GB2158611A - Servo error reduction apparatus for disc record playback system - Google Patents

Abstract

The system, when for an optical disc (10), includes a first servo loop (42, 44, 46, 60, 52, 32) to maintain, in the presence of eccentricity in a rotating disc, a light beam on a selected data track on a surface (20) of the disc (10), and a second servo loop (40, 48, 50, 60, 54, 31) to maintain an objective lens (33) at the proper distance from the disc surface. Both servo loops share a common error reduction circuit (60). 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. <IMAGE>

Description

SPECIFICATION Servo error reduction apparatus for disc record playback system This invention relates generally to a disc playback system including a servo mechanism having apparatus for reducing servo errors. Illustratively, the disc record is an optically readable disc and the apparatus in such a system reduces errors, such as errors caused by eccentricities of the disc, and in focusing a light beam on a disc surface and in following a selected data track on the disc surface with the focused light beam. Reference will be made hereinafter, by way of example, to an optical disc playback system.

High density optical recording systems which may be used for recording and playing back information are known in the prior art. For example, U.S. Patent No. 4,097,895, entitled "MULTI-LAYER OPTICAL RE CORD," issued on June 27, 1978, to F.W. Spong, relates to an optical disc record/playback system wherein data are recorded on the surface of a recording medium. In a Spong system the thermal energy of a focused high intensity light beam causes variations in the optical properties on the surface of the recording medium. For example, in one system the thermal effects of a laser beam form pits in an absorptive coating on the surface of an optical disc.

In the Spong system, approximately 10'1 bits of information can be recorded on one side of a disc-shaped record medium having a thirty centimeter diameter.

In an optical playback apparatus, the disc record is rotated on a turntable while a light beam scans a surface of the record having information recorded thereon. A system of optics directs the light beam onto the information bearing surface. In order to read out the information recorded at high densities on the disc surface, as when the Spong system is used, the light beam must be precisely directed to the center of the track of recorded pits, and it must be focused on the disc record surface so that the light spot covers only a very small area. In the most advantageous arrangement, the size of the light spot is diffraction limited.

qProblems associated with physical limitation of the disc and of the playback system contribute to the difficulties in maintaining the light spot directed on the track of pits and in maintaining the light spot in focus on the disc surface. Eccentricities in a recorded data track result from the mechanical positioning of the disc after removal and remounting, when the disc mounting hole is not perfectly centered with respect to the data track. Another eccentricity problem which alters the track position profiles of previously recorded tracks is caused by surface errors resulting from distortion and deformation of the surface structure by thermal effects, inter alia. A third cause of track eccentricity may be due to a wobbling motion of the turntable shaft in its bearings.

It is known to compensate for eccentric motion of the data track relative to a fixed tracking beam by use of a servo loop in the tracking system. The eccentric motion tends to repeat on adjacent tracks at every disc revolution. It is also known to anticipate the error at several positions about the disc, and to apply a appropriate compensating signal to the servo system. This permits a reduction in the bandwidth requirement for the servo system. U.S. Patent No.4, 138,741, entitled "DISC ECCENTRICITY COMPEN SATING SYSTEM," issued February 6, 1979, to L.V.

Hedlund et al., discloses a system in which a reference track is read, and a "signature" waveform corresponding to the tracking error signal is generated and stored. This waveform is then applied to the electronics controlling the disc tracking system during subsequent read operations. U.S. Patent No.

4,365,324, entitled "ECCENTRICITY CONTROL DE VICE", issued December 1982, injects an eccentricity compensating signal to the tracking system, which signal has been computed by an open loop control circuit counting the number of signal track traversals detected at periodic locations during one revolution of the disc.

Whereas the Hedlund et al. system provides adequate tracking error compensation for discs fabricated on aluminum or other metallic substrates, in which the distortion caused by thermal effects tends to be uniform over a radial cross-section, it is believed to be inadequate for plastic discs. Optical discs made on plastic substrates are subject to distortion due both to thermal effects as well as stresses of rotation, and this distortion tends to be anisotropic. Hence, for the case of a plastic disc, a tracking compensation signal based on a reference track at a first radial distance may be entirely inappropriate for a track at a different radial distance, and the Hedlund et al. system may compensate deleteriously, rather than beneficially. It is clear that, in the case of plastic discs, a compensating tracking signal, based on a single reference track, is not sufficient.

In addition to the problems associated with tracking the recorded data, there are also problems in maintaining focus on the disc surface. Focus problems in maintaining focus on the disc surface. Focus problems may result from imperfections associated with the disc, turntable, or rotating drive mechanism which may displace the information bearing surface of the rotating disc with respect to a fixed point. As was the case in tracking problems, disc surface irregularities may result from deformations caused by thermal or rotational stresses, as well as from nonuniform thickness during fabrication.

