A scanning device producing a first and a second scanning spot which move in different directions and an apparatus containing that device which uses the first spot for reading and/or recording and the second spot to monitor such reading and/or recording

A scanning device for repeatedly scanning a scanning surface of a record carrier (e.g., a tape) by means of a first and a second scanning spot produced by respective scanning beams. The scanning spots perform synchronous movements along a first and a second scanning path, respectively, having different given scanning directions, as a result of which, the distance, viewed in a direction transverse to the scanning paths, between the first and the second scanning spot changes during scanning. The scanning device can be used in a recording and/or read apparatus which further includes a detection system for converting radiation emanating from the second scanning spot into a corresponding detection signal, and a measurement circuit which derives a position measurement signal indicative of deviation of track pitch based on a deviation in a relationship between the detection signal and a reference signal indicative of the position of the first scanning spot on the first scanning path. Other circuits can also be employed to derive a speed measurement signal indicative of the speed of the first scanning spot in a direction transverse to the track direction and a quality measurement signal indicative of the quality of an information pattern at the location of tracks traversed by the second scanning spot. If during reading a third scanning spot is used such that the second and the third scanning spot are always disposed symmetrically relative to the first scanning spot, corresponding detection signals can be obtained which can be used for deriving an inclination measurement signal indicative of the angle between the track direction and the first scanning path.

BACKGROUND OF THE INVENTION 
The invention relates to a scanning device having an optical scanning 
system comprising a unit for repeatedly scanning a scanning surface of a 
record carrier by means of a first and a second scanning beam, the first 
and the second scanning beam forming a first and a second scanning spot on 
the scanning surface, which scanning spots move synchronously over the 
record carrier along a first and a second scanning path, respectively. The 
invention further relates to a recording and/or read apparatus comprising 
such a scanning device. 
Such devices are known from U.S. Pat. No. 4,901,297. By means of the 
scanning device disclosed therein, the recording surface of a 
magneto-optical record carrier in the form of a tape, which surface 
corresponds to the scanning surface, is scanned by two scanning spots in a 
direction transverse to the longitudinal direction of the record carrier 
tape. In order to realise this scanning, two parallel laser beams are 
projected onto the record carrier via facets of a rotating polygonal 
mirror. The scanning spots formed on the recording surface by the two 
laser beams follow parallel scanning paths. The tape is moved in the 
longitudinal direction so that the recording surface is scanned along 
parallel scanning paths which are such that the path scanned by the first 
scanning spot is scanned by the second scanning spot during the next scan. 
The recording surface is erased during scanning by the first scanning 
spot. Subsequently, information is recorded in this erased part of the 
recording surface during scanning by the second scanning spot. This 
results in a track pattern of parallel information tracks whose track 
directions extend transversely of the longitudinal direction to the tape. 
During recording, the track pitch (distance between the centres of two 
adjacent tracks) is not controlled. As a result of inevitable variations 
in parameters which influence the track pitch, for example variations in 
tape transport speed, the track pitch of the resulting track pattern will 
vary. Therefore, it is desirable that during recording information about 
the track pitch is available for the purpose of track pitch monitoring 
and/or control. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a device which enables 
track-pitch information to be derived from radiation emanating from a 
scanning spot. To this end, a scanning device of the type defined in the 
opening paragraph is characterised in that the device further comprises a 
unit for bringing about a movement of the second scanning spot relative to 
the first scanning spot in a direction transverse to the direction of the 
first scanning spot, the position of the second scanning spot with respect 
to the first scanning spot being related to the position of the first 
scanning spot on the first scanning path. 
The device in accordance with the invention enables information to be 
recorded by means of the first scanning spot. During recording, the second 
scanning spot is moved in a direction transverse to the direction of the 
path of the first scanning spot, and during this movement it will pass 
information tracks formed during previous scans by means of the first 
scanning spot. Radiation emanating from the second scanning spot is 
modulated by the information in the tracks passed. The degree of 
modulation is maximal at the instant at which the centre of the scanning 
spot coincides with the centre of an information track. The modulation 
decreases as the distance between the centre of the scanning spot and the 
nearest information track increases. Because there is a fixed relationship 
between the location of the first scanning spot on the first scanning path 
and the position of the second scanning spot relative to the centres of 
the tracks already recorded, a deviation of the actual track pitch from a 
desired track pitch can be detected on the basis of a deviation in a 
relationship between the degree of modulation of the second radiation beam 
and the position of the first scanning spot on its scanning path. 
The scanning device can be used not only for recording information, but 
also for reading the recorded information. In that case, a deviation in 
the previously-mentioned relationship indicates that the centre of the 
first scanning spot does not coincide with the centre of the track being 
scanned. In other words, a deviation in that relationship is indicative of 
a tracking error. 
Another embodiment of the scanning device is characterised in that the unit 
for bringing about the above-mentioned movement comprises a deflection 
unit which is rotatable about an axis of rotation and has a reflecting 
surface, via which surface the first and the second radiation beam are 
directed to the record carrier. The surface is such that the central axis 
of the second radiation beam, at the location where this beam impinges on 
the reflecting surface, is not disposed in a plane perpendicular to the 
axis of rotation, and the central axis of the first radiation beam, at the 
location where this beam impinges on the reflecting surface, is not 
parallel to the central axis of the second radiation beam. This embodiment 
has the advantage that it can be realised in a simple manner. 
The deflection unit can be, for example, a galvanometer mirror. However, a 
polygonal mirror is to be preferred. 
