Abstract:
A method is described for detecting a mechanical shock in a disc drive apparatus ( 1 ) of a type comprising: scanning means ( 30 ) for scanning record tracks of a disc ( 2 ) and for generating a read signal (S R ); actuator means ( 50 ) for controlling the positioning of at least one read/write element ( 34 ) of said scanning means; a control circuit ( 90 ) for receiving said read signal (S R ), deriving at least one error signal (RE) from said read signal, and generating at least one actuator control signal (S CR ) on the basis of said error signal. The method comprises the steps of: -determining a shock sensitivity function (S SHOCK ) which describes the relationship between shocks (OExT) and said error signal; determining or at least approximating an inverse shock sensitivity function (S-l SHOCK ) as the inverse of said shock sensitivity function (S SHOCK );—and applying said inverse shock sensitivity function (Sl- SHOCK )on said error signal.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates in general to a disc drive apparatus for writing or reading information into or from a storage disc. Although the gist of the invention also applies to magnetic discs, the present invention specifically applies to optical discs, for which reason the invention will hereinafter be described for optical discs, while the corresponding disc drive apparatus will also be indicated as “optical disc drive”.  
       BACKGROUND OF THE INVENTION  
       [0002]     As is commonly known, an optical storage disc comprises at least one track, either in the form of a continuous spiral or in the form of multiple concentric circles, of storage space where information may be stored in the form of a data pattern. Optical discs may be read-only type, where information is recorded during manufacturing, which information can only be read by a user. The optical storage disc may also be a writeable type, where information may be stored by a user. For writing information in the storage space of the optical storage disc, or for reading information from the disc, an optical disc drive comprises, on the one hand, rotating means for receiving and rotating an optical disc, and on the other hand optical means for generating an optical beam, typically a laser beam, and for scanning the storage track with said laser beam. Since the technology of optical discs in general, the way in which information can be stored in an optical disc, and the way in which optical data can be read from an optical disc, is commonly known, it is not necessary here to describe this technology in more detail.  
         [0003]     For rotating the optical disc, an optical disc drive typically comprises a motor, which drives a hub engaging a central portion of the optical disc. Usually, the motor is implemented as a spindle motor, and the motor-driven hub may be arranged directly on the spindle axle of the motor.  
         [0004]     For optically scanning the rotating disc, an optical disc drive comprises a light beam generator device (typically a laser diode), an objective lens for focussing the light beam in a focal spot on the disc, and an optical detector for receiving the reflected light reflected from the disc and for generating an electrical detector output signal.  
         [0005]     During operation, the light beam should remain focussed on the disc. To this end, the objective lens is arranged axially displaceable, and the optical disc drive comprises focus actuator means for controlling the axial position of the objective lens. Further, the focal spot should remain aligned with a track or should be capable of being positioned with respect to a new track. To this end, at least the objective lens is mounted radially displaceable, and the optical disc drive comprises radial actuator means for controlling the radial position of the objective lens.  
         [0006]     In many disc drives, the objective lens is arranged tiltably, and such optical disc drive comprises tilt actuator means for controlling the tilt angle of the objective lens.  
         [0007]     For controlling these actuators, the optical disc drive comprises a controller, which receives an output signal from the optical detector. From this signal, hereinafter also referred to as read signal, the controller derives one or more error signals, such as for instance a focus error signal, a radial error signal, and, on the basis of these error signals, the controller generates actuator control signals for controlling the actuators such as to reduce or eliminate position errors.  
         [0008]     In the process of generating actuator control signals, the controller shows a certain control characteristic. Such control characteristic is a feature of the controller, which may be described as the way in which the controller behaves as reaction to detecting position errors.  