A servo system which controls the distance between an objective lens and a rotating disc is disclosed in U.S. Patent No.4,418,405, entitled "LENS POSITIONING CONTROLLER FOR OPTICAL PLAYBACK APPARATUS," issued November 29, 1983, to W.E. Barnette et al. The Barnette et al.

apparatus, however, has no provision for anticipating the error, even though, as is the case for tracking errors, focus errors tends to repeat on adjacent tracks at every disc revolution.

The invention relates to a system for playing back information recorded on a surface of a rotating disc medium. In that system, each of one or more parameters, which relate to the medium, is to be urged from a detected condition toward a desired condition during continuing rotation of the medium.

To that end, there is provided servomechanism means, which includes: (a) sensing means responsive to condition of parameters for producing respective, continuing error signals, which represent deviation of parameter conditions from desired conditions, (b) measuring means responsive (i) to the error signals and (ii) to a position signal representing the angular rotational position of the rotating medium for producing for said parameters respective continuing control signals, and (c) control means responsive to the control signals for changing the conditions of parameters toward the respective, desired conditions thereof.

According to the invention; the position signal comprises trains of pulses generated during respective rotations of the medium, each pulse within each train marking passage of the medium through a respective angular position of the medium; and the measuring means includes: encoding means responsive upon the occurrence of pulses within each train for encoding the level of each continuing error signal into respective error data words; one or more data storage means, each corresponding to a respective parameter, with each data storage means having locations corresponding to a respective angular position of the medium and to a respective pulse within each train of pulse from the position signal, and with each location in each data storage means storing a respective historical data word; calculating means operative upon the occurrence of each pulse in each train for combining (i) each error data word with (ii) an historical data word taken from a respective location of a respective data storage means into (iii) a control data word; and decoding means operative upon the occurrence of each pulse in each train for converting each control data word in a respective one of the continuing control signals.

The present invention will be more fully understood from the following detailed description of an illustrative embodiment, referring to the accompanying drawings, in which Figure lis a block diagram of an illustrative optical disc playback system according to the present invention; Figure 2is a detailed block diagram of an illustrative error reduction apparatus of Figure 1; and Figure 31S a flow diagram indicating the operation of the error reduction apparatus of Figure 2.

Referring to Figure 1, there is shown an optical disc playback system. Record disc 10, mounted on turntable 12 and held in position by hold down device 14, is rotated by drive motor 16. The depiction of hold down device 14, which may imply a clamping device, is merely functional. In practice, disc 10 may alternatively be positioned by a vacuum chuck.

Encoder 18, illustratively an angular incremental encoder affixed to the shaft 17 of drive motor 16, generates electrical pulses corresponding to the position of shaft 17 and thus provides positional information regarding disc 10. Two signal outputs of 18 are coupled to process controller 56. The first is the index pulse signal that is generated once per revolution and the second is the angular segment pulse signal. There are, for example, 1,024 equallyspaced pulses, or segments, per revolution.

Disc 10 may be of a type described in U.S. Patent No.4,222,071, entitled "SENSITIVITY INFORMATION RECORD", issued on September 9, 1980, to A.E. Bell.

Disc 10 has information recorded on at least one surface 20. In accordance with one technique of recording by ablation, the material of the recording layer is elevated to an ablation temperature. In the recording process the material of the recording layer vaporizes or melts forming a pit therein. With suitable modulation of the intensity of the light beam in accordance with the recording signal, as successive regions of the disc record pass through the light beam path, an information track may be formed comprising pits in region where the material is ablated separated by undistrubed regions of the recording layer (that were not subject to exposure by the high intensity beam).

The data recorded on surface 20 of disc 10 are read by the optical disc playback system shown in Figure 1. The monochromatic light output of laser 22 is passed via polarizer 24 to a polarizing beam splitter 26. Polarizer 24 effects a polarization of the laser output in a direction that permits pasage of the light through beam splitter 26. A lens 28 forms the light passed by beam splitter 26 into a beam which passes through quarter-wave plate 30 to galvanometercontrolled mirror 34 which reflects the beam through an objective lens 33, causing the beam to be focused on the information-bearing surface 20 or rotating disc 10. in the read mode of operation, as described above, the intensity of the output of lasser 22 is set at a constant level which is safely below the level causing ablation of the material of the absorptive layer.

