Method and apparatus for recording signals on a record carrier, measurement method and measurement device for use in the recording method and the recording apparatus, and record carrier

An apparatus and method for recording an information signal on a record carrier by means of a radiation beam which produces optically detectable marks in parallel track portions thereon having a substantially constant track pitch (q). The write intensity of the radiation beam is adjusted to an optimum level which results in a width (p) of the optical marks which corresponds to the track pitch (q). The optimum intensity level is determined by recording different test signals in adjoining track portions at various write intensities of the radiation beam, reading-out the recorded test signals, and detecting therefrom the write intensity at which signal components of one test signal no longer occur in another recorded test signal. Recording at the optimum write intensity results in a minimum block error rate (BLER) upon read-out of the recorded information signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of recording signals on a record 
carrier of a rewritable type in which information patterns of optically 
detectable marks are formed on the record carrier in substantially 
parallel track portions of a specific track pitch, the track portions 
being scanned by a radiation beam to form the marks. 
The invention further relates to a recording apparatus for recording 
signals in parallel track portions of a specific track pitch on a record 
carrier of a rewritable type, which recording apparatus comprises scanner 
for scanning the track portions by means of a radiation beam to form an 
information pattern of optically detectable marks in the track portions, 
which pattern corresponds to the signals. 
The invention still further relates to a measurement technique and an 
associated measurement device for advantageous use in the recording method 
and the recording apparatus. 
Finally, the invention also relates to a record carrier provided with 
substantially parallel adjacent track portions of substantially constant 
track pitch, which track portions exhibit an information pattern of 
optically detectable marks. 
2. Description of Related Art 
Such a general recording method, recording apparatus and record carrier are 
known, inter alia, from "Philips Technical Review", Vol. 42, No. 2, pp. 
28-47. That publication describes a magneto-optical recording apparatus. A 
problem in magneto-optical recording is that the read reliability of the 
recorded signal is susceptible to variations in the recording conditions, 
such as for example, the recording velocity and the intensity of the 
scanning beam. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a recording method and 
recording apparatus of the type defined in the foregoing in which the read 
reliability of the recorded signal is improved. 
As related to the method, this object is achieved by a method which is 
characterized in that the dimensions of the recorded marks in a direction 
perpendicular to the track portions direction substantially correspond to 
the track pitch. 
With respect to the apparatus, this object is achieved by apparatus wherein 
the recording apparatus is adapted to record marks whose dimensions in a 
direction perpendicular to the track portions direction substantially 
correspond to the track pitch. 
The invention is based on the recognition of the fact that the read 
reliability is functionally related to the dimensions of the recorded 
marks, an optimum read reliability being obtained when the dimensions of 
the marks in a direction perpendicular to the track portions direction are 
equal to the track pitch. Preferably, the dimensions of the marks are 
adjusted by adjusting the write intensity, for these dimensions depend 
greatly on the write intensity used in recording. Thus, the write 
intensity is very suitable for adjusting these dimensions. 
An illustrative embodiment of the method, in which the write intensity can 
be optimized simply, is characterized in that the method comprises a 
measurement method. In that measurement method, a first test signal is 
recorded with a maximum write intensity of the radiation beam in a 
specific track portion. Then, in the specific track portion and situated 
at opposite sides of the specific track portion, a second test signal, 
which can be distinguished from the first test signal, is recorded with 
different write intensities between a minimum intensity and the maximum 
intensity. After the second test signal has been recorded, it is read from 
the specific track portion to check whether the second test signal being 
read contains signal components corresponding to the first test signal. On 
the basis of the results of the check, an optimum write intensity is 
selected which is situated in the boundary region between the intensity 
range for which signal components corresponding to the first test signal 
are present in the second test signal being read and the intensity range 
for which the signal components corresponding to the first test signal are 
substantially absent in the second test signal being read, the write 
intensity being adjusted to the optimum write-intensity value after the 
measurement method has been carried out. 
