Abstract:
A method is disclosed for reading information from an optical disc ( 2 ) containing tracks ( 11, 21 ) with 2D-SCIPER coded information. The method comprises the steps of: generating at least one light beam ( 32 ); focussing the light beam ( 32 ) in a focal spot (F) on an information layer of the optical disc ( 2 ); controlling the radial position of the focal spot (F) such that the focal spot (F) covers pits ( 10; 20 ) of two adjacent tracks ( 11; 21 ). The optical centre ( 42 ) of the focal spot (F) follows a trajectory ( 45 ) which is radially offset with respect to a halfway line ( 44 ) at a position exactly halfway between the said two adjacent tracks ( 11; 21 ). According to this method, the disturbing non-linear intersymbol interference is removed from the multi-level eye-pattern of 2D-SCIPER, yielding much better distinguishable signal levels.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates in general to a disc drive apparatus for reading information from an optical storage disc; hereinafter, such 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 that consists of physical marks and the absence of those marks for both bit-types in the case of binary modulation. 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 writable type, where information may also 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]     A data pattern representing the information stored on the optical disc is typically a pattern of oblong pits, the pits being arranged successively, defining a track. This track results from the sequential writing mechanism when writing an optical disc. The pit-marks and the non-marks consist of an integer multiple of a basic length which is called the channel bit-length T. In conventional optical storage, the information is encoded in the lengths of successive marks and non-marks measured in units of T. This is the well-known domain of runlength-limited coding (RLL) with the EFM code for CD and the EFMPlus code for DVD.  
         [0004]     Conventionally, information was coded by setting the length of the pits and/or the distance between adjacent pits. As a consequence, the location of pits would vary depending on the information content. In a more recent development, pits are arranged at fixed locations, and information is coded by setting the positions of the front edge and rear edge with respect to a fixed nominal centre of the corresponding pit. Such coding system is indicated as Single Carrier Independent Pit Edge Recording (SCIPER). A more elaborate description of this system is given in U.S. Pat. No. 6,392,973.  
         [0005]     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 light reflected from the disc and for generating an electrical detector output signal. The intensity of the reflected light as received by the detector depends on the interference of the incident light by the pit-structures on the disc; such interference can for instance be destructive so that less light is being reflected leading to a smaller detected signal on the photo-detector; thus, intensity variations of the reflected light, translated into electrical signal intensity variations by the optical detector, correspond to pit edge positions and hence to the information recorded on the disc.  
         [0006]     As mentioned in said publication U.S. Pat. No. 6,392,973 (see for instance  FIG. 9A  of said publication), the focal spot may be aligned with a track, so that light intensity variations are caused by pits of one track only. However, it is also possible that the focal spot is positioned to cover two adjacent tracks (see for instance  FIG. 9B  and  FIG. 9C  of said publication), so that light intensity variations are caused by pits of two adjacent tracks.  
         [0007]     In the system described in said publication U.S. Pat. No. 6,392,973, the pits are arranged according to a rectangular layout, i.e. pits of adjacent tracks are arranged next to each other. In an even more recent development, a pit layout has been proposed where the pits are arranged according to a hexagonal pattern, i.e. a pit of one track is arranged in between two pits of the adjacent track (see, for instance, F. Yokogawa, INSIC Optical Storage Roadmap, Signal Processing and Gray-Scale Section Report, January 2003). Such system is indicated as 2D-SCIPER.  
         [0008]      FIG. 1  schematically illustrates a configuration proposed for the case of physical parameters that relate to the Blu-Ray Disc format. A first row of pits is indicated at  11 , a second row of pits is indicated at  21 . The first row  11  defines a first track, and the second row  21  defines a second track. Pits in the first row  11  are indicated as first row pits  10 ; individual first row pits  10  are distinguished from each other by addition of a letter a, b, c, etc. Similarly, pits in the second row  21  are indicated as second row pits  20 , and individual second row pits  20  are distinguished from each other by addition of a letter a, b, c, etc. Each pit  10 ,  20  has a predefined, fixed nominal centre or central point  12 ,  22 . The central points of all first row pits  10  define a first track centre line  13 ; the central points of all second row pits  20  define a second track centre line  23 . The distance between the track centre lines  13  and  23  of two adjacent tracks  11  and  21  is indicated as track pitch TP. In the proposed configuration, related to physical parameters for Blu-ray Disc (with numerical aperture NA=0.85 and a blue laser with wavelength of 405 nm), the track pitch TP is approximately 205 nm.  
