CA1196101A - Optical disc player - Google Patents
Optical disc playerInfo
- Publication number
- CA1196101A CA1196101A CA000421102A CA421102A CA1196101A CA 1196101 A CA1196101 A CA 1196101A CA 000421102 A CA000421102 A CA 000421102A CA 421102 A CA421102 A CA 421102A CA 1196101 A CA1196101 A CA 1196101A
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- tracks
- stylus
- optical pickup
- disc
- error signal
- Prior art date
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Abstract
OPTICAL DISC PLAYER
ABSTRACT OF THE DISCLOSURE
In an optical disc player for optically reading information recorded in substantially parallel tracks on the surface of a rotated record disc, an optical pick-up device includes an objective lens through which a light beam is to be focused at the surface of the record disc for forming an optical pick-up stylus, and such device is shiftable, as a unit, in a radial direction of the record disc for determining the track to be scanned by the optical pick-up stylus, a focusing servo control is responsive to defocusing of the light beam at the record disc surface for moving the objective lens in the direction to restore correct focus of the light beam at the record disc surface, and a gain control is provided for reducing the gain with which the focusing servo control responds to defocusing when the optical pick-up stylus moves transversely across a plurality of the tracks on the record disc, for example, in the searching or random access mode of operation of the player, whereby to reduce vibratory movements of the objective lens and an undesirably harsh noise that would otherwise result from the defocusing incident to relatively rapid transverse movement of the optical pick-up stylus across a number of the tracks.
ABSTRACT OF THE DISCLOSURE
In an optical disc player for optically reading information recorded in substantially parallel tracks on the surface of a rotated record disc, an optical pick-up device includes an objective lens through which a light beam is to be focused at the surface of the record disc for forming an optical pick-up stylus, and such device is shiftable, as a unit, in a radial direction of the record disc for determining the track to be scanned by the optical pick-up stylus, a focusing servo control is responsive to defocusing of the light beam at the record disc surface for moving the objective lens in the direction to restore correct focus of the light beam at the record disc surface, and a gain control is provided for reducing the gain with which the focusing servo control responds to defocusing when the optical pick-up stylus moves transversely across a plurality of the tracks on the record disc, for example, in the searching or random access mode of operation of the player, whereby to reduce vibratory movements of the objective lens and an undesirably harsh noise that would otherwise result from the defocusing incident to relatively rapid transverse movement of the optical pick-up stylus across a number of the tracks.
Description
(~
~6 BACK~ROUND OF THE INVENTION
Fiel~ o the Invention This invention relates generally to apparatus for optically reading information recorded ln substantially parallel tracks on a record medium surface and, more particularly 7 iS directed to improvements in an optical disc player in which a light beam is directed against a rotated record disc for optieally reading information recorded in tracks thereon.
Description of the Prior Art An optical disc player has been proposed for optically reproducing an information signal, such as, for example, a frequency modula~ed video signal or a pulse ~ode modulated (PCM~ audio signal, recorded in ~he form of numerous small pits arranged in a row in a spiral signal path on a rotated record disc. Such optical disc player employs an optical pick~up device or assel~bly including a laser generating a light beam passed through an objective lens by which the light beam is focused at ~he disc surface to form an optical pick-up stylus for scanning any one of the record tracks d~fined by success-ive turns of the spiral signal path. The objective lens is movable in the direction of its optical axis by an elec~ro-magne~ically applied force in response to defo-cuslngof the light beam at the record disc surface, whereby to provide a focusing servo control for maintaining correct focus of the light beam at the disc surface. The known optical pick-up device further includes an angularly
~6 BACK~ROUND OF THE INVENTION
Fiel~ o the Invention This invention relates generally to apparatus for optically reading information recorded ln substantially parallel tracks on a record medium surface and, more particularly 7 iS directed to improvements in an optical disc player in which a light beam is directed against a rotated record disc for optieally reading information recorded in tracks thereon.
Description of the Prior Art An optical disc player has been proposed for optically reproducing an information signal, such as, for example, a frequency modula~ed video signal or a pulse ~ode modulated (PCM~ audio signal, recorded in ~he form of numerous small pits arranged in a row in a spiral signal path on a rotated record disc. Such optical disc player employs an optical pick~up device or assel~bly including a laser generating a light beam passed through an objective lens by which the light beam is focused at ~he disc surface to form an optical pick-up stylus for scanning any one of the record tracks d~fined by success-ive turns of the spiral signal path. The objective lens is movable in the direction of its optical axis by an elec~ro-magne~ically applied force in response to defo-cuslngof the light beam at the record disc surface, whereby to provide a focusing servo control for maintaining correct focus of the light beam at the disc surface. The known optical pick-up device further includes an angularly
-2- ~ ''7 c--movable mirror interposed in the path of the laser light beam for deflecting the optical pick-up stylus in a direction trans~erse of the tracks by an electro-magnetically applied force in response to detection of a tracking error, thereby to provide a tracking servo control tending to maintain the optical pick-up stylus in correct tracking relation to the track being scanned thereby.
The known optical disc playPr is further provided with a mechanism for shiftin~ the optical pick-up device, as a unit, in the direction of the radius of the record disc. In the normal reproducin~ mode, the shifting mechanism causes continuous gradual shifting of the optical pick-up device in the radial direction o the record disc so that the optical pick-up stylus will continuously scan the successive tracks or turns of the spiral pa~h in which the informat'ion is recorded on the record disc as the latter is rotated at a predetermined constant linear velocity or at a constant angular velociLy. Further, in 2 searching or random access mode of operation of the optical disc player, the shif~ing mechanism is suitably controlled to rapidly shift the optical pick-up device in the radial direction of thP-record disc for rapidly moving the optical pick-up stylus to a designated or selected radial position on the disc. During such operation of ~he shifting mechanism in the searching or random access mode, it will be apparent that the optical pick-up stylus moves transversely across a plurality of the successive record tracks on the disc at a relatively high speed.
Operation of the above described known optical disc pl~yer in the searching or random access mode results in an undesirably harsh noise p~oduced by vibratory move-ment of a portion of the struc~urQ supporting the objectlve lens for the focusing servo control. More particularly, when ~he optical pick-up s~ylus is moved ~apidly across a plurality of the record tracks, as during a searching or random access operation of the optical disc player, the focusing servo control responds to sensed rapid variations in the focus of the laser light beam and thereby causes a vibrational movement of ~he objective lens and of its supporting structure to produce ~he undesirably harsh noise.
OBJECTS A~D SUMMARY OF TH~ INVE.NTION
Accordingly, it is an object of ~his invention to provide an improved apparatus for optically reading information recorded in substantially parallel tracks on a record medium surface, and which avoids the above-mentioned problem encountered with the prior art.
More specifi~ally, it is an object of this invention to provide an improved optical disc player having a focusing servo control and being also capable of operation in a searching or random access mode, and which avoids the generation of undeslrable noise in response ~o the focusing servo control during operat-ion iTI the searching or random access mode.
A further object of the invention is to provid~
an improved optical disc player, as aforesai.d, in which gain of the focllsirl~ servo control is reducecl when the optical pick up stylus moves trans~ersely across a plurallty of the ~ecord tracks, whereby to ensure that undesirable noises do not result from the focusing servo control durlng operation in the searching or random access mode.
In accordance with an aspec~ of this invent;on, in an apparatus for optically reading information recorded in substantially parallel tra~ks on a record medium surface~
for example, in successive turns of a spiral path on a record disc surface, optical p1ck-up means inclu~es objective lens means through which a light beam is to be focused at the record medium surface for forming an optical pick-up stylus and such optical pick-up means is shiftable, as a unit, transverse to the tracks for determining the tracks to be scanned by the optical pick-up stylus, focus-ing control means is responsive to defocusing of thP
light beam at the record medium surface to move the object-ive lens means in the direction for restoring correct focus o~ the light beam at the record medlum surface, and gain control means is provided for reducing ~he gain with which the focusing control means responds ~o the defocusing when the optical pick~up stylus moves transversely across a plurality of the tracks on the record medium suriace.
Preferably, in accordance with ~he invention, the focusing control means provides a focusing error signal correspondin~ to the defocusing of the light beam and being supplied to a focusing coil means associated with the objective lens mea7ls ~or moving the latter ln the direction o~ its optical axis, and the ~ain control means includes variable gain means applying the focusing error signal ~o the focusing coil means and ~etecting means for detecting when the opt:Lc~l pick-up stylus ~oves trans-versely across a plurality of the tracks and providing a corresponding det~cted output ~o the variable gain means hy which the gain of the latter is reduced.
In one embodi~ent of the invention, a repro-duced signal corresponding to information recorded in a track being scanned by the optical pick-up stylus is applied to the detecting means which detects variations in the amplitude of such reproduced signal for providing the previously mentioned detected output when the optical pick-up stylus moves from one ~o another of the tracks in moving transversely across a plurality of such tracks.
In another embodiment of ~he invention, a tracking ~rror signal is generated in correspondence to deviation of the point of impingement of the optîcal pick-up stylus fron a transversel.y centered position on one of the record tracks and, in response to such tracking error signal,.the optical pick-up stylus is deflected relative to the remainder of the optical pick-up means in a direction transverse of the tracks so as to tend to restore the point of impingement to the transversely centered position. In this embodiment of the invention, movement of the optical pick-up stylus transversely across a plurality of the tracks is detected on ~he basis of the level or amplitude of the tracking error signal relative to a predetermined reference voltage or level.
