Patent Publication Number: US-2010118676-A1

Title: Optical disc drive with delayed layer jump

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to the field of optical recording. More particularly, the present invention relates to an optical disc drive apparatus for handling multiple layer optical discs; hereinafter, such disc drive apparatus will also be indicated as “optical disc drive”. The present invention relates in general to any type of disc, but particularly to DVD and BD. 
     BACKGROUND OF THE INVENTION 
     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 writable 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 scanning 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. 
     The optical scanning means comprise an objective lens for focusing the light beam in a focal spot on the disc. During operation, the focal spot should remain aligned with a track or should be capable of being displaced from a current track 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. Further, the light beam should remain focused on the disc. To this end, the objective lens is arranged axially displaceable, and the optical disc drive comprises focal actuator means for controlling the axial position of the objective lens. 
     The information track is physically arranged in a storage layer of the disc. Apart from discs having only one storage layer, there have been developed discs having two storage layers above each other. It is noted that a disc may even have three or more storage layers, but the present invention will hereinafter be explained for the case of a double-layer disc. 
     SUMMARY OF THE INVENTION 
     When writing information to the disc, the information is usually written consecutively in one track. However, it may be that this track does not have sufficient space to continue writing. When the available space is full, a jump must be made to another location, either in the same track or in the track of a different layer. In the following, a jump to a location in a different layer will be indicated as a “layer jump”. 
     Likewise, it is possible that a layer jump must be made when reading, because the user gives a command that requires reading information from a layer different from the current one. 
     Executing a layer jump requires displacing the objective lens over a relatively large distance and, in order to do this fast, it requires a relatively large focus actuator drive signal. A common supply voltage for the actuators is 12 V. However, a recent development is towards lower voltages, for instance 5 V. Now a problem may occur in a situation when there is some disturbance on the focus actuator drive signal. If the disturbance has a relatively high value, it may be that there is insufficient margin for the actuator drive signal. The actuator signal may clip, and the jump may be not correctly performed. 
     An object of the present invention is to reduce this problem. 
     According to the present invention, disturbances on the focus actuator drive signal are monitored and compared to a threshold. If the disturbance level is too high, layer jumps are inhibited. 
     Further advantageous elaborations are mentioned in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: 
         FIG. 1  schematically illustrates an optical disc drive; 
         FIG. 2  is a graph showing actuator signals as a function of time; 
         FIG. 3  is a graph showing actuator signals as a function of time, together with a threshold level; 
         FIG. 4  is a flow diagram illustrating steps taken in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  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 BD. 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 . 
     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. 1 , 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 . 
     The light beam  32  passes a beam splitter  33  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  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 of the disc  2 , which spot F normally is circular. In an enlargement, the figure shows that the disc  2  comprises two recording layers  61 ,  62  arranged in parallel. 
     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. 
     For achieving and maintaining a correct focusing on the desired recording layer, said objective lens  34  is mounted axially displaceable, while further the actuator system  50  also comprises a focal actuator  52  arranged for axially displacing the objective lens  34  with respect to the disc  2 . Since axial actuators are known per se, while further the design and operation of such axial actuator is no subject of the present invention, it is not necessary here to discuss the design and operation of such focal actuator in great detail. 
     It is noted that means for supporting the objective lens with respect to an apparatus frame, and means for axially and radially 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. 
     It is further noted that the radial actuator  51  and focal actuator  52  may be implemented as one integrated 2D-actuator. 
     The disc drive apparatus  1  further comprises a control circuit  90  having a first output  91  coupled to a control input of the radial actuator  51 , having a second output  92  connected to a control input of the focal actuator  52 , and having a third output  94  coupled to a control input of the motor  4 . The control circuit  90  is designed to generate at its first output  91  a control signal S CR  for controlling the radial actuator  51 , to generate at its second control output  92  a control signal S CF  for controlling the focal actuator  52 , and to generate at its third output  93  a control signal S CM  for controlling the motor  4 . 
     The control circuit  90  further has a read signal input  95  for receiving a read signal S R  from the optical detector  35 . 
