NO149188B - DEVICE FOR READING A RECORDING BEAR WITH OPTICAL READABLE INFORMATION STRUCTURE - Google Patents

Description

The invention relates to an apparatus for reading

of a recording medium on which e.g. video and/or audio information in an optically readable, track-shaped information structure, which apparatus comprises a radiation source,"a lens system for projecting the radiation from the radiation source to a radiation-sensitive detection device-

via the recording medium, which detection device transforms the reading beam from the radiation source modulated by the information structure into an electrical signal, as well as a centering detection device which is connected to an electronic circuit to derive a control signal for correcting the centering of the reading beam on the part of the track to be read .

With a centering detection device shall

here is understood a radiation-sensitive detection device which delivers an electrical signal which gives an indication of the deviation between the center of the reading spot of the radiation projected onto the recording medium and the center of the track part which

read..

In ,,Philips,, Technical Review" Volume 33, No. 7,

Pages 186^189, an apparatus for reading a plate-shaped, round record carrier is described. A color television program is stored on the recording medium. The information structure comprises a helical track consisting of a number of depressions pressed into the recording medium, and the luminant information is contained in the frequency of the depressions, while the color and sound information is contained in a variation of the length of the depressions. A reading beam is focused on the information structure to a reading spot, the dimensions of which are in the order of magnitude of the recesses. By moving the recording medium in relation to the reading beam, this is modulated in accordance with the stored information. A radiation-sensitive information detection device converts the modulation in the reading beam into an electrical signal. In an electronic circuit, the signal is processed so that it is suitable for feeding a color television receiver.

When reading the recording medium, it must be ensured that the center of the reading spot is always projected onto the center of the track being read, otherwise the degree of modulation of the signal delivered from the information detection device will be too small and crosstalk can occur between one and the other bordering tracks. Therefore, the position of the reading spot must be detected and corrected continuously.

For this purpose, the known apparatus has an auxiliary detection system with two partial beams which are focused on the edge of a track part which is read. A separate auxiliary detector is used for each partial beam. The difference between the output signals from these auxiliary detectors gives an indication of the degree of centering of the reading beam in relation to the part of the track being read. In addition to the optical elements that are necessary for the actual reading of the information, the known device thus includes a number of auxiliary optical elements that are necessary for detecting centering errors.

The purpose of the invention is therefore to provide a reading device where centering errors can be detected, using a minimum of additional optical elements,

This is achieved according to the invention by the fact that the centering detection device and the information detection device consist of two or four radiation-sensitive detectors which are arranged in the beam path from the information structure in separate quadrants in a two-axis coordinate system whose origin lies in the optical axis of the objective system, whose X-axis extends 1 the track direction and if Y axis runs perpendicular to the track direction, that the outputs from two detectors on the same side of the Y axis are connected to both a subtraction circuit and a summation circuit, that the output from the subtraction circuit and the summation circuit is connected to the input of a multiplication circuit, and that the output from the multiplication circuit is connected with a filter which only passes frequencies lower than the frequency corresponding to twice the mean frequency of the information structure, which filter supplies a control signal for correcting the centering of the reading beam.

With the expression "detectors" which are arranged in the beam path from the information structure, it is to be understood that the detectors lie in a plane in which the different order of deflection of the reading beam modulated by the information structure is sufficiently distinct, i.e. in a plane which is sufficiently far from the image of the information struk.tour,

With the expression "if the X-axis extends in the direction of the track and if the Y-axis runs perpendicular to the direction of the track," it is to be understood that the imaginary projections of these axes on the information structure extend in the direction of the track resp. across the track direction.

The invention is based on that fact

that when reading the information structure which acts as two-dimensional; deflection grating, centering errors will result in additional phase shift between a partial beam of zeroth order and partial beam of higher order. These phase shifts can be detected in the beam path from the information structure by means of suitable placement of the detectors. According to the invention, a reference signal is obtained using the same detectors and this signal is used to derive a control signal to correct the centering of the reading spot in relation to the part of the track to be read. The advantage here is that the reference signal and the signal that gives an indication of centering error are affected at the same way in case of any disturbances in the reading system, such as optical noise or vibrations of the optical elements in the reading device. As a result of the way in which the signals are processed, namely by means of so-called synchronous detection, the resulting control signal for centering correction will be independent of these disturbances.

