Note: Descriptions are shown in the official language in which they were submitted.
PHN. 10.121
The invention relates to a record carrier having
an information structure which comprises optically readable
information areas arranged in information tracks, adjacent
information track portions c~iffering from each o-ther in tha-t
they comprise information areas of a first phase depth and
information areas oE a second phase depth respectively. The
invention also relates to apparatus for reading such a record
carrier.
Such a record carrier and apparatus for reading
this record carrier are described in the Applicants'
Canadian Patent 1,137,628 - issued December 14, 1982
(PHN 9083). In the record carrier described therein the :Eirst
phase depth is preferably approximately ~ rad. and the second
phase depth approximately 2 ~ rad..
When the inf:orma.tion structure is scanned with a
read beam, this beam is split into a zero-order subbeam and
a plurality of higher order subbeamsA The phase depth is
defined as the difference between the phase of the zero order
subbeam and the phase of one of the first order s~bbeams ~hen
the centre of the read spot formed on the information struc~
ture coincides with tlle centre of an information area. In
said Canadian Patent 1,137,62~ supra (P~IN 90~3) it i.s
demonstrated that if the informati.on areas of each time two
adjacent informa~ion tra.ck poXtions have different phase
depths, these track portions can be arranged more closely to
each other tllan in the case that the information structure
comprises information. areas which all have the same phase
depth. The information content of a record may then for
example be doubled, without any significant increase in
cro~s-talk between adjacent track portions.
However, the information track portions of dif-
ferent phase depths should ~hen be read in different
manners~ The infor~ation trac~ portions with the gxeatex
phase depths are read by determining the variation of the
total intensity of the radiation received from the record
,~
PHN. 10.121 2
carrier and passing through the pupil o~ the read objective.
This is the so-called integral or central aperture read
method. The information track portions with the smaller
phase depth are read by determining the difference of the
intensities in two tangentially different halves of the pupil
of the read objective. This is the so-called differential
read method.
It has been found that when an information track
portion with the greater phase depth is read by the integral
method there is ne~ertheless some cross-talk from an adjacent
information track portion having the smaller phase depth.
It is the object of the present invention to eli-
minate this residual cross-talk. In accordance with a first
aspect oE the invention the record carrier is therefore char-
acterized in that the diffexence between the first and thesecond phase depth is substantially _~ rad.
When the difference in phase depths is thus chosen,
the desired cross-talk reduction can be achieved by applying
an additional electronic phase shift of one detector signal
or of both detector signals.
It is possible to adapt only the greater phase
depth, for example, to ma]ce it 7 7~ rad., and to maintain
the smaller phase depth a-t the ~alue oE 2 ~ rad. as speciEied
in Canadian Patent l,137,628 (PHN 9083). The information
track portions ~ith the greatex phase depth should then be
read in accordance with the integral method and the informa-
tion track portions With the smaller phase depth in accordance
with the differential method As the two read methods have
different optical transfer functions ("modulation transEer
function"; "M.T~F."), the alternate use of the two read
methods could become perceptible in the signal which is
ultimately supplied by the read apparatus. Moreover, the
information areas with lower spatial frequencies can
PHN 10121 3 10.9.1981
no longer be read in an optimum manner when the differential
method is used.
Preferably, the lnformation areas are therefore di-
mensioned so that they can all be read by means of the inte-
- 5 gral method. The preferred embodiment of the record carrier
is characterized in that the first phase depth is approximately
J ~r rad and the second phase depth approximately 31T rad..
4 4
The two phase depths may be realized in different
lO manners, for example by areas with different refractive in-
dices. Suitably, the information areas comprise pits or hills.
The advantage of this is that the record carriers can be
manufactured in large ~uantities using pressing techniques.
In this case of information areas in the form of hills or
pits the phase depth is related to a geometrical depth or
heigth. In the case of pits or hills with steep walls the
phase depth is mainly determined by the geometrical depth or
heigth. If the walls of the pits or hills are less steep,
the phase depth is also determined by the angles of inclina
20 tion of said walls.
In accordance with a further ~2ac~ristic feature of
the record carrier consecutive track portions within one
information track differ from each other in that they com-
prise information areas oE the first phase depth and informa-
25 tion areas of the second phase depth respectively. Thisenables the visual efEect of transitions betw~en the two
types of information areas ;n the signal which is ul-timately
supplied by the read apparatus to be reduced.