Light reflected from the information surface 20 is returned via elements 33, 34,30 and 28 to beam splitter 26. Since the return light beam has made two passes through quarter-wave plate 30, its polarization has been altered to a direction which results in relection of the return light beam by beam splitter 26 onto a device for converting light energy variations into an electrical signal, shown in Figure las photodetector 38.

To provide optical scanning on surface 20 of rotating disc 10, translation stage 36 is moved radially across disc 10 at a rate in accordance with the requirements of the desired playback mode.

Translation stage 36 efects a movement of elements of the optical system, namely, beam splitter 26, lens 28, quarter wave plate 30, galvanometer 32 and its mirror 34, objective lens 33 and its positioning driver 31, and photodetector 38. By means of a signal from a translation stage driver (not shown), stage 36 may be moved (i.e., coarse tracking movements) such that the light beam through lens 33 falls on surface 20 of disc 10 to within a few tracks of a desired position.

Fine tracking movements may be achieved by deflecting the light beam exactly to a desired track by a conventional galvanometer-controller mirror 34. Galvanometer 32 causes mirror 34 to move about an axis that is parallel to the reflecting surface 20 of disc 10 so that a light spot that is formed by lens 33 may be guided along a selected track as disc 10 rotates. Mirror 34 is actuated by galvanometer 32 in response to control signals which are supplied by galvanometer driver 52.

Light beam 35 is focused on surface 20 of disc 10 by focusing objective lens 33. Lens positioner 31 moves lens 33 in the direction shown by doubleheaded arrow 37 to maintain properfocus. Lens positioner 31 responds to the focus drive signal generated by driver 54, and may be of a type disclosed in the aforementioned Barnette et al.

patent.

The intensity of light falling upon photodetector 38 alternates between minimum and maximum levels as the successive pit and undisturbed regions of the information surface 20 pass through the path of the focused beam. The minimum intensity level for light reaching photodetector 38 is obtained when an undisturbed region of the absorptive layer of disc 10 is in the focused beam path, while the maximum intensity level for light reaching the photodetector 38 is obtained when a pit is in the focused beam path.

The output of photodetector 38 comprises pulse code modulated waves which vary in consonance with the pit edge spacing. Variations of the detected beam intensity are representative of the original signal encoded on the disc surface 20 during the recording operation. The data signal provided by photodetector 38 is applied to playback electronics (not shown) for processing its informational content; the data signal is also applied to focus error signal generator 40 and tracking error signal generator 42.

The tracking error signal is passed through filter 44 which attenuates high frequency noise that is independent of disc angular position. The filtered tracking error signal is applied to amplifier 46 which adjusts the signal level so as to be handled by error reduction circuit 60. Circuit 60 acts on the tracking error signal, in a manner to be described in detail in conjunction with Figs. 2 and 3, to thereby provide a signal to galvanometer driver 52 which, in turn, provides the drive signal to galvanometer 32.

The focus error signal is passed through filter 48 which, like filter 44, attenuates high frequency noise that is independent of disc angular position. The filtered focus error signal is applied to amplifier 50 which adjusts the signal level so as to be handled by error reduction circuit 60. Circuit 60 acts on the focus error signal, in a mannerto be described in detail in conjunction with Figs.2 and 3, to thereby provide a signal to focus driver 54 which, in turn, provides the drive signal to focusing lens positioner 31.

Reference is now made to Figure 2 which depicts the error reduction circuit 60 of Fig. 1 in greater detail. The elements of Fig. 2 which are of a type described in FIG. 1 are given identical designation numerals. In this figure, error reduction circuit 60 is shown as including switches 62 and 76, analog-todigital converter (A/D) 64, arithmetic logic unit (ALU) 68, memory storage devices 70 and 72, data bus 66, digital-to-analog converter (D/A) 74, and sampleand-hold circuits 80 and 82.

The two error signals provided from amplifiers 46 and 50 are applied to input terminals of switch 62, which selectively steers one of the input signals to its output terminal under the control of process controller 56, via a signal applied to the control (C) input terminal of switch 62. The selected switch input signal is applied to A/D 64 which converts the input analog signal to illustratively, an eight-bit binary signal (although other numbers of bits could be used in other embodiments) and presents it to data bus 66. The timing and sample-and-hold functions typically associated with A/D functions may be implied but are not explicitly shown.

The digital data represented by the binary output signals of A/D 64 are operated on by ALU 68 in conjunction with a first memory device 70 IT MEM ORY), associated with the tracking error, and a second memory device 72 (F MEMORY), associated with the focus error. After the operations of ALU 68 are complete, which operations will be fully discussed in conjunction with Fig. 3, ALU 68 causes a data word to be applied via data bus 66 to DIA 74 where the digital data word is converted to an analog signal. This signal is selectively switch through switch 76, under the control of process controller 56, via a signal applied to the control (C) input terminal of switch 76, to either of the sample-and-hold circuits 80 and 82, and from these to galvanometer driver 52 or focus driver 54.