The scanning device in accordance with the invention is particularly 
suitable for use in recording and/or read apparatus. An embodiment of such 
a recording and/or read apparatus, comprising a scanning device in 
accordance with the invention for reading and/or writing information from 
and/or in tracks of a recording surface (corresponding to the scanning 
surface), which tracks have directions substantially corresponding to the 
direction of the scanning path followed by the first scanning spot, 
further comprises a unit for bringing about a movement of the recording 
surface relative to the optical system in a given direction of movement, a 
detection system for converting radiation emanating from the second 
scanning spot into a corresponding detection signal, and a unit for 
deriving a position measurement signal on the basis of a deviation in a 
relationship between the detection signal and a reference signal which is 
indicative of the position of the first scanning spot on the first 
scanning path. 
A deviation in the previously-mentioned relationship can be identified 
simply by a shift of the detection signal relative to the reference 
signal. Such a shift can be detected by detecting phase shift of the 
detection signal relative to the reference signal. However, it is also 
possible to detect a shift in time of the detection signal relative to the 
reference signal. 
A further embodiment of the apparatus is characterised in that the 
apparatus includes a unit for deriving from the detection signal a speed 
measurement signal which is related to the period of the detection signal. 
The number of tracks passed by the second scanning spot per unit of time 
is indicative of the speed of movement of the first scanning spot in a 
direction transverse to the track direction. The period of the detection 
signal, which is related to the time between two track crossings, is, 
therefore, indicative of the speed of the first scanning spot in a 
direction transverse to the tracks. 
Since during recording the second scanning spot passes recently recorded 
information tracks, it is possible to derive the quality of the recorded 
tracks from the detection signal. An embodiment by means of which this is 
realised is characterised in that the apparatus includes a unit for 
deriving from the detection signal a quality measurement signal which is 
indicative of a quality of the tracks traversed by the second scanning 
spot. The quality measurement signal can be used, for example, for 
adjusting recording parameters to values for which the quality of the 
tracks complies with a given standard. 
A further embodiment of a scanning device in accordance with the invention 
is characterised in that the device comprises means for directing a third 
radiation beam to the scanning surface so as to form a third scanning spot 
on the scanning surface. This embodiment of the scanning device has the 
advantage that the desired information can be determined on the basis of 
two scanning spots. If the position of the second and the third scanning 
spot at opposite sides of the scanning path is selected to be, for 
example, symmetrical relative to the first scanning spot, this has the 
advantage that during recording the second or the third scanning spot 
moves in an area already provided with tracks. The last-mentioned 
embodiment of the scanning device having at least three scanning spots 
also has additional advantages when used in read apparatus. A first 
advantageous embodiment of a read apparatus comprising the last-mentioned 
embodiment of the scanning device, for reading and/or writing information 
from and/or in tracks of a recording surface (corresponding to the 
scanning surface), which tracks have directions substantially 
corresponding to the direction of the scanning path followed by the first 
scanning spot, further comprises a first detection system for converting 
radiation emanating from the second scanning spot into a corresponding 
detection signal, a second detection system for converting radiation 
emanating from the third scanning spot into a corresponding detection 
signal, and a unit for deriving a position measurement signal on the basis 
of a deviation of the first and/or the second detection signal. Since the 
second and the third scanning spot are situated symmetrically relative to 
the first scanning spot, a tracking error will result in opposite phase 
shifts in the detection signals relative to the reference signal. The 
phase difference between the first and the second detection signal is, 
therefore, indicative of the tracking signal. In this case, it is not 
necessary to generate a reference signal which is indicative of the 
position of the first scanning spot on the first scanning path. 
A second advantageous embodiment of a read apparatus comprising the 
last-mentioned embodiment of the scanning device, for reading and/or 
writing information from and/or in tracks of a recording surface 
(corresponding to the scanning surface), which tracks have directions 
substantially corresponding to the direction of the scanning path followed 
by the first scanning spot, further comprises a first detection system for 
converting radiation emanating from the second scanning spot into a 
corresponding first detection signal, a unit for determining a first 
deviation in a relationship between the first detection signal and a 
reference signal which is indicative of the position of the first scanning 
spot on the first scanning path, a second detection system for converting 
radiation emanating from the third scanning spot into a corresponding 
second detection signal, a unit for determining a second deviation in a 
relationship between the second detection signal and the reference signal, 
and a unit for deriving from the first and the second detection signal an 
inclination measurement signal on the basis of a combination of the first 
and the second deviation. This embodiment makes advantageous use of the 
fact that if the direction of the scanning path and the direction of the 
tracks do not correspond, the deviations in the relationship between the 
detection signals and the reference signal are no longer the opposites of 
one another as in the case of corresponding directions of the scanning 
path of the first scanning spot and the tracks. The sum of the deviations 
in the relationships between the detection signals and the reference 
signal is, therefore, indicative of the angle between the direction of the 
tracks and the direction of the scanning path of the first scanning spot. 
The inclination measurement signal can be used for monitoring and/or 
controlling this angle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an optical recording and/or read apparatus comprising a first 
embodiment of a scanning device in accordance with the invention. The 
scanning device shown comprises an optical scanning system formed by a 
light path 1, a rotatable polygonal mirror 5, a deflection mirror 6 and a 
focusing objective 7. The light path 1 may be of a type customary in 
optical or magneto-optical recording and/or read equipment. Such a light 
path comprises a beam generating unit for generating three radiation beams 
2, 3 and 4, whose directions differ slightly from one another and of which 
two beams (2 and 4) are situated symmetrically relative to a central 
scanning beam (3). 