         [0009]     Each pit  10 ,  20  has width PW, measured perpendicularly to the corresponding track centre line  13 ,  23 . In the proposed configuration, the pit width PW is in the range of approximately 80-100 nm (for the physical parameters related to Blu-ray Disc).  
         [0010]     The central points of successive pits  10  of one track  11  are displaced with respect to the central points of successive pits  20  of the adjacent track  21 , such that a radial projection of a central point of a pit  10  onto the adjacent track  21  corresponds to a position substantially exactly halfway between the two central points of two successive pits  20  of said adjacent track  21 . Thus, the central points of the pits  10 ,  20  together define a hexagonal lattice.  
         [0011]     The distance between the central points of successive pits  10 ,  20  of one track  11 ,  21 , i.e. measured in the tangential direction or track direction, is indicated as pit pitch PP. In the proposed configuration, the pit pitch PP is approximately 237 nm. In order to take into account that consecutive tracks do not have the same length (the length difference being 2π-TP), the pit pitch PP is slightly increased from one track to the next in order to maintain the hexagonal arrangement. When the pit pitch is increased to such extent that the track can contain one or more additional pits at the original pit pitch, a new “zone” in the format can be initiated, hereby maintaining the local density also at larger radii of the disc. Thus, the disc contains a plurality of radial zones, the number of pits in each track differing from zone to zone.  
         [0012]     Each pit has a first edge  14  and a second edge  15 , as illustrated for pit  10   a . The distance between the first edge  14  and the corresponding centre point  12  of the corresponding pit  10   a  is indicated as front distance DF, while the distance between the second edge  15  and the corresponding centre point  12  of the corresponding pit  10   a  is indicated as rear distance DR. For each edge  14 ,  15 , there are three possible edge positions, so that the front distance DF can take three predefined values; the same applies to the rear distance DR. Particularly, in the proposed configuration, the front distance DF can take the values 44.5 nm, 59.5 nm, 74.5 nm; the same applies to the rear distance DR. Thus, each pit edge  14 ,  15  defines a coded ternary symbol, i.e. a symbol which can take three values, which will hereinafter be indicated as 0, 1, 2.  
         [0013]     The tracks  11 ,  21  are scanned with an optical beam having a wavelength of about 405 nm (like in the BD system), the beam being focussed to a substantially circular spot  40  having a spot diameter SD. The scan direction is indicated by arrow V in  FIG. 1 . The optical beam is directed such that the spot  40  covers two adjacent tracks  11 ,  12 .  FIG. 1  illustrates, that the optical spot  40  covers four symbols simultaneously: the front and rear edges of a pit  10   c  of one track  11 , the rear edge of a pit  20   b  of the adjacent track  21 , and the front edge of a pit  20   c  of the adjacent track  21 . These symbols are indicated as S 1 , S 2 , S 3 , S 4 , respectively. It should be clear that, if the optical spot is displaced over a distance corresponding to half the pit pitch PP, the optical spot  40  again covers four symbols simultaneously, now the front and read edges of a pit of said adjacent track  21  and the rear edge and the front edge of successive pits of the first track  11 .  
         [0014]     An advantage of such coding scheme is that very high data densities are possible. However, a difficulty arises in the process of decoding the read signal received from the optical detector. Since the optical spot covers four symbols simultaneously, while each symbol can take three values, there are  81  possibilities of combination. For the amount of light reflected from the optical spot  40 , it makes a difference whether, for instance, symbol S 1 =2 while all other symbols are zero, or, for instance, symbol S 3 =2 while all other symbols are zero, or whether S 1 =S 2 =1 and S 3 =S 4 =0, or whether S 3 =S 4 =1 and S 1 =S 2 =0. More particularly, when scanning such four-symbol configuration, there are 81 possibilities for the output signal to be expected, as illustrated by  FIG. 2 . However, the signal waveform that is obtained for the integrated symbol value S 1 +S 2 +S 3 +S 4  (being 2 in the above example) should reflect only 9 different signal levels (since S 1 +S 2 +S 3 +S 4  can range from 0 to 8).  FIG. 2  is a graph containing all 81 possibilities for the output signal; such graph is indicated as a multi-level “eye-pattern”. The eye-pattern of  FIG. 2  illustrates that distinguishing between the 81 signal possibilities is very difficult. This can be seen as the fuzzy clustering of levels to the 9 basic levels referred to above: this can be explained as systematic amplitude jitter on the signal levels that is induced by the asymmetry of the different cases that would need to lead to the same signal level since the integrated symbol value S 1 +S 2 +S 3 +S 4  is identical for these cases. Thus, the chances on decoding errors are relatively high.  
