Apparatus and method for reproducing data from multilayered disks

Abstract: A method for moving the focus of an object lens from one layer to another in a disk having data recorded on a plurality of layers comprises: a first step for accelerating an object lens in a first direction; a second step for detecting that a focus error signal surpasses a first voltage and for accelerating the object lens in a second direction different from the first direction; a third step for detecting that the object lens has moved for a predetermined duration of time and for accelerating the object lens in the first direction; and a fourth step for detecting that the focus error signal surpasses a second voltage and for starting focus servo control. The method and apparatus can reliably catch the just-focus point of a layer and reduce the time required for movement from one layer to another.

Description: This invention relates to an apparatus and a method for reproducing digital data, for example, recorded on two or more layers of a multi-layered disk. Optical disk reproducing apparatuses have been used for reading out digital image data recorded on optical disks. Data recorded on a disk is digital and contains a large amount of information. It often occurs, therefore, that the entirety of a desired unit of data cannot be recorded on a single disk. To cope with the problem, a multi-layered disk having a plurality of recording layers on a single disk has been proposed. Upon reproducing data from an optical disk, the disk is rotated in a predetermined direction by a spindle motor after focus servo control and tracking servo control. Consequently, focus servo control is executed by moving an object lens in a pickup in a face-to-face relation with the disk so as to close the servo loop at the zero-cross of an S-shaped curve of a first detected focus error. If this is applied to a multi-layered optical disk having two or more layers, in order to move focalization of the object lens from one layer to another, it is necessary to perform a focus servo control of a destined layer by conducting another focus search after canceling the focus servo control which has been effective hitherto. During focus search with a multi-layered optical disk, a number of S-curves corresponding to the number of layers appear in the focus error signal. When the second layer is to be focalized, focus servo control may be performed at the second S-curve. Similarly, for focalization to the fourth layer, focus servo control may be performed at the fourth S-curve. Such focus search, however, takes a long time for movement from a layer to another. EP-A-0 517 490 discloses an apparatus for reproducing data from a multi-layer optical medium in which a focus error signal is used to control lens movement to focus on different ones of the layers. This invention provides a disk reproducing apparatus for reproducing data recorded on a plurality of layers of an optical disk, the apparatus comprising: first comparator means for comparing a focus error signal with a first voltage; second comparator means for comparing the focus error signal with a second voltage; third comparator means for comparing a reproduced RF signal with a third voltage; means responsive to a result of comparison by said third comparator means to control operations of said first and second comparator means; drive voltage supplying means for selectively supplying a first or a second object lens drive voltage in response to results of comparison supplied from said first and/or second comparator means; and means for driving an object lens with said object lens drive voltage supplied from said drive voltage supplying means. This invention also provides a disk reproducing method for reproducing data recorded on a plurality of layers of an optical disk, the method comprising the steps of: comparing a focus error signal with a first voltage; comparing the focus error signal with a second voltage; comparing a reproduced RF signal with a third voltage; in response to the comparison of the reproduced RF signal with the third voltage, controlling operations of said comparisons with said first and second voltages; selectively supplying a first or a second object lens drive voltage in response to results of comparison with said first and/or second voltages; and driving an object lens with said object lens drive voltage. In embodiments of the invention, when focalization is changed from a layer to another of a multi-layered disk, the object lens is first accelerated up to the rising of the S-curve in the focus error signal corresponding to the destined layer, passing over the just focus point, then accelerated in the opposite direction, and again accelerated in the initial direction, thus to establish focalization to the destined layer. The invention will now be described by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of a multi-layered disk reproducing apparatus; Figs. 2A to 2J are timing charts of a focus jump operation from a first layer to a second layer in the multi-layered