The radiation beams 2, 3 and 4 are directed to a scanning surface via the 
polygonal mirror 5, the deflection mirror 6 and the focusing objective 7, 
which scanning surface is formed by the surface of a record carrier 9 in 
the form of a tape of a magneto-optical or optical type. (Hereinafter this 
surface will also be referred to as a "recording surface"). The radiation 
beams 2, 3 and 4 are focused by the focusing objective 7 to form tiny 
scanning spots on the recording surface of the record carrier 9. Since the 
directions of the three radiation beams differ, the positions of the 
scanning spots will also be different. A part 10 of the recording surface 
where the radiation beams are incident on the recording surface is shown 
to an enlarged scale. In the part 10, a first scanning spot formed by the 
radiation beam 3 bears the reference numeral 11. A second and a third 
scanning spot produced by the radiation beams 2 and 4, respectively, are 
referenced 12 and 13, respectively. 
The polygonal mirror 5 has reflecting facets 8a, . . . , 8g and is rotated 
about an axis 17 by drive means, not shown, which are described in detail 
in, for example, U.S. Pat. No. 5,171,984 and EP-A-0,459,586, to which U.S. 
Pat. No. 5,191,221 corresponds, herewith incorporated by reference. The 
polygonal mirror 5 is positioned in such a manner relative to the light 
path 1 that as the polygonal mirror 5 rotates about the axis of rotation 
17 the radiation beams 2, 3 and 4 successively impinge on one of the 
facets 8a, . . . , 8g, as a result of which the recording surface is 
scanned repeatedly by the scanning spots 11, 12 and 13, the scanning spots 
11, 12 and 13 being moved synchronously over the recording surface along 
scanning paths 16, 18 and 19, respectively (see FIG. 2). Since the 
radiation beams 2 and 4 are in a symmetrical position relative to the beam 
3, the scanning spots 12 and 13 produced by the radiation beams 2 and 4 
are situated symmetrically relative to the scanning spot 11 produced by 
the radiation beam 3. 
The polygonal mirror 5 has the shape of a truncated pyramid whose inclined 
faces form the facets 8a, . . . , 8g and whose axis of rotation 17 
intersects the base in its centre of gravity and extends perpendicularly 
to this base. This means that the facets 8a, . . . , 8g are inclined 
relative to the axis of rotation 17 of the polygonal mirror 5. As a result 
of this inclination, the scanning paths followed by the scanning spots 11, 
12 and 13 are not parallel but intersect one another, as is shown in FIG. 
2. The reason for this will be explained with reference to FIGS. 3 and 4. 
FIG. 3 shows the polygonal mirror 5 in a position in which the radiation 
beams 2, 3 and 4 impinge on the facet 8a substantially in its centre. The 
positions of incidence of the beams 2, 3 and 4 define a direction 
indicated by an arrow 31. An arrow 30 indicates a direction of a line 
intersecting a plane perpendicular to the axis of rotation 17 and the 
surface of the facet 8a. 
FIG. 4 shows the polygonal mirror 5 in a position in which the radiation 
beams 2, 3 and 4 impinge on the facet 8a in the proximity of an edge 42. 
The positions of incidence of the beams 2, 3 and 4 define a direction 
indicated by an arrow 40, which deviates from the direction of an arrow 41 
indicating the direction of a line intersecting a plane perpendicular to 
the axis of rotation 17 and the surface of the facet 8a. 
The positions relative to one another for the position of the polygonal 
mirror 5 shown in FIG. 3 correspond to the positions of the scanning spots 
11, 12 and 13 shown in FIG. 2. The positions of the scanning spots 11', 
12' and 13' shown in FIG. 2 correspond to the position of the polygonal 
mirror shown in FIG. 4. 
The variation of the positions of the scanning spots relative to one 
another results 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 with respect to the 
first scanning spot 11 are related to the position of the polygonal mirror 
5, and hence, to the position x of the first scanning spot 11 on the first 
scanning path 16. 
The embodiment described above employs a deflection means which is 
rotatable about an axis of rotation and which is formed by the polygonal 
mirror 5 having facets 8 which are inclined relative to the axis of 
rotation 17 in order to obtain synchronous movements of the scanning spots 
11, 12 and 13 in such a manner that the scanning spots 11, 12 and 13 are 
moved relative to one another in the y direction transverse to the 
scanning directions, and the position of the scanning spot 12 and the 
scanning spot 13 with respect to the scanning spot 11 is related to the 
position x of the scanning spot 11 on the first scanning path 16. 
However, such movements of the scanning spots can also be obtained by other 
deflection means rather than a polygonal mirror having facets which are 
inclined relative to the axis of rotation. It will be obvious to those 
skilled in the art that instead of the polygonal mirror 5 other rotatable 
deflection means comprising a reflecting surface reciprocated about a 
centre position, such as for example a galvanometer mirror, can be used 
for generating such synchronous scanning movements. 
In addition, it is not necessary for the reflecting surface of the 
deflection means to be inclined relative to the axis of rotation. When a 
rotatable deflection means is used, it is important only that, firstly, 
the central axis of the second radiation beam, at the location where it 
impinges on the reflecting surface of the deflection means, is not 
situated in a plane perpendicular to the axis of rotation of the 
deflection means, and, secondly, the central axes of the first and the 
second radiation beam are not parallel. 
Synchronous movements of the scanning spots can also be obtained by other 
means. For example, it is possible to use a separate deflection means for 
each beam, which means perform. mutually synchronised movements, with the 
corresponding scanning paths on the scanning surface scanned by the 
synchronous movements of the deflection means intersecting one another. 