The above, and other objects, Eeatures and advantages of this invention, will be apparent in the following detalled description of preferred embodiments thereon which is to be read in connection with the (~,) ('~`1 accompallying dr~wings :~orming a part hereof, and wherein the same reference nu~erals are employed to identify like parts in the sP~eral ~iews.
BRIEF DESCRIPTION OF THE D~AWINGS
Figs. lA, lB and lC are schematic views illustratin~ optical componen~s o an optical pick-up device for an optical disc player of a type to ~Jhich the present invention may be advantageously applied, and as seen from one side, from below and from the other side, respectively;
Fig. 2 is a schematic view showing the relative positional arrangement of photo-detectors in a light receiving device or transducer assembly included in the optical pick-up aevice of Figs. lA-lC;
Figs. 3A, 3B and 3C are schematic views showing various different focusing conditions, respec~ively, for t~e o~tical pick-up device of Figs. lA, lB and lC;
Figs. 3'A, 3'B and 3'C are schematic views showing patterns of the spots formed by the light beam on the photo-detectors included in ~he transducer assembly of Fig. 2 for the various focusing conditions shown on Figs. 3A, 3B and 3C, respectively;
Figs. 4A, 4B and 4C are schematic views to which reference will be made in explaining the d~tection of tracking errors;
Fig. 5 is a block diagram schematically illustrating a control circuit of an optical disc player according to one embodiment ~ the present lnvention;
() ( Figs. 6A, 6B and 6C are waveform diagrams to which ~efe~ence will be made in explaining the operation of the cont~ol circui.t o~ Fig. 5;
Figo 7 is a block diagram showing a control circuit of an optical disc player according to another embodiment of the present invention; and Fig. 8 is a waveform diagram to which reference will be made in explaining the op~ration of the control circuit of Fig. 70 DESCRIPTION ~F THE PREF~RRED EMBODIMENTS
Referring to the drawings in detail, and initially to Figs. lA, lB and lC, it will be seen that an optical disc player of a type to which this invention may be applied includes an optical pick-up device dispcsed under a rotated optical record disc 11 (Fig. 15~ and being comprised of a laser-l emitting a laser light beam subs~an-tially horizontally through a grating 2 and a spot lens 3 for reflection by a ixed upstanding mirror 4 ~nd then again by a fixed upstanding mirror 5 so a~ to follow a substantially U-shaped path in a horizontal plane. After reflection from mirror 5, the laser ligllt beam passes through a Wollaston prism 6 and a quarter-wave plate 7 for reflection at an upstanding mirror 8 which is suitably mounted for angular displacemen~s about a vertical axis, for example, as indicated by ~he double-headed arrow a on Fig. lB, so as to achieve tracking servo control, as hereinafter described in detail. The light beam reflected from angularly movable mir~or 8 is further reflected by a fixed lnclined mirror 9 so as to be directed upwardly ~36~
through an obj ective lens 10 (Fig. lC) wh;ch is intended to focus the laser light beam at a surface of record disc 11 there~bove and ~hereby e~lp~oy ~he focused light beam as an optical ~ick-up stylus for reading information recor~ed in the record tracks on such surface of disc 11. The laser light beam thus made to impinge against disc 11 is reflected from the surface of ~he latter, and is modula~ed with the informatlon signal recorded in the track being scanned on disc 11. The reflected light beam is returned in a path extending ~hrough objective lens 10 for reflection from fixed mirror 9 and then movable mirror 8 so as to enter quarter-wave plate 7. ~uarter-wave plate 7 and Wollaston prism 6 cooperate to cause the returning or reflected laser light beam to follow a returning path which is separa~e from the path of the incident light beam traveling toward disc 11, and which causes the re~.urn-ing light beam to enter a cylindrical lens 12 (Figs. lA
and lB) after being reflected by mirrors 5 and ~, in that order. The returning laser light beam passing through cylindrical lens 12 is guided to a light receivin~ device or transducer assembly 13 operating as a photo-detector responsive to the returned laser light beam to provide a reproduced information signal corresponding to the infor-mation recorded in that track being scanned on disc 11.
As shown particularly on Fig. 2, light receiving device 13 may desirably comprise four photo-diodes 14a, 14b, 14_ and 14d arranged to form four light receiving quad~
rants which, as hereinafter described in detail, cooperate to provide the reproduced signal correspondin& to the information recorded in the track bein~ scanned, and also f`l ( ;`
to indicate the relation o:~ the focus of the laser light beam ~o the reflecting surface o record disc ll, I.ight receiying device 13 is al~o show~ to include two additional photo-diodes 14e and 14f ar~anged effectively in fr~nt and in back of, respectively, the ~,roupin~ or pattern of photo-diodes 14a - 14d and positioned at opposlte sides of the median of such pattern. As he~einafter described in detail, photo-diode~ 14e and 14f are provided to detect tracking errors, that is, deviations of the point of impingement of ~he focused laser light beam or optical pick-up stylus ~rom a transversely centered position in respect to the record track being scanned on disc 11.
The detection of focusing errors will now be d~scribed with reference ts Figs. 3A, 3B and 3C and Figs. 3'A, 3'B and 3'C. ~ore particularly, when the laser li~ht beam is precisely focused at ~he reflecting surface of optical record disc 11, as indicated on Fig. 3B, the returned laser light beam forms a circular light spot on the photo-diodes 14a 14d assumed ~o be posi~ioned in the plane indicated by a dotted line 15 on Flg. 3B, with the center of such circular light spot being coincident with the intersection of the contiguous edges of the quadrants formed by photo-diodes 14a, 14b, 14c and 14d, as shown on Fig. 3'B. Thus, when objective lens 10 correctly focuses the laser light beam precisely at the reflecting surface of disc 11, all four photo-diodes 14a, 14b, 14c and 14d receive equal amounts of light so that photo-diodes 14a, 14b, 14c and 14d then have output6 S , Sb, S and ~ - - - a c Sd, respectively, of precisely the same level.
-10~
( ( When defocusing occurs by reason of an increase in the distance to the surface of disc 11 from objective lens 10, that is, when the laser light ~eam forming the optical pick-up stylus is ~ocused in front of the surface on disc 11 having the information recorded thereon, as in Fig. 3A, the astigma~ism of cylindrical lens 12 causes the light spot formed by the laser light beam on photo~
diodes 14a - 14d at plane 15 to be elliptical, for example, with the major axis of the elliptical light spot extending in the direction between thc photo-diodes 14b and 14d, as shown on Fig. 3'~. Thus, in the event of the defocusing illustrated on Fig. 3A, the amount of light falling on photo-diodes 14b and 14d is greater than the amount of light falling on photo-diodes 14a and 14c.
Gn the other hand, when defocusing occurs by reason of the disc 11 being too close to objective lens 10 so that the laser light beam forming the optical pick up stylus is focused behind the surface of disc 11 on whic~ the information is recorded, as shown on Fig. 3C, the light spot formed on photo-diodes 14a - 14d is again elliptical but, in this ~ase, the major axis of the elliptical light spot extends in the direction between photo-diodes 14a and 14c, as sho~ on Fig. 3'C. Thus, in the case of the defocusing condition illustrated on Fig. 3C, ~he amount of light applied to photo-diodes 14a and 14c is greater than the amount of light applied to photo-diodes 14b and 14_.
As shown diagrammatically on Fig. 2, the outputs Sa, S~, Sc and Sd of photo-diodes 14a, 14b, 14c and 14d, respectiYely, are supplied to an operational !
circuit 50 which performs operations expressed by the following e~uations:
(Sa ~ Sc) - (Sb -t Sd~ = SF
Sa + Sb ~ Sc ~ Sd ~ SR ~ (2).
As a result of the operation indicated in equation (1~, circuit 50 provides a focusing error signal SF which is zero when the laser light beam is focused precisely at the surface of record disc 11, and which has positive and nega~ive polarities when the defocu~ed condi~ion of ~he laser light beam results from the surface of disc 11 being too close to objective l~ns 10, as on Fig. 3G, or too far from objective lens 10, as on Fig. 3A, respectively. As hereinafter described in detail, the focusing error signal SF is employed for effecting movements of objective lens 10 in the direction of its optical axis so as to achieve focusing servo control, that is, to maintain the focus of the laser light beam precisely at the surface of disc 11 on which the information signal is recorded.
It will be appreciated that the reproduced information signal SR is modulated by the information signal recorded on the surface of disc 11, bu~ is essentially unchanged by the focused cond~tion of the laser light beam.
For the detection of tracking errors in the optical disc player provided with the optical pick-up device of Figs. lA and lB, grating 2 produces, from the laser light beam issuing from laser 1, two auxiliary li~ht beams in addition to the rnain li~,ht beam forming ~he optical pick-up stylus. The main and auxil-lary light beams are directed parallel to e~ch other both on the w3y to the surface of disc 11 and on the return of the reflected light beams to light recieving device or transducer assembly 13. lhus ! as shown on Figs. 4A, 4B and 4C, the auxîliary ligh~ beams form auxil.iary light spots 15b and 16c on the surface of disc 11 at positlons which a~e in advance and behind, respectively, considered in the direction along a record track 17, the position of a main light spot 1-6a formed on the surface of disc ll by the main light beam Dr optical pick-up stylus. ~urther, as shown, auxiliary light spots 16b and 16e deviate slightly, in opposite directions, from a line passing through the center of main light spot 16a parallel to the direction along record track 17. Accordingly, the positional rela~ion-ships of the two auxiliary light spots 16b and 16c to signal track 17 vary in accordance with the tracking condition of the main light beam or optical pick-up stylus relative to signal track 17.