disk reproducing apparatus; Fig. 3 is a flow chart of a soft-ware control of a focus jump operation from the first layer to the second layer in the multi-layered disk reproducing apparatus; Figs. 4A to 4J are timing charts of a focus jump operation from the second layer to the first layer in the multi-layered disk reproducing apparatus; Fig. 5 is a flow chart of a soft-ware control of a focus jump operation from the second layer to the first layer in the multi-layered disk reproducing apparatus; and Figs. 6A to 6J are timing charts of a focus jump operation from the first layer to the third layer of the multi-layered disk reproducing apparatus. A multi-layered disk reproducing apparatus embodying the invention is described below with reference to the drawings. Fig. 1 is a block diagram of the multi-layered optical disk reproducing apparatus for reproducing data from an optical disk. Numeral 11 denotes a quartered detector comprising four detectors (11A, 11B, 11C and 11D) which are based on an astigmatic process. The quartered detector 11 detects whether an irradiated laser beam is exactly focalized on the optical disk or not. Detection signals of the detectors 11A and 11C are supplied to an adder 12, and those of the detectors of 11B and 11D to an adder 14. The sum signal (A+C) output from the adder 12 is supplied to one of input terminals of an adder 20 and one of input terminals of a comparator 16 through an amplifier 13. The sum signal (B+D) output from the adder 14 is supplied to the other terminal of the adder 20 and the other terminal of the subtracter 16 via an amplifier 15. Supplied from the subtracter 16 is a ((A+C)-(B+D)) signal as a focus error signal ERR which enters a phase compensation circuit 17, one of input terminals of a comparator 22, and one of input terminals of a comparator 25. Applied to the other terminal of the comparator 22 is a comparison voltage V2. The comparison voltage V2 has a value higher than noise near 0V. The comparator 22 performs zero-cross detection of the plus side, and outputs the detection signal to a switch 23 when the focus error signal ERR is equal to or higher than the comparison voltage V2. Similarly, a comparison voltage V3 is applied to the other input terminal of the comparator 25. The comparison voltage V3 has a value lower than noise near 0V. The comparator 25 performs zero-cross detection of the minus side, and outputs a detection signal to a switch 26 when the focus error signal ERR is equal to or lower than the comparison voltage V3. The focus error signal, phase-compensated by the phase compensation circuit 17, is supplied to one of selective terminals of a switch 18. Supplied to the other selective terminal of the switch 18 is a focus search drive voltage from a switch 19. The switch 19 is supplied with a focus search drive voltage V+ at one of selective terminals and a focus search drive voltage V- at the other selective terminal. When the output signal of the phase compensation circuit 17 is selected by the switch 18, the focus servo loop for the optical disk is on. On the other hand, when the output signal of the switch 19 is selected, the focus servo loop is off, focus jump operation is done. In response to the signal output from the switch 19, an object lens provided in the pickup moves. When the focus search drive voltage V+ is selected at the switch 19, the object lens is accelerated upward. When the focus search drive voltage V- is selected, the object lens is accelerated downward. A reproduction RF signal (A+B+C+D) output from the adder 20 is supplied to one of input terminals of a comparator 21. Applied to the other input terminal of the comparator 21 is a comparison voltage V1. The comparator 21 compares the reproduction RF signal with the comparison voltage V1. When the reproduction RF signal is equal to or higher than the comparison voltage V1, a focus OK signal FOK indicating focalization being proper is output to a switch 23 and a switch 26 as a control signal. When the switch 23 is turned on with the supply of the focus OK signal FOK, a zero-cross detection signal FZC+ output from the comparator 22 is supplied to a CPU 24. Similarly, when the switch 26 is turned on with the supply of the focus OK signal FOK, a zero-cross detection signal FZC-output from the comparator 25 is supplied to the CPU 24. That is, switches 23 and 26 behave to gate the zero-cross detection signals FZC+ and FZC- by using the focus OK signal FOK. This aims preventing malfunctions by noise at ranges where the reproduction RF signal exhibits a low signal level. The CPU 24 is a typical one with a timer counter. During focus jump operations, the focus search drive voltage V+ or V- supplied from the switch 19 is selected, and the selected drive voltage is output from the switch 18 to accelerate the object lens upward or downward. Changeover of the switch 19 is controlled by a drive signal DRV from CPU 24 on the basis of the zero-cross detection signal FZC+ or FZC- supplied from the comparators 22 and 25. Focus search drive voltages V+ and V- used