OPTICAL RECORD CARRIER AND APPARATUS FOR READING THE RECORD CARRIER

Abstract: The recording surface of an optical record carrier (1) is divided into first (A) and second annular zones (B) which are situated concentrically around a centre of rotation (4). The annular zones (A) are provided with groups of radially aligned first control structures (13), whilst the annular zones (B) are provided with groups of radially aligned second control structures (14). An optical scanning device for optically scanning the structure of the information carrier (1) at the location of the scanning spot (54) projected on the record carrier by the scanning device comprises a detection circuit (70) which in the case of a radial movement of the scanning spot (54) detects first and second control structures (13; 14) and derives a measurement signal Vm which indicates the passage of the boundaries between the zones A and B during a radial movement of the scanning spot 54. The measurement signal Vm is very suitable for the control of a system for rapidly addressing the scanning spot 54 to one of the annular zones (A; B).

Description: The invention relates to an optical record carrier which is or can be provided with information structures arranged along tracks, which tracks are situated substantially concentrically around a centre of rotation, the carrier comprising first and second annular zones which can be distinguished from each other and which are situated concentrically around the centre of rotation. The invention also relates to a method for reading a record carrier which is or can be provided with an information structure arranged along tracks, which tracks are situated substantially concentrically around a centre or rotation, the carrier comprising first and second annular zones which can be distinguished from each other and which are situated concentrically around the centre of rotation, in which method a read radiation beam is directed to a radiation sensitive detection system via the record carrier, the reading radiation beam is focussed on the record carrier, a signal indicating whether the scanning spot passes a first or a second zone is derived from a detection signal delivered by the radiation sensitive detection system. The invention further relates to an apparatus for performing the method, which apparatus comprises drive means for rotating the record carrier about the centre of rotation, a radiation source producing a reading radiation beam, an objective system for focussing the reading radiation beam to form a scanning spot on the record carrier and subsequently directing the reading radiation beam to a radiation-sensitive detection system, the scanning spot being radially movable in the plane of rotation of the record carrier under control of a radial actuating device, and the detection system for reading the information structure converting the modulation of the read beam produced by the structure on the record carrier into an electric signal. Such a record carrier method and apparatus are known from GB-A-1516285 corresponding to Netherlands Patent Application 7510035. In the known record carrier the first zones differ from the second zones in that the track spacing in the first zones differs from that in the second zones. The known apparatus comprises an auxiliary radiation source for producing and additional address beam which projects an address spot on the record carrier, which spot covers a plurality of adjacent tracks. When the address spot and a scanning spot which is locked to the address beam move in a radial direction in such a way that they pass a number of annular zones the different track spacings cause a first-order diffraction sub-beam of the additional address beam to move to and fro over associated detectors. This to-and-fro movement is detected to provide a counting signal which indicates how many annular zones have been traversed by the address spot. This enables a specific area to be addressed rapidly and correctly, resulting in the very fast access time to locate an arbitrary track. However, the known record carrier has the disadvantage that an additional auxiliary radiation source is necessary for addressing the areas. Moreover, the varying track spacing complicates the manufacture of the record carriers. In addition, in the case of a blank still uninscribed record carrier without preformed grooves addressing is impossible owing to the absence of tracks. It is the object of the present invention to provide means enabling a rapid addressing of the annular zones in such a way that the aforementioned drawbacks are mitigated. In accordance with a first aspect of the invention a record carrier as defined in the opening paragraph is characterized in that the record carrier is provided with first and second control structures, and which control structures has a shape which differs from the information structures, the first zones being provided with groups of radially adjoining first control structures and the second zones being provided with groups of radially adjoining second control structures in order to distinguish between the first zones and second zones. In accordance with a second aspect of the invention the method as defined in the foregoing is characterized in that the record carrier is provided with first and second control structures which can be distinguished from each other and from the information structure, the first zones being provided with groups of radially adjoining first control structures and the second zones being provided with groups of radially adjoining second control structures in order to distinguish between the first zones and second zones, the method is further characterized by distinguishing components of the detection signal corresponding to the intersections with the detected radiation beam when the scanning spot passes the first and second control structures, and deriving on basis of the distinguished components a measurement signal which indicates whether a first or second