CA1094684A - Apparatus for reading a record carrier with an optically readable information structure - Google Patents
Apparatus for reading a record carrier with an optically readable information structureInfo
- Publication number
- CA1094684A CA1094684A CA270,465A CA270465A CA1094684A CA 1094684 A CA1094684 A CA 1094684A CA 270465 A CA270465 A CA 270465A CA 1094684 A CA1094684 A CA 1094684A
- Authority
- CA
- Canada
- Prior art keywords
- circuit
- detectors
- read
- axis
- information
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
- G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B7/0901—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following only
- G11B7/0906—Differential phase difference systems
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B11/00—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor
- G11B11/10—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field
- G11B11/105—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field using a beam of light or a magnetic field for recording by change of magnetisation and a beam of light for reproducing, i.e. magneto-optical, e.g. light-induced thermomagnetic recording, spin magnetisation recording, Kerr or Faraday effect reproducing
- G11B11/10502—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field using a beam of light or a magnetic field for recording by change of magnetisation and a beam of light for reproducing, i.e. magneto-optical, e.g. light-induced thermomagnetic recording, spin magnetisation recording, Kerr or Faraday effect reproducing characterised by the transducing operation to be executed
- G11B11/1053—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field using a beam of light or a magnetic field for recording by change of magnetisation and a beam of light for reproducing, i.e. magneto-optical, e.g. light-induced thermomagnetic recording, spin magnetisation recording, Kerr or Faraday effect reproducing characterised by the transducing operation to be executed to compensate for the magnetic domain drift or time shift
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B11/00—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor
- G11B11/10—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field
- G11B11/105—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field using a beam of light or a magnetic field for recording by change of magnetisation and a beam of light for reproducing, i.e. magneto-optical, e.g. light-induced thermomagnetic recording, spin magnetisation recording, Kerr or Faraday effect reproducing
- G11B11/10532—Heads
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B11/00—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor
- G11B11/10—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field
- G11B11/105—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field using a beam of light or a magnetic field for recording by change of magnetisation and a beam of light for reproducing, i.e. magneto-optical, e.g. light-induced thermomagnetic recording, spin magnetisation recording, Kerr or Faraday effect reproducing
- G11B11/1055—Disposition or mounting of transducers relative to record carriers
- G11B11/10576—Disposition or mounting of transducers relative to record carriers with provision for moving the transducers for maintaining alignment or spacing relative to the carrier
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
- G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B7/0946—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following specially adapted for operation during external perturbations not related to the carrier or servo beam, e.g. vibration
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N5/00—Details of television systems
- H04N5/76—Television signal recording
- H04N5/7605—Television signal recording on discs or drums
Landscapes
- Engineering & Computer Science (AREA)
- Multimedia (AREA)
- Signal Processing (AREA)
- Optical Recording Or Reproduction (AREA)
- Holo Graphy (AREA)
Abstract
ABSTRACT:
An apparatus as described for reading a record carrier on which information, for example video and/or audio information is stored in an optically readible track-shaped information structure. A deviation between the centre of a read spot which is projected on the information structure and the centre line of a track to be read can be detected with the aid of at least two detectors which are disposed in the far field of the information structure in different quadrants. With the aid of the same detectors a reference signal is obtained which is used for deriving a control signal for correcting the position of the read spot rela-tive to the track to be read.
An apparatus as described for reading a record carrier on which information, for example video and/or audio information is stored in an optically readible track-shaped information structure. A deviation between the centre of a read spot which is projected on the information structure and the centre line of a track to be read can be detected with the aid of at least two detectors which are disposed in the far field of the information structure in different quadrants. With the aid of the same detectors a reference signal is obtained which is used for deriving a control signal for correcting the position of the read spot rela-tive to the track to be read.
Description
N
~/M:1:/(, OIiI~/(' I '
~/M:1:/(, OIiI~/(' I '
2'J.~ 7fi Apparatus for reading a record carri~r with an opl"ica:l:Ly reada'ble inforlllation structure.
The invention relates to apparatus for reading a record carrier on which in~ormation, for example video and/or audio inormation, is stored in an optically readable -track-shaped information struc-ture, which apparatus comprises a radiation source, an objective system for pas-sing radiation emitted by the radiation source to a radiation-sensitive information detection I system _~a the record carrier, wh.ich d.etection system converts a read. beam which is supplied by the radiation source and modulated by the infor-mation structure i.nt'o an electrical signal, and whi~h apparatus furthermore comprises a . centring deteGtion system which is connected to an electronic circuit for deriving a control signal for correcting the centrin.g of the read beam relative to a track portion to be read.
' A centring detection system is to be understood to mean a radiation-sensitive detection sy.stem which supplies an elec-trical signal whicll provides an indication about the deviation between the centre of a read spot of radi.ation wh:ich is projected on -the record I , carrier and the centre of a track portion to be ¦ '25 ' read.
,' . .
I ~2-0 g4 6 8 4 I'IIN ~29~) ~n "Philips' Technical Revie~" 33 ~To. 7, pages 186 - 189, an apparatus for reading a disc-shaped round record carrier is described.
On this record carrier a colour television programme is stored. The information structure comprises a spiral track which consists of a multitude of pits which are pressed into the record carrier, the luminance information being contained in the frequency of the pits, wllilst 10- the chrominance and audio informa-tion is con-tained in a variation of the leng-ths of the pits.
At the information structure a read beam is focused to a radiation spot whose dimensions are of the order of magnitude of those of the pits. By moving the record carrier relative to the read beam, this beam is modulated in accordance with the stored information. A radiation-sensitive information detection system converts the modu-lation of the read beam into an electrical signal.
! 20 In an clectronic circuit the signal is processed so that it is suitable to be applied to colour te~evision recei-ving apparatus.
When reading the record carrier care must be taken that the centre of the read spot of --radiation is always projected in the centre of the track to be read, because otherwise the modulation depth of the signal supplied by the information ¦ detection system becomes too small and crosstalk may occur between adjacent tracks. Therefor, the position of the radiation spot will have to be detected a~d corrected continually.
The invention relates to apparatus for reading a record carrier on which in~ormation, for example video and/or audio inormation, is stored in an optically readable -track-shaped information struc-ture, which apparatus comprises a radiation source, an objective system for pas-sing radiation emitted by the radiation source to a radiation-sensitive information detection I system _~a the record carrier, wh.ich d.etection system converts a read. beam which is supplied by the radiation source and modulated by the infor-mation structure i.nt'o an electrical signal, and whi~h apparatus furthermore comprises a . centring deteGtion system which is connected to an electronic circuit for deriving a control signal for correcting the centrin.g of the read beam relative to a track portion to be read.
' A centring detection system is to be understood to mean a radiation-sensitive detection sy.stem which supplies an elec-trical signal whicll provides an indication about the deviation between the centre of a read spot of radi.ation wh:ich is projected on -the record I , carrier and the centre of a track portion to be ¦ '25 ' read.
,' . .
I ~2-0 g4 6 8 4 I'IIN ~29~) ~n "Philips' Technical Revie~" 33 ~To. 7, pages 186 - 189, an apparatus for reading a disc-shaped round record carrier is described.
On this record carrier a colour television programme is stored. The information structure comprises a spiral track which consists of a multitude of pits which are pressed into the record carrier, the luminance information being contained in the frequency of the pits, wllilst 10- the chrominance and audio informa-tion is con-tained in a variation of the leng-ths of the pits.
At the information structure a read beam is focused to a radiation spot whose dimensions are of the order of magnitude of those of the pits. By moving the record carrier relative to the read beam, this beam is modulated in accordance with the stored information. A radiation-sensitive information detection system converts the modu-lation of the read beam into an electrical signal.
! 20 In an clectronic circuit the signal is processed so that it is suitable to be applied to colour te~evision recei-ving apparatus.
When reading the record carrier care must be taken that the centre of the read spot of --radiation is always projected in the centre of the track to be read, because otherwise the modulation depth of the signal supplied by the information ¦ detection system becomes too small and crosstalk may occur between adjacent tracks. Therefor, the position of the radiation spot will have to be detected a~d corrected continually.
-3~
10~4684 ]'li~ 8~
29.2.1~76 For this purpose the apparatus described in the cited article comprises an auxiliary system with ~hich two sub-beams are produced, which are focused at the edge oI a track portion to be read. For each of the sub-beams a separa-te auxiliary detector is provided. The difference between the out-put signals of these auxiliary de-tectors pro-vides an indication of the degree of centring of the read beam relative to a track portion to be read. In addition to the op-tical elements required for the actual read-out, -the l~lown apparatus comprises a number of opl;ica:L auxi-liary elements necessary for detecting a centring error.
lt is an object of the present application to provide a read apparatus in which centring errors can be detected using a minimum number of additional optical elements.
The~apparatus in accordance with the invention is therefore characterized in that the centring detection system and -the information detection systnm are constituted by an even number of at Least two and at most four radiation-sensi-tive detectors which are situated in the far ¦ field of the information struc-ture in the indi-_4_ ., 09~684 I'ilN ~;""() 2~. 2 . 1~7G
.
vicluRl quadrants of an imaginary X-Y coordina-te sys-tem whose origin is disposed on the optical axis of -the objective system and whose X-axis offec-tively extends in. the track direction and whose Y-axis effectively extends transversely to the track direc-tion, that the outputs of t-o detectors whi.ch are disposed at the same side of the Y-axis are cor~lected to both a subtractor circuit and an adder circuit, that a mult:iplier circuit is provided to whofie inputs signals de-rived from the subtractor circuit and from the adder circuit are applied and that the output of` the multiplier circuit is connected to a filter circuit which only transmits frequencies lower than the frequency which corresponds to ¦ twice the average spatial frequency of the information structure in the track direction, -at the output of which filter circuit a control signal for correcting the centring of the read beam is obtained.
The phrase "the detectors are situated in the far field of the informati.on structure" is to be understood to mean that these -del;ectors are located in a plane in which the different dif`~raction orders of the read beam formed by the! infolrmation s-tructure are suffi-~ ciently disti.nct; i.e. in a plane which is suf-I ficiently far from the image of the information structure.
' . -5-`I .
l':llN ~,_9
10~4684 ]'li~ 8~
29.2.1~76 For this purpose the apparatus described in the cited article comprises an auxiliary system with ~hich two sub-beams are produced, which are focused at the edge oI a track portion to be read. For each of the sub-beams a separa-te auxiliary detector is provided. The difference between the out-put signals of these auxiliary de-tectors pro-vides an indication of the degree of centring of the read beam relative to a track portion to be read. In addition to the op-tical elements required for the actual read-out, -the l~lown apparatus comprises a number of opl;ica:L auxi-liary elements necessary for detecting a centring error.
lt is an object of the present application to provide a read apparatus in which centring errors can be detected using a minimum number of additional optical elements.