An illustrative embodiment of the apparatus, in which the write intensity 
is optimized automatically, is characterized in that the apparatus 
comprises a measurement device comprising a reader for reading the 
recorded signals with the aid of a radiation beam; a test-signal-generator 
for generating a first test signal and a second test signal which can be 
distinguished from the first test signal; controller for causing the first 
test signal to be recorded in a specific track portion when a specific 
maximum value of the write intensity is reached, for subsequently causing 
the second test signals to be recorded in the specific track portion and 
in track portions situated at opposite sides of the specific track 
portion, with a number of different write-intensity values situated 
between a minimum intensity and the maximum intensity, and for causing the 
second test signals recorded with different write-intensity values to be 
read from the specific track portion; a detector for detecting signal 
components corresponding to the first test signal in the second test 
signals being read; a selector for deriving from the detected signal 
components an optimum write intensity which is situated substantially at 
the boundary between the intensity range for which signal components 
corresponding to the first test signal appear in the second test signals 
being read and the intensity range for which the signal components 
corresponding to the first test signal are substantially absent in the 
second test signals being read; and an adjuster for adjusting the write 
intensity to the optimum value after the optimum write intensity has been 
determined. 
The foregoing illustrative embodiments of the method and the apparatus 
advantageously utilize the fact that at the instant at which the 
dimensions of the marks correspond to the track pitch the signal 
components of the first test signal disappear from the read signal. 
The illustrative embodiments with automatic write-intensity adjustment are 
particularly suitable for use in recording apparatus in which the 
dimensions of the marks strongly depend on the write intensity, such as, 
for example, in magneto-optical recording apparatus. However, the 
invention is not limited to magneto-optical recording but may also be 
applied to other recording principles, such as, for example, recording on 
rewritable record carriers of the "phase-change" type, employing a record 
carrier whose structure can be changed from amorphous to crystalline and 
vice versa upon scanning with a radiation beam, depending on the 
irradiation method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1a and 1b show an embodiment of a record carrier 1 of a rewritable 
type, FIG. 1a being a plan view and FIG. 1b showing a small part in a 
sectional view taken on the line b--b. The record carrier has a pattern of 
track portions, which define substantially concentric information areas 
intended for recording information in the form of information patterns of 
optically detectable marks. The track portions pattern may comprise, for 
example, a continuous spiral servo track 4 defining the centers of the 
information areas. However, these concentric information areas may also be 
defined by, for example, a structure of servo patterns as described in 
Netherlands Patent Application NL-A 8702905. For the purpose of recording, 
the record carrier 1 comprises a recording layer 6 provided on a 
transparent substrate 5 and coated with a protective layer 7. The 
information layer 6 is composed of a material suitable for magneto-optical 
recording. However, it is to be noted that the information layer 6 may 
alternatively consist of other materials such as, for example, a 
"phase-change" material, whose structure can be changed from amorphous to 
crystalline and vice versa by suitable irradiation methods. 
FIG. 2 shows an embodiment of a magneto-optical recording apparatus 10 for 
recording information on the record carrier 1. The recording apparatus 10 
comprises a turntable 11 and a drive motor 12 to rotate the record carrier 
1 about an axis 13. An optical write/read head 14 of a customary type 
suitably for magneto-optical recording and reading is arranged opposite 
the rotating record carrier 1 and directs a radiation beam 15 towards the 
recording layer 6. The recording apparatus 10 comprises a customary 
tracking device, not shown, for keeping the beam 15 directed at the servo 
track 4, a focusing means for keeping the radiation beam 15 in focus on 
the recording layer 6, and customary addressing device for locating a 
specific address, a for example, as described in European Patent 
Application EP-A 0265904 and Netherlands Patent Application NL-A 8800151. 
Opposite the read/write head 14, at the other side of the record carrier 
1, a magnetic field modulator (MFM) 16 is arranged to generate a 
magnetic-field H which is oriented substantially perpendicularly to the 
recording layer 6 in the area of the recording layer 6 which is exposed to 
the radiation beam 15. The magnetic-field modulator 16 is rigidly 
connected to the read/write head 14 via a physical connecting member 17. 
The write/read head 14 and the magnetic-field modulator 16 are radially 
movable relative to the record carrier by means of an actuating system 18, 
the connecting member 17 ensuring that the magnetic-field modulator 16 
remains always positioned directly opposite the read/write head. The 
magnetic-field modulator 16 is of a type for which the direction of the 
generated magnetic field can be modulated in conformity with a bivalent 
write signal Vm. Such a magnetic field modulator is described 
comprehensively in, inter alia Netherlands Patent Application no. 8702451, 
herewith incorporated by reference. 
