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Description  |
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BACKGROUND OF THE INVENTION
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 differing from
each other in that they comprise information areas of a first phase depth
and information areas of 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 it are described in the
Applicants' Netherlands Patent Application No. 78 03517 corresponding to
abandoned U.S. Application Ser. No. 419,640, filed Sept. 17, 1982. In the
record carrier described therein the first phase depth is preferably
approximately .pi.rad . and the second phase depth approximately 2.pi./3
rad.
When the information structure is scanned with a read beam, this beam is
split into a zero-order subbeam and a plurality of higher order subbeams.
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 subbeams when the
centre of the read spot formed on the information structure coincides with
the centre of an information area. In said Netherlands Patent Application
No. 78 03517 it is demonstrated that if the information areas in each of
two adjacent information track portions have different phase depths, these
track portions can be arranged more closely to each other than in the case
where 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 cross-talk
between adjacent track portions.
However, the information track portions of different phase depths should
then be read in different ways. The information track portions with the
greater phase depths are read by determining the variation of the total
intensity of the radiation received from the record carrier and passing
through the pupil of 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.
SUMMARY OF THE INVENTION
It has been found that when an information track portion with the greater
phase depth is read by the integral method there is nevertheless some
cross-talk from an adjacent information track portion having the smaller
phase depth.
It is the object of the present invention to eliminate this residual
cross-talk. In accordance with a first aspect of the invention the record
carrier is therefore characterized in that the difference between the
first and the second phase depth is .pi./2 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 make
it 7.pi./6 rad., and to maintain the smaller phase depth at the value of
2.pi./3 rad. as specified in the Netherlands Patent Application No. 78
03517. The information track portions with the greater phase depth should
then be read in accordance with the integral method and the information
track portions with the smaller phase depth in accordance with the
differential method. As the two read methods have different optical
transfer functions ("modulation transfer 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 no longer be read in
an optimum manner when the differential method is used.
Preferably, the information areas are therefore dimensioned so that they
can all be read by means of the integral method. The preferred embodiment
of the record carrier is characterized in that the first phase depth is
approximately 5.pi./4 rad. and the second phase depth approximately
3.pi./4 rad.
The two phase depths may be realized in different manners, for example by
areas with different refractive indices. Suitably, the information areas
may be pits or hills. The advantage of this is that the record carriers
can be manufactured in large quantities using pressing techniques. In the
case of information areas in the form of hills or pits, the phase depth is
related to the 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 inclination of said walls.
In accordance with a further characteristic feature of the record carrier,
consecutive track portions within one information track differ from each
other in that they comprise information areas of the first phase depth and
information areas of the second phase depth respectively. This enables the
visual effect of transitions between the two types of information areas in
the signal which is ultimately supplied by the read apparatus to be
reduced.
For correct timing of the desired electronic phase shift during read-out of
the record carrier, in accordance with a further characteristic feature,
the record carrier may contain a pilot signal in addition to an
information signal, which pilot signal identifies the transitions between
information areas of the first phase depth and information 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 containing information areas of two different
phase depths, which apparatus comprises a radiation source producing a
read beam, an objective 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 structure one on each
side of a line which is effectively transverse of the track direction, the
outputs of the two detectors being connected to an adder circuit, is
characterized in that at least one of the detectors is connected to the
adder circuit via a phase-shifting element, which element introduces, in
the detector signal to, a phase shift of constant magnitude.
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 introduce a phase shift which is
equal to the difference between the two phase depths, viz. a phase shift
of approximately .pi./2 rad.