The~apparatus in accordance with the invention is therefore characterized in that the centring detection system and -the information detection systnm are constituted by an even number of at Least two and at most four radiation-sensi-tive detectors which are situated in the far ¦ field of the information struc-ture in the indi-_4_ ., 09~684 I'ilN ~;""() 2~. 2 . 1~7G
.
vicluRl quadrants of an imaginary X-Y coordina-te sys-tem whose origin is disposed on the optical axis of -the objective system and whose X-axis offec-tively extends in. the track direction and whose Y-axis effectively extends transversely to the track direc-tion, that the outputs of t-o detectors whi.ch are disposed at the same side of the Y-axis are cor~lected to both a subtractor circuit and an adder circuit, that a mult:iplier circuit is provided to whofie inputs signals de-rived from the subtractor circuit and from the adder circuit are applied and that the output of` the multiplier circuit is connected to a filter circuit which only transmits frequencies lower than the frequency which corresponds to ¦ twice the average spatial frequency of the information structure in the track direction, -at the output of which filter circuit a control signal for correcting the centring of the read beam is obtained.
The phrase "the detectors are situated in the far field of the informati.on structure" is to be understood to mean that these -del;ectors are located in a plane in which the different dif`~raction orders of the read beam formed by the! infolrmation s-tructure are suffi-~ ciently disti.nct; i.e. in a plane which is suf-I ficiently far from the image of the information structure.
' . -5-`I .
l':llN ~,_9
4 6 8 4 2~.2.1 The phrase : "that the X-axis e:rfectively extends in the track direction and the Y-axis effectively extends transversely to the track direction", is to be understood to mean.that the imaginary projections of these axes on the information structure extend in the track direction and transversely to the track d.irection re SpQ ctively.
The inven-tion is based on the recognition that when reading the information structure, which behaves as a two-dimensional diffraction grating, centring er:rors :result in additional phase shifts between a ~ero-order sub-beam and higher order sub-beams. These pha.se shifts can be detected in the far field of the information structure with the aid of suitably arranged detectors. In accordance with -the invention, a reference signal is obtain.ed with the aid of the same detectors, which signal is employed for deriving the control signal for correcting the centring of the read spot relative to a track portion to be read. The advantage of thLs is that the reference signal and the signal which provides an indication of centring errors are affected in tl?e same way by possible dis-turbances in the read system, such as optical noise or vib~.ations of the elements in the read I
10'34684 PIIN ~290 ~9 . ;~. 197G
- appara-tus. Owing to the manner in which said signals are processed! nRmely Vi'd so-called synchronous detection, the resulting con-trol signal for centring correc-tion is independent of said disturhances.
A further advantage is that the applicability oi` the invention is not limited to one specific phase depth of the information structure. Phase depth is to be understood to mean the phase difference be-tween the zero-order sub-beam and the first order sub-beams caused by the details of the information structure.
If the in.formation structure is reflecting and consists of pi.ts which are pressed into the record carrier surface, which pits a~e ~/4 deep.
~ being the wavelength of the read beam, the ¦ phase depth is ~ . The invention is also appli-¦ cable to amplitude structures whose phase depths may be assumed to be also ~ .
However, the selected phase depth of the information structure does dictateh~w the actual information is preferably read, i.e.
whether the sum of the ou-tput signals of the detectors at one.side of the Y-axis should be added to or subtracted from the s~ml of the out~
put signals of the detectors at the other side of the Y-axis.
. -7-~, ~09468~ PHN 8290 In principle, the concept underlying the invention can be implemented using two detectors only. By using four detectors a better signal-noise ratio can be obtained for the information signal and for the centring error signal.
It is to be noted that it has been proposed previously in UOS. Patent 4,006,293 -Bouwhuis et al - February 1, 1977 (PHN 7919) to detect centring errors with the aid of one additional detector which is disposed in the far field of the information structure.
Alternatively, two additional detectors may be used. However, the last-mentioned detectors are situated in the same quadrants of the above-mentioned imaginary coordinate system, and the output signals of these detectors are not sub-tracted from each other for determining a centring error. For a dynamic detection of the centring errors in the previously proposed read apparatus the track portion to be read and the read spot should periodically be moved relative to each other txansversely to the track directic~n during reading. This demands an adaptation of either lthe record carrier or the read apparatus.
:I'IIN ~,./9() 109~684 The illVe:rl tiOIl will I10W be descril~ed in 1110rC detail with reference to the ~Irawing, iIl wI~ich :
Fig. 1 shows an embodimen-t of an apparatus in accordance with the invention, Fig. 2, 6, Ga, 7 and 8 show examples of radiation-sensi-tive detection systems employed in this apparatus, and they also illus-tra-te how the signals supplied by these systems are processed, and Fig. 3, 1l and 5 illustrate the principle of -the invention.
Fig. 1 shows a round disc-shaped record carrier 1 in radial cross-section. The information structure :is assumed to be reflecting.
The information tracks are designated 3. ~
radiation source 6, for example a helium-neon laser, emits a read beam b. The mirror 9 reflects this beam tow~rds an objective system 11, which is schematically represented by a single lens. The path of the read beam b includes an auxiliary lens 7, which ensures that the read beam completely fills the pupil of the objective system. Then, a radiation spot of minimal dimensions is formed on the plane 2 of the information structure.
The read beam is reflected by the information s-l;ructure and during rotation of the record carrier about a spindle 5 which extends through a central opening 4, it is time modulated .
!
g 1094~;84 I'I[N
2~ 7G
in accordallce wi-th tlle inL'ormatioll s-tored in the track to be read. The modu:lated read beam passes again through the objecti~re system and is reflected by the mirror ~ in the direction of the beam which is emi-tted by tlle source. The radiation path of the read beam includes elements for separating the paths of the modulated and the unmodulated read beam.
These elements may for example comprise an assem-bly of a po:Larisation sensitive dividing prism and a ~/4 plate. For the salie of simplicity it is assumed in Fig. 1 that said means are consti-tuted by a semi-transparent mirror 8. This mirror rcflects a part of the modulated read beam to a radia-tion sensitive cletection system 12.
I The optical details o~ the infor-¦ mation structure are very small. For example, I the width of a track is 0.5/um, the track dis-j 20 tance 1.2/um, and the average period of the information areas, which are assumed to be pits, hereinafter is 3/um for a disc-shaped record carrier on which a thirty-minute television programme is stored within a ring witll an inner diameter of 12cm and an outer diameter of 27 cm.
Therefore, the read spot showld remain very accurately centred on the track to be read.
' l:'llN ~2~,() 10~4~84 ~9-'-'. 1()7fi In order to ena~l.e centring errors to be detected, ln accordance wi-th th.e invention, the detec-tion system 12 consists of for exalnple four radiation-sensitive detectors, as is shown in Fig. 2, The four de-tectors 13, 14-, i5 and 16 are di.sposed in four differen-t quadrants of an X-Y coordinate system. If a track portion to be read is projected on thc detection system the longitudinal direction and the lateral direction of the track portion are pa:rallel to the X-axis and the Y-axis respectively.
The four d.etectors are for example disposed in the plane U in which an image of the exit pupil of the objective system is formed by means of an auxiliary lens 23. For the sake of simplicity only the image (a~) of a point a of the exlt pupi.l is sho~ by dashed lines in Fig. 1. The detectors 1.3, 14, 15, and 16 may also 20 . be disposed in an other pl.ane, provided tha-t the sub-beams which are diffracted in various orders by the information structure are suffi-ciently distinct.
~ As is further shown in Fig. 2, ! 25 the output sigrnals of the detectors 13 and 16 I are subtractecl from each other by means of the .1 ..
~ N ~;; ;' () 0 ~0~4~;84 ~ ~ . 2 . 1 ~ 7 G
subt;:ractor c:ircuit 17 and t1le OUtp~lt sigllals o:L -t;he de-lectors 11~ alld 15 by means of the sub-tractor circuit 18. The output signals of the subtractor circuits 17 and 18 are sub-tracted from each other in -the subtractor circuit 19. l`he outpu-t of this circuit is connected -to one of the i.nputs of a multiplier circuit 22. The output signal of the circuit 22 is applied to a low-pass filter 23. At the output of this ~ilter the desired signal S is obtained, as will be explained hereinafter.
For reading the information containecl on the record carrier~ for example a televisi.on programme, the output signals of the detec-tors 13 and 16 can be added to each cther with the aid of the adder circllit 21 and the output signals of the detectors 14 and. 15 by means of t~e adder circuit 20. The output signals of the adder circuits 20 and 21 can be subtracted from each other in the circuit 24. The information signal S24, which appears at the OUtp1lt of the sub-tractor circuit 24, is decoded in an electronic CirGUit 26, known ~ se, and the decoded signal, I if ~ television programme is s1;ored, is rendered visible and audible with the aid of conventional tele~ision receiving apparatus 27. Furthermore, :
10~4684 ~9.2.;~7G
the sig~nal S21l is appliccl to a second input of the mllltiplier circuit 22.
Now the physical backgrounds of the inven-tion will be discussed in fur-tller detail. The information s-tructure of the record carrier, which inforrnatioll structure consists of tracks whicll iIl their turn comprise a multi-tude of areas and in-termediate areas, of which the areas may take the form of pitsS may be regarded as a two-dimensional diffraction grating. This grating divides the read beam b into a ~ero order sub-beam, a number of f`irst order sub-beams and a number of higher order sub-beams.
A part of the radiation of` the sub-beams passes through the pupil of the objective system 11 and coul~ be concentrated in the image plane of the inf`ormation structure. In this image plane the individual diffraction orders are not separated. However, in the plane of the exit pupil of the objective system, or in a plane in which an image of said exi-t pupil is f`ormed, the diffraction orders are more or less separated. F:igures 3 and ~ show the situation in the plane of the exit pupil.
c 25 In Fig. 3 the circle 30 with the centre 3g represents ths cross-s~cCion of the 0!~4684 I'IIN ~ )0 2~.2.197G
Zel'O order sub-bealn b(0,0) in the plane of the exit pupil o~ the objective system. The circ:Les 31, 32, 33, and 31i respectively re-present the cross-sections of the diagonally di~fracted sub-beams b(~ l), b(-1,~
b(-1, -1), and b(+1, -1).
Besides in the diagonal direc-tions, the information struc-ture also diffracts the read beam in the track direction and in the direction transverse to the track direction.