in this example are equal in absolute value. In CPU 24, a focus OPEN signal is generated, and it is supplied to the switch 18 as a control signal. Accordingly, the phase compensation circuit 17 is selected by the switch 18. The signal selected by the switch 18 is transferred through a drive amplifier 27 to a focus drive coil 28 with one end grounded. The drive amplifier 27 is an N-time amplifier for phase compensation. When the focus OPEN signal becomes the HIGH level and the switch 18 selects the phase compensation circuit 17, the focus servo loop is on. When the focus OPEN signal becomes the LOW level and the switch 18 selects the switch 19, the focus servo loop is off, and the focus jump operation is performed. Figs. 2A to 2J are timing charts for focus jump from a first layer to a second layer. At Fig. 2A, taking time T on the horizontal axis and the lens height x on the vertical axis, the moving track of the object lens is illustrated. As stated above, for changing focalization from the just focus point of the first layer to the just focus point of the second. layer, the object lens is first moved to once overshoot the just focus point of the destined second layer, and thereafter moved back to be focalized at the just focus point of the second layer. The speed of the object lens during these movements is shown at Fig. 2B where the lens speed v lies on the vertical axis. As shown at Fig. 2A, the movement from the just focus point of the first layer to the just focus point of the second layer starts at the point of time t1. That is, at time t1, the control is changed from the focus servo loop control to the focus jump control. In the period T1 from time t1 to time t2, the object lens is accelerated upward. Time t2 is the point where the S-curve of the focus error signal ERR of the second layer, when rising, surpasses the comparison voltage V2. After the object lens passes time t2, it is accelerated downward, passing a time point t3 where the object lens intersects with the just focus point of the second layer. The period of time from the downward acceleration of the object lens to the intersection of the object lens with the just focus point of the second layer is designated by T2. The object lens is further accelerated downward, and passes time t4 where the lens speed is zero, taking the same period of time as T1 in which the object lens was accelerated upward. The period of time from time t3 where the object lens intersects the just focus point of the second layer to time t4 where the speed of the object lens becomes zero is determined as time (T1-T2). The object lens, after passing time t4, is accelerated downward by time t5 (in the period of time T3), and then controlled to stop at the just focus point of the second layer. Time t5 is a point where the object lens is distant from the just focus point of the second layer by one half (x2) of such distance at time t4 (x1). Then the distance x1 is x1 = α ( T1 - T2 ) 2 /2 Similarly, the distance x2 is x2 = αT3 2 /2 where α is the acceleration. CPU 24 calculates the point of time t5 (or the period of time T3) from Equations (1) and (2). αT3 2 /2 = 1/2 x α( t1 - t2 ) 2 /2 Equations (1) and (2) are related as shown by Equation (3) which can be rewritten T3 = ( T1 - T2 )/ 2 Among digital data recorded on the first layer of the optical disk, the reproduction RF signal is read out by time t1 as shown at Fig. 2C. Since the digital data recorded on the optical disk relies on the presence or absence of pits, the reproduction RF signal exhibits the waveform as illustrated. In order to effect focus jump from the first layer to the second layer, the object lens must first be accelerated upward. Therefore, the focus OPEN signal (Fig. 2G) is set to the LOW level, and the drive signal DRV (Fig. 2H) is set to the HIGH level. The focus OPEN signal shown at Fig. 2G maintains the LOW level to the end of the control for focus jump. When the drive signal DRV becomes the HIGH level, the object lens moves upward. Accordingly, the signal level of the reproduction RF signal is lowered. When the reproduction RF signal becomes lower than the comparison voltage V1, the focus OK signal FOK becomes LOW. When the signal level of the reproduction RF signal again goes high and surpasses the comparison voltage V1, the focus OK signal FOK becomes HIGH. This is because the focus jump causes the object lens to begin to move out of focalization on the first layer toward focalization on the second layer. After that, the focus jump to the second layer is completed at time t6, and digital data recorded on the second layer of the optical disk is read out. If a focus error signal ERR (Fig. 2E) is generated in the minus side and surpasses the comparison voltage V3, then the zero-cross detection signal FZC- shown at Fig. 2J becomes HIGH. When the focus error signal in the minus side approaches 0V and becomes smaller than the comparison voltage V3, the zero-cross detection signal FZC- becomes LOW. When the focus jump from the first layer to the second layer is performed, the just