Optical disc drives

Abstract: An optical disc drive is provided for driving at a constant linear velocity an optical disc (1) on which pre-grooves are formed for providing wobble signals frequency modulated by predetermined data signals. Until data signals obtained upon demodulating the wobble signals are taken out correctly, the reproduced wobble signals are compared with a predetermined reference signal, so that a spindle motor (2) for rotating the optical disc (1) is controlled so as to be driven at an approximately constant linear velocity. After it has been detected that the data signals obtained upon demodulating the wobble signals are taken out correctly, the spindle motor (2) is driven in a controlled manner on the basis of data signals obtained upon demodulating the reproduced wobble signals, so that the spindle motor (2) is controlled so as to be driven at a constant linear velocity.

Description: This invention relates to optical disc drives. For controlling the rotation of an optical disc at a constant linear velocity (CLV) using an optical disc drive, it might be contemplated to use an optically recordable disc on which pre-grooves wobbled by a constant frequency signal are formed, and to use a spindle servo so that wobble signals derived from the pre-grooves are at a constant frequency. The recording data previously recorded on an optically recordable disc used in an embodiment of the present invention are divided into frames each of 75 Hz, as in the case of a compact disc (CD), and the absolute time from the leading end of the spiral recording track, corresponding to the data of each frame is indicated as shown in Figure 1, as absolute time data DT AT formed by 42 bits. The absolute time data DT AT are constituted by a 4-bit sync code DT SYNC , minute data DT MIN and second data DT sec each composed of 8-bit binary coded decimal (BCD) data, 8-bit frame data DT FRM and 14-bit cyclic redundancy check (CRC) data DT CRC computed using a predetermined polynominal generating the cyclic redundancy check. The absolute time data DT AT are formed by a non-return-to-zero (NRZ) code having a bit rate of 3.15 kbPS or 42 bits x 75 frames = 3150 (bPS). The absolute time data DT AT are demodulated as a biphase mark at the bit clock of 6.3 kHz, while the demodulated biphase mark signals are frequency modulated to form wobble signals consisting of FM signals having a sub-carrier frequency of 22.05 kHz. In this manner, when producing a master disc for an optically recordable disc, there are formed pre-grooves that are wobbled in a direction normal to the spirally extending recording tracks on the master disc on the basis of these wobble signals, so the pre-grooves are wobbled on the basis of the absolute time information. The optically recordable disc, on which the signals have been formed in the above described manner, is hereinafter referred to as being in absolute time in pre-groove (ATIP) format. In an optical disc drive employing an optically recordable disc of ATIP format as the recording medium, since the wobble signals in the pre-grooves are frequency modulated signals, a spindle servo is affected thereby and there is jitter in the rotational speed. To overcome this problem, spindle servo may be applied so that the bit clock of the biphase mark signals obtained upon demodulating the wobble signals will be at a predetermined frequency to provide for rotation control to produce rotation free of jitter. However, even in this case, because the phase lock of the bit clock regenerating circuit for the biphase demodulation is disengaged at the start of disc rotation and at track jumps by the optical head, accurate bit clocks cannot be produced. According to the present invention there is provided an optical disc drive in which an optically recordable disc having pre-grooves formed therein for providing a wobble signal frequency modulated by predetermined data signals is driven at a constant linear velocity; characterized by: first spindle servo means for controlling a spindle motor to rotate said optical disc at a substantially constant linear velocity on the basis of a comparison of the reproduced wobble signal with a predetermined reference signal; second spindle servo means for controlling said spindle motor to rotate said optical disc at a constant linear velocity on the basis of a comparison of clock bit signals obtained upon demodulating said reproduced wobble signal and said reference signal; switching means for selecting said first spindle servo means or said second spindle servo means; and detection means for detecting whether said clock bit signals are derived correctly, and in dependence on said detection controlling said switching means. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a diagrammatic view of an ATIP format adopted in an optically recordable disc; Figure 2 is a block diagram showing the overall construction of a magneto-optical disc drive to which the present invention is applied; Figure 3 shows the structure of a photodetector for detecting servo signals employed in the disc drive; Figure 4 is a waveform diagram showing biphase mark signals demodulated in the disc drive; Figure 5 is a block diagram showing the construction of a spindle control circuit employed in the disc drive; Figure 6 is a block diagram of portions of a modified embodiment of the present invention; and Figure 7 is a block diagram of portions of another modified embodiment of the present invention. In the block view of Figure 2, there is shown a magneto-optical disc drive employing a magneto-optical optically recordable disc 1, on which pre-grooves are formed according to the ATIP format. The disc 1 is driven in a predetermined direction by a spindle motor 2 about a central spindle 3. A laser light L₀ from an optical head 4 is irradiated on the pre-groove on the disc 1, while a modified magnetic field is applied to the pre-groove by a magnetic head 6 which is controlled by a magnetic head driving circuit 5. Input signals S IN are supplied to an analogue-to-digital (A/D) converter 9 during recording, via an input amplifier 7 and a low-pass filter 8 so as to be converted at the A/D converter 9 into input digital data DG IN . These digital data DG IN are eight-to-fourteen modulated in an eight-to-fourteen modulation (EFM) encoder 10 so as to be supplied as the magnetic field modulation signal DG EFM to the magnetic head driving circuit 5. The magnetic head driving circuit 5 then drives the magnetic head 6 to apply the modulated magnetic field to the disc 1, while the laser light L₀ is irradiated on the disc 1 by the optical head 4 from below, for recording signals according to the magnetic field modulated signals DG EFM on the disc 1. In this disc drive, as the laser light L₁, wherein light power L₁ <L₀, is irradiated during reproduction by the optical head 4 on the pre-grooves on the disc 1, the returning light undergoes Kerr rotation in accordance with the pattern of the direction of magnetization recorded on the disc 1, so that a reproduced output RF is produced from a photodetector for RF detection, enclosed in the optical head 4. The reproduced output RF is supplied as the reproduced signal S RF via an RF demodulator 11. The reproduced signals S RF are eight-to-fourteen demodulated at an EFM decode/error correction circuit 13 and converted into output digital data DG OUT so as to be supplied as output signals S OUT via a digital-to-analogue (D/A) converter 14, a low-pass filter 15 and an output amplifier 16. During recording and reproduction, the return light from the disc 1 resulting from the laser light L₀ or L₁ emitted by the optical head 4 is detected by a photodetector D for servo signal detection, which is shown in Figure 3, and which is provided separately from the aforementioned photodetector for RF detection, and the detection signal S₁ obtained at the photodetector D for servo signal detection is supplied to an operational amplifier 12. As shown in Figure 3, the photodetector D for servo signal detection is made up of four elements D₁, D₂, D₃ and D₄ divided in the track direction and in the direction orthogonal to the track direction. In the operational amplifier 12, the sum of the outputs from the elements D₁ and D₄ is subtracted from the sum of the outputs from the elements D₂ and D₃ to produce a focusing error signal S FE , while the sum of the outputs of the elements D₁ and D₂ is subtracted from the sum of the outputs of the elements D₃ and D₄ to produce a differential output signal S TE . The differential output signal S TE is also a tracking error signal and is referred to as the tracking error signal S TEO when it is supplied to an optical head servo circuit 17. The optical head servo circuit 17 is supplied with focusing error signals S FE and tracking error signals S TEO produced at the operational amplifier 12, and produces a tracking control signal, sled motor control signal CNT SM and a focus control signal CNT FO , all of which perform fine and coarse tracking and focusing control of the optical head 4. A track jump control signal CNT HD for shifting the optical head 4 to a desired track position for accessing is produced by a system controller 18, and supplied to the optical head servo circuit 17. On the other hand, the differential output signal S TE obtained from the operational amplifier 12 is also supplied to an absolute time reproducing/processing section 19. In this section 19, the differential output signal S TE is introduced to a bandpass filter 20 having a centre frequency of 22.05 kHz and a predetermined pass band, where the wobble signal S WB contained in the differential output signal S TE is extracted and