Ilence,also (~1, 0) and (-1, 0) order sub-beams are obtainecL solely OWill.g to the pits in a tracls portion to be read, and a~so (0, -~1) and (0, -1) order sub-beams solely owing to the grating structure transverse -to the track direc-tion. In Fig. 4 the circles 41, ~12, 43 and 44 represent the cross-sections of -the sub-beams b(~1, 0), b(0, ~1), b(-1, 0), and b(0, -1) at the location of the exit pupil of the objective system.
The X-axis and the Y-axis of Fig.
3 and ll correspond to the X-axis and the Y-axis of Fig. 2. The distance e from the centres 36, 37, 38, 39, llG ancL 48 of the circles 31, 32, 33, 34, 42 and 44 to t;he X-axis is determined by ~ /q, where q is the spatial period of the information ..
~3 1094~84 ~ . 2.-1~7~
structure in -the di:rection transverse to the -track direction and A the waveleng-th of the read beam b. The period q may be assumed to be constan-t. The c].istance f from the centres 36, 37, 38, 39, ~5 and 47 to the Y-axis is de-termined by ~ /p, p representing the local spatial period of the,pits in a track porti,on to be read.
For determining a centring error use is made of the changes in phase of the first-order diagonal sub-beams relative to the zero order sub-beam.
In the hatched areas in Fig. 3 the various first-order diagonal sub~beams b(+l, +1), b~-1, +1), b(~ 1), and b(+l, -1) overlap the zero-order sub-beam b(0, 0) and interference occurs. The phase of the first-order diagonal ~ sub-beams varies with higrh frequency owing to I the movement of the read spot over the infor-mation structure in the track direc-tion,~and ¦ with low frequency owing to a possi.ble movement ¦ of the read spot in a direction transverse to ~ tha track direction. Tnis gives rise -to inten-i sity variations in the exit pupil, or the effective ex:it pupil, of the objective system, which variat:ions can be detected with for example -the Idetec.tor arrangement of Fig. 2.
' , 'i --1 5--:~ ;
l'llN ~.''()() ~094G84 ~ en the cent:re of -the read spo-t coincides wi-th the oentre of a pit, a spe-ciric phase difference ~, is ob-tained be-tween a firs-t-order sub-'beam and a zero~order sub-beam.
The value O:r ~ depends on the shape of the informa-tion structure, in the case of a pit struc-ture it depends mainly on the phase depth of the pits. When the read spot moves from a first pit to a second pit the phase of for e~Y-ample the first-order sub-beam b(+l 5 +1) relative to the zero-order beam incrcases continuously with 2 ~ . Therefore it may be assumed that as the read spot moves in the track I direction the phase of the first-order sub-beam ¦ 15 b(-~1, +1) relative to the zero order sub-beam changes by ~ t. Here, ~ is a temporal frequency which is determined by the spatial frequency of the pits in a track portion to be read and by the speed with which the read spot moves over this track portion. Also in the case of a movement of the read spot transverse to the track direction the phase of the first-order sub-beamb(+1~ -~1) relative to the zero-order sul>-beam will change. l`his phase shift may be 25represented by 2 ~ . r, where r is the I
-- , -,~ .
10~684 ~ .; . i~ "
distance between tlle centre Or tlle -read spo-t and t,he centre Or the tracl~ por-tion -to be read.
The phase ~ (+ 1, +1) of the various diagonal first-order sub-beams relative to the zero order sub-beam may consequen-tly be represented by :
1, +1) = ~ + ~ -t + 2 ~ r ~ ~ ~ t + 2 ~ -0 (~ ~ t - 2 ~ r ~ (+1, ~ + ~ t - 2 ~ -The intensity variations in the exit pupil of the objective system owing to interference of the firs-t-order diagonal sub-beams with the zero-order sub-bealil is converted into electrical signals by the detcctors 13, 14, 15 and 16. The time-dependent output signals S13~ S14' 15' 16 detectors 13, 14, 15, and 16 may be represented by :
S13 = A cos ( ~' + ~ t + 2 ~ qr ) S1LI = A cos ( ~ - ~ t + 2 ~ - ) S15 = A cos ( ~ - ~ t - 2'~ ~r S16 = A cos ( ~ + ~ t - 2 ~ ~r where A is a constant.
I
~0~684 ! I'i l M ~ o As is show:n in Fi.g. 2, the signals S13 and S16 are subtracted from each ot;her and so are the signa~s S1~l and S15. The si.gnals at the outpu-ts of the sub-tractor circuits 17 and 18 are given by :
S17 = S13 - S16 = -B sin ( ~ ~ ~ t) sin (2 ~ q ) S18 = S1l - S1s = -B sin ( ~ - ~ t) sin (2 ~ ~q ) f where B is ag~ain a positive con.stant. Tlle signals S17 and S18 are subtracted from each other in the 1 10 subtractor circuit 19. The output si.gnal S19 may be ! represented by :
S19 = -C cos ~ . sin (2 ~ Qr) . sin ~-t ~ ~ where C is again a positive constant. The colnponent f si.n 2 ~ q is an odd func-tion of ~ r, so that the ~ 15 signal S19 contains informa-tiDn about both the ; magnitude and the direction of a centring error of the read spot relative to a track portion to be read~
The component sin ~ t varies in time depending on the information stored in the track portion, but is dependent oi` a centring error ~ r.
As i.5 shown in Fig. 2 the output signals of the detectors 13 and 16 are added to each other in the circuit 21. The terms (~ t in the 109~684 signals S13 and S16 have the same sign, whilst the sign of the term 2 1~ ~qr in the signal S13 is oppo~ite to that of this term in the signal S16. As a result, the variation in the sum of the signals S13 and S16 owing to centring errors will be substantially smaller than this variation in the signal S17. The sum signal S13 + S16 is mainly determined by the first orders which are diffracted in the track direction. This sum signal may be written as:
S21 = S13 + S16 = D cos ( ~ + ~ t) (1 + m cos 2~a r) where m is a constant smaller than 1, so that the sign of S21 cannot change owing to a centring error. Similarly, the sum signal S14 + S15 may be S20 = E cos (~ t) (1 + m cos 2 ~qr).
The signals S20 and S21 are applied to the subtractor circuit 24, at the output of which circuit the following signal is obtained.
S24 = -F sin ~ [1 + m cos (2~aqr)] sin ~ t After multiplication in the circuit 22 this yeilds:
S22 = Slg x S24 = G cos ~ sin ~ sin(2~qr) [1 + m cos(Z ~ aqr)] sin2~ t The component sin2 ~t may be written as -- :L9 -1094684 I'iiN 8J~() -2- - 2 cos (2 ~ t) ; and the componen-t sin as -1- sin (2 ~ ), so that :
S22 - sin 2 ~ K( ~ r) sin (2 ~ ~r ) L1 - cosf2 ~t)~
where K = ~ G L + m cos (2 ~ and is always positive. In the above expressions D, F, F and G are positive constants. The signal S22 is finally passed through the low-pass fil-ter 23, which only transmits frequencies lower than the frequency 2 ~? , so that a signal Sr = sin 2 ~ K ( ~ r) sin (2 ~ ~q ) is obtained.
~s ~ is determined by the phase depth, which is constant for a specific record carrier~ sin 2 is also a constant.
Consequently, the signal Sr is an odd function of the centring error ~ r, so that by means of the described detector arrangement and the described signal processing both the magni-- tude and the direction of the centring error is det~cted. The signal Sr may be used for correcting the position of the read spot relative to the -track portion to be read in a manner known ~ se, for example by tilting the mirror 9 in the direction of 1;he arrow 10 (compare Fig. 1).
- The components K ( ~ r) sin (2~r) may be wrltten as :
.
.. , '.
-ZO-~og 46~ 4 1'1 LN ~290 29.2. 1~7G
G [ sin(2~ 2 sin(4~ . In Fig. 5 the functions sin(2 ~ ) and 2 sin (4 ~ - ) are represented by the dashed curves 11 and 12 and the sum function by an unin-terrupted curve 13.
This reveals that in the range around ~ r = 0, which is important for servo control, the signal m2 sin (4 ~ ~r ) augments the signal sin (2 ~ r);
the slope of the curve 13 around ~ r = 0 is steeper than that of the curve l1.
It is to be noted that in the hatched areas in Fig. 4, the sub-beams which are diffracted in the X-direction overlap the sub-beams which are diffracted in the Y-direction.
The output signals of the detectors 13, 14, 15 and 16 are therefore not only determined by the interference between the zero-order sub-beams and the first-order sub-beams which are diffracted in the diagonal direction but also by the interference bet~een the first-order sub-beams which are diffracted in the track direction and in the direction transverse thereto, insofar these beams fall within the pupil of the obJec-` ~ tive system.
I`he difference in phase between 2~ for example the! sub-beam b(+1, 0) and the sub-beam b(0, +1) may be represented by :
~0~'~684 :I'IIN 8,- 90 ~ t ~ 2 ~ ~ r . In thls phase clif~erence the phase angle ~ no longer occurs, because both the sub-beam b(~1, 0) and the sub-beam b(0, +1) e~hibit a phase angle ~ relative to the zero order sub-beam b(0, 0). In-ter-ference of the sub-beams b(~1, 0), b(0, ~1), b(-1, 0) and b(0, -1) yields the following signals at the outputs of the detectors 13, 14, 15 and 16 :
S~13 = cos ( ~ t 2 ~ q S~ = cos (- ~ t - 2 ~ q S~15 = cos (- ~ t + 2 ~t q St16 = cos (+ ~ t ~ 2 1~ q These signals are processed in a similar way as the signals S13, S14, S15 and S16, i~e-:
S~17 = S~13 ~ S~16 = ~B~ sin ~ -t . sin(2~ r ) S 18 S 14 - Sl15 = -Bt sin ~ t ~ sin(2~ ~ r ), and S'19 = Sl17 - Sl18 = C~ sin 2 ~ ~qr sin ~ t a-re determined, where Bl and C~ are positive constants. The signals S19 and Sl19 do not counteract each other but amplify each other, so that the clescribad centring error detection is possible i!f -C cos ~ is positive, or cos is negative. .
109-~684 PllN ~0 29.2.19~
So far only first oder sub-beams have been discussed. It is obvious that the information structure will also diffract radiation in higher orders. I-lowever, the radiation energy in the higher diffraction orders is substantially small, and the diffraction angles are such that only a small portlon of the higher-order sub-beams falls within the pupil of the objective system. Therefore, the ¦ 10 inf]uence of the higher-order sub-beams may be ~ neglected.
