Note: Descriptions are shown in the official language in which they were submitted.
PIIN. 8721.
VMI/1~JM/COBB.
24-4-1977.
Apparatus for reading a radiation-reflecting record
carrier.
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The invention relates to an apparatus for
reading a radiation-reflecting record c~rrier on which
information is stored in an opticall-y readable track-
~; wise arranged information structure, which apparatus
comprises a radiation source which prod~lces a r2ad beam,
an objective system for passing the read beam to a
rad.iation-sensitive de-tection system vla the record
carrier, which system converts the read 'beam which has
been rnodulated by the information structure into an
electri.cal signal, and an opto-elec-tronic focussing-
~'~ ' error detection system for determining a deviation
between the desired and the actual position of the plane
of focussing of the objective system, which focllssirlg-
error detection system comprises an astigmatic elem~ent
;~ 15 and a radiation-sensitive detector9 which detector
compr.ises for sub-detectors which are dispos~d in ~our
different quadrants of an imaginary X-Y coordinate
system, the X and th~ Y-a~'is being di.sposed at an angle
of 45 ~ith the astigrnatic focal lines of the astigmatic
20 ` el~me~t.
Such apparatus is described in Gerrnan
Patent Application No. 2,501,124 which has been laid
open for public inspection. I`his apparatus ls for example
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PHN. 8721.
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employed for reading a record carrier ~n which a
(colour) television programme is stored. The information
structure then consists of a multitude of areas alter-
nating wlth intermediate areas arranged in a spiral
track, which areas and intermediate areas influence
a read beam in different manners. The information is
for exan.ple contained in the lengths of the areas and
those of the intermediate areas. In order to obtain a
sufficiently long playing time with a record carrier
of limited size, the details of the informa-tion struct-
`; ure have to be very small. For e~ample, if a 30-minute
television programme is stored on one side of a disc-
shaped round record carrier in an annular area with an
outer radius of approx~mately 15 cm and an inner radius
of approximately 6 cm, the width o~ the tracks will be
approx. 0.5 /um and the average length of the areas and
o~ the intermediate areas will be approx. I /um.~
In order to enable these minute details
to be read an objective system with a fairly large
numerical aperture is to be used. Howe~er, the depth of
focus of such an objective system is small. ~s in the
read apparatus the distance between the plane of the
information structure and the objective system ma-y var~
by an amount greater than the depth of focus~ provisions
must be made so as to enable these variations -to be
detected and the focussing to be corrected.
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PHN. 8721.
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In accordance with the said German Patent
Application for this purpose a beam of radiation which
has been reflected by -the record carrier is made astig-
matic, in that a cyliIldrical lens is arranged in the
path of this beam behind the obJective system. Between
the focal lines of the astigmatic system constituted
by the objective system ancl the cylindrical lens a
radiation-sensitive detector which consists of four
sub-detectors is disposed. When the posi-tion of the
plane of the information struc-ture relative to the
objective system changes, the shape of the image formed
on the sub-detectors also changesO This change in shape
~ can be detected by combining the output signals of the
;~ ~ sub-detectors in a suitable manner.
15~ ~ ~ When the known read apparatus is used
practical problems may arise. The radiation distribution
across the composite detector also depends on the dif-
fraction of the radia-tion beam by the details of the
nform~tion structure. ~irst of all~ diffraction is ob-
20 ~ talned in the longltudlnal direction~of a track to be
read,~owing to the sequence of areas and intermediate
; areas. The variations in the radiation distribution
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across the composite detector owing to this dlffraction
havc a high frequcncy relative to the focussing errors~
` Z5 .so that the influence of this diffractio~ on the focus-
si~g control signal can be eliminated electronically.
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PHN. 8721.
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Furthermore, diffraction occurs in a
d:irection transverse to the track direction. This
diffraction depends on the position of the radiation
spot formed on the record carrier relative to the
centre of a track to be read. During assembly of the
read apparatus the focussing-error detection system
will have to be adjusted so that when the centre of the
radiation spot coincides with the centre of a track to
be read and the focussing is correct, the focussing
error signal is zero. However, if during read-out of
the record carrier the read spot moves from the centre
of a track to be read towards its edge, the radiation
distribu-tion across the composite detector will change
; ~ independen-tly of the focussing. This gives rise to an
erroneous focussing error signal, 50 that the focussing
of the objective system is adjusted incorrectly.