zone has been passed. In accordance with a third aspect of the invention the apparatus as defined in the foregoing is characterized in that the apparatus comprises a detection circuit for deriving a measurement signal from first and second components of the electric signal corresponding to the modulated reading radiation beam as it scans the first and the second control structures respectively, which measurement signal indicates whether a first or second zone is passed by the scanning spot during a radial movement of the scanning spot. The use of different control structures which are distinguishable from each other and from the information structure enables the scanning spot to be moved over the record carrier along a path which in each of the annular zones being passed intersects at least one group of control structures. As the control structures are scanned by the scanning spot an electric signal is generated which differs from the electric signals generated during scanning of the information structures. Since the control structures are radially aligned with each other the angle at which the scanning spot intersects the group of control structures hardly affects the waveform of the electric signal generated during scanning of the control structures, enabling a reliable detection signal suitable for controlling the addressing of the annular zones to be derived from the electric signal. In one embodiment the record carrier is characterized in that the control structures are arranged along a substantially concentric tracks in such a way that along the tracks the structure alternately comprises control structures and information structures. This embodiment has the advantage that during manufacture of the disc the control structure can be formed simply at the same time as the information structure. In another embodiment the record carrier is characterized in that over the entire width of the annular zones the control structures are situated at locations which are defined by at least a group of radii issuing from the centre of rotation and having equal angular spacings. This record carrier has the advantage that during each radial displacement of the scanning spot, the disc being rotated through an angle larger than the angle between two radii during the time in which the scanning spot passes an annular zone, the scanning spot always describes such a path that it intersects at least one group of adjacent control structures in each annular zone. In a further embodiment the record carrier is characterized in that the first control structure comprise an optical detectable area on a first predetermined position whilst a second predetermined tangential position within the first control structure is not provided with an optical detectable area, the second control structure comprises an optical detectable are on the second predetermined position, whilst the first predetermined tangential position is not provided with an optical detectable area. This embodiment has the advantage that the first and the second control structures can be distinguished from one another by differential detection, for which the magnitude of the radiation beam reflected when the first and second position are scanned are compared with each other, so that the correct distinction between the first and the second control structures is substantially independent of variations in read-beam intensity. An embodiment of the apparatus which distinguishes between the control structures by means of differential detection is characterized in that the apparatus comprises a synchronizing circuit for deriving from the electric signal first and second control signals which are synchronous with the incidence of the scanning spot on the first and the second tangential position within respectively, a sample-and-hold circuit for taking first and second samples of the elctric signal in response to the first and second control signals, and a circuit for comparing the first and the second samples with each other and for generating the measurement signal depending on the result of the comparison. Embodiments of the invention will now be described in more detail, by way of example, with reference to Figs. 1 to 7, in which Figs. 1 and 1a show a record carrier in accordance with the invention, Fig. 2 schematically illustrates the mechanical construction of the scanning device for reading a record carrier in accordance with the invention, Fig. 3 shows an example of a path followed by a scanning spot during a radial displacement in order to clarity the invention, Fig. 4 shows an electrical block diagram of the detection circuit in the apparatus in accordance with the invention, Fig. 5 shows a number of signals generated in the detection circuit and the synchronizing circuit, Fig. 6 shows an example of a path followed by a scanning spot during a radial displacement and a measurement signal Vm generated during such a scanning process, Fig. 7 shows a detailed circuit diagram of the synchronizing circuit and the detection circuit. Fig. 1 shows a record carrier 1 in accordance with the invention, which is divided into first annular zones A and second annular zones B which alternate with one another and which are situated concentrically around a centre of rotation 4. Each annular zone comprises a plurality of tracks, for example sixteen, which are situated concentrically or spirally around the centre of rotation 4, along which tracks an optically readable structure is arranged. A part 6 of the record carrier 1 is shown to a highly enlarged scale, in which part 6 the tracks bear the reference numeral 5. Each track is divided into groups (9, 10), each group (9, 10) comprising fifteen elementary areas 8 which all extend over an equal number of radii (which groups of elementary areas will be briefly referred to hereinafter as "bytes"). The sequence numers "1" to "15" of the elementary areas within the bytes are also indicated in Fig. 1. The bytes are divided into information bytes 9 and the control