A further advantage is that the applicability of the invention is not limited to a specific phase depth of the information structure. The phase depth here is to be understood as the phase difference between a partial beam of the zeroth order and partial beams of the first order. effected by the details of the information structure. If the information structure is reflective and consists of depressions pressed into the surface of the recording medium, which depressions are X/4 deep, •

and X is the wavelength of the reading beam, the phase depth is equal to Tf , The invention is thus applicable to amplitude structures whose phase depth can be assumed to be ir.

The selected phase depth for the information structure, however, determines how the information structure should preferably be read, i.e. whether the sum of the output signals from the detectors on one side of the Y-axis should be added to or subtracted from the sum of the output signals from the detectors on the other side of the Y-axis .

In principle, according to the invention, only

two detectors. By using four detectors, a better signal-to-noise ratio can be achieved for the information signal

and for the centering error signal.

It should be noted that it has already been proposed

in Norwegian patent application no. 760631 to detect centering errors by means of an additional detector, which is arranged in the radiation path from the information structure. Alternatively, two additional detectors can be used. However, these latter detectors are arranged in the same quadrants in the above-mentioned imaginary coordinate system and the output signals from these detectors are not subtracted from each other to detect a centering error. For dynamic detection errors in the previously proposed apparatus, the track part that is read and the reading

the spot periodically moved relative to each other across the track direction during the reading. This requires an adaptation of either the recording medium or the reading device.

The invention will be explained in more detail below with reference to the drawings.

Fig. 1 schematically shows an embodiment

of an apparatus according to the invention.

Fig. 2, 6, 6a, 7 and 8 show examples of radiation-sensitive detection devices used in this apparatus and also show how the signals are provided in the individual parts. Fig. 3, 4 and 5 show the principle of the invention. Fig. 1 shows a round, plate-shaped recording medium 1 in radial cross-section. The information structure is assumed to be reflective and has the form of tracks 3. Em radiation source 6, e.g. era heliuitf-neon laser supplies a reading beam b. A mirror 9 reflects this beam towards a lens system 11, which is schematically shown only as a single lens. The path for the reading beam b comprises an auxiliary lens 7, which ensures that the reading beam in its entirety falls on the lens system's light opening. A radiation spot with minimal dimensions is thus formed in plane 2 in the information structure.

The reading beam is reflected by the information structure and rotates the record carrier about the shaft 5 which extends through a central opening 4, and is time-modulated in accordance with information stored in the track being read. The modulated reading beam again passes the objective system and is reflected by a mirror 9 in the direction of the beam delivered by the radiation source. The radiation path of the readout beam includes elements to separate, the paths of the modulated and the unmodulated readout beam. These elements can, for example, comprise a unit consisting of a polarization-sensitive dividing prism and an A/4" plate. For the sake of simplicity, it is assumed in Fig. 1 that this unit consists of a partially penetrating mirror 8. This mirror reflects part of the modulated reading beam onto a radiation-sensitive detection device 12.

The optical details of the information structure

are very small. With a track width of 0.5 µm and a track spacing of 1.2 µm, and an average period of the information area assumed to be 3 µm indentations on a flat plate recording medium, the information can contain a thirty minute television program in an annular area with an inner diameter of 12 cm and an outer diameter of 27 cm. Therefore, the reading spot must be very precisely centered in the track to be read,

In order to be able to detect centering errors in accordance with the invention, the detection device 12 consists of e.g.

of four radiation-sensitive detectors as shown in fig. 2.

The four detectors 13, 14, 15 and 16 are arranged in four different quadrants in an X-Y coordinate system. If a track-

part that is read is projected onto the detection device, the longitudinal and transverse direction of the track part is parallel to the X-axis or The Y axis.