         [0015]     It is an objective of the present invention to provide a method for reading 2D-SCIPER coded information which reduces the chances on decoding errors.  
         [0016]     More particularly, it is an objective of the present invention to provide a method for reading 2D-SCIPER coded information such that the eye-pattern of possible read signals shows improved, clearly distinguishable levels.  
       SUMMARY OF THE INVENTION  
       [0017]     According to an important aspect of the present invention, the centre of the optical spot is radially offset with respect to a position exactly halfway two adjacent tracks. In a preferred embodiment, two optical spots are used, one being offset on one direction, the other being offset in the opposite direction, the magnitude of both offsets preferably being substantially equal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     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:  
         [0019]      FIG. 1  schematically illustrates a 2D-SCIPER configuration;  
         [0020]      FIG. 2  is a graph illustrating the eye-pattern for the 2D-SCIPER configuration of  FIG. 1 , for the normal case where the centre of the optical spot follows a trajectory located exactly halfway two adjacent tracks;  
         [0021]      FIG. 3  schematically illustrates an optical disc drive apparatus;  
         [0022]      FIG. 4  schematically illustrates track following details in accordance with prior art;  
         [0023]      FIG. 5  schematically illustrates track following details in accordance with the present invention;  
         [0024]      FIG. 6  is a graph illustrating the eye-pattern resulting in accordance with the present invention;  
         [0025]      FIG. 7  schematically illustrates track following details in accordance to a preferred embodiment of the present invention.  
         [0026]      FIG. 8  schematically illustrates a 2D-SCIPER configuration in accordance with the present invention; and  
         [0027]      FIG. 9  schematically illustrates a system for deteching optical read signals and for processing the optical read signals. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0028]      FIG. 3  schematically illustrates an optical disc drive apparatus  1 , suitable for reading information from an optical storage disc  2  containing 2D-SCIPER coded information. The optical disc  2  comprises at least one track (not shown in  FIG. 3  for sake of simplicity), either in the form of a continuous spiral or in the form of multiple concentric circles, of storage space where information is stored in the form of a 2D-SCIPER data pattern. Defining a pit parameter as the number of data pits per 360° track revolution, the disc  2  typically comprises a plurality of radial zones, all tracks within one zone having the same pit parameter, and the tracks in adjacent zones having different pit parameters.  
         [0029]     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 . For receiving and holding the disc  2 , the disc drive apparatus  1  may comprise a turntable or clamping hub  6 , which in the case of a spindle motor  4  is mounted on the spindle axle  7  of the motor  4 .  
         [0030]     The disc drive apparatus  1  further comprises an optical system  30  for scanning tracks of the disc  2  with an optical beam. 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 optical path of light beam  32  will be indicated by a character a, b, c, etc added to the reference numeral  32 .  
         [0031]     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 objective lens  34  is designed to focus the light beam  32   b  in a focal spot F on an information layer (not shown for sake of simplicity) of 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 .  
         [0032]     During operation, the light beam should remain focussed and should follow the tracks. To this end, the objective lens  34  is arranged displaceable in axial and radial directions, and the optical disc drive apparatus  1  comprises an actuator system  52  arranged for displacing the objective lens  34  with respect to the disc  2 . Since actuator systems are known per se, while further the design and operation of such actuator system is no subject of the present invention, it is not necessary here to discuss the design and operation of such actuator system in great detail.  
         [0033]     It is noted that means for supporting the objective lens with respect to an apparatus frame, and means for displacing 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.  
         [0034]     The disc drive apparatus  1  further comprises a signal processing circuit  90  having a read signal input  91  for receiving a read signal S R  from the optical detector system  35 . The signal processing circuit  90  is designed to process the read signal S R  in order to derive a data signal S D  and to provide this data signal S D  at a data output  92 . The signal processing circuit  90  is further designed to process the read signal S R  in order to generate control signals S C  for the actuator system  52 , and to provide these control signals S C  at a control output  94 .  
         [0035]      FIG. 4  schematically illustrates track following details in more detail as compared to  FIG. 1 , for the prior art situation. In  FIG. 1 , the centre of the optical spot F is indicated at  42 . A broken line  43  indicates the spot trajectory, i.e. the path followed by the optical spot centre  42 ; in accordance with prior art, the spot trajectory  43  is located exactly halfway between the centre lines  13  and  23  of two adjacent tracks  11  and  21 . With such spot trajectory, the eye-pattern of  FIG. 2  results.  