In the present example, in which 1,024 signal pulses are generated by encoder 18 to process controller 56 per revolution of disc 10, tracking memory 70 and focus memory 72 may, appropriately, each comprise 1,024 storage locations for data words having width equal to the width of data bus 66, i.e., eight bits, in the present example. Process controller 56 responds to the pulse signals from encoder 18to generate addressing signals to tracking memory 70 and focus memory 72. Thus, each data word in each memory 70, 72 corresponds to an angular position on disc 10, referenced to a fixed index position determined by the index pulse.The contents of each of these memory words corresponds to a direction and magnitude of a drive signal necessary to maintain beam 35, reflected by galvanometer-controller mirro 34, directed on the desired data track of disc 10, for the case of tracking memory 70, and to maintain lens 33 in proper focal alignment with surface 20 of disc 10, for the case of focus memory 72.

In the present illustrative example of the invention, during each period corresponding to one of the (illustratively) 1,024 angular segments of disc 10 identified by signal pulse from encoder 18 ALU 68 receives, via a properly steered input through switch 62, a data word comprising digital signals corresponding to the tracking error, algebraically sums this received data word with the data word in tracking memory 70 corresponding to the particular angular position of disc 10, stores the combined data word back in the same location in memory and applies the combined data word to D/A 74 where it is steered through the proper output terminal of switch 76 into sample-and-hold circuit 80, which samples the tracking signal at the proper time via XT' and couples it to tracking galvo driver 52. Then, during the same period, switches 62 and 76 are activated to steer the focus error,signal and focus driver 54 into the loop with error reduction circuit 60, and the identical process is repeated using the stored data of focus memory 72, causing a focus signal to be clocked into sample-and-hold circuit 82 at the proper time via F' resulting in a drive signal out of focus driver 54. This dual-function error signal handling process is repeated continually for each angular position of disc 10 during every rotation thereof.

Figure 3is a detailed flow diagram of the aove process, including the initiaiization of the storage location contents of memories 70 and 72. The steps of Fig. 3 may be executed within process controller 56, which may typically be a microprocessor, via control signals coupled to ALU 68 and memories 70 and 72. This error handling process would represent a very small fraction of the task-handling capability of a typical microprocessor, and might conveniently be incorporated within the functions of the main controller of the optical disc system.

The procedure is entered at step 101 during which all of the (illustratively) 1,024 storage locations of both the tracking memory 70 and focus memory 72 are cleared, i.e., set equal to zero. Then, at step 102, the procedure idles awaiting a pulse from encoder 18 indicating the once-per-revolution index mark.

The corresponding position on disc 10 is referred to as position 0 and is associated with storage locations 0 in memories 70 and 72. Thus, in step 103, the memory indexing register, X is set to zero.

Steps 104 through 108 include the procedure for handling the tracking error. At step 104, the tracking error, i.e., the digitized data word representing the difference between the expected galvanometer position and its actual position is read onto data bus 66 and into ALU 68. At step 105, this data word is combined with the contents of storage location X of tracking memory 70 to form a data word referred to as TSUM. (Initially, during the first revolution of disc 10, the contents of storage location X of memory 70 will be zero.) At step 106, the procedure asks if the system mode calls for an update of the contents of tracking memory 70. During the initial revolution of disc 10, the answer will always be "yes," in order to set initial values into memory 70.In addition, it would be highly advantageous, and, indeed, one of the features of this illustrative example of the invention, to keep memory 70 dynamically updated, continuously during the data retrieval operation. Thus, step 107 will almost always be entered, and the updated error data word, TSUM, will be stored into location X in tracking memory 70. TSUM is then presented to DIA 74 where it causes the appropriate galvanometer drive signal to be generated by driver 52. This completes the tracking error handling for the Xth position.

Steps 109 through 113 correspond exactly to steps 104through l08withregardtohandlingthefocus error. These steps (109 through 113) are executed after switches 62 and 76 are positioned to read the focus error signal and to generate the focus drive signal.

At step 114, the procedure awaits the next timing pulse from encoder 18. When it is received, the value of X is incremented at step 115 and, if X is not equal to 1,024, the tracking error handling procedure for the next position of disc 10 is entered at step 104. If X is incremented to 1,024, indicating that the index mark is present, the X register is reset to zero at step 103, and the tracking error handling procedure ensues.