The four detectors are e.g. arranged in a plane U, in which an image of the aperture of objective

the systern is formed by means of an auxiliary lens 28. For simplicity, the image (a<1>) is only shown as a point in fig. 1. The detectors 13, 14, 15 and 16 can also be arranged in a

second plane, provided that the sub-beams which are deflected in different order by the information structure are sufficiently distinct.

As shown in fig. 2, the output signals from the detectors 13 and 16 are subtracted from each other by means of a subtraction circuit 17 and the output signals from the detectors 14 and 15 are subtracted from each other by means of a subtraction circuit 18. The output signals from the subtraction circuits 17

and 18 are subtracted from each other in a subtraction circuit 19. The output signal from this circuit is supplied to an input in a multiplication circuit 22. The output signal from the circuit 22 is supplied to a? low-pass filter 23. The desired signal Sr appears in the output of this filter, as will be explained in more detail below.

For reading the information in the recording medium, e.g. a television program, the output signals from the detectors 13 and 16 can be summed using a summing circuit 21 and the output signals from the detectors 14 and 15 are summed using a summing circuit 20. The output signals from the summing circuits 20 and 21 can be subtracted from each other in a subtracting circuit 24. The information signal $ 24 ' which occurs at the output of the subtractor circuit 24, is decoded in an electronic circuit 26 known per se and the decoded signal is made visible and audible with the help of an ordinary television receiver 27. The signal S24 is further supplied to a second input in the multiplication circuit 22.

The information structure on the recording medium consists of tracks that contain a number of areas and intermediate areas, where the areas have the form of depressions and can be considered a two-dimensional deflection grid. The grating divides the reading beam b into a partial beam of the zeroth order and a number of partial beams of the first order and a number of partial beams of a higher order. Part of the radiation from the partial beams passes the opening in an objective system 11 and can be concentrated in an imaging plane in the information structure. In this imaging plan, the individual sub-beams are not separated from each other. However, in the plane of the exit aperture of the objective system or in a plane in which an image of the exit aperture is formed, the various sub-beams are more or less separated. Fig. 3 and 4 show the situation in the plan of the exit opening.

In fig. 3, the circle 30 with center 35 represents a cross-section of the partial beam b (0,0) of the first order in the plane of the objective system's output opening. Circles 31,

32, 33 and 34 represent the cross section of the diagonally deflected partial beams b(+l, +1), b(-l, +1) b(-l, -1), and b(+l, -1).

In addition to the partial beams deflected in the diagonal direction, there are also partial beams in the track direction and across the track direction, namely partial beams of (+1, 0) order and (-1, 0) order, which are due to the depressions in the track being read, and also (0 , +1) order and (0, -1) order which is only due to the lattice structure across the track direction. Fig. 4 shows circles 41, 42, 43 and 44 which represent the cross section of the partial beams b(+l, 0), b(0, +1), b(-l, 0) and b (0, -1) in the plane of the lens system exit aperture.

The X-axis and Y-axis in fig. 3 and 4 correspond to the X-axis and Y-axis in fig. 2. The distances from the centers 36, 37, 38, 39, 46 and 48 for the circles 31, 32, 33, 34", 42 and 44

from the X-axis is determined by X/q, where q is the period of the information structure in a direction transverse to the track direction and is the wavelength of the reading beam b. The period q can be assumed to be constant. The distance f from the centers 36, 37, 38, 39, 45 and 47 to the Y-axis is determined by X/p, where p represents the period of the depressions in the track being read.

To determine a centering error, changes of the phase of the diagonal sub-beams of the first order are used

relative to the partial beam of the zeroth order.

In the shaded areas in fig. 3 overlaps

the different diagonal subbeams of first order b(+l, +1), b(-l, +1), b(-l, -1), and b(+l,--l) the subbeam of zeroth order b( 0' 0) and interference occurs. The phase

•for the diagonal sub-beams of the first order vary with high frequency which is due to the movement of the reading spot over the information structure in the track direction, and a low fre-

due to a possible movement of the reading spot in a direction transverse to the track direction. This results in intensity variations in the output aperture or the effective output aperture of the lens system, which variations can be detected e.g. by means of the detection device on

fig. 2.

