Abstract:
The present invention relates to a method and a circuit for recovering information contained in a wobble track of an optical storage medium. One aim of the invention is to describe a method within an appliance for reading from and/or writing to optical storage media, which can correct disturbing data signal components in the wobble signal even when the swept frequency and the lowest signal frequency are close to one another. According to the invention, this object is achieved in that the signals from two detector halves of a photodetector which is used for reading the data contained in a track on an optical storage medium are provided with weighting factors which are adjusted dynamically during operation of the appliance for reading from and/or writing to optical storage media. In order to adjust the weighting factors, the obtained swept-frequency signal is linked to a data signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit, under 35 U.S.C. § 365 of International Application PCT/EP02/08706, filed Aug. 5, 2002, which was published in accordance with PCT Article 21(2) on Feb. 27, 2003 in English and which claims the benefit of German patent application No. 10140325.9, filed Aug. 16, 2001. 
   The present invention relates to a method and a circuit for recovering the information contained in a wobble track of an optical storage medium. 
   FIELD OF THE INVENTION 
   Methods such as these are used, for example, in appliances for reading from and/or writing to optical storage media with wobble tracks, in order to obtain address information from the wobble tracks or to use the wobble frequency to produce a write clock. 
   BACKGROUND OF THE INVENTION 
   In general, in optical storage media which are in the form of discs and are suitable for reading from and/or writing the embossed tracks are formed such that they represent an interleaved spiral or concentric circles. Especially in the case of optical storage media which are suitable for writing to, the embossed tracks additionally are wobbled in a specific form, in order to find specific positions on the medium. This means that the track is not embossed in an approximately straight line, but in serpentine lines. By way of example, the shape of these serpentine lines can contain address information which is used to identify a specific position on this optical storage medium. Various methods are used for coding, examples of which include frequency modulation or phase modulation. Furthermore, the wobble signal may also be used for rotation speed information or for presetting a write data rate. 
   Normally, the modulation shift of this track wobble is kept small, so that there is no noticeable effect on the tracking control and the read quality of the data signal. The modulation shift is thus kept in the order of magnitude of a few percent of the track separation. Furthermore, the modulation frequency is designed to be in a frequency band which is typically above the upper cut-off frequency of the tracking regulator, but is below the lowermost signal frequency of the data signal. However, the small modulation shift means that the signal-to-noise ratio of the wobble signal obtained from it is relatively low. Nevertheless, the coded information and the fundamental frequency should be capable of being coded and reconstructed reliably, in order to allow reliable reading and writing. Disturbance noise components must therefore be effectively suppressed. 
   U.S. Pat. No. 5,717,679 discloses a system which is able to correct the noise components in the wobble signal resulting from any eccentricity of the wobble track. The circuit which is specified for this purpose uses variable-gain amplifiers in order to compensate for different illumination levels of two detector halves. The system is based on the CD-R technique, which uses a wobble frequency of 22.05 kHz. Since the lowest signal frequency of the data signal is 934 kHz, those data signal components which are likewise present in the wobble signal can easily be removed by means of a low-pass filter, as is also disclosed in U.S. Pat. No. 5,717,679. One disadvantage of this known system is that low-pass filtering is impossible when using wobble frequencies which are close to the lowest signal frequency, as is the case, for example, with DVD technology (wobble frequency 825 kHz). Disturbance data signal components therefore cannot be removed with the disclosed system at high wobble frequencies. 
   One aim of the invention is to describe a method within an appliance for reading from and/or writing to optical storage media, which can remove disturbance data signal components in the wobble signal even when the wobble frequency and the lowest signal frequency are close to one another. 
   According to the invention, this object is achieved in that the signals from two detector halves of a photodetector from which a wobble signal is obtained and which is used for reading the data contained in a track of an optical storage medium are provided with weighting factors which are adjusted dynamically during operation of the appliance. In order to adjust the weighting factors, the data signal component in the wobble signal that is obtained is linked to the data signal. Dynamic adjustment of the weighting factors has the advantage that the data signal components in the wobble signal are always suppressed optimally, even if any changes occur in the illumination level of the photodetector during operation. 