21ore particularly, in the correc~ ~racking condition shown on Fig. 4A, and in which main light spot 16a is positioned precisely on signal track 17, equal fractional amounts of both auxiliary light spots 16b and 16c extend onto the opposite~longitudinal edge portions of signal track 17, for example, one-half of auxiliary light spot 16b traces one edge portion of track 17 and one-half of auxiliary light spot 16c traces the opposite edge portion of track 17. When a tracking error occurs, for example, when main ].ight spot 16a departs or deviates toward the left from the center of track l7, the full area of auxiliary light spot 16b scans track 17, while the vther auxiliary light spot 16c moves completely off
The known optical disc playPr is further provided with a mechanism for shiftin~ the optical pick-up device, as a unit, in the direction of the radius of the record disc. In the normal reproducin~ mode, the shifting mechanism causes continuous gradual shifting of the optical pick-up device in the radial direction o the record disc so that the optical pick-up stylus will continuously scan the successive tracks or turns of the spiral pa~h in which the informat'ion is recorded on the record disc as the latter is rotated at a predetermined constant linear velocity or at a constant angular velociLy. Further, in 2 searching or random access mode of operation of the optical disc player, the shif~ing mechanism is suitably controlled to rapidly shift the optical pick-up device in the radial direction of thP-record disc for rapidly moving the optical pick-up stylus to a designated or selected radial position on the disc. During such operation of ~he shifting mechanism in the searching or random access mode, it will be apparent that the optical pick-up stylus moves transversely across a plurality of the successive record tracks on the disc at a relatively high speed.
Operation of the above described known optical disc pl~yer in the searching or random access mode results in an undesirably harsh noise p~oduced by vibratory move-ment of a portion of the struc~urQ supporting the objectlve lens for the focusing servo control. More particularly, when ~he optical pick-up s~ylus is moved ~apidly across a plurality of the record tracks, as during a searching or random access operation of the optical disc player, the focusing servo control responds to sensed rapid variations in the focus of the laser light beam and thereby causes a vibrational movement of ~he objective lens and of its supporting structure to produce ~he undesirably harsh noise.
OBJECTS A~D SUMMARY OF TH~ INVE.NTION
Accordingly, it is an object of ~his invention to provide an improved apparatus for optically reading information recorded in substantially parallel tracks on a record medium surface, and which avoids the above-mentioned problem encountered with the prior art.
More specifi~ally, it is an object of this invention to provide an improved optical disc player having a focusing servo control and being also capable of operation in a searching or random access mode, and which avoids the generation of undeslrable noise in response ~o the focusing servo control during operat-ion iTI the searching or random access mode.
A further object of the invention is to provid~
an improved optical disc player, as aforesai.d, in which gain of the focllsirl~ servo control is reducecl when the optical pick up stylus moves trans~ersely across a plurallty of the ~ecord tracks, whereby to ensure that undesirable noises do not result from the focusing servo control durlng operation in the searching or random access mode.
In accordance with an aspec~ of this invent;on, in an apparatus for optically reading information recorded in substantially parallel tra~ks on a record medium surface~
for example, in successive turns of a spiral path on a record disc surface, optical p1ck-up means inclu~es objective lens means through which a light beam is to be focused at the record medium surface for forming an optical pick-up stylus and such optical pick-up means is shiftable, as a unit, transverse to the tracks for determining the tracks to be scanned by the optical pick-up stylus, focus-ing control means is responsive to defocusing of thP
light beam at the record medium surface to move the object-ive lens means in the direction for restoring correct focus o~ the light beam at the record medlum surface, and gain control means is provided for reducing ~he gain with which the focusing control means responds ~o the defocusing when the optical pick~up stylus moves transversely across a plurality of the tracks on the record medium suriace.
Preferably, in accordance with ~he invention, the focusing control means provides a focusing error signal correspondin~ to the defocusing of the light beam and being supplied to a focusing coil means associated with the objective lens mea7ls ~or moving the latter ln the direction o~ its optical axis, and the ~ain control means includes variable gain means applying the focusing error signal ~o the focusing coil means and ~etecting means for detecting when the opt:Lc~l pick-up stylus ~oves trans-versely across a plurality of the tracks and providing a corresponding det~cted output ~o the variable gain means hy which the gain of the latter is reduced.
In one embodi~ent of the invention, a repro-duced signal corresponding to information recorded in a track being scanned by the optical pick-up stylus is applied to the detecting means which detects variations in the amplitude of such reproduced signal for providing the previously mentioned detected output when the optical pick-up stylus moves from one ~o another of the tracks in moving transversely across a plurality of such tracks.
In another embodiment of ~he invention, a tracking ~rror signal is generated in correspondence to deviation of the point of impingement of the optîcal pick-up stylus fron a transversel.y centered position on one of the record tracks and, in response to such tracking error signal,.the optical pick-up stylus is deflected relative to the remainder of the optical pick-up means in a direction transverse of the tracks so as to tend to restore the point of impingement to the transversely centered position. In this embodiment of the invention, movement of the optical pick-up stylus transversely across a plurality of the tracks is detected on ~he basis of the level or amplitude of the tracking error signal relative to a predetermined reference voltage or level.
The above, and other objects, Eeatures and advantages of this invention, will be apparent in the following detalled description of preferred embodiments thereon which is to be read in connection with the (~,) ('~`1 accompallying dr~wings :~orming a part hereof, and wherein the same reference nu~erals are employed to identify like parts in the sP~eral ~iews.
BRIEF DESCRIPTION OF THE D~AWINGS
Figs. lA, lB and lC are schematic views illustratin~ optical componen~s o an optical pick-up device for an optical disc player of a type to ~Jhich the present invention may be advantageously applied, and as seen from one side, from below and from the other side, respectively;
Fig. 2 is a schematic view showing the relative positional arrangement of photo-detectors in a light receiving device or transducer assembly included in the optical pick-up aevice of Figs. lA-lC;
Figs. 3A, 3B and 3C are schematic views showing various different focusing conditions, respec~ively, for t~e o~tical pick-up device of Figs. lA, lB and lC;
Figs. 3'A, 3'B and 3'C are schematic views showing patterns of the spots formed by the light beam on the photo-detectors included in ~he transducer assembly of Fig. 2 for the various focusing conditions shown on Figs. 3A, 3B and 3C, respectively;
Figs. 4A, 4B and 4C are schematic views to which reference will be made in explaining the d~tection of tracking errors;
Fig. 5 is a block diagram schematically illustrating a control circuit of an optical disc player according to one embodiment ~ the present lnvention;
() ( Figs. 6A, 6B and 6C are waveform diagrams to which ~efe~ence will be made in explaining the operation of the cont~ol circui.t o~ Fig. 5;
Figo 7 is a block diagram showing a control circuit of an optical disc player according to another embodiment of the present invention; and Fig. 8 is a waveform diagram to which reference will be made in explaining the op~ration of the control circuit of Fig. 70 DESCRIPTION ~F THE PREF~RRED EMBODIMENTS
Referring to the drawings in detail, and initially to Figs. lA, lB and lC, it will be seen that an optical disc player of a type to which this invention may be applied includes an optical pick-up device dispcsed under a rotated optical record disc 11 (Fig. 15~ and being comprised of a laser-l emitting a laser light beam subs~an-tially horizontally through a grating 2 and a spot lens 3 for reflection by a ixed upstanding mirror 4 ~nd then again by a fixed upstanding mirror 5 so a~ to follow a substantially U-shaped path in a horizontal plane. After reflection from mirror 5, the laser ligllt beam passes through a Wollaston prism 6 and a quarter-wave plate 7 for reflection at an upstanding mirror 8 which is suitably mounted for angular displacemen~s about a vertical axis, for example, as indicated by ~he double-headed arrow a on Fig. lB, so as to achieve tracking servo control, as hereinafter described in detail. The light beam reflected from angularly movable mir~or 8 is further reflected by a fixed lnclined mirror 9 so as to be directed upwardly ~36~
through an obj ective lens 10 (Fig. lC) wh;ch is intended to focus the laser light beam at a surface of record disc 11 there~bove and ~hereby e~lp~oy ~he focused light beam as an optical ~ick-up stylus for reading information recor~ed in the record tracks on such surface of disc 11. The laser light beam thus made to impinge against disc 11 is reflected from the surface of ~he latter, and is modula~ed with the informatlon signal recorded in the track being scanned on disc 11. The reflected light beam is returned in a path extending ~hrough objective lens 10 for reflection from fixed mirror 9 and then movable mirror 8 so as to enter quarter-wave plate 7. ~uarter-wave plate 7 and Wollaston prism 6 cooperate to cause the returning or reflected laser light beam to follow a returning path which is separa~e from the path of the incident light beam traveling toward disc 11, and which causes the re~.urn-ing light beam to enter a cylindrical lens 12 (Figs. lA
and lB) after being reflected by mirrors 5 and ~, in that order. The returning laser light beam passing through cylindrical lens 12 is guided to a light receivin~ device or transducer assembly 13 operating as a photo-detector responsive to the returned laser light beam to provide a reproduced information signal corresponding to the infor-mation recorded in that track being scanned on disc 11.