For a correct timing of the desired electronic
30 phase shit during read-out of the record carrier, in accor-
dance with a further characteristic feature, the record
carrier may contain a pilot signal in addition to an informa-
tion signal, which pilot signal identifies the transitions
between information areas of the first phase depth and infor~
35 mation areas of the second phase depth and vice versa.
In accordance with a second aspect of the invention
an apparatus for reading a record carrier containina informa
tion areas of two different phase depths, which apparatus com-
PHN. 10.121 4
prises a radiation source producing a read beam, an objec-
tive system for focusing the read beam to a read spot on the
information structure, and two radiation-sensitive detectors
which are disposed in the far field of -the information struc-
ture one on each side of a line which is effectively transverse of the track direction, the outputs of the two detec-
tors being connected to an adder circuit, is characteri~ed
in that at least one of the detectors is connected to the
adder circuit via a phase-shifting element, which element sub-
jects the detector signal to a phase shift of constant magni-
tude.
If the two phase depths of the information areas
have been selected so that the entire information structure
can be read by means of the integral method, the phase
shifting element should i.ntroduce a phase shift which is
equal to the difference between the two phase depths, viz. a
phase shift of approximately 2 rad.
Alternati~ely, the two phase depths may be selected
so that one -type of information areas is adapted to be read
with the integral method, whilst the other type of information
areas is adapted to be :read with the differential method.
A read apparatus Which is adapted to read such a reco.rd car-
rier is characteriz.ed in that the outputs of the two detectors
are also connected to a sub-tractor circuit, that the outputs
of the adder circuit and the subtractor circuit are connected
to a signal processing circuit ~ia a switching element, and
that a control input of the switching element is connected to
an electronic circuit in. which a switching signal is derived
from the signal which is read from the record carrier~ This
apparatus is not only suitable for reading an information
structure in which phase depths of ~ rad. and ~ rad.
occur, but may also be used for reading the record carrier
wllich is described in Canadian Patent 1,137,628 (PHN 9083),
i~e. a record carrier with phase depths of ~ rad. and of
2 rad. In that case a phase-shifting
PHN 10121 5 10.9.1981
element is included only in cne of the connections between
the detectors and the adder circuit, whilst the detectors are
connected directly to the subtractor circuit. In apparatus
for reading a record carrier with phase depths of 7~ rad,
5 6
and of 27~ rad,at least one detector is connected both to the
adder circuit and the subtractor circuit via a phase-shifting
- element. In the two last-mentioned apparatus the phase-
lO shifting element introduces a phase shift of approximately
7~ rad..
For reasons of symmetry it is preferred, both in an
apparatus which solely employs the integral read method and
in an apparatus which employs both the integral read method
15 and the differential read method, to connect each of the
detectors vi a a p ha s e sh i f t i n g eleme r,t
the adder circuit only or to both the adder circuit and the
subtractor circuit. Said element should then introduce
phase shifts which are equal but of opposite sign. In the
20 apparatus which only employs the integral read method, the
phase-shifting elements should moreover be ad~ustable in such
a way that the signs of the two phase shifts can be changed.
In order to ensure that the cross-talk reduction in
accordance with the invention is still operative at smaller
25 spatial frequencies of the information areas, the detectors
are preferably each disposed against an edge oE the effective
pupil of the objective system. The efEective pupil is to be
understood to mean the image of the pupil in the plane of
the two detectors.
30The invention will now be described in more~detail
with reference to the drawing. In the drawing :
Figure 1 is a plan view of a part of a first embodi-
ment of a record carrier,
Figure 2 is a tangential sectional view of said
35 record carrier,
Figure 3 is a radial sectional view of said record
carrier,
PHN 10121 6 10.9.1981
Figure 4 is a plan view of a part of a second
embodiment of a record carrier,
Figure 5 is a tangential sectional view of said
- record carrier,
Figure 6 is a radial sectional view of said recorcl
carrier,
- Figure 7 shows an embodiment of a read apparatus,
Figure 8 shows the arrangement of the detectors
relative to the various diffraction orders,
Figure 9 shows a first version of the electronic
circuit for processing the detector signals,
Figure 10 shows a second version of said electro-
nic circuit,
Figure 11 shows a third version of said electronic
lS circuit, and
Figure 12 represents the waveform of a radial error
signal in an embodiment of a servo system for controlling the
radial position of the read spot.