When the center of the reading spot coincides

with the center of a recess, a definite phase difference ip is obtained between a partial beam of the first order and a partial beam of the zeroth order. The value of ty depends on the shape of the information structure and, in the case of a structure with dimples, depends mainly on the phase depth of the dimples

the nings. When the reading spot moves! from a first recess to a second recess, the phase will e.g. of part-

the beam of the first order b(+l, +1) in relation to the partial beam of the zeroth order increases continuously by 2 11. It must therefore be assumed that when the reading spot is moved in the track direction, the phase of the partial beam of the first order b(+l, +1) relative to the zeroth-order partial beam is changed by uit, where w is the instantaneous frequency which is determined by the frequency of the depressions in the part of the track that is read and by the speed with which the reading spot is moved over the part of the track. Also in the case of a movement of the reading spot across the track direction, the phase of the partial beam of the first order b(+l, +1) in relation to the partial beam of the zeroth order will change. This phase shift can be expressed by 2 11. gr where Ar is the distance between the center of the reading spot and the center of the part of the track that is read -

The phase t±1, + 1) for the different diago-

nal partial beams of the first order in relation to the partial beam of

The zeroth order can therefore be expressed by:

The intensity variations in the output aperture of the objective system due to interference between diagonal sub-beams of first order and sub-beams of zero order are converted into electrical signals by means of detectors 13, 14, 15 and 16. The time-dependent output signals S^» ^14'<S>15 and S16 from detectors 13,14,15 and 16 can be expressed with:

where A is a constant.

As shown in fig. 2, the signals S^ and S^g are subtracted from each other and so are the signals S^ and S^j.. The signals at the output of the subtraction circuits 17 and 18 with : where B is again a positive constant. The signals S^ and S^g are subtracted from each other in a subtraction circuit 19 and the output signal S,Q from this circuit can be expressed by:

where C is again a positive constant. The component sin 2 ir is a different function of Ar, so that the signal S^^ contains information about both the size and the direction of the centering error for the reading spot in relation to the track being read. The component's location varies in time depending on the information

which is stored in the track part, but depends on the centering error Ar.

As shown in fig. 2, the output signals from detectors 13 and 16 are summed in a summing circuit 21. The expression ojt in the signals S, . and S,, have the same sign, while

Year 13 16

the sign of 2 ir in the signal S,_ . , .

3 for 2 tt q 3 13 is opposite the same

expression in the signal S1<r. As a result, the reaction in

ib

the sum of the signals S,_ and S1/r as a result of centering 1.ilo

error be significantly smaller than the variation in the signal S^. The sum of the signals S^ ^°9s^g is mainly made up of the partial beams of the first order which are deflected in the track direction. This signal sum can be expressed:

where m is a constant less than 1, so that the sign of S2^ cannot change as a result of centering errors. Similarly, the sum of the signals S^4+ S^,. is expressed :

The signals S^q and S2^ are supplied to the subtraction circuit 24, at the output of which the following signal appears:

After the multiplication in circuit 22, this becomes:

The component sin<2>to t can be expressed:

' h~h cos- (2 lot) ; and the component sin ty cos4* can be expressed as ^ sin (2 ty ), so that : and are always positive. In the above expressions, D, E, F and G are positive constants. The signal S22 finally passes through a low-pass filter 23, which only lets through frequencies that are lower than the frequencies 2 aj , so that a signal is obtained: S r = sin 2 ty K ( A r) sin (2 ^ -^-^ ) . Since to is determined by the phase depth which is constant for a particular recording medium, sin 2ty is also constant.

Consequently, the signal Sr is a different function of the centering error Ar, so that with the help of the described detector device and the described signal processing, both the size and direction of the centering error are detected. The signal Sr can also be used for correcting the position of the reading spot in relation to the part of the track being read on

in and of itself known way, e.g. by tilting the mirror 9 in the direction of the arrow 10 (see fig. 1).