As shown particularly on Fig. 2, light receiving device 13 may desirably comprise four photo-diodes 14a, 14b, 14_ and 14d arranged to form four light receiving quad~
rants which, as hereinafter described in detail, cooperate to provide the reproduced signal correspondin& to the information recorded in the track bein~ scanned, and also f`l ( ;`
to indicate the relation o:~ the focus of the laser light beam ~o the reflecting surface o record disc ll, I.ight receiying device 13 is al~o show~ to include two additional photo-diodes 14e and 14f ar~anged effectively in fr~nt and in back of, respectively, the ~,roupin~ or pattern of photo-diodes 14a - 14d and positioned at opposlte sides of the median of such pattern. As he~einafter described in detail, photo-diode~ 14e and 14f are provided to detect tracking errors, that is, deviations of the point of impingement of ~he focused laser light beam or optical pick-up stylus ~rom a transversely centered position in respect to the record track being scanned on disc 11.
The detection of focusing errors will now be d~scribed with reference ts Figs. 3A, 3B and 3C and Figs. 3'A, 3'B and 3'C. ~ore particularly, when the laser li~ht beam is precisely focused at ~he reflecting surface of optical record disc 11, as indicated on Fig. 3B, the returned laser light beam forms a circular light spot on the photo-diodes 14a 14d assumed ~o be posi~ioned in the plane indicated by a dotted line 15 on Flg. 3B, with the center of such circular light spot being coincident with the intersection of the contiguous edges of the quadrants formed by photo-diodes 14a, 14b, 14c and 14d, as shown on Fig. 3'B. Thus, when objective lens 10 correctly focuses the laser light beam precisely at the reflecting surface of disc 11, all four photo-diodes 14a, 14b, 14c and 14d receive equal amounts of light so that photo-diodes 14a, 14b, 14c and 14d then have output6 S , Sb, S and ~ - - - a c Sd, respectively, of precisely the same level.
-10~
( ( When defocusing occurs by reason of an increase in the distance to the surface of disc 11 from objective lens 10, that is, when the laser light ~eam forming the optical pick-up stylus is ~ocused in front of the surface on disc 11 having the information recorded thereon, as in Fig. 3A, the astigma~ism of cylindrical lens 12 causes the light spot formed by the laser light beam on photo~
diodes 14a - 14d at plane 15 to be elliptical, for example, with the major axis of the elliptical light spot extending in the direction between thc photo-diodes 14b and 14d, as shown on Fig. 3'~. Thus, in the event of the defocusing illustrated on Fig. 3A, the amount of light falling on photo-diodes 14b and 14d is greater than the amount of light falling on photo-diodes 14a and 14c.
Gn the other hand, when defocusing occurs by reason of the disc 11 being too close to objective lens 10 so that the laser light beam forming the optical pick up stylus is focused behind the surface of disc 11 on whic~ the information is recorded, as shown on Fig. 3C, the light spot formed on photo-diodes 14a - 14d is again elliptical but, in this ~ase, the major axis of the elliptical light spot extends in the direction between photo-diodes 14a and 14c, as sho~ on Fig. 3'C. Thus, in the case of the defocusing condition illustrated on Fig. 3C, ~he amount of light applied to photo-diodes 14a and 14c is greater than the amount of light applied to photo-diodes 14b and 14_.
As shown diagrammatically on Fig. 2, the outputs Sa, S~, Sc and Sd of photo-diodes 14a, 14b, 14c and 14d, respectiYely, are supplied to an operational !
circuit 50 which performs operations expressed by the following e~uations:
(Sa ~ Sc) - (Sb -t Sd~ = SF
Sa + Sb ~ Sc ~ Sd ~ SR ~ (2).
As a result of the operation indicated in equation (1~, circuit 50 provides a focusing error signal SF which is zero when the laser light beam is focused precisely at the surface of record disc 11, and which has positive and nega~ive polarities when the defocu~ed condi~ion of ~he laser light beam results from the surface of disc 11 being too close to objective l~ns 10, as on Fig. 3G, or too far from objective lens 10, as on Fig. 3A, respectively. As hereinafter described in detail, the focusing error signal SF is employed for effecting movements of objective lens 10 in the direction of its optical axis so as to achieve focusing servo control, that is, to maintain the focus of the laser light beam precisely at the surface of disc 11 on which the information signal is recorded.
It will be appreciated that the reproduced information signal SR is modulated by the information signal recorded on the surface of disc 11, bu~ is essentially unchanged by the focused cond~tion of the laser light beam.
For the detection of tracking errors in the optical disc player provided with the optical pick-up device of Figs. lA and lB, grating 2 produces, from the laser light beam issuing from laser 1, two auxiliary li~ht beams in addition to the rnain li~,ht beam forming ~he optical pick-up stylus. The main and auxil-lary light beams are directed parallel to e~ch other both on the w3y to the surface of disc 11 and on the return of the reflected light beams to light recieving device or transducer assembly 13. lhus ! as shown on Figs. 4A, 4B and 4C, the auxîliary ligh~ beams form auxil.iary light spots 15b and 16c on the surface of disc 11 at positlons which a~e in advance and behind, respectively, considered in the direction along a record track 17, the position of a main light spot 1-6a formed on the surface of disc ll by the main light beam Dr optical pick-up stylus. ~urther, as shown, auxiliary light spots 16b and 16e deviate slightly, in opposite directions, from a line passing through the center of main light spot 16a parallel to the direction along record track 17. Accordingly, the positional rela~ion-ships of the two auxiliary light spots 16b and 16c to signal track 17 vary in accordance with the tracking condition of the main light beam or optical pick-up stylus relative to signal track 17.
21ore particularly, in the correc~ ~racking condition shown on Fig. 4A, and in which main light spot 16a is positioned precisely on signal track 17, equal fractional amounts of both auxiliary light spots 16b and 16c extend onto the opposite~longitudinal edge portions of signal track 17, for example, one-half of auxiliary light spot 16b traces one edge portion of track 17 and one-half of auxiliary light spot 16c traces the opposite edge portion of track 17. When a tracking error occurs, for example, when main ].ight spot 16a departs or deviates toward the left from the center of track l7, the full area of auxiliary light spot 16b scans track 17, while the vther auxiliary light spot 16c moves completely off
3~
track 17, as shown on ~lg. 4B. Gonver5ely, when the tracking er~or causes ~ain light spo~ 16a to deviate toward the right from the cent~r of record track 17, ~hen the full area of auxiliary light spot 16c traces or scans track 17, while auxiliary li.ght spot 16b moves completely off record track 17, as shown on Fig. 4C.
The auxlliary light beams forming auxiliary light spots 16b and 16c, after being reflected by disc 11, are guided to photo-diodes 14e and 14f, respectively, of light receiving device or transducer assembly 13. The corresponding outputs Se and Sf from photo-diodes 14e and 14_, respectively, are supplied to an operational circui~
51 (Fig. 2) which performs an operation expressed by the below equation:
Se - S = ST __ --(3).
The resulting tracking error signal ST obtained rom operational circuit 51 is zero when correct tracking is achieved, as on Fig. 4A. However, in the e~ent of a tracking error, as shown on Fig. 4B or 4C, tracking error signal ST has a level and polarity corresponding to the amount and direction of the tracking error. As hereinafter further described, tracking error signal ST is employed or angularly moving mirror 8 so as to achieve tracking servo control, that is, to maintain main light spot 16a substantially centered in respect to the ~rack 17 being scanned.
~ eferring now to Fig. 5, it will be seen that, in an optlcal disc player according ~o one embodiment of the p~esent inventian and which employs an optical pick-up device as described above with reference ~o Figs. lA-lC, and 2, the focusing e~ror signal SF from operational circuit 50 (Fig. 2~ is supplied ~o an input terminal 18 and therefrom through a phase compensatin~ circuit 19, a switching circuit 20 (inthe condition shown on Fig. 5) and a variable gain amplifier 21 to a coil 22 which ~orms part of a linear motor provided for moving objective lens 10 in the direction of its optical axis for achieving the previously descri.bed focusing servo control.