The read apparatus may also comprise a
posltlonal-error detection system for determing a
deviation between the centr0 of a radiation spo t formed
on the record carrler and the centre of a track to be
read, and a servo-syst0m for correct:lng the position
of the centre of` the read spot. Th~ positional error
;~ ~ si~nal also depends on the focussing of th0 obJ0cti~e
syste~n. If the focussing LS incorr0ct th0 position of
the radiation spot will also b0 adjusted incorr~ctly~
As a result of this the~ focussing error in its turn
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PHN. o721.
24-Ll-1977.
increases again. It may then happen that both the
; servo-system for focussing and the servo-system for
correcting the position of the radiation spot relative
to a track to be read become deranged.
When the optical rsad unit, on its way
to a specific passage in the programme stored, is moved
radially across the record carrier with a high speed,
the edges of each track which is traversed will give
rise to a change in the focussing error signal. The
servo-system .for the focussing then intervenes unneces-
sarily and with a high frequencyO When the focussing
of tha objective system is corrected to moving said
system, the high-frequency control signals which are
~pplied to the drive means of the objective system,
give rise to an annoying acoustic "rattling". Moreover,
the drive means than consumes additi.onal power.
As the width of a track is smaller than
the distance between the tracks the radiation spot will
on the average be shorter at the tracks than hetween
the tracks, when the read unit is rapidly moved trans-
versely to the track direction. As a resul-t of this
-the average focussing error signal will neither be zero
in the case o.f a correct focussing. Consequently, the
~ focussing will be adjusted incorrectly on the averag~c.
The influence of the diffraction o~ the
radiation beam transverse to the track di.rec-ti.on becomes
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PHN. 8721.
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in particular apparent when a track portion -to be read
which is projected on the surface of the sub-detectors,
- hereinafter to be referred to as -the "effective track
direction~, makes an angle of 45 with the X and the
Y-axis of the composite detector.
It is an obJect of the present invention
to provide an embodiment of the known read apparatus
in which the ~if~raction in a direction transverse to
the track direction has no influence on the focussing
error signal. For this the apparatus in accordallce with
the invention is characterized in that one of the a~es
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of the co-ordinate system in whose quad~ants the sub-
- detectors are disposed, is parallel to the effective
track direction, and that the astigmatic focal lines
are disposed at an angle of 45 relativ~ to the effect-
ive track direction.
The invention will now be described in
more detail with reference to the drawing, in which
Fig. 1 shows the read apparatus in accord-
20 ~ ance with the invention, and
~igs 2a, 2b and 2c show how the shape of
the spot imaged on the detector varies as a function o~
the focussing.
In the apparatus of Fig. 1 a round disc-
2~ shaped reco*d carrier is designated 1. The :inf'ormatiorl
structure is fo-r example a phase structure and compI1i ses
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PHN. 8721.
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a multitude of concentric or quasi^concentric tracks
7, which tracks consist of a sequence of areas ~ and
intermediate areas t. The areas may for example be dis-
posed a-t an other level in the record carrier -than the
intermediate areas, The information may for example
be a (colour) television programme, but alternatively
other in~ormation such as a multitude of different
imagss, or digital information.
The record carrier is illuminated by a read
beam 3 which is obtained from a radiation source 4,~or
example a c.w.-laser. An objective system, which for
simplicity is represented by a single lens 5, focusses
the read beam to a read spot V on the plane of' the
tracks 2. The focal length of the auxiliary lens 6
has been se1ected so that the pupil of the objective
s~stem is adequately filled. The read beam is reflect-
ed by the record carrier and is then modulated in ac
cordance with the information ~stored in a track portion
to be read. ~or separating the incident (unmodulated)
and the reflected (modulated) read beam the radiation
path includes a beam splitter 8, for example in the
form of a partly -transparent mirror. The beam splitter
directs the modulated read beam towards a radiation-
sensitive detector 9. This detector is cOnnected to
an electronic circuit 10 which derives a high frequeney
information signal~Si and, as wlll be explained herei.n-
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24-LI-1977.
after, a focussing error signal S~ of lower frequency.