bytes 10, the control bytes being shown as hatched portions. The control bytes 10 are arranged in pairs at locations which are defined by a group of radii 11 which issue from the centre of rotation 4 and which have the same angular spacing, so that the control bytes of adjacent tracks are radially aligned with each other. (Hereinafter the pairwise arranged control bytes will be referred to briefly as "control words"). The information bytes 9 are provided with an optically readable information structure of the customary type, in which a plurality of elementary areas 8 comprise a pit 12 having a reflectivity which is lower than that of its surrounding. In such information structures it is customary that only a limited number of possible distributions of pits 12 over the elementary areas 8 are permissible. In the present example only those distributions are permissible in which 4 pits are distributed over the 15 available elementary areas 8 of information bytes 9. The control words 13 in the annular zones A are provided with a control structure which differs from the permissible information structures by the distribution of the number of pits over the available positions. In the present example the control structures in the zones A differ in that the pits are so distributed over the available 30 positions that one pit 18 is situated in the elementary area of the sequence number "3" in the first control byte and one synchronization pit 16 situated in the elementary area of the sequence number "12" of the second control byte. The control words 14 (see Fig. 1a) in the areas B differ from the control words in the areas A by a control structure in which one pit 19 is situated in the elementary area of the sequence number "5" of the first control byte and one synchronizing pit 20 is situated in the elementary area of the sequence number "12" of the second control byte. The control structures cannot only be used for distinguishing between the areas A and B but also for other control purposes. For example, the control words may comprise a groove 17 for tracking purposes. This is illustrated by means of a longitudinal sectional view of a control structure in Fig. 1a. This structure comprises a relief structure on a substrate 21 carrying a metal layer 22. The metal layer is covered with a transparent coating 22. The depth of the groove 17 and the depth of the pits 19 and 20 are designated D1 and D2 respectively. In practice, the values of D1 and D2 generally correspond to one quarter and one half of the wavelength of the light of the read beam. As a result of the differences in depth the beam is modulated in such a way that the modulations by grooves and pits can be distinguished from each other and from the modulations or by other parts of the surface. The flat portions in the relief, which are situated in the control structures between the groove and the synchronizing pits 16 or 20 may be used for focus control during reading. Moreover, on account of the fact that the control words are situated at equal angular spacings from each other, a clock signal can be generated by means of the control words, which clock signal corresponds to the centres of the elementary area 8. For a more detailed description of the aforementioned controls reference is made to United States Patent 4 779 253. Fig. 2 shows an apparatus for reading the information carrier 1. In this Figure the reference numeral 50 represents a drive mechanism of a customary type for rotating the information carrier 1 with a constant angular velocity about the centre of rotation 4. A laser 51 generates a read beam 52 which is projected and focussed on the information carrier 1 via a semi-transparent mirror 51a and an objective 53, the beam 52 forming a scanning spot 54 on the information carrier. The read beam 52 is reflected by the information carrier 1, causing the intensity of the read beam 52 to be modulated by the structure of the information carrier at the location of the scanning spot 54. Through the objective 53 and the objective 59 the reflected and modulated read beam 52 is projected onto an optical detector 55 of a customary type, the detector 55 generating electric signals representing the intensity of the incident read beam 52. The signals generated by the detector 55 are processed in a manner as described comprehensively in the aforementioned United States Patent 4 779 253, to form signals which are suitable for tracking control, focus-control of the beam and for the recovery of the information represented by the information structure. As these control systems fall beyond the scope of the present invention they will not be described herein. The objectives 53 and 59, the semi-transparent mirror 51a, and the detector 55 are incorporated in system 57 which can be moved in a direction transverse to the track direction by means of an actuating device 58 of a customary type. During a such a radial movement the scanning spot follows a path which intersects a number of annular zones A and B, the scanning spot 54 cooperating with the control structures at regular intervals. Fig. 3 shows an example of a path 60 along which the scanning spot 54 intersects the annular zones A and B in the case of a radial displacement of the system 57. The intersections of the path 60 with the radii 11 issuing from the centre of rotation 4 represent the intersections of the path 60 with the control structures. An intersection 61 of the control structure in one of the annular zones A is shown to a highly enlarged scale. The signal Vdet which is generated by the detector 55 during the intersection 61 and which represents the intensity of the incident beam is also shown. An intersection 62 with a control structure in one of the annular zones B and the signal Vdet produced during this intersection is also shown in Fig. 3. As can be seen in Fig. 3, the detection signal during intersection with the control structures in the annular zones A and the detection signal during intersection with the