Optical scanning device for scanning a record carrier with a scanning spot which deviates in a direction transverse to the scanning direction by an amount less than a trace pitch because of vibration

An optical scanning device for scanning a tape-like record carrier. The device includes an optical system for focusing a radiation beam at the record carrier, which radiation beam causes a scanning spot to occur on the record carrier. The device also includes a rotary polygon mirror which causes the scanning spot to be displaced transversely to the direction of the tape with a specific repetition rate. The tape-like record carrier is moved with a certain velocity in the longitudinal direction of the tape relative to the scanning device. In this manner, the record carrier is scanned in accordance with a track pattern formed by a longitudinal path of substantially parallel tracks which have a substantially constant track pitch and a track direction transverse to the longitudinal direction of the record carrier. During operation, vibrations occur which cause displacements to occur of the scanning spot over the recording layer transverse to the scanning direction. The relation between velocity and repetition rate is selected such that the amplitude of the displacements of the scanning spot, which displacements have a frequency exceeding the repetition rate, is smaller than the track pitch. As a result, the track pitch throughout the length of the tracks remains substantially constant without use of a controller.

BACKGROUND OF THE INVENTION 
The invention relates to a device for scanning a layer of a medium in 
accordance with a track pattern formed by a longitudinal path of 
substantially parallel tracks which have a substantially constant track 
pitch and have a track direction transverse to the longitudinal direction 
of the path, the device comprising an optical system for focusing a 
radiation beam at the layer, while the radiation beam causes a scanning 
spot to develop on the layer, scanning means for causing the scanning spot 
to be displaced with a specific repetition rate over the layer along a 
scanning path that has a specific scanning direction, driver means for 
causing the medium to be displaced relative to the scanning means with a 
certain velocity in a direction transverse to the scanning direction, in 
which device, when operative, vibrations occur which cause the scanning 
spot to be displaced over the layer in a direction transverse to the 
scanning direction, which displacements of the scanning spot have an 
amplitude that exceeds the track pitch. 
Such a device is known from U.S. Pat. No. 4,901,297. In the known device a 
path of parallel tracks on a tape-like record carrier is realised, which 
record carrier is displaced relative to an optical scanner. The optical 
scanner scans the record carrier repeatedly in a direction transverse to 
the direction of displacement of the tape-like record carrier. 
For the tracking during recording the known device comprises a fine 
adjustment which corrects the position of the scanning spot in a direction 
transverse to the direction of the track in response to a detected 
tracking error, so that the recording scanning spot is maintained 
substantially in the middle of the track. During this operation the 
tracking error is derived with respect to a predetermined track pattern. 
The disadvantage of prior-art device is that a recording, during which the 
distance between the middles of two successive tracks (also denoted track 
pitch) remains substantially constant over the whole length of two 
successive tracks, is only possible if the record carrier used already has 
a prearranged track pattern. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a device of the type defined in 
the opening paragraph in which the track pitch between successive tracks 
remains the same throughout the length of these tracks during recording. 
The device according to the invention is thereto characterized in that the 
relation between velocity and repetition rate is selected such that the 
amplitude of the displacements of the scanning spot, which displacements 
have a frequency exceeding the repetition rate, is smaller than the track 
pitch. 
In the device according to the invention the fact is advantageously 
utilized that the displacements of the scanning spot which are caused by 
annoying vibrations diminish above a certain limit frequency to values far 
below the track pitch. 
By scanning with a frequency for which the track pitch exceeds the 
amplitude of the scanning spot displacements caused by the vibrations 
there is achieved that, seen in longitudinal direction of the track, the 
intertrack distance remains substantially the same without the need for 
readjusting the position of the scanning spot. The displacement of the 
scanning spot due to low-frequency vibrations which have an amplitude 
exceeding the track pitch by far, is uniformly distributed over a very 
large number of tracks and, therefore, only affect the track pitch to a 
very small extent. 
An embodiment for the device is characterized in that the device comprises 
control means for controlling the velocity in dependence on a measuring 
signal. 
This embodiment is advantageous in that the average track pitch (which is 
proportional to the average velocity) is constantly kept at a defined 
value. The velocity is preferably controlled in response to a measuring 
signal which is indicative of the distance between the middles of 
successive tracks (track pitch). An attractive embodiment for the device 
in which this is realised is characterized in that the electro-optical 
measuring means comprises an optical system for focusing a radiation beam 
at the track pattern, the track pattern converting the incoming radiation 
beam into a zero.sup.th order beam and first-order beams and a detection 
means for deriving as the measuring signal a detection signal that is 
indicative of the angle between the zero.sup.th order beam and one of the 
first-order beams. 
A further embodiment for the device in which the velocity is controlled 
based upon the distance between successive tracks is characterized in that 
the optical means comprises means for focusing a satellite beam together 
with aforesaid scanning beam at the recording layer, the satellite beam 
causing a satellite scanning spot on the recording layer which spot is 
located at a predefined position relative to the former scanning spot, the 
electro-optical measuring means comprising a detection system for deriving 
the measuring signal based upon the radiation coming from the satellite 
scanning spot. 