         [0009]     Position errors may, in practice, be caused by different types of disturbances. The two most important classes of disturbances are: 
    1) disc defects     2) external shocks and (periodic) vibration    
 
         [0012]     The first category comprises internal disc defects like black dots, pollution like fingerprints, damage like scratches, etc. The second category comprises shocks caused by an object colliding to the disc drive, but shocks are mainly to be expected in portable disc drives and automobile applications. Apart from the difference in origin, an important distinction between disc defects on the one hand and shocks and vibration on the other hand is the frequency range of signal disturbances: signal disturbances caused by disc defects are typically high-frequency, while shocks and vibrations are typically low-frequency.  
         [0013]     A problem in this respect is that adequately handling disturbances of the first category requires a different control characteristic than adequately handling disturbances of the second category. Conventionally, the controller of a disc drive has a fixed control characteristic, which is either specifically adapted for adequately handling disturbances of the first category (in which case error control is not optimal in the case of disturbances of the second category) or specifically adapted for adequately handling disturbances of the second category (in which case error control is not optimal in the case of disturbances of the first category), or the control characteristic is a compromise (in which case error control is not optimal in the case of disturbances of the fist category as well as in the case of disturbances of the second category). As long as a controller applies linear control technique, there is always a compromise between low-frequency disturbance rejection and high-frequency sensitivity to measuring noise.  
         [0014]     In the state of the art, it has already been proposed to change the gain of the controller, depending on the type of disturbance experienced. For instance, reference is made to U.S. Pat. No. 4,722,079.  
         [0015]     In order to be able to implement a controller having variable gain, it is necessary to determine which class of disturbance is at hand. To this end, it is known to use a separate shock sensor, measuring acceleration. Said U.S. Pat. No. 4,722,079 describes a system where an optical read signal is processed to determine disturbance class, but this system requires a 3-beam optical system.  
         [0016]     A general objective of the present invention is to provide a method for reliably detecting whether a position error is caused by a mechanical shock, which method also allows for determining the strength and shape of the shock.  
         [0017]     Further, it is an objective of the present invention to provide a method for reliably detecting whether a position error is caused by a mechanical shock, which method is hardly sensitive to disc defects.  
       SUMMARY OF THE INVENTION  
       [0018]     According to a first important aspect of the present invention, a mathematical model is determined or at least approximated, which model is a frequency-domain description of the relationship between mechanical shocks and a resulting optical error signal, e.g. radial error signal, focus error signal, etc. This model is considered as being a transfer function, wherein the shock is an input variable, and wherein the resulting optical error signal is an output variable.  
         [0019]     According to a second important aspect of the present invention, an inverse transfer function is calculated, being an inverse function of said transfer function. This inverse transfer function has the optical error signal as an input variable, and has the shock as output variable.  
         [0020]     It is noted that a distinction between shocks in vertical direction (i.e. parallel to the disc rotation axis) and shocks in horizontal direction (i.e. perpendicular to the disc rotation axis) can be made by taking into account the focus error signal or the radial error signal, respectively. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     These and other aspects, features and advantages of the present invention will be further explained by the following description with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which:  
         [0022]      FIG. 1A  schematically illustrates relevant components of an optical disc drive apparatus;  
         [0023]      FIG. 1B  schematically illustrates an embodiment of an optical detector in more detail;  
         [0024]      FIG. 2A  schematically illustrates the optical disc drive apparatus as a mass-spring system;  
         [0025]      FIG. 2B  schematically illustrates the characteristics of this mass-spring system used for deriving a model;  
         [0026]      FIG. 3  shows graphs of magnitude and phase of a radial transfer function of external shocks/vibration to radial error signal as a function of frequency;  
         [0027]      FIG. 4  is a block diagram schematically illustrating a shock detector circuit according to the present invention;  
         [0028]      FIG. 5  is a block diagram schematically illustrating a preferred embodiment of the shock detector circuit according to the present invention;  
         [0029]      FIG. 6  shows graphs illustrating simulation test results of the shock detector circuit according to the present invention. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0030]      FIG. 1A  schematically illustrates an optical disc drive apparatus  1 , suitable for storing information on or reading information from an optical disc  2 , typically a DVD or a CD. For rotating the disc  2 , the disc drive apparatus  1  comprises a motor  4  fixed to a frame (not shown for sake of simplicity), defining a rotation axis  5 .  
