Motor servo circuit for disc reproduction apparatus

Abstract: In a LD player, a phase comparison circuit compares the phases of a horizontal synchronizing signal reproduced from a LD with a reference synchronizing signal to generate a phase error signal having a pulse width according to the phase difference. After the phase error signal is amplified by an equalizer amplifier, this phase error signal is supplied to a motor driving circuit, whereby phase servo control is carried out for the spindle motor. After the start of the reproduction of the disc, a variation amount of the phase error signal during a period corresponding to a predetermined number of rotation of the disc is detected as the eccentricity amount of the rotation of the disc. The duty ratio of the phase error signal is digital-modulated according to the eccentricity amount, whereby the servo gain of the entire servo circuit is corrected to an optimum value.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to motor servo circuits for disc reproduction apparatus, and more particularly, to a motor servo circuit controlling mainly the phase of a spindle motor used for rotating a disc in a reproduction apparatus of optical discs such as laser discs (referred to as LD hereinafter) and compact discs (referred to as CD hereinafter). Description of the Background Art A conventional reproduction apparatus of an optical disc such as a LD and a CD is provided with a motor servo circuit for servo controlling the phase and speed of a spindle motor for the rotation driving of an optical disc. Fig. 1 is a block diagram schematically showing a structure of such a motor servo circuit in a laser disc player (LD player) as an example of an optical disc reproduction apparatus. Referring to Fig. 1, a LD1 is loaded on a chucking device 4 constituted by a turntable and a clamper provided at the top end of a spindle 3 of a spindle motor 2 at the time of reproduction of LD. LD1 is held by chucking device 4 and rotated. The information recorded on the signal face of LD1 is read by an optical pickup 5 to be supplied to a signal processing circuit not shown. Spindle motor 2 is provided with a FG pulse generator not shown. This FG pulse generator generates a FG pulse signal having a frequency proportional to the rotating speed of spindle motor 2. The FG pulse signal is supplied to one input terminal 10 of a speed comparison circuit 9. The other input terminal 11 is supplied with a reference FG signal REFFG of a reference frequency provided from an oscillation circuit not shown. At the start of the reproduction operation of LD1, spindle motor 2 is actuated by a speed error signal provided from speed comparison circuit 9. More specifically, speed comparison circuit 9 compares the frequency of the FG signal supplied to input terminal 10 with that of the REFFG signal supplied to input terminal 11 to generate a speed error signal having a pulse width proportional to the difference therebetween. The speed error signal is applied to a motor driving circuit 16 via a switch circuit 12 connected to the side of contact 12b at the initial state of the rotation drive of spindle motor 2 and via an equalizer amplifier 13 that will be explained later on. Motor driving circuit 16 is responsive to the output of equalizer amplifier 13 to actuate the rotation of spindle motor 2. When the rotating speed of spindle motor 2 approaches a predetermined rotating speed by the rotation drive of motor driving circuit 16, the rotating speed of spindle motor 2 is captured to the aforementioned predetermined speed by a speed servo according to the aforementioned speed error signal. When the rotating speed of spindle motor 2 started to be captured to a predetermined speed, a condition wherein the recorded information can be read out from the signal face of LD1 is established. The horizontal synchronizing signal in the video signal read out by optical pickup 5, i.e. the PBH signal is separated by a signal processing circuit not shown and supplied to one input terminal 7 of phase comparison circuit 6. The other input terminal 8 is supplied with a reference horizontal synchronizing signal REFH having a reference phase provided from an oscillation circuit not shown. Phase comparison circuit 6 compares the phase of the PBH signal supplied to input terminal 7 with that of the REFH signal supplied to input terminal 8 to generate a phase error signal having a pulse width proportional to the magnitude of shift of the phase of the PBH signal from the reference phase. The PBH signal separated at the signal processing circuit not shown is supplied to a lock detecting circuit 57. Lock detecting circuit 57 detects the period of the PBH signal to generate a speed lock detection signal a for providing the same to the control input of switch circuit 12, when the detected period is within ±20% range of a predetermined period of 63.5 micro seconds. Switch