An embodiment for the device which is not only suitable for information 
recording but also for reading the recorded information is characterized 
by an actuator that intervenes in the optical system and causes the 
scanning spots to be displaced in a direction transverse to the tracks 
within a predefined range of displacement, and signal generating means for 
generating a position signal which is indicative of the position of the 
scanning spots in the scanned area, control means which may be brought to 
a first and a second state and which, in the first state, cause the 
velocity to be controlled in response to the measuring signal and, in the 
second state, cause the actuator to be controlled in response to the 
measuring signal and the velocity to be controlled in response to the 
position signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The scanning device shown comprises an optical scanning system formed by a 
lightwave 101, a rotary polygon mirror 105 and a focusing objective 107. 
The lightwave 101 may be of a type as is customarily used in optical or 
magneto-optical recording and/or reading devices. Such a lightwave 
comprises generating means for generating radiation beams 102. The 
radiation beam 102 is focused via the polygon mirror 105 and the focusing 
objective 107 at a layer of the tape-like medium 109, for example a record 
carrier having a radiation-sensitive recording layer which layer undergoes 
an optically detectable change under the influence of the radiation coming 
from the radiation beam 102. This recording layer may be of a 
magneto-optical or optical type. The radiation beams 102 are focused by 
the focusing objective 107 to a very small scanning spot 111 on the 
recording layer of the record carrier 109. An area 110 on the recording 
surface where the radiation beam 102 hits the recording layer is shown in 
detail. In the area 110 shown in detail the scanning spot 111 shown is 
caused by the radiation beam 102. 
The polygon mirror 105 has reflecting facets 108 and is rotated around an 
axis 117 by customary driving means (not shown) which are extensively 
described, for example, in U.S. Pat. No. 5,171,984 and European Patent 
Application 0 459 586, which documents are deemed incorporated herein by 
reference. The polygon mirror 105 is positioned relative to the lightwave 
101 so that, on rotation of the polygon mirror 105 around the axis of 
rotation 117, always a next facet of the facets 108 is hit by the 
radiation beam 102, so that the recording layer is recurrently scanned by 
the scanning spot 111 and the scanning spot 111 follows a scanning path 
116. The repetition rate fh of the scanning along the scanning path 116 is 
equal to the number of revolutions per minute (r.p.m.) of the polygon 
mirror 105 times the number of facets 108 of the polygon mirror 105. 
The embodiment shown in FIG. 1 further includes displacement means for 
displacing the record carrier 119 relative to the optical system with a 
velocity v in a direction y transverse to the direction of the scanning 
path 116. These displacement means may be of a customary type shown 
diagrammatically in FIG. 1 and comprising a reel 114 driven by a motor 115 
for winding the tape-like record carrier 109 which is transported by this 
reel in the direction y that corresponds to a longitudinal direction of 
the tape-like record carrier 119. The direction y is indicated by an arrow 
119 in FIG. 1. 
The recording device described above realises ever successive tracks which 
carry effects on the scanning path 116 scanned by the scanning spot 111 on 
a recording layer of the record carrier 109. In this manner a pattern of 
parallel tracks is realised, a new track being written each time the 
scanning spot 111 scans the recording layer. FIG. 2 shows by way of 
illustration the track pattern thus obtained, the tracks being designated 
by reference character 120. The length of the tracks is referenced 1 and 
the distance between the middles of the tracks, also termed track pitch, 
is denoted p. With a given velocity v and a repetition rate fh the track 
pitch is equal to v/fh. 
If the device shown in FIG. 1 is in operation, there will be oscillations 
due to, among other things, the drive of the record carrier 109, which 
oscillations cause displacements to occur of the recording layer 111 
underneath the scanning spot 111. Especially displacements in the 
direction transverse to the tracks 120 are detrimental, because they 
affect the track pitch of successive tracks 120. The displacement of the 
scanning spot due to these oscillations is therefore to be smaller than 
the track pitch. The disturbance of the track pitch of successive tracks 
120 is exclusively caused by oscillations having frequencies of which the 
values are of the order of the repetition rate fh or higher than this 
repetition rate. 
FIG. 3 shows by way of illustration for a customary combination of driving 
mechanism and polygon scanner, the amplitude z of the displacement of the 
scanning spot 111 caused by vibrations plotted against frequency. 
As appears from FIG. 3 the amplitude strongly decreases with high 
frequencies. With frequencies exceeding 10 kHz the amplitude has dropped 
to values below 0.1 micrometer. 
With a customary track pitch of 1-2 micrometers, the effect of the 
vibrations on the track pitch between successive tracks 120 will be 
negligibly small in the case of repetition rates exceeding 10 kHz. If the 
repetition rate is selected to have an amplitude z that is small relative 
to the track pitch, the vibrations have only a minor effect on the 
distance between the successive tracks. Preferably, the repetition rate is 
selected to exceed a limit frequency for which the amplitude z is 20% of 
the track pitch. It will be evident that the effect of the vibrations 
decreases as the repetition rate is situated higher above this limit 
frequency. 
If the repetition rate is selected to exceed this limit frequency, a path 
of parallel tracks 120 is obtained for which the distance between each 
pair of successive tracks remains substantially constant throughout the 
length of the track. 