transmitted to a frequency demodulator 21. The wobble signal S WB is demodulated in the frequency demodulator 21, and biphase mark signals S BP of Figure 4 produced in this manner in the frequency demodulator 21 are supplied via a low-pass filter 22 to a binary circuit 23 of a comparator circuit configuration where the biphase mark signals are formed into corresponding binary signals which are then supplied to a biphase demodulator 24, and to a bit clock regenerating circuit 25 of a phase locked loop configuration. The biphase demodulating circuit 24 demodulates the biphase mark signal S BP on the basis of the 6.3 kHz bit clocks CK BIT produced in the bit clock regenerator 25 to produce the aforementioned absolute time data DT AT shown in Figure 1, which absolute time data DT AT are transmitted to a CRC error detector 26 and to the system controller 18. The CRC error detector 26 detects CRC errors by having reference to the CRC data DT CRC contained in the absolute time data DT AT obtained at the biphase demodulating circuit 24, and transmits the CRC error flag FG CRC indicating the result and its CRC error detection clock CK CRC to the system controller 18. When the input CRC error flag FG CRC indicates the absence of the CRC error, the system controller 18 indicates the absolute time data DT AT in, for example, a display section 27, while controlling the magneto-optical disc drive in its entirety, using the access command signals entered by an operating section 28 or the absolute time data DT AT . In this embodiment, the disc 1 is controlled in its rotation speed at constant linear velocity during the steady-state operation by the spindle motor 2 and a spindle control circuit 29 in accordance with the 6.3 kHz bit clock CK BIT produced in the bit clock regenerating circuit 25, while being controlled so as to be rotated at a substantially constant linear velocity using wobble signals S WB produced at the band-pass filter 20 of the absolute time reproducing section 19, when the phase locked loop of the bit clock regenerating circuit 25 is not in a correct phase lock state, such as at the start of rotation of the disc 1 or at the time of track jumps by the optical head 4, and hence correct bit clocks CK BIT cannot be produced. As shown in detail in Figure 5, the spindle control circuit 29 is so arranged and constructed that a first spindle servo signal SB SP1 obtained at a first spindle servo circuit 30 is supplied to a first terminal a of a switching circuit 32, a second spindle servo signal SB SP2 obtained at a second spindle servo circuit 31 is supplied to a second terminal b of the switching circuit 32, and the output of the switching circuit 32 is transmitted to the spindle motor 2 via a low-pass filter 33 as the spindle motor control signal CNT SP . The first spindle servo circuit 30 receives wobble signals S WB produced at the band-pass filter 20 of the absolute time reproducing/processing section 19, at a 1/12 frequency divider 30A, and supplies the 1/12 frequency divided signal as a comparison signal to a phase detector 30B. The 1/4 frequency divided 1.8375 kHz signal, produced in a 1/4 frequency divider 30C receiving a 7.35 kHz reference clock CK SB or CK WF from the system controller 18 via the EFM encoder 10 at the recording time or via the EFM decoder/error correction circuit 13 at the reproducing time, is also supplied to the phase detector 30B as the reference signals. The phase detector 30B compares the above comparison signal and reference signal and transmits the results of comparison such that the frequency of the wobble signal S WB equal to 22.05 kHz or 1.8375 kHz x 12, to the first terminal a of the switching circuit 32 as the first spindle servo signal SB SP1 . Then, using the first spindle servo signal SB SP1 as the spindle motor control signal CNT SP , the spindle control circuit 29 controls the spindle motor 2 to effect servo control so that the wobble signal S WB produced from the disc 1 has the frequency of 22.05 kHz, to control the disc 1 so as to be rotated at a substantially constant linear velocity. The second spindle servo circuit 31 receives the 6.3 kHz bit clocks CK BIT produced at the bit clock regenerating circuit 25, at a 1/3 frequency divider 31A, and supplies the 1/3 frequency divided signal as the comparison signal to a phase detector 31B. Also a 2/7 frequency divided 2.1 kHz signal, produced at a 2/7 frequency divider 31C receiving a 7.35 kHz reference clock CK SB or CK WF transmitted from the system controller 18 via the EFM encoder 10 at the recording time, or via the EFM decoder/error correction circuit 13 at the reproducing time, is also transmitted to the phase detector 31B as the reference signal. The phase detector 31B compares the above comparison and reference signals and transmits the results of comparison such that bit clock CK