In these Figures similar elements always bear the
20 same reference numerals.
Figures 1, 2 and 3 show a first embodiment of a
record carrier in accordance with the invention. Figure 1 is
a plan view, Figure 2 a tangential sectional view taken on
the line II-II' in Figure 1, and Figure 3 a radial sectional
25 view taken on the line III-III' in Figure 1 of the record
carrier. The information is contained in a multitude of in-
formation areas ~, or example pits in the substrate 6. These
areas are arranged in accordance with tracks 2. Between the
information areas 4 intermediate areas 5 are interposed. The
3n tracks 2 are spaced by narrow lands 3. The spatial frequency,
and as the case may be the lengths, of the areas is determi-
ned by the information.
The areas of the adjacent information tracks have
different phase depths. As is shown in Figure 3, the pits of
35 a first track, a third track etc. are therefore deeper than
the pits 4' of the second track, the fourth track etc. The
geometrical depths of the pits 4 and 4' are designated d1 and
. d2. Owing to the different depths the first track, the third
P~N 10121 7 10.9.1981
track etc. can optically be distinguished from the second
track, the fourth track etc. This enables said tracks to be
packed more densely.
In a practical embodiment of a record carrier in
- 5 accordance with the invention the radial period of the in-
formation tracks was 0.85/um, the width of these tracks was
0.5/um and the width of the lands 3 was 0.35/um.
The information carrying surface of the record
carrier can be made reflecting, for example by vacuum-depo-
10 sition of a metal layer 7 such as aluminium, on said surface.
It is to be noted that the size of the areas inthe Figures 1, 2 and 3 has been exaggerated for the sake of
clarity.
Figure 4 is a plan view of a part of a second em-
bodiment of a record carrier in accordance with the invention.
This Figure shows a larger part of the record carrier than
Figure 1, the individual information areas being no longer
discernible. The information trac]cs are now divided into
portions a and b, the portions a comprising information areas
20 of greater phase depth (deeper pits) and the portions _ in-
formation areas of smaller phase depth.
In Figure 5, which is an enlarged tangential sec-
tional view of a track taken on the line V-V' in Figure 4,
the pits of the depth d2 are again designated 4' and the
25 pits of the depth d1 are designated 4.
Figure 6 is a radial sectional view, taken on the
line VI-VI' in Figure 4, of the second embodiment of the
record carrier.
In Figures 1 through 6 the information areas have
30 perpendicular walls and the phase depth is dictated by the
geometrical depth of the information areas. In practice the
information areas will have oblique walls. The phase depth
is then also determined by the angles of inclination of said
walls.
Figure 7 shows an embodiment of an apparatus for
reading a record carrier. The round disc-shaped record carrier
is shown in a radial sectional view. The information tracks
thus extend perpendicularly to the plane of the drawing~ It
P~IN 10121 8 10.9.1981
is assumed that the information structure is disposed on the
upper side of the record carrier and is reflecting, so that
reading is effected through the subs-trate 6. The information
structure may furthermore be covered wi-th a protective layer
6. The record carrier can be rotated by means of a spindle 16,
which is driven by a motor 15.
A radiation source 10, for example a helium-neon
- laser or a semiconductor diode laser, produces a read heam 11.
A mirror 12 reflects this beam to an objective system 13,
which is schematically represented by a single lens. The path
of the read beam includes an auxiliary lens 14, which ensures
- that the pupil of the objective system is filled in an opti-
mum manner. A read spot V of minimal dimensions is then
formed on the information structure.
The read beam is reflected by the information
structure and, as the record carrier rotates, is modulated in
accordance with the sequence of the information areas in the
information track to be read. By moving the read spot and the
record carrier relative to each other in a radial direction,
20 the entire information surface can be scanned.
The modulated read beam again traverses the ob-
jective system and is again reflected by the mirror 12. The
radiation path included means for separating the modulated
and the unmodulated read beam. These means may for example
25 comprise a polarization-sensitive splitter prism and al/4
plate (wherein ~ is the wavelen~th of the read beam). For the
sal~e of simplicity it is assumed in Figure 7 that the said
means are constituted by a semitransparent mirror 17. This
mixror reflects the modulated beam to a radiation-sensitive
30 detection system 20.
