METHODS OF ACCESSING AN OPTICAL RECORDING TRACK

Abstract: Lens (20) is accelerated on a coarse tracking carriage (2) while enabling a lens position servo, which moves the carriage to return the lens to a neutral position on the carriage; after the beam spot (BS) becomes fast enough, the carriage is accelerated and decelerated until the beam spot arrives near a desti­ nation track; then the lens is decel­ erated on the carriage to stop on the destination track while not stop­ ping at each track to cross. A sec­ ond method is: the carriage is first accelerated while the fine tracking servo (3) is enabled to trace a start track; after the carriage becomes fast enough, the fine tracking servo is disabled, thus the beam spot is moved by a lens lock force which returns the lens to the neutral posi­ tion on the carriage; the carriage is further accelerated and decelerated, next the lens is decelerated, respec­ tively by the same ways as in the first method.

Description: This invention relates to a method of track accessing in an optical recording apparatus and, in particular, to a novel technique for making a beam spot quickly cross a plurality of tracks while the number of tracks the beam spot has crossed is accurately checked. In a known optical recording disk system, as shown in Fig. 1A, data is written in a recording track on a recording medium, e.g. a disk 1, by a beam spot focused thereon and the data is read out using reflected light therefrom. The system is constituted as follows: the disk 1 is rotated around its axis by a motor 1a. An optical head 2 is coarsely driven across a radial direction of the disk by a motor (not shown) under control of a carriage controller 5. The optical head 2 is constituted so that light emitted from a semiconductor laser 24, as a light source, is introduced via a lens 25 and a polaroid beam splitter 23 into an object lens 20. This light is focused by the object lens 20 to form a beam spot BS projected onto the disk, and light reflected from the disk is input via the object lens 20 by the polaroid beam splitter 23 onto a 4-division light receiver 26. The optical head 2 may be referred to as a carriage. On the disk, very many tracks are formed with a pitch of a few micrometers (e.g. 1.6 µm) along the radial direction; accordingly, even a little eccentricity of the disk, typically 100 µm peak to peak, may cause a large displacement of any given track; moreover, any undulation of the optical disk 1 causes a displacement of the focus point of the beam spot. Therefore, it is necessary for the beam spot position to be controlled to less than 1 µm so as to cope with such displacements and/or undulations. For this purpose, there are provided a focus-actuator (focus coil) 22 for moving the object lens 20 on the optical head 2 along the axial direction of the lens so as to change the location of the focusing point, and a track-actuator (tracking coil) 21 for moving the object lens 20 (i.e. along left and right directions in the Figure) so as to change the location of the beam spot along the orthogonal (i.e. radial) direction of the tracks. Corresponding to this, there are also provided a focus servo controller 4 for generating a focusing error signal FES from the light signal of the light receiver 26 to drive the focus actuator 22, and a fine tracking servo controller 3 for generating a tracking error signal TES from the light signal of the light receiver 26 to drive the track actuator 21. Fine tracking control is performed, as shown in Fig. 1B and 1C, by utilizing a diffraction phenomenon of the beam spot BS caused by a coaxial or spiral guide groove (track) 10 preformed on the optical disk 1. That is, a variation in the distribution of reflected light detected by light receiver 26, caused by light diffraction at the track 10 depending on the position of the beam spot BS relative to the track 10, is utilized in order to acquire the beam spot's position error from the track 10. For example, when a known push-pull method having a four-division light receiver which consists of four photo diodes 26a, 26b, 26c and 26d is employed, the reflected light distributions at the light receiver 26 are as follows. When the beam spot P₁ deviates toward the left-hand side of the track 10 as shown in Fig. 1C, the distribution is as shown in Fig. 1D; when the beam spot P does not deviate from the track 10 (i.e. is on track), the distribution is as shown in Fig. 1E; and when the beam spot P₂ deviates toward the right-hand side of the track 10, the distribution is as shown in Fig. 1F. The value {(a+b)-(c+d)}, obtained in the fine tracking servo controller 3 from the outputs a to d of the photo diodes 26a to 26d, forms a tracking error signal TES shown in Fig. 1G, which then drives the tracking actuator 21. The latter drives the object lens 20 in the left or right directions, and thus the beam spot can be controlled to follow the track 10 regardless of the eccentricity and undulation of the optical disk 1. To make the beam spot cross several tracks to access a particular track (referred to below as track jumping), there have been employed two basic methods. The first method is to move the beam spot by moving the optical head (carriage) 2, having the lens 20 or a galvano mirror (not shown in the Figure) mounted thereon, by a large amount in the radial direction, such as 120 mm. The second method is to move the beam spot by means of a fine tracking mechanism, which is provided to make the beam spot follow a track by moving the lens 20 or galvano mirror (both referred to as a "lens" from now on, for simplicity) on a carriage in small amounts in the radial direction to compensate for eccentric movement of the track, while the carriage is fixed or by following the lens by a lens position servo control (see below), if the number of tracks to be jumped is less than one hundred for example. These two methods are used generally in combination. The number of tracks crossed by the beam spot is acquired by counting zero cross transitions in a zero cross signal TZCS, this signal being generated from a tracking error signal which is originally provided for the fine tracking servo control of the lens. However, as already mentioned, the optical disk may rotate with an eccentricity as much as 100 µm peak to peak, having a frequency of 30 Hz for 1800 revolutions per minute, for example. The amount 100 µm corresponds to about 60 tracks having a pitch of 1.6 µm. Accordingly, in the first method a wrong zero cross signal is generated whenever the radial speed of an eccentrically-rotating track exceeds the beam spot's radial speed, resulting in an error in counting crossed tracks. Therefore, an ID (identification) number recorded in each track has to be read out after track jumping is finished, and then according to the read out track ID number, the beam spot location must be adjusted as necessary by moving the lens to reach the destination track. Consequently, track jumping takes a long time. In the second method, the counting of crossed tracks is correct if the track crossing is done track by track. In this track-by-track jump method, a track actuator 21 drives the object lens 20 to move the beam spot BS from a present track 10a to a destination track 10d. As shown in Figs. 2A and 2B, repeated single-track jumps are carried out, i.e. a sequence of an acceleration current i a applied to the track actuator for an acceleration, followed by a deceleration current -i a for a deceleration to stop, for each track. After each single track jump, the beam spot BS cannot immediately follow the track due to mechanical characteristics of the system; accordingly, a settling period has to be provided to allow the beam spot to capture the track by the fine servo (i.e. to allow the tracking error signal TES to reach almost zero). It takes a relatively long period, such as 2.5 ms, for the beam spot to cross to an adjacent track and to stop at the centre of the track. Accordingly, the period required for jumping a great number of tracks becomes 2.5 ms multiplied by the number of tracks to be jumped. Such a long period cannot satisfy present requirements for fast accessing of optical disks. EP-A-0 090 379 discloses a data track searching device having a rough searching mode and a fine searching mode. In the rough searching mode, a sledge (carriage) is moved whilst a fine tracking element is disabled. To minimise inertial effects in the rough searching mode, the movement of the sledge is carried out by equal acceleration and deceleration phases, without any special control of the sledge at the start or target tracks. EP-A-0 138 245 discloses reading apparatus for a video-disc player in which a light beam spot is made to jump over tracks during a field blanking interval of the video signal, using a mirror. By braking the jump in stages it is ensured that the jump always ends before an address code of the desired field. The address code is read to check that the correct track has been reached. All independent claims are delimited with respect to EP-A-0 090 379. According to a first aspect of the present invention, there is provided a method of track jumping for positioning a light beam spot on a destination track of a recording medium in an optical recording apparatus, the recording medium having eccentric tracks, the optical recording apparatus having a carriage and a fine tracking device mounted on the carriage for moving the beam spot, the method including the steps of:- (d) detecting light reflected from the recording medium to generate a tracking error signal, and counting a number of zero crossings, indicating crossing of tracks by said beam spot, in the tracking error signal since the track jumping was initiated; and (e) accelerating/decelerating the fine tracking device according to a difference between the counted number of crossed tracks and said instructed number of tracks to be jumped, to reach the destination track; characterised by the prior steps of:- (a) accelerating the fine tracking device, while enabling a position servo which moves the carriage so that the fine tracking device returns to its neutral position on the carriage, until the beam spot speed becomes faster than a radial speed of the eccentric tracks; (b) further accelerating the carriage until said beam spot reaches a track whose track number is calculated, by a program prepared in advance, according to an instructed number of tracks to be jumped; and (c) decelerating the carriage until the beam spot speed becomes slow enough for the fine tracking device to seek the destination track. According to a modification of the above method, step (a) is modified as follows:- (a1) accelerating the carriage while enabling a fine tracking servo of said fine tracking device so as to keep the beam spot on a start track at which the beam spot was initially located, until the carriage speed exceeds said radial speed of the eccentric tracks; and (a2) disabling the fine tracking servo and enabling a lock, by which the fine tracking device returns to a neutral position on the carriage, so that the beam spot is accelerated. According to a second aspect of the present invention, there is provided a method a method of track jumping for positioning a light beam spot on a destination track of a recording medium in an optical recording apparatus, the recording medium having eccentric tracks, the optical recording apparatus having a carriage and a fine tracking device mounted on the carriage for moving the beam spot, the method comprising the steps of:- (i) detecting light reflected from the recording medium to generate a tracking error signal; (ii) detecting zero crossings, which indicate that the beam spot has crossed a track, in the tracking error signal and measuring the time interval between successive zero crossings while the beam spot is moving; and characterised by the further steps of:- (iii) obtaining a time difference between the time interval between successive zero crossings and a predetermined time interval; and (iv) accelerating or decelerating the fine tracking device for moving the beam spot according to the time difference, so as to make the time interval between successive zero crossings essentially equal to the predetermined time interval, whereby the beam spot is moved at a predetermined speed without stopping at each track. A method embodying the invention may enable fast track jumping over a long distance, without stopping at each track, while the number of tracks the beam spot has crossed is accurately counted; therefore, fast and accurate track jumping can be accomplished. Reference is made, by way of example, to the accompanying drawings, in which:- Fig. 1A schematically illustrates servo mechanisms in an optical recording disk apparatus; Figs. 1B to 1G explain a principle of a fine tracking servo control; Fig. 2 explains a prior art track-by-track jumping method; Fig. 3 shows a block diagram of an optical disk apparatus to which the invention may be applied; Fig. 4A schematically illustrates an optical head (carriage); Figs. 4B and 4C schematically illustrate the structure of a lens lock and a lens position servo; Fig. 5 is a block diagram of electrical circuits of an optical disk apparatus to which the present invention is applied; Fig. 6 shows detailed circuits of a head circuit in Fig. 5; Fig. 7 shows detailed circuits of a TES generating circuit, a total-signal generating circuit, an AGC circuit, a zero-cross detector circuit and an off-track detector circuit of the block diagram of Fig. 5; Fig. 8 shows detailed circuits of a phase compensation circuit, a return signal generating circuit, a lock-on switch, a driver circuit and a power amplifier of the block diagram of Fig. 5; Fig. 9 explains a principle of a track access method of the present invention; Fig. 10 shows a flow chart of a preferred embodiment of the present invention; Fig. 11 shows a timing chart of a preferred embodiment of the present invention; Fig. 12 shows a flow chart of a second preferred embodiment of the present invention; Fig. 13 explains a track-by-track speed control method which may be applied to the present invention; Fig. 14 shows a flow chart of the track-by-track speed control method of Fig. 13; and Fig. 15 shows timing charts of the track-by-track speed control method. Preferred embodiments of the present invention will now be described, referring to cases where a fine tracking device consists of a lens. First of all, an optical head, which is a carriage having the lens thereon, is explained. In Fig. 4A, light emitted by a semiconductor laser 24 is formed into a parallel light beam by a collimator lens 25a; the cross-section of the light beam is corrected to be a true round (i.e. circular) cross-section by a true-round correction prism 25b; the light beam is transmitted to a polarization beam splitter 23; further projected onto an object lens 20 via a 1/4 wavelength plate 25c; and focused as a beam spot BS on an optical disk 1 (Fig. 1A). Reflected light from the beam spot on the optical disk 1 is led to the polarization beam splitter 23 via the object lens 20 and 1/4 wavelength plate 25c, and then projected onto a four-division light receiver 26 via a focusing lens 27. The object lens 20 is installed at the end of a main body 28 of an actuator rotatable around an axle 28a, and a slit (aperture) 28b is provided on the other end of the main body 28. The main body 28 is provided with a coil holder 28c. The coil holder 28c has a focus coil 22 wound around it and spiral tracking coil 21 provided on its side, and a magnet 28d is provided surrounding the coil 28c. When a current is applied to the focus coil 22, the actuator 28 having the object lens 20 thereon moves along the lens's axis, i.e. up or down in the X direction in the Figure, thus varying its focusing point; and when a current is applied to the actuator coil (the tracking coil) 21, the actuator 28 rotates around the axle 28a in the direction marked α, thus varying its position in a direction orthogonal to the tracks, i.e. the radial direction. For the slit 28b on another end of the actuator 28, there is provided a position sensor 29 on the carriage (lens head) 2. The position sensor 29 is arranged so that a light emitter 29e and four-division light receiver 29 are facing and aligned with each other via the slit 28b as shown in Figs. 4B and 4C. The slit 28b is provided with a window W, through which the light beam from the light emitter 29e is received by the four photo diodes 29a to 29d of the light receiver 29. So, as shown in Fig. 4B, the received light distribution upon the photo diodes 29a to 29d varies depending on the amount of the displacements along the α and X directions of the actuator 28. Therefore, a lens position signal LPS indicating the lens's displacement on the carriage 2 in the track's radial direction and a focus position signal for the focus direction FPS are acquired from the outputs A, B, C and D of each of the four photo diodes 29a to 29d in a similar way to those of focusing and tracking servos, as follows: LPS = (A+C) - (B+D) FPS = (A+B) - (C+D) These lens position signals LPS and FPS exhibit an S shape form for the displacements X and α from each essentially centre position, which is referred to hereinafter as a neutral position, of the stroke and rotation of the lens on the carriage. Each signal becomes zero at each neutral position, as shown in Fig. 4B. Thus, an electrical restoring force returning the lens toward each neutral position can be generated from these signals. This function is referred to as a lens lock and a focus lock, and its original purposes are to prevent the lens from floating when the carriage is travelling and to prevent the lens from shifting too much from the centre of the coverage of the fine tracking servo. The lens position signal LPS may also be negatively fed back to the carriage controller 6 to move the carriage so that the lens returns to its neutral position on the carriage. This may be referred to as a double servo control, a lens position servo control or simply a position servo control. The purpose of this position servo control is also to prevent the range of the fine servo control from exceeding the allowable range of the lens on the carriage. Next, referring to the circuit diagram in Fig. 5, an embodiment of the present invention will now be explained. An operation controller is provided by a micro processor (MPU) 7, which receives a track jump instruction from a controlling processor (not shown) and stores an instructed number of tracks to be jumped. The MPU 7 then transmits instructions to the tracking servo controller 3, the focus controller and the carriage controller 5 by running a program pre-installed in the MPU. The MPU 7 also receives a track zero-cross signal TZCS from the zero cross-detector 34a and an off-track signal TOS from an off-track detector 34b so that the beam spot location is controlled by the use of a lens forwarding signal FWR, a lens reversing signal RVS, and a lock-on signal LKS. A head circuit 6, as shown in Fig. 6, includes an RF-composing circuit 60 for composing an RF signal RFS from the outputs a to d of the four-division light detector 26, an amplifier 61 for amplifying the outputs a to d of the four photo diodes 26a to 26d so as to output servo outputs SVa to SVd, an LP-generating circuit 62 for generating the lens position signal LPS from the outputs A to D of the four photo diodes 29a to 29d of the position sensor 29, the four-division light detector 26 and a constant voltage power source 63 for the position sensor 29. In the head circuit 6, as shown in Fig. 6, the constant voltage power source 63 is composed of a Zener diode ZD, a condenser C, and a resistor R, and is connected to four photo diodes 26a to 26d of the four-division light receivers 26 as well as to the four photo diodes 29a ∼ 29d of the position sensor 29, and supplies a power source voltage V thereto. The LP-generating circuit 62 composed of a differential amplifier performs a differential amplification, i.e. (A+C) - (B+D), of the outputs A ∼ D of the photo diodes 29a ∼ 29d so as to generate the lens position signal LPS; the RF-generating circuit 60 is composed of a high-pass filter, and adds the outputs of the four photo diodes 26a ∼ 26d via differentiator condensers C₁ so as to be output to an operational amplifier 60. Furthermore, the amplifier 61 is composed of amplifiers 61a ∼ 61d connected respectively to the photo diodes 26a ∼ 26d and outputs servo outputs SVa ∼ SVd. Returning back to FIG. 5, a TES (tracking error signal) generating circuit 30 generates the tracking error signal TES from the servo outputs SVa ∼ SVd of the amplifier 61 (61a ∼ 61d). A total-signal generating circuit 31 adds the servo outputs SVa ∼ SVd so as to generate a total-signal DSC which indicates a total reflection level. An AGC (Automatic Gain Control) circuit 32 divides the tracking error signal TES by the total-signal (total reflection level) to perform the AGC in reference to the total reflection level, so that a variation of the projecting light intensity or the reflection coefficient is compensated. In a phase compensation circuit 33 the tracking error signal TES is differentiated and then added with a proportional component of the tracking error signal TES so as to advance the phase. A zero-cross detector 34a detects a zero-cross transition of the tracking error signal TES so as to output the track zero-cross signal TZCS to MPU 7. An off-track detector 34b detects that the tracking error signal TES becomes larger than a predetermined level Vo in a positive direction or lower than a predetermined level -Vo in a negative direction, i.e. indicating an off-track state, and then outputs an off-track signal TOS to MPU 7. A servo switch 35 closes a servo loop when the servo signal SVS is output from the MPU 7, as well as opens the servo loop when the servo signal is disabled. A return signal generating circuit 36 generates, from the lens position signal LPS acquired from the LP-generating circuit 62, a return signal RPS which generates a return force on the actuator 28 toward the neutral position on the carriage as shown in FIG. 4B. A lock-on switch 37 is closed by the lock-on signal LKS from MPU 7 so as to introduce the return signal RPS to the servo loop, and opens by the disabling of the same so as to cut the introduction of the return signal to the servo loop. A driver circuit 38 generates a forward acceleration signal +V corresponding to the lens forwarding signal FWR and a backward deceleration signal -V corresponding to the lens reversing signal RVS, as well as for outputting a sum of the outputs from the servo switch 35 and the lock-on switch 37. A power amplifier 39 amplifies an output of the driver circuit 38 so as to deliver a track driving current TDV to the track actuator coil 21. Details of the tracking servo controller 3 are shown in FIG. 7 and FIG. 8. As shown in FIG. 7, the TES generating circuit 30 includes an adder amplifier 300 for adding the servo outputs SVa and SVb via input resistors r₁ and r₂ respectively, an adder amplifier 301 for adding the servo outputs SVc and SVd via input resistors r₃ and r₄ respectively, and an adder amplifier 302 for reducing the output -(SVa+SVb) of the adder amplifier 301 from the output -(SVc+SVd) of the adder amplifier 300, and then the differential amplifier 302 outputs the tracking error signal TES {= (SVa+SVb) - (SVc+SVd)} . The total-signal generating circuit 31 includes an adder amplifier 310 for adding each servo output SVa ∼ SVd via input resistors r₅ ∼ r₈, and outputs the total-reflection level signal DCS (= SVa + SVb + SVc + SVd) . The AGC circuit 32 includes an operational amplifier 302 for receiving the tracking error signal TES, a first FET (Field Effect Transistor) 321 for dividing and controlling the input to the first operational amplifier 320 according to the output of the operational amplifier 320, a second operational amplifier 322 for receiving the total-reflection level signal DCS for controlling FET 321, and a second FET 323 for voltage-dividing the input to the second operational amplifier 322, thus, the first FET 321 is controlled by the total-reflection level signal DCS, which is an output of the operational amplifier 322, so as to control the gain of the operational amplifier 320, then an AGC-controlled tracking error signal TES/DCS is acquired from the output of the operational amplifier 320. The second FET 323 is provided in order to compensate the non-linearity of the first FET 321 so as to accomplish a linear characteristic. The zero-cross detector 34a is composed of a comparator 340 for comparing the zero-cross potential with the tracking error signal TES from the AGC circuit 32, so as to output the track zero-cross signal TZCS. The off-track detecting circuit 34b includes a first comparator 341 for comparing the tracking error signal TES with the predetermined level Vo and then outputting a "High" level output when TES > Vo, and a second comparator 342 for comparing the tracking error signal TES with the predetermined level (-Vo) and then outputting a "High" level output when TES < -Vo, and outputs a sum of the outputs from both the comparators 341 and 342 as the off-track signal TOS. The phase compensating circuit 33, in which a differentiation circuit composed of a resistor r g and a condenser C g and a proportion circuit formed of a resistor R g are connected to an operational amplifier 330, outputs to the servo switch 35 a signal of a sum of the differentiation of the track error signal TES of the AGC circuit 32 and its proportion, i.e. a signal TCS which is a tracking error signal added with a phase-advancing component. The return signal generating circuit 36 includes an operational amplifier 360 for amplifying the lens position signal LPS from the LP-generating circuit 62 and a phase compensating circuit 361 for phase-compensating the output of the operational amplifier 360, and outputs the return signal RPS. The lock-on switch 37 includes a first switch 370, which is closed when the lock-on signal LKS is output, for adding the return signal RPS to the signal TCS of the servo switch 33; an inverter circuit 371 for inverting the lock-on signal LKS; and a second switch 372 which becomes closed when the inverted lock-on signal LKS is output, i.e. the lock-on signal is disabled. The driver circuit 38 includes an adder amplifier 380; a first switch 381 which is enabled by the lens forwarding signal FWR to deliver the forward driving signal +V to the adder amplifier 380; and a second switch 282 which is enabled by the lens reversing signal RVS to deliver the backward driving signal -V to the adder amplifier 380, and outputs driving signals (voltages) +V, -V and a sum of the control signals TCS, RPS from the adder amplifier 380. In the summation of the signals TCS and RPS, the signal RPS is set weaker than the signal TCS. The power amplifier 39 amplifies the output of the driving circuit 38 by serial two-stage amplifiers 390 and 391 so as to apply a positive or negative track driving current TDV into the tracking actuator coil 21. Operation of the above-described embodiment is hereinafter explained. First of all, the concept of the track access of the present invention is described with FIG. 9. In the above-described embodiment the lens is moved as the fine tracking means on the carriage 2; therefore, the track access by moving the fine tracking device, i.e. the lens 20 in this embodiment, on the carriage is referred to hereinafter as a lens seek. A track seek carried out by moving the carriage 2 is referred to hereinafter as a carriage seek. At first, the carriage 2 is stationary relative to the disk. The range (a) in FIG. 9 is for a lens seek acceleration, in which the beam spot, i.e. lens 20, is accelerated on the carriage from a start track, on which the beam spot is projected when the track jumping is instructed, driven by the fine tracking controller 3, until the speed of the beam spot exceeds the maximum radial speed Vec of the eccentric tracks. During this range (a), the fine tracking servo control is disabled, and the lens position servo is kept enabled. Therefore, the carriage follows the beam spot by the function of the lens position servo so that the lens returns to the neutral position on the carriage. Then, the lens position signal which was once increased becomes close to zero. Now the speed of the beam spot by the lens seek is adequately faster than the radial speed of the eccentric tracks varying typically at 30 Hz. Accordingly, the number of the tracks the beam spot has crossed can be accurately counted from the track zero-cross signal TZCS. The range (b) is a range of the carriage seek where the beam spot acceleration is carried out by further accelerating the carriage until the beam spot reaches almost half of the number of tracks to jump. The range (c) is a range of the carriage seek where a beam spot deceleration is carried out by decelerating the carriage until the beam spot speed becomes slower than a limit speed Vls, below which the lens seek deceleration is surely possible, but faster than the radial speed Vec of the eccentric tracks. During the ranges (b) and (c), the beam spot speed is faster than the radial speed Vec of the eccentric tracks; therefore, the number of the crossed tracks can be accurately counted. The range (d) is for deceleration of the beam spot by the lens seek, in which the beam spot, i.e. lens 20 is decelerated on the carriage driven reverse-ward by the fine tracking controller 3, so that the beam spot reaches a destination track. During the range (d), the crossed number of the tracks can be accurately counted because the beam spot speed is faster than the radial speed of the eccentric tracks. Operation of the circuits where the present invention is embodied is described hereinafter in detail referring to a block diagram in FIG. 5, a flow chart in FIG. 10 and a timing chart in FIG. 11. The beam spot has been tracing a start track by the fine tracking servo control and the position servo control, and then is instructed to jump toward the inner direction of the disk; therefore, the inner direction is referred to hereinafter as a forward direction, and the outer direction as the reverse direction. In FIG. 10, in the step (1) the fine tracking servo is disabled; the lens position servo is kept enabled; and a lens seek acceleration denoted with the range (a) is initiated by disabling the lens reversing signal RVS, which moves the beam spot reverse-ward, i.e. outward, and enabling the lens forwarding signal FWR, which moves the beam spot inward, to be input to the lens driving circuit 38 so that the lens 20 is accelerated on the carriage 2 toward the inner direction. Owing to the lens position servo, the carriage 2 automatically moves to follow the beam spot. The amount of acceleration/deceleration may be controlled by the duration of the signal FWR or RVS, according to the beam spot speed measured from the interval of track zero-cross signal TZCS. In the lens seek, the beam spot speed easily becomes faster than the radial speed of the eccentric tracks because the lens system movable on the carriage is originally designed to be light enough to move fast for the fine tracking control. There is a track-by-track speed control method, as one of the preferable methods of the lens seek. In this method, the beam spot speed crossing each track is watched and controlled, i.e. accelerated or decelerated, by the measurement of the interval of track zero-cross signal TZCS, so that the beam spot travels at an essentially constant speed without stopping at each track. Details of this method will be described later. At the end of the step (1), i.e. at the end of the range (a) of FIG. 9, the lens position signal is approximately zero, i.e. the lens 20 is approximately at the neutral position on the carriage 2. In the step (2), it is discriminated whether the beam spot speed is adequately higher than the radial speed of the eccentric tracks. This discrimination is carried out by MPU 7 by judging whether the beam spot speed exceeds the maximum radial speed Vec of the the eccentric tracks according to the time interval of the track zero-cross signal TZCS while the beam spot crosses a single track. If the judgement is YES, the step (3) is executed, but if the judgement is NO, the step (2) is repeated. In step (3), the carriage seek denoted with the range (b) is initiated by the disabling of the lens position servo control and by applying a signal INR, which accelerates the carriage toward the inner direction, to the carriage tracking control circuit 52. During this step, the lens position servo control is disabled by disabling the lens position servo switch signal PSS, and the lens lock is enabled by outputting the lens lock signal LKS. In step (4), which is carried out concurrently with the step (3), the carriage seek is discontinued by enabling the lens lock. In the step (5), it is discriminated whether the beam spot has reached, by the carriage seek of the range (b), a track where the carriage deceleration is to be started. The track where the carriage deceleration is to be started is typically chosen at approximately half of or a little less than the number of tracks to jump usually for the case where the acceleration speed or the deceleration speed, i.e. the acceleration or deceleration, is of the same degree. Accordingly, when the natures or the conditions, such as friction, etc., are different, the optimum location of the track where the carriage is to be decelerated becomes different from the half of the track jump number. Therefore, the track where the carriage deceleration is to begin may be calculated by a program installed in MPU 7 in advance. If the discrimination is NO, the step (5) is repeated. In the step (6), when the discrimination in the step (5) is YES, the carriage 2 is decelerated, shown in the range (c) of the carriage seek, by disabling the signal INR and applying the signal OUT, which decelerates the carriage toward the outer direction, to the carriage tracking controller 2. In the step (7), it is discriminated whether the beam spot speed has reached a predetermined speed which is slower than the limit speed Vls but faster than the maximum radial speed Vec of the eccentric tracks. If the discrimination is NO, the step (7) is repeated. In the step (8), when the discrimination in the step (7) is YES, the carriage seek deceleration is discontinued by disabling the signal OUT. In the step (9), the lens seek deceleration is started, as shown in the range (d), by the disabling of the lens lock signal LKS and outputting the lens position servo signal PSS. During this lens seek, the beam spot is decelerated by applying the lens reversing signal RVS to the lens driving circuit 38 so that the lens moves on the carriage toward the reverse direction of the track jumping, i.e. outward. Because of the lens position servo, the carriage is automatically decelerated to follow the beam spot according to the lens position signal generated by this lens seek. During the lens seek, the number of the tracks the beam spot has crossed is accurately counted from the track zero-cross signal TZCS. If the beam spot speed at the beginning of this lens seek deceleration is slow enough for the lens to be able to stop at a destination track, the beam spot is kept moving at the constant speed until the beam spot reaches a predetermined track, typically one track prior to the destination track. Strictly speaking, it is moved until a half track prior to the destination, as explained later about a deceleration period tc in FIG. 15. And then, the lens is decelerated as mentioned above by the lens reversing signal RVS to stop at the destination track. If the beam spot speed at the beginning of this lens seek is not slow enough for the lens to be able to surely stop at a destination track, the beam spot is decelerated by decelerating the lens by the lens reversing signal RVS gradually or step by step until the beam spot speed becomes slow enough for the lens to stop at a destination track. The track-by-track speed control can be preferably employed in the lens seek of the range (d) as well. Though in the description of this preferred embodiment the lens position servo is enabled in the step (9), this is not an absolute requirement to achieve this lens seek deceleration for the range (d), because the carriage can be stopped by other means, for example by the signal OUT, than this lens position servo. Though in the description of this preferred embodiment the lens lock is disabled in the steps (1) and (9) disabling the lens lock is not an absolute condition for the respective steps to achieve the operation of the invention. This is because the lens lock as well as the lens position servo is designed weaker than the lens seek signal; accordingly, the lens lock or the lens position servo may be continuously kept enabled from the start through all the steps, including a case where a spring or a rubber damper is employed as the lens lock means, then there is no way to disable the lens lock. A second preferred embodiment of the present invention is hereinafter described, referring to a flow chart in FIG. 12. The beam spot has been tracing a start track by the fine tracking servo control and the position servo control; and then is instructed to jump toward the inner direction of the disk. In the step (1), when the track jump is instructed, the carriage is accelerated by the output of the signal INR of the instructed direction and by the disabling of the lens position servo and the lens lock while the fine tracking servo is kept enabled for the beam spot to keep the start track, at which the track jump is initiated. Accordingly, the lens position signal is increased. In the step (2), it is discriminated whether the lens position signal, which now corresponds to the amount of the carriage's travel, indicates adequately larger than a predetermined amount of the radial displacement of the eccentric tracks. This predetermined amount is chosen much larger than the maximum radial displacement of the eccentric tracks. When the carriage has traveled this predetermined amount, the carriage speed is already larger than the maximum radial speed Vec of the eccentric tracks. If the discrimination is NO, the step (2) is repeated. In the step (3), when the discrimination in the step (2) is YES, the fine tracking servo is disabled and the lens lock is enabled. Accordingly, the lens lock force returns the lens to the neutral position on the carriage, causing the beam spot to move. At this time, the beam spot speed is high enough to accurately count the number of tracks the beam spot has crossed, eliminating the effect of the eccentric track rotation. In the step (4) the carriage acceleration is continued while the lens lock is enabled. In the step (5), it is discriminated whether the beam spot has reached a track where the carriage deceleration is to be started. The track where the carriage decelerated is to be started is typically chosen at approximately half of the number of tracks to jump usually for the case where the acceleration speed or the deceleration speed, i.e. the acceleration or deceleration, is of the same degree. Accordingly, when the natures or the conditions, such as friction, etc. of the apparatus, are different, the optimum location of the track where the carriage deceleration is to be started becomes different from the half of the track jump number. Therefore, the track where the carriage deceleration is to begin may be calculated by a program installed in the MPU 7 in advance. If the discrimination is NO, the step (5) is repeated. In the step (6), when the discrimination in the step (5) is YES, the carriage 2 is decelerated by the disabling of the signal INR and by the output of the signal OUT to the carriage tracking controller 2, in the same way as the step (6) for the range (c) of the carriage seek of the first preferred embodiment. In the step (7), the beam spot speed is discriminated in the same way as the step (7) of the first preferred embodiment. If the discrimination is NO, the step (7) is repeated. In the step (8), when the discrimination in the step (7) is YES, the carriage seek is discontinued by the disabling of the signal OUT. And, the lens seek deceleration is started in the same way as the range (d) of the previous embodiment, by the disabling of the lens lock signal LKS and preferably by enabling the lens position servo. During this lens seek deceleration, the beam spot is decelerated by applying the lens reversing signal RVS to the lens driving circuit 38 so that the lens moves on the carriage toward the reverse direction of the track jumping, i.e. outward in this case. Because of the lens position servo, the carriage is automatically decelerated to follow the beam spot according to the lens position signal generated by this lens seek. During the lens seek deceleration, the number of the tracks the beam spot has crossed is accurately counted from the track zero-cross signal TZCS. If the beam spot speed at the beginning of this lens seek is slow enough for the lens to be able to stop at a destination track, the beam spot is kept moving at the constant speed until the beam spot reaches a predetermined track, typically one track prior to the destination track; and then, the lens is decelerated to stop at the destination track. If the beam spot speed at the beginning of this lens seek deceleration is not slow enough for the lens to be able to surely stop at a destination track, the beam spot is decelerated by decelerating the lens gradually or step by step until the beam spot speed becomes slow enough for the lens to to stop at a destination track. The track-by-track speed control can be preferably employed in this lens seek as well. Though in the description of the second preferred embodiment the lens position servo and the lens lock are disabled in the step (1), and the lens lock is enabled in the step (3), whether the lens position servo or the lens lock is enabled or disabled is not an absolute condition, but preferable, for the respective step to achieve the operation of the invention. This is because the lens position servo as well as the lens lock is designed weaker than the fine tracking servo, accordingly, they may be continuously kept enabled from the start through all the steps, including a case where a spring or a rubber damper is employed as the lens lock means and there is no way to disable the lens lock. A preferred embodiment of the track-by-track speed control method preferably referred to as a method of the lens seek in the above-described two embodiments is hereinafter explained, referring to a principle diagram in FIG. 13, a circuit block diagram in FIG. 5, a flow chart in FIGs. 14 and a timing chart in FIG. 15, where a case of a cycle of an acceleration from a start track, a constant speed travel over a plurality of tracks and an arrival at a destination track is described as an example. This method is used while the carriage is stationarily fixed or following the beam spot by means of the lens position servo control. The circuit constitution for this method is essentially same as that used for the above-described two embodiments in FIG. 5. MPU 7 further includes a timer 7a and in the memory 7b there are stored: a predetermined acceleration period ta; a target period tb for a single track crossing corresponding to a target speed; a predetermined deceleration period tc; a number D of tracks to jump; a number D₀ of tracks for which the target speed is switched; an acceleration/ deceleration control pulse time t for the track-by-track speed control; a processing time tp; a high speed target time tbh which corresponds to a high target speed; a low speed target time tbl which corresponds to a low target speed. FIG. 13 shows a relation of the track number the beam spot is located at and the time elapsed on the X axis, and the beam spot speed, tracking error signal TES, track zero-cross signal TZCS and acceleration/ deceleration signal on the Y axis. When the MPU 7 is instructed from an upper processor to start the track access, if the number of track jumping instructed "n" is larger than (D₀ + 2) , a target speed is set at a high target speed Vh up to the (n - D₀ - 2) th track which is near the destination track as shown in FIG. 13; and after the (n - D₀ - 2) th track the target speed is set at a low target speed Vl, where D₀ is a number chosen for the beam spot to easily stop at the destination track from the low target speed while crossing D₀ tracks, allowing enough margin considering the inertia, etc., of the movable part of the lens system. The number D₀ is referred to hereinafter as a speed switching track number. The amount of the high target speed Vh is chosen as an available highest speed at which the lens control is possible, in order to enhance the high speed capability. The amount of the low target speed Vl is chosen as a speed from which the beam spot can stop at the destination track while crossing a single track, for example, approximately 200 us per track. When the instructed track jump number n is less than (D o + 2), the target speed is set at only Vl as shown in FIG. 13 B. Dotted lines in FIG. 13 show an actual speed Vr of the beam spot when the beam spot is controlled by the target speeds. Referring to a flow chart in FIGs. 14, the operation is hereinafter explained. It is assumed now that the servo-on signal SVS from MPU 7 is enabling the servo switch 35, and the disabled lock-on signal LKS is disabling the lock switch 37, until the track jump instruction is received. In other words, it is assumed that the fine tracking servo loop is closed so that the track actuator 21 is servo-controlled by the control signal TCS from the tracking error signal TES, accordingly the beam spot is tracking the track 10a. In the step (1), at this state, a track jump procedure shown in FIG. 14 starts when a track jump instruction as well as its track jump number D is given from the upper processor to MPU 7. In this embodiment, the track jump is instructed toward the inner direction. At first, MPU 7 subtracts [2] from the given track jump number D (= n), and stores it in the memory 7b. The subtraction of this [2] is because an acceleration is made during the travel toward the second track (10b of FIG. 15 and a deceleration is made for stopping at the destination, D-th track; therefore, these two times of track crossings are subtracted from D and a track-by-track speed control is carried out (D - 2) times for the residual tracks. Next, MPU 7 compares the track number D (= n - 2) with the speed switching track number D₀; then, if D > D₀, this is the case of FIG. 13A, where the target speed tb is set with the high target speed Vbh; and the acceleration period ta is set with tah for the acceleration up to the speed Vh. If D ≦ D₀, this is the case of FIG. 13B, where the target speed tb is set with the low target speed Vbl; and the acceleration period ta is set with tal for the acceleration up to the speed Vl. Next, MPU 7 disables the servo signal SVS to disable the servo switch 35, so as to open the servo loop of the tracking error signal TES. Thus, the servo control of the tracking error signal TES is inhibited. This is necessary to smoothly control the travel of the beam spot as instructed by MPU 7, and to acquire an accurate actual speed from the track error signal TES. Next, MPU 7 checks which direction the track jump is directed to, i.e. toward the outer direction or inner direction along the radius of the optical disk 1, and then, if the direction is toward the outer, a procedure for the outer direction is executed. The procedure for the track jump toward the outer side are almost the same as the track jump process toward the inner side shown in the step (2) and the following. In the step (2), MPU 7 enables the lens forwarding signal FWR for the acceleration period ta so as to apply the acceleration voltage +V for the period ta. That is, after outputting the lens forwarding signal FWR, MPU 7 sets the timer 7a with a complement of the acceleration period ta in the memory 7b, and then starts the timer 7a. The timer 7a being set with the complement of the acceleration period ta counts the clock pulses, and generates a count-up signal when the period ta is counted. When the timer 7a generates the count-up signal, MPU 7 terminates the timer 7a so as to disable the lens forwarding signal FWR. Therefore, the first switch 381 of the driver circuit 38 is enabled for the period ta; the acceleration voltage +V is applied to the adder amplifier 480 for the period ta; an acceleration current is applied as the track driving current TDV from the power amplifier 39 to the track actuator 21 for the period ta; accordingly, the object lens 20 (i.e. the beam spot) is accelerated up to the high target speed Vh. In the step (3), MPU 7 observes the track zero-cross signal TZCS and detects a timing when the track zero-cross signal TZCS changes from "0" state to "1" state, i.e. a rise transition of the track zero-cross signal TZCS, and then starts measuring an interval of the track zero-cross signals TZCS. That is to say, MPU 7 reads out the target period tb (tbh or tbl) corresponding to the target speed Vh or Vl respectively, from the memory 7b; sets a complement of the target period tb in the timer 7b; and then starts the timer 7b. In the step (4), MPU 7 observes the track zero-cross signal TZCS and detects the rise transition, i.e. from "0" state to "1" state, of the track zero-cross signal TZCS ; and then stops the timer 7a so as to terminate measuring the interval of the track zero-cross signals TZCS. The timer 7a being set with the complement of the target period tb generates a count-up signal when the period tb is counted. Therefore, if the time interval T between a rise transition, at which the timer 7a is started, and the next rise transition, at which the timer is terminated, is T ≧ tb (i.e. the actual speed is equal to or slower than the target speed), the timer 7a has been already counted up when the timer 7a is terminated. On the contrary, if T ≦ tb (i.e. the actual speed is faster than the target speed), the timer 7a is not counted up yet when the timer 7a is terminated. In the step (5), MPU 7, therefore, checks whether the timer 7a is counted up. On finding a count-up, which means the actual speed is equal to or slower than the target speed, the lens acceleration is performed. The timer 7a continues to count until the timer 7a terminates even after counting up; therefore, MPU 7 reads out a count-over amount Δt and calculates therefrom a control pulse period t. Then, MPU 7 outputs the lens forwarding signal FWR for the lens acceleration; sets a complement of the control pulse period t in the timer 7a; and initiates the timer 7a. On counting the control pulse period t, the timer 7a counts up; therefore, MPU 7 terminates the timer 7a on counting-up of the timer 7a so as to disable the lens forwarding signal FWR. Thus, the first switch 381 of the driver circuit 38 is enabled for the period t; the acceleration voltage +V is applied to the adder amplifier 480; the acceleration current as the track driving current TDV is applied from the power amplifier 39 to the track actuator 21 for the period t; accordingly, the lens 20 is accelerated. If the actual speed is equal to the target speed, then Δt = 0, accordingly t = 0; therefore, the first switch 381 is enabled only for a moment. In the step (6), on the other hand, when the timer 7a has not counted up yet, MPU 7 discriminates that the actual speed is faster than the target speed, and decelerates the lens. For this purpose, MPU 7 reads out the counted number n of the timer 7a. The count-up number N of the timer 7a is known in advance; therefore, the difference (tb - T) = Δt between the target period tb and the interval (period T) of the rise transitions of the track zero-cross signals TZCS is acquired from (N - n), so as to calculate the control pulse period t. Then, MPU 7 enables the lens reversing signal RVS for lens deceleration; sets a complement of the control pulse period t in the timer 7a; and then initiates the timer 7a. On counting up of the timer 7a, MPU 7 stops the timer 7a and disable the lens reversing signal RVS. Therefore, the second switch 382 of the driver circuit 38 is enabled for the period t; the deceleration voltage -V is applied to the adder amplifier 480; the deceleration current as the track driving current TDV is applied from the power amplifier 39 to the track actuator 21 for the period t; thus the lens 20 is decelerated. In the step (7), as a rise transition of the track zero-cross signal TZCS has been detected in step (4), MPU 7 renews the track jump number D in the memory 7b to (D - 1) after the step (5) or the step (6). And, MPU 7 checks whether D is [0] or not. Next, MPU 7 discriminates whether the renewed track jump number D is less than the speed switching track number D₀ or not. That is, if the remaining track jump number D is less than the speed switching track number D₀ (i.e. D ≦ D₀), and excluding the case where the target speed is set with the low target speed Vl already, the target speed tb is renewed with the low target speed Vl. If not, (i.e. D > D₀), the target speed is kept to remain at the high target speed Vh. Next, MPU 7 discriminates whether D = 0 or not. If D ≠ 0 is true, it means that the beam spot has not arrived at the track prior next to the target track yet. If D ≠ 0 is not true (i.e. D = 0), the beam spot has arrived at the track prior next to the target track. In the step (8), if D ≠ 0 is true, the measurement of the interval of the track zero-cross signals and the above-described track-by-track speed control is carried out again. For this purpose, MPU 7 sets the target period tb in the timer 7a; initiates the timer 7a; and returns to the step (4). At this time, since the timing when the rise transition was detected in the step (4) it has elapsed over the control pulse period t and the process period tp needed for the processes in the steps (5) or (6) (7). Therefore, the target period tb in the timer 7a is set to (tb - t - tp). In the step (9), thus repeating the track-by-track speed controls, the beam spot arrives at the track prior next to the destination track, and then it is discriminated that D = 0. Now, MPU 7 instructs the deceleration of the lens to stop. That is, observing the track zero-cross signal TZCS, MPU 7 discriminates whether a fall transition, i.e. from "1" to "0", of the track zero-cross signal TZCS, takes place indicating a timing to begin the lens deceleration. The fall transition in the track zero-cross signal TZCS indicates that the the beam spot crosses to an adjacent track. On detecting the fall transition of the track zero-cross signal TZCS, MPU 7 outputs the lens reversing signal RVS so as to start the lens deceleration; sets a complement of the predetermined deceleration period tb in the timer 7a; and then initiates the timer 7a. When the timer 7a counts up, i.e. the period tc is counted, MPU 7 disables the lens reversing signal RVS and outputs the servo-on signal SVS, then the track jumping is terminated after a predetermined adjusting period has elapsed. That is to say, the second switch 382 of the driving circuit 38 is enabled for the period tc; the deceleration voltage -Vc is applied to the adder amplifier 480 for the period tc; a deceleration current as a track driving current TDV is applied from the power amplifier 39 to the track actuator 21 for the period tc; and accordingly the lens, i.e. the beam spot, is decelerated to stop. Then, the servo switch 35 is enabled to close the servo loop of the tracking error signal TES; the track driving current TDV according to the control signal TCS of the phase compensation circuit 33 is applied from the power amplifier 39 to the track actuator 21; accordingly, the fine tracking servo control is started. When this servo switch 35 is enabled, the beam spot is located on the destination track and its speed is zero; therefore, the track servo control can be smoothly started. Though in the description of the preferred embodiment of the track-by-track speed control the lens lock is kept disabled, the track-by-track speed control can be also achieved while the lens lock is kept enabled. This is because the lens lock is weaker than the signal of the lens seek. For a case where the lens lock is mechanically made with a spring or a rubber damper, there is no way to disable the lens lock. Details of the timing chart of FIGs. 15 are hereinafter described. FIGs. 15 show waveforms when a track-jumping carried out from track 10a to the fifth-next track 10f. At first, disabling the fine tracking servo; outputting the lens forwarding signal FWR; the acceleration current is applied for the period tah so that the beam spot reaches the target speed Vh. Next, a time interval T₁ between the second and third rise transitions of the track zero-cross signal TZCS is measured by enabling the timer 7a so that the actual speed is obtained, and then is compared with the target period tbh which corresponds to the target speed Vh. If tbh > T₁, it means the actual speed is faster than the target speed Vh according to the step (6); therefore, the lens reversing signal RVS is output for the period t which corresponds to the speed difference (tb - T₁); accordingly, a deceleration current is applied to the actuator coil 21 for the period t so as to retard the actual speed to the target speed Vh. Next, it is attempted to measure the interval between the third and fourth rise transitions of the track zero-cross signal TZCS; however, the measurement of the interval after the the third rise transition of the track zero-cross signal TZCS is impossible because the track-by-track speed control (the steps (5), (6) and (7)) is being carried out after the third rise transition of the track zero-cross signal TZCS as mentioned above. Therefore, after entering the track-by-track speed control, the timer 7a is enabled; the interval till the fourth rise transition is measured; and the target period tb is set (tb - t - tp) in order to meet this interval. In this example, this interval T₂ is equal to the target period tb (= tb - t -tp); therefore, an acceleration/deceleration is not actually carried out according to the step (5). In a similar way, the track-by-track speed control is carried out between the fourth and fifth rise transitions. In this case the interval T₃ is larger than (tb - t -tp), i.e. the actual speed is slower than the target speed Vh; therefore, an acceleration is carried out for the period t. Thus, when the beam spot controlled at the high target speed Vh arrives at the (n - 4) th track (10n - 4), the target speed is switched to the low target speed Vl, because the beam spot is now close to the destination track. Further in a similar way, at the (n - 3) th and (n - 2) th rise transitions of the track zero-cross signal TZCS the track-by-track speed control is carried out at the low target speed Vl. A deceleration current is applied for a period tc in synchronization with the n-th fall transition of the track zero-cross signal TZCS so as to decelerate the lens (i.e. the beam spot) to stop. Immediately after this, the fine tracking servo control is enabled. The track jump number D is also renewed by the rise transition of the track zero-cross signal TZCS. Track jumping toward the outer direction is almost the same as mentioned above in the steps (2) to (9) for the inward jump, because the difference is only in that the direction is reversed. That is, all the procedures are carried out in the same way as those for the jump toward the inner direction except: the lens forwarding signal FWR via the forward switch 381 is replaced by the lens reversing signal RVS via the reverse switch 382 in step (2) and (5); and the lens reversing signal RVS via the reverse switch 382 is replaced by the lens forwarding signal FWR via the forward switch 381 in the steps (6) and (9); the detection of the rise transitions of the track zero-cross signal TZCS in the steps (3) and (4) are replaced by the detection of the fall transitions; and the discrimination of TZCS = 0 in step (9) is replaced by the discrimination of TZCS = 1. As described above, the beam spot BS, after being initially accelerated is controlled at the high target speed Vh, thus the beam spot travels at the possible highest speed; and decelerated to a low target speed Vl, which is easy for the beam spot to surely stop, in the vicinity of the destination track, then the beam spot is surely positioned at the destination track, while the long distance quick track jump is achieved without stopping at each track. For the case where a great number of tracks, for example a thousand tracks, are to be jumped, a prior art jump requires 40 ms for the coarse seek (carriage seek) including the period required for entering the fine tracking servo, 1 ms for the confirmation by reading the track ID, and 6 ms for a fine seek (lens seek) for 20 tracks, consequently 47 ms in total; however, according to the present invention an accurate track jump is achieved e.g. with 2 ms for the 10 tracks of the lens seek acceleration, 30 ms for the coarse seek, and 6 ms for the 20 tracks of the lens seek deceleration, consequently 38 ms in total, without requiring the 1 ms for the confirmation by reading the track ID. The track-by-track speed control is carried out, as described already, by measuring the time interval between rise transitions of the track zero-cross signal TZCS. This speed control, i.e. the measurement of the time interval, is carried out easily with an addition or modification of a program without modifying the hardware. Because the actual speed is compared with the target speed according to the target period loaded in the memory, the compared result is given from the existence of the count-up signal. Therefore, the complicated comparison procedure is simplified to reduce the process steps, resulting in a fast operation. Thus, the track-by-track speed control of the present invention is beneficial for an access of less than about a hundred tracks. Though in the above-described embodiment the target speeds are set with two speeds, high and low, the target speeds can be set with more than two speeds. Furthermore, the target speed can be a single speed as long as the beam spot can stop surely at the destination track without being affected by the eccentric movement of the tracks. Though in the above-described embodiments the tracking error signal TES is generated by means of a push-pull method, other known methods may be of course used to generate the tracking error signal. Though in the above-described embodiments the detection of the beam spot speed difference is carried out by loading the timer 7a with a target period, the detection of the beam spot speed difference can be carried out also by measuring a time interval of the transitions in the track zero-cross signal TZCS by the timer 7a and then comparing it with the target period. Furthermore, the timer can be constituted with a software timer. Or, the gain of the feedback loop of the speed control during the track jump may be varied. Though in the above-described embodiments the explanations are made with the cases where the fine tracking servo control is carried out by moving a lens along the radial direction of the tracks, it is apparent that the present invention can be applied to a fine tracking servo of a galvano mirror type. In the above-described embodiments the embodiments are made using a positive logic; however, it is apparent that a negative logic is also applicable to an embodiment of the present invention. Though the descriptions are made for the optical disk apparatus the present inventions are applicable to an optical card apparatus, etc., using an optical memory media having a known track, and not only to a reflection type but also to a transparent type as well.

Claims: A method of track jumping for positioning a light beam spot (BS) on a destination track (D) of a recording medium (1) in an optical recording apparatus, the recording medium having eccentric tracks, the optical recording apparatus having a carriage (2) and a fine tracking device (20) mounted on the carriage for moving the beam spot (BS), the method including the steps of:- (d) detecting light reflected from the recording medium (1) to generate a tracking error signal (TES), and counting a number of zero crossings, indicating crossing of tracks by said beam spot (BS), in the tracking error signal since the track jumping was initiated; and (e) accelerating/decelerating the fine tracking device (20) according to a difference between the counted number of crossed tracks and said instructed number of tracks to be jumped, to reach the destination track (D); characterised by the prior steps of:- (a) accelerating the fine tracking device (20), while enabling a position servo (5) which moves the carriage so that the fine tracking device returns to its neutral position on the carriage, until the beam spot speed becomes faster than a radial speed (V ec ) of the eccentric tracks, said radial speed (V ec ) being a maximum rate of displacement of the tracks along a nominal radius of the disk, resulting from eccentricity of the disk; (b) further accelerating the carriage (2) until said beam spot reaches a track whose track number is calculated, by a program prepared in advance, according to an instructed number of tracks (n) to be jumped; and (c) decelerating the carriage (2) until the beam spot speed becomes slow enough for the fine tracking device (20) to seek the destination track (D). A method according to claim 1, wherein the step (b) further comprises the substeps of:- (b1) measuring a time interval (T) between zero crossings in said tracking error signal (TES), for each track crossing, while said beam spot (BS) is moving; (b2) obtaining a time difference (Δt) between said time interval (T) of zero crossings and a first predetermined time interval (t b ) which corresponds to a predetermined speed faster than said radial speed (V ec ) of said eccentric tracks; and (b3) accelerating or decelerating said beam spot (BS) by accelerating/decelerating said fine tracking means (20) according to said time interval difference (Δt) so as to make said time interval (T) of said zero crossings essentially equal to said first predetermined time interval (t b ), without stopping said beam spot at each track. A method according to claim 1 or 2, wherein step (a) is modified as follows: (a1) accelerating the carriage (2) while enabling a fine tracking servo (3) of said fine tracking device (20) so as to keep the beam spot (BS) on a start track at which the beam spot was initially located, until the carriage speed exceeds said radial speed (V ec ) of the eccentric tracks; and (a2) disabling the fine tracking servo (3) and enabling a lock, by which the fine tracking device (20) returns to a neutral position on the carriage (2), so that the beam spot (BS) is accelerated. A method according to claim 1 or 2, wherein the step (e) further comprises the substeps of:- (e1) measuring a time interval (T) between zero crossings in said tracking error signal (TES), for each track crossing, while said beam spot (BS) is moving; (e2) obtaining a time difference (Δt) between said time interval (T) of zero crossings and a second predetermined time interval (t b ); and (e3) accelerating or decelerating said beam spot (BS) according to said time difference (Δt) so as to make said time interval (T) of said zero crossings essentially equal to said second predetermined time interval (t b ), without stopping said beam spot at each track. A method according to claim 4, wherein steps (e1) to (e3) are repeated with a series of lengthening second predetermined time intervals (t b ) and a corresponding series of instructed tracks to be jumped to, such as to make the beam spot (BS) progressively approach said destination track (D). A method according to claim 5, wherein the final second predetermined time interval is chosen long enough, and the final instructed track to be jumped to is chosen near enough to said destination track, for the beam spot (BS) to be able to stop surely at said destination track (D). A method according to any preceding claim, wherein said number of tracks up to which the step (b) is continued is programmed to be approximately half said instructed number of tracks to be jumped to reach said destination track (D). A method according to any preceding claim, wherein the step (e) is carried out while said position servo (S) is enabled, said position servo keeping said fine tracking device (20) at a neutral position on said carriage (2) by moving said carriage according to a position signal, said position signal becoming zero when said fine tracking device (20) is at said neutral position on said carriage (2) and becoming positive or negative when deviating from said neutral position to one direction or to the opposite direction respectively. A method of track jumping for positioning a light beam spot (BS) on a destination track (D) of a recording medium (1) in an optical recording apparatus, the recording medium having eccentric tracks, the optical recording apparatus having a carriage (2) and a fine tracking device (20) mounted on the carriage for moving the beam spot (BS), the method comprising the steps of:- (i) detecting light reflected from the recording medium (1) to generate a tracking error signal (TES); (ii) detecting zero crossings, which indicate that the beam spot (BS) has crossed a track, in the tracking error signal (TES) and measuring the time interval between successive zero crossings while the beam spot is moving; and characterised by the further steps of :- (iii) obtaining a time difference (Δt) between the time interval (T) between successive zero crossings and a predetermined time interval (t b ); and (iv) accelerating or decelerating the fine tracking device (20) for moving the beam spot (BS) according to the time difference (Δt), so as to make the time interval (T) between successive zero crossings essentially equal to the predetermined time interval (t b ), whereby the beam spot is moved at a predetermined speed without stopping at each track. A method according to claim 9, wherein said steps (iii) and (iv) are repeated for a series of lengthening predetermined time intervals (t b ) and corresponding series of instructed tracks to be jumped to, so as to make the beam spot (BS) progressively approach the destination track (D). A method according to claim 10, wherein the final predetermined time interval is chosen long enough, and the final instructed track to be jumped to is chosen near enough to the destination track, for the beam spot to be able to stop surely at the destination track (D). A method according to claim 9, 10 or 11, wherein the steps (iii) and (iv) are carried out while said carriage (2) is stationarily fixed. A method according to claim 9, 10 or 11, wherein the steps (iii) and (iv) are carried out while a position servo (5) is enabled, said position servo keeping said fine tracking device (20) at a neutral position on said carriage (2) by moving said carriage according to a position signal, said position signal becoming zero when said fine tracking device (20) is at said neutral position on said carriage (2) and becoming positive or negative when deviating from said neutral position to one direction or to the opposite direction respectively. A method according to any preceding claim, wherein said time interval (T) between zero crossings in said tracking error signal (TES) is measured by an interval of adjacent two same-type transitions of zero crossings, said transitions being from one polarity to an opposite polarity, said zero crossings being generated whenever the beam spot (BS) passes the centre of a track.

Inventors: YANAGI SHIGENORI, YANAGI, SHIGENORI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1988
Main classification: G11B7
IPC codes: G11B7