   SUMMARY OF THE INVENTION 
   According to the invention, the signals from the two detector areas of the photodetector are provided with mutually opposing weighting factors. This has the advantage that the amplitude of the wobble signal obtained from the difference between the signals from the two detector areas is not influenced by the weighting factors. 
   According to the invention, the data signal is digitized before being linked to the data signal component of the wobble signal, so that the linking process is carried out as a synchronous demodulation process. The advantage of using a digitized data signal is that the synchronous demodulation represents multiplication by ±1, and, technically, this can be carried out easily. 
   The wobble signal is advantageously normalized before determining the data signal component. This may be done, for example, by means of an averaged sum signal or the signal from one detector half. In consequence, the amplitude of the wobble signal is less dependent on the light power of a light source in the optical scanner, or reflection on the optical storage medium. The reaction time of the control loop to dynamic adjustment of the weighting factors is then likewise less dependent on these variables. One advantage of using an averaged sum signal is that such an averaged sum signal is generally already available in appliances for reading from and/or writing to optical storage media. 
   In a further method according to the invention, the amplitude of the wobble signal is kept constant, by the signals from the two detector halves being normalized separately by means of their respective averaged sum signal. 
   According to the invention, disturbance data signal components in the wobble signal are removed by means of a circuit which uses one of the methods mentioned above. 
   A method according to the invention or a circuit according to the invention is advantageously used for recovering the information contained in a wobble track in an optical storage medium in an appliance for reading from and/or writing to optical storage media. 

   
     BRIEF DESCRIPTION OF THE FIGS. 
     The invention will be explained in the following text with reference to advantageous exemplary embodiments and using  FIGS. 1 to 9 . Combinations of advantageous exemplary embodiments are, of course, within the applicability area of the invention. In the figures, identical reference symbols denote the same components and signals. In the figures: 
       FIG. 1 : shows an arrangement for obtaining a wobble signal according to the prior art; 
       FIG. 2 : shows an arrangement according to the invention for automatic adjustment of the weighting factors; 
       FIG. 3 : shows a modification of the arrangement corresponding to  FIG. 2 ; 
       FIG. 4 : shows an arrangement corresponding to  FIG. 2  with amplitude regulation of the data signal; 
       FIG. 5  shows an arrangement corresponding to  FIG. 2  with the data signal being normalized; 
       FIG. 6 : shows an arrangement corresponding to  FIG. 2 , with the wobble signal being normalized; 
       FIG. 7 : shows an arrangement corresponding to  FIG. 2  with amplitude regulation of the wobble signal; 
       FIG. 8 : shows an arrangement corresponding to  FIG. 2 , with the signals of the two detector halves being normalized; and 
       FIG. 9 : shows an appliance for reading from and/or writing to optical storage media, which has an arrangement according to the invention for automatic adjustment of the weighting factors. 
   

   DETAILED DESCRIPTION 
     FIG. 9  shows an appliance for reading from and/or writing to optical storage media  34 , which has an arrangement  38  according to the invention for obtaining information INF contained in a wobble track  36  on the optical storage medium. The scanning beam  40  which is emitted from a light source  30  is collimated by a collimator  31 , and is diverted by means of a beam splitter  32 . An objective lens  33  focuses the scanning beam  40  onto the wobble track  36  of a layer  35  of an optical storage medium  34  which carries information. The scanning beam  40  reflected from the layer which carries the information is collimated by the objective lens  33  and is imaged on the photodetector  1  by means of an imaging unit  37 . A tracking error signal TW′ and the information INF are obtained from the signals A, B, C, D from the photodetector  1  by means of an arrangement  38  according to the invention for recovering the information INF contained in a wobble track  36  of an optical storage medium  34 . The tracking error signal TW′ is supplied to a tracking regulator  39 , which itself ensures that the scanning beam  40  moves as close as possible to the track centre of the wobble track  36 . 