focus point of the second layer becomes nearer than the just focus point of the first layer. Therefore, a focus error signal ERR is generated in the plus side with reference to the just focus point of the second layer. As shown at Fig. 2E, when the focus error signal ERR surpasses the comparison voltage V2 (time t2), the zero-cross detection signal FZC+ becomes HIGH, and the drive signal DRV (Fig. 2H) becomes LOW. Therefore, the focus search drive voltage V- (Fig. 2F) is supplied to the focus drive coil 28. That is, the object lens is accelerated downward. When the focus error signal ERR becomes lower than the comparison voltage V2, the zero-cross detection signal FZC+ becomes LOW. The drive signal DRV (Fig. 2H) again becomes HIGH at time t5 where the object lens is distant from the just focus point of the second layer by a distance which is half the distance at time t4, and the focus search drive voltage V+ (Fig. 2F) is supplied to the focus drive coil 28. The supply of the focus search drive voltage V+ continues until the object lens reaches the just focus point of the second layer (point of time t6). After time t6, digital data on the second layer is reproduced in the ordinary way. However, the time when the object lens reaches the just focus point of the second layer does not necessarily coincide with t6, affected by the gravity, and the point of time where the focus error signal ERR becomes lower than the comparison voltage V3 is determined as time t6. Note that no affection is taken into consideration because the acceleration of the object lens is larger than the gravity acceleration. Also note that the focus search drive voltages V+ and V- need not equal in absolute value. In such cases, if the absolute value of the focus search drive voltage V- is M times the focus search drive voltage V+, then Equation (4) becomes T3 = ( T1 - T2 ) x M /2 Fig. 3 shows a process of control by soft-ware for moving the object lens from the just focus point of the first layer to the just focus point of the second layer. In upward acceleration of the object lens in step 31, since the focus search drive voltage V+ is supplied to the focus drive coil 28 by setting the drive signal HIGH and the focus OPEN signal LOW, the object lens is accelerated upward. In step 32, after the data of the counter is replaced with zero, the control proceeds to step 33. At to whether the zero-cross detection signal FZC+ is HIGH or not in step 33, if FZC+ is HIGH, the control proceeds to step 34; however, if FZC+ is low, the control does not proceed to step 34 until FZC+ becomes HIGH. For downward acceleration of the object lens in step 34 after the zero-cross detection signal FZC+ becomes HIGH, the drive signal DRV is set LOW, and the focus search drive voltage V- is supplied to the focus drive coil 28 to accelerate the object lens downward. Then the control proceeds to step 35. In step 35 the value of the counter is held in an address labeled with T1 (the address labelled T* is hereinbelow called register T*), and in step 36 the value of the counter is again set zero. The register T1 holds the value of a period of time counted after the object lens is accelerated upward until the zero-cross detection signal FZC+ becomes HIGH, and the counted value is taken as the period of time T1. The value to be counted may be time, or any other amount equivalent to time, such as clocks of the apparatus, may be selected to be held in the register. As to whether the zero-cross detection signal FZC+ is LOW or not in step 37, if FZC+ is LOW, then the control proceeds to step 38; however, if FZC+ is HIGH, the control does not proceed to step 38 until FZC+ becomes LOW. In step 38 the value of the counter is held in register T2 A . As to whether the zero-cross detection signal FZC- is HIGH or not in step 39, if FZC- is HIGH, the control proceeds to step 40; however, if FZC- is LOW, the control does not proceed to step 40 until FZC-becomes HIGH. In step 40 the value of the counter is held in register T2 B , and in step 41 the average value of register T2 A and register T2 B is held in register T2. In next step 42, a result of operation of (T1-T2)/√2 is held in register T3. In step 43, it is detected whether the sum of register T1 and register T3 equals the value of the counter or not. If they are equal, the control proceeds to step 44; however, if not, step 43 is repeated until the sum of T1 and T3 becomes equal to the value of the counter. After that, as to upward acceleration of the object lens in step 44, the drive signal DRV is set HIGH, and the focus search drive voltage V+ is supplied to the focus drive coil 28 to accelerate the object lens upward. Detection of whether the zero-cross detection signal FZC- is HIGH or not in step 45 is a chattering step, and the control proceeds to step 46 when the zero-cross detection signal FZC- is detected to be HIGH. As to whether the zero-cross detection signal FZC- is LOW or not in step 46, if it is determined that FZC- is LOW and that focalization on the just focus point of the second layer is