circuit 12 responds to the speed lock detection signal a to be switched to the side of contact 12a. As a result, the phase error signal provided from phase comparison circuit 6 is supplied to equalizer amplifier 13 via switch circuit 12. Equalizer amplifier 13 is constituted by an integration and amplifying circuit 4, and a phase compensation circuit 15. Integration and amplifying circuit 14 implements a low pass filter processing for the applied phase error signal, and amplifies the obtained signal. Phase compensation circuit 15 functions to compensate for phase delay by 90° of the phase error signal associated with the low pass filter processing by integration and amplifying circuit 14. The phase error signal from equalizer amplifier 13 is supplied to motor driving circuit 16. In response, motor driving circuit 16 drives the rotation of spindle motor 2 to capture the rotation phase of spindle motor 2 to a reference phase. At the time of LD reproduction, there is periodical time base variation in the rotation of the LD caused by eccentricity of the LD itself, or that of the chucking device of the LD player. Therefore, circuit constants of the above mentioned equalizer amplifier 13 are set to values so that the servo characteristics of the entire phase servo circuit constituted by phase comparison circuit 6, equalizer amplifier 13, motor driving circuit 16 and spindle motor 2 can capture the rotation phase of spindle motor 2 to a reference phase and suppress periodical time base variation of the LD rotation caused by various eccentricity. The magnitude of eccentricity (eccentricity amount) of the above mentioned disc rotation differs greatly depending on the reproduced discs and is not uniform. It is therefore desired that various servo characteristics of the phase servo circuit are prepared so as to conform to the wide range of eccentricity amount. However, this is difficult to realize due to limitations on circuit configuration. In conventional practice, the circuit constants of equalizer amplifier 13 were selected to implement a constant servo characteristics suitable for suppressing an intermediate servo characteristics between servo characteristics of great eccentricity amount of rotation and that of small eccentricity amount, i.e. a standard magnitude of eccentricity. If the actual eccentricity amount of the rotation of the disc is greatly shifted from this standard eccentricity amount, the capture of the rotation phase of spindle motor 2 to a reference phase becomes difficult with the phase servo characteristics set in advance to deteriorate the accuracy of phase servo control. If it becomes difficult to suppress the time base variation of rotation, the time base variation of the reproduced video signal will not come within the possible correction range of a time base correction (TBC) circuit (not shown) provided for subjecting the reproduced video signal to time base correction. This means that accurate signal processing of the reproduced video signal is not always possible. This problem is particularly significant in two side reproduction type LD players. Such a two-side reproduction type LD player is disclosed in U.S. Patent No. 4,839,881, for example, wherein an optical pickup reads out information in the signal face of the lower side of the LD mounted on a chucking device, and then moves to the upper side of the LD to read out information on the signal face thereof. The eccentricity amount at the time of lower side face reproduction and upper side face reproduction differ due to reasons such as the lower and upper sides of the LD not accurately bonded together in a two-side reproduction type LD player. Therefore, the phase servo characteristics realized to suit the eccentricity amount of the lower side reproduction do not always suit the eccentricity amount of the upper side reproduction. In such a case, time base correction of the reproduced signal from the signal face of the upper side could not be carried out accurately to result in a disturbed reproduced picture. Fig. 2 is a block diagram showing another example of a conventional motor servo circuit for a LD player. Speed comparison circuit 9, switch circuit 12 and lock detecting circuit 57 shown in Fig. 1 are not depicted in Fig. 2. The phase servo circuit of Fig. 2 differs from the phase servo circuit of Fig. 1 in that the former comprises a gain switching circuit 18 provided between equalizer amplifier 13 and motor driving circuit 16, and a system control circuit 19 implemented by a microcomputer for controlling gain switching circuit 18. The basic operations of the phase servo circuit of Fig. 2 is similar to that of the phase servo circuit of Fig. 1, except for the output of equalizer amplifier 13 (referred to as the first phase error signal hereinafter) supplied to gain switching circuit 18, whereby the level of the first phase error signal varies according to