In addition, it is to be noted that the deflection means themselves need 
not be movable. Deflections of radiation beams with a varying angle can 
also be obtained with stationary deflection means, such as for example 
acousto-optical deflection means. 
Finally, it is to be noted that the radiations beams can also be generated 
by means of a so-called laser array. Such an array mat comprises a 
plurality of different laser rows. The lasers of each row are oriented in 
another direction. By each time activating a following laser in a row, the 
beam-generating source is moved, which results in a movement of the 
scanning spot produced by the corresponding radiation beam. By 
synchronising the activation of the lasers in the various rows, a 
synchronous movement of the scanning spots corresponding to the radiation 
beams generated by means of the laser rows is obtained along different 
paths. 
The embodiments of the scanning device described above are particularly 
suitable for use in a recording apparatus which by means of the radiation 
beam 3 records effects which are readable by means of a read radiation 
beam. Such an effect may cause, for example, a variation of the intensity 
of reflected radiation of the read beam, such as for example a recess or 
pit producing destructive interference in the radiation. Such an effect 
may for example alternatively comprise a magnetic domain having a given 
direction of magnetisation, causing a change of the polarisation of the 
radiation. 
To record effects, the intensity of the radiation beam 3 (of FIG. 1) during 
scanning is usually switched between a write level which is high enough to 
produce an effect. and a read level which is not high enough to produce an 
effect. It is to be noted that in the case of magneto-optical recording, 
the effects can also be obtained by means of a magnetic field of varying 
strength, which is applied at the location of the recording surface part 
scanned by a write beam. The intensity levels of the radiation beams 2 and 
4 are inadequate to produce optically detectable effects in the recording 
surface in order to preclude the undesired formation of effects. 
In addition to the scanning means, the embodiment of a recording and read 
apparatus in accordance with the invention shown in FIG. 1 comprises 
transport means for realising a movement of the record carrier 9 relative 
to the optical system in the direction y transverse to the direction of 
the scanning path 16. These transport means may be of a customary type 
shown diagrammatically in FIG. 1 and comprising a reel 14 driven by a 
motor 15 for taking up the record carrier tape 9, which is thus moved in 
the direction y (which corresponds to a longitudinal direction of the 
record carrier tape 9). The direction y is indicated by an arrow 19 in 
FIG. 1. 
The recording apparatus described above records successive tracks of 
effects on the record carrier 9 at the location of the scanning path 16 
scanned by the scanning spot 11. Thus, a pattern of parallel tracks is 
recorded, a following track being written each time that the scanning spot 
11 scans the recording surface. 
By way of illustration, FIG. 5a shows tracks 75 obtained in this manner. 
The tracks 75 have further been numbered as tracks -7, . . . , -1, 0. The 
position along the scanning path 16 is indicated by the quantity x and the 
corresponding position of the polygonal mirror 5 by the quantity phi, 
which represents the position of the facet used for scanning relative to 
its centre position in degrees. There is an unambiguous relationship 
between the position of the polygonal mirror 5 and the position x of the 
scanning spot 11 on the scanning path 16. The positions of the scanning 
spot produced by the radiation beam 3 for three different values of phi 
(phi=-22, phi=0 and phi=22) are referenced 11', 11 and 11". The positions 
of the scanning spots formed by the radiation beams 2 and 4 for the three 
above-mentioned values of phi are referenced 12' and 13', 12 and 13, and 
12" and 13". The scanning spot 11 moves along the path 16, and the 
scanning spots 12 and 13 move along the paths 18 and 19, which cross the 
path 16. 
When the scanning spot 11 travels from position x=-x.sub.1 mm to x=0 mm, 
the scanning spot 13 moves past a number of tracks, and the scanning spot 
12 moves over a part of the recording surface where no tracks 75 have been 
recorded yet. When the scanning spot 11 moves from position x=0 mm to 
x=+x.sub.1 mm, the scanning spot 13 moves past a number of tracks 75, and 
the scanning spot 13 moves over a part of the recording surface without 
any tracks 75. At the locations where the scanning spots 12 and 13 wholly 
or partly coincide with one of the tracks 75 the radiation reflected from 
the record carrier 9 will be modulated in accordance with the pattern of 
effects or information pattern in the track 75. The degree of modulation 
corresponds to the degree in which the scanning spot coincides with the 
track 75. 
The recording apparatus, in accordance with the invention, shown in FIG. 1 
further comprises detection systems of a type known per se for converting 
the radiation emanating from the scanning spots 12 and 13 into detection 
signals S1 and S2 which correspond to the modulation produced in the 
reflected radiation by the information pattern. In the embodiment shown in 
FIG. 1, the reference numerals 70 and 71 denote detection systems for 
converting the radiation which returns from the scanning spots 12 and 13 
into the light path 1 via the focusing objective 7, the deflection mirror 
6 and the polygonal mirror 5. The detection systems 70 and 71 may be of a 
generally known type and fall beyond the scope of the invention, for which 
reason they are shown only diagrammatically. 
The recording apparatus shown in FIG. 1 further comprises means for 
generating a reference signal S3 indicative of the position of the 
scanning spot 11 on the scanning path 16, and a measurement circuit 72 for 
deriving from the detection signals S1 and S2 and the reference signal S3 
one or more measurement signals Sp, Sv and/or Sa for monitoring and/or 
controlling the recording process. How the measurement signals Sp, Sv, and 
Sa are derived will be explained hereinafter. 