         [0036]      FIG. 5  is a drawing comparable to  FIG. 4 , but now showing track following details in accordance with the present invention. Dotted line  44  is a line which is located exactly halfway between the centre lines  13  and  23  of two adjacent tracks  11  and  21 ; in the following, this line will be indicated as halfway line  44 . It is noted that in prior art the spot trajectory coincides with the halfway line  44  (see  FIG. 4 ). A broken line  45  indicates the spot trajectory in accordance with the present invention. It is clearly shown that the spot trajectory is radially displaced (offset) with respect to the halfway line  44 . The radial offset of the spot trajectory  45  is indicated as RSTO. A very suitable value for RSTO, which appears to be optimal and which is, therefore, preferred, is RSTO=0.1·TP (for the considered (quasi) hexagonal arrangement of pits, the track pitch TP corresponds to 0.5·√3·PP). This value applies for the chosen parameters of the 2D SCIPER storage system (relative to the scaled distances with scaling factor λ/(2NA), with λ being the wavelength of the laser light. If the (relative) storage density changes, also the optimum value of the radial displacement RSTO will change accordingly.  
         [0037]      FIG. 6  is a graph comparable to  FIG. 2 , illustrating the eye-pattern which results with a radial spot trajectory offset of 0.1·TP. The horizontal axis represents the distance D, measured parallel to the track direction, between the spot centre  42  and a point of reference. This point of reference (D=0) is located halfway between two pits (for instance: between pits  10   b  and  10   c ) of the first track  11 , i.e. the track towards which the optical spot F is offset. The vertical axis represents signal magnitude, in arbitrary units. It can clearly be seen that, around D=0, which is the ideal sampling phase of this eye-pattern, the signals to be expected can take only one of nine distinct, sharp levels, which are easily distinguishable. Thus, the improvement over the prior art (compare  FIG. 2 ) is clear.  
         [0038]     It is noted that  FIG. 6  shows the eye-pattern resulting from a combination of four symbols associated with two half-pits of the first track  11  and one pit of the second track  21  (for instance rear edge of pit  10   b , front edge of pit  10   c , and both edges of pit  20   b ), ignoring all other pits and pit edges. The situation becomes more complicated if more pits are taken into account. Equalization can reduce the effect of intersymbol interference of pits that are beyond the range of the first neighbours. Nevertheless,  FIG. 6  clearly illustrates that a combination of four symbols as mentioned can be decoded more reliably than in prior art, if the centre of the optical spot is displaced as mentioned. This implies that the systematic intersymbol interference which has lead to the fuzzy levels in the eye-pattern of  FIG. 2  has been compensated by shifting the radial position of the laser spot.  
         [0039]     Again, it is noted that  FIG. 6  shows the eye-pattern resulting from a combination of four symbols associated with two half-pits of the first track  11  and one pit of the second track  21  (for instance rear edge of pit  10   b , front edge of pit  10   c , and both edges of pit  20   b ). For reading a combination of four symbols associated with two half-pits of the second track  21  and one pit of the first track  11  (for instance the symbols S 1 , S 2 , S 3 , S 4  as illustrated in  FIG. 1 ), the situation is opposite. Improving the readout of such combination of four symbols in accordance with the invention is achieved when the optical spot is radially offset in the opposite direction, i.e. towards the second track  21 .  
         [0040]     In principle, it is possible to implement the present invention with only one optical spot. Then, reading the combination of two tracks  11  and  21  will involve two scan revolutions, one revolution with the optical spot being offset in a direction towards the first track  11 , and the second revolution with the optical spot being offset in the opposite direction. For correctly decoding the information recorded in the pits of both tracks, the readout signal of the first revolution should be buffered in a track memory, and should be re-read from this track memory during the second revolution for suitable combination with the readout signal of the second revolution: the signal of the first and second scans are properly multiplexed so that decoding and signal processing can produce the symbol values. Or, the readout signal of both revolutions should be stored for later processing.  
         [0041]     Preferably, however, the present invention is implemented with two optical spots, one optical spot being offset in a direction towards the first track  11 , and the second optical spot being offset in the opposite direction, as schematically illustrated in  FIG. 7 , where two optical spots F 1  and F 2  are shown, having respective spot centres  42  and  46  substantially displaced from each other in track direction. The optical centre  42  of the first optical spot F 1  is radially offset towards the first track  11  (RSTO 1 ), while the optical centre  46  of the second optical spot F 2  is radially offset in opposite direction towards the second track  21  (RSTO 2 ), both offsets preferably having equal magnitude (|RSTO 1 |=|RSTO 2 |).  