The apparatus 10 further comprises a control circuit 19 for controlling the 
write/read head 14 and the actuating system 18 and for controlling the 
generation of the write signal Vm. When information is recorded, the servo 
track 4 is scanned with the radiation beam 15 whose intensity is set to a 
write intensity which is adequate to heat the part of the recording layer 
6 which is scanned by the radiation beam to a temperature near the Curie 
temperature of the material of the recording layer 16. At the same time, 
the write signal Vm, and hence the generated magnetic field H, is 
modulated in conformity with the information to be recorded, so that a 
pattern of marks in the form of magnetic domains in conformity with the 
write signal Vm is obtained in the part of the servo track 4 being 
scanned. The domains which are formed can be detected optically, as will 
be described in detail hereinafter. 
By way of illustration, FIG. 3 shows the write signal Vm, the corresponding 
magnetic field H and the resulting pattern of magnetic domains having 
different directions of magnetization as a function of time. The domains 
of different directions of magnetization bear different reference 
numerals, namely 30 and 31. The center of the servo track in which the 
pattern is recorded is represented diagrammatically by a line 4'. The 
pattern of magnetic domains 30 and 31 can be read with the read/write head 
14, which for this purpose scans the pattern with a beam of linearly 
polarized light. Upon reflection of the radiation beam the direction of 
polarization of the beam is rotated in a direction which is dictated by 
the direction of magnetization of the part of the recording layer 6 being 
scanned. This results in a modulation pattern of variations of the 
direction of polarization corresponding to the pattern of magnetic domains 
30, 31 being scanned. This modulation is detected in a customary manner in 
the read/write head 14, for example, by means of a Wollaston prism, 
photoelectric transducers, and an amplifier which converts the output 
signals of the photoelectric transducers into a read signal V1 which is 
representative of the pattern being read, as is described, inter alia, in 
Netherlands Patent Application NL-A 8602304. 
One of the principal aspects of recording is the reliability with which the 
recorded information can be read. A known parameter to express the read 
reliability for recorded CD signals is referred to as the "block error 
rate" (BLER). The parameter BLER specifies the number of EFM blocks per 
unit of time in which one or more errors have been detected during 
reading. 
FIG. 4 shows patterns of domains 30 and 31 formed in a plurality of 
adjacent track portions of the servo track 4. The centers of the servo 
track portions bear the reference numeral 4'. The track pitch, i.e., the 
distance between the centers 4' of the servo track portions, bears the 
reference letter q. The dimensions of the domains in a direction 
perpendicular to the track portions direction is indicated by means of the 
letter p. Hereinafter, the domain dimensions in this direction will be 
referred to briefly as the "domain width". 
FIG. 5 gives the BLER value as a function of the domain width p. It is 
found that within a range from pmin to pmax the BLER value substantially 
assumes a constant minimum value, while outside this range the BLER value 
increases rapidly. The domain width p which is equal to the track pitch q 
is found to be situated in the center of the range between pmin and pmax. 
In accordance with the invention, the domain width p during recording is 
selected to be equal to the track pitch q. In that case, the read 
reliability is least susceptible to domain-width variations, which are 
inevitable on account of the various tolerances in the recording system. 
Hereinafter, the domain width p corresponding to the track pitch q will be 
referred to as the optimum domain width po. The domain width can be 
adjusted simply by adapting the write-energy intensity of the radiation 
beam 15. 
FIG. 6 gives the domain width p as a function of the write intensity E when 
the track 4 is scanned with a specific scanning velocity during recording. 
The write-intensity values corresponding to the domain width pmin, po and 
pmax are Emin, Eo and Emax, respectively. This makes clear that the 
optimum domain width can be adjusted by adjusting the write intensity to 
the corresponding value Eo. For a specific record carrier, it is possible, 
for example, to determine the value of the optimum write intensity in 
advance. Thus, before information is recorded on this record carrier, it 
is then possible, in principle, to adjust the write intensity of the 
recording apparatus to this value. 
However, this presents the following problems: 
1) There is a substantial spread in radiation sensity of the recording 
layers on different record carriers, even if they are made of the same 
magneto-optical material. This is caused by the generally adopted method 
of depositing the recording layer, such as, for example, sputtering. 