As already stated, the scanning spots 11, 12 and 13 perform synchronous 
movements. Variations in the distances between the scanning spots viewed 
in the direction y transverse to the scanning directions are related to 
the position of the polygonal mirror 5 and, consequently, to the position 
of the scanning spot 11 on the first scanning path 16. For a given value 
of phi the centres of the scanning spots 12 and 13 coincide with the 
centres of the previously written tracks 75. The given values of phi for 
which this is the case depend on the distance of the scanning path 16 from 
the track 75 passed by the scanning spot 12 or 13. Since the distances 
between the tracks 75 already formed have a constant value equal to the 
track pitch, the values of phi for which the centres of the scanning spots 
pass the centres of the tracks 75 depend on a distance dy between the 
scanning path 16 and the centre of the last recorded track 75 (the track 
having the track number -1 in FIG. 5a). This means that the maximum and 
minimum modulations of the detection signals occur at predetermined 
positions of the polygonal mirror 5. 
By way of illustration, FIG. 5b shows the detection signal S1 as function 
of phi and as a function of the position x in the case that the distance 
dy from the scanning path 16 to the centre of the adjacent track, 
represented as a line 80, corresponds to a desired track pitch. FIG. 5c 
shows the detection signal S2 as a function of phi for a part of the 
scanning path 16 in the case that the distance dy corresponds to the 
desired track pitch. FIGS. 5b and 5c further show the signal envelopes S10 
and S20 of the detection signals S1 and S2. These signal envelopes S10 and 
S20 have substantially sinusoidal shapes representing the degrees of 
modulation of the corresponding detection signals S1 and S2. The maxima of 
each of the signal envelopes S10 and S20 represent the positions for which 
the modulation of the detection signal is maximal. These are the positions 
for which the centre of the corresponding scanning spot 12 and 13 
coincides with the centre of one of the tracks. 
As is shown in FIGS. 5b and 5c, there is a relationship between the 
detection signals S1 and S2 and phi (and, as a consequence, the position 
x). This relationship depends on the distance dy. If this distance varies, 
the positions where the maxima and minima of the signal envelopes S10 and 
S20 occur will change. This is because the centres of the scanning spots 
12 and 13 then coincide with the centres of the tracks 75 at other 
positions x of the scanning spot 11. For example, when the distance dy 
decreases the position values for which the maxima and minima of the 
signal envelope S10 occur will change in a positive direction (hereinafter 
also referred to as "lag") and the position values for which the maxima 
and minima in the signal envelope S20 occur will change in a negative 
direction (hereinafter also referred to as "lead"). Conversely, when the 
distance dy increases the position values for which the maxima and minima 
of the signal envelope S10 occur will change in a negative direction 
(lead) and the position values for which the maxima and minima in the 
signal envelope S20 occur will change in a positive direction (lag). 
Accordingly, a deviation in the relationship between the detection signals 
S1 and S2 from the relationship corresponding to a value of the distance 
equal to the desired track pitch is indicative of a difference between the 
distance dy and the desired track pitch. In this respect, it is to be 
noted that because of the symmetrical position of the scanning spots 12 
and 13 relative to the scanning spot 11 the influence of a variation of dy 
on the relationship between the detection signal S1 and the position x is 
opposite of the influence on the relationship between the detection signal 
S2 and the position x. 
The deviation in the relationship between the detection signals S1 and S2 
and the reference signal S3 is determined by means of the measurement 
circuit 72. 
The reference signal S3 may comprise, for example, a position signal whose 
signal value corresponds to the position of the deflection means 
(polygonal mirror 5 in the present embodiment), and hence, to the position 
x of the scanning spot 11. Alternatively, the reference signal S3 may 
comprise a pulse-shaped signal whose edges correspond to given positions 
of the deflection means, for example those positions for which the 
detection signal S1 or the detection signal S2 has its maxima or minima at 
a desired value of dy. 
The reference signal S3 is obtained from a position detection device 73. 
The position detection device may form part of a control system for 
controlling the speed and/or position of the polygonal mirror 5. Drive 
circuits in which information signals are available to indicate the 
position of a driven object are generally known and are, therefore, not 
described in detail. 
The measurement circuit 72 may comprise a circuit which detects a (phase or 
time) shift of the detection signals S1 and S2 relative to the reference 
signal. The measurement circuit 72 may, for example, be of a type which 
determines for which values of x the maxima and minima of the detection 
signals occur, on the basis of the detection signals S1 and S2, and a 
reference signal S3 representing the position of the polygonal mirror 5, 
and subsequently compares the values determined with positions laid down 
for a relationship belonging to the desired value of the distance dy. Such 
a circuit can be realised, for example, by means of a program-controlled 
circuit loaded with a suitable program for carrying out the above 
operations. 
Alternatively, the measurement circuit 72 may comprise a circuit which 
determines whether there is a phase difference between the detection 
signals and a pulse-shaped signal whose edges indicate the positions at 
which maxima and the minima in the detection signals should occur. For 
this purpose various kinds of phase comparison circuits can be used. 
FIG. 6 shows an example of the measurement circuit 72 in greater detail. 
The measurement circuit 72 has an input 81 for receiving the detection 
signal S1. A switch 82 controlled by a signal S4 connects the input 81 to 
an input 93 of a signal processing circuit 84. The processing circuit 84, 
converts the detection signal received at its input into a binary signal 
S', which can assume a first logic value to indicate that the scanning 
spot corresponding to that detection signal is situated substantially on 
one of the tracks 75, and which can assume a second logic value to 
indicate that the corresponding scanning spot is situated substantially 
between two tracks 75. The signal processing circuit 84 may be of a 
conventional type, which is also referred to as an off-track detector. 