the type of the reproduced LD. The output of gain switching circuit 18 (referred to as the second phase error signal hereinafter) is supplied to motor driving circuit 16 to implement capture of the rotation phase of spindle motor 2 to a reference phase. The inertia force required for the rotation of spindle motor 2 is different for a LD having a diameter of 20cm and a LD having a diameter of 30cm. If phase servo control is carried out with the same servo gain in all cases according to just the first speed error signal, an appropriate phase servo control may not be obtained due to the servo gain being too high or too low. A status signal (referred to as STAS signal hereinafter) for switching the servo gain is supplied to gain switching circuit 18 from system control circuit 19 according to the type of the reproduced disc, whereby the level of the phase error signal, i.e. the phase servo gain is switched accordingly. Fig. 3A is a circuit diagram showing an example of a gain switching circuit 18. Gain switching circuit 18 of Fig. 3A is constituted by a discrete circuit comprising dividing resistors Ra and Rb for level adjustment, and an analog switch SW turned on/off by STAS signal. Corresponding to the type of the LD to be reproduced, analog switch SW of gain switching circuit 18 is turned on/off according to STAS signal from system control circuit 19 to change the level of the first phase error signal, whereby the first error signal is converted into a second phase error signal. Fig. 3B is a circuit diagram showing another example of gain switching circuit 18. Instead of analog switch SW of Fig. 3A, the circuit of Fig. 3B employs a switch circuit formed of transistors Q 1 and Q 2 , a diode D 1 , bias resistors Rc, Rd, Re, and a bias voltage supply terminal Vcc. Although the examples of Figs. 3A and 3B have the servo gain switched into two stages, it is possible to switch the servo gain into three or more stages. In this case, a multi-contact analog switch SW may be implemented using a digital signal of a plurality of bits as STAS signal. The conventional phase servo control circuit of Fig. 2 requires dedicated gain switching circuits of discrete configuration of a scale according to the number of stages of switching, in order to switch the servo gain according to the type of the disc. If there are many switching stages, the number of components of the gain switching circuit increases, to result in a phase servo circuit of high cost. Patent Abstracts of Japan, Vol. 3, No. 90 (E-128), July 31, 1979 (Sanyo Denki K.K.) discloses a system which determines the eccentricity of a disc by detecting over a period longer than one rotation the variation amount of the phase of a readout synchronization signal. SUMMARY OF THE INVENTION An object of the present invention is to provide a motor servo circuit for a disc reproduction apparatus capable of phase servo control of high precision. Another object of the present invention is to provide a motor servo circuit for a disc reproduction apparatus that can change the phase servo gain to an optimum gain according to the eccentricity amount of rotation of a disc, without changing the discrete circuit structure for phase servo control. A further object of the present invention is to provide a motor servo circuit for a disc reproduction apparatus than can change the phase servo gain according to various factors such as the type of the reproduced disc, the reproduction position, and the reproduction mode, without using a dedicated gain switching circuit of a discrete structure. These objects are solved by the features of the main claim. Further advantages are given by the features of the subclaims. The main advantage of the present invention is that accurate phase servo control can be carried out with a simple and economical circuit structure, because the servo gain of the entire phase servo circuit is switched to an optimum value according to the eccentricity amount of the rotation of the disc, while the circuit constants of discrete servo circuits such as an equalizer amplifier are fixed. Another advantage of the present invention is that eccentricity of disc rotation can be suppressed for each signal face because an optimum phase servo gain is realized for each signal face in a two-side automatic reproduction type LD player. A further advantage of the present invention is that the gain of the entire servo circuit can be switched to an optimum gain with a simple and economical structure without providing separately a gain switching circuit of a discrete structure by digital-modulating the phase or speed error signal according to various factors such as the type of the reproduced disc, the reproduction position and the reproduction mode. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram schematically showing a conventional motor servo circuit of LD player. Fig. 2 is a block diagram schematically showing another example of a conventional motor servo circuit. Figs. 3A and 3B are circuit diagrams showing in detail gain switching circuits of the motor servo circuit of Fig. 2. Fig. 4 is a block diagram schematically showing a motor servo circuit according to an embodiment of the present invention. Fig. 5 is a block diagram showing in detail an eccentricity detecting circuit, a duty modulating circuit and a gain determining circuit. Fig. 6 is a timing chart for explaining the operation of the embodiment of Fig. 4. Fig. 7 is a graph indicating the operation characteristics of the embodiment of Fig. 4. Fig. 8 is a flow chart for explaining the operation of the embodiment of Fig. 4. Fig. 9 is a block diagram schematically showing a modification of the embodiment of Fig. 4. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 4 is a block diagram schematically showing a motor servo circuit according to an embodiment of the present invention. The motor servo circuit of the first embodiment of the present invention of Fig. 4 is similar to the conventional motor servo circuit of Fig. 1 except for the following points. Referring to Fig. 4, phase error signal ERR1 provided from a phase comparison circuit 6 is supplied to an eccentricity detecting circuit 22 and a duty modulating circuit 23. Eccentricity detecting circuit 22 is supplied with a FG signal from a FG pulse generator not shown via a terminal 10. Eccentricity detecting circuit 22 responds to this FG signal to count the predetermined number of rotation of a spindle motor 2 to detect the range of variation of phase error signal ERR1 during that period, i.e. the magnitude of the eccentricity of the rotation of LD1. A correction constant of the servo gain is determined by a gain determining circuit 24 according to the magnitude of eccentricity detected by eccentricity detecting circuit 22. According to the correction constant, duty modulating circuit 23 changes the duty of phase error signal ERR1 to switch the servo gain of the phase servo circuit. The phase error signal having the duty ratio changed by duty modulating circuit 23 is supplied as signal ERR2 to a contact 12a of switch circuit 12. The functions of phase comparison circuit 6, speed comparison circuit 9, switch circuit 12, eccentricity detecting circuit 22, duty modulating circuit 23, and gain determining circuit 24 are implemented by a microcomputer 20. Fig. 5 is a block diagram showing in detail eccentricity detecting circuit 22, duty modulating circuit 23, and gain determining circuit 24 of Fig. 4. Referring to Fig. 5, eccentricity detecting circuit 22 comprises a maximum value latching portion 22a for latching the maximum value of phase error signal ERR1 provided from phase comparison circuit 6, a minimum value latching portion 22b for latching the minimum value of the signal ERR1, a timing control portion 22c for controlling latching portions 22a and 22b, and a subtractor 22d for calculating the difference between the outputs of latching portions 22a and 22b as the eccentricity amount. Gain determining circuit 24 comprises an operation portion 24a for calculating a correction constant α for changing the phase servo gain to an appropriate servo gain, and a memory 24b for storing the correction constant α. The duty modulating circuit 23 comprises a down-counter 23a and an up-counter 23b for counting the clock pulses provided from a clock signal source not shown, and a multiplier 23c. Fig. 6 is a timing chart and Fig. 7 is a characteristic diagram for explaining the operation of the first embodiment. The operation of the first embodiment will be explained hereinafter with reference to Figs. 4-7. After LD1 is mounted on a chucking device 4, spindle motor 2 is actuated by a speed error signal from a speed comparison circuit 9. The rotating speed of spindle motor 2 is captured as mentioned before to a predetermined speed. When the rotating state wherein reproduction of signals from the signal face of LD1 is possible, switch circuit 12 switches to the side of contact 12a according to a speed lock detection signal a from lock detecting circuit 17, as in the conventional example of Fig. 1. One input terminal 7 of phase comparison circuit 6 is supplied with a reproduced horizontal synchronizing signal PBH (Fig. 6 (a)) from a signal processing circuit not shown. More specifically, the lock detecting circuit 17 detects a period of the PBH signal in a manner similar to that of the conventional example. When the detected period comes into a range of ±20% with respect to a predetermined period of 63.5 micro seconds, the lock detecting circuit 17 determines that a speed locked condition is reached and generates the speed lock detection signal a . After this speed lock detection signal a is generated, the lock detecting circuit 17 continues to monitor the PBH signal. When the absolute value of the phase difference between the phases of the PBH