! In order to enable a record carrier to be read with a specific optical system the spatial frequencies in the information struc-ture should be within specific limits. ~ig. 3 and 1~ show the situation in which the spatial I frequencies in the track direction and trans-- ~erse to the track direction, correspond to half the cut-off frequency of the optical read system.
The modulation depth of the information signal S24 is then a maximum and that of the centring error signal Sr is substantially a maximum. ~hen ¦ the spatial frequency of the pits in a track portion to be read increases, the first order sub-beams will be diffracted through a larger angle, i.e. the distance f increases. At a ~ I .
..
.
~09,~4 ~ N ~2~JO
specific spatial frequency, whicll corresponds to the cut-off frequency of the optical sys-tem, there will no longer be an overlap of the first-order sub~beams with the zero-order sub-beam and of the first-order sub-beams mutually. Then there will no longer be any interference in the area covered by the de-:tectors, and it is no longer possible to derive an information signa]. As the distances from the centres 36, 37, 38and 39 to the ' centre 35 are proportional to ~ ~ the highest spatial frequency of the pits in a track for which a centring error signal can be derived will be slightly lower, for example 15 lower than the highest spatial frequency for which an information signal S24 can be ob-tained. On the other hand, if the spatial fre-quency of the pits approximates to zero, the distance f will also approximate to zero. The various first order sub-beams are than no longer separated, so that it is no longer possible to obtain an information signal. The lowest value of the spatial frequency of the pits for which a centring error signal can be derived is slightly smaller than that value of which an information signal can still be obtained. The lower limit for the spatial frequency on the record carrier for ~094684 ~ 290 ~9.~197(>
wll;ch a centring error signal can still be obtained is that spatial frequency for which i-t is still possible to derive an information signal.
It is eviden-t that the same remarks can be made with respect to the spatial frequency of the information struc-- tures in the direction transverse to the track direction.
~lence, there is an optimum value for the spatial frequency of the information struc-ture in the track direction and in the direc-tion transverse thereto for which an optimum centring error signal is obtained.
However, there is a wi~e range of spatial frequencies around the optimum value within which it is possible to derive an information signal and a centring error signal with a satisfactory signal-to-noise ratio.
In the apparatus of Fig. 2 the signals from the left-hand and the right-hand part of the exit pupil are subtracted from each other -f`or deriving the information s:ignal (S2~ his apparatus is in parti-cular suitable for reading a record carrier with a small phase depth or a small pit-depth.
- .
~09468~ N ~
In the expression for S sin 2 y' tllen reaches an extreme value for ~ = ~ , whereas cos ~ is then nega-tive.
For reading a record carrier with a greater phase depth it is preferable -to add the signals of -the left-hand and the righ-t-hand part of the exit pupil to each other. For this purpose, the subtractor circult 24 in the arrangement of Fig. 2 might be replaced by an adder circuit 2L
(represented by dashed lines iIl Fig. 2).
In addition between tliis adder circuit and the multiplier circui-t a phase~-shif-ting circuit, for e~ample a differentia-ting network 25 (re-presented by dashed lines in I`ig. 2) must be included. However, it is then also possible to add the output signals of the detectors 13 and 15 and those of the detectors 14 and 16 to each other. The signal processing circuit may then be simplified, as is shown in Fig. 6.
In the circuit arrangement in accordance with Fig. 6 the following signals are determined :
60 13 + S15 = Bl cos~ cos ( ~ t + 21r ~ r S61 = S14 ~ S16 = B1 cos~ cos ( ~ t - 2~ q rllN ~J(;)o 10~?4684 2~).2.1~)l6 62 S60 S61 = -C1 cos ~1~ sin(2'~ ~ l ) sin (~) t S63- = S60 ~ S61 = D1 cos\i>~l ~ m cos(2~r )~ cos ~ t The components in the equations for S62 and S63 which vary with ~ t are ~ phase shif-ted relative to 0ach other, so that again one of the signals S62 and S63 should be passed -through a phase-shifting circuit. This circult could be a difI`erentiating network.(Cornpare -the element 25 in Fig. 2). However, preferably the phase shif-ting circuit 64 takes the form of a so-called phase-locked loop.
Fig. 6a shows the circuit diagram of such a circuit arrangement. The reference numeral 66 designates an oscillator which provides a cosine function at its output 67 and a sine functic-n at its output 68. The output 67 is connected to a first input of a frequency comparator 65 in which the frequency of the oscillator 66 is compared with the frequency of the signal cos (A~ t, whose phase is to be shifted through 90. The output signal of the comparator 65 is fed bac]k to the oscillator, so that the frequency of this oscillator becomes equal to that of the signal cos ~) t. A sine function with the desired frequency (~ is then obtained at the output 68 of the oscillator.
--27.--;
10!~46t~4 I'IIN ~,~0 2~ 7 Consequently, the circuit 64 co.nverts the signal S63 into a signal S64 = -E . cos ~ [1 + m.cos(2 ~ q )~ sin ~ t Multiplication of the signals S62 and S6l~ and filtering of the product signal yields Sr = ~ Z) ; cos2 ~ . sin (2 ~ ~ r where K1 is again a function of ~ z and is always positive. B1, C1, D1 and E1 are positive constants. Sr is again an odd func-tion of the centring error ~ z. cos2 ~ is maximum for a phase depth ~ so that the apparatus of Fig. 6 is suitable for reading a record carrier with a large phase depth, and also for a record carrier with an ampli-tude structure, whose phase depth may be assumed to b~ ~ radians.
Instead of using the entire exit pupil it would also be possible to use only half the exit pupil. An arrangement which is suitable for this is shown in Fig. 7. From the preceding it will be evident that the following equations are valid for the arrange-ment of Fig. 7 :~
~0 9 ~G 8 4 I~IIN f',~
~ 7~) 17 S13 S16 = -B2 sin ( ~ -~ ~ t) . sill(2'~ ~ r) S21 S13 ~ S16 = D2 cos ( I ~ ~ t) .L1~-m cos(2~ r)~
After a phase shiL't through ~ /2 of S21 multi-plication of the phase shifted signal by S17 thus yields:
S22= G12 - -2- COS 2(~ -t wt)'¦ 5in(2~r) . Ll~m cos ~2~ q )~ -Filtering this signal yields the signal Sr, which may be wrltten as :
Sr K2 ( ~ r) . SiIl 2r~ ~ r B2, D2 and G2 are positi~e constants. K2 is a positive function of ~ r, so that S is an odd function of ~ r. This expression for Sr does not contain any function of ~ , so that the apparatus of Fig. 7 is suitable for reading both rbcord carriers with a small phase depth and re-cord carriers with a large phase depth.
Tt is to be noted that when signals from the entire exit pupil are used, the signal-to-noise ratio for the information signal and ' the centring error signal is better than in the j case that only the signals from half the exit pupil are used.
..
~ ' .
~09~6f.~4 I' I I N o '-' ~) () 2~ 7G
The invention has been described by way of ex~mple with reference to a round ~isc-shaped record carrier with a radia-tion-reflecting information structure. It will be obvious that it is also possible to read radiation-transmitting rccord carriers with an apparatus in accordance with ~e invention.
The record carrier need not be round and disc-shaped, but may also be a record carrier in the form of a tape with a multitude of information tracks.
In respect of the information structure, it is to be noted that the only requirement is that this structure should be 1~ readable by optical means. This structure may be a pit structure, a black white structure, or for example, a magneto-optical structure.
Apart from a television programme tlle record carrier may for example also store digital in-formatinn for a computer.
For determining centring errors use is made of the pattern of the in-terference lines in the pupil of the objective system, which pattern is produced by interference between 2~ the zero-order sub-beam and the first-order sub-beams. T:he phase of the line pattern relative to the detectors is determined by the -30- .
~094684 Pll:N 82'Jo 2~.2. 1~7G
degree in which -the read spot is centred relative to a trac]~ to be read. The spatial frequency of the line pattern, however, is determined by the degree in which the read beam is focused at the plane of the infor-mation structure. ~or large focusing errors this period is small and for small focusing errors this period is large. The man1ler in which the focussing is corrected is irrele-vant for the present invention and is there-fore not discussed. However, it is to be noted that the focusing errors may influence the choice of the shape of the detectors in Fig. 7.
It has been assu~ed hereinbefore that the detectors 13 and 16 are rectangular detectors. The response of a rectangular de-tector to a line-shaped intensi-ty pattern is a curve whlch varies in accordance with sin , 1 being equal to the spatial period of the line pattern. This response curve has a value of zero if this spatial period equals the width of the detector. I~i that case this detector will always "view" one period of 1;he line pattern independently of the phase of the line pattern, and thus , ~0~3~684 I'l[N 8~90 2~.2 197G
independently of the centrin~. When for larger focusing errors the spatial period of the intensity pattern becomes smaller than the width of the detector, the response curve will h~ve a negative portion. This means that the servo system for centring might move the read spot in the wrong direction and that a possible centring error would increase. ~hen using rectangular detec-tors there is a risk, owing to the occurrence of focusing errors, that the ser~-o system for centring causes the centre of the read spot not to remain on the cen1;re line of a track portion to be read but to be projected at a fixed ~istance from said centre line.
This problem may be overcome by using triangular detectors instead of rec-tangular detectors. Fig. 8 shows a couple of such detectors 13~ and 16~ which may replace the detectors 13 and 16 of Fig. 7. The res-ponse curve of ths triangular detectors is in accordance with ( _ )2 and conse-~uently has no negative portion.
It is evident that the said problem does not occur if the read apparatus is provided with a servo-control wlqich ensures that the read spot always remains correctly focused at the information structure.
The inven-tion is based on the recognition that when reading the information structure, which behaves as a two-dimensional diffraction grating, centring er:rors :result in additional phase shifts between a ~ero-order sub-beam and higher order sub-beams. These pha.se shifts can be detected in the far field of the information structure with the aid of suitably arranged detectors. In accordance with -the invention, a reference signal is obtain.ed with the aid of the same detectors, which signal is employed for deriving the control signal for correcting the centring of the read spot relative to a track portion to be read. The advantage of thLs is that the reference signal and the signal which provides an indication of centring errors are affected in tl?e same way by possible dis-turbances in the read system, such as optical noise or vib~.ations of the elements in the read I
10'34684 PIIN ~290 ~9 . ;~. 197G
- appara-tus. Owing to the manner in which said signals are processed! nRmely Vi'd so-called synchronous detection, the resulting con-trol signal for centring correc-tion is independent of said disturhances.