The components K( A r), sin (2ir ■^<r>— can be written :

Ari q

,n. Year. ,m sin (4 ir )l. In fig. 5 is represented1G|j5in(2Tr—)+^(4 q J On fi9'representeS

the functions sin (2 tt ^-) and?sin (4 " ^ )<a>vthe dashed curves 1^and 12and the sum function of the solid curve 1^«

This shows that in the area around A r = 0, which is important for power steering, the signal ? sin (4 ir ^<r>—) increases the signal sin (2 ir Ar ) ; and the slope of the curve 1^ around A r = 0"

is steeper than for curve 1,.

It should also be noted that in the shaded areas in fig. 4, the partial beams deflected in the X direction are overlapped by the partial beams deflected in the Y direction. The output signals from the detectors 13, 14, 15 and 16 are therefore not only determined by the interference between partial beams of the first order and partial beams of the zeroth order, which are deflected in the diagonal direction, but also by the interference between partial beams of the first order which are deflected in the track direction and partial beams from first order that are deflected across the track direction, as long as these rays fall within the aperture of the objective system.

The phase difference between e.g. the partial beam b(+l, 0) and the partial beam b(0, +1) can be expressed by : u> t + 2 ir— — In this expression the phase angle ty no longer appears, because both the partial beam b (+ 1, 0) and partial str -the eel b(0, +l) has the same phase angle 4* in relation to the partial beam of the zeroth order b(0, 0), Interference between the partial beams b(+ 1, 0), b(0, +1),

b(-1, 0) and B((0, -1) give the following signals from the detectors 13, 14, 15 and 16:

These signals are processed in the same way as the signals S^' S14, S 15 and Slg, i.e.:

where B<1>and C are positive constants. The signals Sig and S<1>^g do not oppose each other but reinforce each other, so that the described centering error detection is possible if

-C cos is positive or cos \ p is negative.

So far, only partial beams of the first order have been treated. It is clear that the information structure will also deflect the radiation in a higher order. However, the radiation energy in these higher-order deflections is so small and the deflection angles are such that only a small part of these partial beams fall within the opening of the objective system. Therefore, the effect of these higher-order partial beams can be neglected.

In order to enable the reading of a recording medium, especially an optical system, the frequency of the information structure must lie within certain limits. Fig. 3

and 4 shows the situation where the frequency in the track direction and perpendicular to the track direction corresponds to half of the upper limit frequency for the optical reading system. The degree of modulation of the information signal S24 is then maximum

and the degree of modulation of the centering error signal Sr is also maximum. When the frequency of the depressions in the part of the track that is read increases, the partial beams of the first order will be deflected at a larger angle, i.e. the distance f increases. At a particular frequency which corresponds to the upper limit frequency of the optical system, there will no longer be any overlapping between the partial beams of the first order and the partial beams of the zeroth order, and between the partial beams of the first order with each other. There will then no longer be any interference in the area covered by the detectors and it is no longer possible to derive an information signal. When the distance from the centers 36, 37, 38 and 39 to the center 35 is proportional to "\le2 + f<2>," the highest frequency for the depressions in the groove for which a centering error-

signal can be deduced to be somewhat lower, e.g. 15% lower than the highest frequency for an information signal S2ij. On

on the other hand, if the frequency of the indentations approaches zero, the distance f will also approach zero. The different partial beams of the first order are then no longer separated, so that it is no longer possible to obtain an information signal. The lowest value of the frequency of the depressions at which a centering error signal can be derived is somewhat less than the value at which an information signal can still be obtained. The lower limit frequency for the frequency of the recording medium at which the centering error signal can still be obtained is the frequency at which it is still possible to derive an information signal.

It is clear that the same considerations can be made for the frequency of the information structure in the direction across the track direction.

There is thus an optimal value for the frequency of the information structure in the track direction, and in the direction across the track direction, at which an optimal centering error signal can be obtained. However, there is a wide range for the frequency around the optimum value within which it is possible to derive an information signal and a centering error signal with a satisfactory signal-to-noise ratio.