     FIG. 1  shows an arrangement for obtaining a wobble signal TW, which is used by a decoding unit  8  to decode the information INF contained in the wobble track  36  of an optical storage medium  34 , according to the prior art. The basis for obtaining the signal is the linking of the signals A, B, C, D from a photodetector  1 . This makes use of the characteristic that the scanning beam  40  which strikes the optical storage medium  34  as shown in  FIG. 9  causes an effect which makes it possible to use the push-pull tracking control method. This effect is based on the principle that a diffraction effect occurs at the edges of the tracks  36 , so that not only is a vertical beam (zeroth order) reflected in the direction of the photodetector  1  from the memory layer  35  which carries the information, but also higher-order beams, which are not reflected precisely at right angles to the surface of the memory layer  35 . In this case, an objective lens  33  is generally used to collect the reflected zeroth order and ±1 st  order beams and to image them on a photodetector  1 , which is subdivided into at least two areas  1 A+ 1 D,  1 B+ 1 C. In the process, destructive interference of different intensity is formed in the overlapping area between the zeroth order and ±1 st  order as a function of the tracking position, and this is evaluated in the form of a tracking error signal TW′. The resultant tracking error signal TW′is thus referred to as the push-pull tracking error signal. 
   In order to obtain this tracking error signal TW′, the output signals A, B, C, D from the photodetector  1  are first of all amplified by means of amplifiers  2 , and are then linked, for example as shown in  FIG. 1 . The photodetector  1  is typically subdivided into four areas  1 A,  1 B,  1 C,  1 D, in order to obtain a focusing error signal (not shown) at the same time, in addition to the tracking error signal TW′. However, in order to obtain the tracking error signal TW′ using the push-pull effect, it is sufficient to subdivide the photodetector  1  into a right-hand half  1 B+ 1 C and a left-hand half  1 A+ 1 D, and to subtract the output signals from these two detector halves from one another. In the case of a four-quadrant detector, this is done by first of all carrying out the logic operations (A+D) and (B+C) using two adders  4 ,  5 . The difference signal (A+D)−(B+C) is then formed by means of a differential amplifier  6 . The difference signal (A+D)−(B+C) obtained in this way is then supplied to a tracking regulator  39 , as the tracking error signal TW′. The tracking regulator  39  for its part ensures that the scanning beam  40  is moved as close as possible to the track centre of a predetermined track  36 . 
   In order to decode the information INF contained in the wobble tracks  36  of an optical storage medium  34 , or in order to form a write clock, the wobble signal TW is supplied to a decoding unit  8  which, by way of example, emits address information and/or a write clock. The wobble signal TW is itself formed by filtering from the push-pull tracking error signal TW′, by means of a filter  7 . 
   On the other hand, the data signal HF which represents the information content of the optical storage medium  34  is formed from the output signals A, B, C, D from the photodetector  1  by addition in an adder  3 . In order to allow detection by addition of the photodetector signals A, B, C, D, the information is stored by writing light/dark contrasts or by embossing so-called pits on the optical storage medium  34 . 
   If the scanning beam  40  is following the centre of an embossed track  36 , the scanning beam  40  is reflected at the layer  35  of the optical storage medium  34  which carries the information, such that, ideally, a round light spot is imaged onto the photodetector  1 , at whose sides the already mentioned interference resulting from the push-pull effect is observed. The total intensity of this light spot is modulated by the brightness contrast of the area illuminated by the scanning beam  40 . 
   Since the data is stored by means of structures which cause brightness differences, the intensity of the light spot is thus modulated such that it corresponds to the data on the memory layer  35 . Ideally, this is carried out in a synchronized manner on the two detector halves  1 A+ 1 D,  1 B+ 1 C. Since the tracking error signal TW′ and the wobble signal TW derived from it are derived from the difference (A+D)−(B+C) between the signals from the detector halves  1 A+ 1 D,  1 B+ 1 C the data signal component caused by the brightness contrast is cancelled out during the subtraction process by the differential amplifier  6 . However, if the imaging of the scanning beam  40  on the photodetector  1  is not ideally axially symmetrical, then a data signal component AS′ is superimposed on the desired signal component which represents the wobble track  36 . This results in it not being possible to evaluate those signal components which are caused by the wobble track  36  as well as before, so that errors occur in the address evaluation. 