2) The influence of the scanning velocity on the optimum write intensity Eo 
is considerable. This poses a problem particularly if the recording 
velocity for different recording apparatus vary substantially, as for 
example in recording apparatus for CD signals, in which the permissible 
recording velocity is between 1.2 m/s and 1.4 m/s. 
3) In practice, accurately determining the actual radiation power is very 
problematic. The spread between power meters is of the order of magnitude 
of 10%. Moreover, different adjustment conditions may give rise to 
additional errors. 
4) Finally, the shape of the scanning spot formed on the recording layer 6 
by the radiation beam and the wavelength of the radiation also influence 
the optimum write intensity. 
These problems mean that the variation of the optimum write intensity is so 
large that it is impossible to guarantee that when the write power has 
been adjusted to the predetermined power the domain width will be situated 
within the write-intensity range given in FIG. 5, in which the BLER value 
is small. 
A method and a recording apparatus in accordance with the invention, 
enabling the optimum write intensity to be adjusted reliably and simply, 
will now be described with reference to FIG. 7, in which the reference 
numerals 4a', 4b' and 4c' denote the centers of three adjacent track 
portions of the track 4. In the first step of the method, a predetermined 
first pattern of magnetic domains 30 and 31, for example, a periodic 
pattern of the frequency f1, is recorded with a maximum write intensity E1 
in the center track portion, as is illustrated in FIG. 7a. The write 
intensity E1 is selected so as to ensure that the corresponding domain 
width p1 is larger than the track pitch. Subsequently, a second pattern of 
magnetic domains 30 and 31, which can be distinguished from the first 
pattern, is recorded in all three track portions with a minimum write 
intensity E2 for which the associated domain width p2 is bound to be 
smaller than the track pitch. FIG. 7b shows the result of this recording, 
in which the second pattern is a periodic pattern of a frequency f2 lower 
than the frequency f1 of the first pattern. In the recording obtained, the 
first pattern, which was originally recorded in the center track portion, 
is partly overwritten by the second pattern. 
After recording of the second pattern in the three track portion the center 
track portion is read. Since the originally recorded first pattern is 
still partly present (as is indicated by the reference numeral 70) the 
read signal, will contain signal components corresponding to the first 
pattern in addition to signal components corresponding to the second 
pattern. The presence of the signal components corresponding to the first 
pattern is detected. Subsequently, the write intensity is increased and 
the second pattern is recorded again in the three track portion with this 
increased write intensity. Since as a result of the increased write 
intensity, the width of the recorded domains is larger than in the 
preceding recording of the second pattern, the originally recorded first 
pattern will be overwritten to a larger extent. When the center track 
portion is again read, the signal component in the read signal 
corresponding to the first pattern will have decreased. The method 
increasing the write intensity, of recording the second pattern with of 
increased write intensity, and of reading the center track portion is 
repeated continually. The read-signal component corresponding to the first 
pattern will decrease continually until the write intensity reaches a 
value for which the corresponding domain width has become so large that 
the originally recorded first pattern is overwritten completely. This is 
the case when the domain width p becomes equal to the track pitch q. In 
that case, the spacing between the two patterns recorded in the adjacent 
tracks has decreased to zero. By way of illustration, FIG. 7c shows the 
second in the three track portions for the situation in which the domain 
width p is equal to the track pitch q. As is shown in FIG. 7c, the 
originally recorded first pattern has disappeared completely for this 
track portion width. Thus, the optimum write intensity Eo can be obtained 
by recording the second pattern with increasing intensity and at the same 
time detecting the write intensity for which the signal component produced 
by the originally recorded first pattern disappears from the read signal 
produced by the center track portion. 
By way of illustration, the curve 80 in FIG. 8 gives the variation of the 
read-signal component Ulf corresponding to the originally recorded first 
pattern as a function of the write intensity E. Moreover, this figure 
gives the BLER values determined for the various write intensities. The 
curve 81 represents the variation of the BLER values. As is apparent from 
FIG. 8, the write intensity E for which the signal component Ulf 
disappears is situated substantially in the center of the write-intensity 
range in which the BLER value is minimal. 