Switching circuit 20 is adapted ~o be changed-over from the condition shown on Fig. 5 to a condition in which an output signal o a stand-by circuit 23 is supplied through switching circuit 20 and vari.able gain amplifier 21 to coil 22 in ~lace of ~ocusing error slgnal SF. The foregsing arrangement is provided because ~he maximum range or peak-~o-peak ~alue of focusing error signal SF actually obtainable from light receiving device 13 corresponds to an axial movemen~ of objective lens 10 of only several tens of microns. On the other h~nd, variations in the vertical position of the surface of rotated disc 11, for example, due to deformation of the latter, and hence the extent of defocusing encountered during actual reproducing, may be as much as ~ 1.0 milli-meter, from which it follows that such defocuslng cannot be corrected by the focusing error signal S~,. Therefore, stand-by circuit 23 is provided to reduce the e~tent o the de~ocusing to within a range capable of being ~`?
corrected by the ~ocusing servo control, whereupon the latter becomes operative to precisely maint~in the correot focus. In order to achieve the foregoing operation, focusing error signal SF is also supplied rom anput terminal 18 to an ON/OF~ control circuit 24 which xesponds to the level of the focusing error signal SF or providi~g a switchîng signal by which switching circuit 20 is controlled. More particularly, so long as the level of focusing error signal S~ exceeds a pr~determined ~alue of either polarity corresponding to the range of defocusing that can be corrected by ~he focusing servo control, ~he resulting switching signal from circuit 24 causes change-over o switching circuit 20 from the position shown on Fig. S. Upon such change-ov~r of switching circuit 20, the output of stand-by circuit 23 is applied ~hrough switching circuit 20 and gain con~rol amplifier 21 with the result that a corresponding current supplied to coil 22 is e~fec~ive to move objective lens 10 to a position wi-~hin the effective range of the focusing servo control. When objecti.ve lens 10 is thus moved to a position within ~he effective range of the focusing servo con~rol, the corresponding reduction o~ the value or level of focusing error signal SF causes ON/OFF control circuit 24 to return switching circuit 20 to the condition sho~
on Fig. 5 for disconnectin~ stand-by circuit 23 and causing the focusing servo control to become operative, that is, to determine the amplitude and direction of the current supply to coil 22 in dependence on the level and polarity of focusing error signal SF.
~6~
In the case of an optical disc player 'having an optical pick-up device of the type described above with reference to Figs. lA, lB and lC, it is conventional to provide, in addition to the angular movement of mirror 8 in response to tracking error signal ST for efecting tracking servo control, a mechanism :~or shifting the optical pick-up device, as a unit, in the direction of the radius of the record disc, In the normal reproducing or playback mode using an op~ical r~cord disc having the information signal recorded ~hereon in a spiral path, the shifting ~echanism causes continuous gradual shif~ing or movement of the optical pick-up device in the radial dirertion of the record di.sc so that the optical pick-up stylus will substantially follow the successive tracks or turns of the spiral path on the rotated record disc, and the tracking servo control is employed for correcting those tracking errors resulting from eccentrici~ies in the rotation of the record disc. On the other hand, in a searching or random access ~ode of operation of the optical disc player, the shifting mechanism is suitably controlled to rapidly shift the optical pick-up device, as a unit, in the radial direction of the record disc for rapidly moving the optical pick-up stylus to a designated or selected radial position on the disc.
During such operation o the shi:fting mechanism in the searching or rando~ access mode~ the optical pick-up stylus undergoes periods of rapid transverse movement across large nurnbers of the record tracks on the disc, and the focusing servo control responds to resulting sensed rapid focus variations or defocusing o~ ~,he laser light bearn and thereby causes yibra~iona3. movemen~ of objective lens 10 and o its suppor~ing structure to produce an undesirable harsh noise.
~ enerally, in a~cordance with the present inventionJ the foregoi.ng problem is avoided by reducing the gain with which the focusing servo control responds to defocusin~ in any period in which the optical pick-up stylus undergoes rapid transverse movement across a large number of the tracks on the record medium surface.
In other words, in accordance with this invention, the gain of vari.able gain amplifier 21 (Fig. 5) is reduced when it is detected that thc optical pick-up stylus is moving transversely across a plurality o the record tracks on record disc 11.
In the embodiment of the invention jllustrated on Fig. 5, the movem~n~ o~ the optical pic~-up stylus transversely across a plurality of the tracks on record disc 11 is detected on the basis of variations occurring in the amplitude of the reproduced informa~ion signal S~
from operational circui~ 50 occurring when ~he optical pick-up stylus moves from one to another of the tracks in moving transversely thereacross. More particularly, in the embodiment of Fig. 5, th~ reproduced information signal SR, which is a relatively high frPquency or RF
signal so long as the op~ical pick-up stylus is cont.in-uously scanning along the spiral path on record disc ll, is supplied through an input terminal 25 and a low pass filter 26 to a voltage com~arator 27 and a ~C voltage maintaining circuit 28, Voltage cvmparator 27 compares the voltag~ of the output of low pass ilter 26 with an adjusted D~ reference volta~e ~ro~ circui~ 28, ~nd the result of such comparison is used for controlling the gain of variable gain amplifier 21. ~hus, the a~plification of the focusing error signal s~ in ampllfier 21 ~or application to focusing cQil 22 is controlled.
The operation of the above d~scribed arrange ment embodying this i.nvention will now be described with reference to Fig5. 6A, 6B and 6C~ In the period when the optical pick-up stylus formed by the laser light beam focused at the surface of disc ll by objective lens lO
is tracing or scanning a track on disc 11 and in which, for example a frequency modulated signal ha~ing a carrier frequency of 8 MHz is rPcorded, the reproduced informa~ion signal SR similarly has a high carrier frequ~ncy wi~h an env~lope of almost constant level. However, wheil~the optical pick-up device is shifted radially of the disc in a searchin~ or random access mode of operation so that the optical pick-up stylus moves transversely across a substantial number o the tracks or turns of the spiral path, the amplitude of signal SR varies substantially dependin~ on whether ~he optical pick-up stylus is i~pinging on a track or on a land or portion of the record disc surface between two adjacent tracks where no signal is recorded, whereby to provide the signal SR with a component having a frequency lower than the carrier frequency, as indicated on Fig. 6A. Therefore, at the output of low pass filter 26 there will be obtained the relatively low fre~uency signal component, as shvwn on Fig. 6Bj and which occurs in each period when the optical pic~-up s tylus is rapidly ~noved transversely across a substantial or large number of the tracks on record disc 11, The level of such output of low pass Pilter 26 is co~parecl in ~oltage comparato~ 27 wi~h the reference ~ voltage from circuit 28 and the resulting c~mparison output is applied to a suitable time constant circuit within comparator 27 so that a rectangular pu1~2 si~nal (Fig. 6C) is applied to amplifier 21 for reduclng the gain of the latt~r, for example, by 20 dB, so long ~s the optical pick-up stylus is moving transversely acrDss signal tracks on disc 11.
Since the DC component of the signal S~ may have sligh~ly different levels from one disc to the next, the DC voltage maintaining cîrcuit 28 de~ec~s the DC component of signal SR and suitably adjusts the level of the reference DC
voltage which is supplied from circuit 28 to volta~e comparator 27 for comparison in the la~ter with ~he outpu~ of low pass filter 26.
It will be appreciated from the foregoing that, by reason of the described reduetion in the gain of amplifier 21 whenever the optical pick-up stylus is moving transversely across a substantial number of the signal tracks on disc 11, the current flowing through coil 22 for moving objective lens 10 in effecting the focusing servo control is sufficiently reduced so as to restrict the movements of objective lens 10. Therefore, the previously described undesirably harsh noise is not produced durin~ the period when the optical pick-up stylus is made to Move transversely across a substantlal number of the signal tracks, as during a searchin~, or random access operation of the optical disc player, and ~he focusing s~rvo control i5 otherwise effective to ensure ' ~3~
~he main~enance of the co~rec~ focus of the laser light bea~ or optical pick-up stylus at the surface of rotated disc 11.
Referring now to Fig. 7, it will be seen that, in an optical disc player accordin~ to another emhodiment o~f the present invention, each period during whlch the optical pick-up stylus is moving transversely across a substantial number of the signal tracks on disc 11, and during which the gain of amplifier 21 is reduced, is detected with reference to the tracking error signal ST.
More particularly, in the e~bodiment of Fig. 7, the tracking error signal ST, for exampleS obtained from the operational circuit 51 on Fig. 2, is supplied through an input terminal 29 and a variable resistor 30 acting as ~ level controller, to a phase compensa~ing circui~ 31, and from the latter through a switching circuit 32 and an amplifier 34 to a coil 35 for driving the angularly movable mirxor 8 and thereby effecting the ~racking servo control. In the normal reproducing mode of the optical disc player, the amplitude of tracking error signal ST
~aries periodically in response to tracking errors caused by eccentric rotation of disc 11, and mirror 8 is angularly moved in accordance with the level and polarity of the current flowing through coil 35 in response to tracking error signal STJ and thus tends to eliminate the tracking error.
In Fig. 7, the reference numeral 16 indicates a coil of a conventional ~otor mechanisIa which i.s provided for shiftin~ the optical pick up device, as a unit, in the direction of the radius of disc 11 in response to a signal ~21-supplied ~hereto by way of a swi~ch circuit 37, a low pass filter 38 and an amplifier 39, More par~icularly, switch circui~ 37 is shown to h~ve a movable contact selectiyely engageable with a first fixed contac~ 37a connected with the output of arnpliier 34, a~d second and third fixed con~acts 37b and 37c connected with positive and negative voltage sources 40a and 40b, respect-ively. Switeh circuit 37 is actuable to select for application to coil 36 either tracking error signal ST from ~he output of amplifier 34, or the positive voltage or negative voltage from source 4Da or 40b, respectively, Eor effec~ing a searching or random access operation.
Further, in the embodiment of Fig. 7, a pulse generator 41 is ~ade operative to produce a pulse in response to an output signal from a voltage comparator 42 which compares ~he level of tracking error signal ST from level controller ~0 with the level oE a reference voltage from a suitable source 43 thereo. Such pulse from generator 41 is applied to variable gain arnplifier 21 for reducing the gain of the la~ter, and also to switch circuit 32 for opening the latter.
