Note: Descriptions are shown in the official language in which they were submitted.
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PRE-ETCHED INFORMATION MEDIUM WITH OFFSET MARKS, METHOD
FOR MARING THE MARKS AND METHOD FOR PREPARING A RADIAL
TRACKING ERROR SIGNAL AND READING DEVICE FOR THE
IMPLEMENTATION OF THE METHOD
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an information medium
designed for the recording, along tracks, of optically
readable information, more particularly to an
information medium with pre-etchings including at least
offset marks defining the mean axis of the tracks along
which the optically readable information can be
recorded. The present invention also relates to a
method for making marks that are offset with respect to
the mean axis of the tracks as well as a method for
making a radial tracking error signal for a track
during the reading o an information medium such as
this and an optic reading device to implement the
method for preparing the radial tracking error signal
for tracks.
The French patent No. 2 523 347, filed on behalf
of THOMSON-CSF, notabl~ describes a pre-etched
removable inf ormation medium comprising pre-etched
zones, notably with two marks, offset laterally on
either side of the mean axis of the track to generate
an error signal o~ track deviations and thus achieve
the radial tracking of the track. This type of
pre-etching is presently used on certain digital optica
2 ~ ~~3 ~J ~ 5;'~
disks. However, the inter-track pitch should enable the
etching of offset marks: this restricts the capacity of
the disk and is a particularly penalizing factor for
small-diameter disks. To make it possible to narrow the
pitch of the tracks up to the permissible limit for the
cross-talk that appears between the information signals
recorded on contiguous tracks, the firm Alcatel Thomson
Gigadisc, in the European patent No. 0201093, has
proposed the making of marks that are respectively
offset to the right and to the left of the tracks in
such a way that they are symmetrically arranged with
respect to the inter-track axis located at mid-distance
from the axes of two neighboring track elements. Such a
configuration is shown in figure 1. This figure shows a
very limited part of a standard optic disk. Therefore,
the axes of the tracks la, lb, lc are shown
symbolically by dashed straight lines whereas they are
actually arcs of circles or spirals. Furthermore, in a
known way, the disk has angular zones 4 reserved for
the recording of elements of user information such as
those represented by the marks 10 centered on the axes
and pre-etched zones 5 reserved for the service
functions of the disk such as the servo-linking of the
opticalbeam - to the track, and the focusing, the
synchronization and the access functions. The zones 4
and 5 are made so that, at least locally, they are
radially aligned.
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As shown in figure 1, the pre-etched zones 5 each
have at least two successive marks 23 and 33, offset on
either side of the mean axis of the tracks la, lb, lc.
These marks are used for the radial trac~ing of the
track. To enable the axes of the tracks to be brought
closer to each other, one and the same offset mark,
associated with two contiguous axes, is used in figure
1. Thus, the mark 33 centered on E is used for the
radial tracking of the axis la and lb. In the same way,
the mark 23, centered on D, is used for the radial
tracking of the axis lb and the axis lc. In figure 1,
therefore, two distinct- marks symbolized by dashes have
been replaced by only one mark centered at mid-distance
from the tracks that it should define. The marks 23 and
33 therefore appear as being formed by two partially
overlapping marks.
While the use of such marks, offset on either side
of the mean axis of the tracks, makes it possible to
narrow the pitch of the tracks, it does not enable the
length of the pre-etched zones to be reduced. Now, the
present invention is aimed at overcoming this drawback
by proposing the use, in each pre-etched zone, of only
one offset mark of the two marks used up till now.
For, the elements of information recorded on an
information medium, as described above, are read by
detecting the light that is diffracted by the
information medium and re-enters the objective
generating the optic spot that illuminates the medium
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as is well known to those skilled in the art. The
diffracted light is detected ~y photodetectors that
convert the light signal into an electric signal. If
the light spot T is shifted along the direction
symbolized by the arrow C in figure 1, at the straight
lines AA and BB which go through the pre-etched marks
23 and 33, respectively at the center C, D and E, F of
these pre-etched marks, the curves V and V
A B
respectively, shown in figure 2, are obtained. In this
figure 2, the x-axis Or represents the distance to the
center of the disk. The distances correspondi.ng to the
points C and D on the straight line AA of figure 1 are
represented by the letters C and D on the axis Or while
the distances corresponding to the points E and F of
the straight line BB in figure 1 are represented by the
letters E and F on the same axis Or. The differences in
the distance from C and from D to the center of the
disk, from D and from E to the center of the disk, and
from E and from F to the center of the disk are equal
to one another and are equal to the center distance of
a~es of the tracks. Furthermore, in figure 2 we have
shown a third curve V which is the sum of V and V .