Such an off-track detector may comprise, for example, a series arrangement 
of a band-pass filter 85, an envelope detector 86 and a comparator 87. 
The signal S' is available at an output of the circuit 84 and is applied to 
a phase detector 88. By way of illustration, the signal S' is shown as 
function of phi in FIG. 5d. An output of the phase detector 88 is applied 
to an output 90 of the measurement circuit 72 via an inverter circuit 89 
controlled by a signal S6. 
The measurement circuit 72 further has an input 92 for receiving the 
detection signal S2. The input 92 is connected to the input 93 of the 
circuit 84 by a switch 94 controlled by a signal S5. 
The phase detector 88 further receives the signal S3, which is pulse-shaped 
in the present case and whose edges indicate the positions at which the 
maxima and minima in the detection signals should occur. By way of 
illustration, FIG. 5e shows the reference signal S3 as a function of the 
position phi. 
FIGS. 5f, 5g and 5h further show the signals S4, S5 and S6, respectively. 
The signal S4 has a logic value "1" for 0&lt;phi&lt;.THETA..sub.1. For these 
values of phi, the scanning spot 12 is situated on a part of the recording 
surface already provided with tracks 75, and the detection signal S1 
exhibits a modulation caused by these tracks 75. 
The signal S5 has a logic value "1" for -.THETA..sub.1 &lt;phi&lt;0. For these 
values of phi, the scanning spot 13 is situated on a part of the recording 
surface already provided with tracks 75, and the detection signal S2 
exhibits a modulation caused by these tracks 75. 
The signal S6 has a logic value "1" for -30&lt;phi&lt;0. The edge (signal-level 
transition) at the value phi=0 indicates the boundary between the range in 
which the scanning spot 12 is situated in the area with tracks 75 and the 
range in which the scanning spot 13 is situated in the area with tracks 
75. 
The signals S3, S4, S5 and S6 can be generated in a customary manner by 
means of the position detection device 73. For this purpose such a 
position detection device 73 may be coupled to a shaft of the polygonal 
mirror 5. Such a position detection device 73 coupled to the shaft of the 
polygonal mirror 5 may carry a so-called shaft-position encoder, if 
required in combination with counting circuits. Such position detection 
devices are known per se and are not described in detail because they fall 
beyond the scope of the invention. 
The operation of the measurement circuit 72 will be described in detail 
hereinafter. The polygonal mirror 5 is driven with a constant angular 
velocity so that the value of phi (indicating the position of the facet 
used for deflection) each time covers the range from -30 to 30 degrees. In 
the part -.THETA..sub.1 &lt;phi&lt;0 of the range, the detection signal S2 will 
be transferred to the circuit 84 via the switch 94, which is controlled by 
the signal S5. The phase detector 88 determines the phase difference 
between the reference signal S3 and the signal S' derived on the basis of 
the detection signal S2. This phase difference is 90 degrees for the 
desired value of dy (see also FIG. 5). The phase detector 88 is of a type 
which supplies a phase-difference signal whose (average) signal strength 
is proportional to the phase difference between the signal S' and S3 minus 
90 degrees. Hence, the sign of the (average) signal strength represents 
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, numerous other types of phase detectors may 
be used. The phase-difference signal obtained, which is a measure of the 
deviation of dy, is transferred without any change to the output 90 of the 
measurement circuit 72 via the controllable inverter circuit 89. 
At the instant at which the polygonal mirror passes through the position 
phi=0 the detection signal S2 is blocked by the switch 94 and the 
detection signal S1 is transferred to the input 93 of the circuit 84 via 
the switch S4 controlled by the signal 82. The phase detector 88 now 
determines the phase difference between the reference signal S3 and the 
signal S' derived on the bases of the detection signal S1. As already 
stated, the influence of dy on the detection signal S1 is opposite to the 
influence of dy on the detection signal S2. The inverter circuit 89 
controlled by the signal S6 provides a correction for this. Indeed, at the 
instant (phi=0) at which the detection signal S2 at the input 93 is 
replaced by the detection signal S1 the inverter circuit 89 is activated, 
as a result of which the phase-difference signal applied to the input 90 
is inverted. 
The period T of the signal S' represents the time interval between two 
successive track crossings by one of the scanning spots 12 or 13. If the 
scanning spot 11 used for recording has a motion component in the 
direction y transverse to the tracks 75, this will result in a change of 
the value of T relative to a value Ts, which corresponds to a situation in 
which the position of the scanning spot in a direction transverse to the 
tracks 75 does not change (value of dy remains constant). The difference 
between the actual value of T and Ts consequently represents the speed of 
the scanning spot 11 in the direction y transverse to the tracks 75. Here, 
it is to be noted again that the influence of the speed on the period of 
the detection signal S1 is opposite to the influence of the speed on the 
period of the detection signal S2. 
In order to determine the period T of the signal S' the measurement circuit 
72 may be provided with a circuit 96 of a type known per se. The circuit 
96 may comprise, for example, a time measurement circuit for determining 
the length of the period T and a subtracter circuit for determining the 
difference between the measured value of T and Ts. The circuit 96 supplies 
a difference signal whose sign corresponds to the sign of the detected 
difference to an output 98 via an inverter circuit 97 controlled by the 
signal S6. The inverter circuit 97 controlled by the signal S6 serves to 
correct the difference in influence of the speed of the scanning spot 11 
on the signals S1 and S2. In the foregoing the period of the detection 
signals is determined in order to determine a measure of the speed of the 
scanning spot 11. It will be obvious to those skilled in the art that for 
determining a measure of the speed of the scanning spot 11 another signal 
may be derived which is related to the period of the signal S', for 
example a signal which is indicative of the frequency of the signal S'. 