         [0031]     The disc drive apparatus  1  further comprises an optical system  30  for scanning tracks (not shown) of the disc  2  by an optical beam. More specifically, in the exemplary arrangement illustrated in  FIG. 1A , the optical system  30  comprises a light beam generating means  31 , typically a laser such as a laser diode, arranged to generate a light beam  32 . In the following, different sections of the light beam  32 , following an optical path  39 , will be indicated by a character a, b, c, etc added to the reference numeral  32 .  
         [0032]     The light beam  32  passes a beam splitter  33 , a collimator lens  37  and an objective lens  34  to reach (beam  32   b ) the disc  2 . The light beam  32   b  reflects from the disc  2  (reflected light beam  32   c ) and passes the objective lens  34 , the collimator lens  37  and the beam splitter  33  (beam  32   d ) to reach an optical detector  35 . The objective lens  34  is designed to focus the light beam  32   b  in a focal spot F on a recording layer (not shown for sake of simplicity) of the disc.  
         [0033]     The disc drive apparatus  1  further comprises an actuator system  50 , which comprises a radial actuator  51  for radially displacing the objective lens  34  with respect to the disc  2 . Since radial actuators are known per se, while the present invention does not relate to the design and functioning of such radial actuator, it is not necessary here to discuss the design and functioning of a radial actuator in great detail.  
         [0034]     For achieving and maintaining a correct focusing, exactly on the desired location of the disc  2 , said objective lens  34  is mounted axially displaceable, while further the actuator system  50  also comprises a focus actuator  52  arranged for axially displacing the objective lens  34  with respect to the disc  2 . Since focus actuators are known per se, while further the design and operation of such focus actuator is no subject of the present invention, it is not necessary here to discuss the design and operation of such focus actuator in great detail.  
         [0035]     For achieving and maintaining a correct tilt position of the objective lens  34 , the objective lens  34  may be mounted pivotably; in such case, as shown, the actuator system  50  also comprises a tilt actuator  53  arranged for pivoting the objective lens  34  with respect to the disc  2 . Since tilt actuators are known per se, while further the design and operation of such tilt actuator is no subject of the present invention, it is not necessary here to discuss the design and operation of such tilt actuator in great detail.  
         [0036]     It is further noted that means for supporting the objective lens with respect to an apparatus frame, and means for axially and radially displacing the objective lens, as well as means for pivoting the objective lens, are generally known per se. Since the design and operation of such supporting and displacing means are no subject of the present invention, it is not necessary here to discuss their design and operation in great detail.  
         [0037]     It is further noted that the radial actuator  51 , the focus actuator  52  and the tilt actuator  53  may be implemented as one integrated actuator.  
         [0038]     The disc drive apparatus  1  further comprises a control circuit  90  having a first output  92  connected to a control input of the motor  4 , having a second output  93  coupled to a control input of the radial actuator  51 , having a third output  94  coupled to a control input of the focus actuator  52 , and having a fourth output  95  coupled to a control input of the tilt actuator  53 . The control circuit  90  is designed to generate at its first output  92  a control signal S CM  for controlling the motor  4 , to generate at its second control output  93  a control signal S CR  for controlling the radial actuator  51 , to generate at its third output  94  a control signal S CF  for controlling the focus actuator  52 , and to generate at its fourth output  95  a control signal S CT  for controlling the tilt actuator  53 .  
         [0039]     The control circuit  90  further has a read signal input  91  for receiving a read signal S R  from the optical detector  35 .  