When the polygon mirror 105 is driven, the r.p.m. will generally remain 
constant because the operating conditions affecting the r.p.m. for the 
polygon mirror 105 do not vary or vary only slightly. This is in contrast 
to the drive of the record carrier. Albeit not necessary, preference 
should be given to having the record carrier drive include a feedback 
control for controlling the velocity v. This may be effected, for example, 
by deriving in customary fashion a measuring signal Vm which is inactive 
for the velocity v. FIG. 4 shows by way of example an embodiment in which 
this is realised, in which the elements corresponding to those shown in 
FIG. 1 carry the same reference characters. Reference character 140 
denotes a tape velocity meter of a customary type for deriving the 
measuring signal Vm. The measuring signal is applied to a non-inverting 
input of a comparator circuit 141. An inverting input of the comparator 
circuit 141 is supplied with a reference signal Vmr which indicates a 
desired velocity. The comparator circuit 141 applies an output signal to a 
control circuit 142, which output signal is indicative of the difference 
between the measuring signal Vm and the reference signal Vmr. An output 
signal of the control circuit 142 triggers a control signal for a driving 
circuit 143 to drive the motor 115. The control circuit 142 is one of a 
customary type which produces a control signal in response to the output 
signal of the comparator circuit 141 for which the difference between Vm 
and Vmr remains substantially equal to zero. 
Although the embodiment shown in FIG. 4 is highly satisfactory, it is 
advantageous to have the velocity controlled as a function of a measuring 
signal Vp indicative of the track pitch in lieu of the measuring signal 
Vm. 
For the case where the record carrier 109 is of a type in which the scanner 
records tracks showing a regular structure, FIG. 5 shows an embodiment in 
which the velocity of the record carrier 109 is controlled in response to 
the measuring signal Vp. 
FIG. 5 shows the pans shown in the previously described FIGS. 1 and 4 again 
designated by like reference characters. To derive the measuring signal 
Vp, the device comprises a light source 150 which is, for example, a 
semiconductor laser for generating a radiation beam 151. The radiation 
beam 151 is focused at the track pattern formed by the tracks 120, which 
pattern is at a short distance from the location where the scanning spot 
110 recurrently scans the recording layer 109. As a result of the regular 
structure formed by the track pattern, the radiation beam 150 incident on 
the track pattern is converted into a reflected zero.sup.th order beam 152 
and higher-order beams. From these higher-order beams there is shown only 
a first-order beam 153. The size of the angle a between the n.sup.th order 
beam 152 and the first-order beam 153 depends on the track pitch of the 
tracks 120. By means of a radiation-sensitive detector 154 a signal is 
derived which denotes how much angle a deviates from the angle of the 
desired track pitch. The radiation-sensitive detector 154 is one of a 
customary type producing a signal that denotes the deviation between the 
middle of a radiation-sensitive surface 155 and the spot where the 
radiation beam is incident on the radiation-sensitive surface 155. The 
radiation-sensitive detector is arranged in such a way that in the case 
where the angle a corresponds to the desired angle, the first-order beam 
hits the radiation-sensitive surface 155 of the detector 154 in the 
middle. 
The output signal of the detector 154 is always indicative of a deviation 
between the real track pitch of the tracks 120 and the desired track 
pitch. For that matter, this output signal is used as the measuring signal 
Vp. The measuring signal Vp is applied to the control circuit 142 which, 
in dependence on this measuring signal Vp, derives the control signal by 
which the velocity is maintained at a value for which the track pitch 
between the tracks is kept equal to the desired track pitch. 
FIG. 6 shows an embodiment in which the signal Vp is derived in a different 
manner. In this drawing Figure the parts corresponding to those shown in 
the other drawing Figures are again designated by like reference 
characters. 
In lieu of the light wave 101, the device comprises an adapted light wave 
101a which generates a satellite radiation beam 103 in addition to the 
radiation beam 102. The satellite radiation beam 103 is also focused at 
the recording layer of the record carrier 109 via the objective 107 and 
causes a very small satellite scanning spot 112 on the recording layer of 
the record carrier 109. The intensity of the beam 103 is insufficient to 
cause a change in the recording layer. The beam 103 is focused such that 
with a desired track pitch of the tracks 120 the middle of the satellite 
scanning spot coincides with an edge of a track realised previously. By 
way of illustration FIG. 7 shows a position of the scanning spots 111 and 
112 for the case where the spacing of two successive tracks 120 is equal 
to the desired track pitch. The radiation beam 103 reflected by the record 
carrier 109 is led back to the lightwave 101a via the focusing objective 
107, in which lightwave the reflected beam is directed at a 
radiation-sensitive detector of a customary type which produces a 
detection signal indicating the power of the received radiation. The 
output signal of the detector functions as the signal Vp. FIG. 8 shows by 
way of illustration the power of the signal Vp as a function of the 
distance between the track instantaneously recorded by the scanning spot 
111 and the adjacent track which is scanned by scanning spot 112. The 
signal value Vp0 corresponds to the desired distance p0. The comparator 
circuit 141 compares the signal Vp with a reference signal Vpr which has a 
signal value corresponding to Vp0. The result of the comparison is applied 
to the control circuit 142. The control circuit controls the velocity of 
the record carrier 109 to a value at which the signal Vp and the signal 
Vp2 substantially remain equal to each other. 
The embodiments described above are intended for recording tracks 120. FIG. 
