Optical disc apparatus

Abstract: An optical disc apparatus includes a tracking device which positions the beam light on the recording track of an optical disc in order to carry out at least recording or reproducing of information by moving the beam light in the radial direction of the optical disc, and a detection device for obtaining a position deviation signal of the beam light by receiving reflected light from the surface of the optical disc. In addition, the apparatus includes an optical head which carries out recording or reproducing of information for the recording track of the optical disc, a control device which controls the tracking device in response to the position deviation signal obtained from the detection device, a first device which reads, prior to an actual operation of recording or reproducing, a position deviation signal from the detection device, and stores the resulting data in a memory as eccentricity correction data for the recording track, and a second device which transfers, when actually carrying out recording or reproducing operation, the eccentricity correction data memorized in the memory to the D/A converter and gives the output of the D/A converter to the tracking control device. The D/A converter converts the digital quantity read from the memory to an analog signal, and the analog signal may be applied to a device for driving the optical head.

Description: The present invention relates to an optical disc apparatus which carries out recording or reproducing of information by the tracking of an optical head with respect to a rotating optical disc, and more particularly to an eccentricity correction means of the optical disc. Description of the Prior Art In general, in an optical disc apparatus of the above kind, an optical disc has concentric circular or spiral recording tracks as information recording medium, and the recording or reproducing of information for the recording track of the optical disc is carried out by linearly moving the recording head in the radial direction of the optical disc while rotating the disc. In such an optical disc apparatus, the optical head has to precisely follow the recording tracks of the rotating disc. For this reason, tracking control is ordinarily carried out by using a tracking servo system. Now, in the optical disc apparatus of the above description, when the optical head is moved from an arbitrary recording track which is in rotation to another recording track, tracking is interrupted for a while to move the optical head for a distance which corresponds to the separation between the present and desired tracks, and tracking is resumed. However, the recording tracks themselves of the optical disc are in general not necessarily concentric with respect to the center of the rotation shaft of the motor that drives to rotate the optical disc, and are often eccentric. Because of this, when tracking is interrupted once, the relative position of the optical head with respect to the recording track has a dispersion that corresponds to the eccentricity amount so that when tracking is resumed after the head has been moved, it leads to a drawback that accuracy tends to be lowered by the deviation corresponding to the dispersion. This results in a difficulty in reducing the time for positioning the optical head to the desired track. Further, in the optical disc 1. as shown in Fig. 1, there is given in advance a spiral pre-groove 2 for tracking on the surface of the substrate, in order to facilitate recording and reproducing. Since the recording track is provided in the pre-groove 2, the portion indentical to that of the pre-groove 2 is referred to simply as the track hereinafter. In Fig. 1, for facilitating the understanding, the track pitch is drawn schematically with wide separation. However, the real track is formed with a considerably small pitch of the order of µm. The optical disc 1 is placed on a spindle motor, as will be described later, to be rotated in the direction of the arrow R of Fig. 1. Accompanying the rotation, the beam spot (laser spot) of the optical beam projected from the optical head is traced on the track 2. The beam spot is shifted to track 2 by one track outward for each one rotation of the optical disc 1. Recording of information is made by the change in reflectivity generated by the phase transition or the like in a thin film layer in the track portion of the disc, through irradiation of the beam spot whose intensity is modulated by an information signal. Fig. 2 shows a track control device for causing the optical head 4 to follow the track 2 in recording and reproducing information from the optical disc 1 as in the above. In Fig. 2, a light source is omitted to simplify the track control device. In the optical head 4, reference numeral 5 is an objective lens which lets the beam spot of the light beam to be projected on the optical disc 1, 6 is a driving coil for controlling the position of the objective lens 5 in the radial direction of the optical disc 1, 13a and 13b are photodetector for detecting the intensities of reflected lights ℓ1 and ℓ2, respectively, photodetector 13a being connected to the negative input terminal of a differential amplifier 15a via a sensor amplifier 14a while photodetector 13b is connected to the positive input terminal of the differential amplifier 15 via another sensor amplifier 14b. It should be noted that there is provided an optical system for incidence from the laser diode as will be described later. However, for the sake of convenience of explanation the incidence optical system is omitted in Fig. 2 and the configuration of the reflection optical system is illustrated mainly. If the beam spot is projected correctly on the pre-groove 2 as shown in the figure and the track following is carried out precisely, then the intensities of both reflected lights ℓ1 and ℓ2 become equal and the track difference signal voltage TS output from the differential amplifier 15 becomes zero. If the beam spot is shifted toward the center of the optical disc 1 as shown by the arrow in Fig. 2, the reflected light ℓ1 is reflected from the surface portion of the disc (the area outside the pre-groove 2) with larger intensity, giving larger detected value to the photodetector 13a, and causes the differential amplifier 15 to output a negative track deviation signal voltage - TS. If the beam spot is shifted in the direction opposite to the arrow in Fig. 2, the reflected light ℓ2 has a larger intensity opposite to the above, and there is output a positive track deviation signal voltage + TS from the differential amplifier. The track control is carried out by applying the positive and negative track deviation signal voltages ± TS to the driving coil 6 and controls the position of the objective 5 to have the track difference signal voltage TS to be zero. By applying tracking servo so as to make the track deviation signal voltage TS zero, the beam spot follows the track 2 to carry out the correct recording and reproducing. The optical disc apparatus can carry out the operation of repeatedly reproducing an identical track of one turn, the so-called track-on mode, in addition to the ordinary operation of recording and reproducing as in the above. The track-on made operation can be realized by applying a jump pulse to the driving coil 6 at the timing when the optical disc 1 completed one rotation, to cause the beam spot to jump to the adjacent previous track. In carrying out the track jump, the loop of the tracking servo is open. The solid waveform in Fig. 3 is the output waveform of the track deviation signal voltage TS when the track jump is carried out by passing a current in the driving coil in the direction of the arrow in Fig. 2, and the broken-line waveform is the output waveform for the track deviation signal voltage TS when the track jump is carried out in the opposite direction. The track deviation signal voltage TS is output as a waveform for one cycle every time when the optical head 2 traverses the pre-groove 2. Therefore, the number of waves in the track deviation signal voltage TS corresponds to the number of tracks jumped. Now, when the optical disc 1 is mounted on the spindle motor, it is usual that the central point Cp1 of the optical disc 1 and the center Cp2 of the rotation shaft of the spindle motor do not coincide perfectly, producing eccentricity for the optical disc 1, due to the accuracy of the center hole of the optical disc 1, errors generated in chucking the disc to the spindle moter, and so on. To illustrate the situation schematically, let us approximate the track by concentric circles 2a and 2b. If one assumes that the optical disc 1 is rotated in the direction of arrow R in Fig. 4 and the beam spot from the optical head 4 is projected on a fixed position, then the locus of the beam spot will be a dotted circle which has the rotation center Cp2 of the spindle motor as the center. Thus, for the track 2a, the beam spot coincides with the track 2a at points a and c, but deviates by the amount of eccentricity at points b and d, where it is shifted to the outside of the track 2a at point b while it is shifted to the inside of the track 2a at point d. Fig. 5 shows the waveform of the track difference signal voltage TS on which each of the points a, b, c and d is indicated. If the tracking servo is in effect, the track deviation signal voltage TS is applied to the driving coil 6, and the beam spot is corrected to follow the track 2 so as to be moved to the inside at point b and to the outside at point d. As in the above, even if the optical disc 1 is placed on the spindle motor with eccentricity relative to the rotating center of the motor, when the tracking servo is turned on, the beam spot can be made to follow the track 2 correctly. However, when a track jump is carried out, the loop of the tracking servo is brought to an open state. Because of this, when the beam spot which has been following the track 2a is jumped, for example, to the track 2b, the beam spot deviates from the track 2a in the vicinities of the points b and d. Therefore, there occurs a problem in making a jump from the track 2a to the track 2b in that the track jumping is unstable by the variations in the jumping distance in the vicinities of the point b and the point d. Now, a successive comparison type A/D converter is known as A/D converter used in the prior art optical disc apparatus which can carry out high precision A/D conversion. The A/D converter has a voltage comparison circuit, a successive comparison register, a D/A converter that constitutes the feedback circuit, a clock oscillator which carries out the overall control, and so on. With this converter, a digital quantity which corresponds to the input analog voltage is arranged to be output by the following way. Namely, starting with the least significant bit of the successive comparison register, a converted voltage that corresponds to each one bit is arranged to be output from the D/A converter to compare the converted voltage with the input analog voltage in the voltage comparison circuit. If these two voltages do not coincide, the same procedure is successively repeated for each bit, until agreement is obtained. However, in the successive comparison type A/D converter as in the above, an analog voltage is converted to a digital quantity by the successive comparison of the smallest bit unit. Accordingly, there was a problem in that it is unsatisfactory because of the lack of high speed property of the operation due to a relatively long time required before obtaining the digital quantity. On the other hand, a parallel comparison type A/D converter is known as