Note: Descriptions are shown in the official language in which they were submitted.
N
~/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.
-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
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
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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.
'
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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,
--1 2--
:
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
-13-
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
..
- 1 4-
~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
-- , -
- 1 6 -
,~ .
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.
-17-
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
-18-
109~684
PHN 8290
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 :
-21-
~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. .
-22-
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 .
..
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.
~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
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~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.
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~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
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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 :~
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~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.
..
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~ ' .
~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
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~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
,
--31--
~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.
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