This detection system comprises two radiation-
sensitive detectors 22 and 23, which are disposed in the so-
called "far field of the information structure", i.e. in a
plane in which the centroids of the subbeams formed hy the
information structure, specifically of the zero-order sub-
beam and the first-order cubbeams, are ceparated. The de-
tection system may ~e disposed in the plane 21 in which an
image of the exit pupil of the objective system 13 is formed
P~ 10121 9 10 ~.1981
by the auxiliary lens 18. In Figure 7 the image C' of the
point C of the exit pupil is represented by dashed lines.
The information structure which comprises adjacent
information tracks, which tracks comprise information areas
and intermediate areas, behaves as a two-dimensional diffrac-
tion grating. This grating splits the read beam into a zero-
- order subheam, a plurality of first-order subbeams and a
plurality of higher-order subheams. After being reflected
by the information structure a part of the radiation re-enters
lO the objective systemO In the plane of the exit pupil of
the objective system, or in the plane in which an image of this
exit pupil is formed, the centoids of the subbeams are sepa -
rated from each other. Figure 8 represents the situation in
the plane 21 of Figure 7.
The circle 40 with the centre 45 represents the
cross-section of the zero order subbeams in this plane. The
circles 41 and 42 with the centres 46 and 47 represent the
cross-sections of the tangentially diffracted subbeams of the
orders (+1, 0) and (-1, 0). The X-axis and the Y-axis in
20 Figure 8 correspond to the tangential direction, or the
track direction, and the radial direction, or the direction
transverse of the track direction, on the record carrier. The
distance f from the centres 46 and 47 to the Y-axis is deter-
mined by A/p, where p is the local spatial period cf the
25 information areas in the information trac]c portion to he read
and A the wavelength of the read beam.
For reading the informa-tion use is made of the
phase shifts of -the subheams of the (+1, 0) and ~-1, o)
orders relative to the zero-order subheam. In the hatched
30 areas in Figure 8 said first-order subbeams and zerc-order suh-
beam cverlap, so that interference occurs. The phases of the
first-order subbeams vary with high frequencies as a result
of the tangential movement of the re;~l spot re1ative to the
information track. This results in the intensity variations
35 in the exit pupil, or in the image thereof, which variations
can be detected by the detectors 22 and 23.
When the centre of the read spot coincides with the
centre of an information area a specific phase difference ~
PHN 10121 10 10,9.1981
will occur between the first-order subbeam and the zero-
order subbeam. This phase difference is called the phase
depth of the information area. At the transition of the read
spot from a first information area to a second information
area the phase of the subheam of the (+1, 0) order increases
by 2 r~ . Therefore, when the read spot moves in a tangential
direction the phase of said subbeam relative to the zero~
order subbeam will vary by W t. Here, W is a time frequency
which is determined by the spatial frequency of the informa-
tion area and by the speed with which the read spot travelsover the track.
The phases ~(+1, 0) and ~(-1, 0) of the first-
order subbeams relative to the zero-order subbeam may be re-
presented by :
a (+1, o) -~+ ~t
~ (-1, O) = ~- Wt
The intensity variations as a result of interference of the
first-order subbeam with the zero-order subbeam are converted
into electric si~nals by the detectors 22 and 23. The time-
20 dependent output signals S23 and S22 of the detectors 23 and22 may be represented by :
S23 = B (~) cos (~ + ~ t)
S22 = ~ (~) cos (~ - ~t)
Here B ( ~) is a factor which is proportional to the geome-
25 trical depth of the pits. For ~ it may be assuemd
that B ( ~) is zero.