         [0042]     In  FIG. 7 , the tangential distance (i.e. measured along the direction of the track axes  13  and  23 ) between the two optical centres  42  of the two optical spots F 1  and F 2 , respectively, is shown as being relatively small such that the two optical spots partially overlap. Preferably, said distance is much larger, such that the two optical spots F 1  and F 2  do not overlap. A suitable distance is, for instance, in the order of about 1 μm, without the invention being restricted to this distance. In fact, the two optical spots F 1  and F 2  may be generated by two separate laser sources and two separate optical systems located 180° opposite with respect to the disc rotation axis  5 . On the other hand, in order to save costs, it is preferred that the two optical spots F 1  and F 2  are generated by one common laser, for instance by splitting a laser beam using a splitting device such as a diffraction grating. Also, if the mutual beam distance is in the order of 10 μm, these two beams are focussed by one common optical lens system. Since splitting a beam into two or more beams by using a grating is known per se, it is not necessary here to explain this technique in more detail.  
         [0043]     In  FIG. 7 , the track centre lines  13  and  23  are shown as straight lines. Actually, however, the track centre lines  13  and  23  are curved lines, the radius of curvature of these lines being smaller at an inner radius of the disc and larger at an outer radius of the disc. As a consequence, it may be that an ideal orientation of the two optical spots F 1  and F 2  with respect to each other has to be adapted when going from an inner radius to an outer radius. This can easily be achieved by slightly rotating the splitting device (i.e. diffraction grating). This rotation of the diffraction grating can be controlled by an actuator and related servo-control means.  
         [0044]      FIG. 8  is a drawing comparable to  FIG. 1 , on a smaller scale, showing two track centre lines  13  and  23  and two series of pit centres  12 ( 1 ),  12 ( 2 ),  12 ( 3 ), etc and  22 ( 1 ),  22 ( 2 ),  22 ( 3 ), etc, respectively. These pit centres are projected on the halfway line  44 , giving read locations  61 ( 1 ),  62 ( 1 ),  61 ( 2 ),  62 ( 2 ), etc, read locations  61 ( i ) corresponding to pit centres  12 ( i ) and read locations  62 ( i ) corresponding to pit centres  22 ( i ). It is noted that these read locations define moments in time for sampling the optical read signal SR, which moments will be indicated as sampling moments or sampling phases.  
         [0045]     In the case of “normal” 2D-SCIPER with only one optical spot, the sampling phases  61 ( i ) and  62 ( i ) are scanned intermittently. When the optical spot has reached a first sampling phase  61 ( i ), the optical read signal SR contains information from four symbols which are located in an orientation roughly defining a triangle with its top directed towards the first track  11 , as illustrated at A. When the optical spot has reached a second sampling phase  62 ( i ), the optical read signal SR contains information from four symbols which are located in an orientation roughly defining a triangle with its top directed towards the second track  21 , as illustrated at B.  
         [0046]     In the prior art, where the sampling phases are scanned by only one optical spot, the optical read signals SR are obtained by one optical detector  35  in the order  61 ( 1 ),  62 ( 1 ),  61 ( 2 ),  62 ( 2 ),  61 ( 3 ),  62 ( 3 ), etc. In the present invention, the first sampling phases  61 ( i ) are scanned by the second optical spot F 2 , while the second sampling phases  62 ( i ) are scanned by the first optical spot F 1 . In order to be able to clearly distinguish optical read signals SR 1  obtained by the first optical spot F 1  from optical read signals SR 2  obtained by the second optical spot F 2 , the optical system  30  preferably comprises two independent optical detectors  135  and  235 , wherein the first optical detector  135  receives the light reflected from the first optical spot F 1 , and wherein the second optical detector  235  receives the light reflected from the second optical spot F 2 , as illustrated in  FIG. 9 .  
         [0047]     In view of the tangential distance between the two optical spots F 1  and F 2 , the timing relationship between the readout signals regarding the two sampling phases is shifted. In the illustrated example, the second optical spot F 2  is ahead of the first optical spot F 1 , hence first optical read signals SR 1  obtained by the first optical spot F 1  lag with respect to the second optical read signals SR 2  obtained by the second optical spot F 2 . In order to eliminate this timing difference, the second optical read signals SR 2  may be delayed in a buffer or delay  236  before being processed in a signal processor circuit  190 , as illustrated in  FIG. 9 .  
         [0048]     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.  
         [0049]     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, digital signal processor, etc.