signal and REFH signal reaches the value less than a predetermined value, the lock detecting circuit 17 generates a phase lock detection signal b . The other input terminal of phase comparison circuit 6 is supplied with a reference horizontal synchronizing signal REFH (Fig. 6 (b)). Phase comparison circuit 6 compares the timing (tx 1 , tx 2 of Fig. 6 (a)) of the falling edge of PBH signal with the timing (tr 1 , tr 2 of Fig. 6 (b)) of the falling edge of REFH signal to generate a phase error signal ERR1 (Fig. 6 (c)) of pulse widths t 1 , t 2 corresponding to the difference between the aforementioned timing of the PBH signal from the timing of the REFH signal. After the phase lock detection signal b is generated from lock detecting circuit 17, phase error signal ERR1 is applied to maximum value latching portion 22a and minimum value latching portion 22b of eccentricity detecting circuit 22, and to multiplier 23c of duty modulating circuit 23. The phase error signal ERR1 has the pulse width determined as t 1 = n 1 ·τ, t 2 = n 2 ·τ, ... (Fig. 6 (d)) by counting the internal clock pulses having a period τ (τ < t 1 , t 2 ) of microcomputer 20. The lead or lag of the PBH signal phase to REFH signal phase is represented by adding a positive or negative sign. When timing control circuit 22c of eccentricity detecting circuit 22 detects that LD1 has reached a rotating state possible for reproduction, according to the phase lock detection signal b provided from lock detecting circuit 17, timing control circuit 22c activates maximum value latching portion 22a and minimum value latching portion 22b during the period of spindle motor 2 rotating by a predetermined number of rotation by counting a predetermined number of FG pulses. During the activation period, these latching portions 22a and 22b automatically repeat latching to latch the maximum value data and minimum value data of the pulse width of phase error signal ERR1 during the period of spindle motor 2 rotating for a predetermined number of rotation. The maximum and minimum values of the ERR signal held in latching portions 22a and 22b, respectively, are provided to subtractor 22d. Subtractor 22d provides the difference between these data, i.e. an output corresponding to the magnitude of the variation range of ERR1 signal. The output provided from subtractor 22d corresponds to the eccentricity amount of the LD rotation caused by the eccentricity of the LD itself or the eccentricity of the chucking device. The output from subtractor 22d is provided to operation portion 24a of gain determining circuit 24. Operation portion 24a calculates a correction constant α for implementing an appropriate servo gain for the eccentricity amount according to the predetermined characteristics of Fig. 7, for example. Referring to the characteristics of Fig. 7, the capture of the rotation phase of spindle motor 2 to the reference phase is accelerated by maximizing the correction constant α to the maximum value αmax to increase the phase servo gain when the eccentricity amount is reduced. When the eccentricity amount is increased, the capture to the reference phase is retarded by minimizing the correction constant α to the minimum value αmin to reduce the phase servo gain. This is carried out to suppress current consumption and heat generation. In the graph of Fig. 7, ta and tb in the abscissa, respectively indicate the upper limit value and the lower limit value of the saturation of correction constant α. The correction constant α determined by operation portion 24a is stored in a memory 24b. The predetermined constant α 0 of a predetermined reference servo gain held in memory 24b as the initial value is changed to the newly determined correction constant α. Multiplier 23c of duty modulating circuit 23 functioning as a gain switching circuit has one input supplied with phase error signal ERR1 from phase comparison circuit 6 and the other input supplied with correction constant α from memory 24b. Multiplier 23c multiplies the pulse width of ERR1 signal by α to generate the data having pulse width of α·n 1 ·τ, α·n 2 ·τ, ... and provides the same to the up-counter 23b. Down-counter 23a always counts down internal clock pulses CK of microcomputer 20. When the counted value becomes 0, down counter 23a is self-preset by data corresponding to modulation period T (Fig. 6 (e)) equivalent to the periods of the falling edge tr 1 , tr 2 of REFH signal. This count down operation is repeated. Thus, down-counter 23a repeatedly counts modulation period T. Up-counter 23b is instantly reset in synchronism with the self-preset of down counter 23a and begins right after the reset to count up internal clock pulses CK during the period of the α multiplied pulse width of ERR1 signal. As a result, a phase error signal ERR2 of period T with the pulse width modulated is provided (Fig. 6 (e)) having a pulse width of α·n 1 ·τ, α·n 2 ·τ, ... that