established, the control proceeds to step 47. In step 47, the focus OPEN signal is set HIGH, thus changing the switch 18, terminating the focus jump from the first layer to the second layer, and restoring the focus servo loop. Figs. 4A to 4J show a timing chart for focus jump from the second layer to the first layer. At Fig. 4A, taking time T on the horizontal axis and the lens height x on the vertical axis, the moving track of the object lens is illustrated. As stated above, for changing focalization from the just focus point of the second layer to the just focus point of the first layer, the object lens is first moved to once overshoot the just focus point of the destined first layer, and thereafter moved back to be focalized at the just focus point of the first layer. The speed of the object lens during these movements is shown at Fig. 4B where the lens speed v lies on the vertical axis. As shown at Fig. 4A, the movement from the just focus point of the second layer to the just focus point of the first layer starts at time t1. That is, at time t1, the control is changed from the focus servo loop control to the focus jump control. In the period of time T1 from time t1 to time t2, the object lens is accelerated downward. Time t2 is the point where the S-curve of the focus error signal ERR of the first layer, when rising, surpasses the comparison voltage V3. After the object lens passes time t2, it is accelerated upward, passing a point (time t3) where the object lens intersects with the just focus point of the first layer. The object lens is further accelerated upward, and passes time t4 where the lens speed is zero, taking the same period of time as T1 in which the object lens was accelerated downward. The object lens, after passing time t4, is accelerated upward by time t5, and then controlled to stop at the just focus point of the first layer. That is, as shown at Fig. 4C, digital data recorded on the second layer of the optical disk is read out as the reproduction RF signal by time t1. Since the digital data recorded on the optical disk relies on the presence or absence of pits, the reproduction RF signal exhibits the waveform as illustrated. In order to effect focus jump from the second layer to the first layer, the object lens must first be accelerated downward. Therefore, the focus OPEN signal (Fig. 4G) is set LOW, and the drive signal DRV (Fig. 4H) is set LOW. The focus OPEN signal shown at Fig. 4G maintains the LOW level to the end of the control for focus jump. When the drive signal DRV becomes LOW, the object lens moves downward. Accordingly, the signal level of the reproduction RF signal is lowered. When the reproduction RF signal becomes lower than the comparison voltage V1, the focus OK signal FOK becomes LOW. When the signal level of the reproduction RF signal again goes high and surpasses the comparison voltage V1, the focus OK signal FOK becomes HIGH. This is because the focus jump causes the object lens to begin to move out of focalization on the second layer toward focalization on the first layer. After that, the focus jump to the first layer is completed at time t6, and digital data recorded on the second layer of the optical disk is read out. If a focus error signal ERR (Fig. 4E) is generated in the plus side and surpasses the comparison voltage V2, then the zero-cross detection signal FZC+ shown at Fig. 4H becomes HIGH. When the focus error signal in the plus side approaches 0V and becomes smaller than the comparison voltage V2, the zero-cross detection signal FZC+ becomes LOW. When the focus jump from the second layer to the first layer is performed, the just focus point of the first layer becomes nearer than the just focus point of the second layer. Therefore, a focus error signal ERR is generated in the minus side with reference to the just focus point of the first layer. As stated above, when the focus error signal ERR (Fig. 4E) surpasses the comparison voltage V3 (time t2), the zero-cross detection signal FZC- becomes HIGH, and the drive signal DRV (Fig. 4H) becomes HIGH. Therefore, the focus search drive voltage V+ (Fig. 4F) is supplied to the focus drive coil 28. That is, the object lens is accelerated upward. When the focus error signal ERR becomes lower than the comparison voltage V3, the zero-cross detection signal FZC- becomes LOW. The drive signal DRV (Fig. 4H) again becomes HIGH at time t5 where the object lens is distant from the just focus point of the first layer by a distance which is half the distance at time t4, and the focus search drive voltage V- (Fig. 4F) is supplied to the focus drive coil 28. The supply of the focus search drive voltage V- continues until the object lens reaches the just focus point of the first layer (time t6). After time t6, digital data on the first layer is reproduced in the ordinary way. Fig. 5 shows a process of control by soft ware for moving the object lens from the just focus point of the second layer to the just focus point of the first layer. In downward acceleration of the object lens in step 51, since the focus