         [0040]      FIG. 1B  illustrates that the optical detector  35  may comprise a plurality of detector segments. In the case illustrated in  FIG. 1B , the optical detector  35  comprises four detector segments  35   a ,  35   b ,  35   c ,  35   d , capable of providing individual detector signals A, B, C, D, respectively, indicating the amount of light incident on each of the four detector quadrants, respectively. A centre line  36 , separating the first and fourth segments  35   a  and  35   d  from the second and third segments  35   b  and  35   c , has a direction corresponding to the track direction.  FIG. 1B  also illustrates that, in the case of a four-quadrant detector, the read signal input  91  of the control circuit  90  actually comprises four inputs  91   a ,  91   b ,  91   c ,  91   d  for receiving said individual detector signals A, B, C, D, respectively. Since such four-quadrant detector is commonly known per se, it is not necessary here to give a more detailed description of Its design and functioning.  
         [0041]     It is noted that different designs for the optical detector  35  are also possible. For instance, the optical detector may comprise satellite segments, as known per se.  
         [0042]     In any case, as will be clear to a person skilled in the art, the control circuit  90  is designed to process individual detector signals from the detector segments to derive one or more error signals. A radial error signal, designated hereinafter simply as RE, indicates the radial distance between a track and the focal spot F. A focus error signal, designated hereinafter simply as FE, indicates the axial distance between a storage layer and the focal spot F. It is noted that, depending on the design of the optical detector, different formulas for error signal calculation may be used.  
         [0043]     The control circuit  90  is designed to generate its control signals as a function of the error signals, to reduce the corresponding error, as will be clear to a person skilled in the art. In this case, the control circuit  90  has a variable control characteristic which depends on the type of error. In the case of errors due to disc defects, the control circuit  90  has a first control characteristic specifically adapted to adequately handle disc defects. In the case of errors due to external shocks, the control circuit  90  has a second control characteristic specifically adapted to adequately handle external shocks, which second control characteristic differs from the first control characteristic. Since the exact nature of these control characteristics are no subject of the present invention, while further control circuits with variable gain are known per se, while further the present invention can be implemented in the case of a control circuit having variable gain, it is not necessary here to describe the control characteristics in more detail. For being able to select the first or second control characteristic, the control circuit  90  needs to know the type of error. To this end, the control circuit  90  is provided with a shock recognition section  100 , which receives at least one error signal from the control circuit  90  (radial error signal RE in the exemplary embodiment as illustrated), and which uses this at least one error signal to generate a shock recognition signal SRS for the control circuit  90 . This shock recognition signal SRS may simply be indicative for the presence/absence of a shock; preferably, the shock recognition signal SRS also contains information on the strength and shape of the possible shock.  
         [0044]     According to an important aspect of the present invention, the shock recognition section  100  is designed to calculate the shock recognition signal SRS on the basis of an inverse shock transfer model, as will be explained in the following.  
         [0045]     For illustrating a shock transfer model for the case of radial errors, reference is made to  FIGS. 2A and 2B .  
         [0046]      FIG. 2A  schematically shows a main apparatus frame  3 A of the disc drive apparatus  1 , which is movable with respect to the fixed world W. The spindle motor  4  is coupled to the main apparatus frame  3 A. The disc  2  is coupled to the spindle motor  4 . The disc drive apparatus  1 , in this case, comprises a tilt frame  3 B which is coupled to the main apparatus frame  3 A. An optical pickup unit  3 C is coupled to the tilt frame  3 B. The objective lens  34  is coupled to the optical pickup unit  3 C. A track of the optical disc  2  is schematically indicated as T. A radial error and a focus error are schematically indicated as RE and FE, respectively.  
         [0047]     Alternatively, the disc drive apparatus  1  may comprise a 3D actuator instead of a tilt frame, or the disc drive apparatus  1  may be without tilt facility.  
         [0048]     With reference to  FIG. 2B , which illustrates a 1D-model of the disc drive apparatus  1 , the equivalent mass of tilt frame  3 B will be defined as M 1 , the equivalent mass of optical pickup unit  3 C will be defined as M 2 , the equivalent mass of objective lens  34  will be defined as M 3 , the equivalent mass of disc  2  will be defined as M 4 , and the equivalent mass of spindle motor  4  will be defined as M 5 .  