Alternatively, the two phase depths may be selected so that one type of
information areas is adapted to be read with the integral method, while
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
record carrier is characterized in that the outputs of the two detectors
are also connected to a subtractor circuit, that the outputs of the adder
circuit and the subtractor circuit are connected to a signal processing
circuit via 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 7.pi./6 rad. and 2.pi./3 rad. occur, but may also be
used for reading the record carrier which is described in the previous
Netherlands Patent Application No. 78 03517, i.e. a record carrier with
phase depths of .pi. rad. and of 2.pi./3 rad. In that case a
phase-shifting element is included only in one of the connections between
the detectors and the adder circuit, while the detectors are connected
directly to the subtractor circuit. In apparatus for reading a record
carrier with phase depths of 7.pi./6 rad. and of 2.pi./3 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-shifting element introduces a phase shift of approximately .pi./3
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 and the differential read method, to connect each
of the detectors via a phase shifting element 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
apparatus which only employs the integral read method, the phase-shifting
elements should moreover be adjustable 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 spatial frequencies of the
information areas, the detectors are preferably each disposed against an
edge of the effective pupil of the objective system. The effective pupil
is to be understood to mean the image of the pupil in the plane of the two
detectors.
BRIEF DESCRIPTION OF THE DRAWING
The invention will now be described in more detail with reference to the
drawing wherein:
FIG. 1 is a plan view of a part of a first embodiment of a record carrier,
FIG. 2 is a tangential sectional view of said record carrier,
FIG. 3 is a radial sectional view of said record carrier,
FIG. 4 is a plan view of a part of a second embodiment of a record carrier,
FIG. 5 is a tangential sectional view of said record carrier,
FIG. 6 is a radial sectional view of said record carrier,
FIG. 7 shows an embodiment of a read apparatus,
FIG. 8 shows the arrangement of the detectors relative to the various
diffraction orders,
FIG. 9 shows a first version of the electronic circuit for processing the
detector signals,
FIG. 10 shows a second version of said electronic circuit,
FIG. 11 shows a third version of said electronic circuit, and
FIG. 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 same reference numerals.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1, 2 and 3 show a first embodiment of a record carrier in accordance
with the invention. FIG. 1 is a plan view, FIG. 2 a tangential sectional
view taken on the line II--II' in FIG. 1, and FIG. 3 a radial sectional
view taken on the line III--III' in FIG. 1 of the record carrier. The
information is contained in a multitude of information areas 4, for
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 tracks 2 are spaced by narrow lands 3. The spatial
frequency, and as the case may be the lengths, of the areas is determined
by the information.
The areas of the adjacent information tracks have different phase depths.
As is shown in FIG. 3, the pits of 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 d.sub.1
and d.sub.2. Owing to the different depths, the first track, the third
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 accordance with the
invention, the radial period of the information tracks was 0.85 .mu.m, the
width of these tracks was 0.5 .mu.m and the width of the lands 3 was 0.35
.mu.m.
The information carrying surface of the record carrier can be made
reflecting, for example by vacuum-deposition of a metal layer 7 such as
aluminium, on said surface.
It is to be noted that the size of the areas in the FIGS. 1, 2 and 3 has
been exaggerated for the sake of clarity.
FIG. 4 is a plan view of a part of a second embodiment of a record carrier
in accordance with the invention. This Figure shows a larger part of the
record carrier than FIG. 1, the individual information areas being no
longer discernible. The information tracks are now divided into portions a
and b, the portions a comprising information areas of greater phase depth
(deeper pits) and the portions b information areas of smaller phase depth.
In FIG. 5, which is an enlarged tangential sectional view of a track taken
on the line V--V' in FIG. 4, the pits of the depth d.sub.2 are again
designated 4' and the pits of the depth d.sub.1 are designated 4.
FIG. 6 is a radial sectional view, taken on the line VI--VI' in FIG. 4, of
the second embodiment of the record carrier.
In FIGS. 1 through 6, the information areas have 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.
FIG. 7 shows an embodiment of an apparatus for reading a record carrier.
The disc-shaped record carrier is shown in a radial sectional view. The
information tracks thus extend perpendicularly to the plane of the
drawing. It 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 substrate 6. The information structure may
furthermore be covered with a protective layer 8. 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 beam 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 optimum 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,
the entire information surface can be scanned.
The modulated read beam again traverses the objective system and is again
reflected by the mirror 12. The radiation path includes means for
separating the modulated and the unmodulated read beam. This may, for
example, comprise a polarization-sensitive splitter prism and a .lambda./4
plate (wherein .lambda. is the wavelength of the read beam). For the sake
of simplicity it is assumed in FIG. 7 that said means are constituted by a
semitransparent mirror 17. This mirror reflects the modulated beam to a
radiation-sensitive 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
by the information structure, specifically of the zero-order subbeam and
the first-order subbeams, are separated. The detection system may be
disposed in the plane 21 in which an image of the exit pupil of the
objective system 13 is formed by the auxiliary lens 18. In FIG. 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 diffraction grating. This grating splits the read beam
into a zero-order subbeam, a plurality of first-order subbeams and a
plurality of higher-order subbeams. After being reflected by the
information structure, a part of the radiation re-enters the objective
system. 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 separated from each other. FIG. 8 represents the situation in
the plane 21 of FIG. 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
FIG. 8 corresponds 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 determined by .lambda./p, where p is the local spatial
period of the information areas in the information track portion to be
read and .lambda. the wavelength of the read beam.
For reading the information use is made of the phase shifts of the subbeams
of the (+1, 0) and (-1, 0) orders relative to the zero-order subbeams. In
the hatched areas in FIG. 8 said first-order subbeams and zero-order
subbeam overlap, so that interference occurs. The phases of the
first-order subbeams vary with high frequencies as a result of the
tangential movement of the read spot relative to the information track.
This results in the intensity variations 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 .psi. 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 subbeam of the (+1, 0) order increases by 2.pi..
Therefore, when the read spot moves in a tangential direction the phase of
said subbeam relative to the zero-order subbeam will vary by .omega.t.
Here, .omega. is a time frequency which is determined by the spatial
frequency of the information area and by the speed with which the read
spot travels over the track.
The phases .theta.(+1, 0) and .theta.(-1, 0) of the first-order subbeams
relative to the zero-order subbeam may be represented by:
.theta.(+1, 0)=.psi.+.omega.t
0(-1, 0)=.psi.-.omega.t
The intensity variations as a result of interference of the first-order
subbeam with the zero-order subbeam are converted into electric signals by
the detectors 22 and 23. The timedependent output signals S.sub.23 and
S.sub.22 of the detectors 23 and 22 may be represented by:
S.sub.23 =B(.psi.) cos (.psi.+.omega.t)
S.sub.22 =B(.psi.) cos (.psi.-.omega.t)
Here B (.psi.) is a factor which is proportional to the geometrical depth
of the pits. For .psi.=.pi./2 it may be assuemd that B (.psi.) is zero.
In a first embodiment of a record carrier in accordance with the invention,
the phase depth .psi..sub.1 of the information areas 4 is 7.pi./6 rad. and
the phase depth .psi..sub.2 of the information areas 4' is 2.pi./3 rad. In
the apparatus for reading said record carrier, as is shown in FIG. 9, the
outputs of the detectors 22 and 23 are connected to the phase shifting
elements 24 and 25. The element 24 shifts the phase of the detector signal
S.sub.22 through +.phi. rad, while the element 25 shifts the phase of the
detector signal S.sub.23 through -.phi. rad. The signals S.sub.22 and
S.sub.23 then change to:
S'.sub.23 =B(.psi.). cos {.psi.+(.omega.t-.phi.}=B(.psi.). cos
(.psi.+.omega.t-.phi.)
S'.sub.22 =B(.psi.). cos {.psi.-(.omega.t+.phi.)}=B(.psi.). cos
(.psi.-.omega.t-.phi.)
When the information areas of an information track portion being read have
the greater phase depth .psi..sub.1 =7.pi./6 rad. the signals S'.sub.22
and S'.sub.23 should be added, while if the information areas of the
information track portion being read have the smaller phase depth,
.psi..sub.2 =2.pi./3 rad., the signals S'.sub.22 and S'.sub.23 should be
subtracted from each other. For this purpose, as is shown in FIG. 9, the
signals S'.sub.22 and S'.sub.23 may be applied both to the adder circuit
26 and to the subtractor circuit 27. The outputs of the circuits 26 and 27
are connected to the two input terminals e.sub.1 and e.sub.2 of a switch
28 having one master terminal e. Depending on the control signal S.sub.c
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 circuit 29. In this circuit the read out signal is
demodulated and 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 indicates the positions on the record carrier where a
transition occurs from the information areas of a first phase depth to the
information areas of a second phase depth. If a television signal has been
recorded, one television signal being recorded per information track
revolution, the picture synchronizing pulses or field synchronizing pulses
contained in the actual television signal may be employed for generating
the control signal S.sub.c 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 FIG. 4, the line synchronizing
pulses 32, as shown in FIG. 9, may be extracted from the signal from the
demodulation circuit 29 in the line synchronizing pulse separator 31. In
the circuit 33, which is for example a bistable multivibrator, the pulses
32 are converted into a control signal S.sub.c for the switch 28, so that
said switch is changed over each time after reading one television line.
If each information track of the information structure contains only one
type of areas, the element 31 is a picture synchronizing pulse separator
and the switch 28 is changed over after read-out of each information track
or two television fields.
When point e.sub.2 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:
S.sub.I =S'.sub.23 +S'.sub.22 =2.B(.psi.). cos (.psi.-.phi.). cos
(.omega.t).
If point e is connected to point e.sub.1, reading is effected in accordance
with the so-called differential method. The signal applied to the
demodulator may then be expressed by:
S.sub.D =S'.sub.23 -S'.sub.22 =-2.B(.psi.). sin (.psi.-.phi.). sin
(.omega.t).
The integral method is employed when reading information areas having a
phase depth .psi..sub.1 =7.pi./6 rad. The signal S.sub.I is then a maximum
if cos (.psi..sub.1 -.phi.)=1, i.e. if .phi.=.pi./6 rad. For the
information areas of the phase depth .psi..sub.2 =2.pi./3 rad, cos
(.psi.-.phi.)=0. Thus, when reading in accordance with the integral
method, the information areas of the smaller phase depth are not
"observed". Conversely, when the differential read method is used the
information areas 4' of a phase depth .psi..sub.2 =2.pi./3 rad. will be
read in an optimum manner, for sin (2.pi.-.phi./3) is then 1, whilst the
information areas 4 of the phase depth .psi..sub.1 =7.pi./6 rad. are then
not "observed", for sin (7.pi.-.phi./6) is then 0.
Instead of the two phase-shifting elements 24 and 25 it is alternatively
possible to use the phase shifting element 25 only. If the phase shift
.phi. of said element is selected to be .pi./3 rad., the same result is
obtained.
By means of an apparatus in which one detector signal or both detector
signals are subjected to an additional phase shift, it is also possible to
obtain a substantial improvement of the read-out of the record carrier
described in Netherlands Patent Application No. 78 03517, i.e. of the
record carrier having the phase depths .psi..sub.1 =.pi. rad. and
.psi..sub.2 =2.pi./3 rad.
The apparatus adapted for reading this record carrier is shown in FIG. 10.
The signals from the detectors 22 and 23 are applied directly to the
subtractor circuit 27. In the connections between said detectors and the
inputs of the adder circuit 26 phase shifting elements 24 and 25 are
included, which introduce a constant phase shift of +.phi. rad. and -.phi.
rad. respectively. During differential read-out of the information areas
of the phase depth .psi..sub.2 =2.pi./3 rad., the information areas of the
phase depth .psi..sub.1 =.pi. rad. will produce no cross-talk. The
cross-talk from the information areas of .psi..sub.2 =2.pi./3 rad. during
read-out by the integral method of information areas of .psi..sub.1 =.pi.
rad. can be substantially eliminated if .phi.=.pi./6 rad. As a result of
this phase shift the amplitude of the signal S.sub.I decreases slightly,
but is still sufficiently high. It is alternatively possible to employ the
phase shifter 24 only, which should then introduce a phase shift of .pi./3
rad.
For the values of the phase depths .psi..sub.1, .psi..sub.2 and the phase
shift .phi. specified in the foregoing, the integral read method and the
differential read method must be used alternately. However, these two
methods have different optical modulation transfer functions. If a video
signal is stored on the record carrier, one transfer function will, for
example, cause different grey shades or a different colour saturation in
the ultimate television picture than the other transfer function. In the
case of an audio signal in the form of a frequency-modulated signal,
switching between the transfer functions may become audible as an
undesired frequency.
Furthermore, for reading lower spatial frequencies of the information
areas, the transfer function of the differential method is worse than that
of the integral method.
Suitably, the phase depths .psi..sub.1 and .psi..sub.2 are therefore
selected so that they are symmetrical relative to .pi. rad. The phase
depth .psi..sub.1 is then 5.pi./4 rad. and the phase depth .psi..sub.2 is
then 3.pi./4 rad. The magnitude of the phase shift .phi. is then .pi./4
rad.
FIG. 11 shows a signal processing circuit of an apparatus 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
shift -.phi. and the element 24 a phase shift +.phi., the magnitude of
.phi. being .pi./4 rad. The sign of .phi. should now be changed at the
transition from information areas of the greater phase depth to
information areas of the smaller phase depth and vice versa. When reading
the information areas of the greater phase depth .phi.=+.pi./4 rad. and
when reading information areas of the smaller phase depth .phi.=-.pi./4
rad. For changing the sign of the phase shift .phi. it is again possible
to employ the signal S.sub.c.
The information signal S.sub.I is always given by:
S.sub.I =S'.sub.23 +S'.sub.22 =2.B(.psi.). cos (.psi.-.phi.). cos
(.omega.t).
When the information areas 4 of the phase depth .psi..sub.1 =5.pi./4 rad.
are read, then .phi.=+.pi./4 rad. Then, cos (.psi..sub.1 -.pi./4) is equal
to 1. For the information areas 4' of the phase depth .psi..sub.2 =3.pi./4
rad., cos (.psi..sub.2 -.pi./4) is equal to 0, so that these information
areas will produce no cross-talk. When the information areas 4' are read
.phi.=-.pi./4 rad. and cos (.psi..sub.2 +.pi./4) is 1, whilst cos
(.psi..sub.1 +.pi./4) is 0, so that the information areas 4 of the greater
phase depth are not "observed" and thus produce no cross-talk.
The values for the phase depths specified in the foregoing are no strict
values. Deviations of the order of some degrees are permissible.
It is possible that the difference between the phase depth .psi..sub.1 and
.psi..sub.2 deviates from .pi./2 rad. However, by adapting the electronic
phase shift .phi. it is still possible to ensure that the cross-talk
between adjacent information track portions is minimized.
So far only the tangentially diffracted first-order subbeams have been
discussed. The information structure also diffracts the read radiation in
higher tangential orders and in various radial and diagonal orders. The
information areas, which for the tangential first orders exhibit a
difference between the phase depths .psi..sub.1 and .psi..sub.2 of .pi./2
rad., however, will also exhibit such a phase depth difference for the
higher tangential 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 supplied by the
detectors have a fixed phase 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 first phase depth and the signal from information areas with a second
phase depth can be minimized. It is then assumed that the detector 22 only
receives the beam 42 and the detector 23 only receives the beam 41. At
lower spatial frequencies of the information areas, i.e. at greater
periods p of said areas, the distance f in FIG. 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 beams could then no longer be influenced individually, so
that no cross-talk reduction in accordance with the invention could be
obtained. In order to enable a satisfactory cross-talk reduction to be
realized at lower spatial frequencies, the radiation-sensitive areas of
the detectors, instead of being disposed as closely as possible to each
other and in the centre of the pupil, as is shown in FIG. 8 by the
uninterrupted lines, are arranged as far as possible from each other and
at the edge of the pupil. In FIG. 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 receives only the beam 42
and the detector 23 only the beam 41, is then considerably 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 FIG. 7, the mirror 12 may be mounted for rotation. The axis of
rotation 38 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 forms; it is for example an
electromagnetic element as shown in FIG. 7, or a piezo-electric element.
