RECORDING DISK REPRODUCING APPARATUS

Abstract: A recording disk reproducing apparatus employing simplified digital servo control for lower-cost, more stable operational control of the apparatus. At least one of the focusing servo loop, (10,27) a tracking servo loop (11,28), a single servo loop, (12,30) and a time base servo loop (19,17) employs digital conversion of an analog error signal for processing in a microprocessor (14). The analog-to-digital conversion circuitry (13) has an input-output characteristic which varies in accordance with error signal level, enabling a reduction in the number of bits of A/D output necessary, and reducing cost.

Description: The present invention relates to an optical disk reproducing apparatus. An optical recording disk reproducing apparatus reproduces recorded information on a recording surface of an information recording disk (hereinafter simply referred to as a disk) by using a pickup for radiating a light beam onto the recording surface to detect the recorded information by means of, for example, reflected light from the recording surface. In such a recording disk reproducing apparatus, the pickup is movable in the radial direction of the disk, and the rotational driving of the disk is controlled by a spindle servo device, so that the movement of an information detecting light spot of the pickup relative to the disk is controlled. The apparatus is further provided with a focusing servo device for controlling the position of the focus of the light beam of the pickup so as to form the information detecting light spot on the recording surface of the disk. The apparatus is also provided with a tracking servo device for controlling the light spot so as to make the light spot accurately trace a recording track which is formed on the recording surface, for example, spirally. There has been proposed a disk reproducing apparatus employing a so-called digital servo device for generating an error signal corresponding to a difference between a target value and a control amount on the basis of the output signal of the pickup so as to perform phase compensation, or the like, of the error signal in a digital manner. In such an apparatus, an A/D (analog-to-digital) converter for converting the analog error signal into a digital one is used. The higher the number of bits of the output data of the A/D converter, the higher the resolution, so that highly accurate controlling can be performed. However, as a result, the operational processing becomes complicated and the required processing time increases, so that the sampling frequency cannot be made sufficiently high. Consequently, there is deterioration of the phase characteristic of the servo, making stability poor as well as resulting in an increase in the number of the constituent elements of the A/D converter, thereby increasing the cost. For the foregoing reasons, the conventional disk reproducing apparatus using a digital servo device is disadvantageous in that it is impossible to perform stable control with high accuracy, and in that the cost is high. The basic components of the present invention such as indicated in the first part of claim 1 are disclosed in EP-A-138275. Moreover, GB-A-2134289 discloses the use of a limiter in the servo control system so that when the input to the servo control device exceeds a predetermined level (because of noise on the disk), the level of the input is reduced to that predetermined level. In view of the foregoing disadvantages in the conventional apparatus as described above, it is an object of the present invention to provide a disk reproducing apparatus in which stable control can be attained with high accuracy and in which the cost can be reduced. According to the present invention, optical disk reproducing apparatus comprising a pickup for directing an optical beam onto a recording surface of a disk containing recorded information, and for detecting information recorded on the disk by detecting light reflected from the recording surface; focusing servo means for controlling the position of a focus of the optical beam so as to form an information detecting light spot from the pickup on the recording surface of the disk tracking servo means for controlling the relative position of the information detecting light spot in the radial direction of the disk; spindle servo means for controlling motion of the information detecting light spot relative to the disk; and, analog-to-digital conversion means provided in at least one of the focusing servo means, tracking servo means, and spindle servo means, for analog-to-digital converting an error signal, an output of the analog-to-digital conversion means being subjected to operational processing for generating a driving signal for driving an actuator; is characterised in that: the analog-to-digital conversion means has an input-output characteristic which varies according to the level of the input error signal such that the ratio of a change in the output relative to a change in the error signal is reduced to a non-zero value when the absolute value of the level of the error signal is greater than a predetermined value. Accordingly, the conversion accuracy can be increased without increasing the number of bits of the analog-to-digital converting means in order to perform control with high accuracy. Furthermore, the operational processing can be simplified to attain a favourable phase characteristic, enabling stable control at reduced cost. In the drawings:- Fig. 1 is a block diagram showing the optical disk reproducing apparatus according to the present invention; Fig. 2 is a diagram showing a specific circuit of the D/A converter 13 in the apparatus of Fig. 1; and, Fig. 3 is a graph showing a characteristic of the circuit of Fig. 2. In Fig. 1, after being collimated through a collimator lens 3, laser light emitted from a laser beam source 2 such as a laser diode or the like provided in a pickup 1 is focussed to form an information detecting light spot on a recording surface of a disk 7 through a beam splitter 4, a 1/4 wavelength plate 5 for varying the direction of polarisation, and