         [0049]     The coupling between tilt frame  3 B and main apparatus frame  3 A is represented by an equivalent stiffness k 1  and an equivalent damping d 1 . The coupling between optical pickup unit  3 C and tilt frame  3 B is represented by an equivalent stiffness k 2  and an equivalent damping d 2 .  
         [0050]     The coupling between objective lens  34  and optical pickup unit  3 C is represented by an equivalent stiffness k 3  and an equivalent damping d 3 .  
         [0051]     The coupling between disc  2  and spindle motor  4  is represented by an equivalent stiffness k 4  and an equivalent damping d 4 .  
         [0052]     The coupling between spindle motor  4  and main apparatus frame  3 A is represented by an equivalent stiffness k 5  and an equivalent damping d 5 .  
         [0053]     The X-position of main apparatus frame  3 A is indicated as x0.  
         [0054]     The X-position of M 1  is indicated as x1.  
         [0055]     The X-position of M 2  is indicated as x2.  
         [0056]     The X-position of M 3  is indicated as x3.  
         [0057]     The X-position of M 4  is indicated as x4.  
         [0058]     The X-position of M 5  is indicated as x5.  
         [0059]     In response to sensing a radial error signal RE, the control circuit  90  controls the radial actuator  51 , such that a force F is generated, acting between lens  34  and optical pickup unit  3 C. The characteristic of the control circuit  90  is indicated as control transfer function CTF, while the characteristic of the radial actuator  51  is indicated as actuator transfer function ATF.  
         [0060]     An external shock, acting on the main apparatus frame  3 A in the radial direction, will be indicated as {umlaut over (x)}0 EXT , indicating an acceleration of the main apparatus frame  3 A. It is noted that shock {umlaut over (x)}0 EXT  will be a function of time t. The shock {umlaut over (x)}0 EXT  results in a displacement Δx0 of main apparatus frame  3 A, this displacement also being a function of time. Through the mechanical path from main apparatus frame  3 A via tilt frame  3 B and optical pickup unit  3 C, a displacement Δx3 of objective lens  34  results, which can be expressed as 
 
Δ x 3( s )= H   LENS ( s )·Δ x 0( s ), 
 
 with  
                       H   LENS     ⁡     (   s   )       =       ⁢             d   1     ⁢   s     +     k   1             m   1     ⁢     s   2       +       d   1     ⁢   s     +     k   1         ·           d   2     ⁢   s     +     k   2             m   2     ⁢     s   2       +       d   2     ⁢   s     +     k   2         ·                     ⁢           d   3     ⁢   s     +     k   3             m   3     ⁢     s   2       +       d   3     ⁢   s     +     k   3                       (   1   )             
 
         [0061]     Similarly, through the mechanical path from main apparatus frame  3 A via spindle motor  4 , a displacement Δx4 of disc  2  results, which can be expressed as  
                   Δ   ×   4   ⁢     (   s   )       =           H   DISC     ⁡     (   s   )       ·   Δ     ×   0   ⁢     (   s   )         ,     
     ⁢   with     ⁢     
     ⁢         H   DISC     ⁡     (   s   )       =             d   4     ⁢   s     +     k   4             m   4     ⁢     s   2       +       d   4     ⁢   s     +     k   4         ·           d   5     ⁢   s     +     k   5             m   5     ⁢     s   2       +       d   5     ⁢   s     +     k   5                     (   2   )             
 
         [0062]     The radial error RE can basically be expressed as RE=Δx3−Δx4. Thus, the shock sensitivity S SHOCK  of the system can be written as  
                         S             ⁢   SHOCK       ⁡     (   s   )       =     RE       x   ¨     ⁢     0   EXT                     =       1     s   2       ·       S   CONTROL     ⁡     (   s   )       ·     (         H   LENS     ⁡     (   s   )       -       H   DISC     ⁡     (   s   )         )               ⁢     