A further advantage is that the applicability oi` the invention is not limited to one specific phase depth of the information structure. Phase depth is to be understood to mean the phase difference be-tween the zero-order sub-beam and the first order sub-beams caused by the details of the information structure.
If the in.formation structure is reflecting and consists of pi.ts which are pressed into the record carrier surface, which pits a~e ~/4 deep.
~ being the wavelength of the read beam, the ¦ phase depth is ~ . The invention is also appli-¦ cable to amplitude structures whose phase depths may be assumed to be also ~ .
However, the selected phase depth of the information structure does dictateh~w the actual information is preferably read, i.e.
whether the sum of the ou-tput signals of the detectors at one.side of the Y-axis should be added to or subtracted from the s~ml of the out~
put signals of the detectors at the other side of the Y-axis.
. -7-~, ~09468~ PHN 8290 In principle, the concept underlying the invention can be implemented using two detectors only. By using four detectors a better signal-noise ratio can be obtained for the information signal and for the centring error signal.
It is to be noted that it has been proposed previously in UOS. Patent 4,006,293 -Bouwhuis et al - February 1, 1977 (PHN 7919) to detect centring errors with the aid of one additional detector which is disposed in the far field of the information structure.
Alternatively, two additional detectors may be used. However, the last-mentioned detectors are situated in the same quadrants of the above-mentioned imaginary coordinate system, and the output signals of these detectors are not sub-tracted from each other for determining a centring error. For a dynamic detection of the centring errors in the previously proposed read apparatus the track portion to be read and the read spot should periodically be moved relative to each other txansversely to the track directic~n during reading. This demands an adaptation of either lthe record carrier or the read apparatus.
:I'IIN ~,./9() 109~684 The illVe:rl tiOIl will I10W be descril~ed in 1110rC detail with reference to the ~Irawing, iIl wI~ich :
Fig. 1 shows an embodimen-t of an apparatus in accordance with the invention, Fig. 2, 6, Ga, 7 and 8 show examples of radiation-sensi-tive detection systems employed in this apparatus, and they also illus-tra-te how the signals supplied by these systems are processed, and Fig. 3, 1l and 5 illustrate the principle of -the invention.
Fig. 1 shows a round disc-shaped record carrier 1 in radial cross-section. The information structure :is assumed to be reflecting.
The information tracks are designated 3. ~
radiation source 6, for example a helium-neon laser, emits a read beam b. The mirror 9 reflects this beam tow~rds an objective system 11, which is schematically represented by a single lens. The path of the read beam b includes an auxiliary lens 7, which ensures that the read beam completely fills the pupil of the objective system. Then, a radiation spot of minimal dimensions is formed on the plane 2 of the information structure.
The read beam is reflected by the information s-l;ructure and during rotation of the record carrier about a spindle 5 which extends through a central opening 4, it is time modulated .
!
g 1094~;84 I'I[N
2~ 7G
in accordallce wi-th tlle inL'ormatioll s-tored in the track to be read. The modu:lated read beam passes again through the objecti~re system and is reflected by the mirror ~ in the direction of the beam which is emi-tted by tlle source. The radiation path of the read beam includes elements for separating the paths of the modulated and the unmodulated read beam.
These elements may for example comprise an assem-bly of a po:Larisation sensitive dividing prism and a ~/4 plate. For the salie of simplicity it is assumed in Fig. 1 that said means are consti-tuted by a semi-transparent mirror 8. This mirror rcflects a part of the modulated read beam to a radia-tion sensitive cletection system 12.
I The optical details o~ the infor-¦ mation structure are very small. For example, I the width of a track is 0.5/um, the track dis-j 20 tance 1.2/um, and the average period of the information areas, which are assumed to be pits, hereinafter is 3/um for a disc-shaped record carrier on which a thirty-minute television programme is stored within a ring witll an inner diameter of 12cm and an outer diameter of 27 cm.
Therefore, the read spot showld remain very accurately centred on the track to be read.
' l:'llN ~2~,() 10~4~84 ~9-'-'. 1()7fi In order to ena~l.e centring errors to be detected, ln accordance wi-th th.e invention, the detec-tion system 12 consists of for exalnple four radiation-sensitive detectors, as is shown in Fig. 2, The four de-tectors 13, 14-, i5 and 16 are di.sposed in four differen-t quadrants of an X-Y coordinate system. If a track portion to be read is projected on thc detection system the longitudinal direction and the lateral direction of the track portion are pa:rallel to the X-axis and the Y-axis respectively.
The four d.etectors are for example disposed in the plane U in which an image of the exit pupil of the objective system is formed by means of an auxiliary lens 23. For the sake of simplicity only the image (a~) of a point a of the exlt pupi.l is sho~ by dashed lines in Fig. 1. The detectors 1.3, 14, 15, and 16 may also 20 . be disposed in an other pl.ane, provided tha-t the sub-beams which are diffracted in various orders by the information structure are suffi-ciently distinct.
~ As is further shown in Fig. 2, ! 25 the output sigrnals of the detectors 13 and 16 I are subtractecl from each other by means of the .1 ..
~ N ~;; ;' () 0 ~0~4~;84 ~ ~ . 2 . 1 ~ 7 G
subt;:ractor c:ircuit 17 and t1le OUtp~lt sigllals o:L -t;he de-lectors 11~ alld 15 by means of the sub-tractor circuit 18. The output signals of the subtractor circuits 17 and 18 are sub-tracted from each other in -the subtractor circuit 19. l`he outpu-t of this circuit is connected -to one of the i.nputs of a multiplier circuit 22. The output signal of the circuit 22 is applied to a low-pass filter 23. At the output of this ~ilter the desired signal S is obtained, as will be explained hereinafter.
For reading the information containecl on the record carrier~ for example a televisi.on programme, the output signals of the detec-tors 13 and 16 can be added to each cther with the aid of the adder circllit 21 and the output signals of the detectors 14 and. 15 by means of t~e adder circuit 20. The output signals of the adder circuits 20 and 21 can be subtracted from each other in the circuit 24. The information signal S24, which appears at the OUtp1lt of the sub-tractor circuit 24, is decoded in an electronic CirGUit 26, known ~ se, and the decoded signal, I if ~ television programme is s1;ored, is rendered visible and audible with the aid of conventional tele~ision receiving apparatus 27. Furthermore, :
10~4684 ~9.2.;~7G
the sig~nal S21l is appliccl to a second input of the mllltiplier circuit 22.
Now the physical backgrounds of the inven-tion will be discussed in fur-tller detail. The information s-tructure of the record carrier, which inforrnatioll structure consists of tracks whicll iIl their turn comprise a multi-tude of areas and in-termediate areas, of which the areas may take the form of pitsS may be regarded as a two-dimensional diffraction grating. This grating divides the read beam b into a ~ero order sub-beam, a number of f`irst order sub-beams and a number of higher order sub-beams.
A part of the radiation of` the sub-beams passes through the pupil of the objective system 11 and coul~ be concentrated in the image plane of the inf`ormation structure. In this image plane the individual diffraction orders are not separated. However, in the plane of the exit pupil of the objective system, or in a plane in which an image of said exi-t pupil is f`ormed, the diffraction orders are more or less separated. F:igures 3 and ~ show the situation in the plane of the exit pupil.
c 25 In Fig. 3 the circle 30 with the centre 3g represents ths cross-s~cCion of the 0!~4684 I'IIN ~ )0 2~.2.197G
Zel'O order sub-bealn b(0,0) in the plane of the exit pupil o~ the objective system. The circ:Les 31, 32, 33, and 31i respectively re-present the cross-sections of the diagonally di~fracted sub-beams b(~ l), b(-1,~
b(-1, -1), and b(+1, -1).
Besides in the diagonal direc-tions, the information struc-ture also diffracts the read beam in the track direction and in the direction transverse to the track direction.
Ilence,also (~1, 0) and (-1, 0) order sub-beams are obtainecL solely OWill.g to the pits in a tracls portion to be read, and a~so (0, -~1) and (0, -1) order sub-beams solely owing to the grating structure transverse -to the track direc-tion. In Fig. 4 the circles 41, ~12, 43 and 44 represent the cross-sections of -the sub-beams b(~1, 0), b(0, ~1), b(-1, 0), and b(0, -1) at the location of the exit pupil of the objective system.
The X-axis and the Y-axis of Fig.
3 and ll correspond to the X-axis and the Y-axis of Fig. 2. The distance e from the centres 36, 37, 38, 39, llG ancL 48 of the circles 31, 32, 33, 34, 42 and 44 to t;he X-axis is determined by ~ /q, where q is the spatial period of the information ..
~3 1094~84 ~ . 2.-1~7~
structure in -the di:rection transverse to the -track direction and A the waveleng-th of the read beam b. The period q may be assumed to be constan-t. The c].istance f from the centres 36, 37, 38, 39, ~5 and 47 to the Y-axis is de-termined by ~ /p, p representing the local spatial period of the,pits in a track porti,on to be read.
For determining a centring error use is made of the changes in phase of the first-order diagonal sub-beams relative to the zero order sub-beam.
In the hatched areas in Fig. 3 the various first-order diagonal sub~beams b(+l, +1), b~-1, +1), b(~ 1), and b(+l, -1) overlap the zero-order sub-beam b(0, 0) and interference occurs. The phase of the first-order diagonal ~ sub-beams varies with higrh frequency owing to I the movement of the read spot over the infor-mation structure in the track direc-tion,~and ¦ with low frequency owing to a possi.ble movement ¦ of the read spot in a direction transverse to ~ tha track direction. Tnis gives rise -to inten-i sity variations in the exit pupil, or the effective ex:it pupil, of the objective system, which variat:ions can be detected with for example -the Idetec.tor arrangement of Fig. 2.
' , 'i --1 5--:~ ;
l'llN ~.''()() ~094G84 ~ en the cent:re of -the read spo-t coincides wi-th the oentre of a pit, a spe-ciric phase difference ~, is ob-tained be-tween a firs-t-order sub-'beam and a zero~order sub-beam.
The value O:r ~ depends on the shape of the informa-tion structure, in the case of a pit struc-ture it depends mainly on the phase depth of the pits. When the read spot moves from a first pit to a second pit the phase of for e~Y-ample the first-order sub-beam b(+l 5 +1) relative to the zero-order beam incrcases continuously with 2 ~ . Therefore it may be assumed that as the read spot moves in the track I direction the phase of the first-order sub-beam ¦ 15 b(-~1, +1) relative to the zero order sub-beam changes by ~ t. Here, ~ is a temporal frequency which is determined by the spatial frequency of the pits in a track portion to be read and by the speed with which the read spot moves over this track portion. Also in the case of a movement of the read spot transverse to the track direction the phase of the first-order sub-beamb(+1~ -~1) relative to the zero-order sul>-beam will change. l`his phase shift may be 25represented by 2 ~ . r, where r is the I
-- , -,~ .