S A B
Thus, in the case of an information medium having
pre-etched zones 5 as shown in figure 1, the curve V
of figure 6 is deeply modulated. However, the Applicant
has observed that, if the two curves V and Y are
A B
strictly symmetrical with each other in relation to a
straight line that is parallel to the axis Or and
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represents the mean value of each of them, then the sum
of the two curves V and V is a strai~ht line parallel
A B
to the axis Or.
SUMMARY OF THE INVENTION
S The present invention is aimed at providing a
novel information medium, pre-etched with offset marks,
wherein the structure of the marks makes it possible to
obtain a curve V parallel to the axis Or.
Consequently, an object of the present invention
is an information medium having at least one face
designed for the recording, along the tracks, of
optically readable information, the medium being
provided with pre-etched zones alternating with zones
designed for the recording of said information, the
pre-etched zones having discrete patterns that define
the mean axis of the tracks, wherein, in each
pre-etched zone, the discrete patterns are constituted
by marks centered at mid-distance between two adjacent
tracks and having an optical width, as given by a real
optical system with limited transfer function, in the
direction perpendicular to the axis of the tracks, that
is substantia~ly equal to theoptical width of the zone
located between two successive marks along said
direction and wherein, from one pre-etched zone to the
next one, the marks have equal offsets, with opposite
signs, with respect to mean axes of the tracks.
Thus, in using this specific configuration for the
offset marks of the pre-etched zone of the information
medium, the signal V = V + V , corresponding to the
S A B
sum of the signals detected during the passage on the
offset marks, is a constant. Consequently, if the
signal V is known once and for all, it is enough to
measure V or V to obtain the error signal. Indeed,
A B
the error signal is given by the difference V - V
A B
which, in this case, becomes equal to:
e = V - (V - V ) = 2 V - V . The same would be
A S A A S
obtained in retaining only the other offset marks 33.
In this case, the error signal e = V - 2V .
S B
The present invention is also related to a method
for the making of marks offset with respect to the mean
axis of the tracks on an information medium as defined
here above.
In this case, during the making of a parent
medium; the marks offset on a first side are etched by
deflection of the etching beam during the passage over
a first track, and the marks offset on the other side
are etched by the deflection of the etching beam during
the passage over a second track adjacent to the first
track. Then the above operation is repeated until the
end of the etching of the parent medium.
According to another embodiment, during the making
of a parent medium, the offset marks are etched by
deflection of the etching beam alternately on each side
of the mean axis of the tracks, a first part of the
mark being etched during the passage of the beam on a
first track and a second axially complementary part of
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the mark being etched during the passage of the beam on
a second track adjacent to the first track.
The present invention also relates to a method for
the preparation of an error signal of the radial
tracking of a track by a light spot for the exploration
of the tracks of a information medium as described here
above, wherein, at least once, the sum is obtained of
the signals detected by optical detection means during
the passage of the light spot on two marks, offset
respectively on one side and on the other side, and
this sum is memorized, then the track radial tracking
error signal is prepared by alternately subtracting
sai~ sum from the signal detected during the passage of
the light spot on one of the offset marks and
subtracting the signal detected during the passage of
the light spot on the other offset mark from said sum.
BRIEF DESCRIPTION OF THE DRAWINGS
Other characteristics and advantages of the
present invention shall appear from the following
description of a preferred embodiment of the
information medium pre-etched with offset marks as well
as of an embodiment of a reading device enabling the
reading of the pre-etched information medium ar.d the
obtaining of a radial tracking error signal, this
description being made with reference to the appended
drawings, wherein:
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- Figure 1, already described, gives a schematic
view of a part of a pre-etched information medium with
offset marks according to the prior art;
- Figure 2, already described, shows a curve
giving the amplitude of the electric signals detected
during the passage over the marks as a function of
their distance from the center of the disk;
- Figure 3 is a schematic view of an information
medium pre-etched with offset marks according to the
present invention;
- Figure 4 shows curves giving the amplitude of
the signals detected during the pasage over the offset
marXs as a function of their distance from the center
of the medium;
- Figure 5 is a schematic view of an optical reading
device used with a pre-etched information medium
according to the present invention;
- Figure 6 is a schematic drawing of a circuit
giving the track tracking error signal;
- Figures 7 and 8 are two schematic views of an
information medium pre-etched with offset marks
according to the present invention, explaining
different methods of making the marks.