In a first embodiment o a record carrier in accor-
dance with the invention the phase depth ~1 cf the information
30 areas 4 is 7 r~/6 rad.and the phase depth ~2 of the information
areas 4' is ~ l~3 rad. In the apparatlls forreading said re -
cord carrier, as is shown in Figure 9, the outputs of the
detectors 22 and 23 are connected to the phase shifting ele -
ments 2~ and 25. The element 24 shifts the phase of the detec-
tor S22 through + ~ rad, whilst the element 25 shifts the phaseof the detector signal S23 through - ~ rad. The signals S22
and S23 then change to :
9~l~
PHN 10121 11 10.9.1981
S 23 = B (~).cos~r+ (~ t - ~ = B (~).cos (~y-~ t - ~)
S'~2 = B (~).cos~ - (~ t-~0)~ = B (~).cos (~- ~t - ~)
When the information areas oE an information track
portion being read have the greater phase depth ~P1 = 7~;/6 rad.
- the signals S'22 and S'23 should be added, whilst if the
informatior- areas of the information track portion being read
have the smaller phase depth, ~2 - 2~~/3rad, the signals
S'22 and S'23 should be subtracted from each other. For
this purpose, as is shown in Figure 9, the signals S'22 and
S'23 may be applied both to the adder circuit 26 and to the
subtractor circuit 27. The outputs o the circuits 26 and 27
are connected to the two input terminals e1 and e2 of a
switch 28 haviny one master terminal e. Depending on the control
signal Sc applied to its control input said switch transfers
either the sum signal of the detectors 22 and 23 or the
difference signal of said detectors to a demodulation cir-
cuit 29. In this circuit -the signal read is demodulated and
20 rendered suitable for reproduction with for example a television
set 30.
For controlling the switch 28 a control signal
should be generated. In addition to the actual information
signal the record carrier may contain a pilot signal, which
25 indicates the positions on -the record carrier where a transi-
tion OCCUl-S from the information areas of a first phase depth
to the information areas of a second phase depth. If a tele-
vlsion signal has been recorded, one television signal being
recorded per information track revolution, the picture syn-
30 chronizing pulses or field synchronizing pulses contained in
the actual television signal may be employed for generating
the control signal Sc and no separate pilot signal is needed.
The pilot signal may be needed if an audio signal has been
recorded.
If the information of the lines of a television
picture is contained in track portions a and b in accordance
with Figure 4, the line synchronizing pulses 32, as shown in
Figure 9, may be extracted from the signal from the demodula-
PHN 10121 12 10.9.1981
tion ircuit 29 in the line synchronizing pulse separator 31.
Inthe circuit, 33, which is for example a bistable multivi-
brator, the pulses 32 are converted into a control signal Sc
For the switch 28, so that said switchis changed over each
time after reading one television line.
If each information track of the information struc-
ture contains only one type of areas, the element 31 is a
picture synchronizing pulse separator a,nd the switch 28 is
changed over after read-out of each information track or two
lO television fields.
When point e2 in the switch is connected to point
e, the so-called integral read method is employed. The signal
applied to the demodulator 29 may then be expressed by :
SI = S'23 t S'22 = 2-B (~)-cos (~ -0)-cos (W t)-
If point e is connected to point e1, reading is effected in
accordance with the so-called differential method. The
signal applied to the demodu]ator may then be expressed by :
D S 23 S 22 = -2.B (~ ).sin (~ - 0).sin ( ~ t).
The integral method is employed when reading information areas
20 having a phase depth ~1 = 7 ~/6 rad. The signal SI is then a
maximum if cos (~1 ~ 0) = 1, l.e. if 0 = l~/6 rad. For the
information areas of the phase depth ~2 = 2~C/3 rad,
cos (~ - 0) = 0. Thus, when reading in accordance with the
integral method, the inormation areas of the smaller phase
25 depth are not "observed". Conversely, when the differential
read method is used the information areas ~' of a phase depth
~ 2 = 2 r~/3 rad. wlll be read in an optimum manner, for
sin ( 2~C- 0 ) is then 1, whilst the information areas 4 of
30 the phase depth ~1 = 7~ rad. are then not "observed", for
sin ( 7~- 0) is then 0. 1,
Instead of the two phase-shifting elements 24 and 25
35 it is alternatively possible to use the phase shifting ele-
ment 25 only. If the phase shift 0 of said element is selected
to be ,~/3 rad., the same result is obtained.
By means of an apparatus in which one detector slgnal
PHN 10121 13 10.9.1981
or both detector signals are s~kjected to an additional phase
shift, it is also possible to obtain a substan-tial improve-
ment of the read-out of the record carrier described in
Netherlands Patent Application no. 78 03517, l.e. of the record
carrier having the phase depths ~ rad and ~ 2 = ~ rad..
