Optical disc drive and method of controlling rotation speed of a spindle motor for rotating an optical disc

Abstract: An optical disc drive has a spindle motor for rotating an optical disc; an optical pick-up which can move at least in a radial direction of the optical disc; a sensor for detecting the rotation of the motor; and a spindle servo for controlling the rotation number of the spindle motor based on signals from the sensor. This spindle servo is adapted to be operated at the time of changing the rotation number of the spindle motor, e.g. at the time of changing rotation speed of the spindle motor in multiple steps given by integral multiples of 1 or at the time of changing rotation speed when the optical pick-up moves in the radial direction of the optical disc. According to this optical disc drive, it takes a relatively short time until the rotation speed of the motor reaches the target rotation speed and its rotation becomes stable.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an optical disc drive for playing back or recording and playing back an optical disc and a method of controlling rotation number of a spindle motor for rotating the optical disc. Description of the Prior Art There are known optical disc drives for recording and playing back (reproducing) an optical disc such as a CD (compact disc), CD-ROM, CD-R (CD-Recordable) and CD-RW. These optical disc drives are equipped with a motor (spindle motor) for rotationally driving the optical disc when recording and playing back (reproducing) the optical disc. The motor is driven under the rotation number control (rotation speed control). This control is called as a spindle servo. Such rotation number control is mainly carried out based on the information read out from the optical disc by the optical pick-up. Namely, in the case of optical discs to which no data has yet been recorded, such control is carried out based on WOBBLE PWM signal outputted from the WOBBLE servo controller. This method of the spindle servo is called as a WOBBLE servo. On the other hand, in the case of optical discs to which data has been recorded, such control is carried out based on EFM PWM signal outputted from the CD servo controller. This method of the spindle servo is called as an EFM servo. In these optical disc drives, there is a case that the rotation speed of the optical disc (that is, the rotation number of the optical disc in a predetermined period) is required to change during the rotation of the optical disc. For example, such a change is to be carried out when the optical disc rotating at a reference rotation speed (1 X) is accelerated to the six times rotation speed (6X). In this case, the WOBBLE servo or the EFM servo is once turned off, and then the spindle motor is kicked (that is, accelerated by applying a full level voltage to the spindle motor) in a predetermined time, and after the predetermined time, the WOBBLE servo or the EFM servo is turned on again to perform the spindle servo so that the rotation speed reaches the target rotation speed. However, this method involves the following problem. According to the spindle servo using the WOBBLE servo or the EFM servo, it is not possible to obtain (read out) the information from the optical disc until the increased rotation speed of the optical disc becomes a predetermined speed. Further, it is difficult to increase the gain of the servo since hunching is likely to occur by doing so. Therefore, it takes a relatively long time until the rotation number reaches the target rotation number and the rotation becomes stable (hereinafter, this time is referred to as "time until obtaining stability"). Further, in the optical disc drive, the rotation number of the optical disc differs between the case where the optical pick-up is in the inner region and the case where the optical pick-up is in the outer region (the former case is faster than the later case for 2.5 times at the maximum) in order to obtain the constant linear velocity. In the optical disc drive described above, when the optical pick-up has been moved along the radial direction, the WOBBLE servo or EFM servo is also operated for controlling the rotation number of the motor. Therefore, when the optical pick-up is moved (jumped) from e.g. the outer region to the inner region, there arises the same problem as discussed above. Further, during the increasing of the rotation speed of the motorto a different rotation speed level and during the movement (jump) of the optical pick-up, the spindle servo (WOBBLE servo or the EFM servo) does not work as described above. An optical disc drive of the kind defined by the precharacterizing features of claim 1 is known from the US-A-4542423. The US-Patent 5 590 105 discloses an optical disc drive system provided with a spindle motor, a sensor, a pick-up device, a control means and a switching means for switching between constant rotational speed controlling signal and constant linear velocity (CLV) controlling signal. The US-Patent 5 289 450 discloses switching between CLV servo control process and CAV servo control process. The JP-A-08-31088 discloses switching between CLV control and FG control. The CLV control is carried out using wobble signal obtained from an optical disc. The US-Patent 5 627 811, Fig. 4 discloses that the spindle motor element 23 of an optical disc drive is appropriately controlled or switched between a disc servo unit and a disc servo unit. The disc servo unit performs CLV servo control, and the disc servo unit is driven by the signal from a motor speed sensor. The US-Patent 5 477 522 discloses switching from CLV servo to another servo driven by the signal from a sensor when a rough search (track jump) is carried out. The US-Patent 5 528 574 discloses switching between CLV mode and CAV mode of an optical disc drive. The JP-A-07-287932 discloses switching between CLV mode and CAV mode during track search of an optical disc drive. SUMMARY OF THE INVENTION In view of the above problem, the object of the present invention is to provide an optical disc drive which can obtain the stable rotation in a short time when the rotation speed of the motor is to be changed. This object is attained by the features of claim 1. Further, another object of the present invention is to provide a method of controlling rotation number of a spindle motor which can obtain the stable rotation in a short time when the rotation speed of the motor is to be changed. This object is attained by the steps of claim 9. In this optical disc drive, it is preferred that the rotation number control of the motor before and after the changing of the rotation speed of the motor is carried out by the first rotation number control means. In the optical disc drive, it is preferred that the sensor is constructed from a Hall element which outputs FG signals having a predetermined frequency, and the second rotation number control means includes a FG servo using the FG signals. Further, in the optical disc drive, it is preferred that the first rotation number control means is a WOBBLE servo using WOBBLE PWM signals from the optical disc or an EFM servo using EFM PWM signals. Preferably, the method further comprises the step of switching the second spindle servo means to the first spindle servo means after the rotation number of the spindle motor has reached the predetermined rotation number. In this method, it is preferred that the rotation speed of the spindle motor is adapted to be changed in multiple steps given by integral multiples of 1, and the rotation speed changing step is carried out when the rotation speed of the motor is to be changed between the multiple steps. Further, it is also preferred that the rotation speed of the spindle motor is adapted to be changed depending on the radial position of the optical pick-up with respect to the optical disc, and the rotation speed changing step is carried out when the rotation speed of 1 the motor is to be changed due to the radial movement of the optical disc. According to the method, it is also possible to shorten the time required until the increased rotation of the spindle motor reaches a target rotation number and 1 it becomes stable. Other objects, structures and advantages of the present invention will be apparent when the following description of the preferred embodiment is considered taken in conjunction with the accompanying draw- 2 ings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an illustration which shows a state that the optical disc drive according to the present invention is connected to an computer. Fig. 2 is a block diagram which shows an embodiment of the optical disc drive according to the present invention. Fig. 3 is a timing chart which shows the ENCODE EFM signal from the EFM/CDROM encoder and the ENCODE EFM signal from the laser control section in the optical disc drive according to the present invention. Fig. 4 is a timing chart which shows the ATI P-SYNC signal, the SUBCODE-SYNC signal from the SYNC signal generating/ATIP decoder and the ATIP error signal in the optical disc drive according to the present invention. Fig. 5 is a timing chart which shows the ATI P-SYNC signal, the SUBCODE-SYNC signal from the SYNC signal generating/ATIP decoder and the SUBCODE-SYNC signal from the CD servo controller in the optical disc drive according to the present invention. Fig. 6 is a timing chart which shows the 1T Biphase ATIP timing, the WOBBLE signal and the digitized WOBBLE signal in the optical disc drive according to the present invention. Fig. 7 is a timing chart which shows the BIDATA signal, the BICLOCKsignal and the ATIP-SYNC signal in the optical disc drive according to the present invention. Fig. 8 is a chart which shows a format of an ATIP frame. Fig. 9 is a timing chart which shows the ATIP-SYNC signal and the SUBCODE-SYNC signal in the optical disc drive according to the present invention. Fig. 10 is a timing chart which shows an input signal inputted to the peek/bottom detecting circuit, the amplitudes of the inputted signal (envelope), and the PEEK signal and the BOTTOM signal in the optical disc drive according to the present invention. Fig. 11 is a timing chart which shows the SUBCODE-SYNC signal from the CD servo controller and the C1 ERROR signal in the optical disc drive according to the present invention. Fig. 12 is a timing chart which shows the DATA signal of an audio format, the LRCLOCK signal and the BITCLOCK signal in the optical disc drive according to the present invention. Fig. 13 is a timing chart which shows the SUBCODE-SYNC signal from the CD servo controller, the FRAM SYNC signal, the HF signal (EFM signal) in the optical disc drive according to the present invention. Fig. 14 is a drawing which shows a format of the Q data of 96 bits. Fig. 15 is a drawing which shows one subcode frame. Fig. 16 is a flow chart which shows the operations of the control for rotation number of the spindle motor according to the present invention. Fig. 17 is another flow chart which shows the operations of the control for rotation number of the spindle motor according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to the appended drawings, a detailed description will now be given for the preferred embodiments of an optical disc drive according to the present invention. Fig. 1 is an illustration which shows an optical disc drive 1 of the present invention in a connected state with a computer, and Fig. 2 is a block diagram which shows an embodiment of the optical disc drive 1 of the present invention. The optical disc drive 1 shown in these drawings is a CD-R drive for recording and playing back an optical disc (CD-R/CD-Recordable) 2. In optical discs 2 of the type of CD-R, there is formed a spiral pre-groove (WOBBLE) which is not shown in the drawing. The pre-groove meanders at a predetermined period (22.05kHz at the reference rotation speed (1X)), and ATIP (Absolute Time in Pre-groove) information (time information) is previously recorded with the pre-groove. The ATIP information is recorded by being biphase modulated and frequency-modulated at a carrier frequency of 22.05kHz. The Pre-groove functions as a guide groove when forming pits and lands (recording pits and lands) for the optical disc 2. Further, the information recorded with the pre-groove is reproduced and then utilized to control the rotation speed of the optical disc 2 and specify a recording position (absolute time) on the optical disc 2. The optical disc drive 1 is equipped with a turntable and a spindle motor 8 for rotating the turntable (that is, for rotating the optical disc), and includes a rotation driving mechanism (not shown in the drawings) for rotating the turntable when the optical disc is loaded thereon. Further, in the vicinity of the spindle motor 8, there is provided a Hall element 9 for detecting the rotation of the spindle motor 8. The Hall element 9 outputs FG signals (in the form of sine wave) is arranged near the spindle motor 8. The cycle of the FG signal corresponds to the rotation number of the spindle motor 8. Further, the optical disc drive 1 includes an optical head (hereinafter, referred to as "optical pick-up") 3 capable of movement along a radial direction of the loaded optical disc 2 (i.e., along a radial direction of the turntable); an optical pick-up moving mechanism (not shown in the drawings) equipped with a sled motor 5 to move the optical pick-up 3, that is an optical pick-up base of the optical pick-up 3 along the radial direction of the turntable; drivers 6 and 11; PWM signal smoothing filters 7 and 12; control means 13; a laser control section 14; an HF signal generating circuit 15; an HF signal gain switching circuit 16; a peak/bottom detection circuit 17; an error signal generating circuit 18; a WOBBLE signal detection circuit 19; a CD servo controller 21; a WOBBLE servo controller 22; a FG signal digitization circuit 23; an EFM/CD-ROM encodercontrol section 24; memories 25, 26 and 29; a SYNC signal generating/ATIP decoder 27; a CD-ROM decoder control section 28; an interface control section 31; clocks 32, 33, 34 and 35; and a casing 10 which houses all these elements. Hereinbelow, the radial direction of the optical disc 2 along which the optical pick-up 3 is capable of moving will simply be referred to as the "radial direction." The optical pick-up 3 includes (but not shown in the drawings) an optical pick-up base equipped with a laser diode (light source) and a photodiode (light-receiving element), and an objective lens (converging lens). The operation of the laser diode is controlled by the laser control section 14. The objective lens is supported by suspension springs provided on the pick-up base and can be moved, with respect to the pick-up base, along the radial direction and the rotational axial direction of the optical disc 2 (i.e., the axial direction of the turntable). In this way, if the objective lens is shifted from its neutral position (central position), the restoring force of the suspension springs will bias the objective lens toward the neutral position. Hereinbelow, the rotational axial direction of the optical disc 2 will simply be referred to as "axial direction." Further, the optical pick-up 3 includes an actuator 4 to move the objective lens respectively in the radial direction and the axial direction with respect to the pick-up base. The control means 13 is generally constructed from a microcomputer (CPU), and it carries out control of the entire optical disc drive 1, including control of the optical pick-up 3 (actuator 4), sled motor 5, spindle motor 8, laser control section14, HF signal gain switching circuit 16, peak/bottom detection circuit 17, CD servo controller 21, WOBBLE servo controller 22, EFM/CD-ROM encoder control section 24, memories 25, 26 and 29, SYNC signal generating/ATIP decoder 27, CD-ROM decoder control section 28, interface control section 31 and the like. Further, addresses, data, commands and the like from the control means 13 are inputted via an address/data bus 36 to the EFM/CD-ROM encoder control section 24, memory 26, SYNC signal generating/ATIP decoder 27, CD-ROM decoder control section 28, interface control section 31 and the like. An separate apparatus (in the present embodiment, a computer 41) can be freely connected to (and disconnected from) the optical disc drive 1 via the interface control section 31, and this makes it possible for the optical disc drive 1 and the computer 41 to communicate with each other. As for the interface control section 31, it is possible, for example, to use an ATAPI (IDE) (ATAPI Standard), SCSI (SCSI Standard) or the like. The interface control section 31 constitutes a transmission means. A keyboard 42, a mouse 43 and a monitor 44 are connected to the computer 41. In this regard, it is to be notedthatthe HF signal generating circuit 15, the HF signal gain switching circuit 16, the peak/bottom detection circuit 17, the error signal generating circuit 18, the WOBBLE signal detection circuit 19, the CD servo controller 21 and the WOBBLE servo controller 22 constitute a signal processing means. Next, the operation of the optical disc drive 1 will be described. While carrying out focus control, tracking control, sled control and rotation control (rotation speed control), the optical disc drive 1 records (writes in) and plays back (read out) information (data) to and from a predetermined track of the optical disc 2. Hereinbelow, the operations carried out during (1) recording, (2) playback (reproduce), (3) focus control, tracking control and sled control, and (4) rotation control (rotation speed control) will be described in this order in this order. First, as shown in Fig. 2, a predetermined COMMAND signal from the control means 13 is inputted into the CD servo controller 21. Further, a predetermined COMMAND signal from the control means 13 is inputted into the WOBBLE servo controller 22. These COMMAND signals transmitted from the control means 13 to the CD servo controller 21 and the WOBBLE servo controller 22, respectively, are signals indicating predetermined commands (e.g., to start controls and the like). Then, a predetermined STATUS signal from the CD servo controller 21 is inputted into the control means 13. Further, a predetermined STATUS signal from the WOBBLE servo controller 22 is inputted into the control means 13. These STATUS signals are in response to the commands mentioned above, namely, such status signals indicate the statuses for the controls mentioned above (e.g., control success, control failure, control being carried out and other statuses). (1) Recording When data (signals) is to be recorded (written in) in the optical disc 2, the pre-groove formed in the optical disc 2 is reproduced (read out), after which the data is recorded in accordance with the pre-groove. When the data (signals) to be recorded on the optical disc 2 is inputted into the optical disc drive 1 via the interface control section 31, such data is inputted into the EFM/CD-ROM encoder control section 24. In the EFM/CD-ROM encoder control section 24, such data is encoded in response to a clock signal (i.e., at the timing of the clock signal) from the clock 34 and then undergoes modulation (EFM modulation) by a modulation method known as EFM (Eight to Fourteen Modulation) to form ENCODE EFM signals. As shown in Fig. 3, these ENCODE EFM signals are formed from pulses each having a predetermined length (period) of any one of 3T - 11T. Further, as shown in Figs. 4 and 5, in the EFM/CD-ROM encoder control section 24, the clock signal from the clock 34 is divided, and a SUBCODE-SYNC signal (used as the second synchronizing signal) formed by a predetermined period pulse is generated. The pulse period of this SUBCODE-SYNC signal (i.e., the time interval between adjacent pulses) is 1/75 second for the case of the reference rotation speed (1X) and 1/450 second for the case of the 6 times rotation speed. During the encoding described above, a synchronizing signal, namely, a SYNC pattern is added to the ENCODE EFM signal based on the SUBCODE-SYNC signal (i.e., on the timing of the SUBCODE-SYNC signal). Namely, a SYNC pattern is respectively added to the portion corresponding to the head portion of each SUBCODE frame. This ENCODE EFM signal is inputted into the laser control section 14 from the EFM/CD-ROM encoder control section 24. Further, an analog WRITE POWER signal (voltage) is outputted from a D/A converter (not shown in the drawings) in the control means 13 and it is then inputted into the laser control section 14. Based on the ENCODE EFM signal, the laser control section 14 switches the level of the WRITE POWER signal from the control means 13 to a high level (H) or a low level (L), and then outputs such signal, thereby controlling the operations of the laser diode of the optical pick-up 3. In particular, during the period of time in which the ENCODE EFM signal is in a high (H) level, the laser control section 14 outputs a high level (H) WRITE POWER signal. Namely, the laser output is increased (to a level for writing in data). Then, during the period of time in which the ENCODE EFM signal is in a low (L) level, the laser control section 14 outputs a low level (L) WRITE POWER signal. Namely, the laser output is decreased (returned to a level for reading out data). Thus, when the ENCODE EFM signal level is in the high (H) level, a pit having a predetermined length is formed in the optical disc 2, and when the ENCODE EFM signal level is in the low (L) level, a land having a predetermined length is formed in the optical disc 2. In this way, data is written (recorded) in a predetermined track of the optical disc 2 with thus formed pits and lands. In the EFM/CD-ROM encoder control section 24, a predetermined ENCODE EFM signal (random EFM signal) is generated in addition to the above-mentioned ENCODE EFM signal. This random EFM signal is used for output adjustment (power control) of the laser when trial writing in a test area is carried out under OPC (Optimum Power Control) procedure. When trial writing in a test area under the OPC procedure, the random EFM signal is inputted into the lasercontrol section 14 from the EFM/CD-ROM encoder control section 24. Further, when trial writing is carried out in a test area during the OPC procedure, 15-step level WRITE POWER signals are generated in the control means 13, and these WRITE POWER signals are outputted from the D/A converter (not shown in the drawings) provided in the control means 13 and then inputted into the laser control section 14. Then, based on the random EFM signal, the laser control section 14 switches the level of the WRITE POWER signals from the control means 13 to a high level (H) or a low level (L) and then outputs such signals, thereby controlling the operations of the laser diode of the optical pick-up 3. This is done for each of the 15-step level WRITE POWER signals. In this way, in the OPC procedure, trial writing in a test area can be carried out with laser light having a 15-step output levels. Further, when writing data in the optical disc 2, laser light at a read-out output level is emitted from the laser diode of the optical pick-up 3 onto the pre-groove of the optical disc 2, and the light reflected therefrom is received by the split photodiode of the optical pick-up 3. The WOBBLE signal shown in Fig. 6 is outputted from this split photodiode. As was mentioned above, this WOBBLE signal has a 22.05kHz frequency signal at the reference rotation speed (1 X), and includes a signal obtained by biphase modulating the ATIP information and further frequency-modulating it at a carrier frequency of 22.05kHz. This WOBBLE signal is inputted into the WOBBLE signal detection circuit 19 where it undergoes digitization. The digitized WOBBLE signal is then inputted into the WOBBLE servo controller 22. In the WOBBLE servo controller 22, the frequency-modulated ATIP information in the WOBBLE signal is demodulated, and the BIDATA signal (biphase signal) shown in Fig. 7 is obtained. This BIDATA signal is a pulse signal having a length of any one of 1T - 3T Further, by biphase demodulating and then decoding this BIDATA signal, it is possible to obtain the ATIP information. Further, in a digital PLL circuit (not shown in the drawings) provided in the WOBBLE servo controller 22, a clock is generated based on the BIDATA signal to obtain the BICLOCK signal shown in Fig. 7. This BICLOCK signal is used in timing of the decoding of the BIDATA signal (described later). The BIDATA signal and the BICLOCK signal are respectively inputted into the SYNC signal generating/ATIP decoder 27. In the SYNC signal generating/ATIP decoder 27, the BIDATA signal is biphase demodulated based on the BICLOCK signal, and the biphase demodulated BIDATA signal is then decoded to obtain the ATIP information. Further, the ATIP-SYNC signal (used as the first synchronizing signal) shown in Fig. 7 is also generated. In this case, as shown in Fig. 7, the ATIP-SYNC signal pulse is generated when the SYNC pattern included in the BIDATA signal is detected. The period of this ATIP-SYNC signal pulse (i.e., the time interval between adjacent pulses) is 1/75 second for the case of the reference rotation speed (1 X) and 1/450 second for the case of the six times rotation speed (6X). This ATIP-SYNC signal is inputted into the control means 13 and the WOBBLE servo controller 22, respectively. Further, the decoded ATIP information is inputted into the control means 13, In this way, the control means 13 gets a position on the optical disc 2 (the absolute time) from this ATIP information. The above-mentioned SUBCODE-SYNC signal from the EFM/CD-ROM encoder control section 24 is inputted into the SYNC signal generating/ATI P decoder 27, after which such SUBCODE-SYNC signal is respectively inputted into the control means 13 and the WOBBLE servo controller 22 from the SYNC signal generating/ATIP decoder 27. Fig. 8 is a chart showing the frame format of the ATIP frame. As shown in this drawing, the frame format of one ATIP frame is comprised of 4-bits for synchronizing signal (Sync); 8-bits for minutes (Min); 8-bits for seconds (Sec); 8-bitsforframes (Frame); and 14-bits for error detection code (CRC: Cyclic Redundancy Code). In the WOBBLE servo controller 22, the ATIP information of each of the ATIP frames undergoes an error detection process (to determine whether or not the ATIP information is wrong). In this ATIP information error detection process, if the results of a predetermined operation performed on the data of the Sync, Minutes, Seconds and other data of the ATIP frame match the error detection code (CRC), such a state is defined as a "normal", and if the results don't match the error detection code, such a state is defined as a "ATIP error". In this case, if it is found that the ATIP information is wrong, namely if an ATIP error is detected, a pulse 51 will be generated in the WOBBLE servo controller 22 as an ATIP error signal, and then it is outputted, as shown in Fig. 4. The ATIP error signal which is formed from the pulse 51 is inputted into a counter (counting means) 131 provided in the control means 13. Then, the number of the pulses of the ATIP error signal is counted by the counter 131, and the number of the pulses of the ATIP error signal is defined as the number of ATIP errors. Now, because this error detection process is carried out on the ATIP information for every ATIP frame, a maximum of 75 ATIP errors can occur in 75 ATIP frames (in one second at the reference rotation speed (1X)). In this connection, a detection means for detecting such ATIP errors is constructed from the WOBBLE servo controller 22. The counted value of the ATIP error is stored in the memory 26 and is also transmitted to the computer 41 via the interface control section 31 to be utilized in examining the optical disc drive 1 (to judge the recording performance of the optical disc drive 1). The ATIP-SYNC signal inputted into the control means 13 is utilized to the timing of renewal of ATIP time. Further, the ATIP-SYNC signal inputted into the WOBBLE servo controller 22 is used for synchronization with the SUBCODE-SYNC signal. The SUBCODE-SYNC signal inputted into the control means 13 is used for compensation (interpolation) of the ATI P time information described hereinbelow and the measurement of the ATIP error described above. Further, the SUBCODE-SYNC signal inputted into the WOBBLE servo controller 22 is used as a reference signal for synchronization in the same manner as the ATIP-SYNC signal described above. In this connection, the synchronization is carried out such that the timing of the SUBCODE-SYNC signal in the EFM data generated when writing data becomes substantially synchronized with the timing of the ATIP-SYNC signal obtained from the optical disc 2. As shown in Fig. 9, the SUBCODE-SYNC signal and the ATIP-SYNC signal are normally allowed to shift up to ±2 EFM frames at the respective position on the entire optical disc 2. (2) Playback When data (signals) are to be played back (read out) from the optical disc 2, the level of the WRITE POWER signal supplied from the laser control section 14 is maintained at a predetermined DC level corresponding to a read-out output, and in this way, the laser output is maintained at the read-out output level. Normally, the read-out output (the output of the main beam) is set to be equal to or less than 0.7mW. Now, when data is being read out from the optical disc 2, laser light at the read-out output is emitted from the laser diode of the optical pick-up 3 onto a pre-determined track of the optical disc 2, and the light reflected therefrom is received by the split photodiode of the optical pick-up 3. Then, electrical currents (voltages) corresponding to the quantity of received light are outputted respectively from each light-receiving portion of the split photodiode of the optical pick-up 3, and these currents, namely, each signal (detected signal) is respectively inputted into the HF signal generating circuit 15 and the error signal generating circuit 18. In the HF signal generating circuit 15, these detection signals undergo addition, subtraction and the like to generate an HF (RF) signal. This HF signal is an analog signal corresponding to pits and lands formed in the optical disc 2. As described above, this HF signal is inputted into the HF signal gain switching circuit 16 and then amplified. The amplification factor of the HF signal gain switching circuit 16 is switched by a gain switching signal from the control means 13. The amplified HF signal (hereafter, referred to as the "HF signal") is respectively inputted into the peak/bottom detection circuit 17 and the CD servo controller 21. Further, a tracking error (TE) signal (which is described under (3) Focus Control, Tracking Control and Sled Control) is inputted into the peak/bottom detection circuit 17. As shown in Fig. 10, in the peak/bottom detection circuit 17, the amplitudes of the inputted signals (envelope), such as the HF signal and the tracking signal, are extracted. The top and bottom of the amplitude are referred to respectively as the "PEAK" and "BOTTOM", wherein the signal corresponding to the tops of the amplitudes is referred to as "PEEK signal", and the signal corresponding to the bottoms of the amplitudes is referred to as "BOTTOM signal". The PEEK signal and the BOTTOM signal are respectively inputted into the A/D converter (not shown in the drawings) in the control means 13, and in this A/D converter such signals are converted into digital signals. These PEAK and BOTTOM signals are used, for example, to measure the amplitude, to adjust the amplitude of the tracking error signal, to calculate the β value in the OPC (Optimum Power Control) procedure, and to determine the presence or absence of the HF signal. In the CD servo controller 21, the HF signal is digitized and EFM demodulated to obtain an EFM signal. This EFM signal is a signal formed by a pulse having a length (period) corresponding to any one of 3T- 11T. Then, in the CD servo controller 21, an error correction (CIRC error correction) which uses an error correction code referred to as a CIRC (Cross Interleaved Read Solomon Code) is carried out twice on this EFM signal. In this case, the first CIRC correction is referred to as a "C1 error correction", and the second CIRC correction is referred to as the "C2 error correction". Further, the case where error correction can not be carried out by the first CIRC correction, namely by the C1 error correction is referred to as a "C1 error", and the case where error correction can not be carried out by the second CIRC correction, namely by the C2 error correction is referred to as a "C2 error". As shown in Fig. 11, if a C1 error is detected during the C1 error correction in the CD servo controller 21 ,a pulse 52 is generated and then it is outputted. The C1 error signal comprised of the pulses 52 is inputted into the counter 131 of the control means 13. Then, the number of the pulses of the C1 error signal is counted (measured) by the counter 131 as the number of C1 errors. Now, because one subcode frame is comprised of 98 EFM frames, a maximum number of 7350 C1 and C2 errors can occur respectively in 75 subcode frames (in one second at the reference rotation speed (1X)). In this regard, it is to be understood that the CD servo controller 21 constitutes a detection means for detecting the C1 error. The counted value of C1 errors is stored in the memory 26 and is also transmitted to the computer 41 via the interface control section 31 to be utilized in examining the optical disc drive 1 (to judge the playback performance or the recording/playback performance of the optical disc drive 1). In the CD servo controller 21, the EFM signal after CIRC error In the CD servo controller 21, the EFM signal after CIRC error correction has been carried out is decoded (converted) into a predetermined format data, namely, a DATA signal. Next, a description will be given for a typical case in which audio data (music data) is recorded on an optical disc 2 and its EFM signal is decoded into an audio format DATA signal. In this connection, Fig. 12 is a timing chart showing an audio format DATA signal, an LRCLOCK signal and a BITCLOCK signal. As shown in this drawing, in the CD servo controller 21, the EFM signal is decoded into a DATA signal comprised of 16-bit L-channel data and 16-bit R-channel data based on a clock signal from the clock 33. Further, in the CD servo controller 21, the BIT-CLOCK signal and the LRCLOCK signal are respectively generated based on the clock signal from the clock 33. This BITCLOCK signal is a serial data transfer clock. Further, the LRCLOCK signal is a signal for discriminating the L-channel data and the R-channel data in the DATA signal. In this case, the high (H) level of the LRCLOCK signal represents the L-channel data, and the low (L) level of the LRCLOCK signal represents the R-channel. Now, in the case where normal data other than audio data is recorded on the optical disc 2, the EFM signal thereof is also decoded into a DATA signal comprised of the above-mentioned 16-bit L-channel data and 16-bit R-channel data. The DATA signal, the LRCLOCK signal and the BITCLOCK signal are respectively inputted into the CD-ROM decoder control section 28. In the case where error correction information such as ECC (Error Correction Code)/EDC (Error Detecting Code) is recorded on the optical disc 2, error correction is carried out for the DATA signal in the CD-ROM decoder control section 28. This ECC/EDC is an error correction code used in a CD-ROM MODE 1 format. With this error correction, it is possible to reduce the bit error rate to a degree of 10-12. Next, in the CD-ROM decoder control section 28, the DATA signal is decoded into predetermined format data for communication (transmission) based on the clock signal from the clock 35, and this decoded data is then transmitted to the computer 41 via the interface control section 31. In the computer 41, for example, this decoded data is encoded, and then this encoded data is recorded (copied) into a predetermined, recording medium (e.g., a hard disc). Further, the FRAME SYNC signal shown in Fig. 13 is generated in the CD servo controller 21. The level of this FRAME SYNC signal becomes high (H) when the HF signal is inputted into the CD servo controller 21 and the EFM signal is being synchronized at a specified period (3T - 11T). Further, if no HF signal becomes inputted (i.e., if there becomes no synchronization of the EFM signal), the level of the FRAME SYNC signal is switched from a high (H) level to a low (L) level at the respective EFM frame unit. In this regard, it is to be understood that, in the case of the reference rotation speed (1X), the length (period) of one EFM frame is 136 µ. sec, and 98 EFM frames form one subcode frame. This FRAME SYNC signal is inputted into the control means 13, and it is used for detecting the termination of the HF signal. Further, a SUBQ DATA signal is inputted into the control means 13 from the CD servo controller 21. This SUBQ DATA signal is a signal which represents Q data in the subcode data. The subcode includes eight types referred to as P, Q, R, S, T, U, V and W. One EFM frame includes a subcode of one byte in which each of P - W data is recorded with one bit, respectively. Now, because one subcode frame is comprised of 98 EFM frames, with each EFM frame having one bit for each P - W data, there are a total of 98 bits for each P - W data in one subcode frame. However, because thefirsttwo EFM frames are used for the SYNC pattern (synchronization signal), there are actually 96 bits for each P - W data. Next, Fig. 14 is a chart showing the format of the 96 bits of Q data. The CONTROL given by Q1 - Q4 (4 bits) shown in this drawing are used to discriminate normal data or audio data. Further, the ADDRESS given by Q5 - Q8 (4 bits) represents the contents of the data in Q9 - Q80 (72 bits). Further, the CRC (Cyclic Redundancy Code) in Q81 - Q96 (16 bits) is used to detect errors (to judge whether or not the data is wrong). Furthermore, from this Q data it is possible to obtain information such as the absolute time information on the optical disc 2, the information of the presenttrack, lead-in and lead-out, the music number, and a TOC (Table of Contents) recorded in the lead-in area. The control means 13 obtains such information from the Q data and then carries out predetermined operations. Further, the SUBCODE-SYNC signal is inputted into the control means 13 from the CD servo controller 21. Now, as shown in Fig. 15, there are 98 bytes of subcode data in 98 EFM frames, and as mentioned above, the SYNC pattern (synchronization signal) is recorded in the two bytes which form the first two EFM frames, namely, SO and S1. When this SYNC pattern is detected, the CD servo controller 21 generates a pulse and then outputs it. Namely, a pulse is generated and outputted for every subcode frame (98 EFM frames). The signal which is formed by this pulse is the SUBCODE-SYNC signal. Further, the SYNC pattern is outputted 75 times every second in the case of the reference rotation speed (1X) 450 times every second in the case of the 6 times rotation speed (6X). Further, in the CD servo controller 21, the Q data is renewed after the SUBCODE-SYNC signal pulse has been detected. Then, the renewed Q data is read into the control means 13. (3) Focus Control, Tracking Control and Sled Control In the error signal generating circuit 18, a focus error (FE) signal, a tracking error (TE) signal and a sled error (SE) signal are respectively generated by carrying out addition and subtraction and the like on the detection signal from the split photodiode. The focus error signal is a signal which represents the amount of displacement of the objective lens along the rotational axial direction away from the focus position (i.e., the amount of the displacement of the objective lens from the focus position) and the direction thereof. The tracking error signal is a signal which represents the amount of displacement of the objective lens along a radial direction from the center of the track (Pre-groove) (i.e., the amount of the displacement of the objective lens from the center of the track) and the direction thereof. Further, the sled error signal is a signal used for sled control, namely, in the sled servo (i.e., the servo for moving the pick-up base of the optical pick-up 3). In other words, the sled error signal is a signal which represents the amount of the displacement of the optical pick-up 3 along a radial direction (i.e., the moving direction of the optical pick-up 3) from the target position (proper position) of the optical pick-up 3 and the direction thereof. The focus error signal is inputted into the CD servo controller 21. Further, the tracking error signal is also inputted into the CD servo controller 21 and into the peak/bottom detection circuit 17, respectively, as described above. Furthermore, the sled error signal is also inputted into the CD servo controller 21. Using these focus error signal, tracking error signal and sled error signal, the optical disc drive 1 carries out focus control, tracking control and sled control for a predetermined track. During focus control, a focus PWM (Pulse Width Modulation) signal for controlling the driving of the actuator 4 along the rotational axial direction is generated in the CD servo controller 21. This focus PWM signal is a digital signal (continuous pulse). The focus PWM signal is inputted into the PWM signal smoothing filter 7 from the CD servo controller 21 and undergoes smoothing, namely, the focus PWM signal is converted into a control voltage (control signal), and it is then inputted into the driver 6. Then, based on such control voltage, the driver 6 applies the focus signal (predetermined voltage) to the actuator 4 to drive the actuator 4 in the rotational axial direction (focus direction). In this case, the CD servo controller 21 adjusts the pulse width (duty ratio) of the focus PWM signal so that the level of the focus error signal becomes zero (i. e., the level is reduced as much as possible), and reverses the code of the focus PWM signal (pulse/minus sign). In this way, the objective lens of the optical pick-up 3 is positioned at the focus position. Namely, the focus servo is operated. Further, during tracking control, a tracking PWM signal for controlling the driving of the actuator 4 along the radial direction is generated in the CD servo controller 21. This tracking PWM signal is also a digital signal (continuous pulse). The tracking PWM signal is inputted into the PWM signal smoothing filter 7 from the CD servo controller 21 and undergoes smoothing, namely, the tracking PWM signal is converted into a control voltage (control signal), and it is then inputted into the driver 6. Then, based on such control voltage, the driver 6 applies the tracking signal (predetermined voltage) to the actuator 4 to drive the actuator 4 in the radial direction (tracking direction). In this case, the CD servo controller 21 adjusts the pulse width (duty ratio) of the tracking PWM signal so that the level of the tracking error signal becomes zero (namely, the level is reduced as much as possible), and reverses the code of the tracking PWM signal (pulse/minus sign). In this way, the objective lens of the optical pick-up 3 is being positioned at the center of the track (Pre-groove). Namely, the tracking servo is operated. Further, during the sled control, a sled PWM signal for controlling the driving of the sled motor 5 is generated in the CD servo controller 21. This sled PWM signal is also a digital signal (continuous pulse). The sled PWM signal is inputted into the PWM signal smoothing filter 7 from the CD servo controller 21 and undergoes smoothing, namely, the sled PWM signal is converted into a control voltage (control signal), and it is then inputted into the driver 6. Then, based on such control voltage, the driver applies the sled signal (predetermined voltage) to the sled motor 5 to rotationally drive the sled motor 5. In this case, the CD servo controller 21 adjusts the pulse width (duty ratio) of the sled PWM signal so that the level of the sled error signal becomes zero (i.e., to reduce the level as much as possible), and reverses the code of the sled PWM signal (pulse/minus sign). In this way, the pick-up base of the optical pick-up 3 is positioned at the target position (proper position). Namely, the sled servo is operated. Further, in addition to tracking control, the tracking error signal is also used, for example, to control the movement of the optical pick-up 3 toward a predetermined track (target track) of the optical disc 2 (i.e., to control track jump operations). (4) Rotation Number Control (Rotation Speed Control) The optical disc drive 1 is constructed so as to be able to change the rotation speed of the spindle motor 8 in multiple steps given by integral multiples of 1 such as 1x, 2x, 4x, 6x, 8x and 12x and the like. This change of the rotation speed is performed by setting the optical disc drive to a rotation speed changing mode. For example, in recording and playing back an optical disc, the rotation speed (rotation number) of the spindle motor 8 is controlled under the condition that rotation speed is set in a predetermined rotation speed (hereinbelow, explanation is made on the assumption that the rotation speed is set in the reference rotation speed of 1X). In this case, as described above, the rotation number (rotation speed) of the spindle motor 8 is controlled so that the linear velocity becomes constant. The methods of controlling the rotation number (rotation speed) of the motor is called as a spindle servo, and the spindle servo includes a method using a WOBBLE PWM (Pulse Width Modulation) signal, namely, a spindle servo which uses the WOBBLE signal (hereinafter, referred to as "WOBBLE servo"); a method using an FG PWM signal, namely, a spindle servo which uses an FG signal (hereinafter, referred to as "FG servo"); and a method using an EFM PWM signal, namely, a spindle servo which uses the EFM signal (hereinafter, referred to as "EFM servo"). These will be described below in the order mentioned above. The WOBBLE PWM signal is a signal for controlling the spindle motor, and it is generated by the WOBBLE servo controller 22. The WOBBLE PWM signal is a digital signal (continuous pulse) having a level of 0-5V. This WOBBLE PWM signal is inputted into the PWM signal smoothing filter 12 from the WOBBLE servo controller 22 and undergoes smoothing, namely, the WOBBLE PWM signal is converted into a control voltage (control signal), and it is then inputted into the driver 11. Then, based on such control voltage, the driver 11 rotationally drives the spindle motor 8. In this case, the WOBBLE servo controller 22 adjusts the pulse width (duty ratio) of the WOBBLE PWM signal so that the frequency (cycle) of the WOBBLE signal becomes the target value (e.g., 22.05kHz at the reference rotation speed (1 X)). In this way, the spindle servo is operated so as to set the rotation number (rotation speed) of the spindle motor 8 at the target value (hereinafter, referred to as "target rotation number"). The FG PWM signal is a signal for controlling the spindle motor, and it is generated by the control means 13. The FG PWM signal is a digital signal having a level of 0 - 5V (continuous pulse). This FG PWM signal is inputted into the PWM signal smoothing filter 12 from the control means 13 and undergoes smoothing, namely, the FG PWM signal is converted into a control voltage (control signal), and it is then inputted into the driver 11. Then, based on such control voltage, the driver 11 rotationally drives the spindle motor 8. On the other hand, a FG (Frequency Generator) signal corresponding to the rotation number (rotation speed) of the spindle motor 8 is outputted from the Hall element 9. This FG signal is digitized by the FG signal digitization circuit 23 and such a digitized signal is then inputted into a frequency measurement (cycle measurement) section 132 of the control means 13. In the frequency measurement section 132 of the control means 13, the frequency (cycle) of the FG signal is measured based on the clock signal from the clock 32. Then, the control means 13 adjusts the pulse width (duty ratio) of the FG PWM signal so that the frequency (cycle) of the FG signal becomes the target value. In this way, the spindle servo is operated to set the rotation number (rotation speed) of the spindle motor 8 at the target rotation number. The frequency of the FG signal is proportional to the rotation number of the spindle motor 8. Therefore, when the rotation speed is set at 6X, the frequency of the FG signal is 6 times of the frequency at 1X. The EFM PWM signal is a signal for controlling the spindle motor, and it is generated by the CD servo controller 21 as described above. The EFM PWM signal is a digital signal having a level of 0 - 5V level (continuous pulse). This EFM PWM signal is inputted into the PWM signal smoothing filter 12 from the CD servo controller 21 and undergoes smoothing, namely, the EFM PWM signal is converted into a control voltage (control signal), and it is then inputted into the driver 11. Then, based on such control voltage, the driver 11 rotationally drives the spindle motor 8. In this case, the CD servo controller 21 adjusts the pulse width (duty ratio) of the EFM signal so that the EFM signal, that is a period of a predetermined pulse selected from the 3T - 11T period pulses becomes the target value at any portions on the optical disc. In this way, the spindle servo is operated to set the rotation number (rotation speed) of the spindle motor 8 at the target rotation number. In the meantime, as described above, the rotation number of the optical disc differs between the case where the optical pick-up is in the inner region and the case where the optical pick-up is in the outer region in order to make the linear velocity constants. For example, the former case is faster than the later case for 2.5 times at the maximum. Therefore, when deciding the target rotation number in performing the spindle servo, information concerning the position of the optical pick-up 3 in the radial direction of the optical disc in addition to the current rotation speed are taken into account. The information concerning the position of the optical pick-up 3 in the radial direction is obtained from the absolute time on the optical disc. This absolute time is obtained from ATIP or Q data of SUBCODE outputted from the ATIP decoder or CD servo controller, and then it is inputted into the control means 13 and then processed and recognized. Next, a description is made with regard to the rotation number control method for changing the rotation number of the spindle motor 8 for changing the rotation number speed mode of the optical disc drive 1. Fig. 16 is a flow chart which shows the operations of the rotation number control of the spindle motor according to the present invention. Hereinbelow, an explanation is made based on the Fig. 16. In this connection, it is to be understood that the following explanation is made based on the case where the spindle motor 8 rotating at 1 X is changed into 6X. First, a rotation speed changing mode is set up (Step 100), and then an initialization concerning the change of the rotation speed is performed (Step 101) This initialization step includes preparation for carrying out the rotation number control of the spindle motor (spindle servo) with the FG servo (e.g. making it possible for the frequency measurement section 132 to perform sampling of the FG signal) and a setting of the EFM servo (or WOBBLE servo) for the six times rotation speed to be switched and the like. Next, the cycle (frequency) of the FG signal corresponding to the target rotation number is computed (Step 102). In performing this computation, the information that the rotation speed to be switched is 6 times rotation speed (6X) is utilized, and information concerning the position of the optical pick-up 3 in the radial direction of the optical disc is also utilized. Then, the target frequency of the FG signal obtained in Step 102 is set in the FG servo (Step 103). Namely, the target FG frequency is incorporated into the computing routine of the FG servo. Next, the spindle servo which has been being performed by the EFM servo or WOBBLE servo is switched to FG servo (Step 104), and the FG servo is performed (Step 105). In this way, the rotation of the spindle motor 8 is accelerated toward the target rotation number. Next, the current rotation number of the spindle motor 8 is obtained from the frequency or cycle of the FG signal inputted into the frequency measurement section 132 to judge as to whether the rotation number reaches the target rotation number (Step 106). When the rotation number of the spindle motor 8 reaches the target rotation number, the spindle servo is returned to the previous spindle servo, that is the EFM servo or WOBBLE servo (Step 107). In this connection, it is to be noted that the rotation number control of the spindle motor 8 described above can be applied to the case where, for example, the spindle motor rotating at the rotation speed at 6X is decreased to the rotation speed at 4X. Hereinafter, a description is made with regard to the rotation number control of the spindle motor in the case where the rotation number of the spindle motor 8 is changed (increased or decreased) in accordance with the movement of the optical pick-up 3 in the radial direction. Fig. 17 is a flow chart which shows the operations of the rotation number control of the spindle motor according to the present invention. Hereinbelow, an explanation is made based on Fig. 17. In this regard, it is to be understood that the following explanation is made based on the example case where the rotation number of the spindle motor 8 is fixed at the reference rotation number (1X). First, a mode setting is made to an optical pick-up (PU) moving mode in which the optical pick-up (PU) 3 can move in the radial direction by driving the sled motor 5 or the like (Step 200), and an initialization is carried out for this rotation speed changing mode (Step 201). This initialization step includes preparation for carrying out a rotation number control of the spindle motor (spindle servo) with a FG servo (e.g. making it possible for the frequency measurement section 132 to perform sampling of the FG signal) and a setting of a spindle servo which will be used in a position to which the optical-pick up 3 is to be moved (hereinafter, referred to as "moved position"), and a setting of the number of tracks to the moved position and the like. Then, the frequency (cycle) of the FG signal corresponding to the rotation number (target rotation number) at the moved position of the optical pick-up 3 is computed (Step 202). In this computation, information concerning the moved position (radial position) of the optical pick-up 3 is used. This information is computed based on the time information. Next, the target FG frequency obtained in Step 202 is set to the FG servo (Step 203). Namely, the target FG frequency is incorporated into the computing routine of the FG servo. Next, the spindle servo which has been being performed by the EFM servo or WOBBLE servo is switched to the FG servo (Step 204), to start movement of the optical pick-up 3 by driving the sled motor 5 and the actuator 4 (Step 205) and perform the FG servo (Step 206). Then, a judgement is made as to whether or not the optical pick-up 3 reaches the moved position (step 207). In the case where it is determined that the optical pick-up has reached the moved position, the sled motor 5 and the actuator 4 are stopped to terminate the movement of the optical pick-up 3 (Step 208). In this case, the judgement as to whether the optical pick-up 3 reached the moved position is made by inputting a track count signal outputted from a track count signal generating circuit not shown in the drawing into the control means 13 and determining as to whether the number of crossing tracks reaches the previously computed number of tracks using the counter function of the control means 13. Neut, the current rotation number of the spindle motor 8 is obtained based on the frequency or cycle of the FG signal inputted into the frequency measurement section 132, and then determination is made as to whether the rotation number reaches the target rotation number (Step 209). When it is determined that the rotation number of the spindle motor 8 has reached the target rotation number, the spindle servo is returned to the previous spindle servo method, that is the EMF servo or WOBBLE servo (Step 210). As described above based on Fig. 16 and Fig. 17, according to this optical disc drive 1, when the rotation number of the spindle motor 8 is to be changed, a spindle servo is carried out by the FG servo which is based on the FG signal not the EFM servo or WOBBLE servo which is based on the information obtained from the optical disc 2. Therefore, the amount of overshoot or undershoot which occurs when acceleration or deceleration in the form of kick or brake is carried out for the spindle motor 8 is relatively small. As a result, a time required until the rotation number of the spindle motor 8 reaches the target rotation number and the rotation becomes stable is relatively short, that is the time until obtaining the stability is relatively short. Further, since the FG servo is working during the changing of the rotation speed or the movement of the optical pick-up, it is possible to know the rotation number before reaching the target rotation number. This makes it possible to shorten access time to reach the target rotation number in comparison with the prior art using the WOBBLE servo or EFM servo. Further, since the spindle servo by the FG servo is performed mainly by the software or the like provided in the control means 13, it is easily possible to perform gain adjustment and recognition of the information concerning the difference between the current rotation number and the target rotation number and the like. The rotation number control of the spindle motor 8 as described above can be applied not only to the cases described above with reference to the switching of the rotation speed level and the radial movement of the optical pick-up 3 but also to other various cases where the rotation speed of the spindle motor is required to change. For example, this rotation number control can be applied to the case where the spindle motor 8 is started to rotate (at the time of build up of rotation). Further, the optical disc drive according to the present invention is applied to not only the CD-R drives described above but also other various optical disc drives for recording and playing back other optical discs such as CD-RW, DVD-R, DVD-RAM or the like and other optical disc drives for playing back optical discs such as CD (compact disc), CD-ROM, or the like. Finally, it should be also noted that even though the optical disc drive of the present invention was described with reference to the embodiment shown in the drawings, the present invention is not limited thereto, and any additions or changes may be made without departing from the scope of the following claims.

Claims: An optical disc drive (1), comprising; a spindle motor (8) for rotating an optical disc (2), the rotation speed of the spindle motor (8) being adapted to be changed in multiple steps given by integral multiples of 1: an optical pick-up (3) which can move at least in a radial direction of the optical disc (2); a sensor (9) for detecting the rotation of the spindle motor (8); rotation speed control means for controlling the rotation speed of the spindle motor (8), characterized in that the rotation control means includes: (i) first rotation speed control means for controlling the rotation speed of the spindle motor (8) based on information obtained from the optical disc (2), and (ii) second rotation speed control means for controlling the rotation speed of the spindle motor (8) based on signals from the sensor (9); and switching means for switching the rotation speed control given by said rotation speed control means between the first rotation speed control means and the second rotation speed control means during the operation of the optical disc drive (1), wherein said switching means being operated at least at a time when the rotation speed of the spindle motor (8) is to be changed between the multiple steps to change the first rotation speed control means to the second rotation speed control means, in which said switching means being also operated to change the second rotation speed control means to the first rotation speed control means at a time when the rotation speed of the spindle motor (8) reaches a predetermined rotation speed at a changed level of the multiple steps. The optical disc drive as claimed in claim 1, wherein the sensor (9) includes a Hall element which outputs FG signals having a predetermined frequency, and the second rotation speed control means includes a FG servo using the FG signals. The optical disc drive as claimed in claim 1, wherein the first rotation speed control means includes a WOBBLE servo using WOBBLE PWM signals from the optical disc or an EFM servo using EFM PWM signals. The optical disc drive as claimed in claim 1, further comprising means for changing the rotation speed of the spindle motor (8) depending on the radial position of the optical pick-up (3) with respect to the optical disc (2), in which the second rotation speed control means is also used when the rotation speed of the spindle motor (8) is to be changed due to movement of the optical pick-up (3) in the radial direction of the optical disc (2). The optical disc drive as claimed in claim 1, wherein said switching means is operated when the rotation speed of the spindle motor (8) is increased between the multiple steps. The optical disc drive as claimed in claim 1, wherein said switching means is operated when the rotation speed of the spindle motor (8) is decreased between the multiple steps. A method of controlling rotation speed of a spindle motor (8) for rotating an optical disc (2) in an optical disc drive (1), the rotation speed of the spindle motor (8) being adapted to be changed in multiple steps given by integral multiples of 1, the method being characterized by comprising the steps of: performing a spindle servo with a first spindle servo means which controls the rotation speed of the spindle motor (8) based on information obtained from the optical disc (2); switching the first spindle servo means to a second spindle servo means which controls the rotation speed of the spindle motor (8) based on signals from a sensor (9) which detects the rotation of the spindle motor (8), said switching means being operated at least at a time when the rotation speed of the spindle motor (8) is to be changed between the multiple steps to change the first spindle servo means to the second spindle servo means; changing the rotation speed of the spindle motor (8) under the spindle servo provided by the second spindle servo means so that the rotation speed of the spindle motor (8) reaches a predetermined rotation speed at a changed level of the multiple steps; and switching the second spindle servo means to the first spindle servo means after the rotation speed of the spindle motor (8) has reached the predetermined rotation speed. The method as claimed in claim 7, wherein the sensor (9) includes a Hall element which outputs. FG signals having a predetermined frequency, and the second spindle servo means includes a FG servo using the FG signals. The method as claimed in claim 7, wherein the first spindle servo means includes a WOBBLE servo using WOBBLE PWM signals from the optical disc or an EFM servo using EFM PWM signals. The method as claimed in claim 7, wherein said optical disc drive (1) further comprises means for changing the rotation speed of the spindle motor (8) depending on the radial position of the optical pick-up (3) with respect to the optical disc (2), in which the second spindle servo means is used when the rotation speed of the spindle motor (8) is to be changed due to movement of the optical pick-up (3) in the radial direction of the optical disc (2). The method as claimed in claim 7 , wherein the switching from the first spindle servo means to the second spindle servo means and the switching from the second spindle servo means to the first spindle servo means are performed when the rotation speed of the spindle motor (8) is increased between the multiple steps. The method as claimed in claim 7 , wherein the switching from the first spindle servo means to the second spindle servo means and the switching from the second spindle servo means to the first spindle servo means are performed when the rotation speed of the spindle motor (8) is decreased between the multiple steps.

Inventors: ANDOH JUNICHI, ANDOH, JUNICHI,, Andoh, Junichi, c/o Mitsumi Electric Co., Ltd.
Applicants: MITSUMI ELECTRIC CO, MITSUMI ELECTRIC COMPANY LTD.
Application year: 1998
Main classification: G11B19
IPC codes: G11B19