In the apparatus of fig. 2 are the signals from the left and right part of the lens system's output aperture

subtracted from each other to improve information sig-

i naiet (S^)• This device is particularly suitable for reading the recording medium with a small phase depth or a small depth of the depressions.

In the expressions for S sin 2 ty, a maximum is then reached

value for ty = —3 ^ TI, where costy is negative.

For reading a recording medium with a greater phase depth, it is preferable to add the signals on the left and right sides of the objective system's output aperture with each other. For this purpose, the subtraction circuit 24 of FIG. 2

replaced with a summing circuit 24' as shown by dashed lines in fig. 2. In addition, a phase shift circuit has been inserted between this summing circuit and the multiplication circuit, e.g. a differentiation network 25, as shown with dashed lines in fig. 2y It is then also possible to sum the output signals from detectors 13 and 15 and from detectors 14 and 16. The signal processing circuit can then be simplified as shown in fig. 6.

In the device in fig. 6 are the following signals

specific :

The components in the expressions for Sg2 and Sg3 which vary with u) t are-j half phase-shifted in relation to each other, so that one of the signals Sg20<3 s63 can pass the phase shift circuit. This circuit can be a differentiation network. (Compare element 25 in Fig. 2). Preferably, however, the phase shifting circuit 64 has the form of a so-called phase-locked loop.

Fig. 6a shows a circuit 64 which contains an oscillator 66 which supplies a cone function to the output 67 and a sine function to the output 68. The output 67 is connected to a first input in a frequency comparison circuit 65, in which the frequency 66 is compared with the frequency of the signal cos w t, the phase of which is to be shifted by 90°. The output signal from the comparison circuit 65 is fed back to the oscillator, so that the frequency in the oscillator is corrected so that it is equal to the frequency of the signal Cos u t. A sine function with the desired frequency w is then obtained at the output 68 from the oscillator.

Accordingly, the circuit 64 transforms the signal Sg^ into a signal:

Multiplication of the signals S62°9s64°9filtering gives the signal:

where 1 is a function of a z and always positive. B^, C^, D^, and E^ are positive constants. Sr is a different function of the centering error A z. cos2if) which is maximum for a phase depth ty = it, so that the device in fig. 6 is suitable for reading a recording medium with a larger phase depth' and also for a recording medium with an amplitude structure whose phase depth can be tt radians. Instead of using the entire exit opening of the objective system, it is also possible to use only half of it. Such a device is shown in fig. 7. From the foregoing, it will appear that the following expression applies to the device in fig. 7 : A

After a phase shift tt/2 for the signal S2^ and multiplication by the phase-shifted signal S.7, one obtains:

Filtering this signal gives the signal Sr, which can be written:

, D2 and G2 are positive constants. K2 is a positive

function of A r, This expression S , n ,

J r, J S r containxngen

function of ty, so that the device in fig. 7 is suitable for reading both recording media with a small phase depth and recording media with a large phase depth.

It should be noted that when the signals from the entire object Tivsys ternet outlet are utilized, the signal-to-noise ratio for the information signal and the centering signal becomes

better than in the case where only the signals from half

of the exit openings are utilized.

The invention is described above under the assumption of a round plate-shaped recording medium with a radiation-reflecting information structure. It is clear that it is also possible to read radiation-compatible record carriers with an apparatus according to the invention. The recording medium also does not have to be round or flat,

but can also be an op drawing carrier in the form of a tape with many information tracks.

With regard to the information structure, it should be noted that it is only necessary that this structure is readable using optical aids. This structure can persist

of depressions in the recording medium, black or white areas or e.g. a magneto-optical structure. Apart from a television programme, the recording medium can e.g. contain digital information for a computer.

To determine the centering error, a

pattern of interference lines in the lens system's light opening, which pattern is produced by interference between parts

the beam of the zeroth order and the partial beam of the first order. The phase

for the line pattern in relation to the detectors is determined by the extent to which the reading spot is centered in relation to the track being read. However, the frequency of the line pattern is determined by the degree of focusing of the reading beam in the plane

for the information structure. Too large focusing errors are

this frequency high and for small focusing errors this frequency is low. The way in which the focus correct-

geres is immaterial to the present invention and shall therefore not be described in more detail. However, it should be noted that focusing errors can affect the choice of the shape of the detectors in fig. 7.