The apparatus adapted for reading this record carrier is shown
in Figure 10.
The signals from the detectors 22 and 23 are applied
lO directly to the subtractor circuit 27. In the connections
between said detectors and the inputs of th^e adder circuit 26
phase shifting elements 24 and 25 are included, which intro-
duce a constant phase shift of ~ 0 rad. and - 0 rad. respec-
- tively. During differential read-out of the information areas 15 of the phase depth ~2 = 2l~ rad., the information areas of
the phase depth ~ rad. will produce no cross-talk. The
cross-talk from the information areas of ~2 = 2~ rad. during
20 read-out by the integral method of information areas of ~1 =
~I rad. can be substantially eliminated if 0 = 1~ rad..As
a result of this phase shift the amplitude of the signal SI de-
creases slightly, but is still sufficiently high. It is al-
25 ternatively possible to employ the ~hase shifter 24 only,which should then introduce a phase shift of l~ rad,
For the values of the phase depths ~ 2 and the
phase shift 0 specified in the foregoing, the integral read
30 method and the differential read method m-st be used alter-
nately. However, these two methods havedifferent optical modula-
tion transfer functions. If a video signal is stored on the
record carrier the one transfer function will for example
cause different grey shades or a different colour saturation
35 in the ultimate television pictu e than the other transfer func-
tion. In the case of an audio signal in the form of a frequen-
cy~modulated s~gna', switching between the transfer functions
m~y become audible as an undesired frequency.
PHN 10121 14 10.9.1981
Furthermore, for reading lower spatial frequencies
of the information areas, the transfer function of the dif-
ferential method is worse than that of the integral method,
Suitably, the phase depths ~1 and ~2 are therefore
selected so that they are symmetrical re]ative to ~ rad..
The phase depth ~1 is then 51~ rad. and the phase depth ~ 2
is then 3~ rad,,The magnitude of the phase shift 0 is then
l~ rad..
Figure 11 shows a signal processing circuit of an appa-
ratus for reading this record carrier. The detectors 22 and
23 are each connected to a phase shifting element 24 and 25
respectively. The element 25 introduces a phase shiEt -
15 and the element 24 a phase shift -~ 0 , the magnitude of 0
being r~4 rad..The sign of ~ should now bech~ed at the transi-
tion from information areas of the greater phase depth to
information areas of the smaller phase depth and vlce versa.
When reading the information areas of the greater phase depth
20 0 = + ~/4 rad.and when reading information areas of the
smaller phase depth ~ = - r~4 rad.,For changing the sign of the
phase shift ~ it is again possible to employ the signal Sc.
The information signal SI is always given by :
I S 23 + S 22 = 2.B (~).cos (~- 0 ).cos (W t).
When the information areas 4 of the phase depth ~1 = 5 ~/4 rad.
are read, then ~ = + ~/4 rad..Then, cos (~ /4) is equal
to 1. For the information areas 4' of the phase depth ~2 =
30 3 ,~/4 rad., cos ( ~2 ~ ~/4) is equal to 0, so -that these
information areas will produce no cross-talk. When the informa -
tion areas 4' are read ~ /4 rad.and cos (~ 2 + J~/4) is
1, whilst cos ( ~1 + ~Z/4) is 0, so that the information areas 4
of the greater phase depth are not "observed" and thus pro-
35 duce no cross-talk.
The values for the phase depths specified in the fore-
going are no strict values. Deviations of the order of some
degrees are permissible.
PHN 10121 l5 10.9.1981
It is possible that the difference between the phase
depth ~11 and ~2 deviate~ from ~L rad..However, by adapting
the electronic phase shift ~ it is still possible to ensure
- 5 that the cross-talk between adjacent information track portions
is mini~ized
So far only the tangentially diffracted first~order
subbeams have been discussed. The information structure also
diffracts the read radiation in higher tangential orders
lO and in various radial and diagonal orders. The information areas,
which for the tangential first orders exhibit a difference be-
tween the phase depths ~1 and ~2 of l~ rad.~ however, will also
exhibit such a phase depth difference for the higher tangential
15 orders and for the radial and diagonal orders. The subbeams
which are diffracted otherwise than in the tangential first
orders will not significantly influence the effect of cross-
talk reduction and need not be further considered.