10~684 ~ .; . i~ "
distance between tlle centre Or tlle -read spo-t and t,he centre Or the tracl~ por-tion -to be read.
The phase ~ (+ 1, +1) of the various diagonal first-order sub-beams relative to the zero order sub-beam may consequen-tly be represented by :
1, +1) = ~ + ~ -t + 2 ~ r ~ ~ ~ t + 2 ~ -0 (~ ~ t - 2 ~ r ~ (+1, ~ + ~ t - 2 ~ -The intensity variations in the exit pupil of the objective system owing to interference of the firs-t-order diagonal sub-beams with the zero-order sub-bealil is converted into electrical signals by the detcctors 13, 14, 15 and 16. The time-dependent output signals S13~ S14' 15' 16 detectors 13, 14, 15, and 16 may be represented by :
S13 = A cos ( ~' + ~ t + 2 ~ qr ) S1LI = A cos ( ~ - ~ t + 2 ~ - ) S15 = A cos ( ~ - ~ t - 2'~ ~r S16 = A cos ( ~ + ~ t - 2 ~ ~r where A is a constant.
I
~0~684 ! I'i l M ~ o As is show:n in Fi.g. 2, the signals S13 and S16 are subtracted from each ot;her and so are the signa~s S1~l and S15. The si.gnals at the outpu-ts of the sub-tractor circuits 17 and 18 are given by :
S17 = S13 - S16 = -B sin ( ~ ~ ~ t) sin (2 ~ q ) S18 = S1l - S1s = -B sin ( ~ - ~ t) sin (2 ~ ~q ) f where B is ag~ain a positive con.stant. Tlle signals S17 and S18 are subtracted from each other in the 1 10 subtractor circuit 19. The output si.gnal S19 may be ! represented by :
S19 = -C cos ~ . sin (2 ~ Qr) . sin ~-t ~ ~ where C is again a positive constant. The colnponent f si.n 2 ~ q is an odd func-tion of ~ r, so that the ~ 15 signal S19 contains informa-tiDn about both the ; magnitude and the direction of a centring error of the read spot relative to a track portion to be read~
The component sin ~ t varies in time depending on the information stored in the track portion, but is dependent oi` a centring error ~ r.
As i.5 shown in Fig. 2 the output signals of the detectors 13 and 16 are added to each other in the circuit 21. The terms (~ t in the 109~684 signals S13 and S16 have the same sign, whilst the sign of the term 2 1~ ~qr in the signal S13 is oppo~ite to that of this term in the signal S16. As a result, the variation in the sum of the signals S13 and S16 owing to centring errors will be substantially smaller than this variation in the signal S17. The sum signal S13 + S16 is mainly determined by the first orders which are diffracted in the track direction. This sum signal may be written as:
S21 = S13 + S16 = D cos ( ~ + ~ t) (1 + m cos 2~a r) where m is a constant smaller than 1, so that the sign of S21 cannot change owing to a centring error. Similarly, the sum signal S14 + S15 may be S20 = E cos (~ t) (1 + m cos 2 ~qr).
The signals S20 and S21 are applied to the subtractor circuit 24, at the output of which circuit the following signal is obtained.
S24 = -F sin ~ [1 + m cos (2~aqr)] sin ~ t After multiplication in the circuit 22 this yeilds:
S22 = Slg x S24 = G cos ~ sin ~ sin(2~qr) [1 + m cos(Z ~ aqr)] sin2~ t The component sin2 ~t may be written as -- :L9 -1094684 I'iiN 8J~() -2- - 2 cos (2 ~ t) ; and the componen-t sin as -1- sin (2 ~ ), so that :
S22 - sin 2 ~ K( ~ r) sin (2 ~ ~r ) L1 - cosf2 ~t)~
where K = ~ G L + m cos (2 ~ and is always positive. In the above expressions D, F, F and G are positive constants. The signal S22 is finally passed through the low-pass fil-ter 23, which only transmits frequencies lower than the frequency 2 ~? , so that a signal Sr = sin 2 ~ K ( ~ r) sin (2 ~ ~q ) is obtained.
~s ~ is determined by the phase depth, which is constant for a specific record carrier~ sin 2 is also a constant.
Consequently, the signal Sr is an odd function of the centring error ~ r, so that by means of the described detector arrangement and the described signal processing both the magni-- tude and the direction of the centring error is det~cted. The signal Sr may be used for correcting the position of the read spot relative to the -track portion to be read in a manner known ~ se, for example by tilting the mirror 9 in the direction of 1;he arrow 10 (compare Fig. 1).
- The components K ( ~ r) sin (2~r) may be wrltten as :
.
.. , '.
-ZO-~og 46~ 4 1'1 LN ~290 29.2. 1~7G
G [ sin(2~ 2 sin(4~ . In Fig. 5 the functions sin(2 ~ ) and 2 sin (4 ~ - ) are represented by the dashed curves 11 and 12 and the sum function by an unin-terrupted curve 13.
This reveals that in the range around ~ r = 0, which is important for servo control, the signal m2 sin (4 ~ ~r ) augments the signal sin (2 ~ r);
the slope of the curve 13 around ~ r = 0 is steeper than that of the curve l1.
It is to be noted that in the hatched areas in Fig. 4, the sub-beams which are diffracted in the X-direction overlap the sub-beams which are diffracted in the Y-direction.
The output signals of the detectors 13, 14, 15 and 16 are therefore not only determined by the interference between the zero-order sub-beams and the first-order sub-beams which are diffracted in the diagonal direction but also by the interference bet~een the first-order sub-beams which are diffracted in the track direction and in the direction transverse thereto, insofar these beams fall within the pupil of the obJec-` ~ tive system.
I`he difference in phase between 2~ for example the! sub-beam b(+1, 0) and the sub-beam b(0, +1) may be represented by :
~0~'~684 :I'IIN 8,- 90 ~ t ~ 2 ~ ~ r . In thls phase clif~erence the phase angle ~ no longer occurs, because both the sub-beam b(~1, 0) and the sub-beam b(0, +1) e~hibit a phase angle ~ relative to the zero order sub-beam b(0, 0). In-ter-ference of the sub-beams b(~1, 0), b(0, ~1), b(-1, 0) and b(0, -1) yields the following signals at the outputs of the detectors 13, 14, 15 and 16 :
S~13 = cos ( ~ t 2 ~ q S~ = cos (- ~ t - 2 ~ q S~15 = cos (- ~ t + 2 ~t q St16 = cos (+ ~ t ~ 2 1~ q These signals are processed in a similar way as the signals S13, S14, S15 and S16, i~e-:
S~17 = S~13 ~ S~16 = ~B~ sin ~ -t . sin(2~ r ) S 18 S 14 - Sl15 = -Bt sin ~ t ~ sin(2~ ~ r ), and S'19 = Sl17 - Sl18 = C~ sin 2 ~ ~qr sin ~ t a-re determined, where Bl and C~ are positive constants. The signals S19 and Sl19 do not counteract each other but amplify each other, so that the clescribad centring error detection is possible i!f -C cos ~ is positive, or cos is negative. .
109-~684 PllN ~0 29.2.19~
So far only first oder sub-beams have been discussed. It is obvious that the information structure will also diffract radiation in higher orders. I-lowever, the radiation energy in the higher diffraction orders is substantially small, and the diffraction angles are such that only a small portlon of the higher-order sub-beams falls within the pupil of the objective system. Therefore, the ¦ 10 inf]uence of the higher-order sub-beams may be ~ neglected.
! In order to enable a record carrier to be read with a specific optical system the spatial frequencies in the information struc-ture should be within specific limits. ~ig. 3 and 1~ show the situation in which the spatial I frequencies in the track direction and trans-- ~erse to the track direction, correspond to half the cut-off frequency of the optical read system.
The modulation depth of the information signal S24 is then a maximum and that of the centring error signal Sr is substantially a maximum. ~hen ¦ the spatial frequency of the pits in a track portion to be read increases, the first order sub-beams will be diffracted through a larger angle, i.e. the distance f increases. At a ~ I .
..
.
~09,~4 ~ N ~2~JO
specific spatial frequency, whicll corresponds to the cut-off frequency of the optical sys-tem, there will no longer be an overlap of the first-order sub~beams with the zero-order sub-beam and of the first-order sub-beams mutually. Then there will no longer be any interference in the area covered by the de-:tectors, and it is no longer possible to derive an information signa]. As the distances from the centres 36, 37, 38and 39 to the ' centre 35 are proportional to ~ ~ the highest spatial frequency of the pits in a track for which a centring error signal can be derived will be slightly lower, for example 15 lower than the highest spatial frequency for which an information signal S24 can be ob-tained. On the other hand, if the spatial fre-quency of the pits approximates to zero, the distance f will also approximate to zero. The various first order sub-beams are than no longer separated, so that it is no longer possible to obtain an information signal. The lowest value of the spatial frequency of the pits for which a centring error signal can be derived is slightly smaller than that value of which an information signal can still be obtained. The lower limit for the spatial frequency on the record carrier for ~094684 ~ 290 ~9.~197(>
wll;ch a centring error signal can still be obtained is that spatial frequency for which i-t is still possible to derive an information signal.
It is eviden-t that the same remarks can be made with respect to the spatial frequency of the information struc-- tures in the direction transverse to the track direction.
~lence, there is an optimum value for the spatial frequency of the information struc-ture in the track direction and in the direc-tion transverse thereto for which an optimum centring error signal is obtained.
However, there is a wi~e range of spatial frequencies around the optimum value within which it is possible to derive an information signal and a centring error signal with a satisfactory signal-to-noise ratio.
In the apparatus of Fig. 2 the signals from the left-hand and the right-hand part of the exit pupil are subtracted from each other -f`or deriving the information s:ignal (S2~ his apparatus is in parti-cular suitable for reading a record carrier with a small phase depth or a small pit-depth.
- .
~09468~ N ~
In the expression for S sin 2 y' tllen reaches an extreme value for ~ = ~ , whereas cos ~ is then nega-tive.
For reading a record carrier with a greater phase depth it is preferable -to add the signals of -the left-hand and the righ-t-hand part of the exit pupil to each other. For this purpose, the subtractor circult 24 in the arrangement of Fig. 2 might be replaced by an adder circuit 2L
(represented by dashed lines iIl Fig. 2).