9 shows an embodiment for a device according to the invention which is 
suitable for both recording tracks 120 and reading the recorded tracks 
120. In this drawing Figure the parts corresponding to parts in other 
drawing Figures are again designated by like reference characters. The 
device shown comprises a modified lightwave 101b which generates a second 
satellite radiation beam 104 in addition to the radiation beam 102 and the 
satellite radiation beam 103. The beams 102, 103 and 104 are focused at 
the recording layer of the record carrier via the focusing objective 107 
while the radiation beams 102, 103 and 104 cause scanning spots 111, 112 
and 113 respectively to occur. The beams 102 and 103 are focused such that 
the mutual positions of the radiation spots 111 and 112 are identical with 
the mutual positions as they occur in the device shown in FIG. 6. The 
radiation beam 104 is focused such that the scanning spot 113 and the 
scanning spot 112 caused by this radiation beam are symmetrical with the 
scanning spot 111. 
The radiation beams 102, 103 and 104 reflected by the record carrier 109 
are led back to the lightwave 101b via the focusing objective 107. In the 
lightwave each of these reflected beams is directed at a 
radiation-sensitive detector. FIG. 10 shows by way of illustration these 
radiation-sensitive detectors which are referenced 210, 211 and 212. The 
detectors are arranged such that the reflected beam 102 hits detector 210, 
the reflected beam 103 hits detector 211 and the reflected beam 104 hits 
detector 212. The detectors 210, 211 and 212 are of a customary type 
producing a detection signal indicative of the power of the radiation 
received by the detector. The signal produced by the detector 211 
functions as the signal Vp. The detection signal produced by the detector 
210 functions as the reading signal V1 which is indicative of information 
available in the track scanned at the scanning spot 111. The detection 
signal produced by the detector 212 is referenced signal Vn. When 
prerecorded tracks 120 are read out, it is possible to recover in 
customary fashion a tracking error signal Vre from the difference between 
the signal Vp and the signal Vn. Thereto the device shown comprises a 
difference amplifier 190. The tracking error signal Vre is applied to a 
control circuit 191 which measures a control signal Vac in customary 
fashion. The control signal Vac is applied, via a selector switch 192, to 
an actuator 193 which is capable, by displacing a component, in this case 
the focusing objective 107, of the displacement of the scanning spots 111, 
112 and 113 in a direction transverse to the direction of the track within 
a specific limited area. The location of the scanning spots within the 
displacement area is determined by a position detector 194 which produces 
a position signal Vpa which indicates the location of the objective 107 
relative to the detector 194. There should be observed that the position 
signal Vpa can be derived not only with the position detector but also in 
a different manner. For example, in the case where the objective 107 is 
spring suspended, the DC component of the signal applied to the actuator 
193 is indicative of the location. The signal Vpa is compared by a 
comparator circuit 195 with a reference signal Vpar whose signal value 
corresponds to the positions of the scanning spots 111, 112 and 113 
approximately in the middle of their displacement area. 
An output signal of the comparator circuit 195, which is indicative of the 
difference between the signals Vpa and Vpar, is applied to the control 
circuit 142 via a selector switch 196. Also the output signal of the 
comparator circuit 141 is applied to the selector switch 196. The selector 
switches 192 and 196 are of a customary type which may be brought to a 
first or a second state in response to a control signal Vs produced by a 
control circuit 197. In the first state the selector switch 192 applies a 
signal Vco having a constant amplitude to the actuator 193. In the first 
state the selector switch 196 passes the output signal of the comparator 
signal 141 on to the control circuit 142. In the second state the selector 
switch 192 passes the output signal of the control circuit 191 on to the 
actuator 193. In the second state the selector switch 196 passes the 
output signal of the comparator circuit 195 on to the control circuit 142. 
In the selector switches 192 and 196 are brought to the first state (the 
state shown), the device is in a state suitable for realising the tracks 
120. This state will further be referenced writing state. The velocity 
control circuit formed by the components 141, 142, 144 and 115 then 
maintains the velocity at a value for which the track pitch corresponds to 
the desired value in similar manner to the circuit shown in FIG. 6. The 
actuator 193 is then supplied with the signal Vca having a constant signal 
value, so that the objective 107 continues to be in a fixed position. 
If the selector switches 192 and 196 are brought to the second state, the 
device is in a state suitable for scanning with the scanning spot 111 
prerecorded tracks 120. In this state, further to be referenced reading 
state, the output signal of the control circuit 191 is applied to the 
actuator 193. The feedback control loop formed by the components 190, 191 
and 193 then functions as a position tuner which tunes the position of the 
objective 107 so that, in response to the tracking error signal Vre, the 
scanning spot 111 continues to be focused at the track to be scanned. To 
avoid the actuator 193 being jammed at the end of its displacement area, 
the velocity of the record carrier is tuned by the feedback control loop 
formed by the components 194, 195, 142, 144 and 115, so that the actuator 
193 remains in the middle of its displacement area on average. Above 
velocity control loop especially reacts to low-frequency interference and 
therefore acts as a coarse adjustment to keep the scanning spot 111 
focused approximately at the track 120 to be scanned. The position tuning 
especially reacts to high-frequency interference, so that with this 
control loop the scanning spot 111 can be held precisely on the track 120 
to be scanned. Since the change of the track pitch is negligibly small in 
a track, the tuning need not be capable of compensating for these 
deviations. The upper limit frequency of the frequency control band of the 
tuning may then also be lower than the repetition rate fs of the scanning. 