It is assumed above that the detectors 13 and

16 has a rectangular shape. The response of a rectangular detector to a linear intensity pattern is a curve

sinx 1

which varies in accordance with — ^^ , where — is equal to the period

x x

for the line pattern. This curve has a value of zero if the period is equal to the width of the detector. In that case, the detector will always see a period of the line pattern, regardless of the phase of the line pattern and is thus independent of the centering. In larger focusing errors, the period of the intensity pattern is smaller than the width of the detector, and the curve will have a negative part. This means that the servo system for the centering must move the reading beam in the wrong direction and that a possible centering error will increase. By using rectangular detectors, there is therefore a risk as a result of behaviour

. of focusing error that the servo system for the centering causes the center of the reading spot not to remain in the center of the track part being read, but to be projected at a certain distance from the center line

This problem can be overcome by using triangular detectors instead of rectangular detectors. Fig. 8 shows a pair of detectors 13' and 16' which can replace the detectors 13 and 16 in Fig. 7. The response curve for the triangular detectors follows (—) 2 and consequently has no negative part.

Obviously, this problem does not occur if the reading apparatus is provided with servo steering which ensures that the reading spot always remains properly focused on the information structure.

Claims (5)

1. Apparatus for reading a recording medium on which e.g. video and/or audio information in an optically readable, track-shaped information structure, which apparatus comprises a radiation source, an objective system for projecting the radiation from the radiation source onto a radiation-sensitive detection device via the record carrier/which detection device transforms the reading beam from the radiation source modulated by the information structure, into an electrical signal, as well as a centering detection device which is connected to an electronic circuit to derive), a control signal for correcting the centering of the reading beam on the part of the track to be read, characterized in that the centering detection device and the information detection device consist of two or four radiation-sensitive detectors which are arranged in the ray path from the information structure in separate quadrants in a two-axis coordinate system (X,Y), whose origin lies in the optical axis of the lens system, whose X-axis extends in the track direction and whose Y-axis runs perpendicular tt in the track direction, that the outputs from two detectors on the same side of the Y-axis are connected to both a subtraction circuit and a summation circuit, that the output from the subtraction circuit and the summation circuit is connected to the input of a multiplication circuit, and that the output from the multiplication circuit is connected to a filter which only passes frequencies lower than the frequency corresponding to twice the mean frequency of the information structure, which filter supplies a control signal for correcting the centering of the reading beam. 2. Apparatus according to claim 1, with four detectors, characterized in that the output from the first subtraction circuit whose inputs are connected to detectors located on one side of the Y-axis, and the output from the second subtraction circuit whose inputs are connected to detectors located on the other side of the Y-axis, is connected to the inputs of a third subtraction circuit which is connected to a first input of the multiplication circuit, and that the output of a first summing circuit whose inputs are connected to detectors located on one side of the Y-axis and the output of a second summing circuit whose inputs are connected to detectors located on the other side of the Y-axis is connected to a fourth subtraction circuit whose output is connected to a second input of the multiplex ion circuit. 3. Apparatus according to claim 1 with four detectors, characterized in that the output from detectors in the first and third quadrant is connected to the summing circuit and the subtraction circuit via a second summing circuit, and the output from detectors in the second and fourth quadrant is connected to the summing circuit and the subtraction circuit via a third summing circuit, and that a phase shift network has been inserted in one of the connections between the summing circuit and one of the inputs in the multiplication circuit and between the subtraction circuit and the other input in the multiplication circuit. 4. Apparatus according to claim 1, with two detectors on one side of the Y-axis, characterized in that one of the connections between the summation circuit and the multiplication circuit and between the subtraction circuit and the multiplication circuit comprises a phase shift network. 5. Apparatus according to claim 4, characterized in that each of the detectors has the shape of a; isosceles triangle whose base line is parallel to the X-axis.