The control circuit of Fig. 7 operates as ~ollows:
In the normal reproducing mode of ~he optical disc player, switch 37 is in the position shown on Fig. 7 and the tracking servo control is effective, in response to the tracking error signal ST, to tend to reduce the latter to zero, that i8, to cause the optical pick up stylus to accurately and continuously scan the successive tracks or turns of the spiral path in which the inforrnation -2~-(`` (~\
signal is recorded on disc 11. I~ will be appreciated that the dis~ance thr~gh which the optical pick-up stylus can be displaced in the radial direction on disc 11 by angular movement o~ mirror 8 is limited~ for example, to no more than 300 microns. Accordingly, in the normal reproducing mode in which switch ci-rcuit 37 has its movable oontac~ engaged with fixed contact 37a, and s~ long as switching circuit 32 is in its closed s~ate, as shown on Fig. 7, tracking error signal ST is also supplied to low pass filter 38 so that the low frequency component of signal ST is further supplied through amplifier 39 to coil 36. Thus, the motor mechanism associated with coil 36 is effective to shit the optical pick-up device gradually in the direction of the radius of the disc 11 in accordance witll the low frequency com-ponent of tracking error signal ST for accommodating the spiral configuration of the path in which the information signal is recorded on disc 11, while th~ supplying of tracking error signal ST to coil 35 effects the tracking servo control by which small tracking errors, for example, due to eccentric rotation of disc 11, are eliminated.
~ hen a searching or random access operation is desired, switch circuit 37 is actuated to en~a~e either its fixed contact 37b or fixed contact 37c and thereby apply a positive or a negative voltage from source 40a or 40b through low pass filter 38 and a~plifier 39 to coil 36, whereby the optical pi.ck-up device is shifted, as a unit, at relatively high speed in the radial direction ~oward or away from the center of disc 11. During such relatively high speed shifting of th2 optical pick-up device, ~he ~3~
tracking servo control tries to operate in the same manner as in the normal reproducing ~od~. Thus, as the optical pick-up device is being shif~ed, ~s a unit, the resul~ing tracking error signal 5T is applied to coil 35 ~or angu-larly moving mirror 8 and thereby causing the optical pick-up stylus to repeatedly trace a signal track on disc 11 until the m~ximllm range of angular movement of mirror 8 has been attained, whereupon the optical pic~-up stylus jumps across a large number of ~he tracks, for example, several hundred tracks or successive turns of the spiral path on disc 11, and then be~ins to repeate.dly trace or scan another of the signal tracks. As a result of the foregoing, in the searching mode of operation of the optical disc player, the signal recorded in the successive tracks or turns of the spiral path on disc 11 is inter-mittently reproduced at int~rvals of several hundred tracks.
Referring now to ~ig. 8, it will be seen that, in the above described searching ~ode of operation~ the average level of the tracking error signal ST incr~ases gradually in the period nT during which the disc 11 under-goes hundreds of revolutions, and further during which mirror 8 is progressively displaced angularly by increasin~
amounts so as to permit the repeated scanning of one track or turn. The variations in the level of tracking error signal ST appearing within the period T which corresponds to a single revolution of disc 11 represent tracking errors due to eccentric rotation of dis~ 11.
In Fig. 8, such variations of tracking error signal ST
due to eccentric rotation have been shown for only a few revolutions of the disc 11, but it will be apparent ~hat ( l ( a similar va~iation wou~d occu~ for each revolution durin~
the period nT. The trackin~ error signal ST shown on Fig. 8 is supplied to voltage comparator 42 for co~parison therein with the reference ~oltage Vt from source 43.
~en the level of tracking error signal ST exceeds ~efer ence voltage Vt in the course of a searching operation, the resulti.ng comparison Olltput from comparator 42 causes generator 41 to provide a pulse to switch circuit 32 for opening the latter and to variable gain amplifier 21 for reducing the gain of such amplifier through which the focusing error signal is supplied to coil 22, The opening of switch circui~ 32 by the pulse from generator 41 cuts off the tracking servo control, that is, interrupts the circuit through which the tracking error signal ST
is supplied to coil 35 with the result that mirror 80 rapidly returns to its neutral position and causes the optical pick-up stylus to move transversely across 10Q
to 200 signal tracks on disc 11 in a very short period, for example, in 10 ~o 20 milliseconds, during which the tracking error signal ST tha~ is obtained from operational circuit 51 is merely a noise component. At the conclusion of the pulse from generator 41, switch circuit 32 again closes, with the result that the tracking servo control is initiated once more for causing the op~ical pick-up stylus to repeatedly trace one o~ the ~racks while the optical pick~up device is further rapidly moved in the radial direction of disc 11 in a con~inuation of the searchin~ operation.
As noted above, the pulse from generator 41 is effective to reduce the gain of variable gain .. . .
amplifier 21 durin~ t~e period in which optical pick-up stylus moves transversely across a large number of tracks at hi~h speed. Thus, in an optical disc player according to the embodi~ent of Fig. 7 ~ as well as according to the embodiment of Fig. 5, the control g~in of the focusing servo control is reduced during each period in which the optical pick-up stylus moves trans-versely across a l~rge number o signal tracks at high speed, as during a searching or random access operation of the optical disc player, whereby to avoid the vibrational movement of the obj ective lens and the unde-sirable harsh noise that would occur as a consequence thereof if ~he focusing servo control was allowed ~o respond to the rapid and repeated defocusing detected during each such period.
Although preferred embodiments of the invention have been described in de~ail herein with reference to the accompanying drawings, it is to be understood that the invention is not l;mited to those precise embodiments, and that various changes and modifications may be effected therein by one sklllcd in the art without departing from the scope or spirit of the invention as defined in the appended claims.
track 17, as shown on ~lg. 4B. Gonver5ely, when the tracking er~or causes ~ain light spo~ 16a to deviate toward the right from the cent~r of record track 17, ~hen the full area of auxiliary light spot 16c traces or scans track 17, while auxiliary li.ght spot 16b moves completely off record track 17, as shown on Fig. 4C.
The auxlliary light beams forming auxiliary light spots 16b and 16c, after being reflected by disc 11, are guided to photo-diodes 14e and 14f, respectively, of light receiving device or transducer assembly 13. The corresponding outputs Se and Sf from photo-diodes 14e and 14_, respectively, are supplied to an operational circui~
51 (Fig. 2) which performs an operation expressed by the below equation:
Se - S = ST __ --(3).
The resulting tracking error signal ST obtained rom operational circuit 51 is zero when correct tracking is achieved, as on Fig. 4A. However, in the e~ent of a tracking error, as shown on Fig. 4B or 4C, tracking error signal ST has a level and polarity corresponding to the amount and direction of the tracking error. As hereinafter further described, tracking error signal ST is employed or angularly moving mirror 8 so as to achieve tracking servo control, that is, to maintain main light spot 16a substantially centered in respect to the ~rack 17 being scanned.
~ eferring now to Fig. 5, it will be seen that, in an optlcal disc player according ~o one embodiment of the p~esent inventian and which employs an optical pick-up device as described above with reference ~o Figs. lA-lC, and 2, the focusing e~ror signal SF from operational circuit 50 (Fig. 2~ is supplied ~o an input terminal 18 and therefrom through a phase compensatin~ circuit 19, a switching circuit 20 (inthe condition shown on Fig. 5) and a variable gain amplifier 21 to a coil 22 which ~orms part of a linear motor provided for moving objective lens 10 in the direction of its optical axis for achieving the previously descri.bed focusing servo control.
Switching circuit 20 is adapted ~o be changed-over from the condition shown on Fig. 5 to a condition in which an output signal o a stand-by circuit 23 is supplied through switching circuit 20 and vari.able gain amplifier 21 to coil 22 in ~lace of ~ocusing error slgnal SF. The foregsing arrangement is provided because ~he maximum range or peak-~o-peak ~alue of focusing error signal SF actually obtainable from light receiving device 13 corresponds to an axial movemen~ of objective lens 10 of only several tens of microns. On the other h~nd, variations in the vertical position of the surface of rotated disc 11, for example, due to deformation of the latter, and hence the extent of defocusing encountered during actual reproducing, may be as much as ~ 1.0 milli-meter, from which it follows that such defocuslng cannot be corrected by the focusing error signal S~,. Therefore, stand-by circuit 23 is provided to reduce the e~tent o the de~ocusing to within a range capable of being ~`?
corrected by the ~ocusing servo control, whereupon the latter becomes operative to precisely maint~in the correot focus. In order to achieve the foregoing operation, focusing error signal SF is also supplied rom anput terminal 18 to an ON/OF~ control circuit 24 which xesponds to the level of the focusing error signal SF or providi~g a switchîng signal by which switching circuit 20 is controlled. More particularly, so long as the level of focusing error signal S~ exceeds a pr~determined ~alue of either polarity corresponding to the range of defocusing that can be corrected by ~he focusing servo control, ~he resulting switching signal from circuit 24 causes change-over o switching circuit 20 from the position shown on Fig. S. Upon such change-ov~r of switching circuit 20, the output of stand-by circuit 23 is applied ~hrough switching circuit 20 and gain con~rol amplifier 21 with the result that a corresponding current supplied to coil 22 is e~fec~ive to move objective lens 10 to a position wi-~hin the effective range of the focusing servo control. When objecti.ve lens 10 is thus moved to a position within ~he effective range of the focusing servo con~rol, the corresponding reduction o~ the value or level of focusing error signal SF causes ON/OFF control circuit 24 to return switching circuit 20 to the condition sho~
on Fig. 5 for disconnectin~ stand-by circuit 23 and causing the focusing servo control to become operative, that is, to determine the amplitude and direction of the current supply to coil 22 in dependence on the level and polarity of focusing error signal SF.