When a fine adjustment (tuning) is used, by which it is impossible to 
compensate for interference above said repetition rate, it is advantageous 
not to make a fine adjustment of the position of the scanning spot during 
the writing state. When the tracks are realised, minor variations in the 
track pitch between different tracks do occur, it is true, which cannot be 
removed by the velocity control loop of the record carrier 109, but such 
variations are of minor importance. It is only important that the track 
pitch between two adjacent tracks remain constant. A fine adjustment which 
reacts to the variations of the track pitch is then redundant. Moreover, 
the measuring signals generally contain noise which may lead to 
unnecessary deviations in the track pitch. 
With reference to FIGS. 11 to 16 an embodiment of the invention will be 
described in which the measuring signal Vp indicative of the track pitch 
is derived in a different manner. In these drawing Figures the components 
corresponding to those occurring in other drawing Figures are denoted by 
like reference characters. 
The scanning device shown in FIG. 11 comprises an optical scanning system 
formed by a lightwave 1, a rotatable polygon mirror 5, a deflecting mirror 
6 and the focusing objective 107. The lightwave 1 may be of a type as is 
customarily used in optical or magneto-optical recording and/or reading 
devices. Such a lightwave comprises generating means for generating 
radiation beams, in this case three radiation beams 2, 3 and 4, whose 
directions show slight mutual differences and of which two beams (2 and 4) 
are symmetrical with a central scanning beam (3). The scanning beams 2, 3 
and 4 are focused at record carrier 109 via the polygon mirror 5, the 
deflecting mirror 6 and the focusing objective 107. The radiation beams 2, 
3 and 4 are focused by the focusing objective 107 to very small scanning 
spots on the recording surface of the record carrier 109. As the 
directions of the three radiation beams are different, the positions of 
the scanning spots differ likewise. The part 110 of the recording surface 
where the radiation beams hit the record carrier 109 is shown enlarged. In 
the enlarged part 110 a first scanning spot caused by the radiation beam 3 
is referenced 11. Second and third scanning spots caused by the radiation 
beams 2 and 4 are referenced 12 and 13, respectively. 
The polygon mirror 5 has reflecting facets 8a, . . . , 8g and is rotated 
around an axis 17 by a customary driving means (not shown), described in 
detail, for example, in U.S. Pat. No. 5,171,984 and EP-A-0.459.586 which 
documents are deemed to be incorporated herein by reference. The polygon 
mirror 5 is arranged relative to the lightwave 1 in such a way that when 
the polygon mirror 5 rotates around the axis of rotation 17, one of the 
facets 8a, . . . , 8g is successively hit by the radiation beams 2, 3 and 
4, so that a recurrent scanning of the recording surface by the scanning 
spots 11, 12 and 13 is realised and the scanning spots 11, 12 and 13 move 
synchronously over the recording surface along scanning paths 16, 18 and 
19, respectively (see FIG. 12). As the radiation beams 2 and 4 are 
symmetrical with the radiation beam 3, the scanning spots 12 and 13 caused 
by the radiation beams 2 and 4 are symmetrical with the scanning spot 11 
caused by the radiation beam 3. 
The polygon mirror 5 has the form of a truncated pyramid whose sloping 
sides form the facets 8a, . . . , 8g and whose axis of rotation 17 
intersects the base in its middle and forms a fight angle to this base. 
Worded differently, the facets 8a, . . . , 8g form an oblique angle to the 
axis of rotation 17 of the polygon mirror 5. These oblique angles cause 
the scanning paths followed by the scanning spots 11, 12 and 13 not to be 
parallel, but to intersect as is represented in FIG. 12. 
The cause of this will be explained hereinafter with reference to the 
drawing FIGS. 13 and 14. 
FIG. 13 shows a polygon mirror 5 in a position in which the radiation beams 
2, 3 and 4 hit the facet 8a approximately in the middle. The spots where 
the radiation beams 2, 3 and 4 hit the mirror determine a direction 
indicated by an arrow 31. An arrow 30 indicates a direction of an 
intersecting line of a plane perpendicular to the axis of rotation 17 and 
the surface of the facet 8a. 
FIG. 14 shows the polygon mirror 5 in a position in which the radiation 
beams 2, 3 and 4 hit the facet 8a near an edge 42. In this position the 
spots where the radiation beams 2, 3 and 4 hit the facet determine a 
direction indicated by an arrow 40 which direction deviates from that of 
an intersecting line of a plane perpendicular to the axis of rotation and 
the plane of facet 8a indicated by arrow 41. 
The mutual positions of the spots where the radiation beams hit the facet 
for the position of the polygon mirror 5 shown in FIG. 13 correspond to 
the positions of the scanning spots shown in FIG. 12 and referenced 11, 12 
and 13. The positions of the scanning spots 11', 12' and 13' correspond to 
the situation shown in FIG. 14. 
The mutual variations of the positions of the scanning spots result in 
displacements of the scanning spots 12 and 13 relative to the scanning 
spot 11 in a direction y transverse to the scanning path 16, the positions 
of the scanning spots 12 and 13 relative to the first scanning spot being 
related to the position of the polygon mirror 5 and thus to the location x 
of the first scanning spot 11 on the first scanning path 16. 