In addition between tliis adder circuit and the multiplier circui-t a phase~-shif-ting circuit, for e~ample a differentia-ting network 25 (re-presented by dashed lines in I`ig. 2) must be included. However, it is then also possible to add the output signals of the detectors 13 and 15 and those of the detectors 14 and 16 to each other. The signal processing circuit may then be simplified, as is shown in Fig. 6.
In the circuit arrangement in accordance with Fig. 6 the following signals are determined :
60 13 + S15 = Bl cos~ cos ( ~ t + 21r ~ r S61 = S14 ~ S16 = B1 cos~ cos ( ~ t - 2~ q rllN ~J(;)o 10~?4684 2~).2.1~)l6 62 S60 S61 = -C1 cos ~1~ sin(2'~ ~ l ) sin (~) t S63- = S60 ~ S61 = D1 cos\i>~l ~ m cos(2~r )~ cos ~ t The components in the equations for S62 and S63 which vary with ~ t are ~ phase shif-ted relative to 0ach other, so that again one of the signals S62 and S63 should be passed -through a phase-shifting circuit. This circult could be a difI`erentiating network.(Cornpare -the element 25 in Fig. 2). However, preferably the phase shif-ting circuit 64 takes the form of a so-called phase-locked loop.
Fig. 6a shows the circuit diagram of such a circuit arrangement. The reference numeral 66 designates an oscillator which provides a cosine function at its output 67 and a sine functic-n at its output 68. The output 67 is connected to a first input of a frequency comparator 65 in which the frequency of the oscillator 66 is compared with the frequency of the signal cos (A~ t, whose phase is to be shifted through 90. The output signal of the comparator 65 is fed bac]k to the oscillator, so that the frequency of this oscillator becomes equal to that of the signal cos ~) t. A sine function with the desired frequency (~ is then obtained at the output 68 of the oscillator.
--27.--;
10!~46t~4 I'IIN ~,~0 2~ 7 Consequently, the circuit 64 co.nverts the signal S63 into a signal S64 = -E . cos ~ [1 + m.cos(2 ~ q )~ sin ~ t Multiplication of the signals S62 and S6l~ and filtering of the product signal yields Sr = ~ Z) ; cos2 ~ . sin (2 ~ ~ r where K1 is again a function of ~ z and is always positive. B1, C1, D1 and E1 are positive constants. Sr is again an odd func-tion of the centring error ~ z. cos2 ~ is maximum for a phase depth ~ so that the apparatus of Fig. 6 is suitable for reading a record carrier with a large phase depth, and also for a record carrier with an ampli-tude structure, whose phase depth may be assumed to b~ ~ radians.
Instead of using the entire exit pupil it would also be possible to use only half the exit pupil. An arrangement which is suitable for this is shown in Fig. 7. From the preceding it will be evident that the following equations are valid for the arrange-ment of Fig. 7 :~
~0 9 ~G 8 4 I~IIN f',~
~ 7~) 17 S13 S16 = -B2 sin ( ~ -~ ~ t) . sill(2'~ ~ r) S21 S13 ~ S16 = D2 cos ( I ~ ~ t) .L1~-m cos(2~ r)~
After a phase shiL't through ~ /2 of S21 multi-plication of the phase shifted signal by S17 thus yields:
S22= G12 - -2- COS 2(~ -t wt)'¦ 5in(2~r) . Ll~m cos ~2~ q )~ -Filtering this signal yields the signal Sr, which may be wrltten as :
Sr K2 ( ~ r) . SiIl 2r~ ~ r B2, D2 and G2 are positi~e constants. K2 is a positive function of ~ r, so that S is an odd function of ~ r. This expression for Sr does not contain any function of ~ , so that the apparatus of Fig. 7 is suitable for reading both rbcord carriers with a small phase depth and re-cord carriers with a large phase depth.
Tt is to be noted that when signals from the entire exit pupil are used, the signal-to-noise ratio for the information signal and ' the centring error signal is better than in the j case that only the signals from half the exit pupil are used.
..
~ ' .
~09~6f.~4 I' I I N o '-' ~) () 2~ 7G
The invention has been described by way of ex~mple with reference to a round ~isc-shaped record carrier with a radia-tion-reflecting information structure. It will be obvious that it is also possible to read radiation-transmitting rccord carriers with an apparatus in accordance with ~e invention.
The record carrier need not be round and disc-shaped, but may also be a record carrier in the form of a tape with a multitude of information tracks.
In respect of the information structure, it is to be noted that the only requirement is that this structure should be 1~ readable by optical means. This structure may be a pit structure, a black white structure, or for example, a magneto-optical structure.
Apart from a television programme tlle record carrier may for example also store digital in-formatinn for a computer.
For determining centring errors use is made of the pattern of the in-terference lines in the pupil of the objective system, which pattern is produced by interference between 2~ the zero-order sub-beam and the first-order sub-beams. T:he phase of the line pattern relative to the detectors is determined by the -30- .
~094684 Pll:N 82'Jo 2~.2. 1~7G
degree in which -the read spot is centred relative to a trac]~ to be read. The spatial frequency of the line pattern, however, is determined by the degree in which the read beam is focused at the plane of the infor-mation structure. ~or large focusing errors this period is small and for small focusing errors this period is large. The man1ler in which the focussing is corrected is irrele-vant for the present invention and is there-fore not discussed. However, it is to be noted that the focusing errors may influence the choice of the shape of the detectors in Fig. 7.
It has been assu~ed hereinbefore that the detectors 13 and 16 are rectangular detectors. The response of a rectangular de-tector to a line-shaped intensi-ty pattern is a curve whlch varies in accordance with sin , 1 being equal to the spatial period of the line pattern. This response curve has a value of zero if this spatial period equals the width of the detector. I~i that case this detector will always "view" one period of 1;he line pattern independently of the phase of the line pattern, and thus , ~0~3~684 I'l[N 8~90 2~.2 197G
independently of the centrin~. When for larger focusing errors the spatial period of the intensity pattern becomes smaller than the width of the detector, the response curve will h~ve a negative portion. This means that the servo system for centring might move the read spot in the wrong direction and that a possible centring error would increase. ~hen using rectangular detec-tors there is a risk, owing to the occurrence of focusing errors, that the ser~-o system for centring causes the centre of the read spot not to remain on the cen1;re line of a track portion to be read but to be projected at a fixed ~istance from said centre line.
This problem may be overcome by using triangular detectors instead of rec-tangular detectors. Fig. 8 shows a couple of such detectors 13~ and 16~ which may replace the detectors 13 and 16 of Fig. 7. The res-ponse curve of ths triangular detectors is in accordance with ( _ )2 and conse-~uently has no negative portion.
It is evident that the said problem does not occur if the read apparatus is provided with a servo-control wlqich ensures that the read spot always remains correctly focused at the information structure.
Claims (5)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for reading a record carrier on which information, for example video and/or audio information, is stored in an optically readable track-shaped information structure, which apparatus comprises a radia-tion source, an objective system for passing radiation emitted by the radiation source to a radiation-sensitive information detection system via the record carrier, which detection system converts a read beam which is supplied by the radiation source and modulated by the information structure into an electrical signal, and which apparatus furthermore comprises a centring detection system which is connected to an electronic circuit for deriving a control signal for correcting the centring of the read beam relative to a track portion to be read, characterized in that the centring detection system and the information detection system are constituted by an even number of at least two and at most four radiation-sensitive detec-tors which are situated in the far field of the information structure in the individual quadrants of an imaginary X-Y coordinate system, whose origin is disposed on the optical axis of the objective system and whose X-axis effectively 29.2.1976 extends in the track direction and whose Y-axis effectively extends transversely to the track direction, that the outputs of two detectors which are disposed at the same side of the Y-axis are connected to both a sub-tractor circuit and an adder circuit, that a multiplier circuit is provided to whose inputs signals derived from the subtractor circuit and from the adder circuits are applied, and that the output of the multiplier circuit is connected to a filter circuit which only trans-mits frequencies lower than the frequency which corresponds to twice the average spatial frequency of the information structure in the track direction, at the output of which filter circuit a control signal for correcting the centring of the read beam is obtained.
2. An apparatus as claimed in Claim 1, which comprises four detectors, characterized in that the output of a first subtractor circuit whose inputs are connected to the detectors situated at one side of the Y-axis and the output of a second subtractor circuit whose inputs are connected to the detectors situated at the other side of the Y-axis are connected to the inputs of a third subtractor circuit whose PHN 8?90 29.2.1976 output is connected to a first input of the multiplier circuit, and that the output of a first adder circuit whose inputs are connected to the detectors situated at one side of the Y-axis and the output of a second adder circuit whose inputs are connected to the detectors situated at the other side of the Y-axis are connected to a, fourth subtractor circuit whose output is connected to a second input of the multiplier circuit.
3. An apparatus as claimed in Claim 1, comprising four detectors, characterized in that the outputs of the detectors situated in the first and in the third quadrant are connected to the said adder circuit and sub-tractor circuit via a second adder circuit, and the outputs of the detectors situated in the second and in the fourth quadrant are connected to the said adder circuit and subtractor circuit via a third adder circuit, and that a phase shifting network is included, in one of the connections be-tween the said adder circuit and one of the inputs of the multiplier circuit and between the subtractor circuit and the other input of the multiplier circuit.
29.2.1976
29.2.1976
4. An apparatus as claimed in Claim 1, which comprises two detectors which are disposed at one side of the Y-axis of the coordinate system, charac-terized in that one of the connections between the adder circuit and the multi-plier circuit and between the subtractor circuit and the multiplier circuit includes a phase-shifting network.