~6~
In the case of an optical disc player 'having an optical pick-up device of the type described above with reference to Figs. lA, lB and lC, it is conventional to provide, in addition to the angular movement of mirror 8 in response to tracking error signal ST for efecting tracking servo control, a mechanism :~or shifting the optical pick-up device, as a unit, in the direction of the radius of the record disc, In the normal reproducing or playback mode using an op~ical r~cord disc having the information signal recorded ~hereon in a spiral path, the shifting ~echanism causes continuous gradual shif~ing or movement of the optical pick-up device in the radial dirertion of the record di.sc so that the optical pick-up stylus will substantially follow the successive tracks or turns of the spiral path on the rotated record disc, and the tracking servo control is employed for correcting those tracking errors resulting from eccentrici~ies in the rotation of the record disc. On the other hand, in a searching or random access ~ode of operation of the optical disc player, the shifting mechanism is suitably controlled to rapidly shift the optical pick-up device, as a unit, in the radial direction of the record disc for rapidly moving the optical pick-up stylus to a designated or selected radial position on the disc.
During such operation o the shi:fting mechanism in the searching or rando~ access mode~ the optical pick-up stylus undergoes periods of rapid transverse movement across large nurnbers of the record tracks on the disc, and the focusing servo control responds to resulting sensed rapid focus variations or defocusing o~ ~,he laser light bearn and thereby causes yibra~iona3. movemen~ of objective lens 10 and o its suppor~ing structure to produce an undesirable harsh noise.
~ enerally, in a~cordance with the present inventionJ the foregoi.ng problem is avoided by reducing the gain with which the focusing servo control responds to defocusin~ in any period in which the optical pick-up stylus undergoes rapid transverse movement across a large number of the tracks on the record medium surface.
In other words, in accordance with this invention, the gain of vari.able gain amplifier 21 (Fig. 5) is reduced when it is detected that thc optical pick-up stylus is moving transversely across a plurality o the record tracks on record disc 11.
In the embodiment of the invention jllustrated on Fig. 5, the movem~n~ o~ the optical pic~-up stylus transversely across a plurality of the tracks on record disc 11 is detected on the basis of variations occurring in the amplitude of the reproduced informa~ion signal S~
from operational circui~ 50 occurring when ~he optical pick-up stylus moves from one to another of the tracks in moving transversely thereacross. More particularly, in the embodiment of Fig. 5, th~ reproduced information signal SR, which is a relatively high frPquency or RF
signal so long as the op~ical pick-up stylus is cont.in-uously scanning along the spiral path on record disc ll, is supplied through an input terminal 25 and a low pass filter 26 to a voltage com~arator 27 and a ~C voltage maintaining circuit 28, Voltage cvmparator 27 compares the voltag~ of the output of low pass ilter 26 with an adjusted D~ reference volta~e ~ro~ circui~ 28, ~nd the result of such comparison is used for controlling the gain of variable gain amplifier 21. ~hus, the a~plification of the focusing error signal s~ in ampllfier 21 ~or application to focusing cQil 22 is controlled.
The operation of the above d~scribed arrange ment embodying this i.nvention will now be described with reference to Fig5. 6A, 6B and 6C~ In the period when the optical pick-up stylus formed by the laser light beam focused at the surface of disc ll by objective lens lO
is tracing or scanning a track on disc 11 and in which, for example a frequency modulated signal ha~ing a carrier frequency of 8 MHz is rPcorded, the reproduced informa~ion signal SR similarly has a high carrier frequ~ncy wi~h an env~lope of almost constant level. However, wheil~the optical pick-up device is shifted radially of the disc in a searchin~ or random access mode of operation so that the optical pick-up stylus moves transversely across a substantial number o the tracks or turns of the spiral path, the amplitude of signal SR varies substantially dependin~ on whether ~he optical pick-up stylus is i~pinging on a track or on a land or portion of the record disc surface between two adjacent tracks where no signal is recorded, whereby to provide the signal SR with a component having a frequency lower than the carrier frequency, as indicated on Fig. 6A. Therefore, at the output of low pass filter 26 there will be obtained the relatively low fre~uency signal component, as shvwn on Fig. 6Bj and which occurs in each period when the optical pic~-up s tylus is rapidly ~noved transversely across a substantial or large number of the tracks on record disc 11, The level of such output of low pass Pilter 26 is co~parecl in ~oltage comparato~ 27 wi~h the reference ~ voltage from circuit 28 and the resulting c~mparison output is applied to a suitable time constant circuit within comparator 27 so that a rectangular pu1~2 si~nal (Fig. 6C) is applied to amplifier 21 for reduclng the gain of the latt~r, for example, by 20 dB, so long ~s the optical pick-up stylus is moving transversely acrDss signal tracks on disc 11.
Since the DC component of the signal S~ may have sligh~ly different levels from one disc to the next, the DC voltage maintaining cîrcuit 28 de~ec~s the DC component of signal SR and suitably adjusts the level of the reference DC
voltage which is supplied from circuit 28 to volta~e comparator 27 for comparison in the la~ter with ~he outpu~ of low pass filter 26.
It will be appreciated from the foregoing that, by reason of the described reduetion in the gain of amplifier 21 whenever the optical pick-up stylus is moving transversely across a substantial number of the signal tracks on disc 11, the current flowing through coil 22 for moving objective lens 10 in effecting the focusing servo control is sufficiently reduced so as to restrict the movements of objective lens 10. Therefore, the previously described undesirably harsh noise is not produced durin~ the period when the optical pick-up stylus is made to Move transversely across a substantlal number of the signal tracks, as during a searchin~, or random access operation of the optical disc player, and ~he focusing s~rvo control i5 otherwise effective to ensure ' ~3~
~he main~enance of the co~rec~ focus of the laser light bea~ or optical pick-up stylus at the surface of rotated disc 11.
Referring now to Fig. 7, it will be seen that, in an optical disc player accordin~ to another emhodiment o~f the present invention, each period during whlch the optical pick-up stylus is moving transversely across a substantial number of the signal tracks on disc 11, and during which the gain of amplifier 21 is reduced, is detected with reference to the tracking error signal ST.
More particularly, in the e~bodiment of Fig. 7, the tracking error signal ST, for exampleS obtained from the operational circuit 51 on Fig. 2, is supplied through an input terminal 29 and a variable resistor 30 acting as ~ level controller, to a phase compensa~ing circui~ 31, and from the latter through a switching circuit 32 and an amplifier 34 to a coil 35 for driving the angularly movable mirxor 8 and thereby effecting the ~racking servo control. In the normal reproducing mode of the optical disc player, the amplitude of tracking error signal ST
~aries periodically in response to tracking errors caused by eccentric rotation of disc 11, and mirror 8 is angularly moved in accordance with the level and polarity of the current flowing through coil 35 in response to tracking error signal STJ and thus tends to eliminate the tracking error.
In Fig. 7, the reference numeral 16 indicates a coil of a conventional ~otor mechanisIa which i.s provided for shiftin~ the optical pick up device, as a unit, in the direction of the radius of disc 11 in response to a signal ~21-supplied ~hereto by way of a swi~ch circuit 37, a low pass filter 38 and an amplifier 39, More par~icularly, switch circui~ 37 is shown to h~ve a movable contact selectiyely engageable with a first fixed contac~ 37a connected with the output of arnpliier 34, a~d second and third fixed con~acts 37b and 37c connected with positive and negative voltage sources 40a and 40b, respect-ively. Switeh circuit 37 is actuable to select for application to coil 36 either tracking error signal ST from ~he output of amplifier 34, or the positive voltage or negative voltage from source 4Da or 40b, respectively, Eor effec~ing a searching or random access operation.
Further, in the embodiment of Fig. 7, a pulse generator 41 is ~ade operative to produce a pulse in response to an output signal from a voltage comparator 42 which compares ~he level of tracking error signal ST from level controller ~0 with the level oE a reference voltage from a suitable source 43 thereo. Such pulse from generator 41 is applied to variable gain arnplifier 21 for reducing the gain of the la~ter, and also to switch circuit 32 for opening the latter.
The control circuit of Fig. 7 operates as ~ollows:
In the normal reproducing mode of ~he optical disc player, switch 37 is in the position shown on Fig. 7 and the tracking servo control is effective, in response to the tracking error signal ST, to tend to reduce the latter to zero, that i8, to cause the optical pick up stylus to accurately and continuously scan the successive tracks or turns of the spiral path in which the inforrnation -2~-(`` (~\
signal is recorded on disc 11. I~ will be appreciated that the dis~ance thr~gh which the optical pick-up stylus can be displaced in the radial direction on disc 11 by angular movement o~ mirror 8 is limited~ for example, to no more than 300 microns. Accordingly, in the normal reproducing mode in which switch ci-rcuit 37 has its movable oontac~ engaged with fixed contact 37a, and s~ long as switching circuit 32 is in its closed s~ate, as shown on Fig. 7, tracking error signal ST is also supplied to low pass filter 38 so that the low frequency component of signal ST is further supplied through amplifier 39 to coil 36. Thus, the motor mechanism associated with coil 36 is effective to shit the optical pick-up device gradually in the direction of the radius of the disc 11 in accordance witll the low frequency com-ponent of tracking error signal ST for accommodating the spiral configuration of the path in which the information signal is recorded on disc 11, while th~ supplying of tracking error signal ST to coil 35 effects the tracking servo control by which small tracking errors, for example, due to eccentric rotation of disc 11, are eliminated.