To simplify the the description, in the figures
the same references are repeated for the same figures.
Furthermore, the present invention has been
described with reference to oblong-shaped pre-etched
marks. However it is clear to those skilled in the art
that other types of pre-etched marks may be used,
notably circular marks.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, and as shown
5 in figure 3, which gives a schematic view of a portion
of a digital optical disk having track axes symbolized by
the references la, lb, lc, on which user information 10
will be recorded in zones 4, the pre-etched zones 5
have only one offset mark, namely the marks 23' for the
pre-etched zone 5 and the marks 33' for the pre-etched
zone 5 . In accordance with the present invention, the
pre-etched offset marks 23' and 33' are chosen so as to
have the sameoptical radial dimension r as the dimension
r' of the radial zone located between these same marks.
Furthermore, the marks are centered precisely at
mid-distance from the tracks between which they are
located, as symbolized by the references C'-D' and
E'-F' which represent the centre of said offset marks,
respectively 23' and 33'. Furthermore, according to the
present invention, from one pre-etched zone 5 to the
next pre-etched zone 5 , the offset marks have offsets,
equal but with opposite signs, with respect to the
means axes of the tracks as shown by the mar~ 23'
centered on C' and the mark 33' centered on E' with
respect to the axis lb.
This information medium is used in an optical disk
reading/recording device as shown schematically in
figure 5. This device is formed by a laser module 51
1 o ~ , ~ 2 7
giving a cylindrical light beam represented by the
arrows f, a polarization separator cube 52 and a
quarter-wave plate 53, a reflective mirror 54 used for
the servo-control of the track deviation, an objective
55 enabling the focusing of the beam f on the disk and
the disk 56 with an axis 57. The beam f reaching the
disk is referenced I-I and the beam diffracted by the
disk is referenced ~-D. The part of the beam D-D which
again penetrates the objective 55 encounters the mirror
54, then the plate 53. This ~eam part f' is then sent
back by the cube 52 to the lens 57 and to a
photodetector device 58 that enables the measurement of
the part f' of the beam D-D returning into the
objective 55.
If we examine the light re-entering the objective
55 when the light spot T gets shifted along the
direction C in figure 3 and occupies different
successive positions on the straight lines A'A' and
B'B' of figure 3, we obtain, at the output of the
0 photodetector 58, the curves V and V respectively,
A B
shown in figure 4.
In this case, it is seen that the curves V , and
V , are strictly symmetrical with each other in
relation to a straight line parallel to the axis O'r'.
This invariance of V results from the fact that the
marks 23' and 33' have the same radial optical dimension
as the radial zone located between two marks 23' or two
marks 33' and that they are centered exactly at
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mid-distance from the tracks between which they are
located as indicated by the references C', E', D', F'
on the axis O'r which corresponds to the position of
the centers of the offset marks represented in figure
3. In fact, this does not mean that the radial optic
dimension of the marks 23' or of the marks 33' is equal
to that of the radial zone located between two marks
23' or two marks 33' when they are observed with an
optical system having an infinite transfer function.
Hence it does not mean that these marks and these zones
have the same geometric dimension. It is enough for the
reading signals, obtained by the use of a realoptical
system, hence an optical system with a limited transfer
function, will have the symmetry properties of those of
figure 4. With an optical system having a limited
transfer function, marks 23 and 33, described in figura
1, may give signals similar to those of figure 4. It is
certain that the closer the radial geometric dimensions
of the marks 23' and 33' are to those of the zones
between two marks 23' or two marks 33', the more
efficient will be the verification of the condition of
the signals of figure 4. However, experience proves
that despite a notable difference in these two radial
dimensions, it is possible to obtain signals that are
close to those of figure 4 and give a highly
satisfactory servo-control.
By using the offset marks of figure 3, it is
possible to achieve radial tracking of tracks. Indeed,
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the error signal e = V - V and this error signal
A' B'
feeds a device enabling the radial shifting of the
light spot on the disk. When an information medium of
the type shown in figure 3 is used, if the error signal
e is computed for the marks 23', we obtain e = V
(V -V ) = 2V - V since V is a constant. In the
S' ~' A' S' S'
same way, when we reach the level of the offset marks
33', e = V - 2V . In fact, V - V + V depends on
S' B' S' A' B'
the local properties of the disk and may vary. However,
experience shows that this signal varies slowly and
that even if it is necessary to know the signal V
this signal should be determined with a lower frequency
than the signal V or the signal V . This makes it
A' B'
possible to preserve only one offset mark per
pre-etched zone 5.