another existing A/D converter which can carry out A/D conversion at high speed. The A/D converter comparises a resistor ladder circuit which generates (2 n -1) reference voltages by subdividing a given voltage into 2 n equal parts when the digital quantity to be output has n bits, (2 n -1) voltage comparators to each of their reference terminals for setting each of the reference voltages generated in the resistor ladder circuit, and a decoder which transmits the outputs of the voltage comparators as an n-bit digital quantity after binary coding conversion. With this construction, an input analog voltage is input parallel to the input terminals of all of the voltage comparators and compared at once with all of the reference voltages, to output from the decoder a digital quantity that corresponds to the input analog voltage. However, in the parallel comparison type A/D converter described above, there are required a large number, (2 n -1), of voltage comparators for the digital quantity of n bits. Because of this, factors for accuracy deterioration such as the dispersion in the threshold voltage, input bias current, input offset voltage, and delay time, for each of the voltage comparators, and the relative accuracy of the resistor ladder circuit, are augmented, giving rise to a difficulty in achieving a high accuracy A/D conversion. EP-A 0 076 347 describes an optical playback machine which plays back data recorded on an optical record in track segments, employs a track intercept apparatus to transversely deflect a scanning light beam during each scan to accurately intercept each selected track segment despite record or optical misalignment. A track intercept control circuit includes storage means for storage of variable values of track alignment data, adjust means for producing an adjustment signal from the stored data and timing means controlling gating circuitry for applying the adjustment signal to a light beam deflection means at the proper point in each scan. EP-A 0 162 702 describes a tracking control arrangement for use in an optical disc player comprising a photodetector for receiving a light beam reflected at an optical disc on which an information signal is recorded in a record track, signal generating circuit for generating a reproduced information signal and a tracking error signal from the output of the photodetector, a modifier for modifying the tracking error signal in response to variations in the total quantity of light of the light beam received by the photodetector, driving means for driving an optical device to maintain the light beam in a proper tracking relation to the record track in response to the modified tracking error signal, a detector for detecting a variation in the reproduced information signal resulting from the condition in which the light beam reflected at a blank area in the record track is received by the photodetector and a gain controller for varying the gain of transmission of the modified tracking error signal from the modifier to the driving means in response to a predetermined detection output of the detector. GB-A-2043964 describes a control loop suitable for use with means for adjusting a control quantity such as the position of an optical read transducer or a magnetic write/read transducer relative to a record medium. The control loop is provided with a control unit for realizing a transfer characteristic having a number of peaks at a fundamental frequency and harmonics thereof. The control unit comprises a memory device for digitally storing a number of samples of the error signal appearing in the control loop during a cycle period equal to the period corresponding to the fundamental frequency, means for comparing the sample stored in the memory device with the value of the error signal one cycle period later and, depending on this comparison, correcting the memory content of the relevant memory location. The variation of the error signal stored in the memory device is furthermore cyclically employed as a control signal for the control device included in the control loop. EP-A-0094313 describes a misposition correction system for correcting misposition errors due to spindle runout and other slowly varying errors in a servo positioning system of a magnetic disk storage device. The system includes means for dynamically measuring misposition error with respect to a data track centerline using an anti-aliasing analog filter, means for digitizing the measured analog signal and for removing selected harmonics of the fundamental-frequency of the resulting cyclic error signal, means for transforming the digitized error signal by a matched digital filter whose transfer function contains independently adjustable DC gain, fundamental-frequency gain, and phase phase lead components thereby to generate a misposition error correction signal that matches the electrical and mechanical response characteristics of the servo system. Further, the system include means to iteratively refine the correction signal by re-applying it to the servo controller when measuring misposition errors with respect to the data track centerline. The system stores separate misposition correction signals for each transducer on the disk so that one of several correction signals can be selected depending on which transducer in the device is selected. The system further includes means for generating a bias force correction signal to correct for non-linear position errors resulting from variations in bias forces acting on the transducer carriage over its range of radial displacement. DE-A-3507976 describes a servo error reduction appparatus for a disk record playback system, which, for an optical disc, includes a first servo loop to maintain, in the presence of eccentricity in a rotating disc, a light beam on a selected data track on a surface of the disc, and a second servo loop to maintain an objective lens at the proper distance from the disc surface. Both servo loops share a common error reduction circuit. The error reduction circuit includes a tracking memory and a focus memory which store at locations thereof the expected (historical) error words whose values represent earlier-measured tracking and focus position errors, respectively, for a large number of locations around the disc. The values expressed in stored error words are combined using an arithmetic logic unit with currently-measured error values into words representing computed error values. The computed error values are used in the respective servo loops to reduce the measured errors. The memories are updated by storing in their locations the computed words containing values of tracking and focus corrections for respective locations around the disc during every disc revolution. DE-A-3136090 describes a tracking control apparatus for use with an optical information reproducing system in which information to be reproduced is formed on a surface of a recording carrier with a substantially circular record track, and a light beam is incident on the surface for reading the information, the apparatus including a photo-detector for detecting the light beam reflected from the recording carrier and generating an information signal and a first tracking error signal, the first tracking error signal being responsive to deviations by the light beam from the center of the record track; a tracking control apparatus receiving the first tracking error signal for adjusting the position of the light beam in response thereto; a measuring device for generating a second tracking error signal indicative of the eccentricity of the record track; and a circuit for supplying the second tracking error signal to the tracking control apparatus to adjust the position of the light beam in response thereto. US-A-4365324 describes an eccentricity control apparatus provided in a system for optically recording and reading information on a rotating disc medium. The information may be recorded on concentric or spiral tracks. The eccentricity control apparatus includes a detector for detecting track crossings as an optical scanning head scans the disc surface. The detector provides pulses representative of track crossings. The number of pulses in a once-around is representative of the amplitude of an eccentricity control signal. The angle measured between a once-around pulse and a minimum pulse rate of the track crossings indicates the phase of the error. An eccentricity error signal which is generated by suitable circuitry is applied to affect the manner in which the optical scanning head scans the disc surface such that number of track crossings is reduced. US-A-4135217 describes a servo system including a recording head to sense servo data on a disk track for fine positioning of the head and an external transducer for generating signals representing the coarse position of the head. A micro-processor stores the coarse position signals which include information as to repeatable error such as wobble of the disk. A compensator maintains information concerning small offsets of the disk due to, for example, temperature variations. During coarse positioning of the head, from one track to a desired track, the stored repeatable error information and stored offset information is used to generate error signals to control movement of a carriage connected to the head, whereby the head follows the repeatable error and offset to arrive coarsely on the desired track. After coarse positioning is complete, fine positioning information from the recording head, which information includes the offset information, and the stored repeatable error information are used to generate error signals to control movement of the carriage for fine positioning purposes. SUMMARY OF THE INVENTION An object of the present invention, in view of the above circumstances, is to provide an optical disc apparatus which enables the optical head to precisely follow the recording track on the optical disc by minimizing the amount of eccentricity between the optical head and the optical disc. Another object of the present invention is to provide an eccentricity correction device in the optical disc apparatus which enables a precise track jump. Still another object of the present invention is to provide an A/D converter which can carry out high accuracy A/D conversion at high speed. The object of the invention is solved by an optical disc apparatus including: optical means for recording or reproducing information on the recording track of an optical disc; means for detecting at least deviation amount of beam spot projected from said optical means on the recording track of the optical disc and generating a track deviation signal based on the deviation amount; means for memorizing a first digital quantity; and means for converting digital quantities read from said memory means to an analog signal, said analog signal being used to drive said optical means; means for calculating a second digital quantity based on the first digital quantity from the memory means and transferring the second digital quantity to the memory means, characterized by means for outputting a difference voltage between the track deviation signal and the analog signal from the converting means; means for comparing the difference voltage with reference voltages, when the digital quantity read from said memory means is of n bits, the comparing means comprises a number 2n of voltage comparators and the reference voltages comprise a plurality of positive and negative reference voltages +Vr and -Vr each of which corresponds to a weighting of each bit in the digital quantity read from the said memory means said calculating means calculating the second digital data based on the digital ouput of the comparing means and the first digital quantity from the memory means and repeating and repeating said calculation of the second digital quantity until the voltage of the track deviation signal becomes smaller than the reference voltage corresponding to the weighting for the least significant bit, said calculating means comprising an addition-substraction data set means for setting the digital output from the voltage comparators, and a full adder for adding the first digital quantity from the memory means to the set digital signal from the addition-subtraction data set means, a result of the adding being transferred to the memory means as the second digital quantity. In this manner, high accuracy A/D conversion is arranged to be accomplished at high speed. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features and advantages of the present invention will be more apprent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings in which: Fig. 1 is a plan view showing an optical disc; Fig. 2 is a side view with partially sectional view which shows the prior-art optical disc; Fig. 3 is a waveform diagram that shows the output waveforms of the track deviation signal voltage in the above optical disc apparatus; Fig. 4 is a plan view showing the eccentricity situation of the prior art optical disc apparatus in the above; Fig. 5 is a waveform diagram which shows the output waveform of the track deviation signal voltage in the eccentricity state; Fig. 6 is an overall block diagram of an optical disc apparatus to which the present invention can be applied; Fig. 7 is a side view showing the linear motor portion; Fig. 8 is a diagram showing an optical disc which is subdivided into 256 sectors in the circumferential direction; Fig. 9 is a block diagram showing the basic configuration of the eccentricity correction device in the optical disc apparatus in accordance with the present invention; Fig. 10 is a circuit diagram for an embodiment of the present invention, with the optical head and other portions omitted; Fig. 11 is a timing chart illustrating the operation of the frequency multiplying circuit in the embodiment; Fig. 12 is a timing chart illustrating the operation of the embodiment; Fig. 13 is a waveform diagram showing the waveform of the analog voltage for eccentricity correction and other waveforms; Fig. 14 is a block diagram showing the basic configuration of the weighted operation type A/D converter in accordance with the present invention; Fig. 15 is a block diagram showing the overall configuration of the weighted operation type A/D converter; Fig. 16 is a circuit diagram for the important part of the D/A converter; Fig. 17 is a circuit diagram for the analog circuit portion in the first half thereof; Fig. 18 is a circuit diagram for the portions of the voltage comparison circuit and the addition-subtraction data set circuit; Fig. 19 is a diagram showing the addition and subtraction data set in the addition-subtraction data set circuit; Fig. 20 is a logic diagram of the full adder; Fig. 21 is a block diagram for the full adder; Fig. 22 is the truth table for the full adder; Fig. 23 is a timing chart for illustrating the operation of the full adder; Fig. 24 is a circuit diagram showing a modification of the voltage comparison circuit and the addition-subtraction data set circuit; Fig. 25 is a block diagram showing a modification of the memory portion; Fig. 26 is a circuit diagram showing the important part of another embodiment of the present invention; and Fig. 27 is a diagram showing the ralationship between the difference voltage and the addition-subtraction data for the embodiment in Fig. 26. DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical disc apparatus to which the present invention can be applied will be described with reference to the drawings. In Fig. 6, 21 is an optical disc, 22 is a driving motor for rotating the optical disc 21, and 23 is an optical head for recording and reproducing. The optical disc 21 has a concentric circular or spiral recording track 300, as shown in Fig. 8, and one rotation of the optical disc is subdivided into 256 sectors of "0 to 255" for data memory. The motor 22 is driven by a motor control circuit 24. A laser oscillator 31 is used as a laser beam light generating means, and an objective 35 concentrates beam light on the optical disc 21 and positions the beam light on the recording track. A vertical driving coil 36 moves the objective 35 in the vertical direction, the direction of the optical axis, and a radial driving coil 37 moves as a tracking means the objective 35 in the radial direction, the direction orthogonal to the optical axis of the optical disc 21. Split beam detectors 41 and 43 carry out photoelectric conversion by receiving reflected light from the optical disc 21. The laser oscillator 31 generates laser beam light by oscillation driven by a driving circuit 44. In this case, when information is recorded on the recording track of the optical disc 21, the laser oscillator 31 generates laser beam light whose intensity is modulated in response to the information to be recorded, and when information is reproduced from the recording track, the laser oscillator 31 generates laser beam light which has a constant intensity. The beam light from the laser oscillator 31 is led, after made into parallel beams of light by a collimator lens 32, to a polarized light beam splitter 33 where it is reflected on the side of the optical disc 21. The reflected beam light is led to the objective 35 through a quarter-wavelength plate 34, and is irradiated on the recording track of the optical disc 21 after being made into a spot-like beam light by the focusing of the objective 35. The reflected light from the optical disc 21 by this light irradiation, is led through the objective 35, the quarter-wavelength plate 34, and the polarized light beam splitter 33 to a half prism 38 where it is split into two systems of light. One of the split reflected lights is focused through a condenser 39 and a knife edge 40 on the light-receiving surface of the light detector 41 where it is photoelectrically converted. Each of the output signals of the light detector 41 is used for the focusing control. The other of the slit reflected lights is focused through a condenser 42 on the light-receiving surface of the light detector 43 where it is photoelectrically converted. Each of the output signals of the light detector 43 is used for tracking control and for reading of recorded information as described later. Each of the output signals of the light detector 41 is supplied to a differential amplifier 45 from whose output it is arranged to be able to obtain a focus deviation signal of the beam light on the optical disc 21. The focus deviation signal output from the differential amplifier 45 is supplied to a focusing control circuit 46. In response to the input focus deviation signal, the focusing control circuit 46 controls the driving coil 36 for the vertical direction, and carries out focusing control by driving the objective 35 in the vertical direction so as to position the beam light that irradiates the optical disc 21 to be correctly focused all the time. Each of the output signals of the light detector 43 is supplied respectively to a differential amplifier 47 whose output is arranged to give a tracking position deviation signal. The tracking position deviation signal output of the differential amplifier 47 is supplied respectively to a tracking control circuit 48 as a tracking control means and an A/D converter 49. In response to the input tracking position deviation signal, the tracking control circuit 48 controls the driving coil for radial direction 37, and carries out tracking control so as to position the beam light to be on the recording track all the time and the beam light precisely follows the recording track, by driving the objective 35 in the radial direction of the optical disc 21. It is to be noted that in the state when the tracking is in progress, servo mechanism is on for not only the objective 35 but also the linear motor control circuit 30 to determine the positioning of the beam light by the use of the objective 35 and the linear motor 25. The A/D converter 49 outputs the input tracking position deviation signal to a data bus 50 by converting it into a digital signal. Further, each output signal of the light detector 43 is supplied respectively to a video circuit 51. The video circuit 51 generates a reproduced signal of recorded information, namely, a video signal reflecting bits within the recording track, by adding each output signal of the light detector 43, and outputs the video signal by binary coding it to the data bus 50 as reading information. Connected to the data bus 50 are the driving circuit 44, the linear motor control circuit 30, and the motor control circuit 24. Also connected to the data bus 50 are a sector counter 52, a CPU which governs the overall control 53, a ROM 54 storing the operating program of the CPU and the like, a RAM 55 as a memory means for memorizing the eccentricity correction data, and a D/A converter 56. The sector counter 52 is actuated by the 256 sector pulses per rotation of the optical disc 21 generated from the motor control circuit 24. A counter simply counts "from 0 to 255," and the counted value is output to the data bus 50 as well as is supplied to one of the input terminals of the selector 57 as an address signal of the RAM 55. The selector 57 operates switching by a hold acknowledge signal from the CPU 53, and its output terminal is connected to the ROM 54 and the RAM 55 via an address bus 58. When a hold pulse is input from a hold pulse generation circuit 59, the CPU 53 becomes a held state, and generates a hold acknowledge signal which indicates that it is being held. The hold acknowledge signal is supplied to the selector 57, RAM 55, D/A converter 56, and hold pulse generation circuit 59. The D/A converter 56 converts the data transferred from the RAM 55 to an analog signal, and supplies the converted output to the tracking control circuit 48 and the linear motor control circuit 30 as bias. The hold pulse generation circuit 59 is a latch circuit which is set by the rising of a sector pulse generated from the motor control circuit 24 and reset by a hold acknowledge signal, and generates a hold pulse which becomes high level and low level in response to the set and reset, respectively. In the apparatus having the construction mentioned above, first, a course access for moving the beam spot is gained by reading position information recorded in advance in the recording track of the optical disc 21 and moving the optical head 23 by the linear motor 25 to the vicinity of the target recording track. Then, by carrying out a precision access by using the objective 35, it becomes possible to move the beam light to the desired recording track. These operations are carried out by the control of the CPU 53. Since such an access operation is already well known, further explanation will be omitted. Next, eccentricity correction of the tracking will be described. First prior to an actual recording or reproducing operation, the optical disc 21 is rotated by the motor 22, and let the focusing operation take place at an arbitrary position on the optical disc 21. In this state, for each sector indicated by the counted value of the sector counter 52, the CPU 53 reads, via the data bus 50, the output of the A/D counter 49 carrying out A/D conversion of the tracking position deviation signal obtained from the output of the differential amplifier 47. After giving a predetermined data