     ⁢   with           (   3   )                   S   CONTROL     ⁡     (   s   )       =     1     1   +       CTF   ⁡     (   s   )       ·     ATF   ⁡     (   s   )                     (   4   )             
 
         [0063]     In the frequency range of interest, i.e. below approximately 200 Hz, H LENS (s) can be approximated as follows:  
                   H   LENS     ⁡     (   s   )       ≈           d   3     ⁢   s     +     k   3             m   3     ⁢     s   2       +       d   3     ⁢   s     +     k   3           =       (         d   3     ⁢   s     +     k   3       )     ·     ATF   ⁡     (   s   )                 (   5   )             
 
         [0064]     The model of formula (3) is validated by experiments, as illustrated by  FIG. 3 , which shows graphs of magnitude (upper graph) and phase (lower graph) of the radial transfer function of external shocks/vibration to radial error signal as a function of frequency, as measured (solid line) and predicted by the model (broken line). In the frequency range of interest (in the automotive branch: between about 10 Hz and about 200 Hz), the correspondence between model and measurement is remarkably good.  
         [0065]     The shock sensitivity S SHOCK  of the system describes how the system behaves in the case of shocks and vibrations having a certain frequency contents (more particularly in this case: the error signal RE resulting from such shock). In other words, S SHOCK  describes RE as a function of shock. Once this shock sensitivity S SHOCK  has been determined, it is possible to calculate the inverse shock sensitivity S SHOCK   −1 . This is a function which describes shock as a function of RE. A processor programmed to calculate the inverse shock sensitivity S SHOCK   −1 , on receiving the error signal RE as an input signal, will generate an output signal Q OUT  which reconstructs the shock {umlaut over (x)}0 EXT . This is the basis of the shock detector proposed by the present invention, as illustrated in  FIG. 4 , which is a block diagram showing a shock recognition circuit  100  having an input  101  for receiving radial error signal RE and having an output  102  for providing the output signal Q OUT , the shock recognition circuit  100  being designed to apply the inverse shock sensitivity S SHOCK   −1  on its input signal RE, for instance as follows: 
 
 Q   OUT   =S   SHOCK   −1 ( RE ) 
 
         [0066]     It is noted that the output signal Q OUT  can be used as shock recognition signal SRS mentioned earlier, or it can be used as basis for further processing to derive a shock recognition signal SRS, for instance by comparing Q OUT  with a predefined threshold level.  
         [0067]     In principle, the shock sensitivity S SHOCK  of the system is constant, and may be determined by a manufacturer for each apparatus individually, or for a specific type of apparatus in general. The same applies to the inverse shock sensitivity S SHOCK   −1 . Information defining the inverse shock sensitivity S SHOCK   −1  may be available to the shock recognition circuit  100  by being stored in an associated memory  200 , for instance as a formula or in the form of a look-up table, as will be clear to a person skilled in the art.  
         [0068]     When studying  FIG. 3 , it can be seen that, in a frequency range of interest, especially below 200 Hz, the radial transfer function has a positive slope of about 20 dB per decade in the amplitude characteristic (upper graph) at a substantially constant phase of about +90° (lower graph). When this behaviour is inverted, a negative slope of about −20 dB per decade in the amplitude characteristic will result, together with a substantially constant phase of about −90°. Such characteristic is associated with an integrating operation. In this respect, it is noted that the high-frequency part of  FIG. 3  is disregarded, in order not to amplify disc defects, and also to avoid causality problems.  
         [0069]     Thus, it appears possible to implement the shock recognition circuit  100  by a simple integrator  105 , possibly followed by an amplifier  106 , as illustrated in  FIG. 5 .  