5. An apparatus as claimed in Claim 4, characterized in that the detectors have the shape of an isosceles triangle whose bases are parallel to the X-axis.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NL7600842 | 1976-01-28 | ||
| NLAANVRAGE7600842,A NL182258C (en) | 1976-01-28 | 1976-01-28 | DEVICE FOR READING A FLAT RECORD CARRIER WITH AN OPTICALLY READABLE INFORMATION STRUCTURE. |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1094684A true CA1094684A (en) | 1981-01-27 |
Family
ID=19825526
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA270,465A Expired CA1094684A (en) | 1976-01-28 | 1977-01-26 | Apparatus for reading a record carrier with an optically readable information structure |
Country Status (21)
| Country | Link |
|---|---|
| JP (1) | JPS5293222A (en) |
| AR (1) | AR211720A1 (en) |
| AT (1) | AT349231B (en) |
| AU (1) | AU505466B2 (en) |
| BE (1) | BE850781A (en) |
| BR (1) | BR7700452A (en) |
| CA (1) | CA1094684A (en) |
| CH (1) | CH613797A5 (en) |
| DE (1) | DE2701539C2 (en) |
| DK (1) | DK146106C (en) |
| ES (1) | ES455360A1 (en) |
| FR (1) | FR2339928A1 (en) |
| GB (1) | GB1534381A (en) |
| IT (1) | IT1074335B (en) |
| MX (1) | MX143161A (en) |
| NL (1) | NL182258C (en) |
| NO (1) | NO149188C (en) |
| NZ (1) | NZ183152A (en) |
| PL (1) | PL107679B1 (en) |
| SE (2) | SE408238B (en) |
| ZA (1) | ZA7711B (en) |
Families Citing this family (25)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS6038771B2 (en) * | 1978-06-19 | 1985-09-03 | パイオニア株式会社 | Servo device for optical information reading device |
| JPS573236A (en) * | 1980-06-10 | 1982-01-08 | Olympus Optical Co Ltd | Optical information reader |
| JPS57111837A (en) * | 1980-12-27 | 1982-07-12 | Olympus Optical Co Ltd | Signal detecting system of optical information reader |
| JPS5891538A (en) * | 1981-11-25 | 1983-05-31 | Hitachi Ltd | Optical disk track tracking device |
| NL8204555A (en) * | 1981-11-25 | 1983-06-16 | Hitachi Ltd | DEVICE FOR RECORDING AND DISPLAYING INFORMATION. |
| NL8105347A (en) * | 1981-11-26 | 1983-06-16 | Philips Nv | Apparatus for the optical scanning of a disc-shaped record carrier. |
| NL8105346A (en) * | 1981-11-26 | 1983-06-16 | ||
| JPS58125242A (en) * | 1982-01-22 | 1983-07-26 | Victor Co Of Japan Ltd | Tracking error detecting system of optical information signal reproducing device |
| JPS58150145A (en) * | 1982-03-03 | 1983-09-06 | Toshiba Corp | Tracking control circuit of optical digital disc player |
| JPS58208944A (en) * | 1982-05-31 | 1983-12-05 | Sony Corp | Generating device of optical head control signal |
| JPS5922242A (en) * | 1982-07-26 | 1984-02-04 | Foster Denki Kk | Circuit for detecting optical controlling signal in information reproducing device |
| JPS59110045A (en) * | 1982-12-13 | 1984-06-25 | Sanyo Electric Co Ltd | Optical pickup device |
| JPS59191144A (en) * | 1983-04-14 | 1984-10-30 | Sony Corp | Tracking servo circuit of optical pickup |
| JPS59198540A (en) * | 1983-04-26 | 1984-11-10 | Asahi Optical Co Ltd | Tracking servo circuit of optical disc player |
| US4740940A (en) * | 1985-03-11 | 1988-04-26 | Matsushita Electric Industrial Co., Ltd. | Optical information read apparatus |
| JPS61248237A (en) * | 1985-04-25 | 1986-11-05 | Sankyo Seiki Mfg Co Ltd | Detection of tracking error signal |
| JP2796592B2 (en) * | 1985-10-17 | 1998-09-10 | セイコーインスツルメンツ株式会社 | Tracking error signal generation circuit |
| JP2824573B2 (en) * | 1985-11-08 | 1998-11-11 | セイコーインスツルメンツ株式会社 | Tracking error signal generation circuit |
| JP2906144B2 (en) * | 1986-04-09 | 1999-06-14 | セイコーインスツルメンツ株式会社 | Optical head control device |
| US4958245A (en) * | 1988-04-07 | 1990-09-18 | Insite Peripherals, Inc. | Apparatus and method for optical servo control with media having information storage and servo control regions of different reflectivities |
| EP0686968B1 (en) | 1994-06-07 | 1999-01-07 | Matsushita Electric Industrial Co., Ltd. | Optical information recording medium, optical information recording method, and optical information reproducing apparatus using the same |
| TW364992B (en) * | 1996-04-25 | 1999-07-21 | Toshiba Corp | Optic data regeneration apparatus and control method thereof |
| DE19723542A1 (en) | 1997-06-05 | 1998-12-10 | Thomson Brandt Gmbh | Device for reading and / or writing to optical recording media |
| JP3544847B2 (en) | 1998-01-16 | 2004-07-21 | 三菱電機株式会社 | Optical information reproducing method and apparatus |
| CN101536096B (en) | 2006-11-16 | 2011-11-16 | 松下电器产业株式会社 | Optical information reproducing device tracking apparatus |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3381086A (en) * | 1962-08-16 | 1968-04-30 | Minnesota Mining & Mfg | Reproduction of television signals from photographic disc recordings |
| FR2235354A1 (en) * | 1973-06-29 | 1975-01-24 | Thomson Csf | Photoelectric optical test sensor - measures displacement of a diffraction screen with constant spacing in the test sensor plane |
-
1976
- 1976-01-28 NL NLAANVRAGE7600842,A patent/NL182258C/en not_active IP Right Cessation
-
1977
- 1977-01-03 ZA ZA00770011A patent/ZA7711B/en unknown
- 1977-01-15 DE DE2701539A patent/DE2701539C2/en not_active Expired
- 1977-01-24 MX MX167802A patent/MX143161A/en unknown
- 1977-01-25 AU AU21616/77A patent/AU505466B2/en not_active Expired
- 1977-01-25 SE SE7700725A patent/SE408238B/en not_active IP Right Cessation
- 1977-01-25 PL PL1977195541A patent/PL107679B1/en unknown
- 1977-01-25 NZ NZ183152A patent/NZ183152A/en unknown
- 1977-01-25 DK DK30777A patent/DK146106C/en not_active IP Right Cessation
- 1977-01-25 GB GB2943/77A patent/GB1534381A/en not_active Expired
- 1977-01-25 CH CH89377A patent/CH613797A5/en not_active IP Right Cessation
- 1977-01-25 NO NO770240A patent/NO149188C/en unknown
- 1977-01-25 IT IT19598/77A patent/IT1074335B/en active
- 1977-01-25 BR BR7700452A patent/BR7700452A/en unknown
- 1977-01-25 SE SE7700725D patent/SE7700725L/en not_active Application Discontinuation
- 1977-01-26 CA CA270,465A patent/CA1094684A/en not_active Expired
- 1977-01-26 AT AT46077A patent/AT349231B/en not_active IP Right Cessation
- 1977-01-26 BE BE174406A patent/BE850781A/en unknown
- 1977-01-26 ES ES455360A patent/ES455360A1/en not_active Expired
- 1977-01-26 JP JP686777A patent/JPS5293222A/en active Granted
- 1977-01-27 AR AR266331A patent/AR211720A1/en active
- 1977-01-27 FR FR7702258A patent/FR2339928A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| FR2339928B1 (en) | 1980-03-21 |
| AR211720A1 (en) | 1978-02-28 |
| NO149188C (en) | 1984-02-29 |
| MX143161A (en) | 1981-03-26 |
| SE7700725L (en) | 1977-07-29 |
| DK30777A (en) | 1977-07-29 |
| DK146106C (en) | 1983-11-21 |
| NL182258B (en) | 1987-09-01 |
| DK146106B (en) | 1983-06-27 |
| PL107679B1 (en) | 1980-02-29 |
| BR7700452A (en) | 1977-10-04 |
| NO149188B (en) | 1983-11-21 |
| NZ183152A (en) | 1980-04-28 |
| JPS5630610B2 (en) | 1981-07-16 |
| IT1074335B (en) | 1985-04-20 |
| JPS5293222A (en) | 1977-08-05 |
| ZA7711B (en) | 1978-08-30 |
| DE2701539A1 (en) | 1977-08-04 |
| AU2161677A (en) | 1978-08-03 |
| CH613797A5 (en) | 1979-10-15 |
| AU505466B2 (en) | 1979-11-22 |
| NL182258C (en) | 1988-02-01 |
| FR2339928A1 (en) | 1977-08-26 |
| AT349231B (en) | 1979-03-26 |
| DE2701539C2 (en) | 1981-11-12 |
| BE850781A (en) | 1977-07-26 |
| GB1534381A (en) | 1978-12-06 |
| NO770240L (en) | 1977-07-29 |
| NL7600842A (en) | 1977-08-01 |
| ATA46077A (en) | 1978-08-15 |
| SE408238B (en) | 1979-05-21 |
| ES455360A1 (en) | 1978-01-01 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CA1094684A (en) | Apparatus for reading a record carrier with an optically readable information structure | |
| US4057833A (en) | Centering detection system for an apparatus for playing optically readable record carriers | |
| CA1096494A (en) | Apparatus for reading a record carrier on which information for example video and or audio information is stored | |
| US4123652A (en) | Apparatus for reading a radiation-reflecting record carrier | |
| CA1137628A (en) | Record carrier with tracks of different phase depths | |
| US4953124A (en) | Opto-magnetic signal reproducing apparatus for reading, by differential detection using a magneto-optical effect, information magnetically recorded on a record medium | |
| US4455632A (en) | Apparatus for optically reading a record carrier with two types of information areas | |
| KR100252600B1 (en) | Optical pickup device, optical disk recording/reproducing apparatus, and method of generating focus error signal | |
| US5416755A (en) | Optical pickup using split beams impinging on different photo-detector areas | |
| US5255263A (en) | Information reading system and record carrier and reading device for use in such a system | |
| US4661944A (en) | Optical recording/playback apparatus having a focusing control system with reduced spot-offset sensitivity | |
| US5404344A (en) | Recording/reproducing optical head producing focusing error signal from zero-th order diffracted light and tracking error signal from first order diffracted light | |
| EP0116467A2 (en) | Optical disc players | |
| JPH04212742A (en) | Optical head and optical storing and reproducing device | |
| KR960016890B1 (en) | Optical scanning apparatus | |
| US5345072A (en) | Focus detection device for reading information from an optical recording medium | |
| CA1109560A (en) | Apparatus for reading a radiation-reflecting record carrier | |
| JP2877952B2 (en) | Optical scanning device | |
| US4712206A (en) | Tracking error signal detecting device with a two-beam system for use in an optical information memory | |
| US4845701A (en) | Optical disc player for optically reproducing recorded information on an optical information disc into an RF signal | |
| US5283772A (en) | Optical head | |
| KR100395587B1 (en) | Recording or playback apparatus for optical information media with different track widths | |
| US5787060A (en) | Optical data recording/reproducing apparatus for producing data and servo signals using different light receiving areas of a light receiver | |
| US5719830A (en) | High-density magneto-optical disk apparatus | |
| JP2594903B2 (en) | Focus error detector |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |