A first differential signal as a difference between a first light-receiving signal obtained by a first light-receiving element and a second light-receiving signal obtained by a second light-receiving element is calculated, and a second differential signal as a difference between a third light-receiving signal obtained by a third light-receiving element and a fourth light-receiving signal obtained by a fourth light-receiving element is calculated. The first differential signal and the second differential signal are supplied to respective FIR filters. An equalization error is formed from output signals from the FIR filters. Tap coefficients for the FIR filters are controlled to minimize the equalization error.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase of International Patent Application No. PCT/JP2015/005785 filed on Nov. 19, 2015, which claims priority benefit of Japanese Patent Application No. JP 2015-016361 filed in the Japan Patent Office on Jan. 30, 2015. Each of the above-referenced applications is hereby incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a playing apparatus and a playing method that are applied to playing of optical media such as optical discs.

BACKGROUND ART

In a case where a multilayer optical disc is played, for example, there is a high possibility that the light amount of a signal reduces to cause an error in reading the signal. As a solution to this problem, there is known a homodyne detection method by which to amplify the detection signal by the use of light interference (refer to Patent Documents 1 and 2).

According to Patent Documents 1 and 2, as a homodyne method for detecting light resulting from interference between signal light and reference light, four sets of signal light and reference light with phase differences of 90 degrees are detected. Specifically, the sets of signal light and reference light with phase differences of 0, 90, 180, and 270 degrees are detected. Each of the detections is performed by detecting the intensity of light resulting from interference between the signal light and the reference light.

Further, Patent Document 3 describes a playing apparatus that uses the homodyne method for optical discs on which signals are recorded in both lands and grooves.

CITATION LIST

Patent Documents

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

Using the homodyne detection in the land-groove system makes it possible to separate a groove RF signal and a land RF signal and read them independently, thereby achieving a narrow track pitch. According to Patent Document 3, there is the need to set the difference in level between the land and the groove to provide a phase difference of 90 degrees between the reflection light from the land and the reflection light from the groove. However, the value of the level difference may fluctuate due to variations among optical discs, or the phase relationship fluctuates microscopically depending on the condition of the optical disc, thereby resulting in the degradation of signal quality.

Therefore, an object of the present disclosure is to provide a playing apparatus and a playing method that employ the homodyne detection method and allow favorable playing of land/groove recording-type optical media.

Solutions to Problems

To solve the foregoing problems, the present disclosure is a playing apparatus including:

an optical system that irradiates a recording medium on which signals are recorded in both a land and a groove with light emitted from a light source to obtain signal light reflecting both recorded signals of the land and the groove, generates reference light from the light emitted from the light source, and generates a set of first signal light and reference light with a phase difference of almost 0 degree from overlapping light of the signal light and the reference light, a set of second signal light and reference light with a phase difference of almost 180 degrees from the same, a set of third signal light and reference light with a phase difference of almost 90 degrees from the same, and a set of fourth signal light and reference light with a phase difference of almost 270 degrees from the same;

a light-receiving unit that receives the set of first signal light and reference light by a first light-receiving element, the set of second signal light and reference light by a second light-receiving element, the set of third signal light and reference light by a third light-receiving element, and the set of fourth signal light and reference light by a fourth light-receiving element;

an arithmetic operation unit that calculates a first differential signal as a difference between a first light-receiving signal obtained by the first light-receiving element and a second light-receiving signal obtained by the second light-receiving element, and calculates a second differential signal as a difference between a third light-receiving signal obtained by the third light-receiving element and a fourth light-receiving signal obtained by the fourth light-receiving element;

a first FIR filter and a second FIR filter to which the first differential signal and the second differential signal are supplied, respectively; and

an equalization error detection unit that is supplied with an addition signal in which output signals from the first and second FIR filters are added up to form an equalization error, wherein

tap coefficients for the first and second FIR filters are controlled to minimize the equalization error.

The present disclosure is a playing method including:

irradiating a recording medium on which signals are recorded in both a land and a groove with light emitted from a light source to obtain signal light reflecting both recorded signals of the land and the groove, generating reference light from the light emitted from the light source, and generating by an optical system a set of first signal light and reference light with a phase difference of almost 0 degree from overlapping light of the signal light and the reference light, a set of second signal light and reference light with a phase difference of almost 180 degrees from the same, a set of third signal light and reference light with a phase difference of almost 90 degrees from the same, and a set of fourth signal light and reference light with a phase difference of almost 270 degrees from the same;

receiving the set of first signal light and reference light by a first light-receiving element, the set of second signal light and reference light by a second light-receiving element, the set of third signal light and reference light by a third light-receiving element, and the set of fourth signal light and reference light by a fourth light-receiving element;

calculating a first differential signal as a difference between a first light-receiving signal obtained by the first light-receiving element and a second light-receiving signal obtained by the second light-receiving element, and calculates a second differential signal as a difference between a third light-receiving signal obtained by the third light-receiving element and a fourth light-receiving signal obtained by the fourth light-receiving element;

supplying the first differential signal and the second differential signal to a first FIR filter and a second FIR filter, respectively;

supplying an addition signal in which output signals from the first and second FIR filters are added up to an equalization error detection unit to form an equalization error; and

controlling tap coefficients for the first and second FIR filters to minimize the equalization error.

Effects of the Invention

According to at least one embodiment, it is possible to play land/groove recording-type optical recording media favorably using the homodyne detection method. The present disclosure eliminates the need to set the optical system in advance according to the individual grooves depths or set a phase offset in signal processing operations. Further, the present disclosure makes it possible to suppress degradation in signal quality resulting from a phase shift due to recording marks and disc conditions in contrast to the optical discs. Note that the advantages described herein are not limited ones but any one of the advantages described in the present disclosure may be applied.

MODE FOR CARRYING OUT THE INVENTION

The embodiments described below are favorable specific examples of the present disclosure with various technically preferable limitations. However, the scope of the present disclosure should not be limited by these embodiments unless the present disclosure is limited otherwise in the following description.

Note that the present disclosure will be explained in the following sequence:

<1. About the conventional homodyne detection method>

<2. About the improved homodyne detection method>

1. About the Conventional Homodyne Detection Method

Prior to the description of a playing method as an embodiment of the present disclosure, the conventional homodyne detection method and the improved homodyne detection method will be explained. As an example, the phase-diversity homodyne detection method will be explained below.

[Optical Recording Medium to be Played]

FIG. 1illustrates a cross-section structure of an optical recording medium1to be played. The rotationally driven optical recording medium1is irradiated with laser light to reproduce recorded signals. The optical recording medium1is a ROM-type (playback-only) optical recording medium on which information is recorded by forming pits (emboss pits), for example.

As illustrated inFIG. 1, the optical recording medium1has a cover layer2, a recording layer (reflection film)3, and a substrate4formed in sequence from the upper side. Here, the term “upper side” here refers to the upper side of the disc surface on which the laser light from the playing apparatus is incident. That is, in this case, the laser light is incident on the cover layer2of the optical recording medium1.

In the optical recording medium1, the substrate4is formed from a resin such as polycarbonate, for example. The upper side of the substrate4has a concave-convex cross section due to the formation of pits on the upper side. The substrate4with the pits is generated by ejection molding with a stamper, for example.

Then, a reflection film made of a metal or the like, for example, is formed on the concave-convex upper side of the substrate4to produce the recording layer3. In this case, the optical recording medium1to be played by the conventional homodyne detection has tracks as pit rows with a general track pitch within the optical limit. That is, the track pitch in the recording layer3is set to be larger than the optical limit of which the theoretical value is represented as “λ/NA/2” (λ represents the playing wavelength and NA represents the numerical aperture of an object lens).

The cover layer2formed on the upper side of the recording layer3is formed by applying an ultraviolet curable resin, for example, by spin-coating or the like, and then performing cure treatment by ultraviolet irradiation. The cover layer2is provided to protect the recording layer3.

FIGS. 2A and 2Billustrate a structure of a recording surface of the optical recording medium1to be played.FIG. 2Ais an enlarged partial plan view of the recording surface, andFIG. 2Bis an enlarged partial perspective view of the recording surface. Note thatFIG. 2Billustrates the surface of the optical recording medium1that is irradiated with laser light for playing. That is, the laser light for playing is applied from the upper side of the drawing. The optical recording medium1has grooves G and lands L. In the description herein, the grooves G are on the side the laser light for playing reaches earlier, that is, on the convex side, and the lands L are on the concave side, as in the case of the Blu-ray disc (BD) (registered trademark).

The optical recording medium1to be played has pit rows formed both in the grooves G and the lands L. When the pit rows are regarded as tracks, a track pitch Tp can be defined as a formation pitch of the lands L and the grooves G as illustrated inFIG. 2B. Setting the track pitch Tp to be narrow beyond the optical limit value improves the density of information recording. For example, when the formation pitch of the grooves G in the optical recording medium1is equal to the track pitch (the formation pitch of the pit rows) in a conventional optical recording medium, the optical recording medium1is nearly doubled in the density of information recording.

The level difference (also called depth as appropriate) between the lands L and the grooves G is designated as d. When the refractive index of the optical recording medium1is designated as n, for example, the depth d is “λ/8/n.” Under the conditions that the playing wavelength λ is 405 nm and n is 1.5, for example, the depth d is about 33 nm.

Here, in the optical recording medium1of the embodiment, the formation pitch of the lands L and the grooves G exceeds the optical limit value, the relationship between a beam spot of playing light formed on the recording surface, the lands L and the grooves G are as illustrated inFIG. 3, for example.

As in the conventional case, the tracking servo control of the object lens is performed on the grooves G or the lands L. In the example ofFIG. 3, a case where the tracking servo control of the object lens is performed on the grooves G is illustrated. In this case, it is understood that there is a mixture of recording information on two adjacent lands L in the playback signal of the groove G as a servo target.

Specifically, when the track pitch becomes narrow in the land/groove recording method, a crosstalk occurs from adjacent tracks. As illustrated inFIG. 4, in the case of playing the grooves, playback signals f(t) of the grooves are mixed with playback signals g(t) of the adjacent lands. When a phase φ of the groove playback signals is 0, a phase Ψ of the lands is 4πnd/λ (Δ represents wavelength and n represents the refractive index of the substrate of the optical recording medium1).

[Homodyne Detection Method in the Phase Diversity System]

In the phase diversity system, four sets of signal light and reference light with phase differences of 90 degrees are used. Specifically, in the phase diversity system, the sets of signal light and reference light adjusted to have phase differences of almost 0, 180, 90, and 270 degrees are detected. Each of the detection is performed by detecting the intensity of light resulting from interference between the signal light and the reference light.

FIG. 5illustrates mainly a configuration of an optical system for use in the phase diversity system. When being loaded into the playing apparatus, the optical recording medium1is rotationally driven by a spindle motor. The optical system includes a laser (semiconductor laser)10as a laser light source for playing. The laser light emitted from the laser10is turned into parallel light via a collimation lens11, and then enters a polarization beam splitter13via a ½ wavelength plate12.

In this case, the polarization beam splitter13is configured to transmit P polarized light and reflect S polarized light, for example. The attachment angle of the ½ wavelength plate12(angle of rotation around an optical axis in the incident plane of the laser light) is adjusted such that the ratio of the light output by passing through the polarization beam splitter13(P polarized light component) to the light output by reflecting on the polarization beam splitter13(S polarized light component) (that is, the ratio of light splitting by the polarization beam splitter13) is, for example, almost 1:1.

The laser light reflected by the polarization beam splitter13is passed through a ¼ wavelength plate14and then is collectively emitted to the recording layer on the optical recording medium1via an object lens15held by a biaxial actuator16.

The biaxial actuator16holds the object lens15to be displaceable in a focus direction (direction of moving toward and away from the optical recording medium1) and a tracking direction (radial direction of the optical recording medium1orthogonal to the focus direction). The biaxial actuator16includes a focus coil and a tracking coil to which a focus drive signal FD and a tracking drive signal TD described later are supplied, respectively. The object lens15displaces in the focus direction or the tracking direction according to the focus drive signal FD or the tracking drive signal TD.

The reflection light from the recording layer on the optical recording medium1enters the polarization beam splitter13via the object lens15and the ¼ wavelength plate14. The reflection light (return light) having entered the polarization beam splitter13is made different in polarization direction by 90 degrees from the light having entered from the laser10side and reflected on the polarization beam splitter13(outward light) by the action of the ¼ wavelength plate14and the action of the reflection on the recording layer. That is, the reflection light enters the polarization beam splitter13as P polarized light. Accordingly, the reflection light passes through the polarization beam splitter13. Note that in the following description, the reflection light reflecting the recorded signal of the optical recording medium1and passing through the polarization beam splitter13will be called signal light.

Referring toFIG. 5, the laser light (P polarized light) having been emitted from the laser10and passed through the polarization beam splitter13acts as reference light in the homodyne detection system. After having passed the polarization beam splitter13, the reference light then passes through a ¼ wavelength plate17illustrated in the drawing, and is reflected on a mirror18, and enters the polarization beam splitter13again through the ¼ wavelength plate17.

In this case, the reference light (return light) entering the polarization beam splitter13is made different in polarization direction by 90 degrees from the reference light as the outward light by the action of the ¼ wavelength plate17and the action of the reflection on the mirror18(that is, S polarized light). Therefore, the reference light as the return light is reflected by the polarization beam splitter13.

FIG. 5illustrates the reference light reflected by the polarization beam splitter13by a dashed arrow, and illustrates the signal light having passed through the polarization beam splitter13as a solid arrow. The polarization beam splitter13emits the signal light and the reference light in an overlapping state in the same direction. In this case, specifically, the signal light and the reference light overlap with their light axes aligned and are emitted in the same direction. Here, the reference light is coherent light.

The overlapping signal light and reference light output from the polarization beam splitter13enter a half beam splitter19. The half beam splitter19splits the incident light into the reflection light and the transmission light at a ratio of almost 1:1.

The overlapping signal light and reference light having passed through the half beam splitter19enter a polarization beam splitter21via a ½ wavelength plate20. Meanwhile, the overlapping signal light and reference light reflected by the half beam splitter19enter a polarization beam splitter23via a ¼ wavelength plate22.

The ½ wavelength plate20and the ¼ wavelength plate22rotate the polarization plane. Therefore, the ratio of light amounts branched by the polarization beam splitter21can be adjusted by combining the ½ wavelength plate20and the polarization beam splitter21. Similarly, the ratio of light amounts branched by the polarization beam splitter23can be adjusted by the ¼ wavelength plate22.

The light amounts branched by the polarization beam splitter21and the polarization beam splitter23are set to be almost 1:1. The light having been reflected by the polarization beam splitter21enters a light detection unit24, and the light having passed through the polarization beam splitter21enters a light detection unit25. The light having been reflected by the polarization beam splitter23enters a light detection unit26, and the light having passed through the polarization beam splitter23enters a light detection unit27.

A light-receiving signal output from the light detection unit24will be designated as I, a light-receiving signal output from the light detection unit25as J, a light-receiving signal output from the light detection unit26as L, and a light-receiving signal output from the light detection unit27as K.

These light-receiving signals I to L are supplied to subtraction circuits31aand31b. The light-receiving signals I and J are supplied to the subtraction circuit31a, and the subtraction circuit31agenerates a differential signal a (a=I−J), and the subtraction circuit31bgenerates a differential signal b (b=K−L).

As illustrated inFIG. 6, the foregoing differential signals a and b are supplied to an arithmetic operation circuit32. The arithmetic operation circuit32has delay circuits33aand33b, multiplication circuits34aand34b, low-pass filters35aand35b, offset (φ) setting circuits36aand36b, and an addition circuit37. The delay circuit33ahas a delay time that is equal to a delay amount generated in the low-pass filter35aand the offset (φ) setting circuit36a. The delay circuit33bhas a delay time that is equal to a delay amount generated in the low-pass filter35band the offset (φ) setting circuit36b. The output of the multiplication circuit34aand the output of the multiplication circuit34bare supplied to the addition circuit37. A playback signal is retrieved from the output of the addition circuit37.

The foregoing playing apparatus can obtain a playback signal under no influence of the component of a phase shift (θ(t)) of the reference light due to the surface deflection of the optical recording medium1or the like as described below.

The light-receiving signals I to L are represented by the equations shown below. The meanings of each of the terms in the equations are as follows:

R: Reference light component

A: Reflection component of a mirror plane (land portion) formed on the recording surface of the optical recording medium

f: Modulated component according to the presence or absence of pits (taking on a positive or negative value)

t: Sampling time

φ: Phase equivalent to pit depth (optical depth) or the like

θ: Light path length difference between the signal light and the reference light (resulting mainly from the surface deflection of the optical recording medium1)

As illustrated inFIG. 7, when the signal surfaces of the object lens15and the optical recording medium1change due to the surface deflection, the light path length of the signal light changes accordingly. Meanwhile, the reference light reflects on the mirror18and thus its light path length does not change. Consequently, the phase difference between the signal light and the reference light shifts from the set value. The component of the phase shift is θ(t).

The differential signal a (=I−J) of the subtraction circuit31aand the differential signal b (=K−L) of the subtraction circuit31bare represented by the following formulas:

In the normal detection method without homodyne detection, as illustrated inFIG. 8A, the DC component of the playback signal appears corresponding to the mirror portion in the background. In the case of the homodyne detection method, as illustrated inFIG. 8B, the DC components corresponding to the mirror portion wind due to the phase θ corresponding to the difference in the light path length of the reference light described above.

To determine the phase θ, the differential signals a and b illustrated inFIG. 8Bare supplied to the low-pass filters35aand35b. The low-pass filters35aand35bcan be used to determine cos θ(t) and sin θ(t) as illustrated inFIG. 8C. Specifically, since, in Mathematical Formulas (5) and (6), f represents the modulated component due to the presence or absence of pits (taking on a positive or negative value), it is considered that the terms with the multiplication by the function f disappear and the terms sin θ and cos θ are left.

Since (tan θ=sin θ/cos θ), θ is determined from (arctan θ=θ) and φ (offset) is set. The multiplication circuit34amultiplies a by cos(φ−θ(t)), and the multiplication circuit34bmultiplies b by sin(φ−θ(t)). Then, the addition circuit37adds up these multiplication outputs. The playback signal obtained from the addition circuit37is represented by the following formula:

As understood from the foregoing mathematical formula, the playback signal becomes stabled without the component of θ(t). Note that as another homodyne detection method, the mirror18may be controlled in position to cancel out the phase difference between the signal light and the reference light resulting from the surface deflection. According to the phase diversity method, however, the configuration for position control of the mirror18can be omitted. Further, it is possible to obtain the playing result in which the component of the signal light is amplified by the component of the reference light. That is, the amplified recorded signal of the optical recording medium1is detected, thereby achieving improvement in SNR. Note that the term phase diversity method refers to the method by which to determine the playback signal by calculating the sum of squares (a2+b2) of the differential signals a and b or the square-root of sum of squares of the differential signals a and b. The term phase diversity method is also used herein for the arithmetic operation in which the multiplication circuit34bmultiplies a by cos(φ−θ(t)) and multiplies b by sin(φ−θ(t)) as described above.

The graph inFIG. 9Billustrates the results of jitter of the playback signals (the playback signals of the grooves or the playback signals of the lands) determined by simulation in a case of changes in the track pitch Tp on the assumption that the optical recording medium for the land/groove recording as described above is played by the optical system illustrated inFIG. 9A. Note that the jitter is one of indexes of playing performance, and any index other than the jitter may be used instead.

As illustrated inFIG. 9A, the laser light from a laser diode41is applied to the signal surface of the optical recording medium1through a lens42, a polarization beam splitter43, and an object lens44. The reflection light from the signal surface is reflected by the polarization beam splitter43, and is supplied to a light detection unit46via a lens45. Accordingly, the playback signal is obtained from the light detection unit46. The playing optical system illustrated inFIG. 9Adoes not use the homodyne detection described above.

The simulation is carried out under the calculation conditions shown below. Note that a playing method capable of reduction in intertrack crosstalk is used on the assumption that there is no surface deflection.

The graph illustrated inFIG. 9Bindicates changes in the value of jitter relative to Tp for each of (Mrr (meaning the mirror, d=0), (d=0.125λ), (d=0.15λ), and (d=0.175λ)). With (Tp=0.22), for example, the jitter can be made small relative to the groove depth except for the mirror. Additionally, the changes in the jitter can be made almost similar even with differences in the groove depth.

FIGS. 10A and 10Bprovide the results of simulation in a case of playing the optical recording medium1for the land/groove recording by the use of the homodyne detection method. As illustrated inFIG. 10A, a mirror47is provided so that the reflection light (signal light) from the optical recording medium1and the reflection light (reference light) from the mirror47are supplied to the light detection unit46via the lens45.

The results of simulation in a case of using the optical system illustrated inFIG. 10Ais illustrated inFIG. 10B. The calculation conditions for the simulation are similar to those in the case ofFIG. 9B. The graph illustrated inFIG. 10Bindicates changes in the value of jitter relative to Tp for each of (Mrr (meaning the mirror, d=0), (d=0.1λ), (d=0.125λ=λ/8), (d=0.15λ), and (d=0.175λ)).

For (Tp=0.15), for example, the jitter can be reduced relative to the mirror. However, the changes in the value of jitter vary depending on the value of the depth d. Specifically, the jitter can be significantly improved in the case of (d=0.125λ=λ/8), whereas the jitter is too large in the case of (d=0.175λ). Further, the value of the jitter cannot be said to be sufficiently favorable in the cases of (d=0.1λ) and (d=0.15λ). In the case of d=λ/8, a phase difference of 90 degrees can be made between the playback signal of the groove and the playback signal of the land, thereby reducing the crosstalk and setting the jitter to a favorable value.

As described above, the capability of obtaining favorable playing performance only in a case of the specific groove depth d constitutes a design restriction on the optical recording medium1. In addition, the value of d=λ/8 is relatively large and cannot be said to be preferable for the surface on which marks are recorded in the lands between the grooves. Further, in a case where d is large, it is difficult to produce an optical disc with the wall surfaces of steps accurate without inclination. Therefore, it is preferable that the value of d is not limited to (λ/8).

2. About the Improved Homodyne Detection Method

To solve this issue, a playing optical system similar to that illustrated inFIG. 5and a playback signal generation circuit similar to that illustrated inFIG. 6are used. The differential signals formed from the light-receiving signals I to L output from the light detection units24to27illustrated inFIG. 5, respectively, are supplied to the playback signal generation circuit configured as illustrated inFIG. 11.

The playback signal generation circuit includes subtraction circuits31aand31band an arithmetic operation circuit38. The light-receiving signals I and J are supplied to the subtraction circuit31a, the subtraction circuit31agenerates a differential signal a (a=I−J), and the arithmetic operation circuit31bgenerates a differential signal b (b=K−L). The differential signal a of the subtraction circuit31aand the differential signal b of the subtraction circuit31bare supplied to the arithmetic operation circuit38.

The arithmetic operation circuit38has delay circuits33aand33b, multiplication circuits34aand34b, low-pass filters35aand35b, offset (Ψ) setting circuits39aand39b, and an addition circuit37. The delay circuit33ahas a delay time that is equal to a delay amount generated in the low-pass filter35aand the offset (Ψ) setting circuit39a. The delay circuit33bhas a delay time that is equal to a delay amount generated in the low-pass filter35band the offset (Ψ) setting circuit39b. The output of the multiplication circuit34aand the output of the multiplication circuit34bare supplied to the addition circuit37. A playback signal is retrieved from the output of the addition circuit37.

The offset (Ψ) setting circuits39aand39bset an offset of phase according to the level difference between the grooves G and the lands L, that is, according to the depth d as described below. The value of the depth d in the optical recording medium1to be played is known in advance, and therefore the offset Ψ can be set.

According to the foregoing improved homodyne method, as described below, it is possible to obtain the playback signal from which intertrack crosstalk is removed under no influence of the component of the phase shift (θ(t)) of the reference light resulting from the surface deflection of the optical recording medium1or the like. As explained above with reference toFIGS. 3 and 4, when the track pitch becomes narrow in the land/groove recording method, a crosstalk occurs from adjacent tracks. As illustrated inFIG. 4, in the case of playing the grooves, playback signals f(t) of the grooves are mixed with playback signals g(t) of the adjacent lands. When a phase φ of the groove playback signals is 0, a phase Ψ of the lands is 4πnd/λ (λ represents wavelength and n represents the refractive index of the substrate of the optical recording medium1).

The playing optical system illustrated inFIG. 5is used to determine the light-receiving signals I to L. As with the foregoing mathematical formulas, the meanings of each of the terms in the equations shown below are as follows:

R: Reference light component

A: Reflection component of a mirror plane (land portion) formed on the recording surface of the optical recording medium

f: Modulated component according to the presence or absence of pits

g: Crosstalk component from adjacent tracks

t: Sampling time

φ: Phase equivalent to mark complex reflectivity and optical depth of guide groove or the like

θ: Light path length difference between the signal light and the reference light (resulting mainly from the surface deflection of the optical recording medium1)

Further, the playback signal generation circuit illustrated inFIG. 11is used to perform arithmetic operations. The differential signal a (=I−J) of the subtraction circuit31aand the differential signal b (=K−L) of the subtraction circuit31bare represented by the following formulas:

As described above, the low-pass filters35aand35bare used to determine cos θ(t) and sin θ(t). Specifically, since, in Mathematical Formulas (12) and (13), f represents the modulated component due to the presence or absence of pits (taking on a positive or negative value) and g represents the component of crosstalk from adjacent tracks, it is considered that the terms with the multiplication by the functions f and g disappear and the terms sin θ and cos θ are left. Since (tan θ=sin θ/cos θ), θ is determined from (arctan θ=θ) and Ψ (offset) is set by the offset (Ψ) setting circuits39aand39b. The multiplication circuit34amultiplies a by cos (Ψ−θ(t)), and the multiplication circuit34bmultiplies b by sin (Ψ−θ(t)). Then, the addition circuit37adds up these multiplication outputs. The playback signal obtained from the addition circuit37is represented by the following formula:

As show in Mathematical Formula (14), the playback signal becomes stabled without the component of θ(t). In addition, the playback signal does not include the playback signal components g(t) of the adjacent tracks and therefore is clear of intertrack crosstalk. Note that the playback signal may be determined by calculating the sum of squares (a2+b2) of the differential signals a and b or the square-root of sum of squares of the differential signals a and b.

The results of simulation in a case of using an optical system similar to the optical system illustrated inFIG. 10Aare illustrated inFIG. 12. The calculation conditions for the simulation are similar to those in the case ofFIGS. 9B and 10B. The graph illustrated inFIG. 12indicates changes in the value of jitter relative to Tp for each of (Mrr (meaning the mirror, d=0), (d=0.1λ), (d=0.125λ=λ/8), (d=0.15λ), and (d=0.175λ)).

As understood from the graph inFIG. 12, the jitter can be reduced for all the values of d except for the mirror. While the jitter can be significantly improved only in the case of (d=0.125λ=λ/8) in the case ofFIG. 10Bdescribed above, the jitter can be significantly improved in a similar manner even with other values of d according to the improved homodyne method.

3. First Embodiment

The improved homodyne method described above makes it possible to remove the influence of a shift in the phase difference θ between the signal light and the reference light and remove the influence of the groove depth d by presetting the offset Ψ according to the groove depth d. The arithmetic operation to this end uses the phase φ (equivalent to mark complex reflectivity and the optical depth of a guide groove). However, the groove depth d and the phase φ vary among discs and need to be set for the individual discs. Further, the groove depth d and the phase φ may fluctuate microscopically depending on the conditions of the optical recording medium (for example, an optical disc). Therefore, there is the possibility that no favorable playback signal can be obtained removing influence of the crosstalk from the adjacent tracks.

The present disclosure is devised in view of this respect. According to the present disclosure, the phase offset is not preset but is automatically corrected by adaptive equalization using a least mean square (LMS) algorithm. A configuration of a first embodiment will be explained with reference toFIG. 13. In the first embodiment, the grooves are subjected to a tracking servo as an example. In this example, the RF signal is read from the groove while suppressing crosstalk from the adjacent lands.

As described above, the differential signal a generated by the subtraction circuit31aand the differential signal b generated by the subtraction circuit31bare converted into digital differential signals by A/D converters51aand51b, respectively. An addition circuit52adds up the output signals from the A/D converters51aand51b, and the output of the addition circuit52is supplied to a phase locked loop (PLL) circuit53. The PLL circuit53forms sampling clocks for the A/D converters51aand51b.

The output signals from the A/D converters51aand51bare supplied to finite impulse response (FIR) filters54aand54bas adaptive equalizers, respectively. The FIR filter54aperforms a partial response (PR) adaptive equalization process based on the differential signal a. The FIR filter54bperforms a PR adaptive equalization process based on the differential signal b.

An output signal ya from the FIR filter54aand an output signal yb from the FIR filter54bare supplied to an addition circuit55. An output signal yc (=ya+yb) from the addition circuit55is input into a Viterbi detector56.

The Viterbi detector56performs a maximum-likelihood decoding process on the equalized signal yc having undergone PR equalization to obtain binary data (RF signal). The used Viterbi detector includes a plurality of states with continuous bits of a predetermined length and branches represented by transitions between the states, and is configured to detect a desired one from all possible bit series in an efficient manner.

In the actual circuit, two registers are prepared for each state: one that is called path metric register for storing a partial response series and a path metric of a signal to the state; and the other that is called path memory register for storing a flow of a bit series to the state. Further, an arithmetic unit called branch metric unit is prepared for each branch to calculate a partial response series and a path metric of a signal in the bit.

The Viterbi detector56can associate various bit series with paths passing through the states one by one. In addition, the path metrics between the partial response series passing through these paths and the actual signal (RF signal) can be obtained by adding up in sequence the inter-state transitions constituting the paths, that is, the foregoing branch metrics in the branches.

Further, the selection of the paths to minimize the path metric can be achieved by comparing the path metrics of two or less branches to each of the states and selecting the paths with smaller path metrics in sequence. Transferring the selection information to the path memory register stores information on representation of the paths to each of the states in bit series. The value of the path memory register is sequentially updated and finally converged to the bit series with the minimum path metric, and then the final result is output.

Further, a PR convolver provided in the Viterbi detector56performs a convolution process on the result of the Viterbi detection to generate a target signal Zk. The target signal Zk is a noiseless ideal signal obtained by convolving the binary detection result. In the case of PR (1, 2, 2, 2, 1), for example, the impulse response for each channel clock is (1, 2, 2, 2, 1). The constraint length is 5. Further, in the case of PR (1, 2, 3, 3, 3, 2, 1), the impulse response for each channel clock is (1, 2, 3, 3, 3, 2, 1).

Then, the Viterbi detector56determines an equalization error et from the equalization signal yc from the addition circuit55and the target signal Zk, and supplies the same to LMS processors57aand57b. The LMS processors57aand57bdetermine adaptively tap coefficients for the FIR filters54aand54bby LMS algorithm operation between the equalization error et and the phase-separated data such that the square of the equalization error et becomes minimum.

FIG. 14illustrates an example of the FIR filter54a(the FIR filter54bis also applicable to the example). The FIR filter54ais a filter with n+1-stage taps including delay elements60-1to60-n, coefficient multipliers61-0to61-n, and an adder64. The coefficient multipliers61-0to61-nmultiply inputs x at respective points in time by tap coefficients C0 to Cn. The adder64adds up the outputs of the coefficient multipliers61-0to61-nand retrieves the addition result as output ya. The tap coefficients are set in advance to initial values.

The tap coefficients C0 to Cn are controlled to perform an adaptive equalization process. To this end, the equalization error et and arithmetic operators62-0to62-ninto which each of the tap inputs are input for arithmetic operation are provided. In addition, integrators63-0to63-nare provided to integrate the outputs of the arithmetic operators62-0to62-n, respectively. Each of the arithmetic operators62-0to62-nperforms an operation of −1*et*x, for example, in which * represents multiplication. The outputs of the arithmetic operators62-0to62-nare integrated by the integrators63-0to63-n, and the tap coefficients C0 to Cn of the coefficient multipliers61-0to61-nare changed and controlled according to the integration results. Note that the integrators63-0to63-nperform the integration to adjust the responsiveness of the adaptive coefficient control.

As described above, according to the first embodiment, it is possible to form the RF signal of the groove with the crosstalk component removed from the difference in signal quality between the groove playback signal and the land playback signal. Further, according to the first embodiment, it is possible to read the groove signal independently, for example, without having to preset the phase offset.

4. Second Embodiment

In a second embodiment, the preset phase offset is used to suppress degradation in signal quality due to fluctuations (perturbation factors). The differential signals a and b are subjected to an arithmetic operation using the preset phase offset. Consequently, the signals represented by the following Mathematical Formulas (15) and (16) can be read independently at the quality only with the phase shift due to the perturbation.

FIG. 15is a configuration example describing the second embodiment. The differential signals a and b are supplied to a phase (θ) extraction circuit71to calculate the phases. Offset setting circuits72and73are provided to output the offset set to each optical disc to be played.