In the foregoing it has been assumed that the signals
20supplied by the detectors have a fixed phace difference which
is determined by the phase depth of the information areas. By
influencing this phase difference with the aid of an electronic
phase shifter, the signal produced by said information areas
can be maximized during read-out of information areas of a
25first phase depth and the signal from information areas with
a second phase depth can be minimized. It ls then assumed
that the detector 22 only receives the beam 42 and the detec-
tor 23 only receives the beam 41. At lower spatial frequencies
of the information areas, i.e. at greater periods p of said
3~areas, the distance f in Figure 8 becomes smaller and the
first-order beams 41 and 42 will overlap each other. The
detector 22 or 23 would then no longer receive radiation of
the respective beam 42 or 41 only, but also radiation of the
beam 41 and 42 respectively. The phases of the first-order
35beams could then no longer be influenced individually, so
that no cross-talk reduction in accordance with the inven-
tion could be obtained. In order to enable a satisfactory
cross-talk reduction to be realized at lower spatial frequen-
Pl-~ 10121 16 10.9.1981
cies, the radiation-sensitive areas of the detectors, in-
stead of being disposed as closely as possible to each
other and in the centre of the pupil, as is shown in Figure
8 by the uninterrupted lines, are arranged as far as possible
5 from each other and at the edge of the pupil. In Figure 8
the last-mentioned positions of the detectors are represented
by the dashed lines 22' and 23'. The limit for the spatial
frequencies at which the detector 22 only receives the beam
42 and the detector 23 only the beam 41, is then considerably
10 reduced.
During reading the read spot should remain accurately
positioned on the centre of the track to be read. For this
purpose the read apparatus comprises a fine control for the
radial position of the read spot. As is shown in Figure 7,
15 the mirror 12 may be mounted for rotation. The axis of rotation
3~ of the mirror is perpendicular to the plane of drawing, so
that by rotating the mirror 12 the read spot is shifted in
the radial direction. The rotation of the mirror is obtained
by means of the drive element 39. This element may take various
20 fcrms ; it is for example an electromagnetic element as shown
in Figure 7, or a piezo~electric element. The drive element is
controlled by a control circuit 50, to whose input a radial error
signal Sr is applied, i.e. a signal which provides an indica-
tion of a deviation of the position of the read spot relative
25 to the centre of the track.
The signal Sr may be generated by means of two detectors
which are disposed in the plane 21, one on each side of a
line which is effectively parallel to the track direction, as
is described in for example German Patent Application No.
30 2,342,906. By subtracting the output signals of these detectors
from each other a radial error signal Sr is obtainedO Thus, an
asymmetry in a radial direction of the radiation distribu-
tion in the pupil is determined. This is the so-called differen-
tial tracking method.
The servosystem may be adapted so that the information
track portions of the greater phase depth, for example ~1 = 5~ 4 rad.,
are followed. In Figure 12 the uninterrupted line represents
the signal Sr as a function of the radial position r of the
PHN 10121 17 10.9.1981
read spot in the case that only these information track por-
tions would be present. If the read spot is situated exactly
on a deep information track portion, i.e. at the locations r,
2r etc., the signal Sr will be zero. The servo system Eor
the trackirg is adapted so that in the case of a negative
value of Sr the tilting mirror 12 in Figure 7 is pivoted
anti-clockwise, so that the centre of the read spot is positioned
exactly on the centre of the deep information track portion
2. For a positive value of Sr the mirror 12 is pivoted clock-
wise. The points D in Figure 12 are the stable points for
the servosystem.
In a record carrier in accordance with the invention
shallow information track portions 2' are located between
the deep information track portions 2. The pointE cn the curve
for Sr, which point corresponds to the centre of the informa-
tion track portion 2', is an unstable point. When the read spot
is situated slightly to the right of the~centre of the informa-
tion track portion 2', i.e. if Sr were positive, the mirror
12 would be pivoted clockwise and the read spot would be
shifted even further to the right. In a similar way in the case
of a deviation to the left of the position of the read
spot,said spot would be shi~ted further to the left. Without
further steps the read spot would not remain positioned on
a shallow information track portion 2', but the read spot
would constantly be controlled towards a deep information
track portion.