search drive voltage V- is supplied to the focus drive coil 28 by setting the drive signal DRV LOW and the focus OPEN signal LOW, the object lens is accelerated downward. In step 52, after the data of the counter is replaced with zero, the control proceeds to step 33. As to whether the zero-cross detection signal FZC- is HIGH or not in step 33, if FZC- is HIGH, the control proceeds to step 54; however, if FZC- is low, the control does not proceed to step 34 until FZC- becomes HIGH. For upward acceleration of the object lens in step 54 after the zero-cross detection signal FZC-becomes HIGH, the drive signal DRV is set HIGH, and the focus search drive voltage V+ is supplied to the focus drive coil 28 to accelerate the object lens upward. Then the control proceeds to step 55. In step 55 the value of the counter is held in register T1, and in step 36 the value of the counter is again set zero. That is, register T1 holds the time in which the object lens is accelerated upward. As to whether the zero-cross detection signal FZC- is LOW or not in step 57, if FZC-is LOW, then the control proceeds to step 58; however, if FZC- is HIGH, the control does not proceed to step 58 until FZC- becomes LOW. In step 58 the value of the counter is held in register T2 A . As to whether the zero-cross detection signal FZC+ is HIGH or not in step 59, if FZC+ is HIGH, the control proceeds to step 60; however, if FZC+ is LOW, the control does not proceed to step 60 until FZC+ becomes HIGH. In step 60 the value of the counter is held in register T2 B , and in step 41 the average value of register T2 A and register T2 B is held in register T2. In next step 62, a result of operation of (T1-T2)/√2 is held in register T3. In step 63, it is detected whether the sum of register T1 and register T3 equals the value of the counter or not. If they are equal, the control proceeds to step 64; however, if not, step 63 is repeated until the sum of T1 and T3 becomes equal to the value of the counter. After that, as to downward acceleration of the object lens in step 64, the drive signal DRV is set LOW, and the focus search drive voltage V- is supplied to the focus drive coil 28 to accelerate the object lens downward. Detection of whether the zero-cross detection signal FZC- is HIGH or not in step 65 is a chattering step, and the control proceeds to step 66 when the zero-cross detection signal FZC+ is detected to be HIGH. As to whether the zero-cross detection signal FZC+ is LOW or not in step 66, if it is determined that FZC+ is LOW and that focalization on the just focus point of the first layer is established, the control proceeds to step 67. In step 67, the focus OPEN signal is set HIGH, thus changing the switch 18, terminating the focus jump from the second layer to the first layer, and restoring the focus servo loop. Figs 6A to 6J show a timing chart for focus jump from the first layer to the third layer as an example of focus jump of two or more layers. At Fig. 6A, taking time T on the horizontal axis and the lens height x on the vertical axis, the moving track of the object lens is illustrated. As stated above, for changing focalization from the just focus point of the first layer to the just focus point of the third layer, the object lens is first moved to once overshoot the just focus point of the destined third layer beyond the just focus point of the second layer and further overshoot the just focus point of the fourth layer, and thereafter moved back to be focalized at the just focus point of the third layer. The speed of the object lens during these movements is shown at Fig. 6B where the lens speed v lies on the vertical axis. As shown at Fig. 6A, the movement from the just focus point of the first layer to the just focus point of the third layer starts at time t1. That is, at time t1, the control is changed from the focus servo loop control to the focus jump control. In the period of time T1 from time t1 to time t2, the object lens is accelerated upward. Time t2 is the point where the S-curve of the focus error signal ERR of the first layer, when rising, surpasses the comparison voltage V2. After the object lens passes time t2, it is accelerated downward, passing a point (time t3) where the object lens intersects with the just focus point of the third layer. The object lens is further accelerated downward, and passes time t4 where the lens speed is zero, taking the same period of time as T1 in which the object lens was accelerated upward. The object lens, after passing time t4, is accelerated downward by time t5, and then controlled to stop at the just focus point of the third layer. That is, as shown at Fig. 6C, digital data recorded on the first layer of the optical disk is read out as the reproduction RF signal by time t1. Since the digital data recorded on the optical disk relies on the presence or absence of pits, the reproduction RF signal exhibits the waveform as illustrated. In order to effect focus jump from the first layer to the third layer, the object lens must first be accelerated upward. Therefore, the