Referring to figure 6, we shall now describe a
device enabling the computation of the error signal. As
shown in figure 6, this device is constituted by
sample-and-hold circuits symbolized by the capacitors
91, 92, 93 and by switches I , I , I , I controlling
1 2 3 4
their inputs. The signals coming from the
sample-and-hold circuits are used to supply a
subtractor 94 having two high-impedance inputs. The
switches I and I are controlled by a signal d while
1 3
the switches I and I are controlled by a signal g.
The signal d is active and closes the switches I and
I for a limited period that is smaller than the period
of passage of the beam on the offset mark 23' when this
1 3 ,~ k '
beam moves along the same radius of the disk as this
mark 23' and it is inactive for the rest of the time.
The signal g is active and closes the switches I and
I for an iimited period time that is smaller than the
time of passage of the beam on the offset mark 33' when
this beam moves along the same radius of the disk as
this mark 33'. As shown in figure 6, the upper channel
100 of the device receives the signal V coming from
the photodetector, while the lower channel 101 receives
the signal V also coming from the photodetector.
Furthermore, the values of the capacitors 91 and 92 are
equal to c and are far smaller than the value of the
capacitor 93 which is itself equal to C. A ratio of 1
between these capacitor values is a reasonable value.
In normal operating mode, the output of the device of
figure 6 gives the following values in turn: -
V -1/2V , 1/2V -V , V -1/2V
A' S' S' B' A' S'
and so on and so forth, giving 1/2 e. Indeed, V
0 represents the mean of V , weighted by all the samples
S'
measured according to a decreasing geometrical
seguence, namely:
V = V ~ aV ~ a V 3 + ...
S' A'n B'n A'n - lR ~ a VB'n - 1
Referring to figures 7 and 8, we shall now
describe different methods of making offset marks
according to the present invention.
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In the case of figure 7, the offset marks 24 or
34, located between the tracks la and lb or lb and lc
are etched in a single stroke by deflection of the
etching beam when it passes over one of the tracks.
Thus the marks 34, located between the tracks la and
lb, are etched by deflection of the etching beam during
its passage over the track lb, while the marks 24
located between the tracks lb and lc are etched in a
single stroke by deflection of the etching beam when it
passes over the track lc. The marks 24 are therefore
attached to the track lc and the tracks 34 to the track
lb. These two marks are used to servo-link the light
spot to the track lb. This embodiment of the offset
tracks may have drawbacks, notably when a track, during
its recording and for a fairly short period, has
locally undergone a deflection due to parasitic
phenomena as shown in figure 7 for the track lc. In
this case, the deviation of the track lc, which has
been deliberately exaggerated in order to make it
visible, introduces an error into the center distance
of axes of the tracks lb and lc and, consequently, the
mark 24 associated with the track lc is shifted with
respect to the track lb. When this mark 24 is
associated with the mark 34, it creates a wrong track
error signal.
To overcome this drawback, the offset marks can be
etched as shown in figure 8. In this case, the offset
marks 23 and 33 are each made on two parts referenced
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231 and 232 and 331 and 332. In this case, the offset
marks are etched by using two successive passages of
the etching beam. Thus, when the etching beam follows
the axis la, the mark 231 may be be etched by
deflecting the etching beam to the right, then the mark
331 will be etched by deflecting the etching beam to
the left. Then, when the reading beam is servo-linked
to the track lb, the part 232 will be etched by
deflecting the reading beam leftwards and the part 332
of the mark 33 provided between the tracks lb and lc
will be etched by deflecting the reading beam
rightwards, and so on and so forth for all the marks.
The figure also shows dashed lines/curves to connect
the axes of the tracks with the axes of the half-marks
generated during the passage of the etching beam on
these very same tracks. In this case, during the use of
the offset marks to achieve the servo-control, an error
signal V is generated by obtaining the sum of the two
signals V and V coming from the sampling
A'1 A'2
respectively of the half mark 231 and of the half mark
232. In the same way, the signal V is generated by
B'
obtaining the sum of the two signals V and V
B'l B'2
coming from the sampling respectively of the half-mark
331 and of the half-mark 332. It is thus possible, at
each instant, to take the average of the effect of
variation in the center distance between axes of the
tracks. This may have advantages inasmuch as the
etching of two half-marks one after the other may
16
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produce a more satisfactory result during the etching
of the parent disk than is the case with the
overlapping of two marks throughout their length.