In the embodiment described above, a deflecting element rotatable around an 
axis of rotation is used in the form of a polygon mirror 5 which has 
facets 8 forming an oblique angle to the axis of rotation 17 to obtain 
synchronous movements of the scanning spots 11, 12 and 13 in a way in 
which mutual displacements of the scanning spots 11, 12 and 13 in the 
direction y transverse to the scanning directions take place, and the 
position of the scanning spot 12 and scanning spot 13 relative to the 
scanning spot 11 is related to the location x of the scanning spot 11 on 
the first scanning path 16. 
Such movements of the scanning spots, however, may also be obtained with 
different deflecting elements from a polygon mirror, which have facets 
forming an oblique angle to the axis of rotation. For a description of 
optional alternatives, reference is made to Belgian Patent Application No. 
09301395, to which co-pending U.S. patent application No. 08/248,946 filed 
May 25, 1994 corresponds. 
For realising tracks containing information patterns, the intensity of the 
radiation beam is generally determined to lie between a writing level 
sufficiently high to cause a change in the recording layer and a reading 
level that is not high enough to cause any effect. There should be 
observed that with magneto-optical recordings the effects may also be 
obtained by means of a magnetic field varying in strength, which is 
realised on the spot on the recording layer scanned by a radiation beam. 
The intensity of the radiation beams 2 and 4 have a level that is not high 
enough to cause optically detectable effects in the recording layer, so 
that undesired effects are avoided. 
By way of illustration, FIG. 15a shows the way in which tracks 75 are 
obtained in this manner. The tracks 75 are further numbered -7, . . . , 
-1, 0. The position along the scanning path 16 is indicated by a magnitude 
x, the associated position of the polygon mirror 5 is indicated by a 
magnitude phi which indicates in degrees the position of the facet used 
for the scanning relative to its central position. There is an unambiguous 
relation between the position of the polygon mirror 5 and the position x 
of the scanning spot 11 on the scanning path 16. The positions of the 
scanning spot which is caused by the radiation beam 3 for three different 
values of phi (phi=-.theta.1, phi=0 and phi=+.theta.1) are referenced 11', 
11 and 11". The positions of the scanning spots which spots are caused by 
the radiation beams 2 and 4 are referenced 12' and 13', 12 and 13 and 12" 
and 13" for the three values of phi mentioned above. The scanning spot 11 
moves along the path 16, whereas the scanning spots 12 and 13 move along 
the paths 18 and 19 intersecting path 16. When the scanning spot 11 is 
displaced from position x=-x.sub.1 mm to x=0 mm, the scanning spot 12 
passes a number of tracks, whereas the scanning spot 13 moves along a part 
of the recording layer in which no tracks 75 have yet been realised. When 
the scanning spot is displaced from position x=0 mm to x=+x.sub.1 mm, the 
scanning spot 13 passes a number of tracks 75, whereas the scanning spot 
12 moves along a part of the recording layer in which no tracks 75 have 
yet been made. At the locations where the scanning spots 12 and 13 
coincide completely or in part with one of the tracks 75, the radiation 
reflected by the record carrier 109 will be modulated in accordance with 
the pattern of effects occurring in track 75, also termed information 
pattern. The degree of modulation corresponds to the degree to which the 
scanning spot coincides with the track 75. The recording device comprises 
detection systems of a type known per se for converting radiation coming 
from scanning spots 12 and 13 into a detection signal that corresponds to 
the reflected radiation modulation caused by the information pattern. In 
the embodiment shown in FIG. 11 reference characters 70 and 71 denote 
detection systems for converting radiation coming from scanning spots 12 
and 13, which radiation returns to the lightwave 1 via the focusing 
objective 107, the reflecting mirror 6 and the polygon mirror 5. 
The detection systems 70 and 71 may be of a general type and do not 
themselves form any part of the invention and are therefore represented 
only diagrammatically. Furthermore, the recording device shown in FIG. 11 
includes means for generating a reference signal S3 which is indicative of 
the position of the first scanning spot 11 on the scanning path 16 as well 
as a measuring circuit 72 for deriving at least the measuring signal Vp 
from the detection signals S1 and S2 and the reference signal S3. The 
derivation of the measuring signals Vp will be explained hereinafter. 
As already stated above, the scanning spots 11, 12 and 13 perform 
synchronous movements. Changes in the interspacing of the scanning spots 
seen in the direction y transverse to the direction of the scannings are 
related to the position of the polygon mirror 5 and hence related to the 
position of the scanning spot 11 on the first scanning path 16. The 
middles of the scanning spots 12 and 13 coincide for predetermined values 
of phi (x) with the middles of the previously formed tracks 75. The 
predetermined values of phi with which this takes place are independent of 
the distance from the scanning path 16 to the track 75 passed by the 
scanning spot (12 or 13). Since the spacing of the prerecorded tracks 75 
has a constant value equal to the track pitch, the values of phi for which 
the middles of the scanning spots pass the middles of the tracks 75 depend 
on a distance dy between the scanning path 16 and the middle of the track 
75 recorded last (track carrying track number -1 in FIG. 15a). This means 
that the maximum and minimum modulations of the detection signals occur 
with predetermined positions of the polygon mirror 5. 