annular zones B differ from each other in that the distances between the peaks 63 in Vdet differ. For intersections with the control structures in zones A the distance between the peaks 63 corresponds to a time interval in which the information carrier is rotated through an angle corresponding to 23 elementary areas 8, whilst for an intersection with the control structures in zones B the distance between the peaks 63 corresponds to a time interval in which the information carrier is rotated through an angle corresponding to 21 elementary areas 8. Since the record carrier 1 is driven with a constant angular velocity the intersections with the control structures in the zones A and B can be distinguished from one another simply by, for example, time measurements. Since only those information structures are permissible in which four pits 12 are distributed over the fifteen elementary areas 8 of the information byte 9 the likelihood is nil that in the case of a radial displacement of the scanning spot over the information structures situated between the control structures the detection signal Vdet comprises two peaks 63 whose distance corresponds to 22 or 24 elementary areas, so that the components of the detection signal corresponding to the intersections with the information structures can be distinguished effectively from the remainder of the detection signal. As is apparent from Fig. 3, at least one group of adjacent control structures within each of the zones A and B will be crossed in the case of a radial displacement of the scanning spot, for which during crossing of each of the annular zones A and B the record carrier 1 is rotated through an angle larger than the angle between two successive radii 11. In that case it is simply possible to detect in the detection signal Vdet a transition from one annular area to the other, for example by comparing the components of the detection signal which represent successive intersections with the control structures. Fig. 4 shows an example of a detection circuit 70, which generates a measurement signal Vm which indicates crossings of the boundaries of successive annular zones. The detection circuit 70 comprises a synchronizing circuit 75 which derives control signals Vb1, Vb2 (see Fig. 5) from the detection signal Vdet, which control signals are synchronized with the instants at which the scanning spot 54 respectively meets an elementary area of the sequence number "3" and "5" of one of the first control bytes. Moreover, briefly after the generation of Vb2 the synchronizing circuit 70 generates a signal Vb3. The control signals Vb1 and Vb2 control a first sample-and-hold circuit 71 and a second sample-and-hold circuit 72 respectively, which is response to the control signals Vb1 and Vb2 respectively sample the detection signal Vdet substantially at the instant at which the centre of the scanning spot 54 respectively passes the centres of the elementary areas 8 of the sequence number "3" and "5" of one of the first control bytes. The outputs of the sample-and-hold circuits 71 and 72 are fed to the positive input and the negative input respectively of a comparator circuit 73. Depending on the result of the comparator circuit 73 produces a logic "O" or a logic "1" signal on its output. The signal on the output of the comparator circuit 73 is applied to a flip-flop 74, which in response to the control signal Vb3 is set to a state corresponding to the output signal of the comparator circuit 73. The signal Vm on the output of the flip-flop 74 is representative of the state of the flip-flop 74. The samples taken by the sample-and-hold circuits 71 and 72 represent the reflectivity of the elementary areas on which the scanning spot 54 is incident in one of the control bytes of the sequence number 3 and 5 respectively. In the annular zones A the reflectivities of the elementary areas of the sequence number 3 are smaller than the reflectivities of the elementary areas of the sequence number 5, so that during the time in which the scanning spot 54 is situated in one of the annular zones A at the instants at which the control signals Vb3 are generated the output signal of the comparator circuit 73 is "0". During the passage of one of the annular zones A this results in the signal Vm also being "0" from the instant at which the first control structure in this zone is scanned by the scanning spot 54. In a similar way it is achieved that during passage of one of the annular zones B the signal Vm is "1" from the instant at which the first control structure in this zone is scanned by the scanning spot 54. During a radial displacement of the scanning spot 5 the detection circuit 70 generates a squarewave signal of which each edge indicates a transition between successive annular zones A and B (see Fig. 5). During a radial displacement of the scanning spot 54 this number of transitions can be counted electronically and compared with a desired number of annular zones to be passed by the scanning spot 54. When the desired number is reached, the radial displacement can be stopped, so that the scanning spot 54 remains addressed to a specific annular zone. The manner in which the signal Vm is processed electronically and the manner in which the radial displacement of the scanning spot 54 is controlled fall beyond the scope of the present Application and are therefore not described. Once the scanning spot 54 is addressed to a specific annular zone the scanning spot 54 can be addressed to a signal track within the annular zone, for example by means of a digital code contained in the tracks and representing the number of the relevant track to be read, for example in a manner as described in the previous Netherlands Patent Application 7212002. By means of this address information the actuating device 58 can be controlled in such a way that the scanning spot 54 is addressed to the desired track. Fig. 7 shows an example of a synchronizing circuit 75. The synchronizing circuit 75 comprises a voltage-controlled