BIT correctly has the frequency of 6.3 kHz (=2.1 (kHz) x 3) as the second spindle servo signal SB SP2 to the second terminal b of the switching circuit 32. Then, using the second spindle servo signal SB SP2 as the spindle motor control signal CNT SP , the spindle control circuit 29 controls the spindle motor to effect servo control so that the bit clock CK BIT extracted from the reproduced biphase mark signal S BP obtained from the disc 1 correctly has the frequency equal to 6.3 kHz to control the rotation of the disc 1 correctly in accordance with the CLV system. Depending on the logical level of the switching control signal CNT SW obtained at the switching control circuit 34, the switching circuit 32 selects the first terminal a when the switching control signal CNT Sw is at the logical level "L", while selecting the second terminal b when the switching control signal CNT Sw is at the logical level "H". A CRC error flag FG CRC having a logical values of "1" and "0" when the CRC error is or is not detected, respectively, is entered from the CRC error detector 26 into a switching control circuit 34, along with the CRC error detection clock CK CRC . In the switching control circuit 34, the CRC error flag FG CRC is entered into an input terminal D of a first D-flip-flop 34A, the output terminal Q of which transmits a non-inverting output to an input terminal D of a second D-flip-flop 34B, the output terminal Q of which transmits a non-inverting output to an input terminal D of a third D-flip-flop 34C. The aforementioned CRC error detection clocks CR CRC are transmitted to clock terminals of a plurality of, herein four, serially connected D-flip-flops 34A to 34D, to which the CRC error flags FG CRC are transmitted sequentially, as described above. The inverting outputs from the inverting output terminals QI of the flip-flops 34A to 34D are transmitted to an AND gate 34E, the logical output of which is transmitted to a reset terminal R of an RS-flip-flop 34F. The non-inverting output from the output terminal Q of the flip-flop 34A is supplied to a set terminal S of the flip-flop 34F. Thus the non-inverting output from the output terminal Q of the flip-flop 34F goes to a logical level "H" at the time when the CRC error flag FG CRC is changed from the value "1" to the value "0", while going to a logical level "L" at the time when the CRC error flag FC CRC detects the state of the value "0" four times consecutively, starting from the state of the value of "1". Thus, depending on the CRC error flag FG CRC produced at the CRC error detector 26, the switching control circuit 34 selects the first terminal a of the switching circuit 32 on consecutive detection of the four CRC errors, to transmit the first spindle servo signal SB SP1 as the above spindle motor servo signal CNT SP , while selecting the second terminal b of the switching circuit 32 on the detection of the absence of the CRC error once to transmit the second spindle servo signal SB SP2 as the spindle motor servo signal CNT SP . The switching control circuit 34 is so arranged and constructed that, since the switching circuit 32 is switched on consecutive detection of the CRC errors four times, the spindle control circuit 29 is not responsive in its operation to CRC errors caused by drop-out or noise produced during steady-state rotation of the disc 1. In the above described magneto-optical disc drive, during the start of rotation of the disc 1 or track jumps by the optical head 4, for example, the phase locked loop of the bit clock reproducing circuit 25 is not in the phase-locked state, so that the bit clocks CK BIT cannot be obtained correctly. As a result, the absolute time data DT AT cannot be reproduced correctly, so that the CRC errors are detected a plurality of times on end. It will be noted that the spindle control circuit 29 transmits the first spindle servo signal SB SP1 as the spindle motor servo signal CNT SP to effect servo control in such a manner that the wobble signal S WB produced from the disc 1 has the frequency of 22.05 kHz, so that the disc 1 can be quickly controlled to rotate at a substantially constant linear velocity. When the disc 1 is controlled to rotate in this manner at the substantially constant linear velocity until the bit clock generating circuit 25 is within the range of being forced into the phase locked state by the phase locked loop, the bit clocks CK BIT can now be produced correctly, as a result of which the absolute time data DT AT can be reproduced correctly, so that it may be detected that there is no CRC error. When this state is reached, the spindle control circuit 29 supplies the second spindle servo signal SB SP2 as the spindle motor control signal CNT SP and performs servo control such that the bit clock CK BIT extracted from the biphase mark signal S BP reproduced from the wobble