The output of the phase extraction circuit71and the output of the offset setting circuit72are supplied to a subtraction circuit74, and a phase of (Ψ−θ) is obtained from the subtraction circuit74. Signal generation circuits76and77generate a sine wave and a cosine wave in synchronization with the phase of (Ψ−θ). The differential signal a and the sine wave from the signal generation circuit76are supplied to a multiplication circuit78, and an output signal from the multiplication circuit78is supplied to an addition circuit80. The differential signal b and the cosine wave from the signal generation circuit77are supplied to a multiplication circuit79, and an output signal from the multiplication circuit79is supplied to the addition circuit80. A signal represented by Mathematical Formula (15) is retrieved from the output of the addition circuit80.

The output of the phase extraction circuit71and the output of the offset setting circuit73are supplied to a subtraction circuit75, and a phase of (φ−θ) is obtained from the subtraction circuit75. Signal generation circuits81and82generate a sine wave and a cosine wave in synchronization with the phase of (φ−θ). The differential signal a and the sine wave from the signal generation circuit81are supplied to a multiplication circuit83, and an output signal from the multiplication circuit83is supplied to the addition circuit80. The differential signal b and the cosine wave from the signal generation circuit82are supplied to a multiplication circuit84, and an output signal from the multiplication circuit84is supplied to an addition circuit85. A signal represented by Mathematical Formula (16) is retrieved from the output of the addition circuit85.

An output signal from the addition circuit80is supplied to a phase locked loop (PLL) circuit86for re-sampling using the playback signal from the groove. An output signal from the PLL circuit86is supplied to the FIR filter54a. An output signal from the addition circuit85is supplied to the FIR filter54b.

As in the first embodiment (FIG. 13) described above, the respective outputs ya and yb of the FIR filters54aand54bare supplied to the addition circuit55, and the output yc of the addition circuit55is supplied to the Viterbi detector56. The equalization error et from the Viterbi detector56is supplied to the LMS processors57aand57b, and the LMS processors57aand57bdetermine adaptively the tap coefficients for the FIR filters54aand54b.

According to the second embodiment described above, when the phase shift due to variable factors occurring in reality is corrected, it can be expected that the signal yb comes close to the crosstalk component from the land and the signal yc comes close to the signal from the groove without the crosstalk component removed. Accordingly, it can be expected to further improve the signal quality by adaptive equalization.

5. Modification Example

FIG. 16illustrates a configuration example of phase extraction. Output signals from photodetectors91,92,93, and94equivalent to the light detection units24to27in the optical configuration illustrated inFIG. 5are processed by subtraction circuits95aand95bto form differential signals a and b. The differential signal a is supplied to a multiplication circuit96, an arithmetic operation circuit97, and an arithmetic operation circuit98. The differential signal b is supplied to a multiplication circuit99, an arithmetic operation circuit97, and an arithmetic operation circuit98.

The arithmetic operation circuit97generates an output of (2a*b), and an output signal from the arithmetic operation circuit97is supplied to a low-pass filter100. The low-pass filter100generates an output c. The arithmetic operation circuit98generates an output of (a*a−b*b), and an output signal from the arithmetic operation circuit98is supplied to a low-pass filter101. The low-pass filter101generates an output d.

The output c of the low-pass filter100and the output d of the low-pass filter101are supplied to an arithmetic operation circuit102. The arithmetic operation circuit102determines the phase θ by performing an operation of (arctan(c, d)/2). The phase θ is supplied to signal generation circuits103and104. The signal generation circuit103generates a cosine wave (cos θ) and supplies the signal to the multiplication circuit96. The signal generation circuit104generates a sine wave (sin θ) and supplies the signal to the multiplication circuit99.

An output signal from the multiplication circuit96and an output signal from the multiplication circuit99are supplied to an addition circuit105. The multiplication circuit96detects synchronously the differential signal a, and the multiplication circuit99detects synchronously the differential signal b. The RF signal is retrieved from the addition circuit105.

The foregoing phase θ corresponds to the difference in light path length between the signal light and the reference light (resulting mainly from the surface deflection of the optical recording medium1). To eliminate the influence of the phase shift, the reference light servo may be used to make the light path length of the reference light physically variable as illustrated inFIG. 17. For example, controlling the position of the mirror18in the optical system illustrated inFIG. 5makes it possible to control the light path length of the reference light.

Referring toFIG. 17, signal light Esig and reference light Eref enter a polarization beam splitter111. The signal light is a signal with a mixture of land signals EL and EG. The optical system illustrated inFIG. 17includes polarization beam splitters114,115, and116, half beam splitters112and113, a (½) wavelength plate122, a (¼) wavelength plate123, a wavelength plate124, and photodetectors131to136. The reference light servo is performed with the use of the outputs of the photodetectors135and136. The wavelength plate124is provided to set the phase difference to (π/8).

When an output signal from the photodetector135is designated as IPD5 and an output signal from the photodetector136is designated as IPD6, the differential value (IPD5−IPD6) is represented by Mathematical Formula (17) as follows:

The reference light servo is performed such that a difference Icalc3 represented by Mathematical Formula (17) becomes zero. This makes it possible to cancel out the phase difference between the signal light and the reference light. In the case where Icalc3 is 0, (θ−φref)=π/2. When an output signal from the photodetector131is designated as IPD1, an output signal from the photodetector132is designated as IPD2, an output signal from the photodetector133is designated as IPD3, and an output signal from the photodetector134is designated as IPD4, the differential values are represented by Mathematical Formulas (18) and (19) as shown below. These differential values are independent signals of the land and the groove.