oscillator 80 whose phase and frequency are such that pulses Vo are generated which are in synchronism with the instants at which the centre of the scanning beam passes the centres of the elementary areas 8 (see Fig. 5). The pulses Vo function as a clock signal for a counter 82. By means of a comparator circuit 83 a pulse-shaped signal Vs (see Fig. 5) is derived from the signal Vdet, which pulse signal serves to reset the counter 82 to zero. It will be evident that the reset signal can also be generated in another way, for example by differentiating the signal Vdet and subsequently generating the reset signal in response to a zero crossing of the differential signal. The counter 82 is constructed in such a way that it supplies a pulse signal Vt (see Fig. 5) at the instant at which the counter has reached the count "20". In response to the signal Vt a pulse generator 84 supplies a pulse signal Vv, whose width substantially corresponds to 5 pulses Vo. The signal Vv and the signal Vs are applied respectively to a first input and a second input of a two-input AND-gate 85. By means of the counter 82 each meeting of the scanning spot 82 with control structures is detected. As already described, the control structures differ from the information structure in that the distance between the peaks 63 in the detection signal Vdet is larger, which does not occur when information structures are scanned. Since the counter 82 is set to zero upon every pulse of the signal Vs the number of pulses counted by the counter 82 represents the time which has elapsed from the instant at which the last peak 63 in the detection signal Vdet has appeared. If this time exceeds a specific value, which indicates that a control structure is scanned (in the present example this is the count "20"), the counter 82 generates the signal Vt, as a result of which a pulse Vv is generated. The count for which the signal Vt is supplied and the width of the pulse Vv are selected in such a way that the instant at which one of the synchronizing pits 16 or 20 is scanned always falls within a time window defined by the pulse Vv, so that on the output of the AND-gate 85 pulses are generated only at instants which correspond to the instants at which the scanning spot 54 meets synchronizing pits 16 or 20. The output signal of the AND-gate 85 is applied to one of the two inputs of a phase detector 86. A signal Vo' is applied to the other input of the phase detector 86, which signal is obtained by means of a frequency divider 81 with a divisor equal to the number of elementary areas corresponding to the angle between successive synchronizing pits 16 or 20. The voltage-controlled oscillator 80, the frequency divider 81, the phase detector 86 and a loop filter 87 together constitute a phase-locked loop by means of which the aforementioned phase and frequency of the signal Vo are obtained for which the pulses Vo are in synchronism with the instants at which the centre of the scanning spot 54 passes the centres of the elementary areas 8. In the present example the frequency divider 81 is formed by a counter. By means of a gate circuit 88 of the customary type the pulse signals Vb1, Vb2 and Vb3 are derived from the count of the frequency divider 81. In the present embodiment the control structures differ from the information structure by a distance between the pits which does not occur in the information structure. Depending on the fact in which of the two types of annular zones the control structures are situated, this distance corresponds to "21" or "23" elementary areas. It is evident that the control structures may differ from the information structures by other distances, for example distances which do not correspond to a multiplicity of elementary areas but, for example, to 1½ or ½ times the length of an elementary area. Further, it is for example possible to identify the control structures by a number of successive pits larger than the maximum permissible number of successive pits in the information structure. It will be appreciated that in fact all those control structures which can be distinguished from each other may be used, provided that these structures are radially aligned with each other and vary mainly in the tangential direction, so that when a group of adjacent structures is crossed the modulation of the scanning beam is substantially independent of the angle at which the scanning spot intersects the control structures. For example, instead of control structures forming rows of pits in a radial direction, it is also possible to employ control structures in which the pits overlap one another and thus constitute elongate grooves which extend in a radial direction. Moreover, it is not necessary that the pits in the control structures are situated in the centres of the tracks. For example, it is also possible within the control structures to arrange a pit to the left of the centre and a pit to the right of the centre. The pits can then also be used for tracking control, so that the groove in the control structure may be dispensed with. In the present example the control structures are situated on a set of radii issuing from the centre of rotation and having equal angular spacings. However, other arrangements are also possible. It is adequate to divide the groups of adjacent distinguishable control structures over the annular zones in such a way that at least one group of control structures is met for a given maximum radial velocity of the scanning spot and a given speed of rotation of the information carrier in each annular zone. Another possibility is to allow a radial displacement over only a limited number of tracks and to select such a distribution of the groups of adjacent control structures that the permissible paths intersect at least one group of control structures within each annular zone. This is possible, for example, by only allowing a radial displacement of the scanning spot within a limited number of record-carrier sectors provided with control structures.