FIG. 9 shows an example of the control circuit 19 of a recording apparatus 
10 in accordance with the present invention, by means of which the optimum 
write intensity can be determined. The control circuit comprises a 
frequency-dividing circuit 90, which in a customary manner derives two 
periodic signals Vt1 and Vt2 of different frequency fc1 and fc2 
respectively from a periodic signal of a frequency fosc. The two periodic 
signals Vt1 and Vt2 supplied by the dividing circuit 90 are applied to a 
first input and a second input, respectively, of a selection circuit 91 
having three inputs. The signal Vi to be recorded is applied to a third 
input of the selection circuit 91. The selection circuit 91 is of a type 
which depending upon a control signal Vse1 selects one of the three input 
signals and transfers the selected input signal to its output. The signal 
on the output of the selection circuit is applied to the magnetic-field 
modulator 16 (of FIG. 2) as the write signal Vm. The control circuit 19 
further comprises a selective band-pass filter 92, which is tuned to the 
frequency fc1 of the signal Vt1. An input 98 of the selective band-pass 
filter 92 is coupled to the write/read head 14 (of FIG. 2) for recording 
the read signal V1. An output signal of the selective filter 92 is applied 
to a peak detector 93 to determine the peak value of the applied signal, 
which has been filtered by means of the, selective filter 92. A signal 
Uc1, which is representative of the peak value, is digitized by means of 
an analog-to-digital converter 94. The digitized peak value is applied to 
a microcomputer 95. The microcomputer 95 is coupled to the selection 
circuit 91 via a signal line 96 to supply the control signal Vse1 to the 
selection circuit 96. 
The microcomputer 95 is also coupled to the read/write head 14 (of FIG. 2) 
to apply a control signal VE for adjusting the intensity of the beam 15. 
The microcomputer 95 further comprises control outputs and inputs (not 
shown) for controlling the search for location of addressed track 
portions, as is described, for example, in Netherlands Patent Application 
NL-A 880015, which is incorporated herewith by reference. The 
microcomputer 95 is loaded with a program for determining the optimum 
write intensity Eo. 
A suitable program will be now described in detail with reference to FIGS. 
10 and 11. FIG. 10 shows the flow chart of the program and FIG. 11 shows 
the track portions of the servo track 4 in which the patterns for 
determining the optimum write intensity Eo can be recorded. These track 
portions comprise three contiguous turns of the spiral servo track 4. The 
address information is recorded in the turns, for example as a preformed 
modulation of the servo track 4, as is described in for example, the 
afore-mentioned Netherlands Patent Application NL-A 8800151. The start 
addresses of the three turns are designated TR1, TR2 and TR3, 
respectively. Halfway along the turn, having the start address TR2, an 
address TR2' is recorded. The end of the third turn is indicated by the 
address TR4. The program, whose flow chart is given in FIG. 10 begins with 
a first step in which the intensity of the write beam E is set to a read 
intensity E0, which is low enough to preclude variations of the 
magnetization in the recording layer 6. Subsequently, the track portion 
indicated by the address TR2 is located in step S2. In step S3, the 
selection circuit 91 is controlled in such a way that the test signal Vt1 
of the frequency fc1 is selected as the write signal Vm. In step S4, the 
beam intensity is set to the maximum write intensity E1, after which 
recording of the signal Vt1 in the form of a pattern of wide domains in 
the track portion indicated by the address TR2 begins. During recording, 
the addresses are read from the track being scanned in step S5. In step S6 
a determination is made based upon the information being read as to 
whether the beginning of the track portion indicated by the address TR3 is 
reached. If not, step S5 is repeated. If the track portion is reached, the 
intensity of the beam 15 is again set to the read intensity E0 during step 
S7. Subsequently, in step S8, a value Es which is representative of the 
write intensity is equalized to the value which is representative of the 
minimum write intensity E2. After this, in step S9, the track portion 
indicated by the start address TR1 is located. In step S10, the selection 
circuit 91 is controlled in such a way that the test signal Vt2 of the 
frequency fc2 is applied to the magnetic-field modulator 16 as the write 
signal Vm. Subsequently, the intensity of the beam 15 is adjusted to the 
intensity specified by the value Es, after which recording of the test 
signal Vt2 in the form of narrow domains begins, During recording, steps 
S12 and S13 are carried out, in which a determination (i.e., a check) is 