a focusing lens 6. After being passed through the focusing lens 6 and the 1/4 wavelength plate 5, reflected light from the recording surface of the disk 7 is made to impinge onto the beam splitter 4 so that the reflected light containing information is separated from the incident light. The reflected light separated by the beam splitter 4 is made to impinge onto a quartering detector 9 through a cylindrical lens 8. The outputs from the quartering detector 9 are respectively supplied to a focusing error signal generating circuit 10, a tracking error signal generating circuit 11, and a demodulating circuit 12. In the focusing error signal generating circuit 10, an analog focusing error signal is generated corresponding to the direction and amount of displacement of the focus of the laser beam emitted from the pickup 1. After being converted to a digital signal by an A/D converter 13, the digital focusing error signal is supplied to a microcomputer 14. In the tracking error signal generating circuit 11, an analog tracking error signal is generated corresponding to the direction and amount of a displacement of the spotlight relative to a recording track, by means of, for example, a three beam method. After being converted to a digital signal by an A/D converter 15, the digital tracking error signal is supplied to the microcomputer 14. In the demodulating circuit 12, an RF signal including recording information is generated on the basis of the output from the quartering detector 9, and subjected to FM-demodulation processing so as to reproduce a video signal. The output of the demodulating circuit 12 is supplied to a spindle error signal generating circuit 16, and at the same time is supplied to a video output terminal 18 as well as to a time base error signal generating circuit 19 through a CCD (charge coupled device) 17. In the spindle error signal generating circuit 16, a spindle error signal is generated corresponding to a phase difference between a reference signal having a predetermined frequency and a horizontal synchronizing signal included in the output of the demodulating circuit 12. In the time base error signal generating circuit 19, an analog time base error signal is generated corresponding to a phase difference between a reference signal having a predetermined frequency and a horizontal synchronizing signal included in the output of the CCD 17. After being converted to a digital signal by A/D converters 20 and 21 respectively, the digital spindle error signal and the digital time base error signal are supplied to the microcomputer 14. In the microcomputer 14, which is constituted by a processor, a ROM (read-only memory), a RAM (random-access memory), etc., the respective digital error signals supplied thereto are processed in accordance with predetermined equalizer characteristics by the processor which is arranged to operate in accordance with a program stored in advance in the ROM, and then supplied to D/A converters 22 through 26. The digitized focusing error signal which is subjected to phase compensation processing is supplied to the D/A converter 22. The output of the D/A converter 22 is supplied to a focusing actuator driving circuit 27 to drive a focusing actuator (not shown) for positioning the focusing lens 6 in the pickup 1 in the direction of the optical axis thereof. The digitized tracking error signal which is subjected to phase compensation processing is supplied to the D/A converter 23. The output of the D/A converter 23 is supplied to a tracking actuator driving circuit 28 so as to drive a tracking actuator for positioning the focusing lens 6 in the pickup 1 in the direction perpendicular to the optical axis thereof. A DC component of the digitized tracking error signal is supplied to the D/A converter 24. The output of the D/A converter 24 is supplied to a slider motor driving circuit 29 so as to send a slider (not shown) for carrying the pickup 1 in the radial direction of the disk 7. The digitized and phase-compensated spindle error signal is supplied to the D/A converter 25. The output of the D/A converer 25 is supplied to a spindle motor driving circuit 30 so as to drive a spindle motor 31 for rotationally driving the disk 7. The digitized and phase-compensated time base error signal is supplied to the D/A converter 26. The output of the D/A converter 26 is used as a control signal for a VCO (voltage-controlled oscillator) 32. The oscillation output of the VCO 32 is supplied to a clock input terminal of the CCD 17 so as to vary a signal delay time in accordance with the time base error signal to correct the error in the time base of the reproduced signal. Fig. 2 is a diagram showing a specific embodiment of the A/D converter 13. In the drawing, a focusing error signal is supplied to a negative input terminal of each of N comparators (N being an odd number), CP₁,..., CP n-1 , CP n , CP n+1 ,..., CP N . (N+1) resistors, R₁,..., R n , R n+1 ,..., R N+1 , are connected in series between reference potential points +V R and -V R . The serial junction points of the resistors R₁ - R N+1 are connected to the respective positive input terminals of the comparators CP₁ - CP N . Therefore, reference voltages different from each other are supplied to the respective positive input terminals of the comparators C₁ - CP N , so that the instantaneous level of the focusing error signal can be discriminated by the respective outputs of the comparators CP₁ - CP N . The respective outputs of the comparators CP₁ - CP N are supplied to a matrix circuit 35. The matrix circuit 35 generates digital data of a predetermined number of bits corresponding to the instantaneous level of the focusing error signal indicated by the respective outputs of the comparators CP₁ - CP N . Of the resistors R₁ - R N+1 serially connected between the reference potential points +V R and -V R , the resistance value of each of the m resistors R₁ - R m successively connected to the reference voltage point +V R and of the m resistors R N+2- m - R N+1 successively connected to the reference voltage point -V R is i times ( i being a real number larger than 1) as