Claims: An optical record carrier (1) which is or can be provided with information structure (9) arranged along tracks (5), which tracks are situated substantially concentrically around a centre of rotation (4), the carrier comprising first (A) and second annular zones (B) which can be distinguished from each other and which are situated concentrically around the centre of rotation (4), characterized in that the record carrier (1) is provided with first (13) and second control structures (14), and which control structures has a shape which differs from the information structures, the first zones (A) being provided with groups of radially adjoining first control structures (13) and the second zones (B) being provided with groups of radially adjoining second control structures (14) in order to distinguish between the first zones and second zones. An optical record carrier as claimed in Claim 1, characterized in that the control structures are arranged along substantially concentric tracks in such a way that along the tracks the structure alternately comprise control structures (13,14) and information structures (9). An optical record carrier as claimed in Claim 1 or 2, characterized in that over the entire width of the annular zones the control structures are situated at locations which are defined by at least one group of radii (11) issuing from the centre of rotation (4) and having equal angular spacings. An optical record carrier as claimed in one of the Claims 1, 2 or 3, characterized in that the first control structure (13) comprise an optical detectable area (18) on a first predetermined position whilst a second predetermined tangential position within the first control structure is not provided with an optical detectable area, the second control structure comprises an optical detectable area (19) on the second predetermined position, whilst the first predetermined tangential position is not provided with an optical detectable area. An optical record carrier (1) as claimed in Claim 3 and 4, characterized in that the first and second detectable position are displayed by a first and a second predetermined angle respectively relative to the radii (11). A method for reading a record carrier (1) which is or can be provided with an information structure (9) arranged along tracks, which tracks are situated substantially concentrically around a centre of rotation (4), the carrier comprising first (A) and second annular zones (B) which can be distinguished from each other and which are situated concentrically around the centre of rotation (4), in which method a read radiation beam (52) is directed to a radiation sensitive detection system (55) via the record carrier (1), thin reading radiation beam (53) is focussed on the record carrier (1), a signal (Vm) indicating whether the scanning spot passes a first or a second zone is derived from a detection signal delivered by the radiation sensitive detection system, characterized in that the record carrier (1) is provided with first (13) and second control structures (14) which can be distinguished from each other and from the information structure (9), the first zones (A) being provided with groups of radially adjoining first control structures (13) and the second zones (B) being provided with groups of radially adjoining second control structures (14) in order to distinguish between the first zones (A) and second zones (B), the method is further characterized by distinguishing components (63) of the detection signal corresponding to the intersections with the detected radiation beam (52) when the scanning spot (54) passes the first (13) and second control structures (54), and deriving on basis of the distinguished components a measurement signal (Vm) which indicates whether a first (A) or second zone (B) has been passed. An apparatus for performing the method as claimed in Claim 6, which apparatus comprises drive means for rotating the record carrier (1) about the centre of rotation (4), a radiation source (51) producing a reading radiation beam (52), an objective system (53,59) for focussing the reading radiation beam (52) to form a scanning spot (54) on the record carrier (1) and subsequently directing the reading radiation beam to a radiation-sensitive detection system (55), the scanning spot being radially movable in the plane of rotation of the record carrier under control of a radial actuating device (58), and the detection system (55) for reading the information structure converting the modulation of the read beam produced by the structure on the record carrier into an electric signal (Vdit), characterized in that the apparatus comprises a detection circuit (70) for deriving a measurement signal (Vm) from first and second components of the electric signal (Vdit) corresponding to the modulated reading radiation beam as it scans the first (13) and the second control structures (14) respectively, which measurement signal indicates whether a first (A) or second zone (B) is passed by the scanning spot during a radial movement of the scanning spot. An apparatus as claimed in Claim 7, characterized in that the apparatus comprises a synchronizing circuit (15) for deriving from the electric signal (Vdet) first (Vm) and second control signals (Vb2) which are synchronous with the incidence of the scanning spot (54) on the first and the second tangential position within respectively, a sample-and-hold circuit (71,72) for taking first and second samples of the electric signal (Vdet) in response to the first (Vm) and second control signals (Vb2), and a circuit (73,74) for comparing the first and the second samples with each other and for generating the measurement signal (Vm) depending on the result of the comparison.

Inventors: VERBOOM, JOHANNES JACOBUS
Applicants: OPTICAL STORAGE INTERNATIONAL HOLLAND
Application year: 1987
Main classification: G11B7
IPC codes: G11B7, G11B21