The signal on the output 90, hereinafter referred to as the position 
measurement signal Sp, is indicative of the deviation of dy relative to a 
desired track pitch. The signal at the output 98, hereinafter referred to 
as the speed measurement signal Sv, is indicative of the speed of the 
scanning spot 11 in the direction y transverse to the tracks 75. The 
position measurement signal Sp and the speed measurement signal Sv can be 
used to monitor whether the position and speed of the scanning spot 11 in 
the y direction remain within specific standards during recording. 
However, it is preferred to use the position measurement signal Sp and the 
speed measurement signal Sv for controlling the y position of the scanning 
spot 11. This can be effected, for example, by adapting the speed of the 
drive means (coil 14 and motor 15 in FIG. 1) in dependence upon a control 
signal derived from the position measurement signal Sp and the speed 
measurement signal Sv by a control circuit 100. 
Generally, such a control circuit will have a limited bandwidth owing to 
the inertia of the drive means (coil 14 and motor 15), so that such a 
control circuit can only compensate for low-frequency deviations of dy. If 
high-frequency variations should also be compensated for, a fast actuator 
can be used, which for example acts upon the optical path of the radiation 
beam and which is also controlled by means of the control circuit 100. One 
possibility is to make the deflection mirror 6 rotatable about an axis 101 
and to drive it with a fast actuator 99. 
Although it is advantageous to control the position of the scanning spot 11 
on the basis of both the position measurement signal Sp and the speed 
measurement signal Sv this is not necessary. For example, it is also 
possible to control this position only in dependence upon the position 
measurement signal Sp. 
In the embodiment described above, the detection signals S1 and S2 are used 
for deriving the position measurement signal Sp and the speed measurement 
signal Sv. However, this is not necessary. The position measurement signal 
Sp and the speed measurement signal Sv can also be derived from only one 
of the two detection signals S1 and S2. It will also be evident that the 
generation of one of the radiation beams 2 and 4 may then be dispensed 
with. 
The detection signals S1 and S2 obtained during recording can also be used 
for determining the quality of the information patterns in recently 
recorded tracks 75 traversed by the scanning spot 12 and/or 13. In order 
to derive a quality measurement signal Sa, the measurement circuit 72 may 
be provided with an analysis circuit 110 having an input connected to the 
input 93 for receiving the detection signal S1 or S2 depending on the 
control signals S4 and S5. In addition, the signal S', which indicates 
whether the scanning spot 12 or 13 is situated substantially on one of the 
tracks 75, is applied to the analysis circuit 110. The analysis circuit 
110 is of a type which performs a quality analysis of a customary type 
during the time that S' indicates that one of the spots is situated 
substantially on one of the tracks 75. For so-called d.c. free signals, 
such a quality analysis may be effected by determining the so-called "duty 
cycle". However, other known quality-analysis methods (such as amplitude 
detection) are also possible. The analysis circuit 110 supplies a quality 
measurement signal Sa which is indicative of the measured quality. The 
quality measurement signal Sa can be used for monitoring the recording 
process. However, it can also be used. to adjust recording which influence 
the quality of the information patterns. 
In the foregoing, it has been described how the detection signals S1 and/or 
S2 can be used advantageously during the recording of information tracks. 
However, it is also advantageous to use these signals during the read-out 
of an information pattern obtained by means of the apparatus described 
above. The pattern of tracks can be read by scanning successive tracks 
with a radiation beam 3 whose intensity has been set to a read level which 
is inadequate to form effects in the recording surface. The presence of 
the effects can be detected on the basis of the radiation beam 3 reflected 
from the recording surface by means of a detection system known per se. 
An embodiment of the recording and read apparatus shown in FIG. 1 which 
employs the detection signals S1 and S2 during reading will be described 
with reference to FIGS. 7a-c, 8 and 9. FIG. 7a shows a track pattern 
comprising tracks 75 obtained by means of the recording and read apparatus 
shown in FIG. 1. The tracks 75 bear the numbers -7, . . . , -1, 0, . . . , 
7. The position along the scanning path 16 is again indicated by the 
quantity x and the corresponding position of the scanning facet of the 
polygonal mirror 5 is again indicated by the quantity phi. FIG. 7a also 
shows the scanning paths 16, 18 and 19 tracked by the scanning spots 11, 
12 and 13, respectively. 
The positions of the scanning spot formed by the radiation beam 3 for three 
different values of phi, i.e., phi=-.THETA..sub.1, phi=0 and 
phi=+.THETA..sub.1, are again referenced 11', 11 and 11". The 
corresponding scanning spots produced by the radiation beams 2 and 4 are 
referenced 12', 12 and 12" and 13', 13 and 13". 
The detection signals S1 and S2 corresponding to the scanning spots 12 and 
13 are shown in FIGS. 7b and 7c, respectively. 
The pattern of scanning spots 12 and 13 shown in FIG. 7a is scanned in a 
manner similar to the pattern shown in FIG. 5a, except that in scanning 
the pattern shown in FIG. 7a the scanning spots are situated in that part 
of the recording surface in which tracks 75 are situated throughout the 
scanning path. This means that the two detection signals S1 and S2 are 
always available simultaneously during scanning of one of the tracks 75 by 
the scanning spot 11. 