Thus, it is seen that error reduction circuit 60 responds to error signals generated by error signal generators 40 and 42 to provide drive signals to tracking driver 52 and focus driver 54. Tracking memory 70 and focus memory 72 permit the storage of the expected error values of the tracking and focus positions, respectively, for a large number of locations around disc 10. Finally, the procedure of Fig. 3, executed by controller 56, which issues appropriate commands to ALU 68 and memories 70 and 72, permits the stored error values to be dynamically updated at each revolution of disc 10, thereby minimizing the error signals generated by generators 40 and 42.

In an earlier paragraph, filters 44 and 48 were described as having low pass characteristics. In addition, using the fixed interval error update procedure described above, filter 44 may be choosen so as to minimize the root-mean-square tracking eror in the presence of noise generated by photodetector 38. In this sense, the error reduction apparatus of the present illustrative embodiment of the invention comprises a Kalman filter with respect to tracking errors. Similarly, filter 48 may be chosen so as to minimize the root-mean-square focus error in the presence of photodetector noise. Hence, the error reduction apparatus may also comprise a Kalman filter with respect to focus errors.

While the principles of the present invention have been demonstrated with particular regard to the illustrative structure of the figures, it will be recognized that various departures from such illustrative structure may be undertaken in practice of the invention. The scope of this invention is thus not intended to be limited to the illustrative structure disclosed herein.

Claims (6)

1. A system for playing back information recorded on a surface of a rotating disc medium where each of one or more parameters relating to said medium is to be urged from a detected condition toward a desired condition during continuing rotation of said medium comprising servomechanism means, which includes:: (a) sensing means responsive to conditions of parameters for producing respective, continuing error signals, which represent deviations of parameter conditions from desired conditions, (b) measuring means responsive (i) to said error signals and (ii) to a position signal representing the angular rotational position of said rotating medium for producing for said parameters respective continuing control signals, and (c) control means responsive to said control signals for changing the conditions of parameters toward the respective desired conditions thereof;; wherein said position signal comprises trains of pulses generated during respective rotations of said medium, each pulse within each train marking passage of said medium through a respective angular position of said medium and said measuring means includes encoding means responsive upon the occurrence of position signal pulses within each train for encoding the level of each continuing error signal into respective error data words, one or more data storage means each corresponding to a respective parameter with each data storage means having locations corresponding to a respective angular position of said medium and to a respective pulse within each train of pulse from said position signal and each location in each data storage means storing a respective historical data word, calculating means operative upon the occurrence of each pulse in each train for combining each error data word with an historical data word taken from a respective location of a respective data storage means into a control data word and decoding means operative upon the occurrence of each pulse in each train for converting each control data word into a respective one of said continuing control signals.

2. A system according to claim 1, wherein; said calculating means includes means for replacing in each location of each data storage means the historical data word stored therein with a respective control data word.

3. A system according to claim 1 or 2 wherein: said encoding means includes: (a) a single analogto-digital converter for producing error data words corresponding to respective parameters, and (b) switching means operative upon the occurrence of each pulse in each of said trains for admitting in sequence said error indicating signals to said analog-to-digital converter, in oder to produce for each pulse in said train a sequence of error data words pertinent to respective parameters; said calculating means responds to each said sequence of data error words by producing a sequence of said control data words which correspond to respective parameters; and said decoding means includes: (c)a single digitalto-analog converter responsive to control data words in each sequence for producing in sequence respective samples of said control signals, (d) sample-and-hold circuits for receiving respective control signals samples and for producing at outputs thereof respective ones of said continuing control signals and (d) additional switching means operative upon the occurrence of each pulse in said train for distributing control signal samples to respective sample-and-hold circuits.

4. A system according to claim 1,2 or 3, which includes a lens for focusing a beam of light on said surface and wherein one of said system parameters is the distance between said lens and said surface, said control means includes means for adjusting said distance, and said error signals include a focus error signal, and said measuring means responds to said focus error signal by producing one control signal for application to said distance adjusting means.

5. A system according to claim 1,2,3 or 4 wherein: said information is recorded on said surface in elongated spaced-apart tracks; means is provided for causing a light beam to follow a selected one of said tracks; one of said system parameters is the accuracy of following a selected one of said tracks; said error signals include a tracking error signal; and said control means includes means for adjusting the position of said focused light beam relative to a width dimension of said selected track, and said measuring means responds to said tracking error signal by producing one of said control signals for application to said tracking adjustment means.

6. A disc record playback system substantially as hereinbefore described with reference to Figures 1 to 3.