         [0070]     However, in a preferred embodiment, also illustrated in  FIG. 5 , the sensitivity of the shock recognition circuit  100  is reduced with respect to disturbances other than shock and vibrations which occur in the same frequency range. An important source for such other disturbances is eccentricity of the disc, leading to disturbances having a frequency equal to the disc rotation frequency. To suppress these disturbances, the shock recognition circuit  100  comprises an additional notch filter  110  coupled in the signal path from the input  101  to the integrator  105 , the notch filter  110  having its central frequency at the disc rotation frequency.  
         [0071]     Further, it is desirable to suppress DC components. To this end, the shock recognition circuit  100  preferably comprises an additional high-pass filter  120  coupled in the signal path from the input  101  to the integrator  105 , the high-pass filter  120  suitably having a cut-off frequency in the range of, for instance, about 1 Hz to about 10 Hz.  
         [0072]     It is noted that, in the case of disc defects, the errors are controlled back to zero. Any remaining error signals do not contain reliable position information. Therefore, in such case, it is preferred to switch off the input signal for the integrator.  
         [0073]     The operation of the shock recognition circuit  100  of  FIG. 5  was tested by simulation. The results of this simulation are shown in  FIG. 6 , which shows graphs of shock (upper graph) and error (lower graph) as a function of time. A real mechanical (radial) shock was applied to the disc drive, and the magnitude of this shock was measured by a shock sensor; the result is shown in curve  61 : it follows that this shock had a magnitude of 0.5 g and a duration of 6 ms. Curves  62  and  63  illustrate the measured radial error and focus error, respectively, resulting from this shock. The radial error was fed as input signal to the simulated shock recognition circuit  100  of  FIG. 5 ; the output signal Q OUT  is illustrated by curve  64 .  
         [0074]     Comparing curve  64  with curve  61  demonstrates that the shock detector circuit proposed by the present invention is capable of detecting quite accurately the occurrence of a shock from a suitable processing of an error signal. Apart from a high-frequency oscillation, which is caused by a gain difference in the filter approximation as used in the simulation, and which can be improved by increasing the model accuracy, the output signal Q OUT  quite accurately reflects the timing and magnitude of the original shock.  
         [0075]     Further,  FIG. 6  demonstrates that the shock detector circuit proposed by the present invention is hardly or not sensitive to disc defects. The disc used in the simulation was provided with a black dot with a diameter of 1.1 mm, which results in large radial and focus errors at time t≈3.31 s (curves  62  and  63 ). Nevertheless, the output signal Q OUT  shows only a minor response at time t≈3.31 s, hardly noticeable, and at least easily distinguishable from the shock response around time t≈3.26 s.  
         [0076]     It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that several variations and modifications are possible within the protective scope of the invention as defined in the appending claims.  
         [0077]     In the above, a model is described for the case of radial errors, caused by a shock in the horizontal direction (i.e. perpendicular to the disc rotation axis). A similar model can be derived for the case of focus errors, which in fact are axial errors, usually caused by a shock in the vertical direction, as will be clear to a person skilled in the art. Likewise, a similar model can be derived for the case of tilt errors. All these errors signals are suitable for use in the present invention.  
         [0078]     In the above description, the inverse sensitivity function is used to reconstruct the acceleration profile which has caused a certain position error. From this reconstructed profile, it is determined whether this profile corresponds to a shock or vibration, or to disc errors. On the basis of this determination, a control characteristic of the control circuit is adapted. The reconstructed acceleration profile can also be used for different purposes, for instance for generating an alarm signal if a severe shock is detected, or to stop playback in case of severe shocks. However, merely reconstructing the acceleration profile, if only for purposes of information or measurement, is already an embodiment of the present invention.  
         [0079]     In the above, the control circuit  90  and the shock recognition circuit  100  are described as separate circuits. However, it is also possible that the shock recognition circuit  100  and the control circuit  90  are integrated into one circuit.  
         [0080]     In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, etc.