       FIG. 2  is a graph illustrating a problem associated with performing a layer jump; the horizontal axis represents time, the vertical axis represents signal level (volt). The focal actuator control signal S CF  is a summation of two contributions. A first contribution, shown as curve  71 , is typically a sine-shaped signal and is required to compensate disturbances in the axial direction such as to keep the beam focused on the recording layer. A second contribution, shown as a block signal  72 , is required to perform a layer jump. This second contribution, which will be indicated as a jump control signal, comprises two signal portions  72   a  and  72   b,  wherein the first signal portion  72   a  results in an acceleration of the objective lens while the second signal portion  72   b,  having opposite sign as compared to the first signal portion  72   a,  results in a deceleration of the objective lens. 
       FIG. 2  also shows the summation of said two signals (curve  73 ).  FIG. 2  also shows a supply voltage Vs from which the actuators are supplied with power. Now, as illustrated at  74 , it can be seen that a problem exists in that the summation of said two contributions (shown in a dotted line) can be larger than the supply voltage Vs. In that case, the actual actuator control signal (given by the solid line) will be less than the intended actuator control signal (given by the dotted line), and the layer jump will not be accurate. 
       FIG. 3  is a graph similar to  FIG. 2 , now illustrating the solution proposed by the present invention. Apart from the disturbance control signal  71  and the jump control signal  72 , the figure shows a threshold level V T  lower than the supply voltage Vs, which is provided to the control circuit  90  by a threshold source  98 . If the absolute value of the disturbance control signal  71  is higher than the threshold level V T , the control circuit  90  prevents the jump control signal  72  to be issued. Assume that it is desired that the jump is executed as from time t 1 , but that the absolute value of the disturbance control signal  71  is higher than the threshold level V T  at time t 1 . In that case, the jump will be postponed until, at time t 2 , the absolute value of the disturbance control signal  71  becomes lower than the threshold level V T . Thus, there is at all times a margin of Vs-V T  for the jump control signal  72 . 
       FIG. 4  is a flow diagram illustrating the steps taken by the control circuit  90  in accordance with the present invention. For this explanation, it is immaterial whether the layer jump is forwards or backwards in the axial direction. 
     In step  81 , the control circuit  90  receives a signal indicating that it should anticipate on performing a layer jump at the next possible occasion. 
     In step  82 , the control circuit  90  reads the focus disturbance signal, and in step  83 , the control circuit  90  compares the focus disturbance signal with the threshold level V T . 
     If it appears that the absolute value of the focus disturbance signal is higher than the threshold level V T , the control circuit  90  returns to step  83 . This situation is continued until it appears that the absolute value of the focus disturbance signal is lower than the threshold level V T , in which case the control circuit  90  in step  84  generates the jump control signal to perform the layer jump. 
     Thus, effectively, a layer jump is delayed until the absolute value of the focus disturbance signal is low enough. 
     It is noted that the above operation is executed when the control circuit  90  anticipates that a layer jump is to be performed soon, but not necessarily immediately. In that case, it is allowable for the control circuit  90  to wait with issuing the layer jump control signal. However, it is also possible that the timing of layer jump is dictated by external conditions, for instance because the disc drive is reading a movie from a disc where the movie information jumps from one layer to another. 
     Summarizing, the present invention provides an optical disc drive  1 , capable of handling optical discs  2  with at least two storage layers  61 ,  62 , comprising an axially displaceable objective lens  34  and a focus actuator  52  for controlling the axial position of the objective lens; 
     a control circuit  90  for generating actuator control signals S CF  for the focus actuator; 
     and a threshold source  98  for providing a threshold level V T  lower than a supply voltage V S  for the focus actuator. 
     The control circuit  90  monitors a focus disturbance signal and delays a layer jump if the absolute value of the focus disturbance signal is too high. 
     Particularly, the control circuit compares the absolute value of the disturbance signals with said threshold level and, if this absolute value is higher than said threshold level, inhibits step  83  the layer jump until the absolute value of the disturbance signals becomes lower than said threshold level. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. 
     Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope. 
     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.