In the reference light servo, a servo signal for controlling the position of the mirror for formation of the reference light is obtained from the value of the difference between the output signals from the photodetectors135and136as described above. Alternatively, the phase θ may be extracted in the configuration illustrated inFIG. 17so that the reference light servo is performed with the extracted phase θ as a servo signal. In that case, there is no need to provide the half beam splitter112, the mirror117, the wavelength plate124, the beam splitter116, the mirror121, and the photodetectors135and136illustrated inFIG. 17.

According to the playing method in the embodiment of the present disclosure described above, the signals recorded with a narrow pitch under the optical limit value are reproduced by the homodyne detection. The relations among the beam spot of the playing light formed on the recording surface, the lands L, and the groove G are as illustrated inFIG. 3.FIG. 3illustrates an example of the case in which the tracking servo control of the object lens is performed on the grooves G.

In this case, it is understood that the information recorded on the land L includes a mixture of the information recorded on two lands L adjacent to the groove G as a servo target. Consequently, although the lands L and the grooves G may be separately readable, it is difficult to reproduce the recorded signals of the lands L in a proper manner.

Note that in the case where the lands L are subjected to the tracking servo, the information recorded on the grooves G has a similar mixture.

However, the tracking control method makes it possible to suppress mixture of information between the lands L and between the grooves G and read the information recorded on the land L and the information recorded on the groove G at the same time in one spot. Such tracking servo with simultaneous reading may be employed.

The embodiments of the present disclosure have been specifically described so far. However, the present disclosure is not limited to the foregoing embodiments but can be modified in various manners on the basis of the technical idea of the present disclosure. For example, the wavelength of the laser light source may not be 405 nm.

Further, the playing optical system is not limited to the configuration illustrated inFIG. 5but may be a homodyne detection optical system to obtain four kinds of light-receiving signals I to L, for example. The homodyne detection optical system has a Wollaston prism that can generate light with each phase differences of 0, 90, 180, 270 degrees.

In addition, the configurations, methods, processes, shapes, materials, numeric values, and the like of the foregoing embodiments can be combined with one another without deviating from the gist of the present disclosure.

Note that, the present disclosure can be configured as follows:

A playing apparatus including:

an optical system that irradiates a recording medium on which signals are recorded in both a land and a groove with light emitted from a light source to obtain signal light reflecting both recorded signals of the land and the groove, generates reference light from the light emitted from the light source, and generates a set of first signal light and reference light with a phase difference of almost 0 degree from overlapping light of the signal light and the reference light, a set of second signal light and reference light with a phase difference of almost 180 degrees from the same, a set of third signal light and reference light with a phase difference of almost 90 degrees from the same, and a set of fourth signal light and reference light with a phase difference of almost 270 degrees from the same;

a light-receiving unit that receives the set of first signal light and reference light by a first light-receiving element, the set of second signal light and reference light by a second light-receiving element, the set of third signal light and reference light by a third light-receiving element, and the set of fourth signal light and reference light by a fourth light-receiving element;

an arithmetic operation unit that calculates a first differential signal as a difference between a first light-receiving signal obtained by the first light-receiving element and a second light-receiving signal obtained by the second light-receiving element, and calculates a second differential signal as a difference between a third light-receiving signal obtained by the third light-receiving element and a fourth light-receiving signal obtained by the fourth light-receiving element;

a first FIR filter and a second FIR filter to which the first differential signal and the second differential signal are supplied, respectively; and

an equalization error detection unit that is supplied with an addition signal in which output signals from the first and second FIR filters are added up to form an equalization error, wherein

tap coefficients for the first and second FIR filters are controlled to minimize the equalization error.

The playing apparatus according to (1), wherein a phase is determined in accordance with a difference in light path length between the first to fourth signal light and the reference light from the first differential signal and the second differential signal, and the first differential signal and the second differential signal have a component of the phase.

The playing apparatus according to (1) or (2), wherein a phase offset is given in advance to the first differential signal and the second differential signal.

The playing apparatus according to any one of (1) to (3), wherein the phase offset is almost equal to (Ψ=4πnd/λ) (n represents refractive index, d represents a level difference between the land and the groove, and λ represents light wavelength).

The playing apparatus according to any one of (1) to (4), wherein the reference light is generated by reflecting the light emitted from the light source on a mirror.

A playing method including:

irradiating a recording medium on which signals are recorded in both a land and a groove with light emitted from a light source to obtain signal light reflecting both recorded signals of the land and the groove, generating reference light from the light emitted from the light source, and generating by an optical system a set of first signal light and reference light with a phase difference of almost 0 degree from overlapping light of the signal light and the reference light, a set of second signal light and reference light with a phase difference of almost 180 degrees from the same, a set of third signal light and reference light with a phase difference of almost 90 degrees from the same, and a set of fourth signal light and reference light with a phase difference of almost 270 degrees from the same;

receiving the set of first signal light and reference light by a first light-receiving element, the set of second signal light and reference light by a second light-receiving element, the set of third signal light and reference light by a third light-receiving element, and the set of fourth signal light and reference light by a fourth light-receiving element;

calculating a first differential signal as a difference between a first light-receiving signal obtained by the first light-receiving element and a second light-receiving signal obtained by the second light-receiving element, and calculates a second differential signal as a difference between a third light-receiving signal obtained by the third light-receiving element and a fourth light-receiving signal obtained by the fourth light-receiving element;

supplying the first differential signal and the second differential signal to a first FIR filter and a second FIR filter, respectively;

supplying an addition signal in which output signals from the first and second FIR filters are added up to an equalization error detection unit to form an equalization error; and

controlling tap coefficients for the first and second FIR filters to minimize the equalization error.

REFERENCE SIGNS LIST