focus OPEN signal (Fig. 6G) is set LOW, and the drive signal DRV (Fig. 6H) is set HIGH. The focus OPEN signal shown at Fig. 6G maintains the LOW level to the end of the control for focus jump. When the drive signal DRV becomes HIGH, the object lens moves upward. Accordingly, the signal level of the reproduction RF signal is lowered. When the reproduction RF signal becomes lower than the comparison voltage V1, the focus OK signal FOK becomes LOW. When the signal level of the reproduction RF signal again goes high and surpasses the comparison voltage V1, the focus OK signal FOK becomes HIGH. This is because the focus jump causes the object lens to begin to move out of focalization on the first layer toward focalization on the second layer. In this example, since the focus jump is to be effected from the first layer to the third layer, as shown at Fig. 6C, focalization is moved from the first layer to the third layer, from the second layer to the third layer, from the third layer to the fourth layer, and finally to the destined third layer, passing the fourth layer another time. After that, the focus jump to the first layer is completed at time t6, and digital data recorded on the third layer of the optical disk is read out. If a focus error signal ERR (Fig. 6E) is generated in the minus side and surpasses the comparison voltage V3, then the zero-cross detection signal FZC- shown at Fig. 6J becomes HIGH. When the focus error signal in the minus side approaches 0V and becomes smaller than the comparison voltage V3, the zero-cross detection signal FZC- becomes LOW. When the focus jump from the first layer to the third layer is performed, the just focus point of the second layer becomes nearer than the just focus point of the first layer. Therefore, an S-curve is of a focus error signal ERR is generated in the minus side with reference to the just focus point of the first layer. After that, a plus-side S-curve based on the second layer is generated, and a minus-side S-curve is also generated. As illustrated, when the object lens passes the just focus point while moving upward, an S-curve of the focus error signal ERR first appears in the plus side and next in the minus side. When the object lens passes the just focus point while moving downward, an S-curve first appears in the minus side and next in the plus side. In this manner, the process of focus jump of two or more layers is exactly the same as the process of focus jump of one layer except that the time T1 is extended to the rising of the S-curve of a focus error signal ERR of a destined layer. The maximum height for movement of the object lens is within the range where the object lens does not hit the optical disk. Although the embodiment has been described setting the time from the start of acceleration of the object lens to the rising of the S-curve of the focus error signal ERR of a destined layer as the time T1 for the first acceleration of the object lens, the time T1 may be set otherwise provided the object lens can be accelerated such that focalization of the object lens moves beyond the just focus point of the destined layer. Moreover, although the embodiment of the soft-ware processing according to the invention employs the address labelled T* as the register T*, it is also possible to use the label T* itself as the register T*. According to embodiments of the invention, focalization to a destined layer can be established in a short time as compared the method of resuming a focus search operation after cancelling the focus servo control. Also, embodiments of the invention can reliably catch the just focus point of a destined layer even with a variety in distance between layers because focus servo control is performed after moving the focalization beyond the just focus point of the destined layer, unlike the method of upward and downward acceleration for a predetermined time. Moreover, embodiments of the invention can perform a reliable servo control after closing the focus servo loop because the speed of the object lens at the just focus point of a destined layer is approximately zero. Having described specific preferred embodiments of the present invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of the invention as defined in the appended claims. In summary, in embodiments of the invention, to move an object lens out of focalization on a layer and to accelerate it upwardly, a drive signal DRV is set HIGH, and a focus OPEN signal is set LOW. After a value a counter exhibits at this time is held in a register T1, the counter is reset at zero. Then a value the counter exhibits upon FZC+ being LOW is held in a register T2 A , and a value the counter exhibits upon FZC- being HIGH is held in a register T2 B . The value of a register T3 is calculated from T1 and the average of T2 A and T2 B , and when the counter value becomes equal to the sum of T1 and T3, DRV is set HIGH. After FZC-becomes equal to the sum of T1 and T3, DRV is set HIGH. After FZC- becomes LOW, OPEN is set HIGH, and the focus jump operation is completed.