~ hen a searching or random access operation is desired, switch circuit 37 is actuated to en~a~e either its fixed contact 37b or fixed contact 37c and thereby apply a positive or a negative voltage from source 40a or 40b through low pass filter 38 and a~plifier 39 to coil 36, whereby the optical pi.ck-up device is shifted, as a unit, at relatively high speed in the radial direction ~oward or away from the center of disc 11. During such relatively high speed shifting of th2 optical pick-up device, ~he ~3~
tracking servo control tries to operate in the same manner as in the normal reproducing ~od~. Thus, as the optical pick-up device is being shif~ed, ~s a unit, the resul~ing tracking error signal 5T is applied to coil 35 ~or angu-larly moving mirror 8 and thereby causing the optical pick-up stylus to repeatedly trace a signal track on disc 11 until the m~ximllm range of angular movement of mirror 8 has been attained, whereupon the optical pic~-up stylus jumps across a large number of ~he tracks, for example, several hundred tracks or successive turns of the spiral path on disc 11, and then be~ins to repeate.dly trace or scan another of the signal tracks. As a result of the foregoing, in the searching mode of operation of the optical disc player, the signal recorded in the successive tracks or turns of the spiral path on disc 11 is inter-mittently reproduced at int~rvals of several hundred tracks.
Referring now to ~ig. 8, it will be seen that, in the above described searching ~ode of operation~ the average level of the tracking error signal ST incr~ases gradually in the period nT during which the disc 11 under-goes hundreds of revolutions, and further during which mirror 8 is progressively displaced angularly by increasin~
amounts so as to permit the repeated scanning of one track or turn. The variations in the level of tracking error signal ST appearing within the period T which corresponds to a single revolution of disc 11 represent tracking errors due to eccentric rotation of dis~ 11.
In Fig. 8, such variations of tracking error signal ST
due to eccentric rotation have been shown for only a few revolutions of the disc 11, but it will be apparent ~hat ( l ( a similar va~iation wou~d occu~ for each revolution durin~
the period nT. The trackin~ error signal ST shown on Fig. 8 is supplied to voltage comparator 42 for co~parison therein with the reference ~oltage Vt from source 43.
~en the level of tracking error signal ST exceeds ~efer ence voltage Vt in the course of a searching operation, the resulti.ng comparison Olltput from comparator 42 causes generator 41 to provide a pulse to switch circuit 32 for opening the latter and to variable gain amplifier 21 for reducing the gain of such amplifier through which the focusing error signal is supplied to coil 22, The opening of switch circui~ 32 by the pulse from generator 41 cuts off the tracking servo control, that is, interrupts the circuit through which the tracking error signal ST
is supplied to coil 35 with the result that mirror 80 rapidly returns to its neutral position and causes the optical pick-up stylus to move transversely across 10Q
to 200 signal tracks on disc 11 in a very short period, for example, in 10 ~o 20 milliseconds, during which the tracking error signal ST tha~ is obtained from operational circuit 51 is merely a noise component. At the conclusion of the pulse from generator 41, switch circuit 32 again closes, with the result that the tracking servo control is initiated once more for causing the op~ical pick-up stylus to repeatedly trace one o~ the ~racks while the optical pick~up device is further rapidly moved in the radial direction of disc 11 in a con~inuation of the searchin~ operation.
As noted above, the pulse from generator 41 is effective to reduce the gain of variable gain .. . .
amplifier 21 durin~ t~e period in which optical pick-up stylus moves transversely across a large number of tracks at hi~h speed. Thus, in an optical disc player according to the embodi~ent of Fig. 7 ~ as well as according to the embodiment of Fig. 5, the control g~in of the focusing servo control is reduced during each period in which the optical pick-up stylus moves trans-versely across a l~rge number o signal tracks at high speed, as during a searching or random access operation of the optical disc player, whereby to avoid the vibrational movement of the obj ective lens and the unde-sirable harsh noise that would occur as a consequence thereof if ~he focusing servo control was allowed ~o respond to the rapid and repeated defocusing detected during each such period.
Although preferred embodiments of the invention have been described in de~ail herein with reference to the accompanying drawings, it is to be understood that the invention is not l;mited to those precise embodiments, and that various changes and modifications may be effected therein by one sklllcd in the art without departing from the scope or spirit of the invention as defined in the appended claims.
Claims (10)
1. In an apparatus for optically reading information recorded in substantially parallel tracks on a record medium surface: the combination of optical pickup means including objective lens means through which a light beam is to be focused at said record medium surface for forming an optical pickup stylus, and being shiftable as a unit, transverse to said tracks for determining the track to be scanned by said optical pickup stylus;
focusing control means responsive to defocusing of said light beam at said record medium surface to move said objective lens means in the direction for restoring correct focus of said light beam at said surface; and gain control means for reducing the gain with which said focusing control means responds to said defocusing when said optical pickup stylus moves transversely across a plurality of said tracks on said record medium stylus.
focusing control means responsive to defocusing of said light beam at said record medium surface to move said objective lens means in the direction for restoring correct focus of said light beam at said surface; and gain control means for reducing the gain with which said focusing control means responds to said defocusing when said optical pickup stylus moves transversely across a plurality of said tracks on said record medium stylus.
2. An apparatus according to claim 1; in which said record medium is a rotated disc and said parallel tracks are constituted by successive turns of a spiral path on a surface of said disc, and the shifting of said optical pickup means, as a unit, is in a radial direction of said disc.
3. An apparatus according to claim 1; in which said focusing control means includes means for providing a focusing error signal in correspondence to said defocusing of the light beam, and focusing coil means associated with said objective lens means for moving the latter in the direction of its optical axis in response to said focusing error signal; and in which said gain control means includes variable gain means through which said focusing error signal is applied to said focusing coil means, and detecting means for detecting when said optical pickup stylus moves transversely across a plurality of said tracks and providing a corresponding detected output to said variable gain means by which the gain of the latter is reduced.
4. An apparatus according to claim 3, in which said optical pickup means further includes means providing a reproduced signal corresponding to information recorded in a respective one of said tracks when said optical pickup stylus scans the respective track; and said detecting means receives said reproduced signal and detects variations in the amplitude of the latter to provide said detected output when said optical pickup stylus moves from one to another of said tracks in moving transversely across a plurality of the tracks.
5. An apparatus according to claim 4; in which said detecting means includes low pass filter means receiving said reproduced signal, DC voltage maintaining means connected to said low pass filter means and producing a reference DC voltage having a level adjusted in correspondence to a DC component of said reproduced signal, and voltage comparator means comparing said reference DC voltage with the output of said low pass filter means and providing said detected output.
6. An apparatus according to claim 3; further comprising tracking control means including means for providing a tracking error signal in correspondence to deviation of the point of impingement of said optical pickup stylus from a transversely centered position in respect to one of said tracks, and means responsive to said tracking error signal for deflecting said optical pickup stylus relative to the remainder of said optical pickup means in a direction transverse of the tracks so as to restore said point of impingement to said trans-versely centered position; and in which said detecting means receives said tracking error signal and detects variations in the amplitude of the latter to provide said detected output when the amplitude of said tracking error signal attains a predetermined level.
7. An apparatus according to claim 6; in which said detecting means includes voltage comparator means having a source of a reference voltage and comparing said amplitude of the tracking error signal with said reference voltage to provide a compared output when said amplitude of the tracking error signal exceeds the level of said reference voltage, and pulse generating means responsive to said compared output for providing a pulse as said detected output and by which said gain of the variable gain means is reduced.
8. An apparatus according to claim 7; in which said tracking control means further includes means responsive to said pulse for rendering inoperative said means for deflecting the optical pickup stylus.
9. An apparatus according to claim 8; in which said means for deflecting the optical pickup stylus includes tracking coil means operative to cause said deflecting of the optical pickup stylus upon supplying of said tracking error signal to said tracking coil means, and said means for rendering inoperative includes normally closed switch means through which said tracking error signal is supplied to said tracking coil means, said switch means being connected with said pulse generating means and made open by said pulse.
10. An apparatus according to claim 9; further comprising drive means operative in response to a drive signal for shifting said optical pickup means, as a unit, transverse to said tracks, and means operable to selectively apply said tracking error signal and voltages of opposed polarity as said drive signal to said drive means.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000421102A CA1196101A (en) | 1983-02-08 | 1983-02-08 | Optical disc player |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000421102A CA1196101A (en) | 1983-02-08 | 1983-02-08 | Optical disc player |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1196101A true CA1196101A (en) | 1985-10-29 |
Family
ID=4124517
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000421102A Expired CA1196101A (en) | 1983-02-08 | 1983-02-08 | Optical disc player |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1196101A (en) |
-
1983
- 1983-02-08 CA CA000421102A patent/CA1196101A/en not_active Expired
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