signals S WB derived from the disc 1 correctly has the frequency of 6.3 kHz, thereby realizing the accurate rotational control of the disc 1. In the above described embodiment of the present invention, the reproduced wobble signal S WB is compared with a prescribed reference signal until the absolute time data DT AT obtained upon demodulation of the wobble signals S WB taken out correctly, so that the spindle motor 2 rotationally driving the disc 1 is driven to rotate in a controlled manner at a substantially constant linear velocity. After it has been detected that the absolute time data DT AT obtained upon demodulation of the wobble signals S WB can be taken out correctly, the bit clocks CK BIT obtained upon demodulation of the reproduced wobble signals are compared with prescribed reference signals to effect the driving of the spindle motor 2 in a controlled manner. In this manner, a magneto-optical disc drive is realized in which the disc 1 can be immediately driven in a controlled manner, and without causing jitters at the time of the start of rotation of the disc 1 or at the time of track jumps by the disc 1. In the foregoing, the first spindle servo circuit 30 and the second spindle servo circuit 31 are employed in which the input wobble signals S WB or the bit clocks CK BIT are frequency divided and phase compared with the frequency divided signals of the reference clocks CK SB or CK WF . However, the present invention is not limited to this specified construction. Thus effects similar to those of the preceding embodiment may also be obtained when, as shown for example in Figure 6, a third spindle servo circuit 40 or a fourth spindle servo circuit 41 is employed in which the input wobble signal S WB or the bit clocks CK BIT is received at a speed counter 40A or 41A, along with a speed reference signal CK 96FS having the frequency of, for example, 4.2336 MHz (=96 x 44.1 kHz), the period of the wobble signal S WB or the bit clock CK BIT is measured at this speed counter 40A or 41A, and the thus produced speed error signal M DS1 or M DS2 is used as the spindle motor control signal CNT SP . Or alternatively, the fourth spindle servo circuit 41 may be annexed to, for example, the second spindle servo circuit 31, including the phase detector 31B, as shown in Figure 7, the spindle servo signal SB SP2 from the second spindle servo circuit 31 and the speed error signal M DS2 from the fourth spindle servo-circuit 41 are summed together at a summing circuit 42, and the sum signal is employed as the spindle motor control signal CNT SP . The above construction allows for more stable and extensive servoeing. Also, in the above embodiment, use is made of a magneto-optical disc in which pre-grooves are formed in ATIP format by wobble signals consisting of frequency modulated absolute time data. However, the invention is not limited to this embodiment, but may be extensively applied to instances wherein an optically recordable disc, such as a DRAW disc having pre-grooves formed thereon for affording wobble signals frequency modulated by prescribed data signals is driven at a substantially constant linear velocity.

Claims: An optical disc drive in which an optically recordable disc (1) having pre-grooves formed therein for providing a wobble signal frequency modulated by predetermined data signals is driven at a constant linear velocity; characterized by: first spindle servo means (30:40) for controlling a spindle motor (2) to rotate said optical disc (1) at a substantially constant linear velocity on the basis of a comparison of the reproduced wobble signal with a predetermined reference signal; second spindle servo means (31:41) for controlling said spindle motor (2) to rotate said optical disc (1) at a constant linear velocity on the basis of a comparison of clock bit signals obtained upon demodulating said reproduced wobble signal and said reference signal; switching means (32) for selecting said first spindle servo means (30:40) or said second spindle servo means (31:41); and detection means (34) for detecting whether said clock bit signals are derived correctly, and in dependence on said detection controlling said switching means (32). A disc drive according to claim 1 wherein said detection means (34) includes a CRC error detecting circuit (26). A disc drive according to claim 2 wherein said detecting means (34) further includes circuit means (34A to 34F) for detecting that said CRC error detecting circuit (26) has detected CRC errors a plurality of times successively.

Inventors: ANDO RYO PATENTS DIVISION, MAEDA YASUAKI PATENTS DIVISION, OBATA HIDEO PATENTS DIVISION, ANDO, RYO PATENTS DIVISION, MAEDA, YASUAKI PATENTS DIVISION, OBATA, HIDEO PATENTS DIVISION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1989
Main classification: G11B19
IPC codes: G11B19, G11B27, G11B20