Although in principle the measurement circuit 72 shown in FIG. 6 can be 
used for deriving the position measurement signal Sp and the speed 
measurement signal Sv during scanning of the pattern shown in FIG. 7a, it 
is more advantageous to adapt the measurement circuit 72 in such a manner 
that both detection signals S1 and S2 are always used for deriving the 
position measurement signal Sp and the speed measurement signal Sv instead 
of only one of the two as in the measurement circuit 72 shown in FIG. 6. 
FIG. 8 shows an example of such an adapted measurement circuit 72. In this 
Figure, those parts which are identical to parts of the circuit shown in 
FIG. 6 bear the same reference numerals. 
A circuit 84', which is identical to the circuit 84, converts the detection 
signal S1 into a binary signal S1'. A phase detector 88', which is 
identical to the phase detector 88, compares the phase of the signal S1' 
with the phase of the reference signal S3. The phase difference detected 
by means of the phase detector 88" is again indicative of the distance of 
the scanning spot 11 relative to the nearest centre of one of the tracks 
75, in the present case the track having the track number 0. Hereinafter, 
the distance between the centre of the scanning spot 11 and the nearest 
centre of one of the tracks 75 will also be referred to as the tracking 
error. Circuits 84" and 88" derive a signal which is indicative of the 
tracking error from the detection signal S2 and the reference signal S3 in 
a similar manner to the circuits 84' and 88". The output signals of the 
circuits 88' and 88" are subtracted from one another by a subtracter 
circuit 120. The influence of a tracking error on the detection signal S1 
is opposite to the influence of the tracking error on the detection signal 
S2, yielding the position measurement signal Sp on the output of the 
subtracter circuit 120. It is to be noted that the reference signal S3 is 
not necessary for deriving the position measurement signal Sp. Since the 
phase shift of the signal envelope S10 caused the tracking error is 
opposite to the phase shift of the signal envelope S20 caused by this 
tracking error, the phase difference between the detection signals S1 and 
S2 will be indicative of the tracking error. Therefore, it is adequate to 
determine the phase difference between the signal envelope S10 and the 
signal envelope S20. 
The speed with which the scanning spot moves in the y-direction can be 
derived by determining the difference between the period of the signal 
envelope S10 and the signal envelope S20. For this purpose the circuit 
shown in FIG. 8 includes a circuit 96' for determining the period of the 
signal S1', a circuit 96" for determining the period of the signal S2', 
and a subtracter circuit 121 for determining the difference between the 
periods determined by the circuits 96' and 96". The speed measurement 
signal Sv, which is indicative of the speed of the scanning spot in the y 
direction, is available on an output of the subtracter circuit. 
If the track pattern is read with a different scanning device than that 
with which it has been recorded, the direction of the scanning path 16 may 
not fully correspond to directions of the tracks 75, as is shown in FIG. 
9. Hereinafter such a situation will be referred to as an inclination of 
the scanning device. Such an inclination may also occur when the speed of 
movement of the recording surface is increased. 
Within a range corresponding to one track pitch, such an inclination can be 
detected on the basis of the sum of the phase difference between the 
signal envelope S10 and the reference signal S3, and the phase difference 
between the signal envelope S20 and the reference signal S3. Outside this 
range, the inclination can be determined on the basis of the number of 
tracks passed per scan and the sum. The number of tracks passed per scan 
provides a coarse indication of the inclination and the sum provides a 
more accurate indication of the inclination than the coarse indication. 
In cases in which the scanning path 16 of the scanning spot 11 is parallel 
to the track directions, the phase difference between the signal envelope 
S10 and the reference signal S3 will have a value which is exactly the 
opposite of the phase difference between the signal envelope S20 and the 
reference signal S3. This opposite relationship will no longer be met if 
the direction of the tracks 75 no longer corresponds to the direction of 
the scanning path 16. 
FIGS. 9b, 9c and 9d by way of illustration show the signals S1, S2 and S3 
in the case that the centre of the scanning spot 11 coincides with the 
centre of one of the tracks 75 but the direction of the scanning path 16 
and the direction of the tracks 75 do not correspond to one another. In 
this case, the signal envelopes S10 and S20 are in phase, which indicates 
that the scanning spot 11 is situated on the centre of one of the tracks 
75. However, the minima and the maxima of the signal envelopes S10 and S20 
no longer coincide with the edges of the reference signal S3. A measure of 
the inclination can therefore be obtained by summation of the phase 
difference between the signal envelope S10 and the reference signal S3, 
and the phase difference between the signal envelope S20 and the reference 
signal S3. 
An inclination measurement signal Ss indicative of the inclination can be 
obtained by means of an adder circuit 122 for adding the output signals of 
the phase detectors 88' and 88". The inclination measurement signal Ss can 
be used for monitoring and/or controlling the direction of the scanning 
path 15 relative to the direction of the tracks 75. To control the 
direction of the scanning path 16, the circuit shown in FIG. 8 may be 
provided with a control circuit which is responsive to the inclination 
measurement signal Ss to drive an actuator 124 by means of which the 
direction of the scanning path 16 can be adjusted. Such an actuator may be 
of a type which influences the scanning system, for example by changing 
the orientation of the axis of rotation 17 of the polygonal mirror 5, as 
described in the afore-mentioned U.S. Pat. No. 4,901,297. However, 
alternatively the direction of the scanning path 16 can be adjusted by 
changing the position of the recording surface.