By way of illustration, FIG. 15b shows the detection signal S1 plotted 
against phi and against the position x for the case where the distance dy 
from the scanning path 16 to the middle of the adjacent track, indicated 
by line 80, corresponds to a desired track pitch. The detection signal S2 
is plotted in FIG. 15c against phi for part of the scanning path 16 for 
the case where the distance dy corresponds to the desired track pitch. 
Furthermore, the envelopes S10 and S20 of the detection signals S1 and S2 
are shown in FIGS. 5b and 5c. These envelopes S10 and S20 approximately 
have a sinusoidal behaviour which expresses the degree of modulation of 
the associated detection signals S1 and S2. The maximum values of each of 
the envelopes S10 and S20 denote the positions for which the modulations 
of the detection signal are largest. They are the positions in which the 
middle of the associated scanning spot coincides with the middle of one of 
the tracks. As shown in FIGS. 15b and 15c, there is a relation between the 
detection signals S1 and S2 and phi (and hence the position x). This 
relation depends on the distance dy. If this distance changes, the 
positions at which the maximum and minimum values of the envelopes S10 and 
S20 are found, will change. For that matter, the middles of the scanning 
spots 12 and 13 coincide with the middles of the tracks 75 with different 
positions x of the scanning spot 11. For example, when the distance dy is 
reduced, the position values at which the maximum values of the envelope 
S10 occur will undergo a change in negative direction (further to be 
referenced postcursing) and the position values at which the maximum and 
minimum values occur in the envelope S20 will undergo a change in positive 
direction (to be referenced precursing hereinafter). Conversely, when the 
distance dy is increased, the position values at which the maximum values 
of the envelope S10 occur will undergo a change in positive direction 
(precursing) and the position values at which the envelope S20 occurs will 
undergo a change in negative direction (postcursing). A deviation of the 
relation between the detection signals S1 and S2 relative to the relation 
belonging to a value of the distance equal to the desired track pitch is 
thus indicative of a difference between the distance dy and the desired 
track pitch. There should be observed that due to the symmetrical position 
of the scanning spots 12 and 13 relative to the scanning spot 11, the 
influence of a change of dy on the relation between the detection signal 
S1 and the position x is contrary to the influence on the relation between 
the detection signal S2 and the position x. 
A deviation in the relation between the detection signals S1 and S2 and the 
reference signal S3 is measured by the measuring circuit. 
The reference signal S3 may, for example, be a position signal whose signal 
value corresponds to the position of the deflecting element (polygon 
mirror in the embodiment shown) and thus by the position x of the scanning 
spot 11. 
The reference signal S3 is obtained from a position detector 73. The 
position detector may be included in a control system for controlling the 
velocity and/or position of the polygon mirror 5. Driver circuits in which 
information signals are available which indicate the position of a driven 
object are widely known and will therefore not be described in detail. 
FIG. 16 shows an embodiment for the measuring circuit 72. The measuring 
circuit 72 has an input 81 for receiving the detection signal S1. The 
input 81 is connected by a switch 82 controlled by a signal S4 to an input 
93 of a signal processor 84 which converts its received detection signal 
to a binary signal S' whose first logical value indicates that the 
scanning spot belonging to the detection signal is substantially located 
in one of the tracks 75 and whose second logical value indicates that the 
corresponding scanning spot is substantially located between two tracks 
75. The signal processor 84 may be of a customary type also referenced 
track loss detector. Such a track loss detector may comprise, for example, 
a series combination of a bandpass filter 85, an envelope detector 86 and 
a comparator 87. 
The signal S' is available on an output of the circuit 84 and is applied to 
a phase detector 88. By way of illustration the signal S' in FIG. 15d is 
shown as a function of phi. An output of the phase detector 88 is 
connected to an output 90 of the measuring circuit 72 via an inverter 
circuit 89 controlled by a signal S6. The measuring circuit 72 further has 
an input 92 for receiving the detection signal S2. The input 92 is 
connected to an input 93 of the circuit 84 via a switch 94 controlled by a 
signal S5. 
The phase detector 88 is further supplied with a signal S3 which in this 
embodiment is pulse-shaped and whose edges indicate the positions at which 
the maximum and minimum values in the detection signals are to occur. 
By way of illustration, FIG. 15e shows the reference signal S3 as a 
function of the position phi. Furthermore, FIGS. 15f, 15g and 15h show the 
respective signals S4, S5 and S6. 
Signal S4 has a logical "1" value for 0&lt;phi&lt;.theta.1. For these values of 
phi the scanning spot 12 is located in a part of the recording layer on 
which tracks 75 have already been realised and the detection signal S1 
shows a modulation caused by these tracks 75. 
The signal S5 has a logical "1" value for -.theta.1&lt;phi&lt;0. For these values 
of phi the scanning spot 13 is located in a part of the recording layer in 
which tracks 75 have already been made and the detection signal S2 shows a 
modulation caused by these tracks 75. 
The signal S6 has a logical "1" value for -30&lt;phi&lt;0. The edge (signal level 
transition) for the value of phi=0 indicates the boundary between the 
section in which the scanning spot 12 is located in the track area 75. 