processing to the output of the A/D converter 49, it is stored in the RAM 55 as eccentricity correction data. Here, the predetermined data processing carried out in the CPU 53 consists, for example, of storing temporarily the data read corresponding to a plural number of rotations of the optical disc 21, and reading the data and taking average for each sector and storing the result again in the RAM 55 as eccentricity correction data. In doing so, a plurality of eccentricity correction data are prepared corresponding to the positions of the optical head 23 for the radial direction of the optical disc 21, for instance, the inner periphery position, an intermediate position, and the outer periphery position. When the eccentricity correction data is obtained in this manner, in the actual operation of recording and reproducing of information, eccentricity correction is carried out by successively reading the eccentricity correction data corresponding to each sector from the RAM 55. Namely, the eccentricity correction is started by the actuation of the hold pulse generation circuit 59. The hold pulse generation circuit 59 generates a hold pulse every time when the sector is switched, and hold the CPU 53 by the hold pulse. The CPU 53 generates a hold acknowledge signal every time when it is held by a hold pulse. By switching the selector 57 by a hold acknowledge signal, the output of the address bus 58 is switched from the CPU 53 to the sector counter 52. At the same time, by enabling the output of the RAM 55 as well as the input of the D/A converter 56, by the hold acknowledge signal, data in the RAM 55 is transferred to the D/A converter 56 while the CPU 53 is being held. This operation is repeated every time when the sector is switched. The address of the RAM 55 is designated by the sector counter 52, and by this relationship a sector and an address are in correspondence all the time. In this case, eccentricity correction data prepared in a plural number in the RAM 55 corresponding to the positions of the optical head 23 for the optical disc 21, are switched and transferred. In addition, considering the response delay of the tracking control circuit 48, control is carried out in this case so as to transfer the eccentricity correction data to the D/A connector 56, by shifting the timing to one earlier than that for the target sector. The output of the D/A converter 56 is sent to the tracking control circuit 48, and is given as a bias for the driving signal of the driving coil for radial direction 37. In this way, it becomes possible to sharply reduce the apparent eccentricity amount. With the eccentricity correction as above, it is possible to reduce the apparent eccentricity amount between the optical head 23 and the optical disc 21, and the beam light from the optical head 23 can be made to follow the recording track precisely. In addition, tracking also becomes easy in moving the optical head 23 to an arbitrary recording track. In particular, by averaging the data for a plural number of rotations of the optical disc 21, it becomes possible to avoid reading errors of the A/D converter 49, or the like. Moreover, by switching the eccentricity correction data in response to the position of the optical head 23 to the optical disc 21, it is possible for the optical head 23 to precisely follow the recording track even for a delicate difference in eccentricity of the position in the radial direction of the optical disc 21. Further, eccentricity correction data are output by considering the response delay in the tracking control circuit 48 so that it is possible to precisely follow the actual eccedntricity in the optical disc 21. It is to be noted that the reading of the tracking position deviation signal and the execution of a predetermined data processing by the CPU 53 are not limited to the embodiments given above. Thus, for example, it may be arranged to read the output of the A/D converter 49 for a plural of times for each sector, memorizing these data temporarily in the RAM 55, and take average of a plurality of data by reading the data for each sector, to store the result in the RAM 55 as the eccentricity correction data. By taking average of a plurality of data within one sector, it is possible to obtain a value for each sector close to the true eccentricity amount, and to improve the data linkage with the preceding and succeeding sectors. Moreover, a predetermined data processing carried out by the CPU 53 is not limited to those of the embodiments in the foregoing. For instance, it may be arranged to temporarily store in the RAM 55 the data for one rotation of the optical disc 21, and read these data and by removing the bias component due to inclination and the like of the optical head 23 from the data, to store them in the RAM 55 as the eccentricity correction data. In this manner, by removing the bias component due to inclination or the like of the optical head 23, it is possible to follow the eccentricity of the optical disc 21 at the most stable position of the optical head 23. As described in detail in the foregoing, according to the present invention, it is possible to provide an optical disc apparatus which can reduce the apparent eccentricity amount between the optical head and the optical disc, and permit the optical head to precisely follow the recording track on the optical disc. Next, the principle of the eccentricity correction device in the optical disc apparatus in accordance with an embodiment of the present invention will be described. In Fig. 9, reference numeral 103 is a spindle motor which rotates an optical disc 101, and 104 is an optical head on which an incidence optical system is disposed in addition to the reflection optical system and others shown in Fig. 2. The optical head 104 may be the same as the optical head 23 in Figs. 6 and 7. Reflected lights ℓ₁ and ℓ₂ from the optical disc 101 pass through a quarter-wavelength plate 111 for two times so that their planes of polarization are rotated by a total of 90°, and are incident upon a cylindrical lens 112 through a beam splitter 109. The reflected light ℓ₁ and ℓ₂ are formed into oblong light bundles by the cylindrical lens 112, and the reflected light ℓ₁ is detected by a photodetector 113a while the other reflected light ℓ₂ is detected by another photodetector 113b. A track deviation signal circuit is formed by the photodetectors 113a and 113b, sensor amplifiers 114a and 114b, and a differential amplifier 115. The output terminal of the differential amplifier 115 is connected on the one hand to a first junction 116a connected via a lens driving circuit or buffer 117 to a driving coil 106. Through the first junction 116a, the track deviation signal voltage TS, a precise access signal, and an analog voltage from a D/A converter that will be described later, are selectively transferred to the side of the driving coil 106. Reference numeral 118 is a linear motor, and the input line of a coarse access signal for moving the optical head is connected via a second junction 116b and a motor driving cirucit (buffer) 119 to the linear motor 118. When in such a case the eccentricity amount of the optical disc 101 is large, where the beam spot has to be moved for a larger distance, the linear motor 118 is driven by the coarse access signal or the like to move the optical head 104 in a desired direction. Further, between the output terminal of the differential amplifier 115 and the junction 116a there are connected various devices having the function of converting the track deviation signal voltage TS to a digital quantity B(4). Namely, to the output terminal of the differential amplifier 115, there is connected a voltage comparison circuit 121 which can be set to the predetermined positive and negative reference voltages ±V ref . The accuracy of conversion of the track deviation signal voltage TS to the digital quantity B(4) is determined by the voltage values of the reference voltages ±V ref . For this reason, the values of the reference voltage ±V ref are set to those values for which a desired accuracy can be achieved as well as an appropriate conversion sensitivity can be realized. Here, the values of the reference voltages ±V ref correspond to the weighting for the least significant bit in the digital quantity B(4). The reference voltages ±V ref and the track deviation signal voltage are compared in the voltage comparison circuit 121, and an addition signal C+ is output if TS > + V ref and a subtraction signal C₋ is output if TS < - V ref . Reference numeral 122 is an addition - subtraction data set circuit which sets digital data B(1) for addition and subtraction based on the addition and subtraction signals C+ and C₋ output from the voltage comparison circuit 121. The output terminal of the addition-subtraction data set circuit 122 is connected to a full adder 124 whose output terminal is connected via a buffer 125 to a memory 126 for memorizing digital quantities. The memory 126 is connected to a latch circuit 127 which latches the data B(3) read from the memory 126 as a digital quantity B(4). One of the output terminal of the latch circuit 127 is connected to the full adder 124 while the other terminal is connected to a D/A converter 128. The output terminal of the D/A converter 128 is connected to the first junction 116a. The data B(3) that corresponds to the eccentricity amount memorized in the memory 126 are composed of eccentricity data for each position on the recording track 102 obtained by subdividing one rotation of the optical disc 101 into desired number of sectors. As a means for obtaining such a digital quantity B(4), use is made of a frequency generator (FG) pulse generated from an unillustrated frequency generator attached to a spindle motor 103. The FG pulses are generated by 76 in number, for example, for one rotation of the optical disc 101. Reference numeral 129 is a frequency multiplication circuit which has the function of doubling, for example, the frequency so that an FG pulse is doubled to a 2FG pulse. The output terminal of the frequency multiplication circuit 129 is respectively connected to the buffer 125, latch circuit 127, memory 126, and an address counter 131. The buffer 125 displays a high impedance when the 2FG pulse is on H level, and when it is on L level, it is in an active state and transfers the calculated data B(2) from the full adder 124 to the memory 126. The latch circuit 127 is in an active state when the 2FG pulse is on H level, and reads the stored data B(3) in the memory 126. When the 2FG pulse is on L level, the circuit 127 holds it as a digital quantity B(4). In addition, the address counter 131 which consists of a loop counter counts the 2FG pulses to control the address of the memory 126, and the eccentricity amount of 152 in number are memorized periodically. To describe the operation of the apparatus, a data B(3) that corresponds to an interim digital quantity is memorized in the memory 126 in the beginning. On the other hand, in the track deviation signal circuit, there are detected amount of deviation and direction of deviation of the beam spot with respect to each position on the track 102. Then, there is output a track deviation signal voltage TS which has an absolute value proportional to the amount of deviation and has a positive or negative sign in response to the direction of the deviation in the outward or inward direction of the track. The track deviation signal voltage TS is compared with the reference voltage ±V ref in the voltage comparison circuit 121. As a result of comparison, if the track deviation signal voltage has a positive sign and its value is greater than +V