large as the resistance value of each of the resistors R m+1 - R N+1- m, and the input-output characteristic of the A/D converter 13 is made non-linear, as shown in Fig. 3. That is, assuming that the gradient of the characteristic curve is α in the case where the input voltage is not higher than the voltage V₁ at the serial junction point between the resistors R m and R m+1 and is not lower than the voltage V₂ at the serial junction point between the resistors R N+1-m and R N+2-m , the gradient of the characteristic curve becomes α/i when the input voltage is higher than the voltage V₁ and when the input voltage is lower than the voltage V₂. In the above-mentioned arrangement, the input dynamic range of the A/D converter 13 can be made large without increasing the number of bits, and the conversion accuracy in the case of a small amplitude of the focusing error signal can be made high. As a result, since the amplitude of the error signal becomes small when a servo loop is in a closed state, it is possible to perform highly accurate control. Since the number of bits is not increased, the operational processing may be performed easily so that the sampling frequency can be made sufficiently high and therefore the servo phase characteristic can be made favourable for stable control. Further, the amplitude of the focusing error signal becomes large in the case of pull-in of the servo so that a quantisation error acting as a disturbance in the servo loop becomes large. However, the pull-in characteristic is hardly influenced by a disturbance and the effect of the quantisation error can be disregarded in practical use. Although in the foregoing description, only the characteristic of the A/D converter 13 is made to be non-linear as shown in Fig. 3 in the above-mentioned embodiment, the characteristic of each of the A/D converters 15, 20 and 21 also can be made non-linear. Further, although the correction of the time base error is performed by the CCD 17 in the above-mentioned embodiment, the present invention is not limited to this structure, but is applicable to the case where the time base error is corrected by using a tangential actuator for moving an information detecting light spot in the extending direction of the recording track.

Claims: Optical disk reproducing apparatus comprising: a pickup (1) for directing an optical beam onto a recording surface of a disk (7) containing recorded information, and for detecting information recorded on the disk by detecting light reflected from the recording surface; focusing servo means (10,27) for controlling the position of a focus of the optical beam so as to form an information detecting light spot from the pickup on the recording surface of the disk; tracking servo means (11,28) for controlling the relative position of the information detecting light spot in the radial direction of the disk; spindle servo means (12,30) for controlling motion of the information detecting light spot relative to the disk; and, analog-to-digital conversion means (13) provided in at least one of the focusing servo means, tracking servo means, and spindle servo means, for analog-to-digital converting an error signal, an output of the analog-to-digital conversion means being subjected to operational processing for generating a driving signal for driving an actuator; characterised in that: the analog-to-digital conversion means has an input-output characteristic which varies according to the level of the input error signal such that the ratio of a change in the output relative to a change in the error signal is reduced to a non-zero value when the absolute value of the level of the error signal is greater than a predetermined value (V1). Apparatus according to claim 1, wherein the analog-to-digital conversion means (13) comprises: a plurality of serially-connected resistors (R₁ - R N+1 ) having different resistance values; a plurality of comparators (CP₁ - CP N ) each having their positive input terminals connected to a voltage division point between said resistors, the error signal being provided at a negative input terminal of each of the comparators; and, a matrix circuit (35) having its inputs connected to respective outputs of the comparators, wherein the matrix circuit generates digital data of a predetermined number of bits corresponding to an instantaneous level of said error signal. Apparatus according to claim 1 or claim 2, wherein the input-output characteristic of the analog-to-digital conversion means has a first gradient (α) of the input-output characteristic within a range defined by first and second input voltage values (V2,V1), and a second gradient (α/i) outside of said range. Apparatus according to any of claims 1 to 3, wherein at least two of the focusing servo means, the tracking servo means, and the spindle servo means include the analog-to-digital conversion means. Apparatus according to any of claims 1 to 3, wherein each of the focus servo means, the tracking servo means, and the spindle servo means includes said analog-to-digital conversion means, the recording disk reproducing apparatus further comprising: microprocessor means (14) for receiving outputs of the focus servo means, tracking servo means, and spindle servo means for providing digital servo control for reproduction of the information recorded on the disk. Apparatus according to any of claims 1 to 5 further including time base servo means (19) for correcting a time base error in the information recorded on the disk and detected by the pickup. Apparatus according to claim 6, the time base servo means including a charge coupled device (17) which receives demodulated information and whose output is coupled to an input of a time base error signal generating circuit (19) in a feedback manner, and a voltage-controlled oscillator (32) coupled between an output of the time base error signal generating circuit and a control input of the charge coupled device.

Inventors: KUSANO, SATOSHI C/O PIONEER ELECTRONIC CORPORATION
Applicants: PIONEER ELECTRONIC CORPORATION
Application year: 1987
Main classification: G11B7
IPC codes: G11B7, H04N5, G11B20, G11B19