   An improvement is achieved if the weighting between the output signals (A+D), (B+C) from the two halves  1 A+ 1 D,  1 B+ 1 C of the photodetector  1  is changed before the subtraction process by the differential amplifier  6 , so that the contrast-dependent components of the alternating light amplitudes are cancelled out on the two halves  1 A+ 1 D,  1 B+ 1 C of the photodetector  1 . 
   To this end, the four photodetector signals A, B, C, D are first of all amplified by means of amplifiers  2 . Two signal elements (A+D), (B+C) are then produced by summation in the adders  4 ,  5  and these reproduce the modulation on the respective halves  1 A+ 1 D,  1 B+ 1 C of the photodetector  1 . Before the subsequent subtraction process, the signal (A+D) is passed through an amplifier  9 K 1  with a variably adjustable gain K 1 , so that the difference signal TW′ is formed in accordance with the following relationship:
 
 TW′=K   1×(   A+D )−( B+C )
 
   The weighting process results in the data signal components which are imaged onto the detector halves  1 A+ 1 D,  1 B+ 1 C as a result of the different modulation being set to the same magnitude before the subtraction process, so that they cancel one another out. This can also be achieved in an equivalent manner by passing the signal (B+C) through an amplifier  9 K 2  with a variably adjustable gain K 2 , corresponding to the following signal calculation:
 
 TW′ =( A+D )− K 2×( B+C )
 
   The two solutions have the common feature that the resultant amplitude of the difference signal TW′ changes as a function of the weighting factor K 1 , K 2  setting. This can be avoided by the two signals (A+D), (B+C) being weighted, and by the weighting factors K 1 , K 2  being matched to one another such that K 2 =1−K 1 . The following signal calculation is thus used:
 
 TW′=K 1×( A+D )−(1 −K 1)×( B+C )
 
   The tracking error signal TW′ normally has any further undesirable signal components, such as low-frequency disturbances caused by residual tracking errors and so on, removed from it by means of a filter  7 , in order to obtain the wobble signal TW which is supplied to the decoding unit  8 . 
   During operation of the appliance for reading from and/or writing to optical storage media  34 , it is possible, however, for a situation to occur caused by heating, ageing or other disturbance variables in which the intensity distribution or position of the image on the photodetector  1  changes. A situation such as this can occur in particular as a result of residual errors in focus control or in tracking control  39 . If the weighting factors K 1 , K 2  are set only once during production of the appliance, it is impossible to compensate for such variables, which vary dynamically. 
   In order to overcome this disadvantage, it is advantageous to adjust the weighting factors K 1 , K 2  automatically during operation such that the disturbance data signal components cancel one another out as well as possible in the subtraction process  6 . 
   An arrangement according to the invention for automatic adjustment of the weighting factors K 1 , K 2 , in which the data signal HF obtained by summation of the signals A, B, C, D from the photodetector  1  is multiplied by the data signal component AS′ from the difference signal TW′ or from the wobble signal TW, the result of the multiplication is integrated, and the result of the integration used to adjust the weighting factors, is shown in  FIG. 2 . 
   The difference signal TW′ obtained in the manner described in  FIG. 1  has low-frequency disturbances removed from it by means of a filter  10  which can pass only the data signal frequency band, and this is supplied to a first input of a multiplier  11 . The wobble signal TW can also be used instead of the difference signal TW′. The second input of the multiplier  11  is supplied with the data signal HF which also has low-frequency disturbances removed from it by means of a filter  10   b . The output signal from the multiplier  11  is integrated by an integrator  12 . The output signal from the integrator  12  controls the first weighting factor K 1 , while the output signal converted by a converter  13  controls the second weighting factor K 2 . The converter  13  is, for example, a divider, an inverter or a functional block which calculates the value 1−x to a value x. Other converters may of course also be used. 