The signals S3, S4, S5 and S6 may be generated in customary fashion by the 
position detector 73. Such a position detector 73 may for this purpose be 
coupled to a spindle of the polygon mirror 5. Such a position detector 
coupled to the spindle of the polygon mirror 5 may comprise a so-called 
pulse disc, possibly in combination with counting circuits. Such position 
detectors may be known per se and do not form part of the invention and, 
therefore, will not be described in detail. 
The operation of the measuring circuit 72 will be further explained 
hereinbelow. The polygon mirror 5 is driven with a constant angle 
velocity, so that the value of phi (indicating the position of the facet 
used for the deflection) is constantly varying in a range from -30 to 30 
degrees. In the sub-range .theta.&lt;phi&lt;0 the detection signal S2 will be 
passed to circuit 84 via the switch 94 controlled by signal S5. The phase 
difference between the reference signal S3 and the signal S' derived from 
the detection signal S2 is determined by a phase detector 88. This phase 
difference is 90 degrees (see FIG. 15) for the desired value of dy. The 
phase detector 88 is of a type producing a phase difference signal whose 
(average) signal strength is proportional to the phase difference between 
the signals S' and S3 minus 90 degrees and thus the sign of the (average) 
signal strength indicates the direction of the deviation of dy relative to 
the desired track pitch. In a simple form such a phase detector may 
comprise a so-called EXCLUSIVE-OR-circuit. However, lots of different 
types of phase detectors can be used. The phase difference signal thus 
obtained, which is a measure for the deviation of dy, is applied unaltered 
to the output 90 of the measuring circuit 72 via the controllable inverter 
circuit 89. 
The moment the polygon mirror passes the position phi=0, the detection 
signal S2 is blocked by the switch 94 and the detection signal S1 is 
passed to the input 93 of the circuit 84 via the switch S4 controlled by 
signal S4. The phase detector 88 detects the phase difference between the 
reference signal S3 and the signal S' obtained in response to the 
detection signal S1. As observed earlier, the effect of dy on the 
detection signal S1 is contrary to the effect of dy on the detection 
signal S2. A correction of this is made by the inverter circuit 89 
controlled by the signal S6. For that matter, the moment (phi=0) at which 
the detection signal S2 on the input 93 is replaced by the detection 
signal S1, the inverter circuit 89 is activated leading to an inversion of 
the phase difference signal available on output 90. 
The period T of the signal S' indicates the time difference between two 
successive track transitions by either scanning spot 12 or 13. 
If the scanning spot 11 used for recording has a component of movement in 
the direction y transverse to the tracks 75, this will result in a change 
of the value T relative to the value Ts which belongs to a situation in 
which the position of the scanning spot in the direction transverse to the 
tracks 75 does not change (constant value of dy). The difference between 
the real value of T and Ts thus indicates the deviation of velocity of the 
scanning spot 11 in the direction y transverse to the tracks 75. There 
should again be observed in this context that the influence of the 
velocity on the phase of the periodic detection signal S1 is contrary to 
the influence of the velocity on the phase of the periodic detection 
signal S2. For determining the difference between the period T of the 
signal S', the measuring circuit 72 may comprise a circuit 96 of a type 
known per se. Circuit 96 may comprise, for example, a timer for 
determining the length of the period T, and a subtracter for determining 
the difference between the values found for T and Ts. Via an inverter 
circuit 97 controlled by signal S6, the circuit 96 passes a difference 
signal which carries a sign corresponding to the sign of the difference 
found to an output 98. The inverter circuit 97 controlled by signal S6 is 
used for correcting the difference between the influence of velocity of 
the scanning spot 11 on the signals S1 and S2. Hereinbefore, the period of 
the detection signals is determined for determining a measure for the 
velocity of the scanning spot. It will be obvious to a person skilled in 
the art that for obtaining a measure for the velocity of the scanning spot 
11, a different signal may be derived which is related to the period of 
the signal S', for example, a signal indicating the frequency of the 
signal S'. 
The signal on the output 90 functions as the measuring signal Vp and is 
indicative of the deviation of the instantaneous track pitch relative to a 
desired track pitch. The signal on the output 98 is indicative of the 
velocity of the scanning spot in the direction y transverse to the tracks 
75. This signal will in the following be referenced measuring signal Vsn. 
The measuring signal Vp, possibly in combination with the measuring signal 
Vsn, can be used for controlling the velocity of the record carrier 109 to 
a value for which the track pitch assumes the desired value. 
This may be effected, for example, by adapting the velocity with which the 
means of displacement (reel 114 and motor 115 in FIG. 1) move in response 
to a control signal derived by a control circuit 100 from the measuring 
signal Vp possibly in combination with the measuring signal Vsn. 
The bandwidth of such a control will generally be limited due to the 
inertia of the driving means (reel 114 and motor 115), so that with such a 
control a compensation can only be made for low-frequency deviations of 
dy. However, in the case of the scanning repetition rate fh selected here, 
a tuning to eliminate the high-frequency deviations may be omitted. 
Although it is advantageous to control the scanning spot and thus the track 
pitch in response to both the measuring signal Vp and the velocity signal 
Vsn, this is not necessary. For example, it is possible to control this 
position only in response to the measuring signal Vp.