Claims: A disk reproducing apparatus for reproducing data recorded on a plurality of layers of an optical disk, the apparatus comprising: first comparator means (22) for comparing a focus error signal with a first voltage; second comparator means (25) for comparing the focus error signal with a second voltage; third comparator means (21) for comparing a reproduced RF signal with a third voltage; means (23,26) responsive to a result of comparison by said third comparator means to control operations of said first and second comparator means; drive voltage supplying means (19) for selectively supplying a first or a second object lens drive voltage in response to results of comparison supplied from said first and/or second comparator means; and means (28) for driving an object lens with said object lens drive voltage supplied from said drive voltage supplying means. The disk reproducing apparatus according to claim 1, wherein said drive voltage supplying means comprises: first object lens drive voltage generator means (V+) for accelerating said object lens in a first direction; and second object lens drive voltage generator means (V-) for accelerating said object lens in a second direction different from said first direction. The disk reproducing apparatus according to claim 2, wherein said first and second object lens drive voltage generator means generate object lens drive voltages which are opposite in polarity and substantially equal in absolute value. The disk reproducing apparatus according to claim 3, further comprising means (24) for obtaining a duration of time (T1-T2) from the point of time (t3) where the object lens passes over a focalized point to the point of time (t4) where the object lens reaches a position at which its velocity is substantially zero. The disk reproducing apparatus according to claim 4, further comprising means for obtaining a first duration of time (T1) which starts at a time (t1) when control is changed from focus servo loop control to focus jump control and finishes at a time (t2) when acceleration changes from said first direction to said second direction, and for obtaining a second duration of time (T2) since a comparison output is supplied from said first comparator means until said object lens passes over the focalized point. The disk reproducing apparatus according to claim 5, further comprising means for obtaining from said first and second duration of time (T1 and T2) a duration of time from a point of time where the object lens moves from the position most distant from the focalized point to a point of time where the object lens passes over a position distant from the focalized point by one half the distance of the most distant position from the focalized point. The disk reproducing apparatus according to claim 2, wherein said first and second voltages are set to values such that one of said voltages is higher than the noise near zero volts and the other of said voltages is lower than the noise near zero volts. A disk reproducing method for reproducing data recorded on a plurality of layers of an optical disk, the method comprising the steps of: comparing a focus error signal with a first voltage; comparing the focus error signal with a second voltage; comparing a reproduced RF signal with a third voltage; in response to the comparison of the reproduced RF signal with the third voltage, controlling operations of said comparisons with said first and second voltages; selectively supplying a first or a second object lens drive voltage in response to results of comparison with said first and/or second voltages; and driving an object lens with said object lens drive voltage. A method according to claim 8, comprising the steps of: accelerating an object lens in a first direction; accelerating the object lens in a second direction different from said first direction in response to a detection that the focus error signal is further than a comparison voltage (V2, V3) from a voltage representing a focalized point; detecting that the object lens has moved in that second direction for a duration of time (t 2 to t 5 ) calculated from the first duration of time (T1) which starts at a time (t1) when control is changed from focus servo loop control to focus jump control and finishes at a time (t2) when acceleration changes from said first direction to said second direction and from the second duration of time (T2) which starts from the time (t2) when the object lens starts to accelerate in said second direction and finishes at a time (t3) when the object lens passes over a focalized point and subsequently accelerating the object lens in said first direction; and starting focus servo control in response to a detection that the focus error signal is closer than a comparison voltage (V2, V3) to a voltage representing a focalized point. A method according to claim 9, wherein the acceleration in said first direction and the acceleration in said second direction are opposite in direction and substantially equal in absolute value. A method according to claim 10, wherein said step of accelerating the object lens in said second direction further comprises a step for obtaining from said first and second durations of time (T1 and T2) a duration of time from a point of time where the object lens moves from the position at which its velocity is substantially zero to a point of time where the object lens passes over a position distant from the focalized point by one half the distance of the most distant position from the focalized position.

Inventors: NISHIKATA MASANOBU, NISHIKATA, MASANOBU, Nishikata, Masanobu, c/o Int. Prop Div, Sony Corp.
Applicants: SONY CORP, SONY CORPORATION
Application year: 1995
Main classification: G11B7
IPC codes: G11B7