In accordance with the invention, for reading a shallow
information traclc or track portion, the signal Sr is inverted
before being applied to the control circuit 50. The inverted
signal Sr is represented by the dashed curve in Figure 12. The
poirlt E on the curve for Sr , which point corresponds to -the
centre of the information track portion 2', is a stable point
and the points D on said curve are unstable points.
In the apparatus in accordance with Figure 7 there
is provided a combination of an inverter 51 and a switch 52.
This enables the signal Sr ~o be applied to the control 50 in
inverted or non-inverted form. The switch 52 is controlled by
the signal S in synchronism with the switch 28 of Figure 9~
PHN. 10.121 18
When a deep information track portion is read the siynal Sr
is not inverted and when a shallow information track portion
is read it is inverted. During read-out of an information
track 2 the heavy portion of the curve for Sr is used and
during read-out of an information track 2' the heavy portion
of the dashed curve for Sr is employed.
It is to be noted that the radial error signal S
contains contributions produced by the information track por-
tions 2 and by the information track portions 2'. As a
result of the different phase depths ~ rad. and ~2 =
3~ rad., these contributions would be in phase opposition.
However, as the information track portions 2' are shifted
relative to the information track portions 2 over a distance
equal to half the radial period of solely the information
track portions 2, the said contributions in the signal Sr will
augment each other.
The detectors for reading the information (22 and
23 in Figure 10) and those for generating the radial error
signal may be combined, in the form of four detectors which
are disposed in the four different quadrants of an ~-Y
coordinate system. For reading the information the signals
from the detectors in the first and the fourth ~uadrant as
well as the signals obtained from the detec-tors in the second
and the third quadrants are added to each other. The sum
signals thus obtained are either added to each other or sub-
tracted from each other as described in the foregoing. For
generating the radial error signal the signals from the
detectors in the first and the second quadrant are added to
each other and so are the signals from the detectors in the
third and the fourth quadrant. The sum signals thus obtained
are subtracted from each other, yielding the signal Sr.
Apart from being used for reading a record carrier
with phase depths ~1 = 5~ rad. and ~2 = 34~ rad., th~
differential tracking method may also be employed for reading
a record carrier with ~1 ~ 7r rad. and ~2 = 23f~rad.. For
the last-mentioned record carriers tracking may also be
realized as for example described in the Applicants'
Cana~dian Patent 987,029 - issued April 6, 1976 tpHN 6296~,
~8~
PHN. 10.12] 19
which has been laid open to public inspection. In addition -to
the read spot, two servo spots may be projected onto the in-
Eormation structure. These spots are positioned so relative
to each other that when the centre of the read spot exactly
coincides with the centre of the information track portion to
be read, the centres of the servo spots are situated at the
two edges of this information track portion. For each servo
spot there is provided a separate detector. The difference
of the signals from said detectors is determined by the mag-
nitude and the direction of the radial positional error ofthe read spot.
When reading a record carrier with the phase
depths ~ rad. and ~2 = ~ rad., a radial error signal
may also be generated by radially moving the read spot and the
information track to be read periodically relative to each
other with a low amplitude, for example o.l times the track
width and with a comparatively low frequency, for example
30 kHz. The signal supplied by the information detectors then
contains an additional component whose fre~uency and phase
depend on the radial position oE -the read spot. The relative
movement of the read spot and the information trac]c can be
obtained by periodically moving the read beam ln the radial
dire~tion. ~lternatively, as is described in -the Applican-ts'
Canadian Patent 1,038,078 - issued September 5, 1978
(PHN 7190), which has been laid open to public inspec-tion, the
information tracks may be undulating tracks. A positional
error signal thus generated should also be inverted when a
shallow track is to be read.
The invention has been described for a reflecting
record carrier. It is also possible to employ the invention
in conjunction with a record carriex having a phase structure
which is read in transmission. If the phase structure com-
prises pits or hills, they should be deeper and higher
respectively than the respective pits or hills of a reflect-
ing record carrier.
Furthermore, the in~ention may also be used forreading a record carrier in thQ form of a tapeO In that case
the expression ~Iradial direction" used in the foregoing
PHN 10121 20 l0.9.1981
I
should read: the direction perpendicular to the track
direction.
ZO
~5