ref , then an addition signal C+ is output, and an addition data is set in the addition-subtraction data set circuit 122. On the other hand, if the track deviation signal voltage has a negative sign and its value is smaller than -V ref , a subtraction signal C₋ is output, and a subtraction data is set in the addition-subtraction data set circuit 122. The addition and subtraction data B(1) is input to the full adder 124. Since there is input an interim digital quantity from the latch circuit 127 in the other input terminal of the full adder 124, the addition - subtraction data B(1) are respectively added and subtracted, from the interim digital quantity. A new data B(2) obtained by such operation is stored in the memory 126 as data B(3) with the timing when the buffer 125 becomes active. In this way, the stored data of the memory 126 is rewritten from an interim digital quantity to a corrected data B(3) based on the track deviation signal voltage TS. Further, the data B(3) is read by the latch circuit 127 with the timing at which the latch circuit 127 becomes active. The digital quantity B(4) held in the latch circuit 127 is converted to an analog voltage by the D/A converter 128 and is applied to the driving coil 106 of the objective 105. A series of processes of the detection of the track deviation signal voltage TS by the track deviation signal circuit, addition and subtraction by the full adder 124, rewriting in the memory 126, and so forth are repeated until the absolute value of the track deviation signal voltage TS becomes smaller than the voltage value of the reference voltages ±V ref in the voltage comparison circuit 121. This operation is carried out for each position of the track 102 by the address control using the address counter 131 and others. In the memory 126, there are stored eccentricity data B(3) in response to the eccentricity amount of the optical disc 101. After that, a digital quantity B(4) in response to the eccentricity data B(3) is converted to an analog voltage by the D/A converter 128. The analog voltage V dac for eccentricity correction and a precise access signal are applied to the driving coil 106 so that the tracking control of high accuracy can be accomplished even when there is an eccentricity in the optical disc 101. The detailed constructions of electrical circuits in the optical disc apparatus will be next described with reference to Figs. 10 - 13. In this embodiment, various data B(1) to B(4) are processed in 8 bits. Figure 10 is a detailed circuit diagram with the optical head and others omitted, Fig. 11(a) to Fig. 11(d) are timing charts for illustrating the operation of the frequency multiplication circuit 129, Fig. 12 is a timing chart for illustrating the operation of the present embodiment, and Fig. 13(A) to Fig. 13(C) are waveform diagrams for showing the eccentricity correction analog voltage and others for illustrating the operation of the above embodiment. First, the circuit configuration will be described by referring to Fig. 10. In the voltage comparison circuit 121 there are provided two voltage comparators 121a and 121b which are set to reference voltages +V ref and -V ref , respectively. The track deviation signal voltage TS is commonly input to the voltage comparators 121a and 121b and is simultaneously compared with the reference voltages ±V ref . In the addition - subtraction data set circuit 122 there is provided only one OR circuit 123, and the output terminals of the voltage comparators 121a and 121b are respectively connected to the two inputs of the OR circuit 123. From the OR circuit 123, the least significant bit, B₁₁ of the addition - subtraction data B(1) = (B₁₈, B₁₇, ..., B₁₁) is output. The other bits B₁₈, B₁₇, ..., B₁₂ are directly output from the comparator 121b. When the track deviation signal voltage TS is greater than the reference voltage +V ref , the output of the voltage comparison circuit 121 has an addition signal C+ of "1" and the addition - subtraction data B(1) = (00000001) = (0 1 H ). On the other hand, if the track deviation signal voltage TS is smaller than the reference voltage -V ref , then the output of the voltage comparison circuit 121 has a subtraction signal C₋ of "1" and the addition - subtraction data is set to B(1) = (11111111) = (F F H ). Table 1 shows the above relationship in a tabular form. The full adder 124 uses ordinary 8-bit system and the addition and subtraction data B(1) output from the addition - subtraction data set circuit 122 is added or subtracted from the digital quantity B(4) input from the latch circuit 127, and the data B(2) obtained by the operation is output to the buffer 125. Namely, the output data B(2) of the full adder 124 is given by B(2) = B(1) + B(4) where the right-hand side represents an addition for TS > V ref and a subtraction if TS < -V ref . When |TS| < V ref is realized, the correction is completed with B(1) = 00 H . The baffer 125 is driven by a 2FG pulse, and displays a high impedance, an open state, when the 2FG pulse is on H level and is in an active state when the 2FG pulse is on L level. The series of operating timings of the buffer 125, memory 126, and latch circuit 127 driven by a 2FG pulse are shown in Table 2. Writing of the data B(3) to the memory 126 is controlled by the address counter 131, and the data B(3) is taken-in with address shifted by 1 each. In order for one turning of the optical disc 101 to be subdivided into 152 parts by 2FG pulses and to be able to memorize the data B(3) with the accuracy of 8 bits, the memory 126 is required to have a capacity of more than 1.2K bits. The frequency multiplication circuit 129 which doubles the FG pulses is composed of an integration circuit consisting of an inverter, a resistor, and a capacitor, and an EXOR circuit. Figures 11(a) to Fig. 11(d) illustrate the doubling operation of the frequency multiplication circuit 129 by which a 2FG pulse in Fig. 11(d) is output by taking an exclusive logical sum of an FG pulse in Fig. 11(b) and the integration voltage in Fig. 11(c). Figure 12(a) is a revolution pulse chart that shows the revolving position of the spindle motor 103, and the timing chart is drawn by assuming that 76 periods of FG pulses are generated during 1 period of the revolution pulse R ev . The FG pulses are doubled by the frequency multiplication circuit 129 to generate 2FG pulses shown in Fig. 12(b), and they are counted by the address counter 131 to control the memory address in Fig. 12(h). Therefore, one rotation of the optical disc 101 is equally subdivided into 152 parts, and the eccentricity data at each of the subdivided position is converted to a digital quantity B(3) to be stored in the memory 126. It will be assumed that the memory 126 is initialized to B(4) = (10000000) = 80 H . The reason for setting the initialization to B(4) = 80 H is to let the analog voltage V dac converted by the D/A converter 128 oscillate in dipolar manner in positive and negative directions with 0 V as the center. In other words, the inner circuit of the D/A converter 128 is so constructed as to have the following relationship between the digital quantity B(4) and the analog voltage V dac that is converted. where V fs means the full scale of the analog voltage V dac . If in the voltage comparison circuit 121 the track deviation signal voltage satisfies TS > +V ref at the first position of the 2FS pulses, an addition signal C+ = 1 and a subtraction signal C₋ = 0 will be output. By this output, an addition data B(1) = 01 H is set in the addition - subtraction data set circuit 122. Since the initial value B(4) = 80 H of the memory 126 is input from the latch circuit 127 to the full adder 124, the addition data B(1) in the above is added to the initial value B(4). The new data after such operation becomes B(2) = 81 H , and this data B(2) is written and memorized in the memory 126 as a data B(3) with the L level timing of the 2FG pulses. In this case, the memory address remains at 1. Suppose that the optical disc 101 is rotated and at the 77th position, for example, of the 2FG pulses the track deviation signal voltage TS satisfies the relation TS < -V ref Then, a subtraction signal C₋ = 1 is output from the voltage comparison circuit 121. By this output, a subtraction data B(1) = FF H is set in the addition - subtraction data set circuit 122. In the full adder 124, a subtraction of the subtraction data B(1) = FF H from the initial value B(4) = 80 H , in other words, the operation of FF H + 80 H = 7F H , is carried out, and a newly calculated data B(2) is output. Since the operation is carried out in terms of 8 bits, the overflow data "1" in the ninth bit will be neglected. The calculated data B(2) = 7F H is written and stored in the memory address 77. When the optical disc 101 makes one rotation, at the 153rd position of the 2FG pulse the memory address returns again to 1. The data B(3) = 81 H written and stored in the previous rotation is read here by the latch circuit 127, and the digital quantity that is to be held becomes B(4) = 81 H . If the track deviation signal voltage at this time satisfies TS > +V ref , the addition signal C+ = 1 and the addition data B(1) = 01 H are obtained, and a new data B(2) = 82 H obtained by the operation is output from the full adder 124 and the data for the memory address 1 is rewritten to B(3) = 82 H . When the optical disc 101 is rotated to the 229th position of the 2FG pulses, the memory address becomes 77, and the data B(3) = 7F H of the previous time is read by the latch circuit 127. Thereafter, similar to the above, addition and subtraction operation are also carried out with 152 shots of the 2FG pulses as one period. If, at all the rotation positions corresponding to the memory addresses from 1 to 152, the absolute value of the track deviation signal voltage TS satisfies the condition |TS| < V ref , then the addition and subtraction data becomes B(1) = 00 H with no need for the generation of change in B(2), B(3), and B(4), so that taking-in of the eccentricity data is completed. Next, referring to Fig. 13(A) to Fig. 13(C), the effects of eccentricity correction in the embodiment will be described. Figures 13(A) and 13(B) represent the amount of displacement of the linear motor 118 with respect to the rotation of the optical disc 101 as scale (SCK) signals. Figure 13(A) and Fig. 13(B) are waveforms that show the amount of displacement before and after the eccentricity correction, respectively. Figure 13(C) is a waveform diagram which shows the analog voltage V dac for eccentricity correction output from the D/A converter 128 as a result of taking-in of the eccentricity data into the memory 126. In Figs. 13(A) to 13(C), the waveforms of the symbol R ev are the signal waveforms that show the rotation position of the spindle motor 103, and the rotation position of the optical disc 101 can be shown by these signals. Before giving an eccentricity correction, the amount of deviation of the beam spot for the track 102 is large so that, as shown in Fig. 13(A), with a large displacement of the linear motor 118, the optical head follows the motion of the track 102. A positive inclination of the displacement indicates an outward displacement and a negative inclination indicates an inward displacement. Therefore, Fig. 13(A) shows that in the first half of the rotation of the optical disc 101 the linear motor 118 moved to the outer periphery side of the optical disc 101, and in the latter half it moved inward. Figure 13(C) shows that when there exists an eccentricity state as above, the eccentricity data is obtained by the memory 126, and the digital quantity B(4) that corresponds to the eccentricity