The drive element is controlled by a control circuit 50, to whose input a
radial error signal S.sub.r is applied, i.e. a signal which provides an
indication of a deviation of the position of the read spot relative to the
centre of the track.
The signal S.sub.r 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. 2,342,906. By subtracting the output signals of
these detectors from each other, a radial error signal S.sub.r is
obtained. Thus, an asymmetry in a radial direction of the radiation
distribution in the pupil is determined. This is the so-called
differential tracking method.
The servosystem may be adapted so that the information track portions of
the greater phase depth, for example .psi..sub.1 =5.pi./4 rad. are
followed. In FIG. 12 the uninterrupted line represents the signal S.sub.r
as a function of the radial position r of the read spot in the case that
only these information track portions 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 S.sub.r will be zero. The servo system
for the tracking is adapted so that in the case of a negative value of
S.sub.r the tilting mirror 12 in FIG. 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 S.sub.r the
mirror 12 is pivoted clockwise. The points D in FIG. 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 point E on the curve for S.sub.r, which point corresponds to the
centre of the information track portion 2', is an unstable point. When the
read spot is situated slightly to the right of the centre of the
information track portion 2', i.e. if S.sub.r 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 shifted 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 track
or track portion, the signal S.sub.r is inverted before being applied to
the control circuit 50. The inverted signal S.sub.r is represented by the
dashed curve in FIG. 12. The point E on the curve for S.sub.r, 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 FIG. 7 there is provided a combination
of an inverter 51 and a switch 52. This enables the signal S.sub.r to be
applied to the control 50 in inverted or non-inverted form. The switch 52
is controlled by the signal S.sub.c in synchronism with the switch 28 of
FIG. 9. When a deep information track portion is read the signal S.sub.r
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 S.sub.r is used and during read-out of an information track
2' the heavy portion of the dashed curve for S.sub.r is employed.
It is to be noted that the radial error signal S.sub.r contains
contributions produced by the information track portions 2 and by the
information track portions 2'. As a result of the different phase depths
.psi..sub.1 =5.pi./4 rad. and .psi..sub.2 =3.pi./4 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 S.sub.r will
augment each other.
The detectors for reading the information (22 and 23 in FIG. 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
X-Y coordinate system. For reading the information the signals from the
detectors in the first and the fourth quadrant as well as the signals
obtained from the detectors in the second and the third quadrants are
added to each other. The sum signals thus obtained are either added to
each other or subtracted 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
S.sub.r.
Apart from being used for reading a record carrier with phase depths
.psi..sub.1 =5.pi./4 rad. and .psi..sub.2 =3.pi./4 rad., the differential
tracking method may also be employed for reading a record carrier with
.psi..sub.1 =7.pi./6 rad. and .psi..sub.2 =2.pi./3 rad. For the
last-mentioned record carriers tracking may also be realized as for
example described in the Applicants' Netherlands Patent Application No. 72
06378 which has been laid open to public inspection and corresponds to
U.S. Pat. No. 3,876,842. In addition to the read spot, two servo spots may
be projected onto the information structure. These spots are positioned
relative to each other so 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 magnitude and the direction of the radial positional
error of the read spot.
When reading a record carrier with the phase depths .psi..sub.1 =7.pi./6
rad. and .psi..sub.2 =2.pi./3 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
0.1 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 frequency and phase depend on the
radial position of the read spot. The relative movement of the read spot
and the information track can be obtained by periodically moving the read
beam in the radial direction. Alternatively, as is described in the
Applicants' Netherlands Patent Application No. 73 14267, which has been
laid open to public inspection and corresponds to U.S. Pat. No. 4,223,347,
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 em | | |