In order to enable focussing errors to be
detected the radiation path behind the beam splitter
8 includes an astigmatic element 11. This element, as
is shown in ~ig. 1, may be a cylindrical lens. It is
also possible to obtain astigmatism in a different
manner~ for example with a plane transparent plate
which is obliquely disposed in the beam or with a lens
which is tilted relative to the beam~ Instead o~ one
focus point an astigmatic system has two astigmatic
focal lines which, viewed in an axial direction, oc-
cupies different positions and ~hich are perpendicular
- to each other. Thus~ the objective system and the cy-
lindrical lens add two focal lines 12 and 13 to the read
spot V~ The radiation-sensitive detector 9 is now ar-
ranged in a plane which viewed along the optical axis
is disposed between the lines 12 and 13, preferably
at the location where the dimensions, in two mutua:l]y
perpendicular direc-tions, of the image spot added to
the read spot V are equal to the highest possib~e degree
in the case of correct focussingO
In order to enable the shape of the spot
V' and thus the degree of focussing, to be detectcd,
the detector 9 comprises four sub-detectors which are
~ disposed in the four q~adrants of an X-Y co-ordinate
~ system. Fig~res 2a, 2b and 2c show arl elevation Or -the
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PHN. 872l.
Zl~ _ 1 977.
four sub-detectors ~, B, C and D in accordance w:ith
the line 2, 2' in ~ig. 1, with the various shapes of
the spot V~ projected on them for different values of
the distance between the objective system and the
plane of the tracks. The X and the Y-axis make an angle
of 45 with the axis 15 of the cylindrical lens, i.e.
with the astigmatic focal lines 12 and 13, whilst the
X-axis is now parallel to the effective track direction.
Fig. 2a represents the situation that the
distance between the objective system and the plane
of the tracks is correct. If this distance is too ~reat,
the focal lines 12 and 13 are nearer the cylindrical
lens 11. The de-tector 9 is -then nearer the focal line
13 than near the focal line 12. The spot V' then has
a shape as is shown in ~g. 2b. If the distance between
the objec-tive system and the plane of the tracks is too
small, the focal lines 12 and 13 are farther from the
cylindrical lens, and the focal line 12 is nearer the
detector 9 than the focal line 13. The spot V~ then
has a shape as shown in Fig. 2c.
T~ the signals supplied by the sub~detectors
A, B. C and D are represented by S~, S~, Sc and SD,
the fOCUssiIIg error signal Sf, is given by;
Sf = (S~ + SCj - (SB ~ SD).
2S~ It is evident that in the situation of Fig. 2a
~ S~ + Sc~ SB + SD~ so that S~ . 0. For the situation of
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Fig. 2b and ~ig. 2c Sf is negative and positive respect-
ively. By adding the signals SA and Sc -to each other,
as well as the signals SB and SD9 and subtracting the
- sum signals thus obtained from each o-ther, an unambi-
- 5 guous focussing error signal is obtained. This signal
can be processed electronically, in a manner known ~
se, to a ~ocussing control signal ~ith which the focus-
sing of the objective system can be corrected~ for e~ample
by moving the objective system relative to the plane of
the tracks ~ith the aid of an electromagnetic coil.
When the radiation spot V is moved in a
direction transverse to the track direction, the amount
of radiatlon on for example the detectors A and B may
increase, relative to that on the detectors C and D,
independently o~ the focussing. Ilowever, as the focus~
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slng error slgnal Sf is derived by subtracting the sig--
nals S~ and S~ from each other, the influence of said
movement on the signal Sf is negligible.
~ Finally, it is to be noted that the beam
b may also be used for reading the information on the
record carrier. The in~ormation signal Si can be obtain-
ed by for example adding the signals of the four sub-
detectors to each other. The variations i.n the radiat'lon
distribution across the'sub-detectors as a result of a
~ mo:~ement of the radiation spot V in a direction trans--
~ ~ verse to the track dLrection then have a substantlall~
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P~IN. 8721.
24-4-1977.
lower frequency than the variations in the radiation
distribution as a result of the sequence of areas and
intermediate areas in a track. By passing the sum sig-
nal SA ~ S~ ~ Sc ~ SD through a high-pass filter the
influence of this movement trans-verse to the track
direction on -the signal Si can be eliminated.
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