amount is output as an analog voltage for eccentricity correction V dac by the D/A converter 128. The objective 105 moves outward for a negative inclination of the analog voltage V dac and moves inward for a positive inclination, so that the direction of following of the objective 105 also coincides with the direction of motion of the linear motor 118 shown in Fig. 13(A). Figure 13(B) shows the amount of displacement of the linear motor 118 when the position of the objective 105 is controlled by the analog voltage for eccentricity correction. From Fig. 13(B) it will be seen that, after eccentricity correction, the amount of displacement of the linear motor 118 is markedly reduced and that the eccentricity correction is being carried out extremely effectively. After the eccentricity correction data is obtained as in the above, by carrying out the tracking control with the addition of the analog voltage V dac for eccentricity correction to the precise access signal, it becomes possible to follow the track 102 with high accuracy by only the movement of the objective 105. In addition, in carrying out the track jumping, the amount of eccentricity correction is identical if the rotation position is the same so that by the addition of the analog voltage V dac for eccentricity correction as it is to the jump pulse an accurate and stable track jumping becomes possible. As described in the foregoing, according to the present invention, digital quantity corresponding to the eccentricity quantity of the optical disc is stored in the memory, and the analog voltage for eccentricity correction obtained by D/A converting the digital quantity is added to driving means of the objective, so that tracking control with high accuracy can be achieved even when an eccentricity exists in the optical disc. Moreover, in carrying out track jumping, an analog voltage for eccentricity correction output from the D/A converter can be applied as it is to the driving means for the objective along with the jump pulse. Therefore, there is an advantage that track jumping can be carried out accurately and stably while applying the eccentricity correction. Next, referring to Fig. 14, the principle of the weighted operation type A/D converter which is used to convert an analog quantity to a digital quantity in the optical disc apparatus of the present invention will be described in association with data for eccentricity correction. In Fig. 14, reference numeral 201 is the input terminal of an input analog voltage V in and a differential amplifier 202 outputs the difference voltage ΔV in between the input analog voltage V in and an analog voltage V dac generated from a D/A converter 203 that constitutes a feedback circuit. Reference numeral 204 is a voltage comparison circuit, and when the digital quantity B(4) is assumed to be of n bits, there are provided a number 2n of voltage comparators 205 and 206. Further, there are prepared a plurality of positive and negative reference voltages +V r and -V r each of which corresponds to the weighting of each bit in the digital quantity B(4). These reference voltages +V r and -V r are set to the respectively corresponding voltage comparators 205 and 206. An addition - subtraction data set circuit 207 contains an addition data set section 207a, a subtraction data set section 207b, and an OR circuit 212 which takes a logical sum of the outputs of both data set sections 207a and 207b. The output terminal of the OR circuit 212 is connected to a full adder 213 whose output terminal is connected via a buffer 214 to a memory 215 for memorizing digital quantities. The memory 215 is connected to a latch circuit 216 for latching the data B(3) read from the memory 215, and the output terminal of the latch circuit 216 is connected to the D/A converter 203. Since the data latched to the latch circuit 216 becomes a digital quantity B(4) which is D/A converted, the output terminal 217 of the digital quantity B(4) is connected to the output terminal of the latch circuit 216. Then, an interim digital quantity initially stored in the memory 215 is read by the latch circuit 216, and an analog voltage voltage V dac proportional to the interim digital quantity is generated. In a differential amplifier 202, the difference is taken between the input analog voltage V in input from the input terminal 201 and the analog voltage V dac generated in the D/A converter 203, and the difference voltage ΔV in given by ΔV in = V in - V dac , is output. The value of the difference voltage ΔV in is proportional to an error in conversion. The difference voltage ΔV in is simultaneously compared with a plurality of reference voltages + V r and -V r in the voltage comparation circuit 204. As a result, if the difference voltage ΔV in has a positive sign and its value is greater than the reference voltage +V r , C₁+ is output from the voltage comparator 205. Then, in an addition data set section 207a of an addition - subtraction data set circuit 207, there is set a binary number for addition data with weighting that corresponds to the absolute value of the difference voltage ΔV in . On the other hand, if the difference voltage ΔV in has a negative sign and its value is smaller than the reference voltage -V r , C₁₋ is output from the voltage comparator 206, and a binary number for subtraction data with weighting that corresponds to the absolute value of the difference voltage ΔV in is set in the subtraction data set section 207b of the addition - subtraction data set circuit 207. The addition data and the subtraction data are logically summed in the OR circuit 212, and the addition subtraction data B(1) are input to the full adder 213. An interim digital quantity is input from the latch circuit 216 to the other input terminal of the full adder 213, so that the addition - subtraction data B(1) is added and subtracted from the interim digital quantity. The new digital quantity B(2) obtained as the result of the above operation is stored in the memory 215 via the buffer 214. The stored data in the memory 215 is rewritten from the interim digital quantity to a digital quantity B(3) corrected based on the difference voltage ΔV in . The correction operation by addition and subtraction of the full adder 213 or the like is repeated until the absolute value of the difference voltage ΔV in becomes smaller than the reference voltage which corresponds to the weighting for the least significant bit, in other words, until the condition -V r < ΔV in < +V r is satisfied. In this way, eventually a digital quantity B(4) A/D converted with high accuracy can be output from an output terminal 217. Next, another embodiment of the electrical circuits mentioned above will be described with reference to Figs. 15 to 23. An A/D converter of 4 bits is used in the following embodiment. Referring to Fig. 15 showing the overall configuration of the A/D converter, a sample hold circuit 218 is connected between an input terminal 201 and a differential amplifier 202. The sample hold circuit 218 is the one for temporarily holding an input analog voltage V in , and is used to avoid an influence on an output digital quantity B(4) due to variations in the input analog voltage V in during A/D conversion. In a voltage comparison circuit 204, there are provided side by side a number 8, which corresponds to twice as many the number of bits, of voltage comparators 205a to 205d and 206a to 206d. Further, a voltage of +16V r is divided by a voltage dividing circuit formed by connecting five resistors with resistances R, 2R, 4R, and 8R in series to generate positive reference voltages with values that correspond to the weightings for the bits, namely, V r , 2V r , 4V r , and 8V r set to the voltage comparators 205a, 205b, 205c, and 205d, respectively. On the other hand, a voltage of - 16V r is divided by another voltage dividing circuit formed by connecting five resistors similar to the above, to generate negative reference voltages with absolute values that correspond to the weightings for the bits, namely, -V r , -2V r , -4V r , and -8V r set to the voltage comparators 206a, 206b, 206c, and 206d, respectively. In an addition data set section 207a of an addition - subtraction data set circuit 207, there are arranged side by side three AND circuits 208a, 208b, and 208c, and one of the two inputs of the AND circuit 208a is directly connected to the voltage comparator 205a and the other input is connected via an inverter 209a to the voltage comparator 205b. For the other two AND circuits 208b and 208c, the situation is similar to the case of the AND circuit 208a. Namely, for the AND circuit 208b, the voltage comparator 205b is connected directly, and the voltage comparator 205c is connected via an inverter 209b. For the AND circuit 208c, the voltage comparator 205c is ocnnected directly, and the voltage comparator 205d is connected via an inverter 209c. For the subtraction data set section 207b, the conditions for connection are similar to the above, with three AND circuits 210a to 210c and three inverters 211a to 211c arranged side by side. For the full adder 213 use is made of disclosed type which will be described later. The full adder adds or subtracts the addition - subtraction data B(1) = B₁₄ to B₁₁ output from the addition - subtraction data set circuit 207, from the digital quantity (B₄₄ to B₄₁) input from the latch circuit 216, and outputs the data B(2) = B₂₄ to B₂₁ obtained after the above operation to the buffer 214. The buffer 214 is driven by a clock pulse CK input from a terminal 219b, and displays a high impedance when the clock pulse CK is on H level, and when the clock pulse is on L level, it becomes active and transfers the obtained data B(2) = B₂₄ to B₂₁ from the full adder 213 to the memory 215. The timings for a series of operations of the buffer 214, memory 215, and latch circuit 216 driven by the clock pulse CK are shown in Table 3. As in the table, ganerally speaking, when the clock pulse is on H level, data is read from the memory 215 and when it is on L level, data is written in the memory 215. Next, referring to Fig. 16 to Fig. 22, the configuration, function, and so on of each circuit portion will be described in more detail. First, referring to Fig. 16, a main portion of the 4-bit D/A converter 203 consists of a ladder type resistor circuit formed by five resistors that have resistance of 2R and three resistors that have resistance of R, and an inverting amplifier formed by an operational amplifier 221 and a resistor with resistance (16/5)R. When voltages V₄₁, V₄₂, V₄₃, and V₄₄ are applied to one end of the respective resistors with resistance 2R, and voltages V₃, V₂, and V₁ appear at the respective connection points of resistors with resistance R, as in Fig. 16, the following relations are formed by Kirchhoff's law. (V₄₃-V₁) / 2R - V₁/R + (V₂-V₁) / R = 0, (V₄₂-V₂) / 2R + (V₁-V₂) / R + (V₃-V₂) / R = 0, (V₄₁-V₃) / 2R + (V₂-V₃) / R - V₃ / 2R = 0 . From Eq. (3) there follows V₄₃ - 5V₁ + 2V₂ = 0, V₄₂ + 2V₁ - 5V₂ + 2V₃ = 0, V₄₁ + 2V₂ - 4V₃ = 0 . By calculating Eq. (5) × 2 + Eq. (6) there is obtained 2V₄₂ + V₄₁ + 4V₂ - 8V₂ = 0 . By multiplying Eq. (4) by 4 there is obtained 4V₄₃ - 20V₁ + 8V₂ = 0 . From Eq. (5) + Eq. (7), V₄₁ + 2V₄₂ + 4V₄₃ - 16V₁ = 0 . By calling as I the current that flows through the feedback resistor in the inverting amplifier one obtains I = V₄₄ / 2R + V₁/R Substituting the value of V₁ obtained from Eq. (9) into Eq. (10), I = (1/16R) . (V₄₁ + 2V₄₂ + 4V₄₃ + 8V₄₄) The analog output voltage in the circuit of Fig. (16) is given by -V out = (16/5) RI = (1/5) (V₄₁ + 2V₄₂ + 4V₄₃ + 8V₄₄) In the above, each of the voltage V₄₁ to V₄₄ in Eq. (12) corresponds to the digital quantity B(4) = B₄₄ to B₄₁ output from the latch circuit 216 as shown in Fig. 15. Therefore, if the voltage of the digital quantity B(4) = B₄₄ to B₄₁ when it is on H level is set, for example, to V cc = 5V, the analog output voltage - V out can be rewritten as the following. -V out = (1/5) (B₄₁ × 5V + B₄₂ × 2 × 5V + B₄₃ × 4 × 5V + B₄₄ × 8 × 5V) = B₄₁ + 2B₄₂ + 4B₄₃ + 8B₄₄ (V) . In the circuit shown in Fig. 16, the analog output voltage has a negative value as shown by Eq.