   The invention is based on the alternating light modulations on the two detector halves  1 A+ 1 D,  1 B+ 1 C being in phase with one another. For this reason, the sum of the output signals A, B, C, D from the photodetector areas  1 A,  1 B,  1 C,  1 D is used to obtain the data signal. The voltage produced in the photodetector  1  is in this case proportional to the intensity reflected from the optical storage medium  34 . 
   A corresponding situation applies to the two detector halves  1 A+ 1 D,  1 B+ 1 C, so that if the weighting K 1 , K 2  is set to be the same in the two branches during the subtraction process by means of the differential amplifier  6 , the data signal components cancel one another out, provided the amplitudes are equal. However, if there is an amplitude difference, then an undesirable data signal component AS′ remains in the difference signal TW′ after the subtraction process, and this is also present in the wobble signal TW after filtering by the filter  7 . This data signal component AS′ is at a phase angle of 0° or 180° relative to the data signal HF, depending on which half  1 A+ 1 D,  1 B+ 1 C of the photodetector  1  receives more reflected light. In the example shown in  FIG. 2 , the phase angle between the data signal HF and the data signal component AS′ in the difference signal TW′ will be zero when the detector half  1 A+ 1 D is illuminated more strongly. When the detector half  1 B+ 1 C is illuminated more strongly, then the negative mathematical sign in the subtraction process for the difference signal TW′ means that the phase angle between the data signal HF and the data signal component AS′ in the difference signal TW′ will be 180°. At the limit, when the modulations on the detector halves  1 A+ 1 D,  1 B+ 1 C are equal or the weighting, K 1 , K 2  is set correctly, the data signal component AS′ in the difference signal TW′ is zero, and thus in the ideal case, no phase angle can be found. 
   This behaviour is made use of by multiplying the data signal HF by the data signal component AS′ in the difference signal TW′. This multiplication results in an output signal whose mathematical sign is positive or negative depending on the phase angle, and whose magnitude is dependent on the amplitude of the data signal component AS′ in the difference signal TW′. The magnitude of the output signal from the multiplier  11  becomes greater the greater the difference between the data signal components in (A+D) with reference to (B+C). The mathematical sign indicates which of the signal components is larger and should be attenuated by appropriate weighting K 1 , K 2 . 
   If the output from the multiplier  11  is connected to an integrator  12 , then the integrator  12  changes its output voltage until the data signal component AS′ in the difference signal TW′ becomes zero. If the output signal from the integrator  12  sets the weighting factor K 1 , K 2  of the one or the two branches to form the difference signal TW′, then this results in a control loop with an integrating response. In this case, the integrator  12  varies the weighting K 1 , K 2  until the output signal from the multiplier  11  becomes zero. 
   The data signal component AS′ in the wobble signal TW is thus likewise zero. Since only the data signal components AS′ are intended to be multiplied by one another, it is advantageous, as shown in  FIG. 2 , to remove the low-frequency components from the input signals to the multiplier  11 . This can be done, for example, by means of high-pass or bandpass filters  10   a ,  10   b , which allow only the data signal frequency band to pass. For reasons of clarity, these filters  10   a ,  10   b  are not shown in the other figures. 
   The advantage of an integrating control loop response is that, after a time which is dependent on the integration time constant, the weighting K 1 , K 2  is always set such that the data signal component AS′ in the wobble signal TW becomes zero. The remaining residual error, that is to say in this case the data signal component AS′ in the wobble signal TW, will always become zero when the control loop has an integrating response. However, the integration time is dependent on the magnitude of the signal at the input of the integrator  12 , that is to say in the case of the weighting factor control loop, from the amplitude of the output of the multiplier  11 . This amplitude is in turn dependent on the amplitude of the input signals to the multiplier  11 , that is to say from the data signal HF and the data signal component AS′ in the wobble signal TW. If, for example, the light power from the light source  30  in the optical scanner or the reflection of the optical storage medium  34  is halved, then the output amplitude of the multiplier  11  is divided by four, which means that the integration time is increased by a factor of 4. 