is α times the pulse width of ERR1 signal. Thus, duty modulating circuit 23 functions as a circuit digital-modulating the duty ratio of a signal of a constant period T. Phase error signal ERR2 obtained in the above manner is applied to equalizer amplifier 13 of Fig. 4, where phase servo control of spindle motor 2 is carried out in a manner identical to that of the conventional example of Fig. 1. The circuit constants of the discrete integration and amplifying circuit 14 and phase compensation circuit 15 forming equalizer amplifier 13 are fixed at this time. Therefore, the phase servo gain according to equalizer amplifier 13 is constant. However, the servo gain of the entire phase servo circuit of Fig. 4 is switched to an optimum servo gain according to the actual eccentricity amount of the disc rotation during reproduction, because the phase error signal itself is converted (digital-modulated) from ERR1 signal to ERR2 signal according to the eccentricity amount of the disc rotation. Fig. 8 is a flow chart for explaining the operation of microcomputer 20 of the above mentioned embodiment. Referring to Fig. 8, at step S1, determination is first made whether the phase servo is locked, i.e. whether the rotation phase of spindle motor 2 is captured to the reference phase. Then, at step S2, determination is made whether spindle motor 2 has rotated by a predetermined number of rotation according to the counted value of the FG pulses. During the period until motor 2 has rotated by the predetermined number of times, the maximum and minimum values of phase error signal ERR1 are latched respectively at steps S3 and S4. If determination is made that spindle motor 2 has rotated by a predetermined number of times at step S2, the difference between the maximum value and the minimum value of the above mentioned phase error signal ERR1 is obtained at step S5, whereby the eccentricity amount of rotation of the disc is detected at step S6. At step S7, correction constant α of the phase servo gain is determined according to the detected eccentricity amount. At step S8, the duty of the phase error signal ERR1 is modulated according to the terminated constant α. With this phase error signal ERR2 having a modulated duty ratio, phase servo control is carried out for spindle motor 2. The above mentioned correction constant α is held in memory 24b and read out to multiplier 23c until the LD mounted on chucking device 4 is removed. The calculation of correction constant α of operation portion 24a may be carried out by preparing in advance a table of a combination of the eccentricity amount and correction constant α according to the characteristics and use this table, instead of specifying an equation having a characteristics of Fig. 7. The characteristics for determining the correction constant may be specified appropriately according to various factors such as the eccentricity characteristics of the disc and the eccentricity characteristics of the chucking device. Although the functions of phase comparison circuit 6, speed comparison circuit 9, switch circuit 12, lock detecting circuit 17, eccentricity detecting circuit 22, duty modulating circuit 23, and gain determining circuit 24 are implemented by a microcomputer 20 in the above described embodiment, they may be implemented by discrete circuits. Thus, according to the embodiment of the present invention, the servo gain of the entire phase servo circuit can be switched to an optimum value according to the actual eccentricity amount of the disc rotation, while fixing the circuit constants of the discrete circuits such as integration and amplifying circuit 14 and phase compensation circuit 15. The embodiment is advantageous when applied to a two-side automatic reproduction type LD player. Fig. 9 is a block diagram schematically showing a modification wherein the above described embodiment is applied to a two-side automatic reproduction type LD player. Referring to Fig. 9, optical pickup 5 is attached to a U-shaped guide member 26 movably. In the initial state of reproduction of LD, optical pickup 5 is placed under the lower side face of LD1 so as to read out signals from the lower side face of LD1 in a manner similar to that of the first embodiment shown in Fig. 4. During such reproduction, correction constant α corresponding to the eccentricity amount of the rotation of the lower side face is calculated, so that an optimum phase servo gain is obtained with respect to the reproduction of the lower side face. After the reproduction of the lower side face of LD1 is finished, optical pickup 5 moves to the position over the upper side face of LD1 (position indicated by broken line) along the guide member 26. Then, optical pickup 5 successively start to reproduce signals from the innermost circumference of the upper side face of LD1. In Fig. 9, a turntable 25 forming chucking device is shown while clamper is not