(13) above so that the D/A converter 203 in Fig. 15 is further connected to a voltage inverting circuit that uses an operational amplifier 222 as will be shown in Fig. 17. From the D/A converter 203 there is output in general an analog voltage V dac (=V out ) as shown by the following equation. V dac = k (B₄₁ + 2B₄₂ + 4B₄₃ + 8B₄₄) . As explained in connection with Eq. (13), it is possible, by properly setting the circuit constants and the value of V cc , to choose k = 1, so that description in what follows will be made on the assumption that k = 1. In this case, for the various values of the digital quantity B(4) = B₄₄ to B₄₁, analog voltage V dac with the following values are obtained. Next, referring to Fig. 17, sample hold circuit 218, differential amplifier 202, voltage comparison circuit 204, and the analog circuit portion including the D/A converter 203 mentioned in connection with Fig. 16 will be described. The sample hold circuit 218 consists of two operational amplifiers 223 and 224, and a switch 225 and a capacitor 226 connected between the operational amplifiers 223 and 224. The switch 225 is driven by (1/4) CK obtained by frequency-dividing the clock CK into four, and the input analog voltage V in is held in the capacitor 226. The differential amplifier 202 is of the ordinary type that consists of an operational amplifier 202a. With respect to the voltage comparison circuit 204, transistors 227a and 227b are connected to the output terminals of the voltage comparators 205a to 205d and 206a to 206d, although these are omitted in Fig. 15. In addition, an analog-logic interface circuit constructed by two diodes D₁ and D₂ and three resistors R₁, R₂, and R₃, is connected to the output terminal of each of the voltage comparators. In contract to the analog circuit portion operated by a power supply voltage, for example, of ±15V, the logic circuit portion that follows the addition - subtraction data set circuit 207 is operated, for example, by a power supply voltage of +5V, so that the ranges of operating voltage for both circuit portions are ordinarily different from each other. For this reason, an interface circuit is used in order to supply signals with normal logical amplitude from the analog circuit portion to the logic circuit portion. To describe the operation of the interface circuit, if ΔV in < + V r and the output of the voltage comparator 205a is on H level, the transistor 227a is turned on, and the voltage V₁ at the input terminal 228a becomes V₁ = V ee + V cesat - 15 (V) . In the above equation, V ee is the emitter voltage of the transistor 227a and V cesat is the saturation voltage, and the voltage V₁ becomes equal to V ee if V cesat can be neglected. On the other hand, if the voltage at the output terminal 228b is V₂ and the voltage at the connection point of the diodes D₁ and D₂ is V₃, voltages V₂ and V₃ are given by V₂ = V₃ - V f2 and V₃ = V f1 , where V f1 and V f2 are the forward voltage drops of the diodes D₁ and D₂, respectively. As in the above, when the transistor 227a is in the on-state, there flow forward currents in the diodes D₁ and D₂ and V f1 and V f2 are nearly equal, so that the voltage V₂ at the output terminal 228 is V₂ ≅ 0 (V). Namely, when ΔV in < + V r , the negative voltage V₁ is cut, and the voltage comparison circuit 204 outputs "0" signal normally. On the other hand, when ΔV in > + V r , the output of the voltage comparator becomes L level, and the transistor 227a is turned off. As a result, the currents that flow in the resistors R₂ and R₃ becomes zero and the voltage V₂ at the output terminal 228b becomes 5V and signal "I" is normally output. In this manner, from the interface circuit, there is obtained a logical output which normally oscillates between 0 V and 5V. Next, referring to Fig. 18 and Fig. 19, the logical process in which the addition - subtraction data B(1) = B₁₄ to B₁₁ is output from the addition and subtraction data set circuit 207 will be described based on the output C₁ to C₄ and C₁₋ to C₄₋ of the voltage comparison circuit 204. First, when the difference voltage ΔV in has a positive sign, every time when it exceeds each of the reference voltages V r to 8V r set for the voltage comparison circuit 204, addition data A₁ to A₄ are output to set the addition - subtraction data B(1) that correspond to each bit in the following way. Namely, when the difference voltage ΔV in exceeds the reference voltage V r to satisfy V r < ΔV in < 2V r , the leas significant bit B₁₁ in the addition - subtraction data B(1) becomes "1". Similarly, when the difference voltage ΔV in exceeds 2V r , B₁₂ becomes "1", and when it exceeds 4V r , B₁₃ becomes "1," and when it exceeds 8V r , the most significant bit B₁₄ becomes "1." Now, the addition data A₁ to A₄ have the following relations with the outputs C₁ to C₄ of the voltage comparators 205a to 205d. A₁ = C₁ . C ₂, A₂ = C₂ . C ₃, A₃ = C₃ . C ₄, A₄ = C₄ . On the other hand, if the sign of the difference voltage is negative and its value becomes smaller than each of the reference voltages -V r to -8V r , subtraction data S₁ to S₄ are output to set an addition - subtraction data B(1) for each bit as in the following. Namely, when the difference voltage ΔV in becomes smaller than -V r , (0001) need be subtracted in the full adder 213 from the digital quantity B(4) so that (1111) is set for the addition - subtraction data B(1). The set value (1111) corresponds to adding 1 to the least significant bit to (1110) which is the inversion of each bit of (0001). In similar manner, when the difference voltage ΔV in becomes smaller than -2V r , one needs to subtract (0010) so that there is set an addition - subtraction data B(1) which is (1110), when it becomes smaller than - 4V r , there is set (1100), and when it becomes smaller than -8V r , there is set (1000). Here, the subtraction data S₁ to S₄ have the following relations with the outputs C₁₋ to C₄₋ of the voltage comparators 206a to 206d. S₁ = C₁₋. C ₂₋, S₂ = C₁₋. C ₃₋, S₃ = C₁₋. C ₄₋, S₄ = C₄₋. For the 4-bit full adder 213 use is made of a disclosed apparatus so that detailed description about its internal configuration and others will be omitted by simply giving its logical diagram in Fig. 20, block diagram in Fig. 21, and the truth table between inputs and outputs. In Fig. 20 to Fig. 22, the input signals A₁ to A₄ correspond to the digital quantity B(4) = B₄₁ to B₄₄, and the other input signals B₁ to B₄ correspond to the addition - subtraction data B(1) = B₁₁ to B₁₄. Next, referring to the timing charts in Fig. 23, the overall operation of the A/D converter shown in Fig. 15 will be described. Suppose that in the beginning the value of the input analog voltage V in that is sample held in the sample hold circuit 218 is 15V + α , and the digital quantity B(4) = (B₄₄, B₄₃, B₄₂, B₄₁) stored in the memory 215 is (0000). The analog voltage V dac generated from the D/A converter 203 in proportion to the digital quantity B(4) is 0 V so that a difference voltage ΔV in = 15V + α is output from the differential amplifier 202. The difference voltage ΔV in = 15V + α is compared in the voltage comparison circuit 204 simultaneously with the reference voltages V r to 8V r , and there is output a result (C₄, C₃, C₂, C₁) = (1111) which corresponds to the difference voltage ΔV in . In the addition - subtraction data set circuit 207, the addition - subtraction data B(1) = (B₁₄, B₁₃, B₁₂, B₁₁) is set to (1000) based on the outputs C₄ to C₁. The full adder 213 adds the addition - subtraction data B(1) which has the value (1000) to the digital quantity B(4), and outputs (1000) as the data B(2) = (B₂₄, B₂₃, B₂₃, B₂₁). Since the buffer 214 becomes active with the timing when the clock pulse CK becomes L level, the data B(2) is written in the memory 215 as a data B(3) = (B₃₄, B₃₃, B₃₂, B₃₁). By the writing of the data B(3), the digital quantity B(14) memorized in the memory 215 becomes (1000). Because of this, the analog voltage V dac output from the D/A converter 203 in proportion to the digital quantity B(4) is changed to 8V. As a result, the difference voltage becomes ΔV in = 15V + α - 8V = 7V + α , the output of the voltage comparison circuit 204 becomes (C₄, C₃, C₂, C₁) = (0111), and based on this output the addition - subtraction data is set to B(1) = (0100). Accordingly, the following binary operation is carried out in the full adder 213. This data B(2) becomes a data B(3) via the buffer 214, and is written in the memory 215. In the next timing, an analog voltage V dac = 12 V proportional to B(4) = (1100) is generated from the D/A converter 203, and the difference voltage becomes ΔV in = 15V + α -12V= 3V + α . As a result, the output of the voltage comparison circuit 204 becomes (C₄, C₃, C₂, C₁) = (0011), the addition and subtraction data becomes B(1) = (0010), the data for writing becomes B(2) = (1110), and the data B(3) = (1110) is stored in the memory 215. In this way, the digital quantity B(4) stored in the memory 215 in the beginning is successively corrected, and at the rising time of the fifth clock pulse CK, a digital quantity B(4) = (1111) which corresponds to the input analog voltage V in = 15V + α is output from the output terminal 217. Suppose that, following the above conversion operation, an input analog voltage V in = 0 V - α is sample-held at the fifth clock pulse CK. Since there is generated from the D/A converter 203 an analog voltage V dac = 15V which is proportional to the digital quantity B(4) = (1111), the difference voltage becomes ΔV in = -15V - α . Due to this difference voltage ΔV in , the output of the voltage comparison circuit 204 becomes (C₄₋, C₃₋, C₂₋, C₁₋) = (1111) and the addition and subtraction data are set to B(1) = (1000). Accordingly, the following subtraction is carried out in the full adder 213. This is an operation which coresponds to a voltage drop of 15V - 8V = 7V. The data B(2) becomes a data B(3) via the buffer 214, and is written in the memory 215. The full adder disregards the carry of the bit. At the next timing, an analog voltage V dac = 7V proportional to B(4) = (0111) is generated from the D/A converter 23, and the difference voltage becomes ΔV in = -7V - α . Due to this difference voltage ΔV in , the output of the voltage comparison circuit 204 is set to (C₄₋, C₃₋, C₂₋, C₁₋) = (0111) and the addition and subtraction data are set to B(1) = (1100). Accordingly, the full adder 213 performs the following substraction. This is an operation that corresponds to a voltage drop of 7V - 4V = 3V. The data B(2) becomes a data B(3) via the buffer 214 and is written in the memory 215. And, at the next timing, an analog voltage V dac = 3V proportional to B(4) = (0011) is generated from the D/A converter 203, and the difference voltage becomes ΔV in = -3V - α . As a result, the output of the voltage comparison circuit 214 becomes (C₄₋, C₃₋, C₂₋, C₁₋) = (0011), the addition and subtraction data becomes B(1) = (1110), the data for writing of the subtracted result becomes B(2) = (0001), and the data B(3) = (0001) is written in the memory 215. As in the above, along with the result of correction operation, the absolute value of the difference voltage ΔV in becomes smaller