made as to whether the track portion having the start address TR4 is 
reached. If not, recording is continued. If the track portion is reached, 
step S14 is carried out, in which the intensity of the radiation beam is 
again set to the read intensity E0. Subsequently, in step S15, the track 
portion indicated by the start address TR2' is located and this track 
portion is read for a specific time interval .DELTA.T. At the end of the 
interval .DELTA.T, the digitized value of Uc1 is read while step S17 is 
performed. In step S18, the value of Uc1 read is compared with a very 
small reference value Umin, which is, for example, 40 dB below the signal 
value corresponding to a non-overwritten test signal Vt1. If the value of 
Uc1 exceeds the value Umin, the value of Es is incremented by an 
adaptation value .DELTA.E in the step S19, and, subsequently the program 
proceeds with the step S9. However, if the value of Uc1 is smaller than 
the reference value Umin, this means that the first test signal has been 
overwritten completely, and, consequently, the intensity value Es 
corresponds to the optimum value Eo. After this, the intensity of the beam 
15 is again set to the read intensity Eo in step S20, and the program is 
terminated. If subsequently the signal Vi is to be recorded, the 
microcomputer 75 controls the selection circuit 91 in such a way that the 
signal Vc is applied to the magnetic-field modulator 16 as the write 
signal, Vm, and the write intensity of the beam 15 is adjusted to the 
optimum value Eo, equal to the most recently obtained value of Es. 
In the present embodiment, the second signal Vt2 is each time recorded over 
the whole length of the three turns of the track 4, designated by the 
start addresses TR1, TR2 and TR3, respectively, after an increase in write 
intensity. However, it is alternatively possible to divide the three turns 
into a number of addressable sectors. Subsequently, the test signal Vt2 is 
recorded within a sector in each of the three track portions for which the 
write intensity of the various sectors can be different, and, moreover, 
the write intensities used for the various sectors are stored in a memory. 
After this, the sectors of the center turn can be read and the read-signal 
components corresponding to the test signal Vt1 can be determined for the 
various sectors. The value of the optimum write intensity Eo can then be 
derived from these measurement results and the stored write intensities. 
In the above-discussed magneto-optical recording apparatus, a 
constant-intensity beam is directed towards the record carrier during 
recording. However, it is to be noted that the invention may also be 
utilized in magneto-optical recording apparatus in which the radiation 
energy is applied in the form of periodic radiation pulses of constant 
intensity, as is described, for example, in Netherlands Patent Application 
NL-A 8703011 or NL-A 8801205. 
It must also be noted that the invention is not limited to magneto-optical 
recording apparatus in which the information is recorded by means of a 
modulated magnetic field. The invention can also be applied to 
magneto-optical recording systems in which the information is recorded by 
first scanning the recording layer with a radiation beam of constant 
intensity during which, the scanned portion of the recording layer is 
exposed to a constant magnetic field, so that a track of uniform 
magnetization is obtained (erasing), and by subsequently reversing the 
direction of the magnetic-field and scanning the track of uniform 
magnetization with a radiation beam whose intensity is modulated in 
conformity with the signal to be recorded, as is described, for example, 
in the above-mentioned "Philips' Technical Review". In such case, the test 
signal Vt2 is suitably not a periodic signal but a d.c. signal, and the 
test signal Vt1 is a periodic signal. 
Moreover, it is to be noted that although the invention is very suitable 
for use in magneto-optical recording it is not limited to this recording 
method. For example, the invention may also be applied to what is referred 
to as "erasable phase-change recording" in which by exposure of the 
recording layer to a radiation beam the structure of the recording layer 
can be changed from amorphous to crystalline or from crystalline to 
amorphous depending on the scanning method used. It is then possible to 
first record a periodic test signal Vt1, which is subsequently overwritten 
by a d.c. test signal Vt2 with a number of different write intensities. 
Finally, it is to be noted that the invention is not limited to the use in 
conjunction with disc-shaped record carriers having a concentric track 
pattern (e.g., a continuous spiral track or separate multiple concentric 
tracks). The invention may also be used in conjunction with record 
carriers on which the information is recorded in straight tracks.