     FIG. 3  shows an arrangement corresponding to that in  FIG. 2 , in which a synchronous demodulator  14 , which has a digital input and an analogue input, is used instead of the multiplier  11 . The data signal component AS′ of the difference signal TW′ is supplied to the analogue input of the synchronous demodulator  14 . The data signal HF is digitized by means of a comparator  15 , and the digitized data signal HFD is then supplied to the digital input of the synchronous demodulator  14 . The weighting factors K 1 , K 2  are set as already described with reference to  FIG. 2 . Firstly, this arrangement has the advantage that the amplitude of the digitized data signal HFD can assume only two fixed values, as a result of which the integration time is less dependent on the light power from the light source  30  in the optical scanner or the reflection of the optical storage medium  34 . On the other hand, the multiplication by the digitized data signal HFD represents a multiplication by ±1, which is technically simple to implement. 
     FIG. 4  shows a further exemplary embodiment according to the invention, corresponding to that shown in  FIG. 2 , in which the amplitude of the data signal HF is kept constant by an amplitude regulator  16 . An analogue multiplier  11  is used in this case. 
     FIG. 5  shows an arrangement corresponding to that in  FIG. 2 , in which the amplitude of the data signal HF is normalized with the aid of the averaged sum signal UMIA. For this purpose, the data signal HF is supplied to an averager  18 , whose output signal UMIA is applied to a normalizer  17 , and is used to normalize the data signal HF. An analogue multiplier  11  is used in this case as well. 
   Both exemplary embodiments have the advantage that the amplitude of the data signal HF is kept constant, so that the integration time is less dependent on the light power from the light source  30  in the optical scanner or the reflection of the optical storage medium  34 . Furthermore, a normalized data signal is generally available in appliances for reading from and/or writing to optical storage media, so that the available signal can advantageously be used. 
   The arrangement shown in  FIG. 6  differs from the arrangement shown in  FIG. 5  in that the difference signal TW′ is normalized, instead of the data signal HF, by the averaged sum signal UMIA. The output signal UMIA from the averager  18  is supplied to a normalizer  19 , which normalizes the difference signal TW′ on the basis of the signal UMIA. 
   The arrangement shown in  FIG. 7  corresponds to the arrangement in  FIG. 4  with the difference that the amplitude of the difference signal TW′, rather than the amplitude of the data signal HF, is kept constant by an amplitude regulator  20 . 
   The advantage of the two arrangements mentioned above is that the integration time is less dependant on the light power from the light source  30  in the optical scanner, or the reflection of the optical storage medium  34 . 
   The signals (A+D), (B+C) from the two detector halves  1 A+ 1 D,  1 B+ 1 C are advantageously normalized separately, by means of their respective averaged sum signal, before amplification by the variable amplifiers  9 K 1 ,  9 K 2  and before subtraction by the differential amplifier  6 , as is illustrated in  FIG. 8 . The sum signal (A+D) from the first detector half  1 A+ 1 D is for this purpose supplied to the averager  21 , whose output signal is supplied to the normalizer  22  and is used to normalize the sum signal (A+D). In a corresponding way, the sum signal (B+C) from the second detector half  1 B+ 1 C is normalized by means of the averager  23  and the normalizer  24 . 
   The advantage of this arrangement is that the amplitudes of the signals (A+D), (B+C) from the two detector halves  1 A+ 1 D,  1 B+ 1 C are completely independent of the reflection and of the light power. 
   One of the arrangements shown in  FIG. 2  to  FIG. 8  is advantageously used in an appliance for reading from and/or writing to optical storage media with wobble tracks as is shown in  FIG. 9 .