in succession as -15V, -7V, -3V, -1V, and 0V, and at the rising of the ninth clock pulse CK, a digital quantity B(4) = (0000) that corresponds to the sample held input analog voltage V in = 0V - α is output from the output terminal 217. In this manner, in carrying out the 4-bit A/D conversion, it needs only four clock pulses CK, realizing a high speed conversion. The conversion time can further be shortened since the number of required clock pulses CK becomes small if the width of variation in the input analog voltage V in becomes small, as can be seen from the table 5. Further, from the number of clock pulses CK in the above table, it will be seen that when the 4-bit A/D conversion is completed, the outputs of the voltage comparison circuit 204 become all "0" since they are (C₄, C₃, C₂, C₁) = (0000), (C₄₋, C₃₋, C₂₋, C₁₋) = (0000). Therefore, it is possible to know the completion of the A/D conversion at an early time by detecting the above result as a CONV signal for completion of conversion. Fig. 24 shows a modification of the portions of the voltage comparison circuit and the addition and subtraction data set circuit in the 4-bit A/D converter. In the modification, there are arranged in a voltage comparison circuit 234 only two voltage comparators 205a and 206a for setting reference voltages +V r and -V r , respectively. In addition, in an addition - subtraction data set circuit 237, one OR circuit 229 alone is arranged, and two input terminals of the OR circuit 229 are connected to the output terminals of the voltage comparators 205a and 206a respectively. The least significant bit B₁₁ in the addition subtraction data B(1) = (B₁₄, B₁₃, B₁₂, B₁₁) is output from the OR circuit 229. The other bits B₁₄, B₁₃, and B₁₂ are directly output from the voltage comparator 206a. When the difference voltage ΔV in is greater than the reference voltage V r , the output C₁ of the voltage comparison circuit 234 becomes "1" and the addition - subtraction data is set to B(1) = (0001). On the other hand, when the difference voltage ΔV in is smaller than the reference voltage -V r , the output C₁₋ of the voltage comparison circuit 234 becomes "1" and the addition - subtraction data is set to B(1) = (1111). Table 6 shows the above relationship. The addition - subtraction data B(1) in the above is added - subtracted in the full adder 213 from the digital quantity B(4) read from the memory 215, the digital quantity B(4) is successively corrected in minimum bit unit, and an A/D converted digital quantity B(4) is output from the output terminal 217. According to the modification, there is an advantage that the configuration of the voltage comparison circuit 234 and the addition - subtraction data set circuit 237 can be made extremely simple. However, A/D conversion is carried out in terms of the correction made in the least significant bit unit of the digital quantity B(4) read from the memory so that its conversion time is somewhat longer compared with the voltage comparison circuit 204 and the addition - subtraction data set circuit 207 shown in Fig. 15. Fig. 25 shows a modification of the memory portion Although in the memory 215 shown in Fig. 15 the address for storing digital quantity is fixed, in the present modification it is arranged that address control can be carried out by providing an address controller 231. When the digital quantity B(4) to be A/D converted is of 4 bits, a digital quantity B(4) can complete A/D conversion with a maximum of four clock times as shown in the timing chart in Fig. 23. Therefore the address controller 231 is driven by (1/4) CK obtained by dividing the frequency of the clock CK into four, and an address data is output to control the address of the memory 215. As a result, the memory 215 can successively store A/D converted data B(3) = (B₃₄, B₃₃, B₃₂, B₃₁) in separate addresses. Figs. 26 and 27 show an embodiment of an 8-bit A/D converter according to the present invention. As shown in Fig. 26, a voltage comparison circuit 204 has the same system as for the 4-bit converter shown in Fig. 15. To the output terminal of a voltage comparator 206a in the voltage comparison circuit 204 for setting a reference voltage - V r , there are connected output lines of upper place four bits, (B₁₈, B₁₇, B₁₆, B₁₅) of the 8-bit addition and subtraction data B′(1) = (B₁₈, B₁₇, B₁₆, B₁₅, B₁₄, B₁₃, B₁₂, B₁₁), and they are expanded to 8 bits at the portion of an addition and subtraction data set circuit 232. When the 8-bit system as in the above is used, it can be realized to reduce the number of required voltage comparators in the voltage comparison circuit. The full adder 213, buffer 214, memory 215, and latch circuit 216 in Fig. 15 are used by simply expanding them for 8 bits although they are not shown in Fig. 26. Accordingly, each data and digital quantity can be represented as follows. B′(2) = B₂₈ to B₂₁, B′(3) = B₃₈ to B₃₁, B′(4) = B₄₈ to B₄₁. Fig. 27 shows the relationship between the magnitudes of the difference voltage ΔV in and the addition and subtraction data B′(1) = B₁₈ to B;i;i1 set in the addition - subtraction data set circuit 232. The addition - subtraction data B′(1) = B₁₈ to B₁₁ shown in Fig. 27 are added or subtracted in the full adder 213, from the digital quantity B′(4) read from the memory 215 to correct the digital quantity B′(4). In this case, for the upper 4 bits, addition and subtraction are carried out with the four bits (00001000) of the addition - subtraction data B′(1) as a unit. As described in the foregoing, according to the present invention, in the voltage comparison circuit, there are set a plurality of positive and negative reference voltages whose values are related to the weightings for the bits in the digital quantity. By generating in the D/A converter an analog voltage proportional to the digital quantity stored in the memory, the difference voltage between the input analog voltage and the analog voltage generated in the D/A converter is simultaneously compared with the plurality of reference voltages in the voltage comparison circuit. Based on the output from the voltage comparison circuit, a binary number with weighting that corresponds to the absolute value of the difference voltage is added to or subtracted in the full adder from the digital quantity read from the memory. The new digital quantity obtained as the result of the above operation is stored in the memory, and the above process is repeated until the absolute value of the difference voltage becomes smaller than the reference voltage corresponding to the weighting for the least significant bit. As a result, there can be obtained an advantage that A/D conversion with high accuracy can be carried out at high speed.

Claims: An optical disc apparatus including: optical means (104) for recording or reproducing information on the recording track (102) of an optical disc (101); means (113, 114, 115) for detecting at least a deviation amount of beam spot projected from said optical means (104) on the recording track of the optical disc (101) and generating a track deviation signal based on the deviation amount; means (126, 215) for memorizing a first digital quantity; and means (128, 203) for converting digital quantities read from said memory means (126, 215) to an analog signal, said analog signal being used to drive said optical means (104), means (207, 213, 214) for calculating a second digital quantity based on the first digital quantity from the memory means (126, 215) and transferring the second digital quantity to the memory means (126, 215), characterized by means (202) for outputting a difference voltage between the track deviation signal and the analog signal from the converting means (203); means (204) for comparing the difference voltage with reference voltages, when the digital quantity read from said memory means (126, 215) is of n bits, the comparing means (204) comprises a number 2n of voltage comparators (205, 206) and the reference voltages comprise a plurality of positive and negative reference voltages +Vr and -Vr each of which corresponds to a weighting of each bit in the digital quantity read from the said memory means (126, 215) said calculating means (207, 213, 214) calculating the second digital data based on the digital output of the comparing means and the first digital quantity from the memory means (126, 215) and repeating said calculation of the second digital quantity until the voltage of the track deviation signal becomes smaller than the reference voltage corresponding to the weighting for the least significant bit, said calculating means comprising an addition-subtraction data set means (207) for setting the digital output from the voltage comparators (205, 206), and a full adder (213) for adding the first digital quantity from the memory means (126, 215) to the set digital signal from the addition-subtraction data set means (207), a result of the adding being transferred to the memory means (126, 215) as the second digital quantity. The optical disc apparatus as claimed in claim 1, wherein said detecting and generating means (113,114,115) outputs the track deviation signal voltage in response to the outward and inward directions of the deviation. The optical disc apparatus as claimed in claim 1, wherein an analog voltage for eccentricity correction is output from the converting means (128) and is applied to means (106) for driving the optical means (104) so that track jumping is carried out while the eccentricity correction is in operation. The optical disc apparatus as claimed in claim 1, wherein said detecting and generating means (113,114,115) has two photodetectors (113a,113b) for detecting reflected light from the optical disc (101) and a differential amplifier (115) which outputs a track deviation signal voltage by receiving signals from both photodetectors (113a,113b). The optical disc apparatus as claimed in claim 4, wherein the converting means (128) is connected to means (106) for driving the optical means (101) and at least the track deviation signal and the analog signal from the converting means (128) are selectively transmitted to the driving means (106). The optical disc apparatus as claimed in claim 8, wherein said calculating means (207, 213, 214) further includes an OR circuit (212) which takes a logical sum of the outputs of the addition and subtraction data set means (207). The optical disc apparatus as claimed in claim 6, wherein said full adder (213) adds or subtracts a binary number corresponding to the absolute value of the difference voltage based on the output signal from the OR circuit (212). The optical disc apparatus as claimed in claim 1, wherein an address controller (231) is disposed to control address data output to said memory means (215) and the A/D converted data are successively memorized in separate addresses. The optical disc apparatus as claimed in claim 1, wherein the data memorized in said memory means (215) are transferred to the converting means (128) with a shifted timing. The optical disc apparatus as claimed in any of claims 1 to 9 wherein the optical disc (21;101) is subdivided into a plurality of sectors for data memory. The optical disc apparatus as claimed in claim 10 wherein the track deviation signal is read into said memory means (126,215) as the first digital quantity for each sector. The optical disc apparatus as claimed in claim 10 wherein reading of the track deviation signal in said memory means (126,215) is carried out for a plural number of times for each sector.

Inventors: NAGANO KATSUMI, WATANABE HIROSHI, NAGANO, KATSUMI, WATANABE, HIROSHI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: G11B7
IPC codes: G11B7