shown. As shown in Fig. 9, since LD1 is mounted on turntable 25 by fitting a spindle member 25a projecting at the center of turntable 25 into a central hole of the lower side face 1a of LD1, a reference surface of chucking is set to the lower side face. Generally, LD1 is formed by bonding two discs together each having signals recorded thereon. Accordingly, eccentricity of the upper side face occurs with respect to the lower side face. As a result, when the signals are reproduced from the upper side face 1b, a large amount of eccentricity occurs corresponding to the total amount of the eccentricity amount of the lower side face caused by chucking device and that of the upper side face caused with respect to the lower side face. According to the embodiment shown in Fig. 9, correction constant α is calculated again when starting the reproduction of the upper side face, so that an optimum phase servo gain is realized with respect to the reproduction of the upper side surface. That is to say, at the time of reproduction, a correction constant is selectively read out according to the reproduced signal face to implement an optimum phase servo gain for the reproduced face, because a correction constant α according to each side is determined and held when information recorded on each of two sides of the LD is first reproduced.

Claims: A motor servo circuit in a disc reproducing apparatus for phase servo controlling a motor (2) which drives the rotation of a disc (1) during reproduction, the disc having information including at least a synchronizing signal recorded thereon on a signal face, said motor servo circuit comprising: phase comparison means (6) for comparing said synchronizing signal reproduced from said disc during reproduction with a reference signal to generate a signal indicating the phase difference therebetween, phase servo means (13, 16) for servo controlling the rotation phase of said motor according to said signal indicating the phase difference, and gain switching means (23) responsive to a gain control signal for changing the phase servo gain of said phase servo means, characterized by eccentricity detecting means (22) for detecting the variation amount of said phase difference during a predetermined period longer than the rotation period of said disc, after the rotating speed of said disc reaches a predetermined speed and reproduction of signals from said disc becomes possible, and gain determining means (24) for determining an optimum phase servo gain according to the eccentricity amount of the rotation of said disc according to said variation amount of said detected phase difference and for outputting said gain control signal in accordance with said determined optimum phase servo gain, said gain switching means (23) being responsive to said gain control signal for maintaining the phase servo gain of said phase servo means as a constant initial servo gain until said optimum phase servo gain is determined by said gain determining means and for changing said intitial servo gain to said determined optimum phase servo gain after said determination. The motor servo circuit according to claim 1, wherein said signal indicating said phase difference is a phase error signal (ERR2) having a pulse width corresponding to the phase difference between the phase of said reproduced synchronizing signal (PBH) and that of said reference signal (REFH). The motor servo circuit according to claim 2, wherein said eccentricity detecting means (22) comprises means (22a, 22b, 22c) for latching the maximum value and the minimum value of said phase error signal during said predetermined period, and means (22d) for calculating the difference between said latched maximum value and minimum value. The motor servo circuit according to claim 3, wherein said gain determining means (24) includes means (24a) for determining an optimum phase servo gain to suppress eccentricity of the rotation of said disc according to the variation amount of said detected phase error signal, using a predetermined relation between said eccentricity amount of the rotation of said disc and said phase servo gain, and means (24b) for holding a signal representing said determined optimum phase servo gain. The motor servo circuit according to claim 4, wherein said gain switching means (23) includes means (23a, 23b, 23c) for digital-modulating the duty ratio of said phase error signal according to said signal indicating said determined optimum phase servo gain. The motor servo circuit according to claim 1, wherein said disc reproduction apparatus is a two-side automatic reproduction type disc reproduction apparatus, and said gain determining means (24) determines an optimum phase servo gain for each side of said disc.

Inventors: OKAMOTO NOBUHIRO, OKAMOTO, NOBUHIRO
Applicants: SANYO ELECTRIC CO, SANYO ELECTRIC CO., LTD.
Application year: 1991
Main classification: G11B19
IPC codes: G11B19